Neptune: From Grand Discovery to a World Revealed: Essays on the 200th Anniversary of the Birth of John Couch Adams (Historical & Cultural Astronomy) 3030542173, 9783030542177

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Historical & Cultural Astronomy Series Editors: W. Orchiston · M. Rothenberg · C. Cunningham

William Sheehan Editor-in-Chief Trudy E. Bell Carolyn Kennett Robert W. Smith Editors

Neptune: From Grand Discovery to a World Revealed Essays on the 200th Anniversary of the Birth of John Couch Adams

Historical & Cultural Astronomy Series Editors:

WAYNE ORCHISTON, University of Southern Queensland, Australia ([email protected]) MARC ROTHENBERG, Smithsonian Institution (retired), USA ([email protected]) CLIFFORD CUNNINGHAM, University of Southern Queensland, Australia ([email protected])

Editorial Board:

JAMES EVANS, University of Puget Sound, USA MILLER GOSS, National Radio Astronomy Observatory, USA DUANE HAMACHER, Monash University, Australia JAMES LEQUEUX, Observatoire de Paris, France SIMON MITTON, St. Edmund’s College Cambridge University, UK CLIVE RUGGLES, University of Leicester, UK VIRGINIA TRIMBLE, University of California Irvine, USA GUDRUN WOLFSCHMIDT, Institute for History of Science and Technology, Germany TRUDY E. BELL, Sky & Telescope, USA

More information about this series at http://www.springer.com/series/15156

Editor-in-Chief William Sheehan

Trudy E. Bell  •  Carolyn Kennett  •  Robert W. Smith Editors

Neptune: From Grand Discovery to a World Revealed Essays on the 200th Anniversary of the Birth of John Couch Adams

Editor-in-Chief   William Sheehan Independent Scholar Flagstaff, AZ, USA

Editors Trudy E. Bell Sky & Telescope Lakewood, OH, USA

Carolyn Kennett Independent Scholar Helston, Cornwall, UK

Robert W. Smith Department of History and Classics University of Alberta Edmonton, AB, Canada

ISSN 2509-310X     ISSN 2509-3118 (electronic) Historical & Cultural Astronomy ISBN 978-3-030-54217-7    ISBN 978-3-030-54218-4 (eBook) https://doi.org/10.1007/978-3-030-54218-4 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Frontispiece. This image, obtained on June 18, 2018, during commissioning of the narrow field mode of the MUSE (Multi Unit Spectroscopic Explorer) spectrograph on the European Southern Observatory’s Very Large Telescope (VLT) array, located high in the Atacama Desert of Chile, is one of the best ever taken from the earth (Credit: MUSE Consortium and ESO)

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“Let us all praise famous men.” In memory of our colleagues, friends, and three great students of Neptune, whose influence and contributions we here gratefully acknowledge: Richard Baum (1930–2016) Bradford A. Smith (1931–2018) Craig B. Waff (1946–2012)

Foreword

The first sighting of the planet Neptune, by Johann Gottfried Galle and Heinrich Louis d’Arrest at the Berlin Observatory on September 23, 1846, was the culmination of one of the great international quests in scientific history. An Englishman and a Frenchman had independently calculated the position of the Uranus-disturbing planet, while a German clinched the achievement by giving Neptune—as it would later be christened—its “first light" of visual recognition. Then, American mathematicians were central in computing its orbit from historical sightings. The discovery seemed to provide a resounding proof for the forensic power of Newtonian gravitation, in so far as Newton’s theory was needed to predict the position of a hitherto unknown body, by means of the gravitational behavior of a known one. But as Dr. William Sheehan so clearly points out, the whole Neptune discovery saga right back to the I840s has been the focus for all manner of controversy. Controversies included Anglo-French exchanges, apparently a hero and villain polarization of the various personalities involved, along with a trail of conspiracy theories. And needless to say, twentieth-century popular science writers have had a field day with it, for—in the “right hands”—Adams’s and Le Verrier’s calculations and their consequences can be crafted into a fine thumping tale, replete with missed clues and opportunities, mutual jealousies, and dark dealings! I personally have had a fascination with the whole affair since my youthful reading of Morton Grosser’s The Discovery of Neptune (1962), which for years was the definitive book on the subject. But then, more recent scholars recognized that the only credible way forward was to return ad fontes, to the primary sources. In particular, these included the vast archive of Sir George Biddell Airy, the Astronomer Royal, now classified under “RGO 6,” including his daily “Journal.” Once stored at the Royal Observatory archive at Herstmonceux Castle, Kent, they now reside in the Cambridge University Library, along with parallel documents now preserved in Cambridge, Paris, Berlin, and elsewhere. And from this scholarly approach, a much more balanced interpretation began to emerge, especially after the missing “RGO 6 Neptune file” was brought back from Chile to where somehow it had wandered, to provide a resounding refutation of the myth that the guilty Airy had destroyed it. ix

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William Sheehan; his fellow editors Trudy E. Bell, Carolyn Kennett, and Robert W. Smith; and their six additional contributors have resolved to follow a true path through the surviving scholarly literature pertaining to the Neptune discovery, to give a modern, balanced evaluation of the whole affair. Each contributor approaches from a particular perspective and places the discovery of Neptune within a precise historical context. The English, French, German, and American contributions are carefully discussed, along with the scientific and other motives displayed by the individual astronomers. Yet, what long puzzled me is why John Couch Adams did not announce his 1845 Neptune calculations to the world in The Times newspaper, as he had done on October 15, 1844, with his calculations pertaining to de Vico’s comet. For this would have secured the unequivocal priority and prestige for the young fellow of St. John’s College, Cambridge. Yet for some reason, he never did so. Nor was it clear why Adams never took his computed planet position to the superbly equipped, self-funded, and independent community of British “Grand Amateur” astronomers, several of whom owned telescopes of much greater power and size than those in the Cambridge University and Greenwich observatories and who had no public or academic duties to occupy their time. Such gentlemen constituted most of the Fellowship of the RAS and of the Royal Society, and Adams must either have met many of them personally or read their journal publications. The first Englishman to sight Neptune, on September 30, 1846, working from data which had reached England from the Continent, was John Russell Hind, employed director of the privately owned South Villa, Regent’s Park, Observatory. Then, reading about the Berlin discovery, William Lassell, FRAS and FRS (1849), the Liverpool Grand Amateur, awaited the next clear night, so that he could direct his mighty 24-inch-aperture, 24-foot-focal-length equatorial reflector to the correct place in the sky. Lassell’s notebooks in the RAS Library suggest that on the night of October 2–3, 1846, he saw the planet immediately, its disk shape making it instantly recognizable. Then, upon what was probably the next good night, October 10, 1846, Lassell not only immediately found Neptune shining brightly but also saw its large satellite, subsequently christened Triton, along with what he (incorrectly) believed to be a ring encircling the new planet. Likewise, Lord Rosse, an astronomical friend of Lassell’s, with his newly operational 72-inch-aperture “Leviathan,” would have seen Neptune at first glance had he been asked to look for it. So why did Adams never tap into this extensive, and wholly accessible, astronomical resource of serious and superbly equipped British “Grand Amateur” astronomers? This book reexamines these and other questions. Bill Sheehan and his colleagues have forged a clear path through almost two centuries of Neptunian history. That history extends from first puzzlement regarding Uranus’s orbital behavior through the brilliant analyses of mathematicians, the controversy surrounding the discovery, and the subsequent myths and conspiracy theories. This book achieves an impartial scholarly appraisal of one of astronomy’s greatest sagas. Allan Chapman

President of the Society for the History of Astronomy London, UK

Introduction: A Century and a Half of Discovery and Controversy

The major planet Neptune, outermost ice giant of the outer Solar System, was telescopically discovered by Johann Gottfried Galle and Heinrich Louis d'Arrest at the Berlin Observatory in September 1846, as a direct consequence of the calculations of the French mathematical astronomer Urbain Jean Joseph Le Verrier predicting where to point the telescope. The find—which instantly expanded the size of the known Solar System by half again—was immediately hailed as one of the most remarkable in nineteenth-century astronomy and indeed in all the sciences. Why was it deemed so important? Thirteen years before, one of the great polymaths of the nineteenth century, William Whewell, had argued that (1833: xiii) Astronomy is not only the queen of the sciences, but, in a stricter sense of the term, the only perfect science; —the only branch of human knowledge in which particulars are completely subjugated to generals, effects to causes … and we have in this case an example of a science in that elevated state of flourishing maturity, in which all that remains is to determine with the extreme of accuracy the consequences of its rules by the profoundest combinations of mathematics, the magnitude of its data by the minutest scrupulousness of observation; in which, further, its claims are so fully acknowledged, that the public wealth of every nation pretending to civilization, the most consummate productions of labour and skill, and the loftiest and most powerful intellects which appear among men, are gladly and emulously assigned to the task of adding to its completeness.

And what more spectacular demonstration could there have been of “the most consummate productions of labour and skill” than the prediction of the existence of, and subsequent discovery of, a major planet? Here was a stunning example of the abilities of the “loftiest and most powerful intellects which appear among men” brilliantly adding to astronomy’s “completeness.” Neptune’s discovery had thereby strengthened the belief among astronomers as well as “men of science” (to use the nineteenth-century term for scientists) of the truth of the established order in the workings of the cosmos. That order was set on the foundations of Newton’s law of universal gravitation, for it was calculations exploiting that law that had disclosed Neptune. When Charles Darwin came to pen the final sentence of his On the Origin of Species by Means of Natural Selection…, xi

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published in 1859, his momentous account of the working of laws in the natural world, it was no accident he argued (1859: 490), “whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and wonderful have been, and are being evolved.” The discovery of Neptune, then, as important as it was, was so for reasons that are not always easy for us to grasp from our early twenty-first-century vantage point. Now Newton’s universal gravitation, which seemed unassailable then, has been replaced by Einstein’s general relativity, and the detection of additional bodies belonging to the outer Solar System (Kuiper belt objects, KBOs) and even of entire systems of planets (exoplanets) orbiting distant stars has become commonplace. It is also hard to recall the international rivalries that caused the discovery to be immediately engulfed in controversies, at first mainly involving fierce disputes about priority. These disputes subsided once a carefully crafted compromise had been negotiated allowing the Frenchman Le Verrier and the hitherto unknown English mathematician John Couch Adams to be regarded as mathematical co-­ discoverers of the planet. The compromise seemed to hold for over a century, with a consensus version of the story widely accepted and reinforced by works published during the centennial year of the discovery (1946). But then, in the 1960s, vital papers went missing. The so-called Neptune file meticulously assembled and kept by the Astronomer Royal George Biddell Airy had been “misplaced” by a highly distinguished stellar astronomer and occasional historian of astronomy, Olin Eggen, who had been referring to it while writing biographies of Airy and James Challis. Though suspicion that he was the one who had absconded with the file was general, he repeatedly denied it, and even the authorities at Cambridge and the Royal Greenwich Observatory were careful not to lay any finger of blame, for fear that exposure of the fact might lead to the file’s destruction. Only after he died in 1998 did one of his graduate students discover it in his flat in Chile. After a more than 30 years’ absence, it was carefully packed up and returned from Chile to England under the personal care of the librarian of the Cambridge University Library, Adam Perkins. Thirty years is a long time, and in the meantime, researchers who had wished to consult the file had not only been inconvenienced, they had found ample room to exercise their suspicions and imaginations. The main question was this: Did members of the British scientific establishment, in fact, rewrite the history and steal Neptune from the French? Without the original documents, and in a post-Watergate world, it did not seem impossible. History, like nature, abhors a vacuum, and a number of authors with somewhat paranoid and conspiracy-favoring tendencies sensed a scandal and exercised rather remarkable ingenuity in working out and promoting sensationalistic revisionist theories. Without the ability to refer to missing documents, it was difficult to refute them. Had they been right, this might well have been turned from one of the great triumphs of nineteenth-century science to a complete dismantling of the reputations of “eminent Victorians” Airy and Challis who, whatever their shortcomings, had always seemed to value in the highest degree traits of hard work, conscientiousness, and moral probity. But they were not right. The file’s recovery put paid to their

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t­heories. A close study of the original documents filled in a few minor elisions but frankly offered very little that was genuinely new. Instead, the remarkable accuracy of the transcript published by the scrupulous and obsessive Airy himself soon after the discovery was confirmed. The file, which had always seemed an unimpeachable source of dates and detail, once again could be referred to with confidence, given secure anchoring to studies of the search for and discovery of Neptune that would otherwise have been completely at sea. The British, though they might have tried to spin things in a favorable way to help ensure that their countryman, Adams, got some share of the credit, did not steal Neptune from the French and then try to cover up their crime. There was no conspiracy. The conspiracy theories need not concern us further here: they have now been thoroughly debunked (Hutchins, 2008: 91–95). Having laid aside these imaginary controversies, we can now turn our attention to the real controversies which remain. For the story, though remaining one of the most celebrated and oft-retold in the history of astronomy, is also one of the most complicated. It has a captivating theme—the mathematical discovery of a planet that had never yet been seen through the telescope. As with any such story, this one has multiple strands. The original documentation of the nineteenth century (including Airy’s “Neptune file”) provides the bedrock evidence, but that documentation is extraordinarily vast. And although that documentation—especially Airy’s account to the Royal Astronomical Society (RAS) which was derived from the file—may be regarded as “eyewitness” in some sense, here, as everywhere else, what is seen depends on the viewpoint of the beholder. Does one look at such documentation with a microscope or a fish-eye lens? What is the appropriate eyepiece with which to detect Neptune in the telescope? There were major negotiations even at the time of the discovery (beginning, not least, with Airy) to allocate “priority” and to establish a synoptic view of what had actually taken place. The international controversy that embroiled British and French astronomers—tentatively resolved by a carefully brokered consensus to share credit between the two co-predictors, Adams and Le Verrier—may not seem as interesting today as it did once. But—as with the controversy in our own time about Pluto’s planetary status—it generated enormous contemporary interest, some of it doubtless of the tabloid variety, at the time. And anyone who is interested in how social and cultural contexts give shape to science will recognize the importance of uncovering some of the layers, as archaeologists or geologists painstakingly uncover the strata that hold clues as to the meaning of the past. Major reassessments of the discovery of Neptune occurred for the centennial in 1946 and sesquicentennial in 1996 (though, in the latter case, these were somewhat marred by the missing file and the resulting conspiracy theories). There was a brief revival of interest again in 2011, when the bicentennial of Le Verrier’s birth was celebrated and the first Neptunian “year” since the discovery was observed. We have now (in 2019) just celebrated the bicentennial of Adams’s birth, and so the timing seems right for another reassessment. In addition to being able to refer again to the original Airy file, which is now in the care of the Royal Greenwich Observatory archives at the Cambridge University Library, many other documents, not available to previous researchers, have been recovered and put in some s­ emblance

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of order. Significantly, these have included not only British accounts—which, because of Airy’s central role and to a lesser but still significant extent that of John Herschel and a few others, had always been rather overrepresented—but also those of French and German astronomers. Here for the Neptune story, we trace origins, the shoulders stood upon, and then move forward. The result has been the introduction of a trove of fresh documentation. In addition, thanks to the efforts of scholars in several countries, many of whom are contributors to the present volume, we now have a much broader perspective from which to view the discovery of Neptune than ever before and a better understanding of what the Neptune story tells us about the practices of science in the mid-­nineteenth century. It is also fair to say that the methodology of historians of science has advanced greatly since 1846 or even 1946. In particular, rather than merely chronicling the past, historians of science have become much more adept at understanding the processes by which scientific knowledge is shaped. Instead of focusing only on individual “heroes,” historians have come to realize the importance of the way that a specific scientific problem like the errant motion of Uranus, which led to the discovery of Neptune, brings together a group of individuals in an interactive network. (An early example of this kind of research is found in Susan Faye Cannon’s paper “Scientists and Broad Churchmen: An Early Victorian Intellectual Network” [Cannon, 1964] which provides a model for Robert W. Smith’s Chap. 7 here. [Also see Smith, 1989.]) The cultures of specific scientific institutions, conditioned by social conditions and the purposes for which they were established, must be considered. The important British institutions were Cambridge University (where Adams did his calculations), Cambridge University Observatory (which had largely been set moving in a positive direction by Airy but where his successor, James Challis, was overwhelmed with responsibilities before undertaking the first telescopic search), and the Royal Observatory, Greenwich (where—as stressed by Professor Allan Chapman [Chapman, 1988]—the public service aspect of time-keeping and supporting navigation precluded a purely speculative research purpose such as searching for a planet). The Paris Observatory and the Bureau of Longitudes were the corresponding centers in France. And in Germany, which was at the time not even a unified country, the Berlin Observatory played the key role of actually detecting the planet in the heavens. Since the network of individuals involved in the calculation and eventual discovery was indeed international and the correspondence remarkably extensive and rapid in those days of letter-writing (and post-1815 peace in Europe), there are many threads of communication across the entire European community of scholars inviting reconstruction. Fortunately, the documentation available is extensive enough to allow “fine-grained" analyses of events, personalities, and contingent developments. Cannon (ibid.: 30), thinking of the middle of the nineteenth century in general, remarked, “we possess the most complete documentation, for selected individuals, not only that ever has existed but that ever will exist.”

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As a characterization of the middle of the nineteenth century, this still seems true. On the basis of this extensive documentation, a number of exhaustive studies have appeared, including Martin J. S. Rudwick’s The Great Devonian Controversy (Rudwick, 1985) and James A.  Secord’s Victorian Sensation (Secord, 2003). Perhaps with the exception of the publication of the Vestiges of the Natural History of Creation, no other scientific event was more vigorously discussed during the first part of the nineteenth century, in Britain at any rate, than the discovery of Neptune. But the discussion was not only British nor even only British and French. The Germans played a much more significant role than has previously been recognized; they were not merely opportunists who happened to have a close prediction and a good map. And Americans were also involved, as the discovery happened just as American astronomy, hitherto woefully lagging behind European achievements, began to come into its own as a first-rank power of astronomical research. We are only too keenly aware of the truth of the old adage Ars longa, vita brevis. The amount of material available to Neptunian scholars is both a blessing and a curse. Such rich materials allow the construction of an almost Dickensian canvas, with an array of characters fully the equal of any found in a Victorian novel. History here challenges narrative skill to the utmost and demands something of the art of the novelist, rather than tacking backward from the known ending of the story, as conspiracy theorists and Whiggish historians do. But tempting as it is, when presented with such inexhaustible resources, to take shortcuts, we have resisted that temptation. In particular, we have resolved to abstain from glib character analyses, facile attribution of motive or blame, and all species of retrospective analyses in general. Rather, we have attempted, as far as humanly possible, to remain faithful to what the scientists themselves thought they were up to at the time. We haven’t avoided what might be referred to as “reverse prediction” entirely, but as a rule, it’s a clumsy device—rather like the appearance of Time between the third and fourth acts of Shakespeare’s The Winter’s Tale—and we have tried to avoid it. We have striven always to represent the perspectives of the individuals involved as they themselves experienced them and of course provided the international point of view needed given the universal (or at least trans-European) significance of Neptune’s discovery. To some extent, no doubt, we have failed or fallen short. But at least it is useful to state clearly our aspirations. A few comments regarding intended audience: Determining this is a struggle for any single author, and the difficulties are multiplied in a multiauthored work. The discovery of Neptune is at once one of the best-known and most attractive subjects in the history of astronomy—thus one which has always had wide popular appeal— but it is also one in which the predictions depended upon highly technical mathematical methods (classical perturbation theory). Thus, we have had to find a compromise between appealing to a broader general audience on the one hand and to a highly sophisticated, scholarly, but potentially small one on the other. We have tried our best to strike the right balance. In general, however, we have thought it better to err on the side of the more general reader—for instance, in Chap. 1, which provides an overview of topics in celestial mechanics which are needed as background to understanding the later chapters. Our success is for the reader to decide.

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In outline, the contents of the book are as follows. Foreword—Allan Chapman Introduction: A Century and a Half of Discovery and Controversy—William Sheehan Chapter 1. Preliminaries to the Neptune Discovery: Newtonian Gravitational Theory—William Sheehan Since the discovery of Neptune represents a high point in the history of celestial mechanics, Isaac Newton’s theory of gravitation is in a real sense a major “hero” of this story. We present some of the backgrounds involving Newton and his contemporaries and explore the way that, in the century after Newton published his Principia in 1687, Newtonian gravitational theory—largely through elaboration into the complex methods of perturbations achieved by a series of brilliant mostly French successors—came to acquire its tremendous prestige. Chapter 2. Planetary Discoveries Before Neptune: From William Herschel to the “Celestial Police”—William Sheehan and Clifford J. Cunningham By the end of the eighteenth century, the Solar System was, at least by present standards, a rather simple affair. The outermost planet was still, as it had been in antiquity, Saturn. The planets, their moons, and a few periodic comets like Neptune that moved in very eccentric orbits round the Sun could be fairly represented as moving according to clockwork. Then in 1781, William Herschel discovered a new planet, Uranus, and at a single stroke doubled the scale of the known Solar System. Guided by a strange but seemingly valid “recipe,” the Titius-Bode law that gave the relative distances of the planets and to which Uranus was quickly accommodated, astronomers began to search for additional planets in the place of a missing number between Mars and Jupiter. Their search bore fruit with the discovery of the first four “minor planets.” Meanwhile, celestial mechanicians rather confidently set out to calculate Uranus’s orbit. By the early 1800s, Uranus was known to be veering off course. Astronomers gradually became aware of a “crisis” in gravitational theory that would be resolved only by the discovery of a more remote planet whose existence could be disclosed only by its disturbances on Uranus. By the early 1820s, elite mathematicians across Europe were aware of the challenge. Chapter 3. John Couch Adams: From Cornwall to Cambridge—Brian Sheen and Carolyn Kennett Of the several mathematicians aware of or attempting to skirmish with the problem of Uranus, only two, unknown to each other, made serious attempts to attack it. They were John Couch Adams and Urbain Jean Joseph Le Verrier. One did so as a matter of private research; one was set upon it. By 1845, Le Verrier had already made his mark in research in his professional post at the Paris Observatory and thereby gained the support of his director, François Arago, who set him full-time on the Uranus problem. Adams, who in contrast to Le Verrier has not yet been the subject of a scholarly biography, grew up in somewhat impecunious circumstances in Cornwall; but he was nurtured, encouraged, and supported by a loving family so

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that, while quiet by nature and inconspicuous because he was poor and diligent rather than a social set rowdy as a Cambridge undergraduate, he was self-assured and ambitious for his mathematics which he saw as a path to helping his hard-­ pressed family. Thus, he possessed assurance and ambition which allowed him to flourish in the very demanding academic milieu at Cambridge and to tackle one of the most challenging and complex problems in astronomy as described in Chap. 4. This chapter’s explanation of Adams’s pre-Cambridge years is thus essential to a full understanding of him and his subsequent achievements. Chapter 4. John Couch Adams: From a Senior Wrangler to the Quest for an Unknown Planet—William Sheehan This chapter explains for the first time the onerous studies of an ambitious Maths Tripos undergraduate at Cambridge and the dominating duties and responsibilities required of him as a junior fellow and tutor, for which he was paid. These duties and responsibilities, which Adams took very seriously and prioritized above all else, constrained his personal research to vacations and inhibited the evolution of his relations and correspondence with senior astronomers such as Cambridge University Observatory astronomer James Challis and the Astronomer Royal George Biddell Airy. As early as 1841, while still an undergraduate at St. John’s College at Cambridge, he became intrigued by the problem of Uranus’s wayward motion, but only in 1843, when he completed his degree with high honors and was appointed a fellow at St. John’s, did he take it up in earnest. Doing almost all the work during the vacations, he completed several calculations which gradually refined his solutions, until in October 1845, he had the solution later to become famous as it identified the position of the planet within two-and-a-half degrees of its discovery position. He then tried, without success, to meet the Astronomer Royal George Biddell Airy at the latter’s residence at Greenwich and to present the results of his researches. The well-known stories of his near miss and subsequent failure to respond to Airy’s “radius vector question” are reanalyzed here in terms of newly discovered documentation and challenge long-held stereotypes of Adams’s relationships with Challis and Airy. Chapter 5. Urbain Jean Joseph Le Verrier: Predictions Leading to Discovery— James Lequeux This chapter, based on the magisterial biography of Le Verrier (Lequeux, 1990), outlines how the French mathematician—older and better established than Adams and with no knowledge of the latter’s existence—set out on his own extensive calculations and arrived at a set of elements and a position for a planet beyond Uranus which he published in June 1846. It was this publication, coming into the hands of Airy, who immediately recognized its similarity to that Adams had dropped off at Greenwich the previous October, that led to a sudden acceleration of activity that included the organization of a search over that summer by Challis with Cambridge’s large Northumberland refractor, the publication of a revised calculation and position by Le Verrier at the end of August, and events set in motion that would lead to the planet’s discovery in Berlin.

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Chapter 6. “That Star Is Not on the Map”: The German Side of the Discovery— Davor Krajnović The planet was discovered at the Berlin Observatory after a short search of the region of the sky indicated by Le Verrier, by Johann Galle and Heinrich d’Arrest on September 23, 1846. They had an advantage over Challis in that they were in possession of the latest and by chance the most relevant one of a set of Berlin star charts that had not yet been published to astronomers elsewhere. Of the multiple threads of the Neptune story, this one is in some ways the least known, and this chapter draws extensively on German primary and published sources to contextualize the discovery within the wider culture of German astronomy during this period. Chapter 7. Clashing Interests: The Cambridge Network and International Controversies—Robert W. Smith Immediately after the announcement of the discovery of the new planet in Berlin and while Airy was still vacationing on the Continent, a tremendous national and international furore erupted when several Cambridge alumni or university grandees—not including Adams and not all astronomers—immediately and without coordination claimed co-discovery for him, along with the right to name the planet. These claims, and the subsequent correspondence underlying them, are clear evidence of an informal “Cambridge network” of Cambridge University alumni, fiercely loyal to and like-minded by their social conditioning and common mathematical training and achievements, who saw this discovery as a particularly vital Cambridge interest and thereby an English one. The activities of the Cambridge network, which significantly were launched only after Neptune had been recognized from Berlin, raise the whole question of what constitutes priority in discovery. The network’s patronage and diverse interests, and eruption into rare and wide public visibility, brought to the fore the somewhat embarrassing question as to why other British astronomers, not associated with Cambridge, were not encouraged to join the search for the predicted new planet. Airy, as Astronomer Royal with an extensive array of correspondents both in Britain and the Continent, was in a unique position and took a complete grip in managing from a national point of view the disastrous situation of the discovery having been made in Berlin on the basis of a French mathematician’s calculations. He largely succeeded in controlling the narrative and assuring that Adams received a negotiated share of the credit. This chapter, documented with much private correspondence revealing how astronomers and participants really felt about these most public events, thus provides the most complete explanation so far offered of the English claims on Neptune, within the contemporary context of social status, deference, Cambridge elitism, nationalism, and international attitudes to discovery and priority in this era of revolution in the sciences, so that the participants are understood within their own milieu. Chapter 8. Neptune Examined: William Lassell, a Satellite, and Neptune’s “Ring”—Robert W. Smith and (the late) Richard Baum William Lassell was one of the greatest of the “Grand Amateurs,” men of wealth and independence who built large telescopes and wielded them largely in pursuit of their

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own personal researches (in contrast to the national observatories whose duties and functions were more circumscribed). He was not a Cambridge man, but he was a close friend of several, especially Sir John Herschel, and was just deploying a new large telescope when the planet came to light in Berlin. Ambitious and wanting to burnish his own credentials, Lassell set out at Herschel’s bidding to try to discover a satellite of Neptune, an important and useful challenge as if successful it would lead directly to calculation of the planet’s mass. The latter was not only of interest in its own right but would provide a check on the calculations of Le Verrier and Adams. He succeeded within a week in discovering Triton, as Neptune’s large satellite was later to become known. Nevertheless, this proof of Lassell’s ability and instrument quality raises the question of why others apart from Challis were not primed to search. While attempting to confirm the existence of Triton, Lassell was vouchsafed impressions of something more—a possible ring of Neptune. As explained here, the latter has nothing to do with the planet’s actual system of spidery rings and was eventually recognized to be nothing more than an illusion produced by spurious optical effects. Chapter 9. Neptune’s Orbit: Reassessing Celestial Mechanics—William Sheehan and Kenneth Young In the immediate aftermath of the discovery of Neptune, the calculation of its orbit was of the utmost importance. Several of the most important investigators were Americans, one of whom was Sears Cook Walker, whose orbit was, in a rather alarming degree, dissimilar to those predicted by Le Verrier and Adams. Walker’s researches led another American, Benjamin Peirce, to question the validity of the European calculations altogether and to maintain that the discovery of Neptune had been a mere “happy accident.” Adams attempted to refute Peirce’s claims, and later, astronomers continued to press the same methods that had led to the spectacular discovery into service in the quest for additional planets. Le Verrier analyzed Mercury’s anomalously precessing perihelion in terms of an inner planet, “Vulcan.” Other astronomers, notably Percival Lowell, extended the purview of celestial mechanics into the outer Solar System and used Adams’s and Le Verrier’s methods to try to track down a planet beyond Neptune. It has only been since about 1990, with the advent of newer methods of doing celestial mechanics, that it has finally become possible to determine the limits of validity of the methods Le Verrier and Adams used. Their calculations were valid—but only within rather specific circumstances that were realized in the decades before and after Uranus’s and Neptune’s conjunctions in 1821 (Lai, Lam, and Young, 1990). Thus, though not exactly in the same sense argued by Peirce, the discovery of Neptune was indeed a “happy accident,” and the approach that led to its discovery was a one-off that has never been repeated. Meanwhile, Vulcan has been shown to be nonexistent, and the anomalies of its motion satisfactorily accounted for by Albert Einstein’s general theory of relativity. Hence, this chapter reassesses for the first time the methods of both Adams and Le Verrier and the unknown coincidences in celestial mechanics that allowed their successful calculations and also helps to explain how their discovery motivated the extraordinary efforts of elite astronomers to reprise this success in the decades that followed.

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Chapter 10. Neptune Visited and the Outer Solar System Revolutionised, 1989–2019—William Sheehan The discovery of Neptune marked the end of one quest—that of satisfactorily explaining, in terms of Newtonian gravitational theory, the wayward motions of Uranus—and the beginning of another, the quest to fill in the map and understand the nature of the contents of the outer Solar System whose icy precincts Neptune, as the outermost of the giant planets, bounds. Though the attempt to find planets beyond Neptune by analyzing remaining “residuals” in the motions of Uranus and Neptune would prove unsuccessful, Clyde Tombaugh, pursuing with dogged determination and thoroughness an empirical photographic survey of the sky in search of Percival Lowell’s putative “Planet X,” did manage to find a small “planet,” Pluto, in 1930, which for a time was thought perhaps to answer to the description. We now know that Pluto is too tiny to have produced any significant disturbances in Uranus or Neptune; its discovery was indeed a “happy accident”—and, as is now known, Tombaugh had merely stumbled upon by far the brightest member of the Kuiper belt of icy objects that swarm beyond Neptune’s orbit. Since 1990, the use of powerful new imaging technologies and gigantic telescopes has rapidly begun to fill in what had hitherto seemed nearly empty spaces with a plethora of icy debris known as Kuiper belt objects (KBOs). They represent a complicated bestiary, and their characteristics provide important clues to the history of how the Solar System came to acquire the structure it has. This brings us to an exciting frontier that has begun to be explored only in the last twenty-some years, in which giant planet migrations in the early Solar System played a role, and which tantalizingly hints at the existence of at least one more giant planet, far beyond Neptune. Neptune was a high point of the history of astronomy of the nineteenth century. If past is prologue, the future chapters of this story remaining to be told in the twenty-first century may well be more exciting than those that have already been told. Flagstaff, AZ, USA  William Sheehan

Acknowledgments

Allan Chapman of Wadham College, Oxford, has very generously written a foreword for the book. What Allan brought to the table long ago was his unique appreciation of Airy’s personality, official role and responsibility, and modus operandi. In the aftermath of Neptune’s discovery, Airy was scapegoated and has been derided by too many historians since (just as Challis has). For more than 30 years, Allan has quietly used every opportunity to emphasize a holistic understanding of the man beyond the Neptune affair which caricatured him and also his very many achievements. We hope that something of Allan’s work in getting us to understand this history and its players, appreciate the science, and perceive stumbles by several of the principal actors as just the human errors we all make has informed what we have written here. Carolyn Kennett and Trudy E.  Bell have been far more than editors and have gilded everything they touched. Carolyn contributed very many thoughtful editing suggestions and query resolutions, especially the vast painstaking work of standardizing the multiauthor text in order to achieve enjoyable flow for the great and complex narrative offered here. Trudy contributed her expertise regarding the role that Neptune played in the early years after its discovery in helping launch American astronomy onto the international stage. She sparked a friendly debate, which feeds significantly into Chap. 9. Our colleague, Roger Hutchins, has generously given of his limited time and unlimited expertise, reading and commenting on the manuscript several times. Bernard J.  Sheehan helped produce the excellent translation into English of James Lequeux’s biography of Le Verrier, which we have followed here. Guy Bertrand and Jacques Laskar at the Institut de mécanique céleste et de calcul des éphémérides (IMCCE) in Paris have shared research in progress and greatly enriched the discussion of the perturbation methods used by Le Verrier. We are grateful to H. M. Lai and C. C. Lam for allowing us to summarize and use figures from their 1990 paper with Kenneth Young, which marked a new era in understanding the limits of the calculations whereby Le Verrier and Adams predicted the position of Neptune. In the rapidly developing area of current research into the contents and evolutionary history of the outer Solar System, Dale P. Cruikshank of NASA Ames, Konstantin Batygin of Caltech (California Institute of Technology), xxi

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and Greg Laughlin of Yale provided indispensable insights. They also generously shared researches in progress without which Chap. 10 could never have been written, as well as making accessible some very esoteric regions of higher mathematics. In addition, much help was provided in finding illustrations, as duly acknowledged in the credits. However, we are especially grateful to Lauren Amundson, archivist at Lowell Observatory, for her kind assistance.

References Cannon, S.A., 1964. Scientists and Broad Churchmen: an early Victorian intellectual network. Journal of British Studies, 4:65–88. Chapman, A., 1988. Private Research and Public Duty: George Biddell Airy and the Search for Neptune. Journal for the History of Astronomy, xix:121–139. Darwin, C., 1859. The Origin of Species. London, John Murray. Hutchins, R., 2008. British University Observatories 1772–1939. Aldershot, Ashgate. Lai, H.M. Lam, C.C., and Young, K., 1990. Perturbations of Uranus by Neptune: a modern perspective. American Journal of Physics, 58(10):946–953. Lequeux, J., 2013. Le Verrier—Magnificent and Detestable Astronomer. New York, Springer. Rudwick, M.J.S., 1985. The Great Devonian Controversy: the shaping of scientific knowledge among gentlemanly specialists. Chicago and London, University of Chicago. Secord, J.A., 2003. Victorian Sensation the extraordinary publication, reception, and secret authorship of Vestiges of the Natural History of Creation. Chicago and London, University of Chicago. Smith, R.W., 1989. The Cambridge Network in Action: the discovery of Neptune. ISIS, 8:395–422. Whewell, W., 1833. Address, Report of the Third Meeting of the British Association for the Advancement of Science; Held at Cambridge. London, British Association for the Advancement of Science:xi–xxii.

Contents

1 Preliminaries to the Neptune Discovery: Newtonian Gravitational Theory ������������������������������������������������������������������������������    1 William Sheehan 1.1 The Road to Neptune����������������������������������������������������������������������    1 1.2 The Inverse Square��������������������������������������������������������������������������    2 1.3 Newton’s Headache: The Theory of the Moon ������������������������������   10 1.4 Stars in the Moon’s Way ����������������������������������������������������������������   14 1.5 A Tide in His Affairs Pulls Newton Back to the Moon������������������   19 1.6 The Calculus ����������������������������������������������������������������������������������   25 1.7 The Inverse Square Law: Crisis and Resolution ����������������������������   27 1.8 The Return of the Comet����������������������������������������������������������������   29 Appendix 1.1��������������������������������������������������������������������������������������������    37 References��������������������������������������������������������������������������������������������������   39 2 Planetary Discoveries Before Neptune: From William Herschel to the “Celestial Police” ��������������������������������������������������������������������������   41 William Sheehan and Clifford J. Cunningham 2.1 A Date with Destiny: William Herschel Discovers Uranus������������   42 2.2 The Search for Pre-Discovery Observations����������������������������������   49 2.3 A Strange Progression Revealed����������������������������������������������������   54 2.4 Uranus Veers as Bouvard Calculates����������������������������������������������   63 2.5 The Stuff of Legend: Airy��������������������������������������������������������������   68 2.6 Hussey’s Brazen Idea����������������������������������������������������������������������   73 2.7 Valz and Nicolai Give Chase, while Wartmann Finds a Suspect ������������������������������������������������������������������������������   76 2.8 Astronomers Gather More Clues����������������������������������������������������   78 References��������������������������������������������������������������������������������������������������   81 3 John Couch Adams: From Cornwall to Cambridge����������������������������   83 Brian Sheen and Carolyn Kennett 3.1 A Cornish Childhood����������������������������������������������������������������������   83 3.2 Schooling of the Young Adams������������������������������������������������������   86 xxiii

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3.3 Adams the Young Astronomer��������������������������������������������������������   88 3.4 Preparations for University ������������������������������������������������������������   94 References��������������������������������������������������������������������������������������������������   98 4 John Couch Adams: From Senior Wrangler to the Quest for an Unknown Planet����������������������������������������������������������������������������  101 William Sheehan 4.1 A New Retelling of an Oft-Told Tale����������������������������������������������  102 4.2 Adams Enters Cambridge ��������������������������������������������������������������  105 4.3 Tale of the Tripos����������������������������������������������������������������������������  106 4.4 Adams Embarks on an Honourable Quest��������������������������������������  109 4.5 Adams and His Tutor����������������������������������������������������������������������  116 4.6 Uranus Looms��������������������������������������������������������������������������������  119 4.7 “A Mass of Comet Reductions I Scarcely Know How to Get Through”������������������������������������������������������������������������������  126 4.8 Adams Calculates����������������������������������������������������������������������������  131 4.9 A Place in the Sky��������������������������������������������������������������������������  141 4.10 The Radius Vector and a Year Without a Conclusion����������������������  143 4.11 The Ball Is Dropped, and a Comet Splits ��������������������������������������  149 Appendix 4.1 A Brief History of Mathematics at Cambridge ����������������   152 References��������������������������������������������������������������������������������������������������  155 5 Urbain Jean Joseph Le Verrier: Predictions Leading to Discovery����������������������������������������������������������������������������������������������  159 James Lequeux 5.1 The Problem of the Motion of Uranus��������������������������������������������  159 5.2 Urbain Le Verrier����������������������������������������������������������������������������  162 5.3 Le Verrier’s Work on Perturbations of Uranus��������������������������������  165 5.4 Neptune Discovered������������������������������������������������������������������������  169 5.5 The Competition ����������������������������������������������������������������������������  175 5.6 Janus, Oceanus, Neptune or Le Verrier? ����������������������������������������  179 References��������������������������������������������������������������������������������������������������  182 6 “That Star is Not on the Map”: The German Side of the Discovery����������������������������������������������������������������������������������������  185 Davor Krajnović 6.1 Introduction������������������������������������������������������������������������������������  186 6.2 Laying the Groundwork������������������������������������������������������������������  188 6.3 Bessel’s Verification of Newton’s Theory of Gravitation ��������������  189 6.4 Bessel’s Unfinished Project������������������������������������������������������������  194 6.5 Charting the Skies ��������������������������������������������������������������������������  197 6.6 The Discovery in Berlin������������������������������������������������������������������  204 6.7 The Aftermath ��������������������������������������������������������������������������������  215 6.8 German Contribution����������������������������������������������������������������������  229 References��������������������������������������������������������������������������������������������������  240

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7 Clashing Interests: The Cambridge Network and International Controversies��������������������������������������������������������������������������������������������  245 Robert W. Smith 7.1 A New Interpretation����������������������������������������������������������������������  246 7.2 What Is a Discovery?����������������������������������������������������������������������  248 7.3 The Cambridge Search: Clandestine or Pragmatic? ����������������������  251 7.4 Other Observers?����������������������������������������������������������������������������  260 7.5 Battles over Credit��������������������������������������������������������������������������  262 7.6 Managing the Discovery: Airy’s Matter of “Delicacy”������������������  271 7.7 Managing the Discovery: John Herschel’s Despair and Realism������������������������������������������������������������������������������������  275 7.8 What Counts as a Publication? ������������������������������������������������������  281 7.9 Exploiting the Discovery and Wider Impact����������������������������������  286 7.10 Americans Assert a Happy Accident! ��������������������������������������������  289 7.11 Conclusions������������������������������������������������������������������������������������  291 References��������������������������������������������������������������������������������������������������  293 8 Neptune Examined: William Lassell, a Satellite, and Neptune’s “Ring”������������������������������������������������������������������������������  297 Robert W. Smith and Richard Baum 8.1 A Satellite����������������������������������������������������������������������������������������  297 8.2 Discovery and Confirmation of a Neptunian Ring��������������������������  300 8.3 The Fading Vision ��������������������������������������������������������������������������  304 8.4 What Happened?����������������������������������������������������������������������������  308 8.5 Conclusions������������������������������������������������������������������������������������  312 References��������������������������������������������������������������������������������������������������  313 9 Neptune’s Orbit: Reassessing Celestial Mechanics������������������������������  317 William Sheehan and Kenneth Young 9.1 Neptune’s Discovery Revisited: A Happy Accident?����������������������  318 9.2 Counterattack����������������������������������������������������������������������������������  326 9.3 Setting the Record Straight: Adams Responds ������������������������������  331 9.4 A New Test of Celestial Mechanics: Vulcan����������������������������������  333 9.5 Other Planets Beyond Neptune? ����������������������������������������������������  337 9.6 The Quest Continues����������������������������������������������������������������������  340 9.7 Searches Far and Wide��������������������������������������������������������������������  352 Appendix 9.1��������������������������������������������������������������������������������������������   354 Appendix 9.2��������������������������������������������������������������������������������������������   357 References��������������������������������������������������������������������������������������������������  358 10 Neptune Visited and the Outer Solar System Revolutionised, 1989–2019����������������������������������������������������������������������  361 William Sheehan 10.1 Grand Tour��������������������������������������������������������������������������������������  362 10.2 Resonances, the Nice Model, and Neptune’s Migrations ��������������  370 10.3 Discovering the Kuiper Belt�����������������������������������������������������������  371

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10.4 Discovery of Exoplanets Revolutionises Celestial Mechanics������������������������������������������������������������������������  375 10.5 A Grand Idea: Jupiter’s “Grand Tack”��������������������������������������������  378 10.6 Grand Tack Created Diversity in the Kuiper Belt ��������������������������  379 10.7 A Major Planet Beyond Neptune?: Twenty-First Century Version������������������������������������������������������������������������������  382 References��������������������������������������������������������������������������������������������������  385 Name Index������������������������������������������������������������������������������������������������������  389 Subject Index����������������������������������������������������������������������������������������������������  397

About the Editors

Lead Editor William Sheehan  retired after 30 years in practice as a psychiatrist, earned his MD from the University of Minnesota (1987). He has written or coauthored more than 20 books about astronomy and history of science, including In Search of Planet Vulcan (with Richard Baum) (1997), the first full-length history of the quest for Le Verrier’s intra-Mercurial planet, and Discovering Pluto: Exploration at the Edge of the Solar System (with Dale Cruikshank) (2018). He is a contributing editor for Sky & Telescope, has twice been Cox Memorial lecturer for the Society for the History of Astronomy, was a 2001 fellow of the John Simon Guggenheim Foundation, and was awarded the Gold Medal of the Oriental Astronomical Association (2004) for his work on Mars. He is a member of IAU’s Working Group on Planetary System Nomenclature. Asteroid 16037 was named in his honor.

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Editors Trudy E. Bell  contributing editor for Sky & Telescope and member of the editorial board for Springer’s Historical & Cultural Astronomy series of books, earned her MA in the history of science and American intellectual history from New York University (1978). Her particular research interest is nineteenth-century US astronomy. The author or coauthor of a dozen books, she has been a senior writer for the University of California High-Performance AstroComputing Center and an editor for Scientific American and IEEE Spectrum magazines. Her journalism and research awards include the David N. Schramm Award from the American Astronomical Society (2006) and the Herbert C.  Pollock Award of the Dudley Observatory (2004 and 2007). Asteroid 323552 was named in her honor. Carolyn Kennett, FRAS  is a writer, researcher, and astronomer who lives in the southwest of England. Until 2020 she was the co-editor of the Society for the History of Astronomy Bulletin. She delivers creative engagement opportunities for history, science, and astronomy as a director of Mayes Creative Ltd. and a director of Cornwall Sea to Stars. She runs her own business, Archaeoastronomy Cornwall.

Robert  W.  Smith  is a professor of history at the University of Alberta in Edmonton, Canada. He has written extensively on the history of astronomy from the eighteenth century to the present day and wrote his first paper related to the discovery of Neptune in 1984. He is the recipient of the 2020 LeRoy E. Doggett Prize for the history of astronomy, awarded by the American Astronomical Society’s Historical Astronomy Division.

About the Editors

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Additional Contributors Richard  Baum  was a leading British historian of astronomy until his death in 2017, specializing in prising out wayward episodes of astronomical history. He was the author of The Planets: Some Myths and Realities, In Search of Planet Vulcan (with William Sheehan), and The Haunted Observatory. He was recipient of the Walter Goodacre Medal (2006) and the Lydia A.  Brown Medal (1988) of the British Astronomical Association.

Allan  Chapman  is a historian of science at Oxford University, with a special interest in the history of astronomy. He has written 13 books and numerous academic articles and made several television programs. He has received honorary doctorates and awards from the Universities of Central Lancashire, Salford, and Lancaster and in 2015 was presented with the Jackson-Gwilt Medal by the Royal Astronomical Society. His books include Dividing the Circle: The Development of Critical Angular Measurement in Astronomy, 1500–1850 (1990, 1995), Astronomical Instruments and Their Users (1996), The Victorian Amateur Astronomer: Independent Astronomical Research in Britain 1820–1920 (1998, 2017), Mary Somerville and the World of Science (2004, 2015), England’s Leonardo: Robert Hooke and the Seventeenth-Century Scientific Revolution (2005), Stargazers: Copernicus, Galileo, the Telescope, and the Church—The Astronomical Renaissance, 1500–1700 (2014), and Comets, Cosmology, and the Big Bang: A History of Astronomy from Edmond Halley to Edwin Hubble, 1700–2000 (2018).

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About the Editors

Clifford J. Cunningham  earned his PhD in history of astronomy at the University of Southern Queensland, where he is a research fellow. He has written several monographs on the history of asteroid research, including Discovery of the First Asteroid, Ceres; Bode’s Law and the Discovery of Juno; Studies of Pallas in the Early Nineteenth Century; and Investigating the Origin of the Asteroids and Early Findings on Vesta. Asteroid 4276 is named in his honor. Davor  Krajnović  received his PhD in astronomy at Leiden University in 2004, with a dissertation on earlytype galaxies. He was a postdoctoral researcher at Oxford University; a junior research fellow at The Queen’s College, Oxford; a fellow at the European Southern Observatory in Garching, Germany; and a postdoctoral researcher at the Leibniz-Institut für Astrophysik Potsdam (AIP), Germany. Since 2019, he is a tenured scientist at AIP, whose current research topics of interest are related to galaxy formation and evolution. In 2013, as a member of the SAURON team, he received the Royal Astronomical Society Group Achievement Award. He was a co-principal investigator of the ATLAS3D Project and currently is co-leading the M3G Survey, investigating the most massive galaxies in the universe. He authored more than 150 scientific papers, and his serious side interest has been the history of astronomy. James  Lequeux  completed his PhD thesis in radio astronomy in 1962 and was an assistant, then associate, professor of physics and astronomy at Paris University until 1966. He was an astronomer from 1966 to 1999 and an invited scientist at the California Institute of Technology from 1968 to 1969. He was also the director of the Marseilles Observatory from 1983 to 1988 and was editor-in-chief of the journal Astronomy & Astrophysics for 15 years. Since his retirement in 1999, he has devoted himself to research and writing about the history of astronomy, and his books include acclaimed biographies of Urbain Jean Joseph Le Verrier, François Aragox, and Hippolyte Fizeau. He is a member of the editorial board for Springer’s Historical & Cultural Astronomy series of books.

About the Editors

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Brian Sheen, FRAS  a retired research chemist, is on the RAS Heritage Committee and a member of their newly formed Policy Group. A long-time John Couch Adams scholar, he is director of the Roseland Observatory in Cornwall and on the Board of Cornwall Sea to Stars. Kenneth Young  emeritus professor of physics at the Chinese University of Hong Kong, obtained his BS in physics and his PhD in physics and mathematics from Caltech. He is a fellow of the American Physical Society. He and his colleague H.  M. Lai and student C. C. Lam authored an influential paper on the perturbations of Neptune on Uranus in 1990.

Chapter 1

Preliminaries to the Neptune Discovery: Newtonian Gravitational Theory William Sheehan

Abstract  The discovery of Neptune stunned the 19th century world. It involved the mathematical prediction of the existence of a major planet never yet seen through a telescope, because of the way it seemed to be pulling on a known planet, Uranus, away from its predicted orbit. When an actual planet was discovered near the predicted position, it was famously hailed as the greatest triumph of Isaac Newton’s theory of gravitation. How did they do it? Largely by standing on the shoulders of giants—a dazzling succession of 18th century mathematicians, many of them French—who had to overcome many roadblocks in solving difficult problems, some of which had given Newton himself headaches. In so doing they developed celestial mechanics, the branch of mathematical astronomy that would later point the way to Neptune.

1.1  The Road to Neptune One might set off from many different starting points to Neptune. Its discovery was perhaps inevitable, though the road taken was certainly not. Most accounts begin with William Herschel’s (1738–1822) telescopic discovery of Uranus in March 1781. For the first time it became clear that a vast frontier of the outer Solar System, unsuspected by the ancients, who had circumscribed it within the orbit of Saturn, the outermost of the bright planets visible to the naked eye. The possibility of still other planets gradually came into the view of astronomers—though the view was not one of sight in the usual sense, but rather of something more akin to feeling. Some body (or bodies) was pulling on Uranus from beyond. The nature of the pull was formulated in terms of Isaac Newton’s (1643–1727) theory of gravitation, and complicated formulae defined the pull in terms of the perturbations of each body in the Solar System acting on every other. These formulae were used by mathematicians

W. Sheehan () Independent Scholar, Flagstaff, AZ, USA © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_1

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to discern the mass, orbital parameters and ultimately the position of the unknown planet in one of the most spectacular journeys ever taken by the human mind. It was a journey that would lead to the discovery of the planet we now know as Neptune in 1846—“with the tip of a pen”, as the French astronomer François Arago (1786–1853) would unforgettably express it (Lequeux, 2015:50). It is useful, however, to begin a bit farther back, with the theory of gravitation itself, from its first conception, to its initial triumphs and disappointments, and finally to its elaboration in perturbation theory. That puts us almost a century farther back from the discovery of Uranus.

1.2  The Inverse Square In August 1684, Edmond Halley (1656–1742), a leading member of London’s Royal Society, set out for Cambridge. He was still a very young man—not yet 30, but already a notable figure, with an unusually ebullient personality given the rather dour, obsessive, hardworking and depressive circle of other geniuses he moved in. He has been described as “witty, strikingly attractive … adventuresome, outgoing, a scholar, a drinking man, the sort of person whose rumoured affairs with women were the subject of talk at the University.” (Manuel, 1968:301). Already, at the callow age of 21, he had left Oxford without taking a degree and impetuously set sail for St. Helena, the isolated, sparsely vegetated rocky island in the Atlantic off the west coast of Africa where Napoleon would later be exiled after Waterloo. There he spent a productive year mapping the southern skies with a 5 ½-foot sextant, a 2-foot quadrant, a pendulum clock, and a 24-foot telescope. His star catalogue, Catalogue Stellarum Australium, published in 1679, won praise from John Flamsteed (1646–1719), the first Astronomer Royal (as the director of the Royal Observatory, Greenwich, was known) who called Halley “our southern Tycho.” (Unfortunately, they soon came to detest each other.) (Fig. 1.1). Fig. 1.1  Edmond Halley, in about 1687, by Thomas Murray (Credit: Royal Society)

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Halley’s trip to Cambridge was a follow on to a meeting that took place in London on the evening of 14 January 1683/84 (Old Style). With two other prominent members of the Royal Society, Christopher Wren (1632–1723), the immediate past-President of the Royal Society who was now largely occupied with architectural projects (notably, St. Paul’s) and Robert Hooke (1635–1703), the Society’s curator and Wren’s sometime architectural associate, they engaged in a lively discussion not about politics or religion but planetary orbits. (Westfall, 1980:402). The subject under discussion had implications for the architecture of the heavens. As Halley afterwards recalled, …and this I know to be true, that in January 1683/84, I having from the sesquialtera (i.e., 3/2 power) proportion of Kepler, concluded that the centripetall force decreased in the proportion of the squares of the distances reciprocally, came one Wednesday to town, where I met with Sr. Christ. Wren and Mr Hook, and falling in discourse about it, Mr Hook affirmed that upon that principle all the Laws of the celestiall motions were to be demonstrated. (Newton, Correspondence, II:442)

Hooke, in other words, claimed to have used an inverse square law of force to explain the planets’ motion in elliptical orbits. The fact of the elliptical orbits had been established by Johannes Kepler (1571–1630), and Kepler had even attempted to explain why they were elliptical by means of magnetic forces emanating from the Sun, though the idea was never accepted (Dreyer, 1953:305). He had also published the crucial sesquialetra proportion (as Halley refers to it) related to the distances and periods of the planets. (Today referred to as Kepler’s harmonic law, or third law of planetary motion, though Kepler himself did not so refer to it.) Doubting Hooke’s claim, Wren offered the prize of a book worth forty shillings to whomever could produce a rigorous mathematical demonstration of this result, but Wren’s stipulated time-limit of two months expired with the prize still unclaimed. That summer, Halley’s father disappeared under rather mysterious circumstances (he was found dead five weeks later, and was probably murdered), and Halley had to attend to these pressing family affairs, which included managing his father’s estate (Cook, 1998:137–140). While dealing with this decidedly unpleasant business, Halley came within range of Cambridge, and taking advantage of proximity, decided to drop in to put the question of the inverse-square to the Royal Society’s consummate mathematician, Isaac Newton. When Halley came to visit, Newton had already been thinking about gravitation for some time. In old age (but Newton in old age is not always a reliable source) he told the antiquarian William Stukeley (1687–1765) (Stukeley, 1936:19–20) that his thinking on gravity was first “occasion’d by the fall of an apple, as he sat in contemplative mood. Why should that apple always descend perpendicularly to the ground [and] constantly to the earth’s centre. There must be a drawing power…” He gave a somewhat fuller account along the same lines to the exiled French Huguenot writer Pierre Des Maizeaux (1666–1745), in which he seemed to suggest (Westfall, 1993:40) that in 1665–66 he had combined a formula for centrifugal force—the “endeavour from the centre of a body revolving in a circle”—with Kepler’s harmonic law to arrive at an inverse-square law for gravity. (The formula for centrifugal force had already been derived by Christiaan Huygens (1629–1695) in 1659.

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Fig. 1.2  Isaac Newton, in 1689, after the Godfrey Kneller portrait in the possession of the Earl of Portsmouth, Farleigh Wallop, Basingstoke, Hampshire (Credit: Wikipedia Commons)

However, before Huygens published the result, in 1673, Newton had found it independently, using a different method, in 1665–66.) (Fig. 1.2). In 1665–66, Newton, then an undergraduate at Trinity College, Cambridge, had left the university because of the Plague. Like other scholars and fellows, Newton was provided with a small subsistence allowance while away, and Newton is presumed to have returned to his mother’s home in Woolsthorpe, Lincolnshire, and there, supposedly in the garden, sat under the apple tree which started his train of thoughts. That he left Cambridge is certain, but there is some evidence that he spent the plague years not at Woolsthorpe but at his uncle’s house in Boothby, the only place he is known to have stayed and where there was a large garden with an extensive copse of trees out back. (Newton, 1989:xi). Famously, he did a back-­of-­envelope calculation in which he tried to compare the force of gravity at the surface of the Earth with the Moon’s “endeavour to recede” as a way of testing an inverse-square law, and found the latter to be “4,000 and more times greater,” not 3,600 as it ought to have been according to an inverse-square law. Newton would later claim that the failure of this calculation was owing to his having used an inaccurate value for the length of a degree of terrestrial latitude. This was true enough, though he could easily have corrected his calculation once better values were available—notably, from Jean Picard’s triangulation between Paris and the clocktower of Sourdon, published in 1671 and evidently known to Newton within the same year. Instead, Newton let the matter drop (and did not use the Picard measure of the Earth to do a recalculation until 1684–5. (Westfall, 1980:420–421). The reason for this seems to be that at this time he was still a disciple of the vortex theory of the French mathematician and philosopher René Descartes (1596–1650), whose central idea was of “matter in motion” and which posited “a universe in which vortices [tourbillons] of matter fill

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space and all moving bodies are borne along in the swirl, the planets … trapped in the solar whirlpool” (Whiteside, 1970a:10). Thus, he would not then have expected the calculation to come out exactly. Rather, if, as he then believed, the motions of the Moon and planets were due to a mixture of gravity and “Cartesius’s [Descartes’] Vortices”, then, as Curtis Wilson has pointed out (Wilson, 2002:206), “the supposition of vortices with their hydrodynamical complexities could hardly fail to give rise to doubts about the mathematical accuracy of … elliptical orbits.” (For more on the Cartesian vortex theory, see: Aiton, 1989:206). Newton’s thinking might have continued to swirl endlessly in Cartesian vortices, then, were it not for the sudden interposition, in the winter of 1679/80, of Robert Hooke. Hooke was a remarkable genius in his own right who has, unfortunately, long been in Newtonian eclipse. No portrait of him exists; perhaps one never did, though the sour relations between the two men is attested by a story, probably unfounded, that Newton as president of the Royal Society got rid of the one (or possibly two) in its possession. (Jardine, 2003:15.) They had corresponded by fits and starts for many years, and Hooke, in November 1679, serving as secretary of the Royal Society and attempting to breathe some life into an organization that had become rather moribund, revived his correspondence with the then notoriously reclusive scholar whose favourite studies were biblical chronology, the Book of Revelation, and alchemy. To pique Newton’s interest Hooke threw out a number of ideas, of which the most fruitful was his suggestion that Newton try, in place of thinking about aether-gravity along Cartesian lines, “compounding the celestiall motions of the planets of a direct [i.e., straight] motion by the tangent & an attractive motion.” About this suggestion, Derek T.  Whiteside, the great student of Newtonian mathematics, says: Had Hooke added in a final rider that Kepler’s area law was to be understood to be exact, he would have been handing Newton on a platter the essence of a complete solution to the problem of planetary motion, since all then remaining to do would have been for him adroitly to employ his mathematician’s skill to good effect, as he was well able to. Newton in reply rather grudgingly thanked Hooke for his letter, and in a spirit of reciprocity sent an attempt to discover the Earth’s rotation by analysing the way a body would fall if dropped from a height above the Earth. He found it to be a spiral. Hooke, however, found that Newton had made an error, and corrected him. Newton was a dangerous man to correct. He took it as a personal affront. Nevertheless, Hooke had done his work by providing the goad which, as Newton later told Halley (Newton to Halley, 27 July 1686; Newton, Correspondence, II:447), “occasioned my finding the Theorem by which I afterward examined the Ellipsis’ of the planetary orbits.” Always a grudging debtor, he admitted that Hooke’s correction of his “fancy” (about aether-gravity) had acted as a dare, a challenge, a stimulus, a diversion—not, however, he insisted, an aid. (Newton, 1989:xiii)

However that may be, Newton now was “hooked”, and henceforth would never again let the matter of the celestial motions drop entirely from his mind. Shortly after the resumption of the correspondence with Hooke, between the late autumn of 1680 and the early spring of 1681, a large comet appeared – it even became bright enough to be visible in broad daylight. Though Newton himself claimed to be an indifferent observer, apparently because of short sightedness, he made a few of his own observations, and was also at pains to correspond with Flamsteed about it.

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At first Newton supposed that there were two comets, one going inward toward the Sun, the other going outward from it. He next observed the comet of 1682 (an apparition of that now known as Halley’s), which was moving retrograde to the sense in which the planets move. This was a reproof to the vortex theory, for it meant that the comet would have been beating against the Cartesian tide, did such exist, and marked the end of Newton’s Cartesian phase. (Westfall, 1993:391–392). When Halley visited him at Cambridge, Newton had by then been in residence at Trinity for two decades, as undergraduate, fellow, and—since 1670—as Lucasian professor of mathematics. The university was then, as now, made up of separate colleges, each with its own set of turreted buildings and its own history. Trinity, founded in the last year of the reign of Henry VIII, brought together two older colleges, King’s Hall and Michaelhouse, with land and tithes taken from the monks for support (it was the only college to fly the royal standard, and also the only one to have a master appointed by the crown). Halley would have entered through the Great Gate at its entrance, and passed into the square Great Court, with its massive stone fountain in the centre of the lawn. Newton’s rooms were on the first floor next to the Great Gate, with windows facing west. He also had his own “elaboratory”, whose furnaces were often burning “in the spring and fall of the leaf”, and where he pursued his passion for alchemy; it was situated next to his garden, probably in the projecting room built between the Great Gate and St. John’s College, with a staircase leading down to the garden as shown in David Loggan’s print which shows Trinity just as it looked when Halley came. (Adrian, 1963). The French Huguenot mathematician Abraham De Moivre (1667–1754) (a colleague and friend of the aforementioned Des Maizeaux) gives a vivid, if in some respects questionable, account of the meeting (Westfall, 1993:403). Halley asked Newton what he thought the Curve would be that would be described by the Planets supposing the force of attraction towards the Sun to be reciprocal to the square of their distance from it. Sr Isaac replied immediately that it would be an Ellipsis, the Doctor struck with joy & amazement asked him how he knew it, why saith he I have calculated it, whereupon Dr Halley asked him for his calculation without any farther delay. Sr Isaac looked among his papers but could not find it, but he promised him to renew it, & then to send it to him.

Newton claimed he could not find the paper but— still according to De Moivre—in order to “make good his promise … fell to work again, [to find that he] could not come to that conclusion which he thought he had before examined with care. However, he attempted a new way which thou longer than the first, brought him again to his former conclusion….” The account glosses over some important details. First, we can give short shrift to what Newton’s great biographer Richard S. Westfall calls (Westfall, 1993:403) “the charade of the lost paper”, since it survives among Newton’s papers. Instead of directly answering Halley’s question, in the terms Halley asked for, Newton wrote a nine-page first draft of a treatise, De motu in corporum in gyrum (On the Motion of Bodies in an Orbit). The draft set out five theorems and five problems dealing with motion in space void of resistance, and then expanded his treatment to cover motion through a resisting medium. As Whiteside has shown (1970a), Newton does not

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actually present a mathematical proof of the proposition that Newton had told Halley he had demonstrated, but does show the converse: given that the orbit is an ellipse, the law of force varies according to the inverse-square of the distance. Moreover, the proof is not easy to follow; probably not even Halley would have understood it. Indeed, in this first draft, “there is not a single indication that Newton believes in universal gravitation, or has deduced any of the consequences of it that differ from those of the simple assumption of an inverse-square field of force.” (Wilson, 1989:253). Indeed, so far Newton still regards Kepler’s harmonic relationship as inexact, and is willing to grant wide scope to the Cartesian vortices. Modest though it is, however, it is rather like premonitory thunder announcing the approaching storm, for as Curtis Wilson remarks (ibid.), “somehow, in this first tract, Newton has become ‘embarked’.” After years of seeming indifference, the majestic problem of the celestial motions has seized him, and seized him irresistibly—it would prove to be the turning point of Newton’s life, indeed of the whole history of science, and hand to his successors the crucial tools that would lead to the discovery of Neptune. Newton produced a second draft of De motu, which is little more than a fair copy of the first in Halley’s hand, before, in the last months of 1684, probably December 1684 (as tentatively dated by Whiteside [1970a]), sequestered in his rooms at Trinity, he began a crucial expansion of the nine-page draft of De motu into a treatise in two books ten times as long that became nothing less than a grand investigation into centripetal forces as they determine orbital motion. (Newton, 1989). This was to become nothing less than “a systematic demonstration of universal gravitation”. (Westfall, 1980:423). His mind was concentrated now, and instead of being, as hitherto, confused by the Cartesian fantasies, his definitions become clear and precise. He defines centripetal force (vs. Huygens’s centrifugal force as “that by which a body is impelled or attracted toward some point regarded as a centre.” Resistance: “that which is the property of a regularly impeding medium.” He had earlier used the term “hypothesis” in stating his propositions; however, he now introduces the term Lex (law), and proposes what will (in Principia) become the definitions and three Newtonian laws of motion. With this maneuver, “Newton effectively embraced the principle of inertia,” says Westfall (1993:167), at which point “the rest of his dynamics fell quickly into place.” Indeed, henceforth all reference to ideas about a material aether as the cause of gravitation are now dropped. Newton now knew, at least as far as the motions of the Moon and planets were concerned, that the resistance in space was nil, and now—at last—formulated the idea of an attraction arising from the universal nature of matter. Now, equipped with some powerful new mathematical tools (Whiteside, 1970b), he was ready to tackle rigorously the problem of the inverse square law that had only been suggested in the calculation of the apple and the Moon twenty years before. (Westfall, 1993:170–171). The turning point in his thinking took place, he later recalled, at almost the exact moment that the Cambridge Alderman Samuel Newton (no relation) proclaimed the deceased Charles II’s brother James Stuart, the Duke of York, King James II of England (February 1685). (Westfall, 1980:423). The gravitational force Newton had first calculated in a back-of-envelope calculation for

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the Earth and Moon so long ago is now posited to exist between any two bodies, and to be proportional to their mass, the amount of matter they contain—here we have one of Newton’s most original concepts–and to become weaker with the increasing distance between them according to the inverse-square law. He has now liberated himself from his earlier Cartesian assumptions and sketches for the first time the powerful conception that would become the soul of the Principia: If in any position of the planets their common centre of gravity is computed, this either falls in the body of the Sun or will always be close to it. By reason of the deviation of the Sun from the centre of gravity, the centripetal force does not always tend to that immobile centre, and hence the planets neither move exactly in ellipses nor revolve twice in the same orbit. There are as many orbits of a planet as it has revolutions, as in the motion of the Moon, and the orbit of any one planet depends on the combined motion of all the planets, not to mention the action of all these on each other. But to consider simultaneously all these causes of motion and to define these motions by exact laws admitting of easy calculation exceeds, if I am not mistaken, the force of any human mind. (Wilson, 1989:253).

What was most extraordinary about his thinking at the time is that he is no longer in thrall to “matter in motion”. He now sees the “aether”, to which he had long subscribed as a mechanistic cause for gravity, as not only implausible but incompatible with the actual celestial motions. Instead he embraces “action at a distance”, for which he would later be heavily criticised by Gottfried W. Leibniz (1646–1716) as a return to “occult” forces. (Alexander, 1956: p. x). In fact, over the course of his career he continued to try various explanations in order to beat off Leibniz and others, and though he would eventually fib that “the cause of gravity is what I do not pretend to know” (Newton, 1756:20), his final conclusion—at least according to the General Scholium of Principia—was that that cause was known, and it was God. Those controversies need not, however, concern us here. (For anyone wishing to follow them in detail, see: Ducheyne, 2011:154–159). At present he had enough to do to work through the mathematical consequences of his idea to worry about final causes. By April 1685, in fact, Newton was ready to move beyond these preliminary sketches (which still survive in manuscript) to draft his great work, Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy), usually referred to simply as the Principia. He spent the next 18 months in near total seclusion and almost mystic abstraction from the world. He was, his live-in servant and sometime amanuensis Humphrey Newton (no relation) (c. 1658–1746), wrote: So intent, so serious upon his Studies, yt he eat very sparingly, nay, oftimes he has forget to eat at all so yt going into his Chamber, I have found his Mess untouch’d of wch when I have reminded him, [he] would reply, Have I; & then making to ye Table, would eat a bit or two standing…. When he has sometimes taken a Turn or two [in his garden], [he] has made a sudden stand, turn’d himself about, run up ye Stairs, like another Archimedes, [and] with an eureka, fallen to write on his Desk standing, without giving himself the Leasure to draw a Chair to sit down. (Westfall, 1993:406)

Newton’s preliminary manuscripts reveal the titanic nature of his struggles; they are more like Beethoven’s draft scores than Mozart’s. In the end Newton emerged largely triumphant. Halley (and Hooke) had opened the door, and Newton had darted through it. Yet in seeing the immortal work through to publication it must

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have required all the tact that still very young man could muster in dealing with Newton’s “nice” (i.e., touchy) personality. He had now replaced Hooke as full-time secretary of the Royal Society, and as such was responsible for keeping up the Royal Society’s voluminous correspondence, taking minutes of its meetings, and editing its Philosophical Transactions. Moreover, since his father’s death, Halley no longer enjoyed the fortune he had once depended on. As a secretary he was granted a salary, which he now needed. Just then, the Royal Society had published Francis Willoughby’s History of Fishes. In doing so, it had greatly overestimated the demand, and Halley had to take part of his salary as secretary in remaindered copies. As the Principia began to work its way through the press, Halley was forced to deal with Hooke’s claim that Newton had appropriated the inverse-square law—and the idea of universal gravitation itself—from him. Newton was so enraged that he threatened for a time to withhold the all-important third book, the System of the World. The first edition of the Principia—complete with Book III—finally appeared in July 1687, with Halley paying the expenses of publication out of his own pocket and furnishing a prefatory poem (in Latin) which, though little less than rapturous, hardly exaggerates the immensity of Newton’s achievement: … O mortal men, Arise! And, casting off your earthly cares, Learn ye the potency of heaven-born mind, Its thought and life far from the herd withdrawn! Nearer the gods no mortal may approach. (Newton, 1934:xiv–xv)

From the astronomer’s point of view, Newton’s triumphs are so numerous as to be hard to list, but include his derivations from the inverse square law of the motions of the planets, his investigation of the shape of the Earth (he found it to be oblate, i.e., flattened at the poles) and his qualitative explanation of the precession of the equinoxes, the grand phenomena discovered by Hipparchus which Newton showed arose from the pull of the Moon on the equatorial bulge of the oblate Earth. (The latter is an explanation so natural that as the 19th century astronomer George Biddell Airy (1801–1892) later remarked (Grant, 1852:22), this part of the Principia “probably astonished and delighted and satisfied its readers more than any other.”) Other problems created greater difficulties. One was the calculation of the motions of comets. As late as the summer of 1686, Newton admitted to Halley that he had not yet succeeded—clearly, he was baffled by the horrendous nature of the geometrical analysis required. He managed, however, to come through in the end, with a workable if cumbersome graphical method with which he demonstrated that the orbit of the comet of 1680 was a parabola. Halley later adapted Newton’s graphical method to make it amenable to arithmetical calculation and proved the remarkable result that the comets which had appeared in 1531, 1607, and 1682 were one in the same, moving in a retrograde orbit around the Sun with a period of 75 years, from which he famously predicted that the comet which now bears his name would return again in 1758. However, by far the most complex problem with which Newton grappled in the Principia was the theory of perturbations. This involves the notorious three-body

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problem—the way that the disturbing influence of one planet moving round the Sun affects that of another. In Newton’s time, “the then undeveloped state of the practical part of astronomy … had not yet attained the precision requisite to make such an attempt possible” in the case of the planets. (Herschel, 1861 art. 602). In the case of the Moon it was otherwise, since the disturber of the Moon’s motion around the Earth is the Sun itself. Indeed, the largest inequalities (irregularities) of the Moon’s motion had been noted as far back as the time of Hipparchus, 2000 years earlier.

1.3  Newton’s Headache: The Theory of the Moon Newton understood that, to a first-order approximation, the Moon’s orbit around the Earth is, of course, an ellipse. This ellipse is, however, perturbed, being alternately compressed and relaxed by the disturbing effect of the Sun, and forced to turn and wobble in various complicated ways. The perturbations cause the Moon to sometimes run ahead, at other times behind the position it would have in an undisturbed elliptical orbit. It was this intricate motion that became the most critical test of the theory of universal gravitation—and Newton well knew it. He conceived of the Moon’s path as a Keplerian ellipse drawn from its ideal curve by the disturbing force of the Sun. Though the effect of this force is always directed toward the Sun, the Sun’s position relative to the Earth and Moon is ever-­ changing. By resolving the force into a radial component (i.e., directed along the line between the Sun and Moon) and a tangential component (perpendicular to the line between the Sun and Moon), Newton was able to qualitatively explain all and quantitatively several of these perturbations at once (Airy, 1884:61ff). However, Newton found one particular problem, the advance of the line of the apsides, especially baffling. (Whiteside, 1976). This was a curious and complicated effect, involving the fluctuation back and forth of the line of the chord, known as the apsides (or apses), joining the perigee and apogee of the Moon’s elliptical orbit around the Earth. (Perigee is the point at which the Moon approaches nearest to the Earth, apogee the point at which it is farthest away.) This motion is notoriously irregular— sometimes it progresses, sometimes it regresses. On the whole the progressive motion wins out, so that the line of the apsides advances about 3.3° per revolution of the Moon, and is carried completely around in a period of about nine years. (An apse is either of two points on an elliptical orbit that are nearest to and farthest from the focus or centre of attraction.) Hipparchus was aware of the effect; even the ancient Antikythera Mechanism, apparently based on Hipparchus’s theory of the Moon, corrected for it. Newton, naturally, tried to account for it on the basis of the inverse-square law of gravitation. It did not seem to work out. Newton approached the problem in several ways. The first involved a theorem (Principia, Book I, proposition 43–45) in which he showed that adding a component of an inverse cube force to the inverse square produces a moving line of the apsides, with no effect on the radial motion. He then used this as an argument for the inverse-square law, since the lines of the apsides of the planets are essentially

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stationary, which would not be the case if the inverse-square were not exact. Turning then to the advance of the line of the apsides of the Moon, he assumed, correctly, that it is produced by the disturbing action of the Sun, and attempted to calculate what the effect on the line of the apsides would be if he subtracted from the attraction of the Earth on the Moon a force 1/357.45 as great, which is the average value of the radial component of the Sun’s perturbing force smeared out over one orbit. His result was an advance in the line of apsides of 1°31′28″ per revolution (see Principia, Bk I, Prop. 45, Corollary 2, all editions)1. Unfortunately, as he finally acknowledged only in the 3rd edition (published in 1726, the year before his death) there was a problem. In his words, “the apse of the Moon is about twice as swift.” Another method appears in Principia, Book III. In Propositions LXV–LXIX, he takes the first steps in the study of the motions of three bodies under mutual gravitational attractions, and to Proposition LXVI he presents 22 Corollaries related to the motion of the Moon. These corollaries Laplace would regard as among the most remarkable in the Principia. This is true; but this section of the book is also among the most obscure, so that, as S. Chandrasekhar has said, though Newton’s astonishing grasp of the entire problem of planetary perturbations and the power of his insight are clearly apparent, this part of the Principia is also among the most difficult to grasp because of the paucity of any real explanation and an apparent attempt to conceal details by recourse, too often, to phrases like “hence it comes to pass”, “by like reasoning”, and “it is manifest that” at crucial points of the argument. This “secretive style” is nowhere present, to the same extent, in the Principia. (Chandrasekhar, 1995:258)

The gist of his argument seems to be as follows. Consider the Earth, Moon, and Sun (T, P, and S), as in the diagram shown here (Fig. 1.3). The orbit of the Moon is taken to be (to a first approximation) nearly circular. Also, the fact that the Sun is at an immense distance from both the Earth and Moon means that the lines SP and ST are parallel. He attempts this calculation (Book I, Prop XLV, Cor. II), and finds (Cor. XVI) “that the mean motion of the line of the

Fig. 1.3  Diagram from Principia, at Book III, Prop. XV, Problem VI, referred to in text (Public Domain)  Here and in the rest of the book, the symbols °, ′, and ″ refer to the angular measurements used by astronomers, and mean degrees, minutes, and seconds. There are 60 seconds of arc in 1 minute of arc, and 60 minutes of arc in 1 degree. There are, of course, 360 degrees in a circle. The moon’s apparent diameter in the sky is roughly half a degree. 1

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apsides will be in a given ratio to the mean motion of the nodes; and both those motions will be directly as the periodical time of the body P, and inversely as the square of the periodical time of the body P.” (Node: the point at which the orbital plane of some celestial body, such as the Moon, intersects with the plane of the ecliptic, the apparent path of the Sun among the stars,) Effectively, Newton is attempting to derive the relationship between four periods that used since antiquity to characterise the motions of the Moon: 1. The sidereal year of Earth, To = 365.257 days 2 . The sidereal month, T1 = 27.32166 days 3. The anomalistic month or time between successive apogees, T2 = 27.55455 days 4. The draconitic month or nodical month, the time between the Moon’s return to the same longitude in its orbit, T3 = 27.21222 days. In terms of the ratio m = T1/To, he discovers the formulae

T2 / T1 − 1 = +3m 2 / 4

and

T3 / T1 − 1 = −3m 2 / 4.

The second formula gives roughly the correct answer for the regression of the nodes when appropriate values are substituted; however, the first, giving the expression for the advance of the line of the apsides, yields only half the correct value, as before. This time, as Newton saw, the calculation fails because it has left out the effect of the perturbing force at right angles to the radius vector. He therefore tried another calculation—probably in the winter or early spring of 1687, just as the first edition of Principia was going to press—that took this into account. It should have been his tour de force. But he never published the calculation. He did make the claim, in the first edition of the Principia, that he had computed the motion of the lunar apsides to be about 40° per year, a value differing only slightly from the observed value of 38°51′51″ found in Flamsteed’s tables (published in his Doctrine of the Sphere).(Flamsteed, 1680). He claims, however, that he declined to publish it since it was “too intricate and burdened with approximations, and moreover insufficiently accurate.” In the later editions of 1713 and 1726, this scholium is changed, with all reference to an attempted computation omitted. It would appear that the great man wanted to erase even the trace of his failure. (We now know, since the manuscript of the attempted calculation first came to light among the Portsmouth papers in the 19th century—they were first studied by John Couch Adams (1819–1892)—that Newton did include both the radial and transverse components of the Sun’s perturbing force. But he also introduced an unjustified step, so that his final result—though it appears close to being correct, as Adams saw—is, however, as later investigators have discovered, “fudged”.) (Fig. 1.4). In fact, Newton never would succeed in giving a quantitatively correct explanation for the advance of the lunar apsides on the basis of the inverse-square law.

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Fig. 1.4 Newton’s diagram in Principia, Section IX; the motion of bodies in movable orbits and the motion of the apsides; at Prop. XLIII, Problem XXX. (Public Domain)

When he later told his younger contemporary John Machin (1680–1751), professor of mathematics at Gresham College, London that “his head never ached but with his study on the Moon” (Machin, 1729:31), he must have been thinking of his struggle with the advance of the lunar apsides. Even the great Newton was fallible: he left this particular Everest of the theory of the Moon unscaled, and it would not be conquered during his lifetime. Two years after the great man’s death in 1727, Machin added, “Neither is there any method that I have ever yet met with upon the commonly received principles, which is perfectly sufficient to explain the motion of the Moon’s apogee.” The other problem that Newton left for future investigators was the perturbations of the planets on one another. He knew, of course, that they would exist as a consequence of the theory of universal gravitation. To recall again what he had said in de Motu (Wilson, 1989:253), “There are as many orbits of a planet as it has revolutions … and the orbit of any one planet depends on the combined motion of all the planets, not to mention the action of all these on each other.” Obviously, to be able to perform these calculations was of the utmost importance as an extension of the theory of gravitation and as a means of perfecting predictions of the motions of the planets. That Newton himself failed to take up the matter was entirely, as Sir John Herschel (1792–1871) later observed, “owing to the then undeveloped state of the practical part of astronomy, which had not yet attained the precision requisite to make such an attempt inviting, or indeed feasible.” (Herschel, 1861, art. 602). The only mutual planetary perturbations recognised in Newton’s time involved the “Great Inequality” of the giant planets Jupiter and Saturn, a puzzle which had already been recognised in the early 17th century and finally unraveled by Pierre Simon de Laplace (1749–1827) on the basis of perturbation theory in 1784, as described below. Newton also worried about a problem that would obsess later astronomers: that of the stability of the Solar System. Deeply religious in his own

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unorthodox way,2 Newton believed that the perturbations among the various bodies in the Solar System would eventually become sufficiently great as to cause the whole thing to collapse into disorder unless God intervened to set it back in order again. (Teeter Dobbs and Jacob, 1985:58). Later astronomers would try to establish the stability of the Solar System without invoking that “hypothesis”.

1.4  Stars in the Moon’s Way But let us return to the problem of the advance of the lunar apsides. Newton’s failure to derive it from the inverse-square law was more than just a bothersome loose end, since the motion of the apsides plays a singularly significant role in the determination of the Moon’s place in its orbit. An incorrect determination of the apogee’s position could lead to an error of as much as 13 or 14° in the place of the Moon itself—an error twice that encountered by astronomers of ancient times who had supposed that the Moon moved uniformly in a circle around the Earth. Though after the publication of the first edition of Principia, which would mark the greatest achievement of Newton’s life, he seems to have returned to his other

Fig. 1.5  Trinity College, David Loggan’s engraving from Cantabrigia Illustrata, 1688. Newton, who was in residence at the time the engraving was made, had first-floor rooms with a balcony in the building joining the Great Gate to the Chapel, facing Trinity Street. His “elaboratory” was in the corner of his well-tended garden, lying between the building and a buttress of the Chapel. (Public Domain)  Though Newton was affiliated as an undergraduate, fellow and Lucasian professor of mathematics at Trinity College, he was not a trinitarian at all but a secret Unitarian, and so an advanced heretic by the standards of the time. 2

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favourite studies, especially alchemy, biblical chronology, and the prophecies of the Books of Daniel and Revelation. During the 1690s, he seems to have again devoted more time to alchemy than to all of these put together; in particular, the winter of 1692 seems to have seen a period of the most frantic activity in his “elaboratory”, and this, which may have involved exposure to heavy metals, has long been suspected to have contributed (though was probably not the only factor) to the mental breakdown he suffered in 1693 (Fig. 1.5). As he later admitted to his friend John Locke (1632–1704), whom Newton during his dark period of mental imbalance had charged with trying to “entangle him with woemen [sic.]”: The last winter by sleeping too often by my fire I got an ill habit of sleeping & a distemper wch this summer has been epidemical put me further out of order, so that when I wrote to you I had not slept an hour a night for a fortnight together & for 5 nights together not a wink. (Newton to Locke, 15 October 1693; Newton, Correspondence, III:284.)

Whatever the cause (and many have been suggested), Newton, for about a year, seems to have been, as Christiaan Huygens reported, “not in his right mind” (Manuel, 1968:222). When he emerged from this crisis, he was not the same Newton. His creative powers were no longer what they had been–though it was not true, as Huygens told his brother Constantyn, that Newton was “a man lost and so to speak dead for research, so I believe, which is deplorable” (ibid). There was certainly a change, inasmuch as, having for thirty years remained satisfied with the life of the mind, as a reclusive scholar, he now began—through his friends—to angle for an important position in society, which meant moving to London. He needed to be “useful”, and attempted to reconstitute himself as Master of the Mint and president of the Royal Society. He was still quarrelsome, but a bit more relaxed in company; he became jowly and pot-bellied, prosperous and vain. He also continued, off and on, to think about the Moon, but no longer as an object of detached curiosity. When he thought about it now, it was with a very practical purpose in mind. The Moon being in some sense a “clock”, it can be used to solve a very prominent earthly problem: that of finding the longitude of a ship at sea, a problem with a long pedigree and obviously of the utmost importance to the defence and prosperity of the great sea-faring British nation, which was just then on the verge of hurling itself into a global empire. Thus, practical concerns fed the development of celestial mechanics, and the solution of the problem of the Moon’s motion prodded the extensions of Newton’s theory that would be needed for the calculations related to Neptune. The determination of one’s position at sea requires knowledge of two coordinates: latitude, one’s position north or south of the equator, and longitude, one’s east-west position on the globe of the Earth. Latitude can be worked out easily in principle, although not always in practice on a rolling ship. For instance, already in the Middle Ages, a Jacob’s staff (also known as a cross staff), consisting of a shaft fitted with an adjustable cross-bar, was standard for the purpose. The observer rested one end on his cheek and attempted to sight along the shaft on the Sun or Polaris while adjusting the length of the length of the bar to find the altitude. (The reader is advised to try it sometime!) Later, the octant was developed for the purpose.

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By contrast, the longitude of a position on the Earth relative to the reference framework of the fixed stars is being constantly shuffled by the Earth’s rotation. Thus, it is a complicated business—so much so that as late as 1711 William Derham (1657–1735), canon of Windsor and a Fellow of the Royal Society of London, in one of his Boyle lectures claimed that, along with the ability to fly and the problem of squaring the circle, longitude of sea was insoluble—it lay “beyond the powers of reasoning that God had given to man.” (Derham, 1713). Without knowing how to determine longitude at sea, intrepid fifteenth and 16th century navigators had depended on jealously guarded proprietary knowledge and maps of sea-routes and dead reckoning. The Portuguese, who pioneered routes around Africa that enabled them to win a lucrative monopoly on pepper, obsessively guarded their navigational logs; to divulge them carried the sentence of death. The Spanish, likewise, kept their maps in a lockbox with two locks, the key to one being kept by the pilot-major, to the other by the cosmographer-major. Human enterprise being what it is, in time the secrets leaked out despite the best precautions. Later trading companies—the British East India Company and the Dutch East India Company, for instance—acquired their own secret maps showing the best routes. But if knowledge and maps of these routes were precious, mastery of longitude at sea would be the pearl of great price, truly—whoever possessed it would gain a decisive commercial advantage over their colonial rivals. Thus, in 1598, King Phillip III of Spain—whose empire, enriched beyond the dreams of avarice by the gold and silver from its New World possessions, was beginning to fade, but was still the most powerful on Earth—offered a magnificent prize of 6,000 ducats (the equivalent of several million dollars in today’s currency) for the successful solution of the problem of longitude at sea. (Sheehan and Dobbins:43). Several schemes were proposed to capture Phillip’s ducats—and later prizes which were offered in later contests as the problem remained unsolved. Necessarily, all these methods involved referencing a standard clock—whether an astronomical one like the Moon or the constantly shifting satellites of Jupiter, or a mechanical clock able to keep accurate enough time even on a swaying ship bound on a long sea voyage. The longitude could be found by comparing the local time on the ship with that at a standard meridian—such as Greenwich or Paris—by comparing the Moon’s observed position with data worked out in advance for the standard station a clock set to local time with one keeping Greenwich or Paris time. For each hour’s difference in time, the longitude difference was 15°. (Sheehan and Dobbins, 2001:14–15). A number of mathematicians and natural philosophers, including Galileo Galilei (1564–1642), Christiaan Huygens, and Robert Hooke, devoted much time to the perfection of pendulum clocks or spring watches, at the end of the 17th century. Although the pendulum clocks of Huygens were good enough to keep time in an observatory—they indicated minutes with greater accuracy than the old foliot clocks could indicate hours, and made possible a new era in precision measures of right ascensions of stars and planets—they were thrown off by the jostling conditions at sea. Until John Harrison (1693–1776) devised his famous marine chronometer in the mid-18th century, no clock could keep time accurately enough at sea to solve the longitude problem, so much of the effort was directed to what was known as the “method of lunars”, which depended on knowing the Moon’s place relative to stars

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that lay along its path, worked out in advance for the standard stations and observed at sea. This method was straightforward in principle but difficult in practice, and requires both an accurate star-map along the Moon’s track and an accurate theory of its motions. (Sheehan and Westfall, 2004:106). A star-map was produced by the Flamsteed, appointed to take charge of the small observatory established at Greenwich Park in 1675 by King Charles II in rivalry with the Paris Observatory founded by his cousin Louis XIV. From its inception, the warrant of the Royal Observatory, Greenwich explicitly spelled out its utilitarian purpose, which was “finding out of the longitude of places for perfecting navigation and astronomy”. Though the Greenwich appointments were not nearly as grandiose as those of the Paris Observatory, Flamsteed made the best of the situation that he could. (Chapman and Bennett, 1996). His quarters were cramped, and he was required to furnish his own equipment, initially consisting only of two clocks, a 3-foot quadrant, telescopes of 7- and 15 -foot focal length, and an equatorial sextant of 7 feet radius. Of this last it has been said: This instrument consisted of an iron frame mounted on a stout polar axis and two subsidiary half-circles that allowed the limb’s movement into any plane. It was made by Thomas Tompion, a leading horologist of the day, and the only person to whom Flamsteed could then apply for careful workmanship. Flamsteed divided the limb himself to every 5 minutes of arc and then, by transversals, to single seconds read off directly by means of a movable index arm. There were two telescopes, both movable by tangent screws which engaged in the racked edge of the limb. (King, 1979:107)

With these meagre appliances, and a mural arc he added in 1681 and used as a meridian instrument but which, because of its weak construction, gave false readings that he had to patiently correct, he had to do a great deal of improvising. When in 1688 his father, a prosperous maltster from Derbyshire, died, leaving him the family property, he experienced an upturn in his fortunes. Now he was able to invest in a 90-foot focal-length object-glass by Borelli, one of Galileo’s disciples, and to reconstruct the iron sextant into a proper 7-foot mural arc. The parts were so skilfully made by his assistant Abraham Sharp (1653–1742) that, as Flamsteed remarked in 1694 (Cudworth, 1889:14), it “was a source of admiration to every experienced workman who beheld it.” It was set up in a “Sextant House” designed for it at the bottom of the garden, and after re-determining his latitude and recalibrating his sextant measures, he set out on the great work of measuring all the right ascensions of stars anew. His method involved choosing certain bright stars as a reference points, and then working out positions of the other stars by determining the relative angular separations from the reference stars. The 3000-star positions in the Historia Coelestis Britannica, accurate often to within 5″ of arc (a figure to keep in mind, as it will prove crucial later) attest to the instrument’s accuracy and Flamsteed’s confidence in the value of telescopic sights. Looking ahead, it was with this instrument that Flamsteed, working on Greenwich hill through the chilly night of 23 December 1690, recorded a star of sixth magnitude, which lay close to omega Tauri, between the Hyades and Pleiades. Later given the Flamsteed number 34 Tauri, it would prove to have been an unwitting—and singularly valuable—sighting of Uranus (Figs. 1.6 and 1.7). In Flamsteed’s day, the stars were taken to form the rigid framework relative to which the restlessly moving bodies of the Solar System could be tracked. Sir John

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Fig. 1.6  Flamsteed and his assistant Thomas Weston at the eyepiece of the Mural Arc. Detail from the ceiling of the Painted Hall in the Old Royal Naval College, Greenwich, by Sir James Thornhill. The painting of the ceiling was completed in 1712 (Credit: William Sheehan)

Herschel, though omitting the mention of proper motions of stars which, though not known in Flamsteed’s day, were certainly known in his, nevertheless affirmed the moral worth of precision that men like Flamsteed worked tirelessly through cold lonely nights to attain. The stars are the landmarks of the universe; and, amidst the endless and complicated fluctuations of our system, seem placed by its Creator as guides and records, not merely to elevate our minds by the contemplation of what is vast, but to teach us to direct our actions by reference to what is immutable in His works. It is, indeed, hardly possible to over-­ appreciate their value in this point of view. Every well-determined star, from the moment its place is registered, becomes to the astronomer, the geographer, the navigator, the surveyor, a point of departure which can never deceive or fail him, the same for ever and in all places; of a delicacy so extreme as to be a test for every instrument yet invented by man, yet equally adapted for the most ordinary purposes; as available for regulating a town clock as for conducting a navy to the Indies; as effective for mapping down the intricacies of a petty barony as for adjusting the boundaries of Transatlantic empires. When once its place has been thoroughly ascertained and carefully recorded, the brazen circle with which that useful work was done may moulder, the marble pillar may totter on its base, and the astronomer himself survive only in the gratitude of posterity; but the record remains, and transfuses all its own exactness into every determination which takes it for a groundwork, giving to inferior instruments—nay, even to temporary contrivances, and to the observations of a few weeks or days—all the precision attained originally at the cost of so much time, labour and expense. (Maunder, 1900:203)

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Fig. 1.7  The Royal Observatory, Greenwich, as it appeared in Flamsteed’s time. Flamsteed House, the original building of the Observatory, was designed by Christopher Wren with the assistance of Robert Hooke. It was built at a cost of £520 (only £20 over budget), and reputedly paid for by selling mouldy gunpowder to the French. As a further cost-saving measure, it largely utilised recycled materials from the foundations of Duke Humphrey’s Tower, the forerunner of Greenwich Castle; because of this, the alignment was 13 degrees from true north, rather to Flamsteed’s chagrin. Also, Flamsteed had to provide instruments and equipment for the observatory at his own cost (Public Domain)

1.5  A Tide in His Affairs Pulls Newton Back to the Moon In undertaking to draw up his star catalogue, Flamsteed’s first priority was, needless to say, the stars near the ecliptic, which were “in the Moon’s way”. These were the stars on which the method of lunars for determining longitude at sea would critically depend. From 1689, when he began to use the mural arc, he also began to take precise measures of the meridian transits of the Moon. The latter would be the measures Newton would come to covet as the means of perfecting his lunar theory. Perfectionist that he was, Flamsteed was slow to publish, and eventually this led to serious strains in his relationship with Newton. As early as 24 February 1692 (when they were still on good terms), Newton was asking for observations, only to be put off. Flamsteed says that he felt he was doing more good in continuing to

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make his observations as long as he had strength rather than use up time preparing them for the press, “to gain a little present reputation”. (Flamsteed to Newton, 24 February 1691/2; Newton, Correspondence, III:203). Newton’s breakdown intervened. By the time Newton had recovered, he began to adopt a more peremptory tone. Beginning in 1694, he made a more concerted effort, even trying a measure of flattery. He assured Flamsteed that when he published his perfected theory of the Moon (which he was still hoping to derive from the theory of gravitation) he would generously acknowledge the Royal Observatory’s accuracy. “For all the world knows”, he wrote, “that I make no observations my self & therefore I must of necessity acknowledge their Author: And if I do not make a handsome acknowledgement, they will reccon me an ungratefull clown.” (The underlinings are apparently Flamsteed’s.) Appealing strongly to Flamsteed’s self-interest, he added: And for my part I am of opinion that for your Observations to come abroad thus with a Theory wch you ushered into the world & wch by their means has been made exact would be much more for their advantage & your reputation then to keep them private till you dye or publish them without such a Theory to recommend them. For such a Theory will be a demonstration of their exactness & make you readily acknowledged the Exactest Observer that has hitherto appeared in the world. But if you publish them without such a Theory to recommend them, they will only be thrown into the heap of the Observations of former Astronomers till somebody shall arise that by perfecting the Theory of the Moon shall discovery your Observations to be exacter than the rest. But when that shall be God knows: I fear not in your life time if I should dye before tis done. For I find this Theory so very intricate & the Theory of Gravity so necessary to it, that I am satisfied it will never be perfected but by somebody who understands the Theory of gravity as well or better than I do. But whether you will let me publish them or not may be considered hereafter. I only assure you at present that without your consent I will neither publish them nor communicate them to any body whilst you live, nor after your death without an honourable acknowledgment of their Author. (Newton to Flamsteed, 16 February 1694/5; Newton, Correspondence, IV:87–88).

Newton had already made this promise to Flamsteed, inscribing on the inside of the back cover of Flamsteed’s leather Diarum Observationum: “Sept. 1. 1694. Received then of Mr fflamsteed two sheets MS of the places of the Moon observed & calculated for years 89 90 & part of 91 wch I promise not to communicate without his consent.” Both promises were later broken. As Newton defined their relationship, Flamsteed was the satellite, and derived the light of his reputation from Newton. Flamsteed, of course, saw it the other way round. It was the perennial struggle for ascendency between observer and theoretician which had already begun to emerge in the late 16th century with Tycho Brahe (1546–1601) and Kepler and would represent a division of labour which, as we shall see, would become crucial in the Neptune story. Newton saw the theory of gravity alone as capable of solving the vexed problem of the Moon’s motion, while from Flamsteed’s perspective, theories were at best only probable; observations only were absolute truth. As he put it, “Theories do not commend observations; but are to be tried by them.” (Flamsteed to Newton, 10 January 1698/9; Newton, Correspondence, IV:303).

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Ultimately, the two positions were not irreconcilable, but Newton and Flamsteed were both self-righteous and intransigent, and inevitably they headed toward a final rupture. Flamsteed continued to hold back his observations until they could be corrected for effects such as those due to refraction and reduced. Newton, infuriated by the delays, rebuked him: “I want not your calculations but your Observations only.” (Newton to Flamsteed, 29 June 1695; Newton, Correspondence, IV:134). Newton added pressure by threatening to drop the theory of the Moon altogether and blaming Flamsteed for it. When this approach failed, he tried a different tack. On 9 July 1695 he reminded Flamsteed that in three important works he had helped him when he was “stuck” and had communicated matters, such as a theory of refraction, “of more value then many Observations.” (Newton to Flamsteed, 9 July 1695; Newton, Correspondence, IV:143). Flamsteed’s response can be judged by his private notes on Newton’s letter (ibid.:144); it was, he says, “hasty artificiall unkind arrogant.” He complained that he had been ill—suffering from severe pains in the head. Newton, unhelpfully, suggested that in that case Flamsteed should follow the example of Dr. John Batteley (1646–1708) who used to “bind his head strait with a garter till the crown of his head was numbed”. Newton, Correspondence, IV:152). (Whether Newton himself followed Dr. Batteley’s prescription in dealing with his own headaches, which must have been severe as he was once more entered upon the lunar theory, is unknown.) A week later, Newton offered to pay Flamsteed’s servant to encourage him to copy more diligently. (Newton to Flamsteed, 27 July 1695; Newton, Correspondence, IV:156–157). The offer was declined. Meanwhile, Newton moved to London, where in 1696, through the patronage of Charles Montagu (1661–1715), 1st Earl of Halifax and then Chancellor of the Exchequer, he became warden of the Royal Mint. Three years later, on the death of Thomas Neale (1641–1699), he became Master of the Mint. Had he wished, he could have occupied the position as a mere sinecure. Newton, however, threw himself into it. For the time being, reforming the coinage and punishing clippers and counterfeiters were higher priorities than solving the problem of the motion of the Moon. (Manuel, 1968:229). But he did continue to work on the lunar theory in the margins of his time, though rather secretively. Thus, in 1699, Flamsteed revealed (in a letter to Dr. John Wallis (1616–1703), which Wallis planned to publish in Latin) that Newton was thus employed as evidenced by the fact that Newton had been soliciting—and he, Flamsteed, had been sending—lunar observations from the Royal Observatory, Greenwich. (Flamsteed to Wallis, 7 January 1698/9; Newton, Correspondence, IV:298). Newton was embarrassed by this divulgation. He not only resented the implication that he was stealing time from his official business (he had an absurdly demanding superego), he also did not want (as he told the diarist Samuel Pepys (1633–1703) at the same time) to have his efforts bruited about prematurely, lest he failed to find a solution. Rebuking Flamsteed, he said: I do not love to be printed upon every occasion much less to be dunned & teezed by foreigners about Mathematical things or to be thought by our own people to be trifling away my time about them when I should be about the Kings business. And therefore I desired Dr Gregory to write to Dr Wallis against printing that clause wch related to that Theory &

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W. Sheehan mentioned me about it. You may let the world know if you please how well you are stored with observations of all sorts & what calculations you have made towards rectifying the Theories of the heavenly motions: But there may be cases wherein your friends should not be published without their leave. And therefore I hope you will so order the matter that I may not on this occasion be brought upon the stage. (Newton to Flamsteed, 6 January 1698/9; Newton, Correspondence, IV:296).

Newton and Flamsteed continued their parley for several more years. At one point, an anonymous informant told Flamsteed (Manuel, 1968:308) that, according to the latest gossip from Oxford, Newton had managed to perfect his law of gravity so far that the lunar theory could be derived solely from it. If so, he would have no further need of Flamsteed’s observations. The rumour, indeed, proved false, and Newton and his confederate Halley would continue to wangle observations from Flamsteed—a rather sad business which would lead to Flamsteed’s eventual humiliation with the unauthorised and error-ridden publication of his work in 1712. As with Newton’s dispute with Hooke over the discovery of the inverse square law and with Leibniz over the invention of the calculus, this episode does not show Newton in a favourable light. His argument vis-à-vis Flamsteed was that, since the Astronomer Royal’s position was publicly supported, the observations he made at The Royal Observatory, Greenwich were public property, and as officers of the Royal Society (Newton became president—and virtual dictator—in 1703), they were within their rights to expedite the entrance of these observations into the public domain. On the other side, Flamsteed had disbursed well over half of his life ‘s salary for computing services, construction and repair of instruments, and other operating expenses, which gave him some right to retain the observations until they had been perfected to his satisfaction. The argument was bitter indeed—perhaps partly because the question it raised was, at the time, rather novel (though not quite unprecedented; it is worth recalling that Kepler had faced the same issues when after Tycho’s death he had had to wrest control of Tycho’s observations from the latter’s heirs.) Eventually, in 1714, Flamsteed was able to get some satisfaction by burning all but ninety-seven pages of three-quarters of the spurious edition, and though he died in 1719, his assistant, Abraham Sharp, was able to see the authorized three-volume Historia coelestis Britannica through the press in 1725. The magnificent Atlas colestis, with maps based on the stars catalogued and with the figures carefully following Ptolemy’s descriptions of them, appeared in 1729. (Cook, 1998:388–392) (Fig. 1.8). Even with the help of 200 of Flamsteed’s lunar observations, Newton was unable to derive the motion of the Moon, from first principles (the inverse square), to a sufficient degree of accuracy to allow the construction of accurate tables. He now abruptly changed tack. Whiteside was the first to sort all this out, in a paper aptly entitled “Newton’s Lunar Theory: from high hope to disenchantment” (Whiteside, 1976:317–328), showing how he now reverted to a heuristic model, based on that proposed as far back as 1638 by his precocious predecessor Jeremiah Horrocks (1618–1641), which involved a rotating Keplerian ellipse of variable eccentricity. This quasi-Horrocksian model was used by Newton as the basis of a series of rules (recipes) for constructing tables of the Moon’s motion which David Gregory

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Fig. 1.8  A page from Atlas Coelestis, published in 1729, showing the constellations of Orion and Taurus. Uranus, unwittingly mapped near ω Tauri, between the Pleiades and Hyades, received the Flamsteed number 34 Tauri from the French astronomer Joseph-Jérôme Lefrançais de Lalande in a revised edition of Flamsteed’s catalogue published in 1783 (Credit: Linda Hall Library of Science, Engineering & Technology)

(1659–1708) published in 1702 as Theory of the Moon’s Motion. (Gregory, 1702.) (Ironically, Newton had taken his quasi-Horrocksian model from none other than Flamsteed, who had brought it down from the Midlands and published it in his Doctrine of the Sphere, the only work Flamsteed published in his lifetime.) (Flamsteed, 1680). The centre of the Moon’s orbit is at F, rotating annually in a small epicycle about D, which in turn rotates about C with twice the average angular speed of the Sun from the Moon’s apogee. This model is purely kinematic and has been stripped of any reference to gravitation. Apart from the Horrocksian basis, Newton in addition added four small terms. No one has ever explained just how Newton derived them, though Newton, in the second edition of Principia, asserts that they were derived from the theory of gravitation; he omits to say how (Fig. 1.9). David Gregory would claim (Gregory, 1702:10) that Newton … has compassed this extremely difficult matter, hitherto despaired of by astronomers; namely, by calculation to define the Moon’s place even out of the syzygies, nay in the quadratures themselves so nicely agreeable to its place in the heavens (as he has experienced it by several of the Moon’s places observed by the ingenious Mr. Flamsteed as to differ from it (when the difference is greatest) scarce above two minutes in her syzygies, or above three

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Fig. 1.9  Newton’s quasi-Horrocksian kinematic model of the Moon’s motion, first introduced in his Theory of the Moon’s Motion (Gregory, 1702) though later modified. The diagram here shows the new epicycle of 1713, in the version suggested by Roger Cotes (1682–1716), where the point F is now the empty focus of the Moon’s ellipse that rotates semiannually about the point D (From: Newton, Correspondence, V: 298) in her quadratures;3 but commonly so little that it may well enough be reckoned only as a defect of the observation.

As against this, Flamsteed, testing the theory against observations, found errors in longitude as high as 8′ or 9′; the errors in latitude were as high as 4′. Similar errors were found in later years by Halley. These errors were serious, as they made the theory useless as a practical basis of the calculation of longitude at sea (a moot point, in any case, without an accurate star catalogue, which Flamsteed had yet to publish). Nevertheless, Halley did succeed, eventually, in making them the basis of a method that was, at least in principle, quite practicable. His idea was to compare tables from Newton’s theory with observations over a complete Saros cycle, the cycle of eclipses that recurs every 18 years, 11 days. As the errors would presumably repeat over successive cycles, they could be used, he hypothesized, in conjunction with the tables to determine accurately the Moon’s longitude for any time. Halley himself, on becoming Astronomer Royal (succeeding Flamsteed) in 1720, set out to make the requisite comparisons; by the time of his own death, in 1742, he had actually completed the task. However, his tables and the tabulated errors were not published until 1749. By then, other approaches—based on analytic solutions of the problem of the Moon’s motion according to the theory of gravitation, or improved chronometers—had become fashionable. (Sobel, 1995). Thus, Halley’s tables and errors were never actually to be applied to the problem of longitude at sea, though a comparison of Halley’s tables and tabulated errors with predictions based on a computerised modern ephemeris show that the method gives an accuracy of a minute of arc—good enough to have merited the £20,000 prize for a solution to the longitude problem as had been famously proposed by Parliament in 1714.. (In today’s values, this would be the equivalent of something like £40,000,000.)  Syzygy is the lining up of three celestial bodies (e.g., Sun, Moon, Earth) as during a solar or lunar eclipse. Quadrature is a configuration in which two celestial bodies form a right angle with a third, as at the moon’s first and last quarter. 3

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Despite the fact that Newton’s theory of the Moon’s motion produced results that were better than anything that had been achieved previously (and as adapted by Halley, much better), it ultimately represented a failed strategy. It was a kinematic theory of the motion of the centre of the Moon’s orbit, and without any foundation in gravitational dynamics. Rather than pointing a way forward, it led in a cul-de-sac. As Curtis Wilson points out (1989:267), “With hindsight we may say that Newton’s effective adoption of Horrocks’s theory, by interfering with ongoing insight into perturbations not actually embraced in that theory, proved ultimately an insurmountable obstacle to him.” It would also prove an insurmountable obstacle to those who tried to follow him as well, for: … the Newtonian theory, considered as a program to account for the celestial motions, remained nearly at a standstill during the first half of the eighteenth century. The perturbations of the Moon were not yet accurately enough known to permit determination of the longitude at sea. The vagaries of Jupiter and Saturn remained unaccounted for. At least some of the blame for this standstill must be placed upon Newton himself. In his later years he could not bear in any way to be seen to be wrong, nor to acknowledge that the solution of problems in the Principia might be improved upon. When advances finally came to be made, they involved a new mode of attack. The perturbations of the Moon were adequately addressed after a new beginning was made from the equations of motion, without prior assumption of the theory of Horrocks. As for the perturbations of the planets, the only systematic method of calculation available before the middle of the eighteenth century was numerical integration, so laborious prior to the age of the electronic computer that it was applied to celestial motions but two or three times in the eighteenth century. The new start here needed was made when Euler in 1747 introduced the method of approximating by trigonometric series. Determination of perturbations now began with a force diagram, but geometry was then left behind, the problem being formulated in the symbolism of the Leibnizian calculus. The symbolic formulation by no means guaranteed the adequacy of the solution found, and it was some forty years before Laplace with an assist from Lagrange at last showed how the major perturbational terms could be identified…. (Wilson, 1989:273)

1.6  The Calculus Ultimately, it would be French mathematicians, rather than British, who would find a way out of the cul-de-sac. Newton, during the remarkable year of creative insights 1665–66, which had included not only the early intimation of gravity reaching to the sphere of the Moon but also his celebrated work with the prism, also outlined a new and wonderful branch of mathematics, which he referred to as fluxions—now generally known as the differential calculus. The decisive event leading to the creation of this beautiful branch of mathematics was the discovery of the remarkable relation between the new problems of mechanics and the old problems of geometry, the latter consisting of drawing a tangent to a given curve and of determining areas and volumes. These problems are at the heart of the Principia, many of whose results could have been derived by Newton using the methods of fluxions. It was long believed—in large part because of Newton’s own later claims, during his priority disputes with Leibniz over the invention of these methods-that the arguments

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used in the central portions of the Principia are essentially fluxional analyses clothed in the heavy guise of traditional synthetic geometry (the latter is what it so difficult for the modern reader to decipher). However, Whiteside, after exhaustive study of Newton’s mathematical manuscripts, concludes that “this beguiling hypothesis eludes substantiation when we dare to test it.” Instead of being fluxional analysis disguised, he concludes, The published state of the Principia—one in which the geometrical limit increment of a variable line-segment plays a fundamental rôle—is exactly that in which it was written. During the two decades preceding 1684, when Newton drafted the first version of his tract De motu corporum in a foretaste of the mighty Principia to come, there is overwhelming evidence of his developing expertise with (and, to be sure, firm preference for) arguments involving one or more orders of the infinitesimally small. To his ever-deepening mastery over the limit-increments of a mathematical variable, in his Principia itself Newton added the fundamental dynamical insight that a continuous force acting over an infinitesimal time may be measured by the second-order deviation which it induces in a body from—or indeed along—its instantaneous path (this is effectively his second axiom of motion); or, equivalently, by the component of the instantaneous increment in the body’s orbital speed which is directed to the centre of force. Both approaches make use of infinitesimal limit-­ increments—of length and velocity respectively—and their effect on the elements of orbit is likewise to be measured. (Whiteside, 1970b:119).

The geometrical limit increment of a variable line-segment would find no successors able to wield it as Newton. Further progress in the examination of what was the fundamental theme of Principia, the motions of bodies in time under the action of central forces, would await their reformulation in terms of infinitesimal calculus. And while Newton being Newton had been secretive about his methods of fluxions (and did not even invent the standard dotted notation for differentials until mid-­ December 1691), in the meantime another mathematician of genius, the German Gottfried Wilhelm Leibniz, had invented it independently. Moreover, he did as Newton had not yet done—publish his result. Leibniz, moreover, invented a much more convenient notation, in which differentials were represented as quotients like dy/dx, which makes them much more effective for certain equations. In Britain, of course, Newton was a god, and the dotted form would continue to be used loyally by his countrymen—Halley, Roger Cotes, and others—for a hundred years, while Continental mathematicians, such as the Swiss Jacques Bernoulli (1655–1705) and Leonhard Euler (1707–1783), had no such scruples, and readily adopted what Laplace would call the “very happy notation” of Leibniz. (Dunham, 2005:22). This would prove crucial to the development of perturbation theory and what Laplace would call “celestial mechanics”. Indeed, during the first half of the 18th century, the singular test of the inverse square law would be the problem of the lunar apsides, as in the nineteenth it would be the wayward motion of Uranus. When Newton himself abandoned his effort to resolve the apsides problem, wrote Craig B. Waff, It seems certain that [his] sole reaction to the apogee motion discrepancy was to question the accuracy of his … calculation. Nowhere, it would appear, did he question any of the physical conditions which he had assumed in his calculations. Newton’s [attitude] in this regard may have been due to his perception that he was only a pioneer—indeed, the pio-

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neer—in the theoretical calculation of perturbation effects; he may have thought that his successors would surely find new and more accurate means of mathematically studying such phenomena, and would consequently be able to account theoretically for the entire observed mean motion of the moon’s apogee. Newton’s hope, that his younger contemporaries Halley and Cotes might push further than he had, is well known. However, it was three Continental mathematicians—Leonard Euler, Alexis-Claude Clairaut, and Jean le Rond d’Alembert—who eventually were the first to do so. (Waff, 1975:48).

1.7  The Inverse Square Law: Crisis and Resolution As Waff suggests, Newton had failed in large part owing to the want of fully adequate analytic tools. However, the analytic tools were even then rapidly being supplied by the efforts of several extraordinary 18th century mathematicians who pioneered the branch of mathematics dealing with the construction and solution of differential equations. Much of this activity was occurring, not in England, but in France. Though Descartes’s vortex theory still reigned supreme in France during the half century or so after the publication of Principia, Newton eventually found an enthusiastic following on the other side of La Manche—Voltaire (1694–1778). As Voltaire himself once put it (Durant, 1961: 246), “the French always come late to things, but they do come at last.” He was born François Marie Arouet (1694–1778), and adopted the pen name Voltaire after his first stint in the Bastille, in 1715, where he had been thrown for writing two lampoons suggesting the Regent, the Duc de Orleans, was plotting to usurp the throne. He was apparently incited by Anne Louise Bénédicte de Bourbon, Duchesse du Maine (1676–1753), who presided over the famous salon at the Court de Seaux. The salons of this period were a uniquely French institution in which, though French society was still dominated by men, women were allowed to play a leading role. (Outram, 2005.) Members of both sexes gathered for intellectual discourse, with women serving as hostesses and choosing and moderating the topics of conversation. By 1725, Voltaire had reached a high point in his success. However, success aroused jealousy, and during a dinner at the chateau of Maximilien-Henri de Béthune, fifth Duc de Sully (1669–1729), a young aristocrat taunted him over his adopted name; a quarrel ensued, Voltaire was beaten, and the following day approached the assailant’s box at the theatre and challenged him to a duel. Once more he found himself in the Bastille. On his release, he was exiled to England, and during his two years there wrote the Lettres philosophiques (Letters Concerning the English Nation). (Voltaire, 1734). Instead of Descartes, he became a disciple of Bacon, Newton and Locke. Present at Newton’s funeral at Westminster Abbey on 28 March 1727, he was discussing the question with others, “who was the greatest man—Caesar, Alexander, Tamerlane, or Cromwell? Some one answered that without doubt it was Isaac Newton. And rightly: for it is to him who masters our minds by the force of truth, and not to those who enslave them by violence, that we owe our reverence.” (Voltaire, 1734: Letter XIII.)

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On returning to France, Voltaire fell in love with Gabrielle Émilie le Tonnelier de Breteuil (1706–1749), marquise Du Châtelet, who had just re-entered society after giving birth to her third child. When they met, the marquise was 28, Voltaire 40. Her husband was much older than she was, and evidently unusually tolerant, for he accepted having Voltaire live with them at their country house at Cirey-sur-Blaise in Haute-Marne. She too had adopted Newtonianism, and was a brilliant mathematician, having studied both mathematics and physics under Pierre–Louis Moreau de Maupertuis (1698–1759) (Beeson, 1992), then under the celebrated mathematical prodigy Alexis-Claude Clairaut. (Zinsser and Courcelle, 2003). No doubt with her help, Voltaire in 1736 wrote a charming (in other words, popular and non-­ mathematical) account of Newton’s discoveries, Éléments de la Philosophie de Newton, which was an instant success. From 1740 onward, no more papers on the vortex theory would appear in the transactions of the Paris Académie des Sciences. (Pannekoek, 1961:298). The marquise herself later (in 1756) published a translation of Principia into French; for many years it was the only French translation. And so, largely through the influence of a marquise and a philosopher, Newton won what have been described as the “Newton wars” in France. (Fara, 2003:126–154). Also, it was French mathematicians—and Leonard Euler, who was Swiss but did most of his work, first, at the Imperial Russian Academy of Sciences at Saint Petersburg and then at the Berlin Academy—who developed the differential equations needed to grapple with the complications of the theories of “Le Grand Newton”. Euler, Alexis– Claude Clairaut (1713–1765), and Jean de le Rond d’Alembert (1717–1783) all succeeded in expressing the three-body problem of the Moon’s motion as three differential equations. They were not, however, able to solve them directly. Clairaut wrote of his equations that anyone “may now integrate them who can…. I have deduced the equations given here at the first moment, but I only applied a few efforts to their solution, since they appeared to me little tractable. Perhaps they are more promising to others.” (Pannekoek, 1989:299). Others, however, found them no more solvable. They remained unsolved, and there is still no exact solution. Under the circumstances, the only choice was to attempt to approach the solution by means of successive approximations. The method involves beginning with the unperturbed orbit of the body, the Moon or a planet, and then calculating the forces and accelerations due to the body perturbing it (in the case of the Moon, the Sun). This produces a slight instantaneous change in the position, leading to a further small increment in forces and accelerations (second-order deviations, so-called), and so on. In principle, one can carry the approximations as far as one wishes. However, “since in the higher orders all these terms react upon one another, their complete computation constituted from the beginning an entangled, nearly inextricable task, demanding years of work—in later times with higher standards, even an entire life of strenuous and careful work.” (Pannekoek, 1989:300). Though daunting, the approach seemed workable to mathematicians of the calibre of Euler and a few others, and by sequential steps even seemingly intractable problems such as that of the lunar apsides seemed soluble. Each investigator used a somewhat different method, but each arrived at the same disappointing result—the same that Newton had arrived at: the line of the apsides appeared to precess at half the observed value. Theory and observation appeared sharply at odds. A moment of truth

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for universal gravitation and the inverse square law had arrived. In September 1747 Euler notified Clairaut: “I am able to give several proofs that the forces which act on the Moon do not exactly follow the rule of Newton.” (Peterson, 1993:133). Seeing that Euler was willing to consider such a strategy, Clairaut also convinced himself of the necessity of abandoning the exact inverse-square law and experimented with replacing it with an inverse-square plus a higher-order term. D’Alembert, considering the same, was in no hurry to publish; “I will be very sorry to overthrow Newton,” he asserted. (Peterson, 1993:135). (To anticipate, same strategy of doubting the inversesquare would be considered when the motions of Uranus seemed intractable.) Two years on, Clairaut finally nerved himself to carry out a more rigorous calculation, taking account of some small terms in the perturbations he had at first ignored. One had to do with the motion of the Sun. Since the Moon’s motion relative to the Earth is so much more rapid than the Sun’s, the position of the latter can, to a first approximation, be regarded as fixed. This had been the approximation Newton had used, and he and his colleagues had also done so. On now looking more deeply into the matter, Clairaut discovered that the effects of the Sun’s motion were not as negligible as assumed. A detailed analysis showed the overall advance of the line of the apsides is produced by the excess of the disturbing effect of the Sun on the Moon at apogee over that at perigee. Near apogee, the line is advancing, but since the Sun’s motion is then in the same direction, the Sun tends to keep up with it, increasing its effect. At perigee, the line is regressing, but the Sun’s motion is in the opposite direction, decreasing its effect. Perturbation thus augments perturbation—adding in the one case, subtracting in the other—and when these second-­ order effects are summed, they are found to contribute just enough to bring the calculated advance of the line of the apsides into exact agreement with the observed value. Theory and observation now agreed. Newton had been rescued. As Clairaut wrote (Waff, 1975:215), “the motion of the apogee is found rather conforming to the observations, without supposing the Moon impelled toward the Earth by any force other than the one which acts inversely as the square of the distance.” “Thus”, notes Robert Grant (1852:46), “a circumstance, which at one time threatened to subvert the whole structure of the Newtonian theory, resulted in becoming one of its strongest confirmations.” Craig B. Waff declares (1975:215), “having resolved a problem whose true solution had escaped Newton and all subsequent investigators, Clairaut can perhaps be forgiven for later proudly asserting that his theoretical determination of the entire mean motion of the apogee per revolution had opened up ‘the road to the theory of the Moon’.”

1.8  The Return of the Comet Flushed with this triumph, Clairaut might well have rested on his laurels. But in 1757, he successfully completed a computation of the lunar and planetary perturbations of the Earth. At once, he was stirred to an even greater challenge, by the ambitious young astronomer Joseph-Jérôme Lefrançais de Lalande (1732–1807). (Until 1752, he used the simple patronym Le Français, when he began to write Le Français

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de la Lande. Then, with the abolition of noble titles during the French Revolution, he called himself, simply, Lalande.) This was to calculate a refined date for the forthcoming return to perihelion of the comet that Halley, dead since 1742, had predicted for 1758–59. Clairaut eagerly accepted the challenge of snaring it within the web of his differential equations. Halley had realised that the comet, whose previous returns to perihelion had occurred in 1531, 1607, and 1682, had made a close approach of Jupiter in 1681. The comet, as a result of the gravitational acceleration of the giant planet, had been forced into a somewhat larger orbit; thus its next return would clearly be delayed, but by how much? Halley’s best guess was that the delay would mean the comet would return about the end of the year 1758 or the beginning of the year 1759. He had added, “Wherefore if according to what we have already said it should return again about the year 1758, candid posterity will not refuse to acknowledge that this was first discovered by an Englishman.” (Halley, 1752). In attempting a refined prediction, Clairaut’s method was brutally straightforward. He would work out the successive positions of the comet in its orbit at intervals of several days, find the distance separating the comet at each of these points from the massive planets Jupiter and Saturn, and then compute the resulting effect on the comet’s motion and period. As with the problem of the lunar apsides, however, problems in gravitational analysis again showed a remarkable propensity to become more intricate than they appeared at first. Clairaut had originally assumed it would be unnecessary to take account of any perturbing effects on the comet except during its close passages by Jupiter and Saturn. He was dismayed to discover, however, that for the calculation to be reliable, their effects would have to be summed up over an entire orbit. Moreover, even Jupiter’s pull on the Sun had to be taken into account, since even a small displacement of the Sun’s position could have a large effect on the size of the comet’s orbit and hence on its period of revolution. This meant that the calculations would have to be carried over two full orbits of the comet—a period of 150 years. This covered the period back to the comet’s two previous returns: 1607 and 1682. To further check on the accuracy of his technique, Clairaut decided to extend the calculation back over one more orbit, to its return in 1531. In all there would be 700 calculated positions of the comet, Jupiter and Saturn, and the effects of the perturbations of the planets on the comet’s orbit. To accomplish all these computations on the short timeline before the comet’s return was far more than could be accomplished by any one man, and Clairaut enlisted the help of Lalande but, at Lalande’s suggestion, of the remarkable Nicole-Reine Étable de la Briere Lepaute (1723–1788), the wife of Louis XV’s clockmaker and a first-rate mathematician in her own right. In 1757, Madame Lepaute was living with her husband in the Palais du Luxembourg. Lalande lived in a house owned by Lepaute at the place de la Croix-rouge, near the Seine, Clairaut in the rue du Coq St-Jean (now rue des Archives); the latter was quite far from either Madame Lepaute or Lalande. (Dumont, 2007). Though not exactly certain, it seems probable that the calculations were carried out at the Palais du Luxembourg. Be this as it may, the triumvirate of Clairaut, Lalande, and Lepaute worked incessantly for six months, usually from early morning until late at night,

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Fig. 1.10  Luxembourg Palace at Sunset. Image by Benh Lieu Song (Credit: Wikipedia Commons)

and sometimes not even pausing from their calculations during meals. Perhaps it was true that as Lalande claimed the strain of these extraordinary efforts led him to contract an illness which changed his personality, which was notoriously difficult in later years. Lalande also criticised Clairaut, whom he called “judicious but weak”, for failing to make any reference to Lapaute’s contribution, for no better reason, he said, than “in order to accommodate a woman jealous of Mme. Lepaute’s merit, and who had pretentions but no knowledge whatever.” (Lalande, 1803) (Figs.  1.10 and 1.11). By 14 November 1758, the calculating triumvirate had reached a tentative result, which Clairaut presented to the Academy of Sciences of Paris. The immense calculations had been worked through completely but only partly for Saturn; the latter were therefore only estimated. They indicated that the comet would reach perihelion within a month either side of mid-April 1759. The uncertainty was owing to doubt about the date of the perihelion passage of 1683. Even then it was already being searched for by Charles Messier (1730–1817), with the 4 ½-foot Newtonian reflector in the rooftop observatory of the palace of Clûny built by Joseph Nicolas Delisle (1688–1768). Messier was not yet well-­ known but would soon go on to become France’s best-known “ferret of comets”. Delisle, whose employee Messier was, and Messier knew of Halley’s prediction but there is no evidence that they took any account of the revisions by Clairaut and his collaborators. During much of the winter of 1758 the skies were cloudy over Paris, but on the clear evening of 21 January 1759, Messier picked up a suspicious object

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Fig. 1.11  A Celestial Triumvirate: Clairaut, Lalande, and Madame Lepaute (Credit: Wikipedia Commons)

in Pisces. It would indeed prove to be the comet whose return Halley had predicted. With some difficulty Messier and Delisle, contained their excitement, wanting to be certain that it was indeed the comet, not merely an unexpected interloper. They followed the object until mid-February, when it disappeared in the evening twilight, not to reappear into the morning sky until April. (Baum and Sheehan, 1997:34–35). Though so far Messier and Delisle apparently thought they had the comet all to themselves, they had been forestalled. A German farmer and amateur astronomer, Johann Georg Paltizsch (1723–1788), who lived at Prohlis, near Dresden, had already picked it up on Christmas night 1758. Throughout April and May 1759, Halley’s comet, as it was known henceforth, was widely observed (Fig. 1.12). The orbit calculated by Lalande and others showed that it had passed perihelion on 13 March—thus the prediction of Clairaut, Lalande, and Lepaute was off by only 33  days. Once again, universal gravitation had proved resoundingly triumphant. “The return of the Comet of 1682 in the time prescribed by the Newtonian theory”, exulted Clairaut, “is one of the brilliant triumphs of physics and marks an epoch in this science.” (Clairaut, 1760:5). It was the 18th century equivalent of the discovery of Neptune. The “lawless” comets had been subordinated to the Newtonian law, the promise that Halley had offered in the tribute he prefixed to the Principia (Newton, 1934:xiv) completely and spectacularly fulfilled: Now we know The sharply veering ways of comets, once A source of dread, nor longer do we quail Beneath appearances of bearded stars.

Clairaut was lionized in all the fashionable salons. The French writer Charles Bossut summarised, “engaged with suppers, late nights, and attractive women, desiring to combine pleasures with his ordinary work, he deprived himself of rest, health, and finally, at the age of only 52, of life.” (Baum and Sheehan, 1997:35). By then, the

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Fig. 1.12  Halley’s comet over Paris in 1759, as painted by Samuel Scott (Credit: Library of Congress)

torch had passed to a younger generation of mathematical astronomers. Clairaut’s solution of the problem of the lunar apsides and spectacular success with Halley’s comet had inspired unshakeable confidence in Newton’s theory of gravitation to account for all the phenomena of the planetary motions. One of the greatest of the younger generation, Pierre-Simon de Laplace, was still only 35 when, in 1784, he solved the greatest problem in planetary perturbations, the aforementioned “Great Inequality” of Jupiter and Saturn. These two giant planets exert marked perturbations on each other, producing irregularities in their motions first recognised by Kepler in 1625. He found that the tables of Jupiter made the mean motion (the rate of motion if it moved in a circular orbit) too slow, and that of Saturn too rapid. In other words, Jupiter seemed to be speeding up, Saturn slowing down. Halley verified this result and attempted to explain them on the basis of their mutual gravitational attractions. Later astronomers showed that the acceleration of Jupiter necessarily would be offset by a retardation of Saturn, but also wrestled with a further, and singularly embarrassing result: The Solar System ought, in the course of ages, lose its two most prominent members, with Jupiter falling into the Sun and Saturn being driven into the depths of space. (Baum and Sheehan, 1997:38) (Fig. 1.13). Laplace began with the well-known fact that Jupiter’s period of revolution around the Sun, 11.86 years, and Saturn’s, 29.57 years, are very nearly in the ratio 5:2. In other words, Saturn completes almost exactly two revolutions for every five completed by Jupiter, which is to say, the periods are nearly commensurable. The 5:2 ratio means that after twenty years (more precisely, after 19.86 years), Jupiter and

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Fig. 1.13  Pierre Simon de Laplace. His greatest result was an illusory one-the apparent stability of the Solar System (Credit: Luestling—commonswiki)

Saturn line up (are in conjunction) in the same position relative to the Sun. However, each conjunction is displaced 120° towards the west from the last one, so that after three successive conjunctions, or 60 years, the planets join up together again in the same part of the sky. (Baum and Sheehan, 1997:39). If the 5:2 resonance were exact, the conjunctions of Jupiter and Saturn would repeat in exactly the same place every sixty years. Then there would be a pull in the same direction each time, and after many revolutions there would be a transfer of momentum and a shift of orbits. However, the Jupiter/Saturn orbital resonance is not exactly 5:2 but much more nearly in the ratio of 72:29. This means that though the conjunctions take place almost 120° apart, each set of three is shifted 8.37° from the last. Rather than an exact resonance, there is only a near-resonance. But near-­ resonance relationships have no dynamic significance. After each cycle, the relative position of the bodies shifts, and so over astronomically short timescales these are quite as random as those of bodies that are nowhere near resonance. Laplace showed that in the case of Jupiter and Saturn, there was an apparent retardation of Saturn, and an apparent acceleration of Jupiter; these had reached their maximum values, 9027.7″ in the case of Saturn and 3856.5″ in that of Jupiter, about the year 1560. The two planets then began to decrease towards the values of their mean motions, which they attained in 1790. Afterwards, the motion of Saturn began to accelerate, while that of Jupiter was retarded. That is, for 450 years Saturn’s orbit is gradually enlarging and Jupiter’s shrinking, then, during the next 450 years, the situation is reversed, with Saturn’s orbit shrinking and Jupiter’s enlarging. The effects cancel out, and there is no net change over the whole period of 900  years. This result was

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reassuring: It seemed that Jupiter and Saturn would remain in the Solar System after all! (Baum and Sheehan, 1997:39). Laplace, playing off a number of results along the same lines by his great rival Jean Joseph Lagrange (1736–1813), proceeded from his solution of the “Great Inequality” to his celebrated theorems that the eccentricities and inclinations of the planets cannot vary except within very narrow limits. These theorems, he believed, proved the long-term stability of the Solar System. In his own words: I have succeeded in demonstrating that whatever be the masses of the planets, in as much as they all move in the same direction, in orbits of small excentricity, and little inclined to each other; their secular inequalities will be periodic, and contained within narrow limits, so that the planetary system will only oscillate about a mean state, from which it will deviate but by a very small quantity; the planetary ellipses therefore always have been, and always will be nearly circular, from whence it follows that no planet has ever been a comet at least if we only calculate upon the mutual actions of the planetary system…. (Laplace, 1809:2,45).

Possessed of this result, Laplace had no need of God to provide a steady hand on the dynamical tiller (or even so much as an occasional tilt at it every so often to wind it up). Indeed, upon presenting the first two volumes of his masterpiece, the Mécanique céleste, to Napoleon in 1799, the First Consul is said to have remarked (according to a diary entry of William Herschel, who was present at the time), “M. Laplace, they tell me you have written this large book on the system of the universe, and have never even mentioned its Creator”, to which the mathematician replied, “I have had no need of that hypothesis.” The words are ambiguous; it is not clear whether Laplace meant to imply that he was an atheist, though Lalande certainly thought he was. (Hahn, 2005:172) (Figs. 1.14 and 1.15). Of his achievement in explaining the intricate planetary dance on the basis of the gravitational theory, Laplace would write: I found that all the observed phenomena of the motions of these two planets adapted themselves naturally to the theory; they before had seemed to form an exception to the law of universal gravitation; they are now become one of the most striking examples of its truth. Such has been the fate of this brilliant discovery, that every difficulty that has arisen has only furnished a new subject of triumph for it, which is the most indubitable characteristic of the true system of nature. (Laplace, 1809:vol. 2, 55–56).

With such confidence, astronomers fully expected that the new planet discovered by March 1781 (to be discussed in Chap. 2) would also follow the theory. According to Laplace, the planet after its discovery was, indeed, moving expressly in the manner demanded by gravitational perturbation theory: The planet Uranus, though lately discovered, offers already incontestable indications of the perturbations which it experiences from the action of Jupiter and Saturn. The laws of elliptic motion do not exactly satisfy its observed positions, and to represent them its perturbations must be considered. This theory, by a very remarkable coincidence, places it in the years 769, 1756, and 1690, in the same points of the heavens, where Le Monnier, Mayer, and Flamsteed, had determined the positions of three stars, which cannot be found at present: this leaves no doubt of the identity of these stars with the new planet. (Laplace, 1809:vol. 2, 58).

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Fig. 1.14  The 18th century’s Newtonian world, consisting of a rather simple Solar System: six planets move in nearly circular paths round the Sun, as various comets crisscross their paths as they travel in more elongated orbits. From: John Theophilus Desaguliers, Course of Experimental Philosophy, Vol. II, 1744 (Public Domain)

On this note of complacency ended the 18th century. Acknowledgements  In addition to my co-editors Robert W. Smith and Carolyn Kennett, I am grateful to the late Craig B. Waff, for providing a copy of his unpublished Ph.D. thesis and many valuable discussions about the problem of the lunar apsides, Derek Whiteside, and Curtis Wilson. Trudy E.  Bell and Larry d’Antonio commented on early material. Greg Laughlin and Jacques Laskar provided valuable inputs on the problem of the stability of the Solar System. Françoise Launay kindly shared her expertise on French mathematical astronomy in the 18th century, especially regarding Clairaut, d’Alembert, Madame Lepaute, and Lalande.

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Fig. 1.15  James Ferguson’s orrery, showing the “clockwork universe” of Newton. The irony is that Newton himself never had a “clockwork universe”, though that is how it came to be known later. In fact, Newton would have been horrified that God had been excluded. From: James Ferguson, Astronomy Explained Upon Sir Isaac Newton’s Principles and Made Easy To Those Who Have Not Studied Mathematics. London, 1756 (Public Domain)

Appendix 1.1 Elements of a Planetary Orbit By way of background, at the heart of the mathematical investigation that led to the discovery of Neptune are various concepts of celestial mechanics, beginning with the basic Keplerian ellipse (the path a planet travels round the Sun at one focus when unperturbed) which is described by six parameters known as elements. The subject matter proceeds to the case where this simple orbit is perturbed by the gravitational attraction of another planet, and so to several other planets, with the resulting orbit defined by a series of complicated formulae. This is known as the forward perturbation problem. In the hands of Laplace, Lagrange, and others, 18th century astronomers had become very skillful in representing the perturbation of planet P’s orbital elements by another planet P′ as a periodic series representing the elements of an orbit as a function of time. Let the orbital elements, e, a, i, Ω, ω, and ν of planet P, which is

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that to be perturbed, be defined as follows, with the orbital elements of the perturber represented by the same symbols accented: e, the eccentricity, defining the shape (elongation) of the ellipse, where e = 0 is a circle and e = 1 is a line; a, the semimajor axis; the sum of the perihelion and aphelion distances divided by 2; i, the inclination, or vertical tilt of the ellipse with respect to the reference plane (e.g., plane of the Earth’s orbit, known as the ecliptic); Ω, the longitude of the ascending node of the ellipse, where the orbit tilts upward relative to the reference point, the First Point of Aries, symbolised by ♈; ω, the argument of the perihelion, defining the orientation of the ellipse in the orbital plane, as the angle measured from the ascending node to the perihelion; ν, the true anomaly at epoch Mo, which defines the position of the body at the epoch, a specific point in time. In addition, to compute the perturbations, it is necessary to know the masses of the two planets, m and m′, and n and n′, the mean motion of the two planets. A significant role in these computations is played by the perturbation function, introduced by Lagrange, which can be developed into a (very complicated) power series in e1, e2, and sin2I/2, where I is the angle between the two planetary orbits. Applications of perturbation theory are mentioned throughout the book, especially as related to Adams’s and Le Verrier’s calculations of the position of the unknown planet disturbing Uranus described in Chap. 4 and 5. The literature is vast, highly technical, and mostly in French, with the most significant contributions including those of Lagrange, Laplace, Pontécoulant, and Tisserand. Most of the more recent treatments have recast the classical methods in terms of vectorial notation. The best single-volume text for English-speaking readers with a good knowledge of differential equations that follows the classical methods is probably still (Moulton, 1914) (Fig. 1.16).

Fig. 1.16  Diagram showing the elements of a planetary orbit (Credit: Wikipedia Commons)

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References Adrian, Lord E.D., 1963. Newton’s Rooms at Trinity. Notes and Records of the Royal Society, 18:17–24. Airy, G.B., 1884. Gravitation: an elementary explanation of the principal perturbations in the Solar System. London, Macmillan Aiton, E., 1989. The Cartesian vortex theory. In: Taton, R. and Wilson, C., Planetary astronomy from the Renaissance to the rise of astrophysics, Part A: Tycho Brahe to Newton. Cambridge, Cambridge University Press:206–221. Alexander, H.G., 1956. The Leibniz-Clarke Correspondence. Manchester, Manchester University Press. Baum, R. and Sheehan, W., 1997. In Search of Planet Vulcan: the ghost in Newton’s clockwork. New York, Plenum. Beeson, D., 1992. Maupertuis: an intellectual biography. Oxford, Voltaire Foundation. Chandrasekhar, S., 1995. Newton’s Principia for the Common Reader. Oxford, Clarendon Press. Chapman, A. and Bennett, J., 1996. Astronomical Instruments and their Users: Tycho Brahe to William Lassell. Aldershot, Variorum Collected Studies. Clairaut, A., 1760. Théorie du mouvement des comètes, dans laquelle on a égard aux altérations que leurs orbits eprouvent par l’action des planètes. Avec l’application de cette théorie à la comète qui a été observe dans les années 1531, 1607, 1682, & 1759. Paris, Michel Lambert. Cook, A., 1998. Edmond Halley: charting the heavens and the seas. Oxford, Clarendon. Cudworth, W., 1889. Life and Correspondence of Abraham Sharp. London, Sampson Low, Marston, Searle and Livingston, Ltd. Derham, W., 1713. Physico-Theology: Or, A Demonstration of the Being and Attributes of God from His Works of Creation: Being the Substance of Sixteen Sermons Preached in St. Mary-le-­ Bow Church, London, at the Honourable Mr. Boyle’s Lectures, in the Years 1711, and 1712. London, W. Innys. Dreyer, J.L.E., 1953. A History of Astronomy from Thales to Kepler. New York, Dover reprint of 2nd ed., 1906. Ducheyne, S., 2011. Newton on action at a distance and the cause of gravity. Studies in History and Philosophy of Science 42, 154–159. Dumont, S., 2007. Un astronome des lumières: Jérôme Lalande. Paris, Vuibert/Observatorie de Paris. Dunham, W., 2005. The Calculus Gallery: masterpieces from Newton to Lebesgue. Princeton and London, Princeton University Press. Durant, W., 1961. The Story of Philosophy. New York, Washington Square Press. Fara, P., 2003. Newton: the making of genius. New York, Columbia University Press. Flamsteed, J., 1680. The Doctrine of the Sphere, grounded on the motion of the earth, and the antient Pythgorean or Copernican system of the world. London, A. Godbid and J. Playford. Grant, R., 1852. History of Physical Astronomy. London, Robert Baldwin. Gregory, D., 1702. A New and Most Accurate Theory of the Moon’s Motion; whereby all here Irregularities may be solved, and her Place truly calculated to Two minutes. London, A. Baldwin. Hahn, R., 2005. Pierre-Simon Laplace, 1749-1827: A Determined Scientist. Cambridge, Mass., Harvard University Press. Halley, E., 1752. Astronomical Tables with Precepts both in English and Latin. Herschel, F.G.W., 1861. Outlines of Astronomy. Philadelphia, Blanchard and Lea, 4th ed. Jardine, L., 2003. The Curious Life of Robert Hooke: the man who measured London. London, HarperCollins. King, H.C., 1979. The History of the Telescope. New York, Dover, reprint of 1955 ed. Lalande, J.J. 1803. Madame Nicole-Reine Etable de la Briere Lepaute. In: Bibliographie astronomique, avec l’histoire del’astronomie depuis 1781 jusqu’à 1802. Paris, De l’Imprimerie de la République:676-687.

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Laplace, P.S., 1809. System of the World, Pond, J. (trans.), 2 vols. London, R. Phillips. Lequeux, J., 2015. Le Verrier—Magnificent and Detestable Astronomer. Sheehan, B. (trans.), New York, Springer. Machin, J., 1729. The Laws of the Moon’s Motion According to Gravity. In: Mathematical Principles of Natural Philosophy, Motte, A. (trans.). London, Hibbert, 2 vols. Manuel, F.E., 1968. A Portrait of Isaac Newton. Cambridge, Mass., Belknap Press of Harvard University Press. Maunder, E.W., 1900. The Royal Observatory Greenwich: a glance at its history and work. London, The Religious Tract Society. Moulton, F.R., 1914. An Introduction to Celestial Mechanics. New  York, Dover, 1970 reprint of 2nd ed. Newton, I., 1756. Four Letters from Sir Isaac Newton to Doctor Bentley Containing Some Arguments In Proof of a Deity. London, J. and R. Dodsley. Newton, I., 1934. Sir Isaac Newton’s Mathematical Principles of Natural Philosophy and his System of the World. Cajori, F. (ed.). Berkeley, University of California Press. Florian Cajori’s revision of Andrew Motte’s 1729 translation. Newton, I., 1959–1977. The Correspondence of Isaac Newton. Turnbull, H.W., Scott, J.F., Hall, A.R. and Tilling L. (eds.), Cambridge, England, published for the Royal Society at the University Press. Six volumes under different editors were published over 18 years. Newton, I., 1975. Isaac Newton’s Theory of the Moon’s Motion (1702) , Cohen, I. Bernard (trans.), Folkestone, Wm. Dawson & Sons. Newton, I., 1989. The Preliminary Manuscripts for Isaac Newton’s 1687 Principia 1684–85: facsimiles of the original autographs, now in Cambridge University Library. With an Introduction by D.T. Whiteside. Cambridge, Cambridge University Press. Outram, D., 2005. The Enlightenment. Cambridge, Cambridge University Press. Pannekoek, A., 1989. A History of Astronomy. New York, Dover, reprint of 1961 ed. Peterson, I., 1993. Newton’s Clock: Chaos in the Solar System. New  York, W.H.  Freeman and Company. Sheehan, W.P. and Dobbins, T.A.., 2001. Epic Moon: a history of lunar exploration in the age of the telescope. Richmond, VA: Willman-Bell. Sheehan, W. and Westfall, J., 2004. The Transits of Venus. Amherst, New York, Prometheus. Sobel, D., 1995. Longitude. New York, Walker. Stukeley, W., 1936. Memoirs of Sir Isaac Newton’s Life. London, Taylor and Francis. Teeter Dobbs, B.J. and Jacob, M.C., 1985. Newton and the Culture of Newtonianism. Atlantic Highlands, NJ, Humanities Press. Voltaire, 1734. Lettres ecrites de Londres sur les Anglois et autres sujets. Basle. Waff, C. B., 1975. Universal Gravitation and the Motion of the Moon’s Apogee: the Establishment and Reception of Newton’s Inverse-Square Law, 1687–1749. Baltimore, Maryland, Johns Hopkins University, Ph.D. dissertation. Westfall, R.S., 1980. Never at Rest: a biography of Isaac Newton. Cambridge, Cambridge University Press. Westfall, R.S., 1993. Life of Isaac Newton. Cambridge, Cambridge University Press. Whiteside, D.T., 1970a. Before the Principia: the maturing of Newton’s thoughts on dynamical astronomy, 1664-1684. Journal for the History of Astronomy 1, 5-19. Whiteside, D.T., 1970b. The Mathematical Principles Underlying Newton’s Principia Mathematica. Journal for the History of Astronomy 1, 116–138. Whiteside, D.T., 1976. Newton’s Lunar Theory: from high hope to disenchantment.Vistas in Astronomy, 19, 317–328. Wilson, C., 1989. The Newtonian Achievement in Astronomy. In Taton, R. and Wilson, C. (eds.), Planetary Astronomy from the Renaissance to the Rise of Astrophysics, Part A: Tycho Brahe to Newton. Cambridge, Cambridge University Press. Wilson, C., 2002. Newton and celestial mechanics. In. Cohen, I.B. and Smith, G.E. (eds.), The Cambridge Companion to Newton. Cambridge, Cambridge University Press. Zinsser, J.P. and Courcelle, O., 2003. A remarkable collaboration: the marquis Du Chatelet and Alexis Clairaut. SVEC (now Oxford University Studies in the Enlightenment) 12:107–120.

Chapter 2

Planetary Discoveries Before Neptune: From William Herschel to the “Celestial Police” William Sheehan and Clifford J. Cunningham

Abstract  From time immemorial, Saturn had marked the outermost visible boundary of the Solar System. Then, in March 1781, the Solar System was suddenly doubled in size with the telescopic discovery of Uranus, by a German immigrant musician and independent  astronomer, William Herschel. Naturally, doyens of celestial mechanics attempted to calculate an orbit for Uranus, based on the inverse-­ square law, whose prestige had been greatly enhanced by its success in solving problems such as the advance of the lunar apsides and the return of Halley’s comet. The calculation, however, proved to be more difficult than expected. Meanwhile, the interest of astronomers was seized by the discovery in 1772 of an empirical numerical relationship (the Titius-Bode law, or Bode’s law as it was known throughout the 19th century). This gave the distances from Mercury to Saturn according to a series of numbers, 4, 7, 10, 28, 52, 100. There was one exception—number 28 appeared as a gap between Mars and Jupiter. When Uranus was discovered, it too fit the scheme, but the gap between Mars and Jupiter remained, and inspired a number of astronomers in Germany to speculate about the possibility that a planet might be found in that location. They began to organize a cooperative search in an effort dubbed the “Celestial Police,” but were forestalled by Giuseppe Piazzi’s discovery on New Year’s Day 1801 of the new planet Ceres, now known to be the largest asteroid and the closest dwarf planet. Later, uncoordinated, efforts by two  members of the Celestial Police additionally yielded Pallas, Juno, and Vesta. Meantime, back in the outer solar system, Uranus refused to follow the orbit calculated for it. After several decades, astronomers began to consider, among alternatives, the possibility that a massive planet even more distant than Uranus might be perturbing it from its course…

W. Sheehan Independent Scholar, Flagstaff, AZ, USA C. J. Cunningham () University of Southern Queensland, Toowoomba, QLD, Australia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_2

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2.1  A Date with Destiny: William Herschel Discovers Uranus William Herschel had a date with destiny, which he met on the night of 13 March 1781. He had just moved into a house at 19 New King Street in the city of Bath, which he had lived in previously and soon regretted leaving since it had an excellent workshop to accommodate the mirror-making activities that had been as much a preoccupation as music, his professional calling, for several years. He was alone that night as his sister Caroline Herschel (1750–1848), still tying things up at their former residence on Rivers Street, had not yet arrived. He set up his most successful telescope so far, a seven-foot telescope with which he had been carrying out a systematic “review of the heavens,” in the southward-facing garden, “behind the house and beyond its walls all open as far as the River Avon,” and placed his eye at the ocular. (Sheehan and Conselice 2015:37) (Fig. 2.1). This “review of the heavens” was actually Herschel’s second. His first, which he had begun in August 1779 had involved examining all the stars brighter than fourth magnitude that were visible at his latitude, in order to determine which of them might be double stars. On the very first night of searching, he discovered what

Fig. 2.1  The seven-foot reflector used by William Herschel to discover Uranus. It is displayed at the William Herschel Museum, Bath. (Credit: William Sheehan)

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previous astronomers had missed—Polaris, the Pole Star, was double. At the time, astronomers believed that double stars were simply chance alignments of stars along a common line of sight, rather than genuinely associated with one another, and that was the reason for Herschel’s interest. (Sheehan and Conselice 2015:37). As first proposed by Galileo Galilei in 1632, it might be possible, in that case, to estimate the distances of stars. (Sheehan and Conselice 2015:37). Assuming the fainter star to be much more distant than the brighter star companioned with it, one might, by carefully measuring the separation between them from opposite sides of the baseline of the Earth’s orbit (say in January and in June), detect a parallax, a slight shift in position that would enable the distance of the brighter star to be calculated. During his first “review”, Herschel had carefully listed all the stars that had appeared double and had measured their separations. Being Herschel, he was not willing to stop there, but decided to take the project further, and so in 1780 set about a much more ambitious “second review”, in which he hoped to examine all the stars that had been listed by John Flamsteed in his great catalogue (see Chap. 1) down to the 8th magnitude. It was not by accident, therefore, that Herschel came to the place in the heavens he did on his “date with destiny”. His logbook shows that on 12 March, he looked at Mars at 5:45 in the morning. “Mars”, he wrote, “seems to be all over bright but the air is so frosty & undulating that it is possible there may be spots without my being able to distinguish them.” At 5:53 he added: “I am pretty sure there is no spot on Mars.” He next looked at Saturn and found, “The shadow of Saturn lays [sic.] at the left upon the ring.” The next entries were made under the date of March 13: Pollux is followed by 3 small stars at about 2′ and 3′. Mars as usual. In the quartile near Zeta Tauri the lowest of two is a curious either nebulous star or perhaps a comet. A small star follows the Comet at 2/3rds of the field’s distance. (Sheehan and Conselice 2015:39).

It is clear from these notes that Herschel had observed Pollux and Mars in the morning, and then, presumably after returning from giving concerts, had recommenced his double star search that evening with the ocular magnifying 227×. He was sleuthing out the Flamsteed stars in the interesting region between M1 (the Crab Nebula) in Taurus and M35, an open cluster in Gemini. Though the telescopes used by astronomers elsewhere showed the stars as (recalling Shakespeare in the Merchant of Venice) “patines of bright gold,” Herschel’s were alleged to have such superior optics that even with magnifying powers much greater than used by other astronomers (including those at the Royal Greenwich Observatory at the time) they purportedly showed single stars as “round as a button.” (This was true at least of his telescopes up until 1786, which were of the “Newtonian” type; after 1786, he applied the “front view” system in all his largest instruments, which seem to have suffered more from optical distortions.) (Mauer and Forbes 1971; Ceragioli 2018). On that particular evening in March, Herschel came across a “star” that was not only “round as a button” but noticeably larger than the other buttons strewn across

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the field. A new planet had “swum into his ken,” to recall John Keats’s famous line written thirty-five years later, but Herschel did not yet realise it. All he could say for certain was that it was not a fixed star. It had a disk, which became even clearer as he boosted the magnifying power to 460x and 932x. Cautiously, he had noted it down as “perhaps a comet.” (Herschel 1912:vol. 1, 30.) Measuring its position with his micrometer, the object’s motion was apparent to Herschel within a few hours, and he continued to track its motion on successive nights. Whatever it was, it was held firmly in the embrace of the Sun’s gravitational force. It was not a nebula. It must, then, be a comet. The paper he wrote about it bears the title, “An Account of a Comet”. Dated 22 March 1781, Herschel presented it the Bath Philosophical Society, and as usual his friend William Watson (1744–1824) communicated it to the Royal Society on 26 April 1781. Herschel gave a few additional details about the discovery. At “between ten and eleven” in the evening, While examining the small stars in the neighborhood of H Geminorum [1 Geminorum], I perceived one that appeared visibly larger than the rest; being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet. (Herschel 1912:vol. 1:30).

Charles Messier, referred to by his sovereign Louis XV (1710–1774) of France as the “ferret of comets”, found Herschel’s achievement almost unbelievable. Sending his congratulations to “Monsieur Hertsthel at Bath,” he exclaimed: “I cannot conceive how you were able to return several times to this star—or comet … since it had none of the characteristics of a comet.” (Lubbock 1933:86). Messier seems to have regarded the discovery as a happy accident. Herschel himself knew better. The discovery was inevitable, it lay in the course of things given the quality of his telescope and the thoroughness of his methods (including the unprecedented use of high magnifying powers for systematic research). As he later explained in a manuscript autobiography: In the regular manner I examined every star of the heavens, not only of that magnitude but many far inferior, it was that night its turn to be discovered. I had gradually perused the great volume of the Author of Nature and was now come to the page which contained [it]. Had business prevented me that evening, I must have found it the next, and the goodness of my telescope was such that I perceived its … disc as soon as I looked at it; and by the application of my micrometer, determined its motion in a few hours. (Ibid, 1933:40)

Herschel obtained a dozen micrometric measures of the object’s diameter from March 17 to April 18. They seemed to show a steady increase in its apparent diameter from 2″.53 to 5″.2, which dovetailed with his expectation that it was then approaching the Earth. Actually—as became evident only after the orbit had been worked out—it was receding, thus getting smaller. (For the parameters of an orbit in celestial mechanics, see Appendix 1.1.) Herschel’s positional measures of the successive places of the supposed comet were also puzzling; they failed to show a regular path for it. In this case, Herschel himself was able diagnose the problem: it was a faulty setting of the micrometer

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Fig. 2.2 William Herschel. Portrait by Lemuel Francis Abbott, 1784 (Credit: National Portrait Gallery)

threads. (It was not the last time, by the way, that observing with defective instrumentation would lead Herschel to erroneous conclusions; a few years later, using his preferred front view arrangement in which light was deflected from a tilted primary mirror out of a hole on the side of the tube, he would detect an apparent “ring” of Uranus, which as we now know was a matter of the astigmatism of this optical system. [Baum 1973:106–119]). It presaged the similar case of the ring of Neptune “discovered” by William Lassell (1799–1880). (See Chap. 8) (Fig. 2.2). By now, Herschel’s discovery was coming under scrutiny by professional astronomers, notably the Rev. Thomas Hornsby (1733–1810) at the Radcliffe Observatory, Oxford, and Nevil Maskelyne (1732–1811), the Astronomer Royal at Greenwich. Hornsby began rather fumblingly; he could not even locate the object for two or three weeks, and when he finally did so on April 14, it was only after receiving further guidance from Herschel. With some embarrassment he realised he had already seen it on March 19 and 30, but had done so “unknowingly”. Still curiously uncometary, it appeared “perfectly sharp upon the edges, and extremely well defined, without the least appearance of any beard or tail.” (Lubbock 1933:40). Hornsby confidently believed, though on what basis is unclear, that Herschel had discovered Lexell’s “lost comet” of 1770 and expressed eagerness in computing the orbit.

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In an era long before electronic computers, even the best methods for computing a comet orbit were tedious and drawn out. However, a new method, which has been described as “the first complete analytical orbit-determination procedure that had a good degree of practicality” (Marsden 1995:183), was read to the French Academy of Science by its author, Laplace, just eight days after Herschel found his object. Owing to the coincidence in timing, it appears that Herschel’s discovery was the first to which Laplace’s method was applied. (Gillispie 1997:289). In outline, the method is as follows. (Moulton 1914:191–199). In the absence of perturbations (i.e., the ideal case), the motion of any body in the Solar System will be determined if one knows six initial conditions of motion, e.g., three spatial coordinates and three components of velocity in relation to the Sun at an initial time of observation. In practice, this means making three observations giving the comet’s right ascensions and declinations at three different times. Next one derives from these a geocentric longitude and latitude and calculates the rate of change and the rate of rate of change. (For the reader familiar with the differential calculus, these are, of course, the first and second time-derivatives.) Though the method works best for parabolic orbits, it can be used, at least in principle, for elliptical orbits. (In practice, the generally poor determination of the derivatives makes this rather difficult.) In addition to Laplace’s pioneering attempt, several others tried to derive the orbit of Herschel’s comet. They were Pierre Méchain (1744–1804) in Paris, the Ragusan Jesuit Roger Joseph Boscovich (1711–1787), then living in Paris, and Anders Lexell (1740–1784), a professor of mathematics at St. Petersburg in Russia but then in London where his attention to the problem may have been drawn by Hornsby. Méchain gave the perihelion distance of the comet as 0.46 AU and the perihelion date as 23 May 1781. Lexell, using a somewhat longer baseline of observations, computed the perihelion distance of 16  AU, with perihelion not to be reached until 10 April 1789. Clearly, the results were inconsistent with one another. (Baum and Sheehan 1997:50–51). Meanwhile, Maskelyne had begun to suspect that what Herschel had found was not a comet at all. Writing on 4 April 1781, he noted that its motion was “very different from any comet I ever read any description of or saw”, to which he added on April 23, it was “as likely to be a regular planet moving in an orbit very nearly circular around the sun as a comet moving in a very eccentric ellipsis.” (Herschel 1912:vol. 1, p. xxx). A more definitive result was obtained on May 8, when Messier’s friend Jean-­ Baptiste Gaspard Bouchart de Saron (1730–1794) demonstrated to the Paris Académie des Sciences that the object was very remote—at least 14 AU from the Sun. If so, then it was not a comet at all but a planet. Boscovich, having juggled several different orbits, concluded that the best approximation was a circular orbit of large radius. Lexell also continued to refine his calculations. Trying to fit the motion to several parabolas, he found perihelion distances from 14 to 18 AU. After combining Herschel’s observation of March 17 with Maskelyne’s of May 11, he settled on a circular orbit with radius 18.93 AU and a rate of motion of only about 4o20′ per year. (Alexander 1965:39) (Fig. 2.3).

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Fig. 2.3  One of the first to attempt to calculate the orbit of Uranus: Roger Joseph Boscovich, S.J. Portrait by Robert Edge Pine, London, 1760 (Credit: Wikipedia Commons)

The planet slipped away into the glare of the Sun in late May. When it emerged again in July, further positions were obtained by Augustin Darquier (1718–1802) (best remembered today for his discovery in 1779 of the Ring Nebula). Johann Elert Bode (1747–1826) of the Berlin Observatory analysed them and did not hesitate to pronounce in the Berliner Jahrbuch that Herschel’s comet was indeed a hitherto unknown planet at a distance from the Sun of 19  AU. (Alexander 1965:32–33). Remarkably, the size of the known Solar System had doubled! Herschel seems to have been one of the last to appreciate the importance of his own discovery. He was like Columbus, the man who having found an authentic New World believed he had simply reached the East Indies—conceiving, as do most of us, that the world holds fewer surprises than is usually the case. As late as November, when Joseph (later, Sir Joseph)  Banks (1743–1820) awarded him the prestigious Copley medal of the Royal Society, Herschel still seemed reluctant to acknowledge the object’s significance, admitting only: “With regard to the new star I may still observe that tho’ we are not sufficiently acquainted with its nature, yet enough has been seen already to shew that it differs in many essential particulars from Comets and rather resembles the condition of Planets.” (Herschel 1912:vol. 1, 95). In fact, by then there could no longer be any doubt. What Herschel had done was nothing less than introduce a new major planet of the Solar System, the first to be added in recorded history.

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Naming the new object proved to be a haphazard affair. While Herschel named it Georgium Sidus to honor his patron King George III of Great Britain, Jean Bernoulli (1744–1807), astronomer at the Berlin Academy, suggested Hypercronius (“above Saturn”), while Bode first proposed the name Uranus. (Alexander 1965:51). Many preferred to call it, simply, “Herschel”. Other names were also set in play—among them, Neptune! (von Zach 1785:48). It was not until the mid-19th century that it was universally called Uranus. The question was raised why Uranus had not been discovered long ago, a question that also dogged the discovery of Neptune. Herschel wrote to Joseph-Jérôme Lalande on 5 September 1782 that “the discovery was not owing to chance” since the planet “must sooner or later fall into my way, as it was that day the turn of the stars in that neighborhood to be examined I could not very well overlook it.” Even though the complex issue of the nature of discovery is beyond the scope of this chapter, a contemporary approach to the question in the context of Uranus is worth considering. The German natural philosopher Georg Christof Lichtenberg (1742–1799) deemed the question as to why Uranus had not been found earlier not very much more reasonable than that of Lelio’s servant in Lessing’s 1749 play The Old Maid, who would have liked to have known why his master’s father was to return today, and not a year earlier or later, which would have been much more understandable to him. Gotthold Lessing (1729–1781) was a German dramatist and writer on philosophy. Invoking a character by Lessing seems especially appropriate as he believed the search for truth is more valuable than the certainty gained by clinging to doctrinaire orthodoxy (which in the case of the planets left no room for additional members). (Cunningham 2016b:259). Once the object’s planetary nature was agreed, mathematical astronomers attempted to fit its observed motion to a circular orbit. The period was calculated as 83 or 84 years. Soon, however, the planet was observed to be getting ahead of the predicted longitude indicating a circular orbit would not do. Instead, though hardly surprisingly, the orbit was elliptical, with an eccentricity variously estimated by Johann Friedrich Hennert (1733–1813) in Utrecht, Placidus Fixlmillner (1721–1791) in Austria, and others as given below. (The current value is included for comparison.) Given the fact that the orbital elements vary slightly over time, the value derived by Laplace was most nearly accurate: Hennert Oriani Méchain Laplace Fixlmillner Modern

1783 1783 1783 1784 1784 2000

0.042664 0.04842 0.04300 0.047587 0.0461183 0.04716771

Were Uranus’s motion due solely to a force directed constantly toward the center of the Sun and inversely proportional to the squares of its distance to the Sun, its orbit around the Sun would be a perfect ellipse, forever immobile, with neither the plane of the orbit nor its axis ever altered. This perfect elliptical orbit would then

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suffice to predict its motion for all time. Of course, the actual case is otherwise as the Sun and Uranus do not exist in isolation. All the planets perturb one another to greater or lesser degree. In order to remain in step with the observed motion the theory of Uranus must take account of these perturbation effects—notably, those due to the two giant planets, Jupiter and Saturn. In order to make out the nature of these perturbations, one needs as long an arc of the planet’s motion along its orbit as can be obtained. In the first few years after its discovery, there were thus considerable uncertainties. However, since Uranus is bright enough to be just visible with the naked-eye (under dark sky conditions such as were found everywhere in the late 18th century and almost nowhere today) it seemed reasonable to suppose that it might have been catalogued unwittingly as a star by one of the star-cataloguers of the past. In that case, astronomers’ chances of computing an accurate orbit increased dramatically.

2.2  The Search for Pre-Discovery Observations In 1784, the Berlin astronomer Johann Elert Bode (1747–1826) took up the question. No one was in a better position than Bode to look into it; he was himself a star cataloguer extraordinaire, who would publish, in 1801, Uranographia, which has been called “the pinnacle of gloriously engraved star-atlases.” (Olson and Pasachoff 2019:42). It included 17,240 stars on 20 large copperplate engravings (Fig. 2.4). He wrote: Now there is the question, as this star is almost visible to the naked eye, why have astronomers not discovered it long ago?.... It is not altogether unusual for former astronomers, who furnish us with more or less complete observations of the stars of the zodiac, to overlook Fig. 2.4  Johann Elert Bode (Credit: Wikipedia Commons)

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W. Sheehan and C. J. Cunningham many 6 and 7 magnitude stars, as I often found by comparison of star catalogues with the sky…. It can also well be that this new star … was seen and entered in catalogues as a fixed star, especially as the places of the smaller stars were usually based on a single observation, and so the singular motion was not recognisable. I have therefore with this purpose compared the star catalogues of Tycho, Hevel, Flamsteed and Tobias Mayer (1723–1762) with the sky and computed where the present star stood in the years of their observations. (quoted in Alexander 1965:81)

Star records going back to Tycho Brahe’s observations of the 1580s and 90s were ransacked by Bode who noted a star that had been seen in 1756 by Tobias Mayer at the Göttingen Observatory. He catalogued it as star 964 in Aquarius. There was no longer a star in this position, but tracking backward, Uranus had been there at the time. A second pre-discovery observation was found by Bode in 1785. It would prove to be the most ancient of all the “ancient” observations. Flamsteed, working through the chill night on Greenwich Hill on 23 December 1690, had recorded a star of the sixth magnitude, which he catalogued as 34 Tau, close to ω Tauri (between the Hyades and Pleiades). (Alexander 1965:85). This was also Uranus—though there was a moment of doubt when William Herschel wrote from Datchet (18 May 1784): I find that you consider that the 34th star in Taurus according to Flamsteed may also be our new planet. But I find in my Journal that I have still seen this star on 13 October 1782 with a telescope of 7 feet which magnifies 460x. It must thus have vanished recently if it no longer stands in its place. But I must remark that as my instrument was not then adjusted, I could not determine accurately this star’s place. (Alexander 1965:86)

This may also have been the fine night on which he “plainly” saw the planet with the naked-eye, which was presumably the first such observation of the planet. Herschel proved to be mistaken, however; he later agreed that there was no longer a star in the position of Flamsteed’s 34 Tau, and that this was indeed the planet. Buoyed by these finds, star observations going back to Tycho Brahe’s observations of the 1580s and 90s were searched as possible repositories of Uranus sightings (von Zach 1785). In 1788, three more pre-discovery observations were found in the registers of the French astronomer Pierre Charles Le Monnier (or Lemonnier) (1715–1799). Laplace was gratified that the new planet seemed to be moving in the manner called for by perturbation theory: The planet Uranus, though lately discovered, offers already incontestable indications of the perturbations which it experiences from the action of Jupiter and Saturn. The laws of elliptic motion do not exactly satisfy its observed positions, and to represent them its perturbations must be considered. This theory, by a very remarkable coincidence, places it in the years 1769, 1756, and 1690, in the same points of the heavens, where Le Monnier, Mayer, and Flamsteed, had determined the positions of three stars, which cannot be found at present: this leaves no doubt of the identity of these stars with the new planet. (Laplace, trans. J. Pond, Laplace 1809:Vol. 2, 58)

About 1810, the German astronomer Friedrich Wilhelm Bessel (1784–1846) found another observation by the Rev. James Bradley (1693–1762), made at the Greenwich Observatory in 1753. By 1820 Johann Karl Burkhardt (1773–1825) at the Bureau des Longitudes unearthed two more by Flamsteed from 1712 and 1715. The largest

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Fig. 2.5  Alexis Bouvard (Credit: Wikipedia Commons)

trove, however, was unearthed among the records of Le Monnier by Alexis Bouvard (1767–1843), who would become the leading expert on the motions of Uranus in the first decades of the 19th century. (Forbes 1982) (Fig. 2.5). Bouvard had been born in 1767, in a hut at Contamines-Montjoie, between Chamonix and Megève, in what was then the Duchy of Savoy (remarkably, the hut still exists). He passed his early life as a shepherd, without means or prospects, but in order to improve his chances in life, he emigrated at the age of 18 to Paris. Attending free lectures in mathematics, he found that he had natural ability for the subject, and soon came to the attention of the great Laplace himself, who employed him as a computer at the Paris Observatory. His assistance proved to be invaluable and it was Bouvard who actually performed the detailed (and laborious) calculations of Laplace’s Foundational work Mécanique céleste. In addition, for many years Bouvard was charged with supervising the calculation of the ephemerides of the Annuaire du Bureau des Longitudes. After the death of John Baptiste Joseph Delambre (1749–1822) in 1822, he succeeded to the directorship of Paris Observatory, and remained in the post until his own death in 1843. (Cruikshank and Sheehan 2018:34). Bouvard published astronomiques tables of Jupiter and Saturn in 1808, but their accuracy was marred by the considerable errors in the masses of the two planets. Shortly after these tables were published, Laplace noticed that the “Grand Inequality” in the motions of those planets, which as he himself had explained are dependent on the fifth power of the eccentricities of the orbits, had been analysed by Bouvard with a wrong sign. Since the Grand Inequality has a marked influence on the values of the elliptic elements, including the mean longitude at epoch and the mean motion, not only would the tables not accurately represent the long-term motions of Jupiter and Saturn but the tables of Uranus also were in urgent need of re-calculation. In 1820, Bouvard set out to do just that. Bouvard had already spent several years in the

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massive work of recomputing the tables of the three giant planets, and as a first step in his work on Uranus considered all the pre-discovery observations unearthed by astronomers from Bode to Burckhardt, of which there were, when he began, half a dozen. Following a hunch, Bouvard wrote (Bouvard 1819:339–340), “I presumed I would find still other observations in the original registers of [Pierre] Le Monnier.” Le Monnier had been a positional astronomer who had filled no fewer than fifteen folio volumes with his observations made between 1736 and 1780. For reasons about to be given, searching through these records must have required a staggering effort, but it was worthwhile, for Bouvard’s success was beyond what he could have imagined. There were, in fact, twelve pre-discovery observations of Uranus, including a remarkable run of six, recorded over the nine-day period 15–23 January 1769. Le Monnier, wrote Bouvard, had he only compared the observations from day to day, would have “the honour of a beautiful discovery.” (Bouvard 1819:339–340). A. F. O’D. Alexander refers (Alexander 1965:89) to “the most amazing fact … that Le Monnier should have recorded Uranus as stars (in Aries) six times in nine days, four of them on consecutive days, without suspecting that he was confronted by a moving planet.” In partial extenuation of the fact, Uranus did happen to lie near one of its stationary points at the time. (A stationary point is defined as one of the points where, in making its annual retrograde loop, the Earth just overtakes and passes the slower-moving Uranus in its orbital motion, so that the apparent motion stalls and reverses direction.) Near such times its motion would have been difficult to descry. Le Monnier was at least as unlucky as he was careless. (See also Forbes 1982) (Fig. 2.6). The reason Bouvard had suspected that Le Monnier’s observing book might contain such a cornucopia had to do with the fact that, since the latter’s main interest had always been in improving tables of the Moon’s motion, he had from the outset been preoccupied with cataloguing the stars along the Moon’s path along the Zodiac. Standing high in the favour of Louis XV, who had placed at his disposal apartments in the former convent of the Capuchins, on the rue St. Honoré, Le Monnier from 1753 had employed in these observations a superb 8-foot mural quadrant made by the British instrument-maker John Bird (1709–1776). (It was identical to an instrument just set up at the Royal Observatory, Greenwich, by the Astronomer Royal the Rev. James Bradley.) In the course of this work, Le Monnier often re-observed the same stars—both the brighter ones and also the “little” ones—and Bouvard reasoned that Uranus was bound to have been recorded from time to time among Le Monnier’s “little” ones. (Cruikshank and Sheehan 2018:36). In Le Monnier’s time, the two positions, of right ascension and declination, were made with entirely separate instruments. The declinations were taken by means of a transit circle, mounted between two stone piers, the right ascensions with the mural quadrant. The latter were made as stiff as possible and mounted on the face of a great stone pier or wall, with a light-weight telescope attached to it that was able to turn about the centre. Being supported on one side only, any flexure or outward bending of either the telescope or circle or both was always feared, as this would introduce errors. By the 18th century the entries included in star catalogues going back to Hipparchus (190 BC–120 BC) and Ptolemy (100–c170)—Ptolemy gives for

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Fig. 2.6  Pierre Charles Le Monnier, circa 1777. Painting by Nicolas-­ Bernard Lépicié, Calouste Gulbenkian Museum, Lisbon (Credit: Wikipedia Commons)

most of the 1028 stars a brief word description of the place of the star in the constellation, a measured ecliptic latitude and longitude, and the star’s brightness as expressed in the 1–6 magnitude scale—had been supplemented by several others: the effects of refraction of the air, which made a star viewed near the horizon appear half a degree from its position as measured overhead, and such effects as precession, aberration of starlight and nutation (the last two discovered by Bradley). Thus, an observation taking only a few minutes to make at the telescope might require a great deal of calculating afterward in order to correct it for these effects. The corrections were referred to as “reductions”, the corrected observations as having been “reduced”. (King 1979:107–108). An astronomer engaged in this kind of work would be expected to be careful and meticulous in the extreme, and yet according to Bouvard (writing in the Connaissance des Tem (later the Connaissance des Temps), Le Monnier’s were found to be “dans le plus grande désordre” (in the greatest disorder). (Bouvard 1821). The handwriting was so poor that it was sometimes impossible to make out what he had written, especially the numbers; the clock seemed to be so irregular that it was difficult to determine its rate; the meridian transits so carelessly recorded that errors of several seconds of arc were not unusual!

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Bouvard’s account of Le Monnier’s sloppy records was disputed by a Baltimore astronomy analyst, Dennis Rawlins, who in 1970 looked into the matter during a visit to the Paris Observatory. Rawlins did not publish the results of his investigation until 1981, when he rather surprisingly claimed that everything was in perfect order and that Le Monnier had been slandered. (Rawlins 1981). Then the question became, if so, what motive would Bouvard have possibly have had? One might imagine that since by then he was struggling to fit the “ancient” observations to an orbit that satisfied both those and the modern ones (from 1781) for Uranus, it would have helped the cause had suspicion been cast on the earlier observers’ accuracy. (Cruikshank and Sheehan 2018:39). As a historian, it is always wise, however, to “trust but verify”, and so while the authors were writing this chapter, Paris Observatory graduate student Guy Bertrand generously offered to search the observatory archives again. His findings supported Bouvard’s version and contradicted Rawlins’s. So it now seems that Bouvard rather than Le Monnier had been the one slandered, and that he deserves great credit for succeeding in what was the equivalent of searching for a needle (or perhaps a few needles) in a haystack (Fig. 2.7). According to Bertrand (personal communication, 26 August 2019), Bouvard was totally right when he said that Le Monnier’s books were in a bad state: the writing is mostly illegible with badly formed letters and many parts are written in very small characters. The sheets are badly organised. The figures were not as bad as the letters, but they are not easy to read either. Above all, the measures do not seem to be very accurate; for instance, he hardly mentions the transits on the different threads. I looked through the 1768–1769 books, which contain eight observations of Uranus recorded as a star. I do not know how Bouvard ever managed to find them, but somehow he did, and at each of the eight observations he wrote URANUS, followed by his initial B, and with the position given by Le Monnier corrected.

Bouvard’s discovery of the pre-discovery observations in Le Monnier’s haystack brought the total to seventeen. They are listed in Table 2.1, along with three more Flamsteed observations found after 1820 by Johann Karl Burckhardt (1773–1825), two Bradley observations found by Hugh Breen, Jr. (1824–1891), a computer at Greenwich, in 1864 and—so far the last—a Flamsteed observation discovered by Rawlins only in 1968 (Forbes 1982). With this tranche of pre-discovery observations added to recent ones made by observers using meridian instruments, Bouvard got underway with his massive calculations. Let us leave him at his calculations for the moment, and take up a different thread of our story—the search for a new planet in the zone between Mars and Jupiter.

2.3  A Strange Progression Revealed In 1772 a German mathematician, Johann Daniel Dietz, called Titius (1729–1796), of Wittenberg, found that if one takes the series 0, 3, 6, 12, 24 and 48 and adds 4 to each term, the resulting set of numbers closely approximates the relative distances

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Fig. 2.7  One of the pages of Le Monnier’s observing log books, showing the general untidiness of his records. Admittedly they were written at night, probably by candlelight, and never retranscribed. Though Le Monnier himself identified three pre-discovery observations of Uranus, Alexis Bouvard unearthed another nine—including six between 15 and 23 January 1769. The 15 January 1769 observation is shown above. Note that Bouvard has added the label “Uranus” and signed his name next to the note on the right upper part of the page (Credit: Guy Bertrand, Observatoire de Paris)

of the planets from the Sun. Though announced in almost the most obscure fashion imaginable, as a footnote to a book he was translating (Charles Bonnet’s Contemplation de la Nature), it attracted the attention of an omnivorous reader, the Berlin astronomer Johann Elert Bode. Bode realised its importance and popularised it (initially without credit to Titius), so that it became generally known (and was so referred to by all 19th century astronomers) as Bode’s law. (In what follows, both “Bode’s law” and “Titius-Bode law” are used; note that they refer to the same relationship.) Titius’s relationship did not arise entirely out of a vacuum. Astronomers had long groped for an understanding of why the planets were at their particular distances from the Sun. Kepler had of course devoted a great deal of attention to the problem, and for a moment thought he had succeeded with his model fitting the planetary orbits within the so-called Pythagorean or Platonic solids well known from classical geometry. Working from such speculative schemes, he had even once said, “Between

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Table 2.1  After William Herschel discovered the major planet Uranus, astronomers raced to look for prediscovery sightings in earlier star charts so they could calculate its orbit. Altogether, 23 prediscovery sightings were discovered in the records of four 17th- and 18th-century observers; 1 by Mayer, 3 by Bradley, 7 by Flamsteed, and 12 by Le Monnier (Data: Forbes 1982; table designed by Trudy E. Bell) Date observation Date observed Observer recognized 1690 Dec. 3 Flamsteed 1785 1712 Apr. 2 Flamsteed by 1820 1714 Flamsteeda 1968 1715 Mar. 4 Flamsteed after 1820 1715 Mar. 5 Flamsteed after 1820 1715 Mar. 10 Flamsteed after 1820 1715 Apr. 29 Flamsteed by 1820 1748 Bradley 1864 1750 Bradley 1864 1750 Le Monnier by 1818 1750 Oct. 14 Le Monnier by 1818 1753 Dec. 3 Bradley 1810 (1813?) 1756 Sep. 26 Mayer 1781 1764 Jan. 15 Le Monnier by 1818 1768 Dec. 27 Le Monnier by 1818 1768 Dec. 30 Le Monnier by 1818 1769 Jan. 15 Le Monnier by 1818 1769 Jan. 16 Le Monnier by 1818 1769 Jan. 20–23 Le Monnierb by 1818 1771 Dec. 18 Le Monnier by 1818 Key to Names in Table Bessel Friedrich Wilhelm Bessel Bode Johann Elert Bode Bouvard Alexis Bouvard Bradley James Bradley Breen James Breen Burckhardt Johann Karl Burckhardt Flamsteed John Flamsteed Le Monnier Pierre Charles Le Monnier Mayer Johann Tobias Mayer Rawlins Dennis Rawlins a Perhaps actually Flamsteed’s assistant, Joseph Crosthwaite b Four consecutive nights

Discoverer Bode Burckhardt Rawlins Burckhardt Burckhardt Burckhardt Burckhardt Breen Breen Bouvard Bouvard Bessel Bode Bouvard Bouvard Bouvard Bouvard Bouvard Bouvard Bouvard

Mars and Jupiter I put a planet.” The Swiss polymath Johann Heinrich Lambert (1728–1777) had made the same conjecture in 1761. Both had intuited what the law made explicit: there was a gap at number 28. The intrigue became even more compelling when Uranus was discovered, for it too fit the scheme. (Cunningham 2016a,

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b, 2017b). As we shall see, it broke down completely in the case of Neptune, however. No one was more intrigued by Bode’s law than the Hungarian astronomer János Ferenc Zach, better known by the German version of his name, Franz Xaver von Zach (1754–1832). Born to a noble family at Pest (now part of Budapest) in 1754, von Zach studied physics in Pest and in Vienna, and taught mechanics for a time at the University of Lemberg (now Lyio, Ukraine). After the University closed down, he moved first to Paris and then, in 1783, to London. There he became acquainted with Herschel and with Count Moritiz von Brühl (1736–1809), ambassador of Saxony, who provided him with an introduction to Ernst II, Duke of Saxe-Gotha (1745–1804). The duke was interested in setting up a private observatory on the mountain of Seeberg, near his castle at Gotha, and von Zach agreed to become the director. Actual construction of the observatory began in 1788, the first observations, made with instruments made by the English instrument-maker Jesse Ramsden (1735–1800), not until 1792. (Cunningham 2016a) (Fig. 2.8). Already von Zach had been captivated by the idea of an unknown planet. In the Astronomisches Jahrbach, founded by Lambert but since the latter’s death in 1777 edited by Bode, von Zach wrote (1785:162), “Of the supposed planet between Mars and Jupiter I will disclose to you my dreamings.” His dreamings included a prediction of the presumed orbital elements of the putative planet. He also saw the need to

Fig. 2.8  Baron Franz Xaver von Zach (Credit: Wikipedia Commons)

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compile a catalogue of the stars along the zodiac to enable a methodical search, which would, he saw, be the only way it would be possible to find it. He put his dreamings on hold for a few years while overseeing the construction of the observatory, but as soon as it was completed, made his first order of business the “revision of the stars of the zodiac.” He continued the plodding effort for a few years but finally was persuaded that it was too great to be accomplished by a lone observer. In 1798, he hosted the world’s first astronomical congress at Gotha, and introduced the idea of searching for the planet between Mars and Jupiter to the astronomers gathered there, who included both Bode and J.J.  Lalande, and discussed the project again at meetings of astronomers at Celle, Bremen and Lilienthal, the following year. By then he was even referring to the planet with a name, “Hera” or “Juno”. So far, however, there had only been preliminaries to organising an actual search. The decisive step forward occurred in September 1800, at yet another meeting of astronomers, held at the observatory of Johann Hieronymus Schröter (or Schroeter) (1745–1816) at Lilienthal, near Bremen. Schröter was the chief magistrate of Lilienthal, part of the Electorate Hannover (then united in the “personal union” between Great Britain and Hannover which was in effect, except for the years when Hannover was under Napoleonic control, until the accession of Queen Victoria in 1837). In addition to his official duties, which appear to have been reasonably light, he was a fanatic collector of telescopes and an obsessive amateur observer of the Moon and planets. (George III was a benefactor, and helped pay for his rather expensive hobby.) His largest telescopes were too large to house in an observatory, and were simply mounted out of doors. Though Europe was now at war and Lalande and other French astronomers were prevented from attending the meeting, six astronomers were able to attend. The sole purpose of the meeting, the like of which had never before been seen in the history of astronomy, was to search for a new planet. (Gerdes 1990). Von Zach, of course, was the driving force. He had travelled some 350 km from Gotha to be in Lilienthal. Of the others present, one was Karl Harding (1765–1834), who had been hired as a tutor to Schröter’s children but was now his assistant. The rest included Johann Gildemeister (1812–1890) and Heinrich Wilhelm Mathias Olbers (1758–1840), who made the journey from Bremen, also 350 km away, and Ferdinand von Ende (1760–1816) (who later suggested to the Olbers the idea that the asteroids originated in a celestial catastrophe), who came from Celle, some 250 km away. Such journeys then took days. This says something about the level of belief these men had in the new planet. The meeting was briefly interrupted (on 20 September) by Adolf Friedrich, a prince of Great Britain and Hannover (1774–1850) and his entourage, who were touring the village to examine the newly built bath houses, but also wanted to see Schröter’s celebrated observatory. The group got down to serious work the next day, and by the time they disbanded, von Zach’s plan had borne fruit. A group of 24 astronomers were to be engaged in a systematic search for the missing planet. Known as the Vereinigten Astronomischen Gesellschaft (VAG) Schröter was to serve as president, and von Zach himself as secretary. Rather than by its rather stodgy official name, the VAG has been remembered in history as the “Celestial

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Police,” a name applied colloquially by von Zach (1801:602-603): “By such a strictly organised Celestial Police (Himmels-Polizei), which had been assigned to 24 Sections (Departments), we finally hope to see a trace of this planet, which had so long escaped our scrutiny, if it did exist and make itself seen.” It should be noted, by the way, that the name “Celestial Police” was not in common use at the time; it does not appear in the German literature before 1844, nor in English before 1886. In soliciting the participation of additional European astronomers, von Zach was always careful to use the more formal, serious, name. In his role as secretary, von Zach was charged with inviting other astronomers to the effort. The complete list of prospective members (including the first six who had attended the meeting in Lilienthal) was: Johann Bode (Berlin), Johann Tobias Buerg (Vienna) (1766–1834), Thomas Bugge (Copenhagen) (1740–1815), Johann Burckhardt (Paris), William Herschel (Slough), Johann Huth (Frankfurt) (1763–1816), Georg Simon Klügel (Halle) (1739–1812), Julius August Koch (Danzig) (1752–1817), Nevil Maskelyne (Greenwich), Daniel Melanderhjelm (Stockholm) (1726–1810), Pierre Méchain (Paris), Charles Messier (Paris), Barnaba Oriani (Milan) (1752–1832), Giuseppe Piazzi (Palermo) (1746–1826), Friedrich Schubert (St. Petersburg) (1789–1865), Jan Śniadecki (Cracow) (1756–1830), Jacques-Joseph Thulis (Marseille) (1748–1810), Johann Friedrich Wurm (Blaubeuren) (1760–1833), Ferdinand von Ende (Celle), Johann Gildemeister (Bremen), Karl Harding (Lilienthal), and Jöns Svanberg (Uppsala) (1771–1851), Wilhelm Olbers (Bremen), Johann Schröter (Lilienthal), and Franz von Zach (Gotha). (Gerdes 1990). Among these 24, the zodiac would be subdivided into search zones covering 15° of longitude, in a band 6° north and south of the ecliptic. Each member was to be responsible for scrutinizing stars to 9th magnitude in the assigned zone. Von Zach was nothing if not enthusiastic, and with a very wide list of contacts across Europe, he worked untiringly to win other astronomers to his cause. Typical is the following letter (von Zach to Wurm, 15 March 1801) which was written to Wurm, a secondary school teacher in Stuttgart who also, incidentally, seems to have been the first to suggest that the Bode-Titius law might apply to unknown planets even beyond Uranus and thus, remotely, anticipated the search for Neptune: Last autumn I was in Bremen, Celle and Lilienthal to visit my friends Olbers, Schröter and Ende. We founded an astronomical society of 24 practical astronomers. La Lande, DeLambre, Méchain, Messier, Burckhardt, Bode, Triesnecker, Buerg, Sniadecki, Svanberg, Olbers, Ende, &c... are members. Our plan is to share the sky among us. Every member gets half a sign in the sky, that is 15° on the ecliptic, with 6° south. and 6° north latitude. Every astronomer has to monitor such a strip. That is his prefecture or area of inspection. Every member starts with drawing a map of his area according to the best catalogues, charting all known stars, revising the sky with only a free standing telescope if all the stars are still there. Now he starts to chart the missing stars only by eye, according to [Francis] Wollaston's suggested manner of a wire system. The stars in the zodiac are already determined well enough, for now we want a list of all stars even the telescopic stars—we are not yet interested in extremely accurately determined RA's and Decl. For now we need to chart all stars and their respective positions as accurately as possible. Every astronomer needs to revise his prefecture as often as he is able and inclined to, see if all stars are still on their spot, none moved or disappeared. Only by this and such a plan can one discover 1st if there is really a planet between Mars and Jupiter, which maybe shows only as a telescopic star. 2nd If it is

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In this same letter von Zach disclosed that with the patronage of Duke Ernst II of Saxe-Gotha, the Seeberg Observatory was being equipped with a new telescope specifically designed for the search. Von Zach further related that Olbers had already drawn up a catalogue of stars down to the 9th magnitude in his assigned zone of the sky (he does not indicate in which area of the sky). Unfortunately, for whatever reason—among those later cited (von Zach 1801) were “the disquiet of war and the ensuing vagaries in land and sea delivery of the mails”—von Zach fell considerably behind schedule. On 1 January 1801, the effort was forestalled. On that date, the Theatine monk Giuseppe Piazzi, director of the observatory in Palermo and one of the astronomers on von Zach’s list but not yet contacted, was revising the stars of Taurus in the star catalogue of the French astronomer Nicolas-Louis de Lacaille (1713–1762) when he came across the small planet, circling in the zone between Mars and Jupiter at just the distance predicted by the Titius-Bode law. It was the minor planet now known as Ceres. (Cunningham 2016a) (Figs. 2.9, 2.10 and 2.11). Fig. 2.9  Giuseppe Piazzi (Credit: NASA—JPL)

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Fig. 2.10 Niccolò Cacciatore, Piazzi’s assistant and unsung co-discoverer of Ceres (Credit: Wikipedia Commons)

For whatever reason, Piazzi was not very forthcoming with information about the circumstances of his discovery nor with his data. As with Johann Gottfried Galle (1812–1910) and the discovery of Neptune, Piazzi had his assistant with him. Niccolò Cacciatore (1770–1841) had been Piazzi’s assistant for the past year, and according to the later account given by the British naval officer Basil Hall (1788–1844), who claimed to have heard it from Cacciatore himself: As Piazzi was at that time engaged in making the noble catalog of the stars, which has since become so well known, he placed himself at the telescope and observed the stars as they passed the meridian, while Cacciatore wrote down the times, and the polar distances, as they were read off by his chief. Certain stars passed the [measuring] wires, and were recorded as usual on the 1 January 1801. On the next night, when the same part of the heavens came under review, several of the stars observed the evening before were again looked at and their places recorded. Of these, however, there was one which did not fit the position assigned to it on the previous night, either in right ascension or declination. “I think,” said Piazzi to his companion, “you must, accidentally, have written down the time of that star’s passage, and its distance from the pole, incorrectly.”

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Fig. 2.11  Palermo Observatory, where Piazzi and Niccolò Cacciatore discovered Ceres (Credit: NASA-JPL) “To this,” said Cacciatore, who told me the story, “I made no reply, but took especial pains to set down the next evening’s observations with great care. On the third night there again occurred a discordance, and again a remark from Piazzi that an erroneous entry had probably been made by me of the place of the star. I was rather piqued at this,” said Cacciatore, “and respectfully suggested that possibly the error lay in the observation, not in the record.” “Under these circumstances, and both parties being now fully awakened as to the importance of the result, we watched for the transit of the disputed star with great anxiety on the fourth night. When lo and behold! It was again wide of the place it had occupied in the heavens on the preceding and all the other nights on which it had been observed.” “Oh ho!” cried the delighted Piazzi, “we have found a planet while we thought we were observing a fixed star; let us watch it more attentively.” (Hall 1841:292–293).

Watch it intently they did, but Piazzi was very sparing about sharing his observations with his colleagues. He announced his discovery to only two  astronomers, Bode in Berlin and Barnaba Oriani in Milan on 24 January, but emulating Herschel with Uranus, described what he had found as a comet, “without tail or envelope.” Only to Oriani, a particular friend, did he share his suspicion that what he had found might be “better than a comet.” (Piazzi to Oriani, 24 January 1801; in Cacciatore and Schiaparelli 1874:49). In fact, there was no doubt that it was a new planet. Piazzi continued to observe until 11 February, when he had to break off because of one of his frequent illnesses (probably brought on by overwork). By then, other astronomers were learning of what he had discovered mainly by rumours emanating from Sicily. Hard data, however, was difficult to come by; Lalande did not receive any actual observations until late May, by which time the new planet had been lost

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in the glare of the Sun; William Herschel learned of it from Bode, some three months before he received official notice. The “Celestial Police” were thrown, at least for a moment, into disarray. Olbers wrote to von Zach on 16 May 1801 complaining that Piazzi had robbed the budding society of the honour of discovering a new planet, while noting that the plan of search would certainly have succeeded, if only it had been allowed to develop. (Cunningham 2016b:267). Von Zach, irritated at Piazzi’s suppression at data, tried desperately to get data from Oriani about the planet Piazzi had “nicked”. The discovery of Ceres essentially rendered moot the efforts of the Vereinigten Astronomischen Gesellschaft, and the organisation quickly became extinct, though on finding out how small Ceres was, individual members decided to continue to search for additional planets in the zone between Mars and Jupiter—with great success. Briefly lost after disappearing into the glare of the Sun, Ceres, with the help of an orbit computed by the Brunswick mathematician Carl Friedrich Gauss (1777–1855), was recovered by von Zach and Olbers on 31 December 1801 just a day short of a year after Piazzi had found it. Then, on 28 March 1802, Olbers discovered another small planet, Pallas. Shortly afterward, on 6 April 1802, von Ende wrote to Olbers (Cunningham 2017b:140), “The possibility of several planets in orbits that we did not even dream of is now established, and we must commit to the diligent study and search for [these planets].” He added that this might revive the former fervour and get the society moving again. Though von Ende and Olbers clearly anticipated that additional planets might be found between Mars and Jupiter, in a tantalising letter to Sir Joseph Banks, president of the Royal Society of London, von Zach even held out the prospect of hunting down quarry much farther afield. On 1 May 1802, he wrote, “Pallas cannot be deemed a Planet, if we understand by Planets, heavenly bodies revolving in little excentric ellipses round the Sun, and pursuant to the law of distances, completed now by the Discovery of Ceres, and extending to the Georgian Planet [Uranus], & perhaps beyond.” By then, others had caught the scent—and had done so not based on speculations from the Bode-Titius law, but from the failure of tables to successfully predict the motion of Uranus. By 1800, it was evident that the best tables available, by Joseph Delambre, were no longer satisfactory, and it was long afterward rumoured that both Lalande and Johann Karl Burckhardt, a leading German computer of orbits, were persuaded that “there existed an unseen planet beyond Uranus.” (Chambers 1889:vol. 1, 253fn).

2.4  Uranus Veers as Bouvard Calculates To pick up where we left off at the end of Section 2.2, when, in 1820, the human calculating machine that was Alexis Bouvard undertook the massive calculation of the tables of Uranus, he could look back at a number of previous disappointing efforts to calculate its orbit. Thus, as had been noted already by Barnaba Oriani in Milan in 1789 and then by Jean Baptise Joseph Delambre the following year, the

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laws of elliptic motion did not exactly satisfy the observed positions of Uranus, and thus, in order to represent them, its perturbations needed to be considered. (Laplace 1809:vol. 2, 35). Delambre was the first to achieve a definite result. Using the perturbation theory of Laplace, and taking account of the pre-discovery observations known up to that time, Flamsteed’s, Mayer’s and three of Le Monnier’s, Delambre published tables in 1790 which accounted to an accuracy of 1′ or 2′ of arc for every observation except those of 1690 and 1790. For this, the Académie des Sciences awarded him its prizes for Uranus theory and planetary tables that year. They gave him the same award in 1792. The calculation of perturbations was not a trivial affair in the days before electronic computers; nonetheless, great advances were made in the last decade of the 18th century, which would be crucial to advancing the 19th century project of finding an even more distant planet. We have a contemporary account of this advance from von Zach: Uranus was discovered in 1781 and its perturbations were calculated in 1789. A decade ago, we needed eight years but today this takes us only months and days. Ceres Ferdinandea was discovered only 13 months ago, and its perturbations have been calculated, compiled in tables and its orbit calculated accordingly. These perturbation equations, as La Lande wrote, were calculated by Dr. Burckhardt in the course of just one day. Eight years ago, there were only four or five capable astronomers in the whole of Europe who could have made such elaborate calculations in several months – today we have more than a dozen young and talented men, who accomplish such a work within days. Unmistakably, pure theory has come to the aid of refined practice to walk hand in hand. (von Zach 1802:394)

Bouvard, of course, was in a much stronger position in 1820 than Delambre had been in 1790. Nevertheless, the calculation proved to be far more difficult than he expected at the outset, and he was soon mired in difficulties. In performing such calculations, one starts with heliocentric longitude of the planet (the Sun-centered position of the planet), deduced from tables. One also needed values for orbital elements—the epoch, the mean sidereal motion, the time since epoch, the perihelion (or aphelion), and the first terms of the equation of the center, all of which were determined empirically. One then included the perturbational terms due to the action of the other planets—for Uranus, those due to Jupiter and Saturn. In the Mécanique céleste, Laplace had developed the perturbation function to the third order in the power series in the eccentricities and inclinations. Later astronomers would carry the development farther—De Pontécoulant and Benjamin Peirce (1809–1880) to the sixth order, Urbain Le Verrier (1811–1877) to the seventh order. The higher the order in the power series the more accurate the calculations but at the expense of having to evaluate increasingly complicated expressions. At the stage at which Bouvard was investigating the problem, the third order was good enough. This gave an expression that, when the appropriate numbers were supplied, produced a linear equation yielding the heliocentric longitude computed from the theory for a given time. One next defined a series of “equations of condition”, in which expressions taken from perturbation theory are set equal to values determined from observations, in order to determine algebraically the values of any unknown quantities sought. Bouvard’s equations of condition compared calculated values obtained

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from perturbation expressions (i.e., tabular values) to those derived from observations for a given time. (Bouvard 1821). When Bouvard carried out his calculation, astronomers had long been aware of the fact that in practice no observation is ever absolutely precise; there are always small errors. It was desirable, when in possession of multiple data points, to determine the most probable value—in other words, to minimise, as far as possible, the errors. The way this was done had grown ever more sophisticated since 1750 when Tobias Mayer first introduced the concept of equations of condition as a new mathematical tool, and successfully applied by Laplace in his successful attempt to grapple with the “Great Inequality” of Jupiter and Saturn. (See Chap. 1) By the early 19th century, astronomers (except in England, where the arithmetical mean continued to be used) had begun to use the result obtained from a least squares analysis of the observations, a new mathematical tool developed independently by Carl Friedrich Gauss and Adrien-Marie Legendre (1752–1833). (Cunningham 2017a:43–47). Precisely what technique Bouvard used is uncertain, but he had these three options to draw upon depending on the data at hand. In the ancient observations, he had only a single value except for 1715, for which he had Flamsteed’s two, and 1769, Le Monnier’s eight. Thus, with these exceptions, a least squares analysis was not feasible. Nevertheless, despite the “imperfections of an instrumental nature and the particular circumstances in which they were made, which detracted from their value”, Bouvard regarded these observations as “precious”, as they gave a great extension of the arc traversed by Uranus since their time. The second series, dating only to 1781, formed an uninterrupted series made with finer instruments and under more favourable conditions of observation. These consisted of all the oppositions and quadratures of Uranus observed under Maskelyne and Pond up to 1815, as well as those of the Paris Observatory registers for 1800– 1819. (Bouvard 1821:xiii). Despite the difference in quality (and quantity) of the observations, Bouvard had every reason to be confident that he could find a single elliptical orbit that would satisfactorily fit all the observations, ancient as well as modern. Using the formulae for perturbations from the Mécanique céleste, and new masses of Jupiter and Saturn, he computed provisional tables of the heliocentric longitude of Uranus for 77 dates on which it had been observed. The differences between the theoretical (computed) positions and the observed positions are known as the “residuals”. Since theoretical and observational errors are both bound to occur, it would be unreasonable to expect the residuals to be zero, but obviously, if the observations and the orbit are both tolerably accurate, they ought to be rather small. But Bouvard’s residuals—shown in the table here—were very disappointing. As he said with an evident sigh, “The values I obtained for the elements were far from corresponding to the hopes I had for the work.” (Bouvard 1821: p. xx) (Table 2.2). It is in the nature of residuals that they do not openly declare whether they come more from the theoretical or the observational side. The residuals for the ancient observations were very large, ranging from +98″.7 (in 1690) to −124″ (in 1750). However, those in the modern observations were far from inconsiderable, and

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Table 2.2  Residuals for Uranus (from Bouvard 1821) Year 1690 1712 1715 1750 1750 1753 1756 1764 1769 1771 1781 1781 1782 1782 1782 1783 1783

Errors +98″.7 +62″.0 −4″.5 −124″.0 −101″.4 −69″.8 −75″.7 −23″.8 +10″.9 +50″.9 +17″.8 +22″.9 +27″.6 +17″.7 +13″.9 +24″.9 +24″.8

Year 1783 1784 1785 1785 1785 1786 1786 1787 1788 1788 1789 1789 1790 1790 1791 1791 1792–7″.6

Errors +16″.7 +19″.4 +6″.7 +6″.5 +12″.5 +11″.7 +10″.5 +12″.1 −8″.1 +8″.9 −1″.4 +23″.5 +14″.8 −16″.9 −12″.6 −5″.1 −7″.6

Year 1792 1793 1794 1794 1795 1795 1796 1796 1797 1797 1798 1799 1799 1800 1801 1802 1802

Errors −13″.0 −19″.5 −13″.3 −18″.5 −20″.5 −25″.8 −3″.0 −35″.0 −18″.9 −37″.9 −37″.6 −53″.4 −45″.4 −25″.8 −44″.3 −49″.7 −34″.2

Year 1803 1804 1805 1806 1806 1807 1807 1808 1809 1810 1811 1811 1812 1812 1813 1813 1814

Errors −44″.5 −27″.8 −38″.2 −23″.1 −8″.5 −26″.5 −0″.9 −14″.1 −19″.6 −5″.9 +9″.3 +1″.0 +0″.1 +22″.0 +16″.6 +42″.4 +27″.9

Year 1814 1815 1815 1816 1816 1817 1818 1819

Errors +41″3 +41″.2 +47″.6 +49″.1 +57″.7 +52″.9 +56″.7 +76″.2

The table here gives the observed positions minus theory; a plus sign means the planet is running ahead, a minus sign behind, the predicted places

appeared to be increasing. Since 1813 they had been 40″, 50″, and—for one observation in 1819—76″. Bouvard could not believe the modern observations could be so flawed. Nor could he find any theoretical errors—such as forgotten terms in the perturbations— to account for the situation. His doubts therefore fell on the ancient observations. Salving his conscience, he pounced on their perceived flaws. “Bradley’s observation was a single one and recorded only in degrees and minutes,” he wrote (1821:xiv); “the same for Mayer’s. It is known that Flamsteed’s instruments were not well made or exactly placed on the meridian. As to Le Monnier’s, we already know from the paper in the C.T. [Connaissance des Tems] for 1821 what to think.” Thus Bouvard managed to convince himself that an error of almost 100″ could be ascribed to Flamsteed’s observation of 1690, despite the fact that his observations were generally accurate to 10″. The observations of Bradley, Mayer, and Le Monnier had to be in error by 40″ or 60″; badly off their usual standard of 5″ or 6″. Perhaps he lost some sleep over it, but at least he finished the job. With the ancient observations suppressed and only the modern ones retained, he repeated the laborious calculations (not quite so laborious, since now he had only 67 equations of condition to evaluate!). He now thought the agreement was reasonably good. It was still hardly perfect, however, with the residuals between the theory and the observations remaining as high as 9″. That is a troubling lapse to an astronomer.

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With the following explanation, Bouvard put the best face forward he could, and sent his Astronomiques Tables into the world: If we combine the ancient observations with the modern ones, the first will be adequately represented, but the second will not be described within their known precise tolerances; while if we reject the ancient positions and retain only modern observations, the resulting tables will accurately represent the latter, but will not satisfy the old figures. We must choose between the courses. I have adopted the second as combining the most probabilities in favor of truth, and I leave to the future the task of discovering whether the difficulty of reconciling the two systems results from the inaccuracy of the ancient observations, or whether it depends on some extraneous and unknown influence which may have acted on the planet. (Bouvard 1821:xiv).

One of the first critical readers of Bouvard’s Tables was the German astronomer Friedrich Wilhelm Bessel. (See Chap. 6). One of the greatest astronomers of the 19th century, Bessel began his career working in an import-export business in Bremen. However, becoming bored with business, he decided on a career change, and in 1806, after Schröter’s assistant Karl Harding moved to the University of Göttingen to become professor of astronomy there, Bessel replaced him at Lilienthal. He distinguished himself, and in 1809, at the age of only twenty-five, he was named director of King Friedrich-Wilhelm III (1770–1840) of Prussia’s new observatory at Königsberg (now Kaliningrad, Russia). (Frommert 2007). Uranus early came into Bessel’s sights, and he obtained a series of positional measures with the meridian circle with which the observatory was equipped between 1814 and 1818. More accurate results followed on the acquisition, in 1819, of a meridian circle by the noted Munich physicist, optician, and instrument-maker Joseph von Fraunhofer (1787–1826). At the time, Bessel’s main project was a revision of the star positions made between 1750 and 1760 by the Rev. James Bradley at the Greenwich Observatory using the Bird quadrant mentioned above. It was while involved in this work that Bessel turned up a pre-discovery observation of Uranus by Bradley, and later discovered that Bouvard had, in computing his Tables, made a mistake in one of the terms relating Saturn’s perturbations of Uranus. Bouvard, as seems to have been characteristic of the man, expressed gratitude in having the error pointed out. (Bessel 1824). As a practical astronomer as well as a first-rate mathematician, Bessel could appreciate the standards achieved by observers such as Flamsteed, Bradley, Mayer, and Le Monnier, and asserted his unwavering faith in them, saying (1848:448), “I have myself … attained the strong conviction that the existing differences … are by no means attributable to the observations.” For a long time, however, he strayed into an unproductive track; instead of assuming the universality of the inverse-square law of gravitation between all masses, he adopted an idea of the Göttingen physicist Johann Tobias Mayer (not to be confused with his father, the great astronomer Tobias Mayer) according to which there was, in addition, some kind of specific attraction dependent on the nature of the masses. (See Chap. 6.) For several years Bessel laboured to account for anomalies in the theory of Uranus on this basis. Eventually, after a series of pendulum experiments on bodies made up of all sorts of materials, including even meteorites, he abandoned this

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unpromising idea. The inverse-square law did not depend on the nature of the materials involved. Unfortunately, Bessel’s pursuit of this deceptive phantom waylaid him from a more useful study of the motions of Uranus. As Heinrich Christian Schumacher (1780–1850) later lamented, Bessel “squandered many hours upon an empty hypothesis, of which he was so fond that I think he would never have given it up, if his pendulum-experiments, with all kinds of bodies, even with meteoric iron, had not all of them given the same results.” (Schumacher to Airy, 22 Dec. 1846:RGO). As astronomers at Greenwich, Königsberg, Cambridge and other observatories in Europe continued to monitor Uranus’s motions, Bouvard continued to refine his calculations. He tried an orbit whose elements included a larger mass, 1/17918 that of the Sun, based on his analysis of the perturbations of Uranus on Saturn. He also calculated a minute annual decrease in the eccentricity, annual increases in the longitudes of the perihelion and ascending node, and a small increase in the inclination, compared to Delambre’s orbit of 1790. At first it seemed that these were the proper correctives. As Airy summed up in 1832, in his report on the progress of astronomy to the British Association for the Advancement of Science, for the first few years after 1821, Bouvard’s Tables represented the observations “pretty well”, to within an accuracy of 9″ or so. Initially, the planet’s actual positions were running slightly ahead of the tabulated positions, but in 1829–30 the tabular and observed longitudes briefly coincided. Thereafter, there was a reversal of sign, with the planet falling behind the tabulated positions. (Airy 1833). As measured at Airy’s own Cambridge University Observatory, in 1832 it was lagging by an intolerable 30″, leading him to conclude (1833:189): “With respect to this planet a singular difficulty occurs…. it appears impossible to unite all the observations in one elliptical orbit, and Bouvard, to avoid attributing errors of importance to the modern observations, has rejected the ancient ones entirely. But even thus the planet’s path cannot be represented truly; for the Tables, made only eleven years ago, are now in error nearly half a minute of space.” As to their explanation, he said only “we still have no clue.”

2.5  The Stuff of Legend: Airy While French astronomers were the uncontested leaders in perturbation theory, under Airy’s leadership, British astronomers had become the main source of the observational data on which the theories of the planets depended. Airy was, by any calculation, one of the giants of British astronomy in the 19th century, and would loom large over the entire Neptune saga. (Chapman 1991; Meadows 1976; Schaffer 1988). He was born at Alnwick, Northumberland, on 27 July 1801. His father was William Airy, a collector of excise taxes; his mother was the daughter of George Biddell, a well-to-do farmer at Playford, near Ipswich. Though when George was born the family lived in Northumberland, but soon afterwards William was transferred to Hereford. On reaching primary school age, George

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was sent to the school at Colchester headed by Byatt Walker (c.1772–1826). He seems to have learned from examining his father’s account books arithmetic, double-­entry bookkeeping, and the use of a slide rule, and at ten, reaching the end of his primary school career, he found himself in the first place. He was, however, never a favourite of his school mates, and was regarded as something of a snob— though partly redeemed himself by his inventiveness in the construction of peashooters and similar devices. Before he had left the primary school, his father was transferred again, to Essex, and from the age of 12, he spent summers with an uncle, Arthur Biddell, who had a farm near Ipswich. The home situation seems to have become strained from 1813, when his father lost his job as an excise tax collector— apparently William had been found guilty of some financial improprieties, a shameful turn of events that seems to have left a deep impression on George’s character and may partly account for his extraordinary scruples about record-keeping. As the family struggled with William’s unemployment and the consequent impoverishment, Airy asked to live with his uncle rather than return to live with his family, and from this point forward Airy’s uncle seems to have taken over the role of his father. George Biddell was, fortunately, not a mere rustic; he had an excellent library, which included books on chemistry, optics, and mechanics, and Airy continued to excel academically at the Colchester grammar school between 1814 and 1819. He was noted for his remarkable memory, and on one occasion repeated in one examination 2,394 lines of Latin verse. With financial support from his uncle, Airy, in 1819, duly entered Trinity College as a sizar (an undergraduate who receives some form of financial assistance in exchange for menial duties), and swept all the mathematics honours. He graduated in 1823 as Senior Wrangler and Smith’s prizeman. He had already begun to pay close attention to astronomy, at first from the side of optics. Already in 1824, he wrote a valuable paper on the achromaticism of eyepieces and microscopes. Being severely myopic himself and wearing the usual concave spectacles for this, he discovered by experiment that his left eye was rendered nearly useless by astigmatism, and in order to correct his own deficiency, designed a concavo-cylindrical lens which opticians still routinely prescribe for the problem today. In 1824 he became a Fellow of Trinity College. (Chapman 1991). His further ascent was meteoric, and became the stuff of legend (see Figs. 4.4 and 7.1 for portraits of Airy). Within two years of becoming a Fellow of Trinity, he was appointed Lucasian professor of mathematics (the chair that Isaac Newton had once held). Just two years after that he became Plumian professor of astronomy. As part of the duties of the latter position, he also took charge of the recently completed Cambridge University Observatory. He did so with a longer view. Cambridge University Observatory historian Roger Hutchins has written: Airy was determined to establish himself as the only candidate to succeed the elderly John Pond as Astronomer Royal [at Greenwich]. To this end he organized his observatory as an exemplar. Instead of the old English method of trying to detect instrument errors and then adjust them to zero, he followed Friedrich Bessel in leaving the instrument stable, rigorously measuring instrument errors, then applying numerical corrections of devised standard formulae to correct for the physical variables of clock, temperature, humidity, and personal error. Differentiating between expert and routine work, Airy also devised simplified stan-

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W. Sheehan and C. J. Cunningham dard procedures on printed forms for each step of the reductions. These could then be performed even by unskilled school-leavers, and readily checked for errors. (Hutchins 2008:88)

At a time when reductions of observations often lagged years behind, Airy completed them within days, thereby greatly increasing their usefulness. At the British Association for the Advancement of Science’s meeting at Cambridge in 1833, he called for all the Royal Observatory, Greenwich’s observations of planets made between 1750 and 1830 to be reduced to a uniform system—and later did so himself. He was personally responsible for the erection of the Northumberland 11 ¾-in. (30-cm) refractor which, though matched by an equal-sized instrument belonging to Sir John Herschel’s friend Sir James South (1785–1867) at Kensington, was far more useful, thanks to its innovative double-yoke equatorial mounting. Airy worked obsessively hard at the Cambridge University Observatory during his seven years there, but even he—despite his comparative youth, his unusual ability, his high level of motivation, and his sheer genius for organization—found the experience “grueling”. Hutchins concludes (2008:89): “His example there could not have been sustained for long with the resources available. Only after promotion did he admit that the overwhelming mass of reductions was incompatible with active lecturing.” Unfortunately, his successor, the Rev. James Challis (1803–1882), stubborn and determined to be his own man, did not heed Airy’s advice—to his eventual discomfiture. Airy’s seven years at Cambridge were the best preparation he could have had for the role he moved up to, that of Seventh Astronomer Royal, late in the year 1835. The previous Astronomer Royal, John Pond (1767–1836), who had served in the role for 25 years, had retired because of ill-health. Pond left serious problems for Airy to solve, though, writes Robert W. Smith (1991:5), “the weight of historical opinion is that his tenure, taken overall, was certainly not a disaster.” New instruments had been acquired, and in line with the Observatory’s original royal warrant to “rectify the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much-desired longitude of places for perfecting the art of navigation,” observations of star positions were being made with increasing accuracy. Observations of the Moon and Sun were indeed kept up, though those of the planets were rather neglected. Pond devoted much of the Observatory’s time and attention to keeping up the Government chronometers for the Hydrographic Office, and though he increased the number of assistants from Nevil Maskelyne’s one to six, he stipulated the men supplied to him in these roles should be “drudges”. To quote his minute on the subject: I want indefatigable, hard-working, and, above all, obedient drudges (for so I must call them, although they are drudges of a superior order), men who will be content to pass half their day in using their hands and eyes in the mechanical act of observing, and the remainder of it in the dull process of calculation. (Maunder 1900:137).

Coupled with the fact that Pond, “either from lack of interest or failing health,” was almost never at the Observatory in later years, this last was, according to a later assistant astronomer at Greenwich, E. Walter Maunder, “a fatal mistake, and one

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which it is very hard to understand how anyone with a real interest in the science could have made”: Men who had the spirit of “drudges”, to whom observation was a mere “mechanical act”, and calculation a “dull process”, were not likely to maintain the honour of the Observatory, particularly under an absentee Astronomer Royal. Pond tried to overcome the difficulty by devising rules for their guidance of iron rigidity. The result was that after his resignation, in 1835, the First Lord and the Secretary of the Admiralty expressed their feeling to Airy … “that the Observatory had fallen into such a state of disrepute that the whole establishment should be cleared out.” (Ibid., 1900:137)

Given Airy’s own obsessive drive and compulsion for hard work, it is hardly surprising that this particular aspect of the administration did not change under him, at least not at first. The dozen or so human computers on the Observatory staff, many assigned to desks in the Octagon Room of the original structure built in Flamsteed’s time, toiled under “sweat shop” conditions. These computers were mere boys (women in the role would not come till later), with the youngest being perhaps 15 or 16. They spent a few years in these positions, after which they either got promoted to the role of a Warrant Assistant as the older men retired, or left for employment in the private sector, often clerking at a bank or for a mercantile concern. The one thing they would not have done was stay on and continue the backbreaking work of serving as computers. That, as Allan Chapman has said (2012:4–8), was strictly “a young man’s trade”. (At Greenwich at the time, women were not even considered for such positions, as they were later in the century at the Harvard College Observatory, where they so greatly distinguished themselves for their stamina and ability.) The main difference between Airy and Pond was that, in contrast with his mostly absentee predecessor, Airy was much of the time at the observatory himself. He was a strong, energetic, aggressive man, and in extreme degree what might now be called a micro-manager. Thus, says Maunder (who knew him personally in later years, though by then he had begun to mellow a little): Great as Airy was, he had the defects of his qualities, and some of these were serious. His love of method and order were often carried to an absurd extreme…. The story has often been told, and it is exactly typical of him, that on one occasion he devoted an entire afternoon to himself labeling a number of wooden cases “empty”, it so happening that the routine of the establishment kept everyone else engaged at the time… His mind had that consummate grasp of detail which is characteristic of great organizers, but the details acquired for him an importance almost equal to the great principles, and the statement that he had put a new pane of glass into a window would figure as prominently in his annual report to the Board of Visitors as the construction of the new transit circle. His son remarks of him that “in his last days he seemed to be more anxious to put letters which he received into their proper place for reference than even to master the contents,” his system having grown with him from being a means to an end, to becoming an end itself. (Maunder 1900:116–117).

Airy had no sooner moved down from Cambridge into the Astronomer Royal’s house on Greenwich Hill—much expanded from the narrow and uncomfortable quarters that Flamsteed had once occupied—than he began to launch the systematic

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institutional reforms of the Greenwich Observatory for which he would become famous, reforms which would see: … the Observatory’s complement and types of instruments, as well as methods of management, … transformed from those characteristic of a pre-industrial society to those of a society that had undergone the second phase of the Industrial Revolution… Under Airy, Greenwich to a large degree resembled a kind of accountant’s office, a very different sort of workplace from that common for astronomical institutions in, say, the 1820s when Airy began his career. (Smith 1991:5)

It is important to emphasise that Airy, who had a very high sense of his own duty as a public servant, never for a moment questioned the purpose of the observatory he was charged with running. The Greenwich Observatory was, as a public institution—and in accordance with its original charter—strictly utilitarian. It existed not for the purpose of pursuing those studies the public might find most interesting, or those which promised the most sensational results. It was rather to carry out the necessary (if onerous) tasks that would not or could not be voluntarily undertaken by others, and to supply, to those who applied for it and who were able to show why they needed it, data of the most fundamental kind. Thus, according to an article Airy wrote for the Penny encyclopaedia in 1838, the Greenwich Observatory was dedicated … to the regular observation of the sun, moon, planets, and stars (selected according to a previously arranged system), when they pass the meridian, at whatever hour of the day or night that may happen, and in no other position; observations which require the most vigilant care in regard to the state of the instruments, and which imply such a mass of calculations afterwards, that the observation itself is by comparison a mere trifle. From these are deduced the positions of the various objects, with an accuracy that can be obtained in no other way… (Airy 1838:442)

First and foremost, the Observatory was committed to the continuance of the record of daily observations of the places of sun, moon, stars, and planets which had commenced under its first Astronomer Royal, John Flamsteed, almost two centuries before. Those who did this work stood little or no chance of making the striking discoveries that so appealed to the popular imagination; they even were actively discouraged from the attempt. When a few years later asteroids began to be discovered in numbers, an assistant was found working with a telescope on his “off-duty” night. Airy asked what business he had to be there on his free night. He was told, “I am searching for new planets.” Apparently, the assistant received a severe reprimand and was ordered to discontinue at once. (Maunder 1900:142). This was entirely characteristic of Airy. In a similar vein, a forty-year relationship with James Glaisher (1809–1903), who had been Airy’s first assistant at Cambridge and who later followed Airy to Greenwich where he took charge of and served with distinction in the Magnetical and Meteorological Department, ended abruptly in 1874, when Airy rebuked his colleague for leaving work 10 minutes early on one occasion! (Dewhirst 2007; in Hockey, 2007, Vol. 1:424). Positional astronomy had made great strides since Flamsteed’s, Halley’s, Le Monnier’s, or even Piazzi’s day. The observer working on the mural quadrants had to measure the right ascensions and declinations as before, tracking the exact

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moment that a star (or planet) drifted across one of multiple evenly spaced wires while listening to a clock beating seconds in order to determine the exact moment the star crossed the meridian. The star or planet’s altitude in the sky affected the observed position, owing to atmospheric refraction—an effect most noticeable near the horizon, where it might amount to half a degree. The particular temperature and pressure at which the observation also affected the measurements, and so had to be carefully recorded and factored in. The observer made his record, with entries in all the columns; then the real work began—that of “reducing” the observations, i.e., correcting for the various sources of error. A single lunar observation might take a computer four hours to reduce. In addition to the sources of error mentioned above, the effects of precession and the aberration of starlight had to be allowed for, and the “personal equation” of the observer had to be estimated and corrected for. (Sheehan 2012). A visitor to the Greenwich Observatory during Airy’s time was apt to see very little looking through telescopes. Instead he would find, as Maunder, who was hired as an assistant by Airy during the latter part of Airy’s long reign, found (Maunder 1900:137), “the official rooms not unlike those of Somerset House or Whitehall, … its occupants for the most part similarly engaged in what is, apparently, merely clerical work. An examination of the big folios would of course show that instead of being ledgers of sales of stamp, or income tax schedules, they referred to stars, planets, and sunspots; but for one person actively engaged at a telescope the visitor would see a dozen writing or computing at the desk.” The reduced data consisted of the raw positions duly corrected by these dozen or so hard-working computers, who stood labouring at their desks or as much as twelve hours a day (with an hour off over lunch), hewing to the specifications of the “skeleton forms” which Airy had first introduced as a student at Cambridge, and of which some thirty were in use at Greenwich by 1835. These skeleton forms had been ingeniously designed as to make the whole process completely rote and mechanical, so that a computer possessing only an elementary knowledge of arithmetic could carry through the intricate and laborious calculations without difficulty. These men were the unsung heroes in the Neptune saga, for it was only because of the valuable data—the reduced positions of Uranus—that the residuals could be worked out whose indications pointed the mathematicians to the unseen world. Ultimately, Airy—despite the ignominy with which he was so long treated for what he failed to do—as superintendent of this enormous effort also deserves credit, since the mathematicians worked up data that he and his staff had provided. (Hutchins 2008:88).

2.6  Hussey’s Brazen Idea By 1832, when Airy pronounced on the failure of Bouvard’s Tables to accurately represent the motion of Uranus, interest in the problem was becoming widespread. Not only were Bessel and Airy engaged; so were François Arago at the Paris Observatory and Sir John Herschel, who was about to leave England for four years

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to survey the southern skies from Cape Town. Airy later recalled this period as one of crisis for Newton’s law of gravitation itself. Speculation was rife. A few astronomers were willing to entertain possibilities such as that Uranus was moving through a resisting medium of some kind, or that it had been struck by a comet at about the time of its discovery and thrown into a different orbit (as a way of accounting for the failure of the pre-1781 observations; obviously no help in the period after 1826 or so). Bessel was trying to explain it in terms of Johann Tobias Mayer’s (1752–1830) idea of a specific attraction producing a modification of the inverse-­square law. Presumably, however, most astronomers still had full confidence in the Newtonian concept, which had emerged triumphant in every test, and expected a solution to emerge from the familiar paradigm. Thus, emerged the increasingly probable hypothesis that an unknown exterior planet might be perturbing Uranus from its course. (Grosser 1962:48–49). That the solution might involve a planet was an explanation too self-evident not to have many fathers. At the Observatoire de l’École Militaire in Paris, Michel-Jean-­ Jérôme Lefrançais Lalande (1766–1839), Joseph-Jérôme Lalande’s “nephew”, Michel’s wife Marie-Jeanne-Amélie (1768–1832), and Burckhardt were finishing observations for J.J. Lalande’s star catalog Histoire Céleste Française, which von Zach later likened to the “Augean stables” due to the vast number of errors it contains among its 50,000 entries. (Cunningham 2016b:164). (A note about Lalande’s “nephew”, who became an astronomer under J.-J. Lalande’s tutelage: he was actually a grandson of Lalande’s uncle. Lalande, who was apparently childless himself, also referred to Michel’s wife Marie-Jeanne-Amélie as his niece or daughter. She was literally Lalande’s daughter, however, as she was born as the result of an extramarital affair.) Lalande and Burckhardt entered a note in their observing logbook on their second sweep of the sky, due to begin in the late summer of 1800. They expressed a hope “to discover a planet beyond Herschel, if one exists.” Ironically, such a planet had been recorded by Michel Lalande himself, twice in 1795, without his realizing it! —That planet was Neptune!). That the deviations of Uranus’s motion might be due to a distant planet was so far an intriguing theoretical possibility. The first person, however, motivated to actually do something about it was the Rev. Dr. Thomas John Hussey (1792–1866), the Church of England rector of Hayes, Kent, who lived at a time when some rectors of establishment churches enjoyed comfortable livings and the leisure to pursue outside interests. Hussey’s private observatory was among the best-equipped in England at the time (with a 6.5-in. Fraunhofer refractor, a 7-foot Herschelian reflector, and a 9.3-in. Newtonian). He achieved contemporary fame as one of the first observers in Britain to catch Halley’s comet on its return in 1835. He also contributed a star chart, Hora XIV, to the Berlin Academy’s great star-mapping project. (See Chap. 6). A successor of von Zach’s plan to map the stars of the Zodiac in order to assist in the discovery of asteroids, this project, international in scope, owed its original impetus to Bessel, but was currently supervised by the director of the Berlin Observatory, Johann Franz Encke (1791–1865). It would amply repay the original hopes for it of assisting in the identification of asteroids; it would also, in a few years’ time, lead to far larger quarry. (Baum 2007:41).

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Hussey’s own observations showed that Uranus was lagging increasingly behind the positions prescribed in Bouvard’s Tables. Suspecting an unknown planet might be the cause, he divulged these suspicions to Bouvard himself during a visit to Paris. Bouvard saw nothing far-fetched in the proposal. He even offered, if he found the time, to undertake the calculations necessary to determine a position for the planet that might aid Hussey in a telescopic search. Unfortunately, as director of the Paris Observatory, he was already greatly overworked, and so nothing more came of it. Instead, on 17 November 1834, Hussey wrote to the leading authority on such matters in England. He was also, in all probability, the one man in all the world who was even busier than Alexis Bouvard. This was Airy, then still at Cambridge University Observatory but soon to be appointed seventh Astronomer Royal at Greenwich (succeeding the Rev. John Pond). “The apparently inexplicable discrepancies between the ancient and modern observations,” Hussey explained (Hussey to Airy, 17 Nov. 1834:RGO), suggested to me the possibility of some disturbing body beyond Uranus, not taken into account because unknown. My first idea was to ascertain some approximate place of this supposed body empirically, and then with my large [9.3-in.] reflector set to work to examine all the minute stars thereabout: but I found myself totally inadequate to the former part of the task…. I therefore relinquished the matter altogether; but subsequently, in a conversation with Bouvard, I inquired if the above might not be the case: his answer was, that, as might be expected, it had occurred to him…. Upon my speaking of obtaining the places empirically, and then sweeping closely for the bodies, he fully acquiesced in the propriety of it, intimating that the previous calculations would be more laborious than difficult…. If the whole matter do not appear to you a chimera, which, until my conversation with Bouvard I was afraid it might, I shall be very glad of any sort of hint respecting it...I am disposed to think that such is the perfection of my equatorial object-glass, that I could distinguish almost at once the difference of light of a small planet and a star. (printed in Airy 1847:385–414).

Airy could almost always be counted on to be prompt—he posted a discouraging reply from Cambridge (Airy to Hussey, 23 November 1834: RGO), “It is a puzzling subject, but I give it as my opinion, without hesitation, that it is not yet in such a state as to give the smallest hope of making out the nature of any external action on the planet…. I am sure it could not be done till the nature of the irregularity was well determined from several successive revolutions.” In fact, though Airy has often been criticised for this reply, he was actually quite right—if the question had been one of working out the elements of the orbit of the perturbing planet, as opposed to simply determining its position in the sky. Then it would indeed have been necessary to observe it through several synodic periods. (The synodic period is defined as the period between a planet’s return to the same angular position relative to the Sun, e.g., a conjunction or opposition.) And yet, in any case, Airy does not seem to have discouraged Hussey entirely, for afterwards Hussey discussed the matter again with Bouvard, who not only repeated his previous belief that the irregularities in Uranus’s motion could be explained by the existence of a perturbing planet but in fact admitted to suspecting two. (Hussey to Airy 1846:RGO). Perhaps Hussey would have pursued the matter further but in 1838 he suffered a severe accident (of unspecified nature). He sold his instruments to Durham University (Chapman 1998:20), thus ending his career in astronomy.

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The facts have tended to disappear into a haze of legend, and later—when it became fashionable to blame Airy for every Neptunian misfortune—The North British Review (1847:210) offered a jibe that descended to an astrological smear in suggesting to the public at large that Airy’s 1834 response to Hussey was no less than despicable, and had perhaps cost Britain then the glorious discovery of the planet lately lost again for Airy’s want of faith. The views of the Astronomer Royal were not in unison with those of Mr. Hussey, and, as if he had been born when Aquarius was in the ascendant, he throws cold water upon the glowing enthusiasm of his friend, and extinguishes for ever his well-founded expectation of adding to Apollo’s lyre another string.

This passage, however unfair in its attitude toward Airy, is at least mythologically erudite. The ancient Greek philosopher Pythagoras was reputed to have added an eighth string to the lyre of Apollo, the god of music, “to produce the overall range of an octave.” (Mathieson 1999:244, 246). The discovery of Uranus in 1781 was seen as adding a seventh string to Apollo’s lyre that had been missing since antiquity; adding Neptune would have completed the octochord of Pythagoras.

2.7  V  alz and Nicolai Give Chase, while Wartmann Finds a Suspect The idea of an unknown planet patrolling the outer Solar System was, by the time Hussey mooted it to Airy and discussed it with Bouvard, one whose time had come. Halley’s comet, which as noted in Chap. 1 had provided such a grand confirmation of the inverse-square law of gravitation, was due to reappear for the second time since Halley’s death in 1742. Astronomers attempted to do as Clairaut had done and compute as accurately as they could from perturbation theory the comet’s date of return. The most accurate would prove to be that of the French nobleman Philippe Gustave Le Douclet, comte de Pontécoulant who a year before the comet returned published the second and third volumes of his massive Théorie analytique système du Monde (and incidentally recast the celestial mechanics of Laplace into the form that would be used by Adams and Le Verrier in their calculations a few years later). In his calculation of the comet’s path, Le Douclet included the perturbative effects of Jupiter, Saturn, and Uranus between 1682 and 1835, and even those of the Earth near the comet’s 1759 perihelion passage. The calculations were completely reworked several times. The last was off by only 3 days in predicting the comet’s November return to perihelion. Small as it was, the discrepancy of 3  days was enough to lead Jean Élix Benjamin Valz (1787–1867) at the Marseilles Observatory and Friedrich Bernhard Gottfried Nicolai (1793–1846) at the Mannheim Observatory independently to posit a retarding force of some kind on the comet’s motion to account for it. Valz opined in 1835, “I would prefer to have recourse to an invisible planet beyond Uranus…. Would it not be admirable thus to ascertain the existence of a body which we cannot even observe?” (Grosser 1962:50–51).

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Nicolai, invoking Bode’s law, went a step farther: “One immediately suspects that a trans-uranian planet (at a radial distance of 38 astronomical units, according to the well-known rule) might be responsible for this phenomenon.” (Grosser, 1962:51). The discrepancy of several days between the actual period of Halley’s comet and that computed using perturbations of the known planets was observed again at subsequent returns (in 1910 and 1986). It has now been accounted for on the basis of nongravitational effects, such as the rocket-like effects of ices vaporizing during the comet’s return to the Sun. (Yeomans 1991:260–262). In 1835, so great was the prestige of celestial mechanics, even this inconsiderable delay was like fingerprints assigning the blame on the culprit responsible. Perhaps that culprit had already been sighted. In May 1835, Niccolò Cacciatore, who had succeeded Piazzi as director of the Palermo Observatory, claimed to have sighted an object of the 8th magnitude moving among the stars of Virgo. (Baum 2007:267–268). Horrible weather then intervened, making it impossible for him to catch it again before it was lost in the evening twilight. Provocatively, Cacciatore judged that its motion had been slow enough to indicate a situation “beyond Uranus”. Though various attempts were made to follow up, Cacciatore’s object was never seen again, and has never been satisfactorily explained. (Cunningham 2017a:112). However, the announcement smoked out a far more interesting document from Louis François Wartmann (1793–1864), a Belgian astronomer at the Geneva Observatory, concerning observations he had made in 1831. As he afterward explained, in the summer of that year the observatory was being refurbished; all the main instruments were placed in storage, while the observatory’s director, Jean Alfred Gautier (1793–1881), retreated to his chateau at Vinzel. Wartmann was left with only a single instrument, a small Fraunhofer refractor. As a diversion, he planned to see if he could “discover” Uranus with this telescope, having only a general knowledge of its location. As a preliminary he used the telescope’s 10x finder-scope to construct a chart of faint stars in Capricorn, in the vicinity of the planet’s path. While thus engaged, on 6 September 1831, he noted that one of his stars, a pale white star of seventh or eighth magnitude, seemed to be moving to the west, an indication of retrograde motion. He recorded a position for that date: 315° 27′ right ascension and −17° 28′ declination. (Uranus at the time was actually at 313°.) After a spell of bad weather, he recorded it again on September 25, confirmed its movement, and examined it with the Fraunhofer refractor, using a magnifying power of 60x, with an inconclusive result. He obtained a few further positions in October and November but by December twilight began to interfere. Subsequent attempts to find what came to be referred to as “Wartmann’s planet” were unavailing. The fact that Wartmann had delayed publishing his observations for four years received a rebuke from Arago. (Baum 2007).

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2.8  Astronomers Gather More Clues Meanwhile, significant progress was being made by celestial mechanicians, as they continued to revise their calculations. Alexis Bouvard planned yet another revision of his tables of Jupiter and Saturn. However, he delegated to his nephew, Eugène Bouvard (1812–1879), the task of revising those of Uranus. In the history of the attempts to tame the wayward movements of Uranus, Eugène Bouvard has been a largely overlooked figure. He, like Alexis, was born in Les Contamines in 1812, and at the age of nine went to Paris to live with his uncle (actually, great uncle), who had just published his Tables of Jupiter, Saturn, and Uranus. From then until his death, Alexis would direct Eugène’s studies and pull strings. In April 1834 Eugène was appointed student-astronomer at the Paris Observatory. When Alexis Bouvard decided to assign Eugène to the Uranus problem, the latter was only 25, and still relatively inexperienced. His main work at the Paris Observatory was as an observer. Alexis hoped the Uranus problem would enable his great nephew to establish a solid reputation, but it proved to be an albatross around Eugène’s neck: his failure to solve the problem would eventually lead him to abandon astronomy altogether. (Cruikshank and Sheehan 2018:47–48). Initially, Eugène took the commission seriously, and went about discharging it in a systematic, methodical way. Though not unwilling to use his uncle’s famous name and reputation as a calling card, he introduced himself to Airy as “a young beginner”, which in point of fact he was: Conscious of the good will that a scientist as famous as yourself must have for a young beginner in the fine career of astronomy, I address myself to you, even though I am personally unknown to you. For a few years I have been in charge of all astronomical observations carried out at the observatory of Paris, with several other young people; but, during the few moments of spare time that my functions allow me, I work at something that I believe to be not without importance. My uncle, whom you had the occasion to see in England in 1826 [not true, as Airy was to point out; they did meet twice in Paris, however], is reconstructing the tables of Jupiter and Saturn, using recently made corrections to their astronomical elements. He has assigned to me the job of reconstructing the tables of Uranus. By consulting your comparisons of observations of this planet with its tabular positions, I note that its differences in latitude are very large and continue to increase. Is that owing to an unknown disturbance in the movements of this star due to a body located beyond it? I do not know, but that at least is my uncle’s idea. I view the solution of the question as being of the greatest importance. However, in order to succeed in it, I need to reduce the observations with the greatest precision, and often lack the means to do so. When I employ the observations of Maskelyne [the former Astronomer Royal] I find myself especially vexed, since I hardly ever can find out the collimation error of his quadrant. My uncle experiences the same difficulties when he reduces the observations of Jupiter and Saturn, but the case is even more critical for me, since it is the latitude that is the most defective in [the theory of] Uranus. … If you would be kind enough to communicate the results [of these reductions] to me, you would render me a great service, and would also infinitely oblige my uncle. As long as the work is done by you or under your direction, I would sooner accept it than re-compute it myself, which would take a long time. Moreover, Sir, you know that in scientific matters it is better to build upon that which has already been done, so long as it has been done by persons of merit….

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I hope, Sir, that my request is not too indiscreet, my only motivation being a desire to do a conscientious work, as will prove to scientists that I am worthy of my name which is also that of my uncle…. Permit me to offer you my gratitude in advance, as well as my well-considered homage and respect. (E. Bouvard to Airy, 6 October 1837: RGO).

Airy replied at once. (Airy to E. Bouvard, 12 Oct. 1837:RGO). He was pleased to report that all the observations of Uranus made at Cambridge University Observatory in 1833–34–35, as well as those made at Greenwich in 1836, already had been reduced. These observations had been made “with the best instruments in existence, and far more completely reduced than any others which are published.” Meanwhile, he had set out upon an even more massive project—the reduction of all the Greenwich planet observations beginning with Bradley’s in 1750: I am carrying on with the reductions with all possible care, in duplicate where it is necessary, taking Bessel’s Tabulae Recomp. [i.e., the Tabulae Regiomontanae reductionem observationum; the Königsberg tables for reducing observations, published in 1830] as the foundation, and am sparing no labor to make the results much as future ages may consider as standard determinations. I am confident that you would find it best, at present, to employ yourself with the various kinds of preliminary work that may appear necessary, but to leave the discussion of the observations till I can supply you with the complete reductions… With respect to the error of the tables of Uranus, I think you will find that it is longitude, which is most defective, and that the excess in latitude is not at present increasing. To show this, I set down a few of my results (each of them the mean of several observations). Heliocentric errors of Bouvard’s tables of Uranus (supposing radius vector correct) Date 1833 Aug. 23 Sep. 22 Oct. 22 1834 Aug. 17 Sep. 15 1835 Aug. 3 Sep. 12 1836 July 22 Oct 12

Longitude Ecliptic Polar Distance +33°.14 +31 .03 +32 .24

+12°.28 +12 .73 +12 .311

+37 .87 +37 .28

+11 .52 +11 .14

+45 .28 +44 .79

+11 .84 +11 .81

+53 .66 +51 .69

+10 .12 +10 .73

You will see by this statement that the errors of longitude are increasing with fearful rapidity, while those of latitude are nearly stationary. In the memoirs upon these subjects, I have included, in the expression for the error in longitude, a term depending on the possible error of the radius vector. I cannot conjecture what is the cause of these errors, but I am inclined, in the present instance, to ascribe them to some error in the perturbations. There is no error in the pure elliptic theory (as I found by examination some time ago). If it be the effect of any unseen

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W. Sheehan and C. J. Cunningham body, it will be nearly impossible ever to find out its place. I beg you to offer my remembrance to your Uncle, whom I never saw in England, but whom I have visited in Paris in two different years.

As in the case of Hussey, Airy’s response seems to have paralysed further initiative. In any event, Eugène seems to have been happy to set the matter aside for the time being, and to patiently wait for the reductions that Airy promised. Meanwhile, there was another important development—and once again, Airy had a hand in it. It was of the utmost importance in Airy’s view, but hardly appreciated by anyone else. Airy had discovered, from the reductions of the Cambridge observations of 1833, 1834, and 1835, and the Greenwich observations of 1836, that in addition to the errors in heliocentric longitude, “the Tabular Radius Vector of Uranus is considerably in error.” (Airy 1838). The values for the radius vector had been computed by the Nautical Almanac office on the basis of Bouvard’s Tables, and in comparing these to the reduced observations Airy found the errors to be as follows: Tabular Errors in the Radius Vector of Uranus (in AU): From observations of 1833 “     “    “  1834 “     “   “  1835 “     “   “  1836

−0.00533 +0.00074 −0.00405 −0.00483

Clearly, there was something wrong with the reductions of 1834; Airy surmised (1838:col. 220) that some difference must have been made in the radius vector used for the computations of the Nautical Almanac office for that year. However, adopting the mean of the four results, the tabular error in the radius vector worked out to −0.00337, the tables gave the radius vector too small by a quantity considerably greater than the Moon’s distance from the Earth, while adopting the means only of the 1833, 1835 and 1836 observations, the error appeared to be nearly equal to the diameter of the Moon’s orbit. This discussion is merely parenthetical at the moment. However, the radius vector problem was later to loom large, and at least if Airy’s various statements on the subject are to be believed, he regarded it as of the first importance. The failure of others to appreciate this would, indeed, be a hinge of fate—and would be alleged as a reason that Britain lost the sole credit for discovering the planet. Acknowledgements  For information on Le Monnier, Guy Bertrand, Observatoire de Paris, has been very generous in sharing his researches in the archives of the Paris Observatory; on Eugène Bouvard, great thanks to Françoise Launay. The Observatoire de Paris has very kindly granted permission to reproduce the page from Le Monnier’s observing logbooks.

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References Correspondence to or from George Biddell Airy is from the RGO: Royal Greenwich Observatory. Anon., 1847. Mr. Adams’s and M. Le Verrier’s Researches Respecting the New Planet Neptune. The North British Review. Edinburgh, W.P. Kennedy. Vol. 7, 207–246. Airy, G.B., 1833. Report on the Progress of Astronomy during the Present Century, Report on the First and Second Meetings of the British Association for the Advancement of Science. London, John Murray. Airy, G.B., 1838. Schreiben des Herrn Airy, Astronomer Royal, an den Heraugsgeber, Greenwich 1838. Febr. astronomische Nachrichten, No. 349, cols. 217–220. Airy, G.B., 1847., Account of Some Circumstances Historically Connected with the Discovery of the Planet Exterior to Uranus, Memoirs of the Royal Astronomical Society, 385–414. Alexander, A., 1965. The Planet Uranus: a history of observation, theory, and discovery. London, Faber and Faber. Baum, R., 1973. The Planets: Some Myths and Realities. Newton Abbot, David & Charles. 116–119. Baum, R. and Sheehan, W., 1997. In Search of Planet Vulcan: the ghost in Newton’s clockwork universe. New York, Plenum. Baum, R., 2007. The Haunted Observatory: curiosities from the astronomer’s cabinet. Amherst, New York, Prometheus. Bessel, F.W., 1824. Anzeige einiger Verbesserungen der Uranus-Tafeln des Herrn Bouvard, Astronomische Nachrichten No. 48; followed by Bouvard’s reply, “Note relative à mes Tables d’Uranus.” Bouvard, A., 1819. Extrait des Registres des observations astronomiques faites par Le Monnier, à l’Observatoire des Capucins, rue Saint Honoré, à Paris. Connaissance des Tems, ou des mouvemens célestes a l’usage des astronomes et des navigatuers, pour l’an 1821. Paris, Bureau of Longitudes, 339–340. Bouvard, A., 1821. Tables astronomiques publiées par le Bureau of Longitudes de France contenant les Tables de Jupiter, de Saturne et d’Uranus contruites d’après la théorie de la Mécanique céleste. Paris, Bachelard et Huzard. Cacciatore, G. and Schiaparelli, G.V., eds., 1874. Corrispondenza Astronomica fra Giuseppe Piazzi e Barnaba Oriani. Milan, University of Hoepli. Ceragioli, R., 2018. William Herschel and the “Front-View” Telescopes; in: The Scientific Legacy of William Herschel (ed. C. Cunningham), 297–364. Cham, Springer. Chambers, G., 1889. A Handbook of Descriptive and Practical Astronomy. Oxford, at the Clarendon Press, 4th ed. Chapman, A., 1991. Britain’s first professional astronomer: George Biddell Airy (1801-1892), Yearbook of Astronomy, Patrick Moore (ed.), 185-205. Chapman, A., 1998. The Victorian Amateur Astronomer: Independent Astronomical Research in Britain, 1820–1920. Chichester and New York, Wiley-Praxis. Chapman, A., 2012. Airy’s Greenwich Staff. The Antiquarian Astronomer, Issue 6, January 2012, 4–8. Cruikshank, D.P. and Sheehan, W., 2018. Discovering Pluto: exploration on the edge of the Solar System. Tucson, University of Arizona Press. Cunningham, C.J., 2007. The Collected Correspondence of Baron Franz-Xaver von Zach, vol. 1. Surfside, Star Lab Press. Cunningham, C.J., 2009. The Collected Correspondence of Baron Franz-Xaver von Zach, vol. 6. Surfside, Star Lab Press. Cunningham, C.J., 2016a. Discovery of the First Asteroid, Ceres. Cham, Springer. Cunningham, C.J., 2016b. Early Investigations of Ceres and the Discovery of Pallas. Cham, Springer.

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Cunningham, C.J., 2017a. Studies of Pallas in the Early Nineteenth Century. Cham, Springer. Cunningham, C.J., 2017b. Bode's Law and the Discovery of Juno. Cham, Springer. Dewhirst, D., 2007. James Glaisher; in Biographical Encylcopedia of Astronomers (ed. T. Hockey), Vol. 1:423–424. Cham, Springer. (Two vols.) Forbes, E.G., 1982. The Pre-discovery Observations of Uranus. In: Garry Hunt (ed.), Uranus and the Outer Planets. Cambridge, Cambridge University Press. Frommert, H., 2007. Friedrich Wilhelm Bessel. In: Biographical Encyclopedia of Astronomers. New York, Springer, Vol. 1:116-117. Gerdes, D., 1990. Die Geschichte der Astronomischen Gesellschaft. Lilienthal, Heimatverein Lilienthal. Gillispie, C.C., 1997. Pierre-Simon Laplace, 1749–1827: a life in exact science. Princeton, Princeton University Press. Grosser, M., 1962. The Discovery of Neptune. Cambridge, Harvard University Press. Hall, B., 1841. Patchwork. London, Edward Moxon. Herschel, W., 1912. Account of a Comet, in Scientific Papers (ed. J.L.E. Dreyer). London, Royal Society and Royal Astronomical Society, vol. 1, 30–38. Hutchins, R., 2008. British University Observatories 1779–1939. Aldershot, Ashgate. Kepler, J., 1619. The Harmony of the World (ed. Aiton, E.  J., Duncan A.M. and Field J.V.), Philadelphia, American Philosophical Society. 1997. King, H.C., 1979. The History of the Telescope, New York, Dover reprint of 1955 edition. Laplace, P.S., 1809. The System of the World (tr. Pond, J.). London, Richard Phillips. Lubbock, C.A., 1933. The Herschel Chronicle. Cambridge, Cambridge University Press. Marsden, B., 1995. Developments in orbit determination, in Planetary Astronomy from the Renaissance to the Rise of Astrophysics, Part B: the eighteenth and nineteenth centuries. Taton, R. and Wilson, C., eds., Cambridge, Cambridge University Press. Mathieson, T. J., 1999. Apollo’s Lyre. Lincoln, University of Nebraska Press. Mauer, A. and Forbes, E.H., 1971. William Herschel’s Astronomical Telescopes. Journal of the British Astronomical Association, 81:284–291. Maunder, E.W., 1900. The Royal Observatory, Greenwich: a glance at its history and work. London, The Religious Tract Society. Meadows, A.J., 1976. The Airy era: the origins, achievement and influence of the Royal Observatory, Greenwich: 1675-1975. Vistas in Astronomy, 20(1-2):197-201. Moulton, F.R., 1914. An Introduction to Celestial Mechanics. New  York, Dover 1970 reprint of 2nd ed. Olson, R.J.M. and Pasachoff, J.M., 2019. Kosmos: the art and science of the universe. London, Reaktion Books. Rawlins, D., 1981. The Unslandering of Sloppy Pierre. Astronomy, 9, 24–28. Schaffer, S., 1988. Astronomers Mark Time: Discipline and the Personal Equation. Science in Context, 2(1):115-145. Sheehan, W., 2012. From the Transits of Venus to the Birth of Experimental Psychology. Physics in Perspective, 15(2):130-159. Sheehan, W. and Conselice, C., 2015. Galactic Encounters: our majestic and evolving star-system, from the Big Bang to time’s end. New York, Springer. Smith, R.W., 1991. A National Observatory Transformed: Greenwich in the Nineteenth Century. Journal for the History of Astronomy, xxii, 5–20. Yeomans, D.K., 1991. Comets: a chronological history of observation, science, myth, and folklore. New York, John Wiley & Sons. von Zach, F.X., 1785. Astronomische Beobachtungen und Nachrichten. Berliner Astronomische Jahrbuch für 1789, 156–163. von Zach, F.X., 1801. Über einen zwischen Mars und Jupiter längst vermuteten, nun wohnscheinlich entdeckten neuen Hauptplaneten unseres Sonnen Systems. Monatliche Correspondenz 3, 592–623. von Zach, F.X., 1802. Resumed news about the new main planet of our solar system, Ceres Ferdinandea. Monatliche Correspondenz, 5, 379–404.

Chapter 3

John Couch Adams: From Cornwall to Cambridge Brian Sheen and Carolyn Kennett

Abstract  John Couch Adams was one of the leading mathematical astronomers in the country and at a very young age (27 in 1846) and a junior Fellow in his college, was a pivotal figure in the hotly disputed discovery of Neptune. Born within one of the more remote areas of England, Cornwall, the region of his birth and his Cornish upbringing were fundamental in understanding the circumstances which would frame his life. His parents were tenant farmers living through a period of decline in rural England and the family income had to stretch to accommodate a burgeoning large family. During his early years Adams attended a local village school, where it was quickly evident that he had mathematical talent. Bookish in nature, he outstripped his tutors’ abilities and was sent to a family run school in Plymouth to further his education during his teenage years. It was clear to the family and close acquaintances that Adams should attend Cambridge University, the leading centre for mathematical studies in Britain and that in itself held challenges due to the expense involved. The large and impecunious family drew together and unstintingly utilised every possible source of income they could to invest in his future by making Cambridge possible for him.

3.1  A Cornish Childhood John Couch Adams was born on 5 June 1819 at Lidcott Farm in Laneast just north of Bodmin Moor, Cornwall (His early letters spell it Litcott). As Adams’s upbringing and childhood experiences were to shape the man he became, it is consequently worth exploring the location in which he spent his formative years. Cornwall forms the most southwestern extent of England and the Cornish are promontory people. B. Sheen Roseland Observatory, Cornwall, UK C. Kennett () Independent Scholar, Helston, Cornwall, UK e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_3

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Ethnically different to the rest of England, recent DNA evidence has shown the Cornish to be a distinct genetic group which relates to the oldest population within the UK, (Callaway, E. 2015) the earliest Neolithic settlers. This is in part due to isolation through geography, as the duchy of Cornwall bounds just one English county, Devon. The Tamar river defines this boundary neatly and other than a few miles of land between the river’s source and the sea, Cornwall in its entirety would form an island. It is a county apart; one which considers itself its own country and from 1844 was granted the legal framework within which to do this. The 1844 Duchy of Cornwall Act declared the land a Duchy and not part of the crown and it’s the only part of the UK to hold this status (Duchy of Cornwall Act, 1844). Access into Cornwall in the early 1800s would have been by dirt track road, navigating past the moorland of Dartmoor and then Bodmin. A southern route would have been by ferry crossing at Plymouth, over the river Tamar and then a long coach journey by road. Travel into the duchy was laborious and at times treacherous. Language was regional and accents would have been strong, although the Cornish language had been effectively side-lined to the far west and south of the region, native dialect still peppered speech with Cornish words and phrases such as Dydh da being used instead of hello. John Bradley (1728–1794) stated during his 1769 Transit of Venus, Board of Longitude funded expedition to Cornwall that he had found himself “in a very odd part of the country.”. (Kennett, 2015:6). You could assume that living in Cornwall would feel as far removed from learned and high society as you could be, but there were pockets of scientific work, often driven by the need for advancement in the tin mining industry. Far west, one man, Malachy Hitchens (1741–1809) had fostered an early centre of scientific learning influencing a number of careers including; Davis Gilbert (1767–1839), Humphrey Davy (1778–1829) (both subsequently became Presidents of the Royal Society) and William Dunkin (1781–1838) (Father to Edwin Dunkin (1821–1898) future assistant of Astronomer Royal, George Biddell Airy). (Kennett, 2017:53). During the time of Adams’s upbringing Cornwall was becoming renowned for its scientific community and organisations such as the Royal Institute of Cornwall (Truro founded 1818), The Royal Polytechnic (Falmouth founded 1833) and the Royal Geological Society (Penzance founded 1814) were at the forefront of these scientific inquiries. (Naylor, 2002:498). Nineteenth century Cornwall was becoming a site of national scientific importance. (Naylor, 2002:494). Unfortunately, Adams was far removed from this influential cluster and visiting his birthplace today it is clear that it was not a centre of excellence of scientific learning. The place of his birth was on the edge of Bodmin Moor near the town of Launceston. A description of the eastern moor area of Cornwall within Daphne du Maurier’s famous 1936 novel Jamaica Inn gives a clue to how remote the location of Adams’s birth and formative childhood years was. Described as an area with no trees, houses or roads, the moor was then and still is now, a bleak part of the world in which people can lose themselves. Little has changed to his home since Adam’s childhood, although there has been the addition of an extra wing. The house can be viewed from the road and is located just to the north of the modern A395. In Adams’s day there would be no sound of manmade machinery and no lorries thundering

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Fig. 3.1  Historic view of Lidcott Farm, Laneast Downs, Cornwall, John Couch Adams’s childhood home (Credit: M. Roos (Private Collection), Painter Unknown)

along the A30 trunk road; at night there would be no light pollution to reduce the splendour of the night sky. It was an ideal location for reflection, thinking and developing theories and it was this combination which helped propel Adams to Cambridge, Neptune and beyond (Figs. 3.1 and 3.2). Within 10 miles of Adams’s childhood home was the town of Launceston. This was the administrative capital of Cornwall until 1835 when the district judge circuit moved it to Bodmin town taking with it the Guildhall and old Goal. Regarded as a “rotten borough” by 1832 it lost one of its two MPs due to the small number of residents being entitled to vote in parliamentary elections, which in turn could leave a significant amount of power sitting with one person or family, making it rotten. A visit to the town by Adams would have found a place in decline. The once glorious and substantial castle, which dominated the town’s skyline, had been left to rot and ruin. Piglets were let to run wild on its slopes and cabbages grown in its interior and it was such a disgrace that by 1842 Queen Victoria had to intervene in its upkeep. (Robbins, 1883:330). The decline in the town was in part due to the mass exodus of the local population. The well-known miner’s exodus from Cornwall started in the 1830s but during the period from the late 1790s the farming community had its own issues leading to mass migration. Competition from imported grain, high rents and poor quality soil would all compound the issue and many tenant farmers were living on the breadline. His parents, Thomas Adams (1788–1859) and Tabitha (Knill Grylls) (1796–1866) were respectable tenant farmers of Lidcott farm Laneast, a step up from general labourers but still living a seasonal existence when one failed harvest could sink people into poverty. Added to this were periods where Adams’s father took to his bed for extended periods due to “fevers”. (Adams, 1892:11). Improvements in life

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Fig. 3.2  Lidcott Farm in modern times, Laneast Downs, Cornwall, John Couch Adams’s childhood home (Credit: John Lyne Photography)

expectancy meant that families had become larger and had more surviving children putting further pressure on a family budget. Adams’s family followed this pattern and he had a number of brothers and sisters born after him in fairly quick succession: Thomas Adams 28 April 1821–1885, George Adams 5 November 1823–1909, Grace Couch Adams 1 September 1828–1891, Elizabeth Grylls Adams 3 November 1831–1846, William Grylls Adams 16 February 1836–1915 and Mary Ann Adams 28 April 1839–1888 (Harrison, 1994:11). Offspring were having to look elsewhere for their future income, as farming was not a sustainable financial option for all the children. A child with a gift such as Adams’s in mathematics would have been encouraged by the whole family as it offered financial stability for them all. Therefore it is no surprise that his exceptional mathematical ability was nurtured by his parents and siblings alike.

3.2  Schooling of the Young Adams His early education highlighted his great potential as a thinker. It started modestly with Ann Dawe, who ran a Dame’s school for local children in her farmhouse kitchen. After a short while he was taken on under the wing of Richard C. Sleep who established a school in the village of Laneast and had a mathematical background.

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One early indication of Adams’s relationship with his teacher and of his character was the ‘Stream incident’. Apparently, the children were bathing in the River Inny at Laneast Bridge one lunchtime and failed to notice the time. Mr. Sleep appeared, cane in hand to chase them back to school. Although Adams could easily have out run the teacher he stopped to “tie his shoe” to allow the others to escape. Adams got on well with Mr. Sleep, they had a love of maths in common, although there were few mathematics books at the school, Adams moved steadily ahead of his teacher, becoming self-taught. (Adams, 1892:13). To achieve this, he used his family’s books and his brother George Adams reminisced about Adams’s love of one particular book saying, “He was constantly at his books, and in his early years Old Moore’s Almanack was a source of great delight.” (Adams, 1892). His Aunt Grace Couch inherited a small estate, Badharlick, but she did not reside there, instead moving in with the family bringing much needed financial support from its rent. She also brought a library containing some books on mathematics and astronomy. These books had originally been owned by Grace’s husband, John Couch, and it is evident that it is from this branch of the family that the interest in science originated. Another book which would have been a firm favourite was the Bible. Adams was brought up in a deeply religious family. The whole family worshipped at the local Anglican Parish Church in Laneast and also at the Wesleyan Chapel. His father was a leading member for the local Methodist Society at Tregeare but was also a parish warden at the local church. (Ibid, 1892:12). This dual faith was not unusual in Cornwall, where Methodism had taken a firm hold since John Wesley’s visits in the 1750s. One of the core values of Methodism is education and Adams would have certainly been encouraged to attain his full potential through Methodist preaching. His Anglican faith allowed him admittance into Cambridge, where dissenters were not accepted as fellows until 1856. (Vernon, 2004:14). The family’s religious habits ingrained from an early age, helped equip Adams for life at Cambridge University and its routines surrounding chapel attendance. By the age of ten Adam’s abilities in mathematics were starting to be recognised wider afield. During a visit to a family friend, John Pearse of Hatherleigh, he impressed a number of esteemed dinner guests with his talent. The following account of the visit was given by Pearse’s son also John Pearse, a banker and solicitor, who met Adams during the visit in the late 1820s. My father had some knowledge of Algebra and on questioning the boy considered him a prodigy. He [John Couch Adams] was invited to spend a few days at Hatherleigh and we three boys walked there together by the short way through the fields. My father invited them to spend an evening at his house, Mr. W C Morris was a neighbouring squire who had received a university education, his friend Mr. Fisher the surgeon and a schoolmaster named Jay, who had taken on Mr. Roberts’ school. Before this triumvirate my father trotted out the wonderful boy in Algebra. The boy and the schoolmaster did not arrive at the same result. The schoolmaster’s work was set before the boy who showed that he was wrong, and where the error was but all in quiet and modest way. The examination of the young mathematician concluded the gentlemen got into an argument on some subject. My brother proposed some amusement for his guest but he was taken up with the argument and said “Stop, let us hear them come to the point.” (Pearse, c1860)

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One sum that was solved on the evening was: What sum of money when divided by 2 the numbers are reversed. The correct answer is £13. 6/- (13/2- = £6 with 20/over). Therefore 26/- divided by 2 = 13. So half £13.6/- is £6. 13/- ! apparently the teacher got it wrong but Adams solved it quickly. (Sheen, 2013) Even at this early stage in Adams’s life it seems that his father realised his destiny did not lay in farming, as during the visit to Pearse’s “Mr Adams told my father that he feared his son would never be fit for a farmer, he was too fond of his books and that he was studying Algebra.” (Pearse, c1860). Pearse agreed saying “If he was my boy, I would sell my hat off my head rather than not send him to college.” (Adams, 1892:15). Adams had really impressed Pearse during his short stay not only with his unusual maths abilities but also with his inquisitive nature and as the following extract shows was to leave with the gift of more precious scientific books to add to his well-thumbed collection. (Pearse, c1860) At my father’s house John Couch Adams saw a pair of globes for the first time in his life and while he was there these globes and a book on the use of the globes were his favourite amusement. On his leaving, my father presented him with a book on the use of the globes, two volumes on “the pursuit of knowledge under difficulties” then just published by the Useful Knowledge Society and some other books all of which Professor Adams recently told my father he still retains.

When the time came for secondary school it was decided that rather than send him to the local Horwell Endowed Boys school in Launceston he would attend his cousin’s school in Plymouth. The Rev. John Couch Grylls (c1793–1854) ran his establishment at Devonport and this meant that Adams had to lodge away from home for extended periods. During this time, he wrote numerous letters to his parents and his siblings and it was within these that we can glean a sense of how he spent his time. One place he was particularly attracted to visiting was the Mechanics Institute and its extensive library. The Mechanics Institute in Devonport was set up in 1825. During the 1820s a minority of children attended school full time and many only to learn the basics in reading and arithmetic, the ethos of the countrywide Mechanics Institute was to provide technical and advanced education to a wide section of the population. The Devonport branch had a membership in excess of 1,000 and members benefited from the large library and a reading room with all the local and national papers. Science lectures were presented and later arts classes were started. (Moseley, B. 2016). It was a breeding ground for scientific and intellectual thought. Clearly such an Institute must have acted like a magnet to the young Adams and studying there would have been a good precursor to Cambridge life (Fig. 3.3).

3.3  Adams the Young Astronomer In common with Adams’s maths interest his early interest in astronomy was sustained by the small collection of family books. These 18th century books became Adams’s early companions. It was certainly unusual for one so young to be able to

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Fig. 3.3  Example of mathematical work undertaken while at School in Devonport. (Credit: Papers of John Couch Adams, Adams Box, 23/16/7. By permission of the Master and Fellows of St John’s College, Cambridge)

read such volumes, often in Latin. One book in the collection was James Ferguson’s (1710–1776) Astronomy explained upon Sir Isaac Newton’s principles, which had been a firm favourite of William Herschel At times the collection was added to, such as in 1834 when he won as a school prize a copy of John Herschel’s Astronomy. This copy is now believed to be the one on display in the St Lawrence House Museum, Launceston.

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As well as books it seems that Adams had an interest in the more practical side of astronomy. Although his father could not understand why he chose to remain out so long, Adams would spend many hours of his time on the moor, learning the various constellations and watching the planets moving amongst them. It is believed that he would lean against the old Cornish Cross that still overlooks the farm. This Cross is not made from local granite moor stone but from Polyphant stone (a type of Greenstone) which is distinctive in colour and been utilised for different artefacts since the earliest of times (Fig. 3.4). As his observational knowledge of the stars grew, he was able at the age of thirteen in 1833 to draw an excellent map of the Solar System. He drew an outer ring of symbols representing the constellations of the plane of the ecliptic (signs of the zodiac) and numbered the days in the month. The outermost planet had two names at that time, Adams labelled the planet “Herschel” after discoverer William Herschel instead of the name “Georgium Sidus” which was being used at that time in the UK. Part of the drawing illustrates the inclination of the planets to the plane of the ecliptic, including asteroids such as Pallas, Ceres and Vesta (Fig. 3.5). His practical skills were also prevalent when he made a simple marine astrolabe. While continuing to experiment with this he also made lines or notches on the windowsill at Lidcott to mark the altitude of the Sun at noon (Clerke, 1901). Unfortunately, the notches cannot be identified today as the sill was removed when the farmhouse was extended.

Fig. 3.4  The Cornish Cross on the moor which Adams would lean against to watch the stars (Credit: Carolyn Kennett)

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Fig. 3.5  John Couch Adams’ diagram of the Solar System drawn when he was 13 years old (Credit: John Couch Adams, 30 January 1833, Papers of John Couch Adams, Adams/Box, 23/2/1-6. By permission of the Master and Fellows of St John’s College, Cambridge)

During the autumn of 1835 while residing in Devonport, Adams was on the lookout for Halley’s comet. This famous comet was spotted during its 19th century apparition in August from Rome. He wrote to his parents on the 17 Oct 1835 about this exciting astronomical once-in-a-lifetime event saying “No doubt you have long before this, discovered the comet. We stayed up a night about 3 weeks ago, in order to discover it, but in vain, and lately neglecting to watch for it, we did not see it till Wednesday night last, when and last night it appeared very plainly.” (JCA to Parents, 17 Oct 1835: AM). He continued to deliberate on the scientific value of the comet’s passing saying “You may conceive with what pleasure I viewed this, the first comet I had ever had a sight, which at its visit 380 years ago threw all Europe into consternation, but which now, affords the highest pleasure to astronomers by proving the accuracy of their calculations and predictions.” (Ibid, 1835). His observations the following year offered another inspiring observational opportunity on 15 May 1836 when there was to be an annular eclipse and his excitement about the forthcoming event is evident in the letter he wrote to his brother Thomas, on the 13 May. (JCA to Thomas, 13 May 1836: AM). I now take my pen to give you a brief description of the large eclipse of the Sun which will take place next Sunday. Eclipses of the Sun, you are aware, only happen at the time of New Moon or at the conjunction of these luminaries and are caused by the Moon being interposed between us and the Sun, and therefore cutting off a part or whole of its light. The

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Adams understood not only the fact that this was annular although almost total at 95% coverage and that different parts of the UK would have a different view. He had possibly read about these ideas in either of his favoured books by James Ferguson and John Herschel. (JCA to Thomas, 13 May 1836: AM). To proceed to that of Sunday, the Sun and the Moon vary their distances from the Earth, and consequently, their apparent sizes vary also. At this time the diameter of the Sun exceeds that of the Moon, and consequently the Moon cannot cover the whole of the Sun’s disc and there will be no total eclipse. For different places, appearances vary, for the Moon being much nearer to us than the Sun, if it appears at one place to cover it, at another it may be so much out of the line. It is evident that to an observer at E, the Moon will appear directly on the Sun, but to one at L [Lidcott] it appears in a different direction and does not touch it at all. So at Edinburgh this eclipse will be annular, that is the Moon will be directly placed on the Sun. At London the eclipse will be large, but not annular, and at this place it will be rather smaller still.

He continues by calculating the time of the eclipse for his family at their home in Lidcott compared to that London. In doing this he demonstrated that he realised Cornwall was 5° west of Greenwich and that translated into the 20 minute time lag. Furthermore the “4 minutes later” means that he also understood the equation of time accurately. As the almanacs only give the time etc. to this eclipse for London and some other remarkable places I have taken some pains to calculate it and herewith send you what I believe has not been done for some time, a calculation of this eclipse for the meridian and latitude for Litcott. This differs about 20 minutes from the time at London, and also consequently the size of it. The following is the result of my computations. The eclipse begins 15th 1 hour 28 minutes after noon. Greatest eclipse 3 hours 0 minutes. Ends 4 hours 22 minutes. Digits eclipsed 10d. The following type (see drawing) will represent it very nearly. The shaded circle represents the Moon the lighter one the Sun, at the greatest eclipse, there will only be the light part visible, the eclipse begins at the place marked “begins” about 129.5 degrees from the vertical, top marked v of the Sun towards the west, and ends at about 21.5 degrees from the vertical towards the east, the whole circle being 360 degrees. The time I have given above is mean or clock time at Litcott, the time by the sundial will be 4 minutes later.

This is impressive as Adams was only 17 at the time and basically self-taught in astronomy. With a flourish, Adams signs off the letter with further astronomical instructions “next Thursday evening between 6 and 7 o’clock a remarkable conjunction of the Moon and the planets Jupiter and Venus, which I wish you would observe. These planets are now approaching each other and will then be very near each other also with the Moon.” (JCA to Thomas, 13 May 1836: AM) (Fig. 3.6). Scientifically significant, this type of eclipse features a flash of bright lights that surround the edge of the Moon just as it is covering the solar disc, due to mountains and valleys on the lunar surface which break up the sunlight emitting from the edge of the Sun. Unlike a total eclipse the Moon is at apogee and too small to completely cover the solar disc, so this effect only occurs at the leading and trailing edges of the Moon as it moves across the Sun. Francis Baily (1774–1844) vice-president of the

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Fig. 3.6  Adams’s letter to his brother Thomas on the 13 May 1836. Showing the diagram of the annular eclipse (Credit: Cornwall Records Office DD AM248)

Royal Astronomical Society had read about this “remarkable phenomenon” and decided to see it for himself. He calculated that the best place would be Jedburgh in Scotland as this was in the area of maximum coverage of the Moon over the Sun, at 95%, and he set up his observatory there. His description of a “string of beads” resonated with astronomers and henceforth the term Baily’s beads entered the eclipse vocabulary. (Baily, 1836:15). The following year Adams had the chance to observe an impressive lunar eclipse on 20 April 1837. Totality was in the evening and lasted one-hour forty minutes. As an evening event it was perfect to catch a large number of observers. Adams was requested by Mr. Bate, a reporter for the Devonport Telegraph, to write an article for the paper. Titled “The Grand Lunar Eclipse”, it appeared in the 29 April edition and Mr. Richards, the Editor of the Devonport Telegraph told Adams that his article had been copied into several London papers. (JCA to Parents, 24 Apr 1837a: AM). By this time Adams was starting to complain that his eyes were not good enough to observe certain events. He wrote to his father asking if some eye water could be brought to him (JCA to Parents, 3 Jun 1837b: AM). During the following week, on 25 April, he watched the planet Mars very near the Moon as it was about to be occulted. He worried that his eyesight had let him down and that he had missed observing the event. (JCA to Parents, 24 Apr 1837a: AM). It is interesting to note that his concerns were possibly unfounded as the Moon, five days after full was so

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bright that no one could see an occultation without using magnification such as binoculars. He was fully aware that his eyesight was not as good as others around him and when Adams was appointed Director of the Cambridge Observatory it was on the understanding that he did not have to do any actual observing. During his trip to observe the Norway Eclipse when in 1851 he downplayed his observational abilities stating, “Not that I had much hope of being able to add anything of scientific importance to the accounts of the many experienced astronomers who were preparing to observe it.” (Adams, 1851:101). His vision did not stop his interest in the practical side of astronomy and he continued to observe throughout his life. What was clear is that he had made a decision his career would not be in observational astronomy, but would lie instead within the realms of mathematics, at which he excelled.

3.4  Preparations for University It had long been the family ambition that Adams would attend University and the family started to make inquiries about Adams attending Cambridge University. While in Devonport his uncle Mr. Grylls approached an acquaintance, Mr. Jarrent, to talk about Adams’s options. Adams wrote home to his parents saying “He does not recommend me to enter as a Sizar but thinks it would be better for me to go as a Pensioner, to one of the small colleges. It need not exceed 60 pounds a year, and if I could obtain a scholarship, as he himself did the first year he was there, the expense would be materially lessened.” (JCA to Parents, 28 Oct 1937:AM). Like many families today, they were starting to weigh up the cost and benefits of sending a child into higher education. Soon after this advice was given in early 1838 Adams returned home, as the Grylls were emigrating to Australia. Once home the family’s landlord Mr. John King Lethbridge (1789–1861) introduced Adams to the curate of Lamerton, Tavistock, George Martyn (1815–1882) [sometimes Martin], who had recently attended Cambridge. Adams was tutored and encouraged by him to prepare for entrance examinations at St John’s College, Cambridge. (Adams, 1892:18). Private tuition from Martyn would not have been cheap and the family were already starting to incur costs related to his further education. It cannot be overstated how instrumental Martyn’s coaching was in preparing Adams for the entrance examinations at Cambridge. A contemporary, future friend and 1840s Cambridge entrant George G. Stokes (1819–1903) was fortunate to receive his pre-Cambridge preparatory education in a formal (and expensive) situation at Bristol College, under the oversee of Principal Jospeh Henry Jerrard (1801–1853), who had previously been a Cambridge graduate. Stokes was expected to learn “differential and integral calculus, statics, dynamics, hydrostatics, optics, conic sections, spherical trigonometry, physical astronomy and the first book of Newton’s Principia.” (Warwick, 2003:87). Stokes spent two years being educated within these topics in the build-up to the critical Cambridge University entrance examinations. Another contemporary of Adams and Cambridge

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entrant, James Joseph Sylvester (1814–1897), was also educated in a formal setting at the Royal Institution School in Liverpool. The school would emulate the curriculum at Cambridge preparing the students for entry into either of the Oxbridge universities. Sylvester’s tutor Thomas Williamson Peile (1806–1882) was an excellent classicist as well achieving 18th wrangler in mathematics when he had sat the Tripos at Cambridge. The students spent five hours in mathematical lessons, while English, geography and history were each taught for one hour a week, while the rest of the full timetable was spent learning the classics. (Parshall, 2006:22). This is the teaching role that Martyn undertook with Adams from early 1838. In comparison to Stokes’s and Sylvester’s full time and more lavish education, Adams was at a disadvantage. That he still went on to excel speaks volumes about his mathematical talents. Adams received Martyn’s tutorship for an extended period as he was residing at home from early 1838 until he left for Cambridge in the autumn of 1839. Going to University was a very expensive business and Adams’s costs absorbed every penny that could be earned by the whole family. (The costs at University will be explored in more detail within Chap. 4, but were initially £35 a term, or half a year’s wages for a skilled tradesman, taken from the UK’s National Archives currency converter). As already stated, his father Thomas was not a well man and he spent a considerable period each year nursing his illness, which at times meant limited resources would be created from the farm itself. The finances needed had to be found from other avenues and Adams had to limit his outgoings by returning home to Cornwall only during the long vacations. By his final year the family had to resort to borrowing money to fund his education. (Adams, 1892:19). Life as a tenant farmer in north Cornwall in the early part of the 19th century was very hard, because of the heavy labour and low prices of produce. However, the Adams family were able to derive additional income from the Badharlick Estate, Badharlick had been bequeathed to Grace Grylls (Adams’s great-aunt) in 1790 after the death of her companion Ann Trewbody. Adams’s mother Tabitha had spent her formative years living with her aunt Grace until her marriage in 1818 to Thomas. When Grace Grylls’s husband died, she went to live at Lidcott, with Adams’s family until her own death in 1836 at the age of 90. By her Will of that year (1836) she left Badharlick to Tabitha and after her to her eldest son John Couch Adams. Consisting of about 100 acres of good pasture, the rent was insignificant as it only amounted to an annuity of £5 while a family relative John Grylls (Grace’s nephew and John Couch Adams’s uncle) was the tenant. (Will of Grace Couch, 26 Jun 1820:AM). Nevertheless, it was in part sufficient to ensure that Adams went to Cambridge but in turn this meant that the other members of the family had to give up the benefits of this additional income and there would be “great difficulty from the resources of the farm to pay for his college bills.” (Adams, 1892:18). Another revenue steam which went towards meeting the costs of sending Adams to Cambridge was that of the sale of manganese ore extracted from a small mine located on Lidcott Farm. Although the standard story as told implies that manganese was found at Lidcott just in time to provide an income for Adams, the truth is that it had been worked intermittently and a good while before Adams needed it. The mine was certainly active for an unknown period prior to 1827 when the lease was

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formerly granted to Thomas Adams’s landlord Lethbridge for a cost of 12/− per ton (English, 1826). What is of interest is that Lethbridge acted like a progressive landlord in that he allowed the Adams family to make profit from the mine in what was essentially on his land. A tenant’s right to farm the surface of the land did not extend to the right to extract minerals from below ground. The mineral rights belonged to the ‘lord of the manor’ and so of course did the profits from the sale of the minerals. At times the mine was not as profitable as one may expect and the local geologist Henry S. Boase (1799–1883) who observed it noted in 1830 that “Laneast Downs is an interesting area to both the geologist and the miner, to the later principally on account of its numerous deposits of manganese ore.” Many of these he wrote, “have been very productive but as far as I could learn the remuneration to the adventurers must be inconsiderable in consequence of the low price of this article and the heavy expense of land carriage over a bad and hilly road.” (Boase, 1832:233). The mine would certainly not be making the family rich (Fig. 3.7). Finally, Adams would have to take on some of the responsibility for the expense of university himself and he did this through teaching. Naturally he was able to charge for his lessons and thus it helped to defray his university expenses. He was a respected teacher who had initially taught his younger siblings and later on the children of his neighbours. During his time in Devonport he also taught the two sons of Admiral Hornby. (Adams, 1892:16). This experience of meeting people of different social levels would have helped Adams with his future integration at Cambridge

Fig. 3.7  The opening to the manganese mine at Lidcott Farm, Laneast Downs, Cornwall, where Adams’s family would extract the resource to help fund his university education (Credit: Carolyn Kennett)

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University. One particular family, the Foot family, were so impressed that they tried to persuade him to emigrate to Australia with them as a private tutor for their children, but by this time he had met Martyn and the decision had been made for him to go and sit the scholarship examination at St John’s College. (Adams, 1892:19). At that time St John’s was considered a “stronghold of mathematical learning,” (Parshall 2006:29), with Trinity being the better college to attend for learning the classics. From 1824 to 1845 St John’s could boast nineteen senior or second wranglers in mathematics, compared to Trinity’s twelve. (Ibid, 2013:29). For the most ambitious mathematical students St John’s was the college to attend. With Martyn’s expertise and encouragement Adams was able to dream about the heights of academic success that he could achieve. It is very likely that he started to consider reaching the highest honours attainable, that of Senior Wrangler. Adams never lost the sense of duty to his family. The appreciation of the monetary support he had received from them must have driven him on to succeed during the coming years, as he felt the need to repay his family. In part his Methodist upbringing helped in his careful management of money, as Methodists are taught to spend money on the necessities in life and not on superfluities. In 1843 he wrote to his brother Thomas, I beg you will listen to what I am going to propose. I feel I should ill fulfil the purposes for which God has raised me to my present position if I did not endeavour to extend to the rest of my family some portion of the advantages which have been so beautifully bestowed on myself. I am anxious therefore, in the first place to give you and George an opportunity for study and improvement which you have furtherto been without…. (JCA to brother Thomas, 11 May 1843:AM).

He would pay for their lodgings and education at a school in Devonport. The financial support Adams gave meant he was able to help his brothers succeed within their field of choice. William Grylls Adams (1836–1915) became an 11th Wrangler and the professor of Natural Philosophy at Kings College, London and an FRS, Thomas a much travelled missionary and official for the Wesleyan-Methodist Missionary Society, while George became a farmer. When Adams’s father Thomas died in 1859, Tabitha gave up the tenancy of Lidcott and moved back to Badharlick, only a short distance away, and stayed there until her death in 1866. Adams had saved enough money to refurbish the house throughout before his mother moved back in. On her death the Badharlick Estate was passed to his brother William Grylls Adams, with Adams’s good wishes, who had been originally in line to inherit it. By the time of his death Adams had accrued a small fortune, which amounted to £32,433 12s. 8d, probate, 2 March 1892. (Hutchins, 2004). This amounts to over 2.6 million pounds in today’s money. During October 1839 Adams set out on his first journey to St Johns College. He had never travelled past Exeter (80km away) and his whole family “went with him to see him on the coach, in the middle of the night in Laneast Downs,” (Adams, 1892:18) where a number of prayers were offered for his safe travels. The journey on the coach called The Traveller was slow, long and very tiresome, taking two days and a whole night before reaching Cambridge. (Ibid, 1892:18). No doubt Adams’s years in Devonport, away from home, helped him cope with the distance from his

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family and he was already used to living apart from family members, so this move would not have been a complete shock. His frequent contact with home through letters meant he did not suffer any ongoing homesickness. He became involved with college life right from the start and his transition from Cornwall to Cambridge life was smooth. Buoyed by a lifetime of praise over his mathematical abilities, from both preceptors and contemporaries alike, his confidence in his own abilities was high and on arrival he sat his scholarship examination in which he excelled. This is a pattern which would continue during his time at Cambridge University, where his determination and competitive spirit would see him reach for the stars. Acknowledgements  The authors would like to thank the owners of Lidcott Farm for their support and access to the manganese mine on Laneast Downs. Descendant Norma Foster has been very generous with her time and discussions over her family knowledge of the Couch Adamses. The authors also thank Dr. Maarten Roos of Lightcurve Films for a still from his online film “Searching for Neptune”, the Curator and Staff of St Lawrence House Museum, the Librarians past and present of the Royal Astronomical Society (Peter Hingley and Sian Prosser), the Librarians of St John’s College, (Kathryn Mckee, Adam Crothers and Fiona Colbert) and Mark Hurn Librarian of the Institute of Astronomy at Cambridge. The author, Brian Sheen appeciated the conversations he had with Les Sleep, the Laneast descendent of Richard Sleep, John Couch Adams’s first teacher. Finally, many thanks go to all those who have supported us in so many ways over many years; Richard Baum, Allan Chapman, Cliff Cunningham, Roger Hutchins, Davor Krajnović, James Lequeux, William Sheehan, Robert W. Smith, Trudy E. Bell, Kenneth Young, and Craig B. Waff.

References Correspondence to or from John Couch Adams is from – AM: Adams family papers, Cornwall Record Office, Kresen Kernow, Redruth. Adams, J.C., 1851. On the Total Eclipse of the Sun, 28 July 1851, as seen at Frederiksvaern. Memoirs of the Royal Astronomical Society. Vol. xxi. (1852). Adams, G., 1892. ‘Reminiscences of our Family’ reprinted in H. M. Harrison, Voyager in Time and Space: The Life of John Couch Adams, Cambridge Astronomer. (Lewes, Sussex: The book Guild, 1994). Baily, F., 1836. On a remarkable phenomenon that occurs in total and annular eclipses of the Sun. Monthly Notices of the Royal Astronomical Society, Volume 4, Issue 2, 9 December 1836, Pages 15–19. Boase, H. S., 1832. Contributions towards a knowledge of the geology of Cornwall. Transactions of the Royal Geological Society Cornwall, 1832, vol. 4, p. 233. Clerke, A.M., 1901. John Couch Adams. Oxford Dictionary of National Biography. Oxford University Press. Callaway, E., 2015. UK mapped out by genetic ancestry, Nature. March 2015.17136. De Maurier, Daphne, (1936) The Jamaica Inn. Gollancz, UK. Duchy of Cornwall Act, (1844) UK Government Legislation, http://www.legislation.gov.uk/ukpga/ Vict/7-­8/65/contents (accessed 2019). English, H., 1826. A compendium of information relating to companies formed for working British mines. London.

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Harrison, H.M., 1994. Voyager in Time and Space. The Life of John Couch Adams, Cambridge Astronomer. The Book Guild, Sussex, England. p. 11. Hutchins, R., 2004. John Couch Adams. Oxford Dictionary of National Biography. Oxford University Press. Kennett, C., 2017. An 18th-century astronomical hub in west Cornwall, The Antiquarian Astronomer, Issue 11, June 2017. Kennett, C., 2015. An astronomer’s summer outing to the Lizard, Cornwall, 1769, The Antiquarian Astronomer, issue 9, July 2015. Lease of Badharlick Estate 19 Aug 1889. Conditions for leasing on a yearly tenancy. Cornwall Records Office catalogue number DD AM44. Moseley, B., (2016) Old Devonport: Devonport Mechanics Institute. http://www.olddevonport.uk/ Mechanics%20Institute.htm (accessed 2019). Naylor, S., (2002) The field, the museum and the lecture hall: the spaces of natural history in Victorian Cornwall. Transactions of the Institute of British Geographers NS 27 494–513 2002. Robbins, A., 1883. Launceston Past and Present. Published by Walter Wershell. Sheen, B., 2013. Neptune: towards understanding how Adams’s upbringing enabled him to predict, then miss the discovery of the century. The Antiquarian Astronomer. Issue 7, p. 71–81. Parshall, K.  H., 2006. James Joseph Sylvester: Life and Work in Letters. Johns Hopkins University Press. Pearse, J., c1860. reprinted from the recollections of the Pearse’s and the Pethybridge’s of Devon and Cornwall in the 19th Century. Unpublished and in Sheen, B. private collection. With the kind permission of the authors Cecilia Jevons and Christopher Knight. Vernon, K., 2004. Universities and the State in England, 1850–1939. Woburn Education Series. pp. 14–15. Warwick, A., 2003. Masters of Theory: Cambridge and the Rise of Mathematical Physics. University of Chicago Press, 2nd Ed, p 87. Will of Grace Couch, 26 Jun 1820. Cornwall Records Office catalogue number DD AM180.

Chapter 4

John Couch Adams: From Senior Wrangler to the Quest for an Unknown Planet William Sheehan

Abstract  As Eugène Bouvard’s attempts stalled to successfully revise his uncle’s Tables of Uranus, a young British mathematician, John Couch Adams, became interested in the problem of Uranus. As an undergraduate at St. John’s College, Cambridge, Adams achieved enormous academic success, including becoming Senior Wrangler and First Smith’s Prizeman. Though he was kept extremely busy with his college work, and could not afford any significant distractions, in June 1841, he became intrigued by the problem of Uranus’s wayward motions, and interested in finding out to what extent the hypothesis of an exterior planet could furnish a solution. The Cambridge mathematics curriculum helped him to develop the world-class expertise he needed to tackle this high-level problem in analysis. Adams’s success in the Mathematical Tripos exam led to his becoming a Fellow of St. John’s in 1844. His duties as a Fellow included lecturing and tutoring, but mainly during the extended vacations, he did take up the Uranus problem, and in October 1845 he attempted to present his famous Hyp I solution to George Biddell Airy personally during ill-fated visits to the Royal Observatory, Greenwich. He failed to meet Airy, and only left a paper summarising his results (including a position for the planet). Contrary to statements in prior historical writings, Airy did not lack interest and—despite being fiendishly busy and distracted at the time—followed up with a query to Adams about the radius vector, which was his own particular hobbyhorse. Despite beginning a new term, and being once more swamped with lectures and tutoring. New research reveals Adams did attempt to respond to Airy. However, the letter was never sent, and the trail went cold. There followed a hiatus until June 1846 when, as described in a later chapter, Urbain Jean Joseph Le Verrier’s independent calculation was published, setting off a race to find the mysterious planet….

W. Sheehan () Independent Scholar, Flagstaff, AZ, USA © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_4

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4.1  A New Retelling of an Oft-Told Tale As one of the best-known stories in the history of astronomy, the pursuit and eventual discovery of Neptune is so familiar that on encountering it again readers are tempted to begin to skim, believing that everything is known that can possibly be known. Many of these retellings are focused on the priority issue: was John Couch Adams, who first computed a position for the planet but did not formally publish it in the autumn of 1845, entitled to the same credit as Urbain Jean Joseph Le Verrier, who published the position that Johann Galle and Heinrich d’Arrest (1822–1875) used to detect the planet on 23 September 1846? Were they co-discoverers? What, in any case, constitutes a discovery? And were the predictions even valid—or merely, as was claimed by some at the time—a “happy accident?” (Fig. 4.1) These are not unimportant questions, and have their place. However, none of them need greatly exercise us here. Nor do we intend to devote any time to assigning credit or blame. Instead the goal is to try to tell what actually happened, and we begin by trying to dispel some of the unconscious pictures that readers are apt to Fig. 4.1  John Couch Adams. A portrait taken at about the time of the discovery of Neptune (Credit: Royal Astronomical Society Library, MSS Add. 94, item 24.)

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bring to the story. Among them is that of a rather relaxed and idyllic academic setting, with walks across fields or along the river Cam, among the well-manicured lawns of quads, and beneath the noble and lofty structures of colleges and spires. In such a setting, one imagines Adams, a gowned figure, perhaps somewhat absentminded, immersed in calculations as he tries to solve what, in hindsight, seems one of the knottiest and most important astronomical problems of the time. Its successful unraveling would be called the “Zenith of Newtonian mechanics.” (Hanson, 1962:359). To continue this rather dreamy thought experiment, one imagines Adams, head bowed in deep lucubrations, headed up Madingley Hill Road to visit his good friend the Rev. James Challis, the Plumian professor of astronomy and director of the Cambridge University Observatory, perhaps for a spot of afternoon tea, as they immerse themselves in details of the Uranus calculations. When, in the autumn of 1845, Adams presents Challis with a calculation containing a position for the planet within a couple of degrees of its actual position, Challis recommends that Adams visit his good friend George Biddell Airy, the Astronomer Royal at the Royal Observatory, Greenwich. Adams, full of youthful idealism and confident of his calculations, and buoyed with his status as Senior Wrangler (the candidate with the highest score on the Cambridge Mathematical Tripos examination), hastens off, and attempts once on the way down to Cornwall (his home) and twice on the way back to meet up with Airy. The first time Airy is simply not at home; the second time Adams finds Airy on a walk, and then at dinner. He is turned away, and leaves feeling rebuffed. On departing, he leaves a brief summary of his results, but instead of immediately turning a telescope to the sky and searching round the position indicated, as anyone but an arrogant and petulant fool would have done, Airy follows up with a pedantic question that Adams feels is too trivial to answer. Thus, the British squander a lead of seven months in searching for the planet, and do not realise the value of what has fallen into their hands until June 1846, when the French astronomer Urbain Jean Joseph Le Verrier publishes a position close to the one Adams had worked out. Challis is now commissioned to search the indicated area of the sky, but proves to be an inveterate bumbler who fails to compare the positions of the stars he maps, and so misses the discovery. It falls instead to Galle and d’Arrest in Berlin, who find it after an hour of searching. Only then does a red-faced Challis confess to his uninspired and humdrum search, while Airy’s arrogant want of faith in a brilliant, young, but shy mathematician, who had only needed a bit of encouragement to have succeeded, is seen as almost villainous. So ensues a row that comes close to becoming an international incident, as the great knights of British and French astronomy joust to achieve a share of honour for their respective national causes. The question of priority has been a major preoccupation of historians ever since. It is an important question, but it needs to be considered in the contemporary context of the discovery, as addressed in Chap. 7. This, rather in caricature, is the oft-repeated story, and being a caricature itself, inevitably leads to caricatures of the principal characters. The caricatures are easy to ridicule. For instance, Isaac Asimov (1979:141) in an essay written for The Magazine of Fantasy and Science Fiction, described the principals as “Nice Guy

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Adams and Nasty Guys Challis and Airy.” Though Challis’s sin was mainly one of omission, Asimov’s accusation against Airy runs to active malevolence: Airy was a conceited, envious, small-minded person who ran the Greenwich Observatory like a petty tyrant. He was obsessed with detail and invariably missed the big picture…. About the only real success Airy had was personal. He was the first to design eyeglass lenses that corrected astigmatism. He was himself astigmatic, you see—literally, as well as figuratively. It was this nasty person that Adams tried to contact…. (Asimov, 1979:147).

If one did not know what Airy looked like, one might well imagine him as a vaudeville villain, with turned-up moustaches, black hat, and cape. However, the caricatures, easy enough to spot here, were not simply conjured out of thin air by Asimov. Most of them were already present in the post-discovery controversies when Adams’s calculations came to light and Airy and Challis’s actions or inactions had to be justified. A few months after the discovery, the Rev. Adam Sedgwick (1785–1873) of Trinity, a close friend of Airy, was heard to refer to Airy and Challis in a common room discussion as “narcotic souls” (Clark and Hughes, 1890:Vol. 2, 288), emphasising what, in retrospect, had seemed a lack of initiative and urgency that cost them a discovery that seemed as if it had almost been begging to be made. (We will say more about all this in Chap. 7.) Because emotions ran so high at the time, and so many things were said in heat (and therefore were vivid and unforgettable), the general story has continued to be repeated ever since— even, for instance, by Morton Grosser in his highly acclaimed and influential book, The Discovery of Neptune (Grosser, 1962), which settled the outlines of the story for a generation. Adams was the bashful scholar whose genius was not recognised by his superiors; Airy the rigid bureaucrat obsessed with order and protocol; Challis a dreamy well-intentioned bumbler. All of these may have a grain of truth; but as a whole, they are not helpful. Allan Chapman is more on point when he wrote (1988:135), “it is futile to speculate what might have happened had certain persons acted differently.” History attempts to get at what actually happened, rather than what might have happened had they happened otherwise. So, we tread forward cautiously, taking inspiration from more nuanced historians such as Chapman determined to avoid acting as judge and jury in casting credit or blame (and who himself has done so much to rehabilitate the reputation of the much-maligned Airy). Instead, in the words of Angus Macintyre (1935–1994), fellow and tutor in Modern History at Magdalen College, Oxford (personal communication, Roger Hutchins), “The historian’s task is to seek an empathy with the actors of the past. We do so by diligently finding all surviving evidence, then immersing ourselves in their milieu, their circumstances.” And so with Adams, Challis and Airy, we attempt to “immerse ourselves in their milieu, their circumstances.” We may safely leave judgement to others. What emerges is the many demands on their time and attention, the extent to which they were all working up to their absolute limits, and the relatively small amount of time they devoted to the Uranus problem before the summer of 1846. Thus the obstacles to their collaboration are defined instead of the open road that appears only with the benefit of hindsight.

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4.2  Adams Enters Cambridge Young John Couch Adams’s Cornish background has already been considered in detail in Chap. 3, where it is made clear that, whatever Adams’s native intelligence, it was only his good fortune in being supplied from financial supports thanks to the manganese lode found at Lidcot farm, the landlord’s generosity in sharing some of the income from this resource with the Adams family, and the fact that, in addition to whatever he was able to learn at the Mechanics Institute at Devenport, Adams was fortunate to receive expert preparation for the Cambridge exams from a local curate and recent graduate of St. John’s, the Rev. Martyn, as described in Chap. 3. Finally, he had been raised in a religious tradition that, despite Wesleyan tendencies, did not impose upon his conscience any difficulty in subscribing to Cambridge’s religious tests (the 39 articles of the Anglican faith). (Non-Anglicans could attend Cambridge, they just could not get their degree). It was the confluence of all these things that gave Adams the opportunity to escape from being numbered among the “many flowers born to blush unseen” of Thomas Grey’s famous elegy (Fig. 4.2). The journey from Cornwall to Cambridge would have been a tiring one. “His family went with him to see him on the coach, in the middle of the night on Laneast Down. He had never gone as far as Exeter, and was a young traveler. To make sure that he should go on right he booked the through journey, and was sent on by a slow coach ‘The Traveller’. It took him two days and a night or more to do the journey.” (Harrison, 1994:18). Perhaps his first impressions of the place were similar to those of William Wordsworth (1770–1850), the poet who a half century before had entered St. John’s with the same “thoughts full of hope.” … up and down I roved; Questions, directions, warnings and advice, Flowed in upon me, from all sides; fresh day

Fig. 4.2  St. John’s College viewed from the back. New Court is to the left (Credit: Wikipedia commons)

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Of pride and pleasure! … I was the Dreamer, they the Dream; I roamed Delighted through the motley spectacle; Gowns grave, or gaudy, doctors, students, streets, Courts, cloisters, flocks of churches, gateways, towers… The Evangelist St. John my patron was: Three Gothic courts are his, and in the first Was my abiding-place, a nook obscure… (Wordsworth, 1850:III, 22–25, 30–34, 46–48)

Young Adams would certainly have taken at least as much notice as Wordsworth of the associations with his hero Newton, who had been undergraduate, fellow, then Lucasian professor of mathematics at Trinity College next door, and had written the immortal Principia in his rooms next to the Great Gate: Near me hung Trinity’s loquacious clock, Who never let the quarters, night or day, Slip by him unproclaimed, and told the hour Twice over with a male and female voice. Her pealing organ was my neighbour too; And from my pillow, looking forth by light Of moon or favouring stars, I could behold The antechapel where the statue stood Of Newton with his prism and silent face, The marble index of a mind for ever Voyaging through strange seas of Thought, alone. (Wordsworth, 1850:III, 53–63)

4.3  Tale of the Tripos The educational curriculum at the university of Newton had always been heavily mathematicised, as might be expected given the high status accorded to its greatest son. Mathematics provided, in the view of the pedagogues of Cambridge (Wilson, 1985:13), “an intense grounding in elementary and advanced mathematics designed to train minds to think clearly and rationally when encountering subjects more cluttered than mathematics.” The more cluttered subjects included, for instance, “mathematised physical subjects, which came to be included in the curriculum as examples of subjects where clarity was possible outside pure mathematics.” By the time Adams arrived at Cambridge, moreover, the mathematics taught there had achieved a great deal of sophistication, and was far more advanced than what had been available in the eighteenth or early nineteenth centuries. (For details, see Appendix 4.1: A Brief History of Mathematics at Cambridge.) The emphasis on mathematics at Cambridge is attested by the fact that even those who intended to pursue careers in other fields were still required to absorb a great deal of mathematics in order to advance academicslly. For instance, no one could sit for the Classical Tripos (founded in the early 1820s) without first attaining honours in mathematics, which meant sitting for and performing well in the

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Mathematical Tripos. (An exemption was granted to the sons of peers, under statutes laid down during the reign of Elizabeth I and not finally revised until the later 1840s.) The “severe studies” of mathematics were seen as providing a “key to success in commercial enterprise, and in other spheres of intellectual activity, including the Church, law, and politics.” (Wilson, 1985:60–61). Rev. Adam Sedgwick (1834:10–12), in A Discourse on the studies of the University, argued passionately that “studies of this kind [mathematics] not merely contain their own intellectual reward, but give the mind a habit of abstraction, most difficult to acquire by ordinary means, and a power of concentration of inestimable value in the business of life,” and it accomplished all this by disciplining the mind by a study of “that symbolical language by which alone [the] laws [of nature] can be fully deciphered….” Indeed—and this is hardly surprising, perhaps, given how very few professional mathematical or science positions there were at the time—most of the honours graduates and even the great majority of Senior Wranglers went on to careers as educators, barristers, churchmen, or government officials, rather than entering upon paths as professionals in mathematics or mathematical science. Any continuation of mathematical interest following upon graduation from the University was generally amateur: If, after leaving their alma mater, alumni of Cambridge pursued mathematics at all, it was typically the pastime for a Sunday afternoon before the week’s professional activities took over. Scattered around the country were pockets of mathematical activity, but it was largely mathematics as a pastime. Some groups did not last, as when the “old clay and pewter days” of the Spitalfields Mathematical Society drew to a close in 1845 as their membership dwindled and a new industrial age beckoned. The Royal Astronomical Society (founded in 1820) which absorbed it, came closest to the interests of mathematicians until the London Mathematical Society was founded in 1865.… [I]n the 1840s a young group of men were emerging with real mathematical ambition. [These] were not content to conduct their mathematical activity in a casual manner or for private enjoyment. (Crilly, 2004:458)

Not everyone found the arguments in favour of such a heavy emphasis on a mathematics and classics curriculum convincing, based as it was in traditional attitudes and procedures and yoked to the Anglican religion so as to be almost sure to stifle originality and creative talent. Alfred Tennyson (1809–1892), the middle-class son of a Somersby vicar, who entered Trinity College in 1827, can hardly have been alone when he complained in a letter to his aunt, “The revelry of the place is so monotonous, the studies of the university so uninteresting.” (Deacon, 1985:16). The monotonous revelry was largely the province of those sons of the aristocracy for whom Cambridge was little more than a diversion, though the majority of aristocrats were likely simply to go to Oxford, where standards were more relaxed and there were more interesting diversions. At Cambridge’s sister university, logic—the favourite discipline of the Medieval Schoolmen —was enjoying a strong second wind among the Tractarians (the Oxford High Church movement). The leaders of this movement, including John Henry Newman (1801–1890) and Edward Bouverie Pusey (1800–1882), were strongly drawn to liturgical and devotional customs borrowing heavily from traditions before the English Reformation (and in contrast to the low church which was very strongly Evangelical, as a result of the revival led by

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John Wesley (1703–1791) (Chadwick, 1960). The old Aristotelian syllogisms were emphasised in place of serious mathematics, and for those making a go at mathematics, Euclid was still the standard mathematical text until nearly the end of the century. Though most of the gentlemen or casual students simply dropped out, a serious student at Cambridge without interest or ability in mathematics must have been a miserable creature indeed. For the student with strong aptitude and inclination to mathematics studies, however, it would have been virtual heaven. From the moment that the student arrived at college, through the almost three years of application to the “severe studies” of mathematics, the goal was ultimately to show successfully in the titanic examination, the Mathematical Tripos, given in the January after the undergraduate completed his third year. The details of the exam’s administration varied somewhat over the years, but at the time Adams was an undergraduate, it was a six-day affair, of which the first day was the Wednesday of the week preceding the first Monday of Lent Term. There were 33 hours of exams in all, of which 24 ½ hours were given over to answering questions from the prescribed textbooks (discussed in more detail below) and the remaining 8½ given over to problems. The exam was given in the Senate House—a beautiful building with a marble floor and ornamented ceiling that had been modeled on the plan of a Greek temple, even in the detail that as Greek temples had no chimneys, and as a stove or fire of any kind might disfigure the building, there was no artificial heat. Accordingly, it could get quite uncomfortably cold in January, and sometimes, it was said, the ink froze in the inkwells. As soon as the exams had been graded, printed copies of the results were enthusiastically tossed from the balcony of the Senate House, and the results read aloud. Those who attained top class marks on the exam and obtained honours were known as “Wranglers,” in an echo of the old Medieval days of disputation, with the man getting the highest marks being the “Senior Wrangler,” the next “Second Wrangler,” and so on. The lowest candidate to get honours obtained the “wooden spoon.” Those that failed or, more usually, did not attempt to sit the exam were referred to as “poll men.” (Gascoigne, 1984). Given that the attainment of mathematical honours was effectively “the only way that a student could show intellectual prowess” (Johnstone, 2016), the announcement of the results of the Mathematical Tripos was followed closely not only in Cambridge itself but across the country. It was a kind of intellectual Epsom or Ascot, with bets being placed on the leading contenders. For that matter, it really was a race: after three years of intense preparation, everything came down, at last, to a 33-hour sprint where questions had to be answered and problems solved against the clock. Though high Wrangler status opened doors, the Senior Wrangler was regarded as a virtual deity, someone deserving—if only the ancient poet still lived—Pindaric treatment. The prestige accruing to the position was immense, with the victors feted with torchlit processions and receiving pride of place in the University’s graduation ceremony. Achievement of Senior Wrangler status was long regarded as “the greatest intellectual achievement attainable in Britain” (Forfar, 1996:1), and deemed “synonymous with academic superiority” (Warwick, 2003:205). This was so much

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so that any graduate’s years in Cambridge were often remembered simply by who had been Senior Wrangler in a given year. Charles Astor Bristed (1820–1874), an American who attended Trinity at the same time Adams was at St. John’s (and he could only afford this lark, or perhaps we should say foray into investigative journalism, as a scion of the wealthy Astor family of New York City) recounted the minor celebrity that attached to the achievement of high marks on the Tripos at the time: The publicity given to College and University Honours … exceed anything of which we have any conception of in our [American] academical institutions…. When an acquaintance of mine, who was related to a member of the Cabinet, wished for a start in the diplomatic line, the statesman’s first advice to him was, “be sure to get a Wranglership.” As to the first men of the year, they are no end of celebrities for the time being. A small biography of the Senior Wrangler is usually published in some local paper near his native place, thence transferred to the Cambridge papers, and from them copied widely into other journals…. But the honor is far from the only point of the temptation. All these Examinations …. bring with them some solid testimonial in the shape of books, plate, or money—more generally the last, and sometimes to a very considerable amount. The two Triposes [i.e., Mathematical and Classical] do so indirectly, as they lead to Small-College Fellowships, and a Fellowship gives an income of from £200 to £400 [per annum]…. Indeed, it is a common saying and hardly an exaggerated one, that a poor student by taking a high degree supports not only himself, but his mother and sisters for life. (Bristed, 1852:146–147).

Commensurate with the prestige attached to success, the Tripos was, compared to present-day exams which are merely hard, a ferocity. Everyone understood that the entire future career of a man—at least an ambitious man, or one aspiring to academic advancement—would be determined by one’s performance on exams over a few freezing days in an unheated hall in January. Not surprisingly, there were many who simply “broke down”. Those who failed disastrously were the “shipwrecks”. Even those who, objectively speaking, had largely succeeded often suffered mental and emotional collapse at the end of the ordeal. (Bristed, 1852.) Sitting the Mathematical Tripos was more than enough for most men, but for those who took the highest honours, there were—immediately after the Tripos results were announced—the examinations for the Smith Prizes to be sat. These included problems in more advanced mathematics than the Tripos, were taken by those with the highest honours and which served to rectify or confirm the arrangement of the first three or four Wranglers. And for the really ambitious, in February those successful in achieving honours in mathematics were eligible to sit for the Classical Tripos, giving them the chance to add honours in classics to their name. (Bristed, 1852).

4.4  Adams Embarks on an Honourable Quest Though Adams is popularly imagined to have been rather dreamy, otherworldly, lost in his thoughts and perhaps given to absent-mindedness, as noted in Chap. 3, he very early received very focused and competent training, and learned to thrive while

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living away from home. Thus, when he arrived in Cambridge, he did so not waiting to be “discovered”, but already possessed of a clear sense of direction. He had extraordinary drive, was highly competitive and eager for honours, and hoped to achieve at the highest levels of excellence of which he was capable—both in mathematics and, apparently at least at first, in classics. One can infer from what he writes to his family members (especially his brother Thomas), that he adapted quickly to the “proper” routine needed for success, which included hanging out with the “steady” set and avoiding the “loose” set. Though he took up residence in Cambridge at the beginning of Michaelmas term in October 1839, he and other students were not formally accepted into their colleges until they had passed their initial entrance examinations. In Adams’s case, this was in mid-December, and the exams ran over a period of 3 days. In a letter to his brother Thomas, he confided his initial impressions of Cambridge, his routines, and his studies in preparation for the examinations: My dear Brother I received your letter this morning, and will make amends as much as possible, for my long neglect by writing immediately. I have very little excuse to offer except that our College examination is rapidly approaching and I am very heavily engaged in preparing for it … My health continues very good, and I think I shall be very comfortable here, though I must confess I don’t much admire the country round; it is very flat. There is scarcely a hill to be seen that deserves the name…. I will now proceed to tell you a little about what we have to attend to, and how we live at College. First, there are prayers in the College Chapel every morning at 7 and evening at 6 during the week, except Sundays when we go at 10 in the morning and a quarter past 6 in the evening. Every one is required to attend at least 9 times every week, twice on Sundays being counted as 3 times. Then there is the dinner in Hall at quarter before 5; at which we are all tolerably regular attendants, and manage to get on very well particularly on Feast days. We generally have some pie or pudding after the meat, and for drink we have either beer which is not very strong, or water, so that you see we are pretty nearly teetotallers. We have also to attend to Lectures every day. On Tuesdays, Thursdays and Saturdays at 8 o’clock in the morning we have Lectures on Thucydides the Greek author wh[ich] we are now reading. The Lecturer who is one of the Tutors of the College, calls on whoever he pleases to translate a passage, then questions them on it, and explains what difficulties may be met with. Then he calls on another to do the same, and so this way goes through the book. On the other days, we are Lectured in Paley or Busby, at 8 o’clock in the morning, and in Algebra at 12 o’clock when we have generally a number of questions given to be done while we are on the spot. All the Lectures last an hour each. With regard to my domestic economy I believe I am very fortunate in having my present bedmaker. She appears to be a very nice old woman, and is I believe what very few of them are (honest). She lights my fire at about 6 o’clock, puts my breakfast in order when I am at chapel, comes 2 or 3 times in the day to see if anything is wanted, lays my tea things during chapel time in the evening etc etc.…. (John Couch Adams to Thomas Adams, 28 November 1839; AM).

In the December exams, which involved readings in Thucydides, Adams distinguished himself. Another student, Archibald Samuels Campbell (1821–1899), who sat for the same exams, later recalled: He and I were the last two at a viva-voce examination; he broke the ice by asking me to come to his rooms to tea. I went and we naturally had a long talk on mathematics, of which I knew enough to appreciate the great talent of my new friend. I was in despair; I had gone

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up to Cambridge with high hopes and now the first man I meet is something infinitely beyond me and whom it was hopeless to think of my beating. (Grosser, 1962:74)

Indeed, the examiner was of the same opinion, telling Adams, at the end of the examination, “If you work, the highest honours are in your reach.” (Ibid.) Adams had already heard similar encouragements by a fellow scholar at the Rev. Grylls’s school, “that your progress will shortly astonish the world and even yourself.” (W.R.D. Gilbert to J.C. Adams, 1835; AM). The three College terms at Cambridge were (and are) called Michaelmas, Lent, and Easter. They were, in Adams’s day, somewhat irregular, but are now determined by statute. The Michaelmas Term to begin on 1 October and consist of eighty days, ending on 19 December. Lent Term shall begin on 5 January and shall consist of eighty days, ending on 25 March or in any leap year on 24 March. The Easter Term shall begin on 10 April and shall consist of seventy days ending on 19 June, provided that in any year in which full Easter Term begins on or after 22 April the Easter Term shall begin on 17 April and end on 25 June. (Statutes and Ordinances 2019, Chapter 2, Section 10).

In Adams’s time, the dates would have corresponded approximately to the same intervals. The vacations between the terms were nominally very long, and the actual ones still longer, so that there were not more than twenty-two weeks of real term time in the year—that is, lectures were delivered and residence required for that period only, and a gownsman not disposed to study had the rest of the year to himself. For a “reading man” like Adams, the vacations were, in fact, the busiest time, there being, wrote Bristed (1852:63), “so much less temptation to idleness when all the idle men were away.” In Adams’s day, the terms were further divided into two parts: after division in the Michaelmas and Lent terms, a student able to make a good plea to the College authorities could go down and take holiday for the rest of the term, having already kept enough of the term to answer the University requirements. Adams, owing both to the cost and difficulty of travel to Cornwall, generally remained in college except during the four-month Long Vacations between Easter term and Michaelmas when he returned to Cornwall to spend time with his family. (The exception was that at the end of his first year, when he allowed himself a walking trip to London with two friends, which included a visit to the Royal Observatory, but afterwards he returned to college to continue to work with his tutor.) (Harrison, 1994). The most important examination of a “Cantab” (Cantabridgean, or person from Cambridge) undergrad’s first year took place at the end of the Easter division, with the conclusion of lectures. Because of the time at which it was administered, it was known as “the May.” The conclusion of this examination effectively marked the beginning of the first Long Vacation, when a student would be free of lectures at least until the beginning of Michaelmas term the following October. Adams took “the May” at the end of Lent term in 1840. It was, he told his brother (JC Adams to Thomas Adams, 8 Jan. 1840:AM), “very important, and very difficult too.” It would include turning a passage of English poetry into Greek iambic verse, about which,

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he said, “I suspect I shall come off badly … as I never did anything of the sort in my life.” The exams also covered the Greek texts he had been studying, Euripides’ Andromache and St. Matthew’s Gospel, and translating a passage in English into Greek prose, which “must all be done in the 2 hours you are in the Examination room, without reference to book, or anything but your own head.” Despite his misgivings, Adams did well enough to win a sizarship. In the 17th century sizars (or sub-sizars, of whom Isaac Newton had been one at Trinity) were attached to a Fellow or Fellow-Commoner, and waited on their tutors and even fetched their assizes of food and drink (“sizes” or “sizings”; hence the term “sizar”) from The Buttery. Thus, there was a certain social stigma attached to the position. By Adams’s time, the service had long since been dropped, and a sizarship was nothing more than a much-needed source of funds. By the time he received notice of his sizarship, Adams’s bill each term was running around £35, so that a sizarship, worth perhaps £150 per annum at the time, went a long way to covering Adams’s (very frugal) expenses. (JCA to parents, 2 April 1840:AM). By the end of the freshman year, the undergraduates had begun to sort themselves out academically. Adams was clearly a serious student, and had managed to see through with great success. No doubt many others—the “coasters” and “shipwrecks”—simply abandoned the enterprise, though those with means could stay on indefinitely, despite devoting little time or effort to their academics. It is important to emphasise that Cambridge at the time was still essentially a “finishing school” for the sons of the wealthy. After the first year’s lectures and exams, they could drop their subject altogether, switch to another subject, or live the life if they had wealthy parents. If so inclined (and many were so inclined), the well-­ heeled undergraduate could spend the entire three years at Cambridge agreeably “in shooting, riding, boating, violence and gambling,” (Hall, 1969:2) and with little effort, still come through with a third-class degree. By the second year, of course, “the ruck falls off rapidly, and the good men settle down to their pace.” (Bristed, 1852:85), It was also at this point that any of the students who hoped for honours found their way to the private tutors or coaches who would help guide their efforts, in an intimate and personal way, with those of the highest reputation bringing in the best men based on the first year’s accounting. No less than others, Adams found himself obliged to enter that scrum. Adams now left the landlady who had kept the rooms he occupied during his first year and, according to rccords in St. John’s College Library, henceforth resided (from Michaelmas Term, 1840 until Lent Term, 1843) in room 1 in D staircase, in First Court; then, in the Lent Term of 1843, he took occupancy of room 1 in F staircase, in Second Court. From the Easter Term of 1843—which included the period in which the calculations were made for which he would be credited with co-­ prediction of a planet beyond Uranus—he occupied room 9 of A staircase in New Court. The latter was a beautiful Gothic revival building with turrets and pinnacles, where he seems to have remained until he left St. John’s in 1852. Though not particularly interesting in itself, this history of lodgings helps to date the well-known verbal sketch of him as an undergrad, penned after his death by a pseudonymous author calling himself “Peregrine”. It surely remembers Adams in

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his first year, since that would have been the only year he would not have been in college and the only one he spent with a landlady: I well remember his appearance. He was a rather small man, who walked quickly, and wore a faded coat of dark green. As I had entered as a pensioner and he as a sizar, we did not sit at the same table in hall, or, indeed, dine at the same hour. In my time, the Fellows’ hall and ours was at four o’clock, the sizars’ (who had some dishes left from the Fellows’ table) at five. Chapel was at six when every member of the College was due, and when some came. Fellow commoners and pensioners saw but little of the sizars as a rule, and I fear they were looked down upon on account of their poverty, except, indeed, those few who were certain to do great credit to the college. The fastest and the vainest man would have been civil to Adams, for he was known to be a pretty certain Senior Wrangler; besides, men bet on him and backed him as they would a racehorse. But he was so exceedingly good, so unusually safe, so utterly and entirely the favourite, that odds were laid on him at three, or even four, to one…. It was said among us that Adams never read more than six hours a day; and in a certain sense, that was perfectly true. In my time … freshmen, and men in their last term, had to “keep” out of college; in other words, they lived in lodgings licensed by the authorities [but] their knockings-on were reported just as strictly as if they had been at college. Oddly enough, I had lodgings in my last term which Adams, somewhat my senior, had inhabited long before…. I naturally asked my landlady if she could tell me anything of the Senior Wrangler’s habits. How long did he read? and so forth. She said that she sometimes found him lying on the sofa, with neither books nor papers near him; but not infrequently he was standing at a desk he had fixed against the wall, and that when she wanted to speak to him, the only way to attract his attention, was to go up to him and touch his shoulder; calling to him was of no use. I found out afterwards that the great man was in the habit, after he had accomplished a certain piece of work at the desk, of thinking the whole matter out on the sofa, and it was there, probably, that his brain work was greatest. (“Peregrine”, The Queen, 11 November 1893).

Someone who read more than six hours a day was considered a “grind”. Details about some of them are found in (Bristed, 1852). For example, Charles Turner Simpson (1819–1902), a Johnian a year ahead of Adams and Second Wrangler behind Arthur Cayley (1821–1895) in the Tripos of 1842, was reputed “a fearfully hard reader, and had once worked twenty hours a-day for a week together at a College examination”. However, he almost broke down before the Tripos, and had to fortify himself with a supply of ether and other stimulants to get through it. (Bristed, 1852:96). Adams, by contrast, was a master of “pace,” and by staying to his six hours a day, allowed himself sufficient time for exercise and relaxation in order to maintain his health and well-being. Adams’s chief recreation was walking. He was a strong walker, and remained so for the rest of his life, including when he became Director of the Cambridge University Observatory. (The path he trod still exists, and is known as “Adams’s Walk”.) Whether as a student at Cambridge he followed the “Senior Wrangler’s Walk”, so-called because shorter than other walks and regarded as the minimum sufficient to maintain health while maximizing time for study, or longer walks such as the “Wranglers’ Walk” and the “Grantchester Grind” we do not know. In addition, Adams is said to have been fond of playing at bowls on the college lawns and card games (he was better at Vingt-et-un than whist). He was never a teetotaler, despite the popularity of the teetotal movement at the time, though he obviously came close.

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His “reading time” would have involved studying closely the mathematics texts already mentioned above (unfortunately he does not seem to have kept a list, or perhaps it has yet to be discovered). We know his classics curriculum better. In Michaelmas term during his second year, he studied the Rev. Dr. Richard Busby’s Introduction to the Study of Scripture and the first nine chapters of Paley’s Evidences of Christianity, in Lent term Busby’s Logic and Essay on the Human Mind, Thucydides and Euripides’ Andromache (again, and Cicero’s de Senectute (on aging and death) and de Annuities. There were as yet no formal university lecture courses, in the modern sense. However, there were numerous lectures, and any Cantab could attend those given at any of the colleges. The second and third-year students (junior and senior sophs) were required to attend lectures given by the most recently appointed fellows. The professors’ lectures were not mandatory, and though savoured by the advanced students, were in general rather poorly attended, presumably because the most serious reading men were too busy preparing for the Tripos, the scholarships, or the college exams to “waste” their time in lectures. (Bristed, 1852:129). Furthermore, the professors’ lectures were often “much criticised for their superficiality” and charged with “failing to satisfy the needs of an honours candidate.” (Smith and Wise, 1989:59). Though unfortunately we do not know which professors’ lectures Adams attended, it is almost certain that among them would have been those of the Rev. James Challis. Senior Wrangler and Smith Prizeman for 1825 and protégé of Airy, Challis was Plumian professor of astronomy and as such responsible for observing and reducing observations at the Cambridge University Observatory, a short walk up Madlingley Rise from St. John’s. Challis’s lectures were an important source of information on the physics topics in the Tripos, and on “astronomy, hydrodynamics and optics.” They were presented, according to historian David B. Wilson (1985:18), to “a class varying from twenty to thirty consisting chiefly of the most advanced students.” Further, “Challis … discussed sound before light,” stating that “many of the laws of aerial vibrations serve to illustrate the theory of Light.” The object of physical optics was “to refer the various phenomena of light to some mechanical action of which a distinct conception can be formed.” According to the undulatory theory, Challis said, “light is an effect of the action of a very elastic medium, as sound is produced by the means of the air.” (Ibid.) Though Challis would, of course, later loom large in Adams’s history, that was only after Adams had achieved the celebrity of being Senior Wrangler and First Smith Prizeman. It seems often to be assumed in previous accounts of the Neptune story that Challis and Adams were intimately familiar with one another even when Adams was an undergraduate, and that they must have spent a great deal of time together. Richarda Airy (wife of Astronomer Royal George Biddell Airy) would, for instance, say in a letter to Sedgwick during the post-Neptune discovery brouhaha that “Adams was [Challis’s] private friend.” (R. Airy to A. Sedgwick, 9 December 1846, RGO).

115 Fig. 4.3  The Rev. James Challis (Credit: Royal Astronomical Society Library. MSS Add. 91. Astronomical scrapbook of Mrs. Hannah Jackson-­ Gwilt, item 98b)

This, however, describes the situation later on. From the moment he took over from Airy, Challis was always desperately overworked, and presumably could have spared little time to foster relationships with undergraduates. Moreover, that was simply not the norm. Rarely, undergraduates did form friendships with professors, as Charles Darwin (1809–1882) had done with John Stevens Henslow (1796–1861), professor of Botany and Mineralogy. But this involved informal instruction, not part of a course. (By the time Adams had become Senior Wrangler, First Smith’s Prizeman and a Fellow, the situation was of course quite different; he was then a colleague if not a peer of Challis, and they did collaborate, for periods of time evidently quite intensively, as will be discussed below.) (Fig. 4.3)

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4.5  Adams and His Tutor Important—or not—as these opportunities were for Cambridge undergraduates, far more so—in fact, indispensable—was the close collaboration a man had with his private tutors or coaches. (Regarding the relationship of undergraduates to tutors, see: Wilson, 1985). A good tutor was an absolute necessity for success on the Tripos. No matter how gifted a student was, success was simply inconceivable without such specialised assistance. At the end of the first year, the best students found their way into the hands of the best tutors. (Needless to say, the best tutors were also the most expensive.) Many were fellows who augmented the income from their fellowships by tutoring as Adams himself would later do. Some did not hold official university positions. They did not receive stipends, nor did they have public lecture rooms. In fact, though the entire Cambridge system was entirely dependent on their services, the tutors were not even officially recognised by the University. (Despite this lack of official recognition, they were virtually licensed to instruct upon a particular branch by a record of high performance in previous examinations.) Thus, a high Wrangler would be considered ipso facto a competent instructor in Mathematics. Because of the importance of the Tripos exam in Cambridge undergraduate education and the inability of any candidate for honours to be successful without a tutor, the system became self-perpetuating: those who achieved high Wrangler status became tutors, and proceeded to tutor the next rounds of students in the methods needed to succeed, and so on. In addition to professional coaches who could earn sufficient income to maintain a very considerable household in Cambridge, many, and perhaps most, fellows of colleges also supplemented their income by tutoring. Adams did not hire a private tutor during his first term, but after Christmas, he is known to have engaged the Rev. Joseph Woolley (1817–1889), who had achieved the highest success of any Johnian as 3rd Wrangler in the 1840 Mathematical Tripos and been named a Fellow and Lecturer at St. John’s at the end of Lent term that same year. At least at first, Adams apparently engaged Woolley as a “half pupil”, that is, in the interests of economy, he took lessons every other day instead of every day. (JC Adams to Thomas Adams, 28 November 1839:AM). The tutor was—at a time when Adams was effusively thankful to his brother for sending him a much-needed pair of new shoes, and before he received his sizarship—a significant added expense, but a necessary one. Adams admitted that “it will cost me £7 a term but I am told that everyone who desires to contend for honours has a private tutor and that I cannot do without one.” Woolley, despite his comparative youth, seems to have been thorough and demanding. “My tutor keeps me at work exceedingly hard,” Adams wrote in February 1840, “and I have nearly as much as I can do, to prepare for his Examinations, and the College Lectures.” (JC Adams to Thomas Adams, 26 February 1840:AM). At the end of Lent term 1840, he sought advice from his parents as to whether he could afford the expense of hiring Woolley’s services during the Long Vacation (May to October 1840). “I am told and know that it must be a

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much greater advantage to have a Tutor for the vacation,” he wrote, “than in term time; not being then interrupted by College lectures etc. and I believe the other expenses attending a residence here for that time, will be very small.” (JC Adams to his parents, 2 April 1840:AM). At some point—the details are a little slippery here—Adams apparently also hired the services of a classics tutor, the Rev. John Frederick Isaacson (1801–1886), a Fellow and Tutor at St. John’s. In 1840, Isaacson had just married Rebecca Stokes (1815–1877) (possibly related to George Gabriel Stokes, the Senior Wrangler and First Smith’s prizeman for 1841), and been appointed rector of Freshwater, on the Isle of Wight. (According to the usual practise of the time, he presumably remained in Cambridge and relied on other curates to do the parish work, and perhaps spent summer vacations on the Isle of Wight. He later settled there.) However, Isaacson’s name makes scarcely any appearance in Adams’s correspondence, possibly because Adams had already by then relaxed his expectations and surrendered the idea of trying for honours in classics. He told his brother at the end of Lent term in his freshman year, “I hear that the College Tutors, throughout the University say that it is a very good year indeed. I think by hard fighting, I shall be able to keep pace, in the Mathematics with most who are at our College. As to the Classics, I shall trouble myself very little with them.” (JC Adams to Thomas Adams, 26 February 1840:AM). This was no doubt a prudent assessment, and one might even be allowed to speculate that had he continued to aspire to honours in classics as well as in mathematics, he would never have found the leisure time, limited as it was, to take up the Uranus calculations as he did. Meanwhile, there was another important development, which further reinforces a sense of his increasingly one-sided commitment to mathematics. Probably before the beginning of his second year, Adams transferred from the capable but still rather young and newly-minted Woolley to a tutor of much greater experience and reputation, John Hymers (1803–1887). As a coach, Hymers was second in reputation only to William Hopkins (1793–1866), the “senior-wrangler maker”, who during his career coached seventeen Senior Wranglers and whose more famous pupils included Stokes, Arthur Cayley, both of whom were Senior Wranglers, and Lord Kelvin and James Clerk Maxwell, both Second Wranglers. Hopkins was a legend, renowned for the “skill and intensity” of his math teaching, as well as his “intimate knowledge of the minds of examiners.” (Leedham-Green, 1996:126). Hymers had been Second Wrangler in 1826, and was one of a handful of individuals who—in contrast to Hopkins, who devoted all his energies to his career as a coach—managed to combine tutoring with a successful college career. Even as a young man, Hymers had been renowned for his acquaintance with the progress of mathematics on the continent, and wrote several important treatises which helped to introduce Cambridge students to the new methods. When invited to act as a moderator for the Mathematical Triposes of 1833 and 1834, he was just thirty years old, had already completed a treatise on analytical geometry in three dimensions, and was writing a treatise on integral calculus which introduced elliptical integrals to English students for the first time. (After a distinguished Cambridge career, he left in 1852 to take up a college living at the rectory of Brandesburton in Holderness.) (Anderson, 1891).

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Adams would not, of course, have been Hymers’s only student, and although we do not know specifically what the arrangement was, group coaching was common. Typically, the mathematics tutor would give “examinations”—that is, assign a student to working examples and problems of the sort that would be encountered in the problems section of the Tripos. As for the “book work”, difficulties were typically sufficiently explained in textbooks—though some tutors considered their own manuscripts better than any of the books, and made their students copy them. To prepare for the ultimate exam in the Senate House in January, students were tasked with practicing their book-work continually in order to get pace as well as accuracy, and to work through the problems. Among the diaries that Adams kept intermittently during his undergraduate years, a significant number have mathematical problems worked on the backs of the sheets. (Adams diaries; SLC). Like most students of the time, and perhaps of any time, Adams sometimes berated his lack of application or tendencies to procrastination (which he regarded as “sins” to be repented of). His diaries, which he kept somewhat irregularly during his undergraduate years, include entries such as, “Had a short paper but took a long time about it, having made myself very imperfectly acquainted with the subject,” or “Had a paper in the Planetary Theory but was, as usual, unprepared.” His strict economy sufficed to cover an annual visit to Lidcot for a month or so during the Long Vacation before the beginning of each academic year, except for the first year, when, as noted earlier, he spent the summer in college. Thus, unflinchingly, frugally Adams prepared himself for the six-day trudge up the mountaintop—the Mathematical Tripos—that loomed ahead in January 1843. As already described, the exam consisted in part of book-work (regurgitation) and in part of problems. For someone like Adams, the book-work would have been a breeze—but others, possessed only of an excellent memory and of exceptional organisational abilities, could also keep the pace. It was the problems which allowed Adams to assert his real superiority. It was afterward recalled that when everyone else was working hard, Adams spent the first hour in looking over the questions, scarcely putting pen to paper. After that he wrote out rapidly the problems he had already solved “in his head.” (Glaisher, J.W.L., 1896:I, xliii, f.n.). Adams’s performance was an unqualified success, and would become legendary in the annals of the Tripos. According to Bristed (1852:125):, With the New Year [1843] came on the great University examinations, which excited the usual interest. The Senior Wrangler this time was Adams of Johns, since celebrated as the other discoverer of Le Verrier’s Planet. He won in a canter, so to speak, having three thousand marks to [Francis Bashforth (1819–1912)] the Second Wrangler’s fourteen hundred, so that there was more numerical difference between them than between the Second Wrangler and the spoon.

This was an incredible achievement, such as had been rarely—if ever—equaled before or since (not even by Airy). Immediately after the results were declared, Adams sat as a matter of course for the Smith’s Prize examinations, and again—as if there could be any doubt after his commanding performance in the Tripos—won in a canter. In addition to the honour accruing to it, the Smith’s Prize was worth £25, a much-needed financial windfall for Adams at the time.

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As usual, newspapers across the country reprted the feat, including, among others, several Cambridge papers, the Yorkshire Gazette, the Leamington Spa Courier, the Sherborne Mercury, the Exeter and Plymouth Gazette, the Salisbury and Winchester Journal and—most notably—the Royal Cornwall Gazette, which noted Adams’s modest background as “the son of a farmer in the parish of Laneast, a tenant and not a very large one.” To this last, Adams responded rather peckishly in a letter to his parents, “By the way I may mention that I was not at all pleased with the long article about me in the Cornwall Gazette. The writer’s intentions were good I have no doubts, but his information on many points, was not very correct as you must have seen.” (JC Adams to his parents, 10 February 1843). As Senior Wrangler and First Smith Prizeman Adams had put himself at least on par with such legendary figures as John Herschel, George Airy, and James Challis. When news of his becoming First Smith’s Prizeman reached Lethbridge, the landlord and the man who had once been skeptical of someone like Adams getting a college education at all but had since become one of his greatest supporters, rode his horse all the way back to Laneast, and on entering the yard of Lidcot farm, took off his hat and shouted, “Adams for ever!” (Sheen, 2013:72).

4.6  Uranus Looms Though as we have seen so far, Adams possessed to an extraordinary degree the Victorian traits of conscientiousness and strict adherence to duty and maintained a disciplined, constant, and narrow focus on his preparations for the Mathematical Tripos, he had meanwhile developed an independent private research interest. At end of his junior soph year, in June–July 1841, while browsing in Elijah Johnson’s secondhand bookstore in Trinity Street (a very short walk from St. John’s), he came across a copy of the proceedings of the Oxford meeting of the British Association for the Advancement of Science in 1832, in which he read Airy’s short “Report on the Progress of Astronomy”. This report would have attracted his attention for at least two reasons. The first was simply that it was by Airy, whose academic accomplishments had led, within three years of his graduation as Senior Wrangler and Smith’s prizeman, to appointment as Lucasian professor of mathematics (the chair Newton had once held), and later to his comprehensive reforms of the Cambridge University Observatory. He had also written the textbooks from which Adams would have learned his lunar and planetary theory (Figs. 4.4 and 4.5). The second reason for Adams’s interest, however, would have been the problem of the motions of Uranus that he now came across, and which was precisely the sort to which his training for the Tripos problems would have prepared him. It says a great deal about Adams’s confidence that, even though he might appear to others as absentminded and diffident (Pym, 1882:vol 2, 90–92), inwardly he was utterly convinced, even at the stage of a junior soph, of his own capabilities. The problem to which Airy called his attention was, of course, that of the unexplained deviation of Uranus from its predicted path. In 1832, this was 30″ of arc in

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Fig. 4.4  George Biddell Airy (Credit: Royal Astronomical Society Library. MSS Add. 91. Astronomical scrapbook of Mrs. Hannah Jackson-­ Gwilt, item 98a)

longitude, and had meanwhile increased to 120″. Though Adams’s serious thinking about the problem may have begun in June or July 1841, he seems already to have encountered the idea of an exterior planet before this. According to a perhaps untrustworthy anecdote recorded long afterward, at a dinner party at Sir John Herschel’s house at Collingwood in December 1847, Adams said that it was reading Mary Somerville’s The Connexion of the Physical Sciences that “first put it into his head to calculate” the orbit of the perturbing planet. The passage in Somerville reads: The tables of Jupiter agree almost perfectly with modern observations; those of Uranus, however, are already defective, probably because the discovery of the planet in 1781 is too recent to admit of much precision in the determination of its motions, or that possibly it may be subject to disturbances from some unseen planet revolving about the sun beyond the present boundaries of our system. If, after the lapse of years the tables formed from a combination of numerous observations should be still inadequate to represent the motions of

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Fig. 4.5 The Northumberland refractor used by Challis in the summer 1846 search for a trans-Uranian planet (Credit: Institute of Astronomy, Cambridge University Observatory)

Uranus, the discrepancies may reveal the existence, nay, even the mass and orbit of a body placed for ever beyond the sphere of vision. (Somerville, 1874:290).

This passage had appeared in editions of Somerville’s popular book since the third in 1836 and by 1840 had reached a fifth edition. Whether or not Adams encountered this passage (and it seems plausible that he did), it certainly shows that the idea of a perturbing planet was very much in the air—and had been for quite some time. Adams’s memorandum, dated 3 July 1841 is as follows (Fig. 4.6): Formed a design … of investigating, as soon as possible, after taking my degree, the irregularities in the motion of Uranus, which are not yet accounted for, in order to find whether they may be attributed to the action of an undiscovered planet beyond it, and, if possible, to determine approximately the elements of its orbit, etc., that would probably lead to its discovery. (SCL).

What Adams does not say, in his memorandum, is almost as interesting as what he does. Committed from the first to the exterior planet idea, he does not—as Airy, Bessel, or (as we shall see in Chapter 5) Le Verrier had done—consider alternative explanations. He says nothing, then or later, about the possibility of a resisting medium as had been inferred by Johann Franz Encke and others from the fact that even after the perturbative effects of the other planets were allowed for, the returns of the short-period (3.3  years) comet named for him were shortened at each

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Fig. 4.6 Adams’s memorandum (Credit: By the permission of the Master and Fellows of St. John’s College, Cambridge)

revolution by half a day. Nor does he mention the “specific action” of gravitational force that had long retarded progress by Bessel, or the possibility mooted by Airy, from the error in the radius vector, that there might be additional terms in the perturbation equations needing to be considered, as explained in Chap. 2. Adams set himself a more straightforward problem: he wanted only to show that by making reasonable (and yet necessarily somewhat arbitrary) assumptions about the elements of the orbit of an exterior planet, the residuals (difference between theoretical and observed positions) in the heliocentric longitude of Uranus could be corrected, and the exterior planet’s position in the sky narrowed sufficiently to allow a search. This remained from beginning to end his great project. It was ambitious, and he could not hope to take it on until after his graduation in January 1843. Thus, the Uranus problem hardly appears as more than a bit of foam on the surface of the largely invisible succession of events that constituted Adams’s undergraduate activity before it disappears again. He must not—and did not—allow himself to be distracted from what he knew was his first duty, which was to prepare for the Tripos. Of course, had Adams not returned to it again, the memorandum of his design would have interested no one. It would have joined the incalculable number of

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Fig. 4.7  Philippe Gustave Doulcet, le Comte de Pontécoulant (Public Domain)

projects that appear in the sights of ambitious and gifted individuals that go nowhere. In Adams’s case, however, it was not forgotten, and remained in the back of his mind. When he jotted his memorandum, he would no doubt have been aware of the fact that he was not yet mathematically sophisticated enough to tackle a problem of such difficulty. One can imagine (though the documentation here is lacking) that during his preparations for the Tripos, he paid close attention to anything that might be helpful in his postponed pursuit. He would certainly have had close at his elbow the three volumes so far published of Philippe Gustave Douclet, le Comete de Pontécoulant’s Système du Monde, the most recent and elegant formulation of celestial mechanics available at the time. It is in these volumes that he found the expressions for perturbation theory that he relied on in his Uranus adventure (Fig. 4.7). Adams was rather careless about recording dates, and so we do not know when, after his graduation, he finally got round to the problem of Uranus again. But we do know that he finished a preliminary phase of the investigation and even completed a first calculation by the spring of 1843. Ralph A. Sampson (1866–1939), a future student of Adams who would edit Adams’s unpublished manuscripts after Adams’s death, summed up all that this entailed: Collecting and reducing observations was one part of [the] task; examining the existing theory was another. Bouvard’s tables were certainly in error in some theoretical points [as] indicated by Bessel and by [Peter Andreas] Hansen (1795–1874) [of the Seeberg Observatory]; Adams verified these errors and included them in his work. The Tables might also be in error by faulty determination of the elliptic elements of the orbit of Uranus, or by errors in the calculation of the inequalities of Uranus due to the other planets; the former cause Adams allowed for by introducing in his equations of condition unknowns that represented corrections to the elliptic elements of Uranus; the latter he disregarded at first, rely-

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ing no doubt on the check offered by the independent computation of Pontécoulant of these inequalities in his Système du Monde… (Sampson, 1904:145)

To break this down, Adams began by taking the observations from 1781 to 1821 directly from Bouvard. Those after 1821 were (Ibid., 144), “collected from the scattered observations published by various persons in the Astronomische Nachrichten and elsewhere; some of the slips of paper remain [among other manuscripts], creased and soiled by his pocket, upon which he had copied out these observations at one of the libraries.” Once this magpie-like phase of data-gathering was completed, the observations were collected and reduced from the Right Ascension and Declination coordinates given by the observers to heliocentric longitude and latitude. This preliminary work was very similar to that which Flemming had done for Bessel before his untimely demise. Even after his graduation, Adams likely could not have spared much time to this effort, since he would have been caught up very quickly in the teaching and tutoring duties of a just-minted Fellow. He might have made some progress during the Lent and Easter terms, but it was not only with the Long Vacation of 1843 that he drafted a first solution. (Adams, 1847:428). This is the first of six calculations, which, for convenience and later reference, are summarised in Table 4.1 on page 130. He was attempting to solve, for the first time, an inverse problem in perturbation theory—in which instead of deriving the effect from the cause, as in the forward problem, he was trying to derive the cause from the effect. It is difficult to give a clear idea of just how intricate this problem is to anyone not familiar with the arcana of celestial mechanics. In merest outline, the problem Adams set for himself may be stated thus. In order to explain the errors in the tables of Uranus, which remained even after the perturbing effects of Jupiter and Saturn were corrected for, an unknown planet is assumed. The elements of Uranus deduced without taking allowance of this unknown’s action would necessarily be incorrect, and produce discrepancies between the theoretical and observed values of Uranus’s longitude, latitude, and radius vector. The discrepancies are partly due to the inaccurate elements of Uranus’s s orbit adopted and partly due to the unknown planet’s perturbing action but—and this is the point—the two are inextricably entangled. They cannot be investigated independently. According to Robert Grant (1814–1892) (1852:169), “It clearly follows … that the elements of Uranus cannot be determined without taking into account the whole effects of planetary perturbation [which includes those of the unknown], and this again implies a knowledge of what is the very object of inquiry, namely, the elements and mass of the unknown planet.” To solve this problem, in all its generality, involves thirteen unknown quantities: the six elements of Uranus’s orbit, the six of the unknown planet, and the mass of the unknown. In this general form, the problem is not soluble. However, it can be made more manageable by adopting some reasonable simplifying assumptions. In his first trial, Adams assumed that the unknown planet’s orbit lay in the plane of the ecliptic; that the orbits of both Uranus and the disturber were circular; and finally, that the mean distance was that suggested by Bode’s law, i.e, that the radius of its orbit was twice the mean distance of Uranus from the Sun. The assumption that the two planets

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move in nearly the same plane alone reduces the number of unknowns to nine. The additional assumptions about the mean distance and the circularity of the orbits reduces the problem to the bare essentials: the unknowns become the mean heliocentric longitude of the disturbed planet and the mean longitude of its epoch, that is at the origin of the time. Solving for these parameters gives the angle between the unknown’s mean longitude and that of Uranus. This angle is zero when the planets are in conjunction. From his study of Adams’s manuscripts, Sampson stated: Perhaps this earliest solution will be considered the most interesting portion of the manuscripts.... It is complete and intelligible, though most difficult to follow. It is written at first straightforwardly, then on the backs of other calculations, and at last squeezed into any corner not already occupied, as if his eagerness to finish would not let him wait to get a few more sheets of paper. (Sampson, 1904:145)

As elucidated by Sampson’s study, it is clear that at this stage Adams had confined himself to utilizing only the “modern” (post–1781) observations, and by skillful algebraical legerdemain succeeded in getting the sought-after angle between the assumed unknown’s mean longitude and that of Uranus. This allowed him to calculate (using the mean motions, which followed from the assumed mean distances) the time of the unknown’s conjunction with Uranus–1822. This was a stunning result, since it was the key to the whole problem and also led, at once, to a theoretical position for the unknown planet. In the whole long history of the Uranus problem, this is the first time anyone had ever obtained such a result. It is worth noting, moreover, how early in Adams’s investigation it was achieved. Given how many assumptions Adams had made (and that about the perturbing planet’s mean distance was, as we shall see, especially far off the mark), his position was actually surprisingly good. Moreover, in reaching it, Adams committed one of his rare mistakes—though he immediately saw it and corrected it. Sampson noted (1904:144) “Adams was a rapid worker when his work was coming to a point, and then and always was able to carry through fabulous computations without error; but here, at the point of writing down the position of the new planet, it is curious to note that he makes a mistake, forgetting to divide by a factor 3… This mistake was corrected upon the manuscript, probably as soon as made, and the resulting place of the planet gives a very close approximation to the correct time of conjunction with Uranus; but owing to the faulty mean motion assumed … the position indicated in 1843 was some 15° in error.” (Actually, it was more like 16° or 17°, corrected for precession and brought up to the epoch of his later published solutions. This is still pretty remarkably good, under the circumstances.) Adams had proved the validity of his basic approach, and could proceed to add complications and refinements at leisure. But he must have felt very encouraged, and at some point—the manuscript is not dated—he attempted another calculation, in which as before, the hypothetical disturbing planet’s orbit was assumed to be circular (and thus still suffered from an incorrect value of the mean motion). However, this time the actual value of the eccentricity of Uranus’s orbit (0.047) was used. Since now eccentricities were being phased in, he employed not only the

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“modern” observations (those since 1781) but also the “ancient” ones going back to Flamsteed’s of 1690. This allowed him to introduce terms containing as unknowns to be solved for the eccentricity and the apse (position of the perihelion) of the disturbing planet. (In the circular orbit, there is no position of perihelion; this enters in only when an elliptic orbit is used.) The trial was very tentative, and Adams does not seem to have attached much weight to it. Indeed, the manuscript containing it is undated. It did, however, convince him that he needed more Uranus data.

4.7  “ A Mass of Comet Reductions I Scarcely Know How to Get Through” So far Adams was working in splendid isolation in his rooms in New Court. In order to get the additional data needed, he decided to approach Challis, who would already have been at least slightly acquainted with Adams from the latter’s attendance at lectures and his success on the Tripos. The Uranus problem had never been of particular interest to Challis (as it had been to Airy), so Adams decided that, rather than tackle him head-on, he would approach him through what seems to have been Challis’s principal preoccupation at the time: comets. A significant share of Challis’s duties involved making and reducing minor planet and comet observations, and when in the summer of 1846 Airy finally pressed him to use the Northumberland to search for a planet, Challis expressed his willingness while adverting to the still outstanding “mass of [comet] reductions which I scarcely know how to get through.” (Challis to Airy, 18 July, 1846; RGO; copy McA 33:1). It should be pointed out that the pressure on Challis, far from being self-imposed, came from the Reverend Richard Sheepshanks (1794–1855), a leading figure at the Royal Astronomical Society, who in that period of comet discoveries and—after a new minor planet was discovered in December 1845, minor planet discoveries as well—approached Challis to use the powerful Northumberland to verify positions and orbits. Challis was under no obligation to do so, but being obliging by nature, felt an obligation to assist. This was largely because the years 1843–44–45–46 were incredibly busy with comets. The Great Comet of 1843 appeared shortly after Adams’s winning performance in the Tripos; it was discovered in the southern hemisphere just before it reached perihelion, when it passed closer to the Sun than any other comet known up to that time, became bright enough to be seen in broad daylight, and had the longest tail recorded up to that time at over two astronomical units in length. It had already begun to fade by the time it became visible to northern hemisphere observers, and traveling in a highly elliptical path will not return again for 600 to 800 years. (Kronk, 1984:35). Another interesting, though not a “Great” comet, was discovered in November 1843 by Hervé Faye (1814–1902) at the Paris Observatory; it had already passed perihelion a month before Faye captured it, and was just slipping out of naked-eye

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visibility. Challis observed it with the Northumberland telescope on 29 November, 8 December, and 16 December. Adams, presumably taking advantage of the lull in his required college duties over the Christmas Vacation, began a collaboration with Challis by computing the comet’s orbital elements, which Challis published in the Monthly Notices of the Royal Astronomical Society on 12 January 1844. Adams perspicaciously commented: the comet may, perhaps, not have been moving long in its present orbit, and that, as in the case of the comet of 1770, we are indebted to the action of Jupiter for its present apparition. In fact, supposing the … elements to be correct … the comet must have been very near Jupiter when in aphelion, and must have suffered very great perturbations, which may have materially changed the nature of its orbit. (Adams, 1844:20)

Independently, the French astronomers Le Verrier and Jean Élix Benjamin Valz had also computed the comet’s orbit, and had reached the same conclusion, Le Verrier showing that the comet had made a close passage by Jupiter in 1841 which had decreased its perihelion distance, and predicted it would next return to perihelion in April 1851. (Grant, 1852:139–141). (When it did in fact return, it would be recovered by none other than the Rev. James Challis.) (Fig. 4.8) Thus, Adams was able to use this comet to prove his usefulness to Challis, and apparently took advantage of the latter’s indebtedness to present a thumbnail of his fledgling Uranus investigation. Documentation is scarce, and though there is no reason to think that Challis was interested in the detail of Adams’s proposal, we do know that on 12 February 1844, Challis and Adams met, for Adams’s diary (JCL) contains the entry, “Prof. Challis brought one of the Notices of the Astr. Socy. containing my results respecting the Comet.” He apparently wanted to express his gratitude to Adams by reciprocating his help with the comet, for the very next day Challis wrote to Airy at Greenwich requesting Uranus data for Adams, … A young friend of mine, Mr Adams of St John’s College, is working on the Theory of Uranus, and is desirous of obtaining errors of the Tabular Geocentric longitude of this planet when near opposition in the years 1818–1826, with the factors for reducing them to errors of Heliocentric Longitude. Are your reductions of the Planetary Observations so far advanced that you could furnish these data, and is the request one which you have any objection to comply with? If Mr Adams may be favoured in this respect, he is rather desirous of knowing whether in the calculation of the Tabular errors any alterations have been made in Bouvard’s Tables besides that of Jupiter’s mass. I have obtained a large number of observations of Faye’s Comet with the Northumberland Telescope, & I expect to be able to observe it to the end of February at least. I remain, my dear Sir, Yours very truly, J. Challis (Challis to Airy, 13 February 1844; RGO)

Airy, in usual fashion, replied immediately (Airy to Challis, 15 February 1844; RGO), “I send all the results of the observations made with both instruments. They have not been finally examined but I believe that they are correct.” Adams now had the data needed to enter more deeply into his Uranus investigation, but at this point, he was again overtaken by his college duties. (In a letter to his parents dated 17 May 1844, he wrote that he was “busy with pupils”. [AM]). Nevertheless, his interest was now rekindled by a minor and little-known figure in

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Fig. 4.8  The Great Comet of 1843 painted by Charles Piazzi Smyth (1819–1900) (Credit: Royal Museums Greenwich and Wikipedia)

the Neptune story, William Hallowes Miller (1801–1880), professor of mineralogy at Cambridge and author of mathematical texts on differential calculus and on hydrostatics and hydrodynamics, who suggested that Adams compete for the mathematical prize question on Uranus that had just been proposed by the Göttingen Academy of Sciences. Adams later recalled that the announcement of the prize was significant: Meanwhile the Royal Academy of Sciences of Göttingen had proposed the theory of Uranus as the subject of their mathematical prize, and although the little time which I could spare from important duties in my college prevented me from attempting the complete examination of the theory which a competition for the prize would have required, yet this fact, together with the possession of such a valuable series of observations [as just received from Airy] induced me to undertake a new solution of the problem. (Adams, 1847:429).

It appears that this calculation—the third so far—made the same assumption as to the mean distance as before, but attempted also to take account of the most important terms depending on the first power of the eccentricity of the disturbing planet. It was thus the first calculation to be considered full weight. Sampson says of it

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(1904:146), “very little was afterwards added which this solution did not recognise, but variations in method of treatment which may seem slight enough render it perhaps the least successful in indicating the planet’s place.” Indeed, the only thing that distinguishes it from the well-known solution communicated to Challis in September were two additional elliptic terms in the theory of Uranus, and two additional terms of his perturbations by the new planet. This calculation (III in Table 4.1) is dated 28 April 1845, and Adams follows up with a comparison with observations on 19 May. The reader may be surprised by the long hiatus in Adams’s work between the spring of 1844 when he learned of the Göttingen prize and these calculations. As usual, the culprit was that Adams’s attention was otherwise directed. He had received another cometary commission from Challis. A new comet was discovered on 23 August 1844 by Father Francesco de Vico (1805–1848) at the Collegio Romano in Rome. Challis observed it on 15, 20, and 25 September with the Northumberland, which furnished the minimum number of observations to determine an orbit. Adams meanwhile having left for his annual vacation at Lidcot, Challis mailed the observations after him, with a request for his younger now-colleague to compute the comet’s orbital elements. (Adams, 1844). Adams put aside whatever else he had in hand to make this his priority; he made several trials in which he found that the orbit was of moderate eccentricity and a period, stated rather too exactly, of 5,636 years. In order to be more certain of his result, he requested further data. Challis sent two more observations on 7 October along with the suggestion, “When you have satisfied yourself about the elliptical form of the orbit will it not be advisable to send a communication to The Times newspaper (the [Royal] Astronomical Society not sitting) in order that we may have the credit in England of first making the discovery.” (Smart, 1947:18) Adams’s notice, “The Orbit of the New Comet,” appeared in The Times for 15 October. It turned out—and not for the last time!—that Adams’s publication failed to secure the credit Challis hoped it would. Before Adams had published, the French astronomers Faye and Le Verrier had—as Adams learned on returning to Cambridge to begin Michaelmas Term—also calculated the elements of the comet’s orbit. Though their conclusions were nearly identical to his, they had published first. Adams wrote to his parents from Cambridge about the Times publication (JCA to parents, 21 October 1844; McA 35:12), “I found soon after my return that the French astronomer had obtained very nearly the same results as myself but earlier in time owing to their observations having been made earlier.” Adams was once more bogged down with his lectures and tutorials, and the Uranus problem, as so often in the previous year, receded into the background. But it did not disappear altogether. Indeed, at the end of 1844, an intriguing new strand had emerged, as he wrote to his brother George thanking him for a telescope and noting that he had just been appointed co-curator of the college observatory: My dear Brother, I should have written to you long before this had I not been so very busy this term to have the time for letters. I am much obliged to you for sending the telescope so carefully. I have not used this, as the weather has not been possibly good enough to try it so much as I

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Table 4.1  Adams’s Calculations for the Planet Perturbing Uranus, 1843–1846 (After Sampson, 1904:151) Epoch 1810.328 I. Undated (but completed during Long vacation 1843)  Mean distance  Eccentricity  Longitude at epoch  Mass (Sun = 1) II. Undated  Mean distance  Eccentricity  Longitude at epoch  Mass III. Dated 28 April 1845  Mean distance  Eccentricity  Perihelion  Mean Longitude at epoch  Mass (Sun = 1) IV. Dated 18 September 1845  Mean distance  Eccentricity  Perihelion  Mean Longitude at Epoch  Mass (Sun = 1) V. Hyp I. Dated October 1845  Mean distance  Eccentricity  Perihelion  Longitude at epoch  Mass (Sun = 1) VI. Hyp II. Completed end of August 1846  Mean distance  Eccentricity  Perihelion  Longitude at epoch  Mass (Sun = 1)

38.4 (assumed) 0.0 (assumed) 253° 28′ 0.00023655

38.4 (assumed) 0.0 (assumed) 263° 16′ 0.000410

38.4 (assumed) 0.19717 248° 8′.3 271° 37′ 0.0001573

38.4 (assumed) 0.1428 320° 30′.2 267° 58′.2 0.172302

38.4 (assumed) 0.16103 315° 57′ 269° 25′.5 0.0001656

37.25 (assumed) 0.120615 299° 11′ 264° 50′ 0.00015003

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could wish, but I think it is a very good glass…. I don’t know whether I ever mentioned that we have a small observatory attached to the college. Mr. Griffin and I have just been appointed joint Curators of it, so I think I now have the opportunity of using a pretty good telescope and several other astronomical instruments. (JCA to George Adams, 18 December 1844; McA 35:12).

This letter reveals that he apparently had enough confidence in his calculations to consider making his own telescopic search for the unknown planet. Perhaps anticipating the danger of relying on others to do the observing for him (and he had had direct experience of this with Challis, who was having trouble keeping his head above water with his comets), he had, as he afterwards recalled (Adams to Airy, 18 November 1846:McA 33:8) a onetime wish to “have our instruments put in order, with the express purpose, if no one else took up the subject, of undertaking the search for the planet myself, with the small means afforded by our observatory at St. John’s.” To further motivate him in this resolve, he added, “I could not expect however that practical astronomers, who were already fully occupied with important labours, would feel as much confidence in the results of my investigations, as I myself did.” The observatory was hardly suited, however, to such an ambitious plan. It contained only a few antiquated instruments—including a 3-foot reflector that could be placed in one of the windows of Shrewsbury Tower in the event anyone wanted to observe with it. (De Clercq and Pederson, 2010 139–140). The plan was never acted upon, and the division of labour between theory and observation would remain, for better or worse, as it had been in the days of Flamsteed and Newton. The fate of Adams’s quest would rest in the lap of the gods—or at least in the hands of Airy and Challis (Fig. 4.9).

4.8  Adams Calculates Soon after finishing the third calculation described above, the Long Vacation began, which would see a great deal of important work. Indeed, things began to pick up decisively beginning with Adams’s attendance at the fifteenth meeting of the British Association for the Advancement of Science, held in the Senate House in Cambridge in June 1845. It was the first time that this august body had met in Cambridge since the third meeting in 1833, and Adams became a member for life. Sir John Herschel received the presidency from his old Cambridge friend and fellow member of the Analytical Society, the now Very Rev. George Peacock Dean of Ely. We know that, though Adams only managed to attend the last part of the meeting (which he called “the feast”), he found everything very inspiring. In a letter to his brother George, he described his “pleasure in seeing for the first time some of our greatest scientific men, Herschel, Airy, Hamilton, Brewster, etc.” (JCA to George Adams, 10 July 1845; McA 35:12). By then, the Long Vacation of 1845 had begun, and though Adams was “working with my pupils as usual,” he added in the same letter that “my labours are much

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Fig. 4.9  St. John’s Observatory, located within the top floor of Shrewsbury Tower to the right of the image, at St. John’s College Cambridge. It contained a few small antique instruments, including a transit instrument, a movable quadrant, an astronomical clock and a 3-foot reflecting telescope that could be placed in a window or on a wooden support especially made for the purpose (Credit: By permission of the Master and Fellows of St. John’s College, Cambridge)

lighter than in term time, having no lectures to give.” (Some idea of what he was covering in his tutorials are found in a week-by-week schedule found among his papers, which, though from the following summer, would no doubt have been very similar in 1845.) (JC). (See Fig. 4.10) At last he had begun what would be his most ambitious attempt to climb the Everest of the Uranus problem, an attempt that would lead, by the time the Long Vacation was winding down, to the solution “published” to Challis in September 1845, and that later known as Hyp I (short for Hypothesis I) given to Airy in October 1845. (These were actually his fourth and fifth calculations, listed in Table 4.1.) The calculations in which he involved himself in July, August and September involved, as always, setting up equations of condition, in which the familiar perturbation formulae based on Pontécoulant were set equal to numerical errors of the tables. The problem, as stated in this way, sounds more straightforward than it actually is. In practice, the mathematician needs not only to have consummate mastery of the theory of planetary perturbations but the skill to employ such artifices as may,

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Fig. 4.10  Schedule of topics in Adams’s hand, to be covered during his tutorials during the Long Vacation of 1846 (Credit: By permission of the Master and Fellows of St. John’s College, Cambridge)

as Grant puts it (1852:169), “lead to the disengagement of the unknown quantities from the complicated expressions in which they are involved.” During his calculations over the Long Vacation of 1845, Adams continued to assume that the disturbing planet lay very nearly in the plane of the ecliptic (like Uranus itself, whose inclination to the plane of the ecliptic was only about three-­ quarters of a degree). Also, and even though he was not very happy about it, he continued to base the mean distance for the perturber based on Bode’s law. This left four elements of Uranus requiring correction: the semi-major axis a, the eccentricity e, the longitude of perihelion ω, and the mean longitude of the epoch ε. (It may be queried why Adams continued to use the mean longitude ε instead of the true longitude ν; there were, however, good reasons for this, as the series expressing the correction of the mean longitude is more convergent than that which gives the correction of the true longitude—in other words, mathematically things were better behaved.)

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What was new was that now the eccentricity e' of the perturber was now a variable. It should be noted that these various parameters are not independent, and taking too large a value for the mean distance (which did in fact happen) would actually be compensated for by a more sizable eccentricity and a larger value of the mass. The upshot is that, despite getting this or that value wrong, his calculation for the parameter he was seeking—the position of the planet in the sky—would remain valid. In symbols, the corrections to the elements of Uranus could be written as differentials: δa, δe, δω, δε. The unknown elements of the disturbing planet, including the mass, were represented by accented symbols, m', a', e', ω', ε'. Thus, Adams was attempting to solve a problem in nine unknowns. In order to do this, he needed to set up a series of equations of condition based on groupings of the residuals. He proceeded as follows. First, the more numerous modern observations were grouped into equidistant three-year periods, forming twenty-one groups for the period 1780–1840. In each of the twenty-one groups, he selected observations either made near Uranus’s opposition, or others that could be combined in such a way that the result would be unaffected by the tabular error of the radius vector which, as Airy had found, “could be quite considerable.” (Adams, 1847:430). As an additional check on the results, he recomputed the principal inequalities produced by Jupiter and Saturn to verify that there were no significant errors in Bouvard’s Tables on which the values of the residuals for the observations up to 1821 were based; there was no difference of any consequence (except an error that had already been discovered by Bessel). Similarly, he recomputed some terms based on recent work by the German astronomer Peter Andreas Hansen, again without any significant effect on the result. After a staggering amount of juggling of the perturbation equations for the mean longitude, Adams was able to form two equations from the modern observations which would “be sufficient for determining the mass of the disturbing planet and its longitude at epoch, if the eccentricity of the orbit were neglected.” (Adams, 1847:439). For the “ancient” observations, the eccentricity could not be neglected. He threw out Flamsteed’s 1690 observation as “unsafe”, and using the residuals for the other ancient observations produced another set of equations of condition that he could solve for the unknown planet’s eccentricity and longitude of the perihelion. By the same process of elimination as before, he reduced this set also to two equations. Combining these two equations to those arrived at from consideration of the modern observations alone, he arrived, after further juggling, from four to three equations, each containing the same set of three unknowns. These equations could be rapidly solved by approximation. Finding that the coefficients of one of the unknowns was small, he found the approximate values for the other two; he then repeated the process iteratively until he was able to satisfy all the equations, “as nearly as we please.” (Adams, 1847:444). This summary is hardly adequate and may not be easy to follow without Adams’s work at one’s elbow; however, it must suffice here, and will at any rate give some idea of the skill and tenacity with which Adams followed these elaborate calculations through to his result. The latter gave values of the mass and eccentricity

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assuming the mean distance of the perturber relative to that of Uranus was a/a’ = 0.5. All of this lies, we must say, like a figure behind a curtain, in back of the following almost innocuously simple statement, “To Challis I communicated the final values for the mass, heliocentric longitude, and elements of the orbit of the assumed planet. The same results, slightly corrected, I communicated in the following month to the Astronomer Royal.” (Adams, 1847:429). The details of Adams’s calculations are given in his November 1846 “Memoir” (Adams, 1847:427–459), of which the above is the merest outline. The reader is forewarned, however, that, as the classical perturbation theory on which it is based is now rather obsolete, it takes a specialist knowledge to follow it in detail. Adams assumed his reader to be this specialist, and so developed his whole argument with a minimum of explications and in the most economical, summary form. If one turns to Adams’s manuscripts for further elucidation of the difficult points, one finds the same problems writ even larger than in his “Memoir.” Sampson, who as one of Adams’s students knew better than anyone the way Adams worked, concluded after his long study of Adams’s manuscripts: It was a lifelong habit of Adams to mature his work in his mind before committing it to paper. As a result, his writings always contain the minimum of explanation… Scores of pages of this work consist of nothing but columns of figures, and these had, in the first instance, to be identified and put into order by a painful process of trial. The task was, moreover, complicated by two complete changes of notation, by the repeated revision and inclusion of fresh terms, by the use of centesimal seconds of arc in the manuscript and sexagesimal seconds in the Memoir, and by the fact some of the processes of the Memoir were afterthoughts, not actually solutions…. [B]ut when these obstacles to a thorough view are overcome, it is truly remarkable to see the essential directness with which this great work was carried to a conclusion, almost without a single error, a futile step, or a change of plan. (Sampson, 1904:146)

It is important to emphasise yet again that Adams accomplished all this virtually alone and unaided, in the margins of his time—mostly during the Long Vacation of 1845. Seen in those terms, it is an achievement that needs to be separated out from the hard fought but ultimately unimportant ‘priority’ debate, on which so much ink has been wasted. Clearly, it must stand as one of the most majestic accomplishments of the human intellect. The calculation just described was actually Adams’s fourth (IV, Table 4.1), and is dated in the manuscripts 18 September 1845. This he presented to Challis as a one-page summary, undated (to the historian, any undated document is problematic), which uses the phrase “New Planet” (used, as noted above, in his 10 July 1845 letter to George) and containing in Challis’s hand the annotation “Received in September 1845.” Except for the error in transcription, which gives for the mean longitude at the end of September 1845 the value 321°40′ instead of 321°30′, this is identical with a paper Sampson found in Adams’s manuscripts. The following is the Challis version (UCL): [Received in September 1845] My dear Sir The Elements of the New Planet I make to be as follows: –

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Mean Dist. Mean Long. at end of Sept’r Long. Per’n. Ecc’y. Mass (Sun’s mass = 1) Geoc. Long. at end of Sept’r d.m’s about 1′ per day

= = = = = =

38.4 321°30′ 321°30′ 0.1428 0.000173 320°30′

I remain, my dear Sir, Yours very truly, J.C. Adams

On receipt of this document, Challis does not seem to have expressed more than polite interest, nor did he seek to discuss the matter further or in great detail with its progenitor. Rather, he did as any overworked person without time to explore such an intricate matter would do, and suggested Adams would do well to run the business by someone less otherwise-preoccupied than himself. (He still had all those comet reductions on his hands after all.) Since Adams was just then about to go on his usual trip to Lidcot, it was decided he might stop by at Greenwich Hill and discuss the problem with the greatest expert on the Uranus problem in the realm, Airy. Whoever originated the idea, Challis, probably on the spot, furnished Adams with the following letter of introduction to Airy, and with letter in hand Adams, pilgrim-­ like, set out. Challis to Airy Cambridge Observatory 1845 Sep. 22 My dear Sir, My friend, Mr. Adams (who will probably deliver this note to you) has completed his calculations respecting the perturbation of the orbit of Uranus by a supposed ulterior planet, & has arrived at results, which he would be glad to communicate to you personally, if you could spare him a few moments of your valuable time. His calculations are formed on the observations you were so good as to furnish him with some time ago, & from his character as a mathematician & his practice in calculation, I should consider the deductions from his premises to be made in a most worthy manner. If he should not have the good fortune to see you at Greenwich he hopes to be allowed to write you on the subject. (RGO).

Now an aspect almost partaking of the theatre of the absurd enters in. Airy had never met Adams, and had scarcely heard of him. Adams, and anyone at Cambridge, regarded Airy as an icon. Adams was at the beginning of his career; the Astronomer Royal firmly established. The Astronomer Royal was a man of excellent memory, exceptional organisation skills, and almost unbelievable productivity, and he had accomplished all he did because of an iron will and inflexible routine. Even as an undergraduate at Trinity, he kept paper beside him so as to record every thought he had, and later had transferred everything to the more permanent organized form of books and diaries he kept. (It is owing to this ingrained habit of his of never destroying a document and devising a plan of easy reference to the huge bulk of his papers that we owe the Royal Observatory Neptune File, which contains many of the primary documents pertinent to the discovery of Neptune.) At Cambridge, he completely reformed the

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observatory and left it on a firm footing to his successor, Challis (together with an unsustainable work load). (Hutchins, 2008:88–89). From there, he had gone to the Royal Observatory, Greenwich, to commence his 45-year-long tenure as its seventh Astronomer Royal, and immediately began to reorganise, restaff, and reequip it in order to accomplish more efficiently what he believed to be its primary work: the observation, reduction and publication on a systematic basis the hundreds of thousands of precise positions of stars and Solar System bodies amassed by his hardworking staff, in service of the practical, utilitarian purpose for which the observatory had been founded: the improvement of the arts of navigation. This included not only astronomical observations but also providing a better understanding the tides, waves, terrestrial magnetism, and deviation of ships’ compasses on which navigation depended. In addition, the British government relied on him as its scientific consultant on such engineering matters as railroad gauges, London sewers, and lighthouse designs. Airy was from head to toe the quintessential Victorian, possessed of an enormous appetite for work and a colourless, Gradgrindian love for facts and what was eminently useful, practical. How did he manage to do all that he did? His son Wilfred Airy (1836–1925) gives at least part of the answer, The ruling feature of his character was undoubtedly Order. From the time he went up to Cambridge to the end of his life his system of order was strictly maintained. He wrote his autobiography up to date soon after he had taken his degree, and made his first will as soon as he had any money to leave. His accounts were perfectly kept by double entry throughout his life, and he valued extremely the order of book-keeping: this facility of keeping accounts was very useful to him. He seems not to have destroyed a document of any kind whatever: counterfoils of old cheque-books, notes for tradesmen, circulars, bills, and correspondence of all sorts were carefully preserved in the most complete order from the time that he went to Cambridge; and a huge mass they formed. To a high appreciation of order he attributed in a great degree his command of mathematics, and sometimes spoke of mathematics as nothing more than a system of order carried to a considerable extent. In everything he was methodical and orderly, and he had the greatest dread of disorder creeping into the routine work of the Observatory, even in the smallest matters…. His strict habits of order made him insist very much upon detail in his business with others, and the rigid discipline arising out of his system of order made his rule irksome to such of his subordinates as did not conform readily to it: but the efficiency of the Observatory unquestionably depended mainly upon it. As his powers failed with age the ruling passion for order assumed a greater prominence; and in his last days he seemed to be more anxious to put letters which he received into their proper place for reference than even to master their contents. (W.A. Airy, 1896:2–3)

And yet this was the man Adams proposed to drop in upon, unannounced and without appointment. As a plan, it was hardly promising. The only justifications for it were: 1) Adams’s complete and understandable naivete about such things and 2) sheer desperation, since he would not, once the Long Vacation ended and the next Michaelmas term began (in latter October), have more than a few scraps of time to devote to the new planet, given the need to be toiling with pupils and lectures again. (And this is in fact what happened.) From this point, then, the oft-told story unfolds, like Fate, with Adams indeed stopping in at Greenwich Hill, only to find Airy away. Creature of rigid routine that

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he was, Airy, like clockwork, invariably took a long working holiday on the Continent every year, during which he visited various savants and important institutions, and this year was no exception. (Chapman, 1988). From 25 August to 26 September, he was in France, traveling with his sister and sister-in-law, and investigating (among other things) the breakwater creating the artificial harbour at Cherbourg. He went on this vacation even though his wife, Richarda (1804–1875), was eight months pregnant. (Though this might have been typical in the 19th century, it still seems remarkable in the 21st century that Airy would have been away at such a time, and possibly speaks volumes for the inflexibility of his routine.) Richarda would afterwards remember seeing Adams’s card “brought into the room.” (That, however, was from a later visit.) Indeed, this first visit by Adams seems to have left no impression at all, prompting historian Allan Chapman’s remark (1988:43), “If Adams was not an arresting or persistent man—as all evidence suggests that he was not—it explains many things.” On this outward-bound visit, Adams dropped off the letter of introduction from Challis, then headed on to Cornwall. He must have missed Airy by only a day or two. The moment Airy returned to Greenwich, he found the letter Adams had dropped off, and wrote at once to Challis suggesting that Adams write to him. (Airy to Challis, 29 September 1845; RGO). Adams did not write, but instead, on his inward-bound trip from Cornwall back to Cambridge, on 21 October 1845, he again dropped by Greenwich Hill. He did so not once but twice that day. He took a risk. It was one worth taking. At Cambridge even a generation later, it was “not always safe to call on a man who is a year above you,” and “men call on one another and leave cards, instead of just running in whenever you want to see a fellow.” (Gifford, 2005:70).) But this was not an upper classman; this was the Astronomer Royal, whom, remember, Adams had never met. Keep in mind, however, that Adams had Challis’s letter of introduction, and with this in hand, Adams could expect to be warmly received—if only time had been on his side. Of course, often timing is everything, and this time timing could hardly be worse. All three men—Adams, Challis, and Airy—were working close to the ends of their tethers. Adams, moreover, had only a narrow window of opportunity to break through, as he would soon be back to lectures and tutorials. Challis was buried in the reductions of an oppression of comets (though so self-absorbed was Adams that he found it surprising that after receiving his unsolicited present of elements and position Challis did not immediately drop all of his other duties and priorities and start a search for the putative planet at once). As for Airy…. Airy was never idle, but at this particular moment was probably as busy as he had ever been. Based on a careful study of Airy’s public and private duties, Chapman points out that, in addition to his usual astronomical responsibilities, Airy had been running all around England for much of the previous year as the Scientific Commissioner of the Rail Gauge Commission, testing trains and interviewing engineers. At the time of Adams’s first visit, Airy had concerns with Richarda’s pregnancy. At 42, she was about give birth to their ninth child, Osmund, who would be born precisely a week after Adams’s second visit. “Now Richarda Airy’s private correspondence makes it clear that her pregnancies had all been difficult and

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dangerous,” Chapman writes (1996:43) “and one suspects that, considering the social delicacy of childbirth in Victorian England, a sudden stranger enthused with a mathematical problem was not especially desirable at the family dinner table.” As if this were not enough, at almost the moment Richarda was going into labour, another difficult situation broke upon Airy and his Chief Assistant the Rev. Robert Main (1808–1878). This involved William Richardson (1796–1872), a long-time assistant of Airy and erstwhile Royal Astronomical Society Gold Medalist. Richardson was interviewed by Airy on 27 October on what was described as a “very serious charge of incest” involving Richardson and his daughter. (Airy, journal, 27 October 1845; RGO). This would lead to Richardson’s suspension from his office and dismissal three days later by order of the Admiralty. It also meant that Airy and Main were down one assistant (Fig. 4.11). Knowing none of these contexts, Adams, returning from Cornwall—and stretching his Long Vacation as far as he conscientiously could—called at Greenwich Hill again, twice in one day. The first time was early in the morning. Airy, as part of the invariable system he had no doubt learned at Cambridge, was out on a walk. It was presumably then, rather than at the earlier visit as she misremembered, that Richarda noted Adams’s card being brought in. Adams seems to have mentioned he planned to come again later, though whether this made an impression or not is hard to know. He did not, however, make an appointment. Instead, he returned late that afternoon, only to find the Airys at dinner. (According to a system as invariable as clockwork, they ate punctually at 3:30 in the afternoon, which seems to have been an anomaly even in Victorian times.) Adams was turned away, presumably by the butler. He now

Fig. 4.11  Flamsteed House, atop Greenwich Hill. This idyllic view shows the scene in 1824; the Rev. John Pond was Astronomer Royal, whose most notable achievement was the installation of the time ball on the roof of the observatory in 1833 (Public Domain)

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dropped into the letter box a summary of his results, presumably drafted in advance against just such an eventuality. It was undated; Airy himself later added “October 1845” to what is presumed to be this document. With nothing more to be done, he scurried off, being already under time pressure to get back to Cambridge. Here is the document received by Airy (RGO): J.C. Adams, Esq. to G. B. Airy [October 1845] According to my calculations, the observed irregularities in the motion of Uranus may be accounted for by supposing the existence of an exterior planet, the mass and orbit of which are as follows:— Mean Dist. (assumed nearly in accordance with Bode’s law) = 0. 38.4 Mean Sidereal Motion in 365.25 days = 1°30’9″ Mean Long., 1st October 1845. = 323°34′ Longitude of Perihelion = 315°57′ Eccentricity = 0.1610 Mass (that of the Sun being unity) = 0.0001656 For the modern observations I have used the method of normal places, taking the mean of the tabular errors, as given by observations near three consecutive oppositions, to correspond with the mean of the times; and the Greenwich observations have been used down to 1830; since which, the Cambridge and Greenwich observations and those given in the Astronomische Nachrichten have been made use of. The following are the remaining errors of mean longitude:— Observation – Theory

1780 1783 1786 1789 1792 1795 1798

″ +0.27 −0.23 −0.96 +1.82 −0.91 +0.09 −0.99

1801 1804 1807 1810 1813 1816 1819

″ −0.04 +1.76 −0.21 +0.56 −0.94 −0.31 −2.00

1822 1825 1828 1831 1834 1837 1840

+0.30 +1.92 +2.25 −1.06 −1.44 −1.62 +1.73

The error for 1780 is concluded from that of 1781 given by observation, compared with those of four or five following years, and also with Lemonnier’s observations in 1769 and 1771. For the ancient observations, the following are the remaining errors:—

1690 1712 1715

″ +44.4 + 6.7 − 6.8

1750 1753 1756

″ −1.6 +5.7 − 4.0

1763 1769 1771

″ − 5.1 + 0.6 +11.8

The errors are small, except for Flamsteed’s observation of 1690. This being an isolated observation, very distant from the rest, I thought it best not to use it in forming the equations of condition. It is not improbable, however, that this error might be destroyed by a small change in the assumed mean motion of the planet.

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This, then, was the famous Hyp I, the “slightly revised” version of the one Challis had received. It was the fifth calculation Adams had made—and the last but one. (See Table 4.1). After the discovery of the planet in September 1846, Adams would publish this and the much reworked solution finished at the end of 1846, Hyp II, his sixth and last. The latter will be discussed in Chap. 7. Arriving back at Cambridge, Adams provided a few additional details about the ill-starred visit to Greenwich in a letter to his parents I just drop a line to tell you of my safe arrival at Cambridge. Father saw that the coach was pretty well laden when I got up, & on arriving at Launceston I found I must take an inside place, the outsides being all taken; I was not sorry for this, however, as the night was rather cold. Nothing worth mentioning occurred on my journey to London, where I arrived, luggage & all, very early on Thursday morning. After waiting till about 2 a.m. for Thomas, I left for Greenwich to call on Mr Airy, who was unfortunately not at home. I left a note for him, however, containing a short statement of the results at which I had arrived, & returned to London, where I found Thomas, who did not come before he had done his day’s work. He is looking very well. We stopped together until half past 7 when he returned to Richmond & not long after I started on the last train to Cambridge where I arrived rather tired. I shall commence lectures tomorrow morning; most of our other lecturers have commenced already, so that I am rather late. … With kindest love to my dear Brothers & sisters & all our family & friends I remain Your affectionate Son J.C. Adams (JCA to parents, 23 October 1845; McA 35:2)

4.9  A Place in the Sky According to Adams’s later account (Adams, 1847:429), the document presented to Airy was a “slightly corrected” version of the Challis document. The two solutions give geocentric longitudes of 320°30′ (for the end of September 1845) and 323°34′ for 1 October 1845, respectively, for the unknown planet. (Neptune actually lay a very discoverable +2.1° and −1.9°, respectively, away.) (Sampson, 1904:152). A more careful comparison of the two solutions, however, shows that they actually give markedly different values for the elements of the unknown planet. (See IV and V, Table 4.1.) The Challis calculation gives a value of the eccentricity of 0.1428, the Airy document of 0.1610. They also contain different values of the position of the apse, the mass, and the longitude. The reason for this was that the first time through Adams had made one of his rare errors, and though a “trifling” one, it required Adams to recompute the whole problem from that point forward, with very different results. (Sampson, 1904:151). According to Baltimore astronomy analyst Dennis Rawlins (1992:13), “Adams worked slowly and cautiously…. It is on the face of it unlikely that, in less than a month … he performed all the necessary calculations that would turn up his term-­ sign miscue—and then recomputed and re-checked this lengthy solution, to his notoriously-perfectionist satisfaction—certain that no more such errors lurked.”

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However, far from being a slow and cautious worker, Adams was (Sampson, 1904:145), “a rapid worker when his work was coming to a point, and then and always was able to carry through fabulous computations without error.” It was well within his power. Moreover, since we cannot know what result Adams might have presented to Airy the first time he stopped by, it is very likely that he continued to work on and refine the computations at Lidcot. According to his brother George: At home much of John’s time … was [always] occupied on long calculations. In the family he was always most cheerful and happy and he thoroughly enjoyed the country life; but he was never idle and did a large part of the work connected with his great problem[s] at home. Night after night I have sat up with him in our little parlour at Lidcot, when all the rest had gone to bed, looking over his shoulder, seeing that he copied, added and subtracted his figures correctly, to save his doing it twice over. On those occasions dear Mother, who would be exhausted with her heavy work, before going to bed would prepare the milk and bread for us for supper before retiring. This I would warm when required and we take together…. Often have I been tired, and said, “It’s time to go to bed, John.” His reply would be, “In a minute,” and go on almost unconscious of anything but his calculations. In his walks on those occasions on Laneast Down, often with me, his mind would be fully occupied in his work. I might call his attention to some object and get a reply, but he would again relapse into his calculations. (Harrison, 1994:20)

Adams no doubt deeply regretted his near miss at Greenwich, and later told Airy, “I was … much pained at not having been able to see you when I called at the Royal Observatory the second time, as I felt that the whole matter might be better explained by half an hour’s conversation than by several letters, in writing which I have always experienced a strange difficulty.” (Adams to Airy, 18 November 1846; McA 33:8) He was confident of his calculations, and could have shown Airy the basis for this confidence, as he explained in the same letter: “Of the accuracy of my calculations I was quite sure, from the care with which they were made, and the number of times I had examined them. The only point which appeared to admit of any doubt was the assumption as to the mean distance, and this I soon proceeded to correct.” Finding, at the end of the Hyp I calculation, “the eccentricity coming out much larger than was probable, and later observations shewing that the theory founded on the first hypothesis as to the mean distance was still sensibly in error, I afterwards repeated my investigation, supposing the mean distance to be about 1/30th part less than before.” (Adams, 1847:429). (This recalculation was to lead to Hyp II. See Chap. 7.) He believed that by placing the results of his calculations in Challis’s and now Airy’s hands, he had effectively “published them,” confiding to Adam Sedgwick (Sedgwick to Richarda Airy, 6 December 1846; McA 33:10), “I did think that the Astronomer Royal would have communicated my results among his correspondents—I took all that for granted & I thought it a publication—etc, etc.” So it all seemed looking back, from the “Land of Might Have Been.”

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4.10  The Radius Vector and a Year Without a Conclusion Though rationally Adams should have known that, in not calling by appointment and only taking his chance on seeing Airy, failure was almost vouchsafed, as usual he seems to have been so self-absorbed that he hardly seems to have anticipated such an outcome. As he later told Sedgwick, “I thought that though [Airy] had been at dinner he would send me a message, or perhaps spoken a word or two to me: but I am now convinced that in fact he never knew of my second call—that the servant had not delivered my message with my card.” (Sedgwick to Richarda Airy, 6 December 1846; McA 33:10). Though Adams would deny that he ever felt he had been snubbed, Airy, creature of rigid routine that he always was, did, in fact, as usual, follow up. Despite Richarda’s giving birth on October 28 and also, the day before, facing the beginning of a major distraction with Richardson’s being taken into custody (Chapman, 1996:43), Airy nevertheless “did his duty.” He wrote to Adams on 5 November 1845: Sir, I am very much obliged by the paper of results which you left here a few days since, showing the perturbations [sic] on the place of Uranus produced by a planet with certain assumed elements.—The latter numbers are all extremely interesting: I am not enough acquainted with Flamsteed’s observations about 1690 to say whether they have such an error, but I think it extremely probable. But I should be very glad to know whether the assumed perturbations will explain the error of radius vector of Uranus. This error is now very considerable as you will be able to ascertain by comparing the normal equations, given in the Greenwich Observations each year, for the time before opposition with the times after opposition. I am, Sir, Your very faithful servant, G.B. Airy (Airy to Adams, 5 November 1845; JCA 1.1.1)

The errors in the radius vector, which he himself had discovered and published on several times over the years, had long been a matter of the keenest importance to Airy. (See Chap. 2). He later, famously, referred to it as the experimentum crucis, and recalled (Airy, 1847:396) how he “waited with much anxiety for Mr. Adams’s answer to my query. Had it been in the affirmative, I should at once have exerted all the influence which I might possess, either directly, or indirectly through my friend Professor Challis, to procure the publication of Mr. Adams’s theory.” That publication, in turn, might well have led to the discovery of the planet. At the height of the storm of public abuse to which he was personally subjected after the planet had been discovered, Airy said: The history of this discovery shews that, in certain cases, it is advantageous for the progress of science that the publication of theories, when so far matured as to leave no doubt of their general accuracy, should not be delayed till they are worked to the highest imaginable perfection. It appears to be quite within probability, that a publication of the elements obtained in October 1845 might have led to discovery of the planet in November 1845. (Airy, 1847:414).

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There is no reason to doubt Airy on this. However, what happened was that Airy waited, and waited in vain. No letter from Adams ever came. This curious silence at the critical moment would be, in retrospect, one of the hinges on which the whole matter would turn, for, as Airy rather ruefully admitted after the planet had been revealed (Airy to Challis, 21 December 1846; McA:33.13), “Adams’s silence was so far unfortunate that it interposed an effectual barrier to all further communication. It was clearly impossible for me to write to him again.” For his part, Adams, though obviously rather embarrassed about his failure to write, seems to have struggled to grasp just why Airy believed the radius vector problem to be so important. As late as 1883 he gave his private assessment to James Whitbread Lee Glaisher (1848–1928) saying that he considered the Astronomer Royal’s query “trivial.” (Smart, 1947:25). As usual, things were rather more complicated than that. It was clear that the tabular radius vector would be increased by any theory which represented the motion in longitude. If, as Adams believed without doubt, a perturbing planet existed, then by simply correcting the elliptic elements so as to satisfy the modern observations as nearly as possible, without taking into account any additional perturbations, Adams was also sure that the corresponding increase in the radius vector would not be very different from that given by his theory. Airy, however, approaching the problem from a more general perspective, did not share this conclusion. He did not doubt the inverse-square law, as is sometimes suggested—no respectable astronomer in England or elsewhere did so by this time—but he did not a priori accept the planet explanation. But the correction of the radius vector was something that had to be demonstrated. After the discovery, Sir John Herschel summarised the difference in positions in a letter to William Whewell (1794–1866): Until the planet was actually seen and shewn to be a planet—there was no discovery…. Certain motions have to be accounted for. A and B unknown to each other frame an hypothesis to account for it as do also C D E & F…. C D E & F however though prior to A, B, in time, do not pursue the hypotheses into consequences—A does— earlier than B—B does subsequently to A but more thoroughly—goes more into detail—finds it satisfies all minutiae. Assuredly A has gone a certain way to establish the hypothesis… But before it can be received as an established hypothesis and called a discovery there is yet a step—it has to be tried whether other rival hypotheses will not do as well or better—till this has been done … there is no discovery…. (John Herschel to William Whewell, 29 December 1846; RS)

In other words, it was not enough to show that a hypothesis, such as that of the planet, might be true; it was also necessary to show that rival hypotheses could not explain the set of known facts as well or better. Agreed, Adams had satisfied the errors in longitude; “it is presumably enough,” Herschel continued, “it might or would have satisfied the radius vector—he explained half the phenomenon—he presumably could and almost certainly, had time been allowed, would have satisfied the other half.” Once again, time was the factor. No sooner had Adams returned to Cambridge after his unsuccessful Greenwich venture but he was caught up again in the usual swirl of duties. In addition, there was another difficulty, which Challis was

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afterwards to raise in Adams’s defense. “I know,” Challis informed Airy in the awkward aftermath of the planet’s discovery, that [Adams] is extremely tardy about writing, & that he pleaded guilty to this fault. He experiences also a difficulty, which all young writers feel more or less, in putting into shape and order, what he has done, and well done, so as to convey an adequate idea of it to other by writing, after receiving your questions it occurred to him that it would be well for him to send you a full account of his methods of calculation, & that he might send the answer at the same time. I believe that nothing but procrastination in fulfilling this intention, was the reason of his not sending an answer at all. (Challis to Airy, 19 December 1846; McA:33:12).

The procrastination was not absolute. This is shown by the fact that Adams did begin a letter, in response to Airy. This draft letter was discovered only in June 2004 by the late historian of astronomy Craig B. Waff, during a visit to the Adams papers in the Cornwall Records Office. Never before published, and of obvious importance, we quote it here in full: St. John’s Coll. Cambridge Novr. 13th. 1845 Sir I must apologise for having called at the observatory the other day at so unreasonable an hour, the reason was that I had only arrived in town that morning & it was necessary for me to be in Cambridge the same day, so that I had no other opportunity. The paper I then left contained merely a statement of the results of my calculation; I will now, if you will allow me trouble you with a short sketch of the method used in obtaining them. My attention was first directed to the anomalies in the motion of Uranus by reading, some time since, your valuable Report on Astronomy. If the action of known planets really proved insufficient to account for the perturbations of Uranus, it appeared to me that by far the most probably hypothesis which could be formed for that purpose, would be that of the existence of an undiscovered planet beyond. If this were the case, I conceived it might be possible to find from an examination of the observed perturbations, the approximate position of the new planet, so as to assist Astronomers in discovering it. The solution of this problem, however, would be clearly impracticable at present, without making some assumption as to the mean distance. Fortunately, Bode’s law supplies us with a value which, at any rate, has a claim to be first tried. Accordingly, using the differences between Obsn. and the Tables given in Bouvard’s Equations of condition as far as they extend, and subsequently those supplied by the Cambridge and Greenwich observns. Together with those in the various numbers of the Astron. Nachrichten, I obtained values of the mass and position of the assumed planet, so as to satisfy all the observations very nearly. The series however, not being sufficiently continuous at one or two points, Professor Challis had the kindness to request you to communicate the results of a few of the Greenwich obsns, to supply the deficiency. For your kindness in sending the whole of the Greenwich obsns. of Uranus, I beg you will accept my warmest thanks. On receiving them, I determined to make them the base of a new investigation, taking into account several quantities which I had previously neglected. The later observations used were the same as before. (Adams to Airy, 13 November 1845:AM)

Adams seems to be struggling to get the right tone here. He begins with diffidence and apologies; “I must apologise”, “I will, if you will allow me, trouble you.” As he presses on, he may have begun to sense a certain ludicrousness in his position: he is writing to the Astronomer Royal on matters pertaining to which the latter was the acknowledged expert. He was struggling to find the right balance between not

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sounding as if he was tutoring or speaking down to Airy on fundamental matters while at the same time not sounding too obsequious or ingratiating. He did not quite succeed. His justification of the distance from Bode’s law seems particularly strained. He admits that a solution was “impracticable” without such an assumption but then argues weakly for the one he adopted, saying only that it has “at any rate … a claim to be tried first.” Even at the time he seems to have been uncomfortable with the Bode’s law distance as it produced an unacceptably high value of the eccentricity, and later, during the summer of 1846, he would attempt to reduce the mean distance in order to correct the eccentricity only to find himself moving closer and closer to the 2:1 resonance position with Uranus, at which point his calculations would break down. After continuing for a few more lines “begging” Airy to thank him for observations, he drops the painful act of composition abruptly. He no doubt hoped to get back to it one day, but it was not to be. It seems, however, that the radius vector problem nevertheless continued to nag at least a little, for among eight dated computations, from the autumn of 1845 until the planet’s discovery, found among Adams’s manuscripts and studied by Sampson, fully three concern the radius vector. This suggests that Adams was indeed trying to solve the “other half” not discharged, in the words of Sir John Herschel. There are indeed two pages, dated 28 November and 24 December, entitled “Additional perturbation of the log of Radius Vector,” where Adams sets up the relevant integral but does not attempt to solve it. (Sampson, 1904:168). A long computation would have been needed, but if it had succeeded, it certainly would have won Airy over. Perhaps Adams’s non-reply simply means that he did not have a good answer for Airy. The embarrassment of Adams’s failure to reply to Airy—which was, to put it into modern terms, rather like the failure of the author of a scientific paper to respond to the comments of a referee—continued after the discovery and indeed for the rest of Adams’s and Airy’s lives. Adams would attempt—always in his characteristically courteous and generous manner—to clarify the situation in various statements over the years (none of which are, however, exactly models of lucidity). In response to these, Airy seems to have remained unmoved. Two months after the discovery of Neptune, he wrote to Challis, by then a champion of Adams’s position: There were two things to be explained, which might have existed independently of the other, and of which one could be ascertained independently of the other: viz., error of longitude and error of radius vector. And there is no a priori reason for thinking that a hypothesis which will explain the error of longitude will also explain the error of radius vector. If, after Adams had satisfactorily explained the error of longitude, he had (with the numerical values of the elements of the two planets so found) converted his formula for perturbation of radius vector into numbers, and if these numbers had been discordant with the observed numbers of discordance of radius vector, then the theory would have been false, not from any error of Adams’, but from a failure in the law of gravitation. On this question therefore turned the continuance or fall of the law of gravitation. This, it appears to me … was a question of vast importance.

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The progress of science almost always depends on questions of this kind. Thus, in chemistry, the Phlogistic theory1 explained the concurring facts of oxidation of metals and vitiation of air, or gaseous formation in water. But did it also account for the weight of the material? No. Then it was false.—Laplace’s notion of forces gave an explanation of the course of extraordinary pencils of light. But did it or could it give an explanation also of the separation of pencils and of their polarization No. Then it was false. The theory of gravitation might have been in the same predicament with regard to Uranus. Adams’ answer would have made this satisfactory…. (Airy to Challis, 21 December 1846; McA:33:17)

Someday a dissertation, or a major paper, will likely be written about the radius vector problem. In the meantime, there are hints about some of the questions an investigation would need to take. Since Adams started with Bouvard’s inequalities (in contrast to Le Verrier, who worked the perturbation function from scratch), he likely would have found it difficult to justify the radius vector as well as the longitude since he had not redone all of the work as Le Verrier had. Adams’s belief that once the longitude was satisfied, the radius vector necessarily has to be satisfied as well was precisely what Airy was asking him to demonstrate. It might be true, of course, but it could not be taken on faith. No matter how great the confidence in the inverse-square law, the specific result regarding the radius vector had to be demonstrated—this was Airy’s position. It was perhaps similar to the case, in an earlier day (see Chap. 1), involving the rate of advance of the lunar apsides, which Newton’s calculation had made just half the true value. This, too, had had to be demonstrated—it could not be taken on faith. When it was—by Clairaut, who took into account a term in an additional perturbation term that Newton had thought negligible and had chosen to ignore—it was a great triumph for perturbation theory and the universal law of gravitation. Adams also neglected a small term which proved to be more significant than he realised. As shown by the Cambridge mathematician J.E. Littlewood (1885–1977), Adams’s transformation from the true longitude to the mean one, though it certainly makes the problem better behaved mathematically, was not, strictly speaking, quite correct. According to Littlewood (Littlewood, 1957:131–132), “On the crucial theoretical point about the [angular motion] Adams was dead wrong…. This is obvious from the point of view of school mathematics…. Adams’s point of view was consistently perturbation theory, but even so, and granted that even a Senior Wrangler type can make a slip, it is an odd slip to make. The numerical confirmation [of the radius vector result by Adams] is a last touch of comedy. In “small contribution” “small” means small in the sense that 15 per cent. is small, not “very small” (Adams’s word). But in any case this “smallness” is an accident of the numerical constants; e.g. it does not happen for a “distant” N[eptune].” 1  Briefly, phlogistic theory postulated that a fire-like element called phlogiston is contained within combustible bodies and released during combustion. However, as observed by the French chemist Antoine-Laurent de Lavoisier (1743—1794) and others in experiments using closed vessels, during combustion some materials gained mass when they were burned, even though they were supposed to have lost phlogiston. These observations and the discovery of oxygen led to the abandonment of the theory by the end of the eighteenth century.

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It follows that Airy had been right on this point. However, the real problem was, as Littlewood intimates, perturbation theory itself, which was the element that Adams and Le Verrier breathed. As the French mathematician Guy Bertrand has written (personal communication, 20 August 2019): The problem in Le Verrier’s and Adams’s research lies in the method. They both use analytical expressions for the perturbations created by an outside planet. But in order to use these expressions, one needs to fix a value for the distance of the perturbing planet (the expressions for the Laplace coefficients that appear in the analytical expressions are quite simply too complex to be used analytically. They both choose twice the distance of Uranus, and then proceeded to minimize the residuals of the longitude of Uranus. The same problems would, however, have entered in if, instead of minimizing the residuals in longitude, they had tried to minimize the errors in radius vector. Again, they would have had to choose an initial value for the ratio of the mean distance, and then interpolated in the neighborhood of this choice.

It turns out, ironically, that the value that was so keenly sought—the longitude of Neptune during the decades on either side of conjunction with Uranus in 1822— does not depend very much on the ratio of the mean distances after all. Thus, even the grossly schematized “first calculation” of Adams, assuming circular orbits, zero inclination, etc.—got the correct position in longitude to within 16 or 17 degrees. On the other hand, it did not follow that much more complicated workings through of the problem such as Hyp I of September 1845, still less his heroic summer 1846 Hyp II calculation (see Chap. 7) resulted in as much of an improvement as one might expect. Indeed, by reducing the mean distance as Adams did (in order to reduce the eccentricity), Adams’s positions veered increasingly wide of the mark. The reason for this, we now know, is that the closer one got to using the actual mean distance of Neptune, the more complicated the problem becomes, as a large number of second-order terms that had been ignored in the interests of simplifying the calculation now have to be included in order to satisfy the observations. In retrospect—and indeed, these issues were not grasped in the mid-19th century, or in fact for many decades after 1846—the uncanny accuracy of both Adams’s Hyp I and Le Verrier’s positions (including the famous 1-degree precision of the latter, which made such an impression at the time) were actually largely matters of chance. According to C.J. Brookes (Brookes, 1970:70), who repeated Adams’s calculations for his dissertation, “the approximate position of the planet could have been predicted with sufficient accuracy to lead to its discovery, for any value of the mean distance between 30 AU and 40 AU.” One further point: other values, such as the eccentricity, longitude of the perihelion, etc., were out of reach of Adams’s and Le Verrier’s methods, being too dependent on the initial choice of the ratio of the mean distances. (Guy Bertrand, personal communication; 20 August 2019). Thus would arise the “happy accident” controversy following the discovery, when it was realised that, though indeed Adams and Le Verrier got close to the value of the longitude, their orbits bore virtually no resemblance at all to the actual orbit of Neptune. But these are undoubtedly abstruse matters, and we have run rather far ahead of ourselves here.

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4.11  The Ball Is Dropped, and a Comet Splits The great year 1845 ended with Adams’s inconclusive radius vector calculations and, more momentously, with the discovery of a planet—not one outside the orbit of Uranus, but one between the orbits of Mars and Jupiter. This was the asteroid Astraea, discovered on 8 December 1845 by Karl Ludwig Hencke (1793–1866), amateur astronomer and postmaster at Dressen, Germany, who had been trying to spot Vesta with a small telescope when he stumbled upon the stranger. (Baum and Sheehan, 1997:61). At Cambridge the quest for the outer planet went strangely quiet. Adams once more found himself buried in his duties of lectures and tutoring, though in January 1846, an astronomical event took place which he could not ignore. The head of Biela’s comet split in two—something that had never happened before in the history of astronomy. Sir John Herschel called it (Herschel, 1849:359) “a phenomenon which struck every astronomer with amazement.” (Fig. 4.12) The comet had been first recorded in 1772 by the French comet-hunters Jacques Leibax Montaigne (1716–c1785) and Charles Messier, and recorded again, though without being recognised as the same comet, in 1805 by the prolific comet-­discoverer Jean-Louis Pons (1761–1831) of the Marseille Observatory. Not until 1826, when Wilhelm von Biela (1782–1856), an army officer serving at the fortress town of Josefstadt, Austria, discovered it again and worked out its orbit, was it recognised to be a periodic Jupiter-family comet. With a period of 6.6 years, its next return was predicted for 1832. It was duly recovered by John Herschel. (It thus became only the third comet—after Halley’s and Encke’s—to have a return successfully predicted.) The comet was seen again at its next return in 1839, but conditions for its observation were extremely unfavourable. Conditions were far better at its next predicted return, and indeed it was recovered again in November 1845 by de Vico. The first to

Fig. 4.12  The two fragments of Biela’s comet shown in February 1846, soon after it had split into two pieces (Credit: E. Weiß, Bilderatlas der Sternwelt, Vienna, J.F. Schrieber, 1888)

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recognise that it had split in two was Matthew Fontaine Maury (1806–1873), the Superintendent of the U.S. Naval Observatory, using the 9.6-inch equatorial, on 14 January 1846. Afterward the comet was kept under extensive surveillance, including by Challis at the Cambridge University Observatory, the two fragments (later designated Comet A and Comet B) strangely alternating in brightness and developing parallel tails as they approached perihelion. (Kronk, 1984:222). Adams was among those who, in Herschel’s phrase, were “struck with amazement” by this strangely fissile object. Already established as a skillful computer of comet orbits, at the end of February he was exchanging letters about the “two Bielas” with the up-and-coming observer John Russell Hind (1823–1895). (Hind to Adams, 28 February 1846; JC). Hind had recently left a position at the Royal Observatory, Greenwich, under Airy to succeed the Rev. W.R. Dawes (1799–1868) as director of George Bishop’s private observatory at Regent’s Park, London. As he had done with de Vico’s comet, Adams gave an exhibition of his skill by solving (Adams, diary entry, 9 March 1846; JC), “the equations for determining the relative distance and velocities of the two heads of Biela’s comet.” He communicated the results to Challis in time for the latter to present that same evening to the Cambridge Philosophical meeting, of which Challis was president at the time. Meanwhile Challis supplied Adams with the latest data while Adams reworked his calculations, and during this period of collaboration, on the night of 10 March, Adams visited the Observatory to view the comet. Perhaps this flurry of extracurricular activity, combined with the usual fatiguing rounds of lecturing and tutoring, took a toll on Adams, for a few days later he records in his diary (14 March 1846; JC), “Did not do much today, feeling rather tired. Walked as far as Observatory with Challis & then went to the Courts.” (By Courts, of course, are meant the three Courts of St. John’s: First Court, Second Court, and New Court.) That same day Challis communicated to the Royal Astronomical Society a brief paper by Adams, “The Relative Position of the Two Heads of Biela’s Comet.” (Adams, 1846:83). At this time, Adams was also making some calculations for Challis on the differential refraction effects for two objects observed near each other in the Northumberland equatorial, similar to those Newton had once done for Flamsteed (Fig. 4.13). And yet, despite the closeness of collaboration between the two men, the author has been unable to discern any evidence of a revival of interest in Uranus, evidence that Adams ever discussed the matter further with Challis. Perhaps this is not so surprising. Challis had a great deal on his plate, and with other priorities seems not to have been particularly interested. For his part, Adams seems to have regarded the Uranus problem as his particular domain. It was neither more nor less important than calculating comet orbits. But at any rate he seems not to have felt any urgency about it. Why should he? To this point, at least, he had every reason to believe that he had the problem to himself. In that, however, he was mistaken. As Airy, who was very much aware of what was going on in Paris but neither Challis nor Adams would have been aware, the problem was actively being pursued by the same man who had beaten Adams into print with the orbit for de Vico’s comet.

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Fig. 4.13  Dome of the Northumberland refractor, as it would have appeared about the time of Challis’s search (G.F. Chambers, Pictorial Astronomy (1905); credit: Truro High School for Girls—Reprographics Dept.)

Indeed, on 10 November 1845, at just about the exact moment Adams was sitting down to pen his abortive reply to Airy about the radius vector, Urbain Jean Joseph Le Verrier was presenting to the Académie des sciences his masterly treatise Premier mémoire sur la théorie d’Uranus. With the assistance of his pupil Émile Gautier (1822–1891), who went over all the calculations in duplicate (“A.R.”, 1891), he examined carefully all the Uranus observations up to 1845, repeated Alexis Bouvard’s calculations, and discovered certain perturbation terms that had been neglected, as well as several outright errors. This required him to redo parts of the calculation. Even so, discrepancies in the motion of Uranus remained, and after considering all the alternatives to account for them—the possibility the inverse square law of gravitation did not hold exactly at such great distances from the Sun, the existence of a resisting medium of some kind, and the perturbing effects of a planet beyond Uranus—he concluded that the last was the most plausible. (See Chap. 5). He at once set himself to tackle the same problem that Adams had been working on for two years, using the discrepancies in the motion of Uranus to work the perturbation problem in reverse, and to find, if possible, the elements of the perturbing body as well as its position in the sky. Thus, with Le Verrier launching a parallel investigation to that of Adams, we have arrived at the point where we must leave Cambridge and cross the Channel (or La Manche as it is known in France) for Paris, and introduce Le Verrier, this man who keeps jumping the queue (la queue).

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Acknowledgements  A great many discussions were held on these topics over the years, and the author wishes to express appreciation to David Dewhirst, James Lequeux, Craig B. Waff, Brian Sheen, Roger Hutchins, Robert W. Smith, Davor Krajnović, and Trudy E. Bell. Kathryn McKee, in Special Collections at St. John’s College, Cambridge, and Adam Perkins, Cambridge University Library, have been most helpful in tracking down original documents during visits to their institutions. Guy Bertrand, Jacques Laskar, Davor Krajnović, and Kenneth Young have provided many technical insights into perturbation theory and the details of Adams’s calculations.

Appendix 4.1 A Brief History of Mathematics at Cambridge Though the Tripos did not encourage “original synthesis, analysis or a developing fountain of knowledge,” and even the present-day Cambridge University Mathematics Department (Johnstone, 2016) admits on its web page that “it may be doubted that a system in which the best students spent three years training to solve problems against the clock represented the ideal way to teach mathematics,” for all its flaws, “the Mathematical Tripos offered the best mathematical foundations for the advancement of physical science available in Britain.” (Smith and Wise 1989:57). According to Bristed (1852:95–96), the top wranglers had to be “remarkably clever,” show “considerable industry,” and “read mathematics professionally”. During the time Adams was there, this system had already succeeded in becoming “the nursery for the great flowering of British physics in the 19th century.” (Johnstone, 2016). To take only a few examples, the two Senior Wranglers ahead of Adams were George Gabriel Stokes (1841) and Arthur Cayley (1842), while the Second Wrangler in 1845, the year of Stephen Parkinson’s (1823–1889) success, was William Thomson (1824–1907), later Lord Kelvin, all of whom went on to great distinction in mathematics or mathematical science. However, it must be emphasized that Cambridge was not, at the time, about research; good research was admired, but the 19th century Cambridge system concentrated on undergraduate teaching, and research “was not viewed as a professional duty and the university was not expected to provide support for it.” (Johnson, 2016). During the time Adams was at Cambridge, the quality of mathematics instruction was first-rate. But this had not always been the case. Throughout the 18th century, it had a been a rather lacklustre affair. This was at least in part owing to the paralysing effects of the near-idolatry in which Isaac Newton was held. The spectacular success of the British hero had had “the fortunate effect of establishing the prestige of mathematics in Britain and Cambridge and the unfortunate effect of blinding British mathematicians to the progress in mathematics elsewhere.” (Johnstone, 2016). While British mathematics lagged, mathematics on the Continent (as described in Chap. 1) had made rapid progress, owing in good part to the adoption by mathematicians such as Euler, Clairaut, d’Alembert, Lagrange and Laplace of the Leibnizian notation dy/dx for differentials instead of the Newtonian notation of dots for fluxions. The methods of course are at bottom equivalent but the Leibnizian form of notation is much more suggestive and convenient to use.

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The standard mathematics texts used at Cambridge in the 18th century and into the nineteenth consisted of a series of sturdy volumes produced between 1795 and 1799 by the Rev. James Wood (1820–1901), President and Master of St. John’s until his death in 1839, and Samuel Vince (1749–1821), Plumian professor of Astronomy and Experimental Philosophy at Cambridge from 1796 until his death in 1821, and known collectively under the title The principles of mathematics and natural philosophy. Needless to say, in all these volumes Wood and Vince used the Newtonian notations wherever later writers would have used differentials. Not until the late 1810s did the inertia of established practice and anti-French sentiments owing to the French Revolution and the Napoleonic wars subside enough to allow the winds of change to blow with ever-increasing fury. A first significant, though premature, effort was made by Robert Woodhouse (1773–1827), a Fellow of Gonville and Caius College (later Lucasian professor of mathematics and Plumian professor at Cambridge Observatory, a position in which he preceded George Biddell Airy and held until his death in 1827). In his Principles of Analytical Calculation in 1803, he advocated for the differential notation. (Woodhouse, 1803). Unfortunately, the book had the misfortune of appearing at almost the exact moment when Cambridge was probably close to its intellectual nadir. As Adam Sedgwick’s biographer Colin Speakman writes (1982:47), “It was conservative, even by its own standards, complacent and singularly remote from any genuine habits of research or intellectual invention.” When serious change began to come, after 1810, it was largely a result of student enterprise rather than faculty initiative. Charles Babbage (1791–1871), an undergraduate at Trinity, even before coming to Cambridge had developed an intense passion for the Continental (i.e., analytic) methods through self-study of the French mathematician Sylvestre François Lacroix’s (1765–1843) textbook, Traité élémentaire de calcul differential et de calcul integral. (Lacroix, 1802). Babbage shared his enthusiasm with John Hudson (1773–1843), his private tutor (and Senior Wrangler for 1797) but Hudson was not encouraging: in his view, questions in Leibnizian form would never appear on the Tripos, so there was no point in learning about it. Realising at once that he would never gain ground for his cause in the college of Newton, Babbage transferred to Peterhouse, where he continued a determined effort, even promoting the differentials in broadsides on the Cambridge town walls. More helpfully, he began a translation of Lacroix’s textbook into English (Lacroix, 1816). Babbage soon found important allies in John Herschel of St. John’s, son of William Herschel, and George Peacock (1791–1858) of Trinity. A mutual friend, Edward Thomas Ffrench Bromhead (1789–1855) (later Sir Edward Bromhead, 2nd Baronet of Thurlby Hall) “…invited a small group to meet in his rooms to discuss the formation of a society whose aim would be to encourage analytical methods in Cambridge. It was then that Babbage, Herschel and Peacock first met each other. Feeling at the meeting was favourable to the formation of a society and the formal inaugural meeting of the Analytical Society took place very shortly afterwards.” (Wilkes, 1990).

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Though Bromhead played no further role, Babbage, Herschel and Peacock devoted themselves with furious energy to “the propagation of D’s; and consigned to perdition all who supported the heresy of dots.” (Snyder, 2011:32). They strove to produce a complete translation, with annotations, of the Lacroix text that Babbage had begun. At the Mathematical Tripos of 1813, Herschel was Senior Wrangler, and Peacock Second Wrangler, which increased the Analytical Society’s influence and prestige, since it was from high wranglers that the moderators who wrote the examination questions on the Tripos were then and afterwards selected. Indeed, in 1817, a year after the Babbage-Herschel-Peacock translation of Lacroix appeared, Peacock, as one of the moderators of the Tripos, included some questions using the Continental mathematics in the examinations. His decision to do so was not entirely without controversy, mainly because the problems were given in general form rather than applied to the solution of actual physical problems. William Whewell of Trinity, Second Wrangler for 1816, wrote to Herschel on 6 March 1817: You have I suppose seen Peacock’s examination papers. They have made a considerable outcry here and I have not much hope that he will be moderator again. I do not think he took precisely the right way to introduce the true faith. He has stripped his analysis of its applications and turned it naked among them. Of course all the prudery of the University is up and shocked at the indecency of the spectacle. The cry is “not enough philosophy.” Now the way to prevent such a clamor would have been to have given good, intelligible, but difficult physical problems, things which people would see that they could not do their own way and which would excite their curiosity sufficiently to make them thank you for your way of doing them. (RS:HS18:158)

Nominated moderator of the Tripos in 1818–19, and further influenced by a visit to Cambridge by the French physicist, mathematician and astronomer Jean-Baptiste Biot (1774–1862), who also argued for the importance of applying the calculus to the solution of practical problems, Peacock used the Leibnizian notation for the 1819 Tripos examinations, and included a number of problems of applied mathematics. The following year, he published a Collection of Examples of the Application of the Differential and Integral Calculus. (Peacock, 1820).) The triumph of Leibniz was now nearly complete, as from 1820, all of the questions on the Tripos were given in the form of differentials, the Newtonian dots having completely disappeared without anyone shedding a tear. Babbage’s close friend Richard Jones (1790–1855) told Whewell, “I hear the old mathematics have died and faded away with scarcely an audible groan before the bright flood of analytical love.” (Snyder, 2011:35). Though as late as 1830, Babbage (1830:1), comparing the British scene with that on the Continent, declared, “We are fast dropping behind. In mathematics we have long since drawn the reins, and given over a hopeless race” by the end of the decade, when Adams came up, the reform of Cambridge mathematics was largely complete. The geriatric texts of Wood and Vince had been (mostly) retired, and newer texts had taken their place. There was George Biddell Airy on trigonometry; Lacroix (as annotated and improved by Babbage, Peacock, and Herschel) on differential and integral calculus. William Whewell had published An Elementary Treatise on Mechanics, A Treatise on Dynamics, and The Mechanical Euclid, on mechanics and hydrostatics.

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In addition, and particularly relevant here, there were Airy’s Mathematical Tracts on the Lunar and planetary Theorie, the Figure of the Earth, Precession and Nutation, the Calculus of Variations and the Undulatory Theory of Optics. In addition, the Cambridge Mathematical Journal had been founded in 1837 by the Scots Duncan Gregory (1813–1844) and Archibald Smith (1813–1872) with the Englishman Samuel S. Greatheed (1813–1887), which gave Cambridge mathematicians a journal of their own to compare with the prestigious Journal für die reine und angewandte Mathematic (Crelle’s Journal) in Germany and the Journal de Mathématiques Pures et Appliquées (Liouville’s Journal) in France. England was finally catching up.

References The locations for correspondence are indicated by the following abbreviations; AM: Adams family papers, Cornwall Record Office, Truro. COA: Cambridge Observatory Archives (at Cambridge University Library). CUL: Cambridge University Library. JC: Adams MSS at St. John’s College, Cambridge. McA: The McAllister Collection of copied letters at St. John’s College, Cambridge. RS:HS: Herschel Papers, Royal Society. RGO: Royal Greenwich Observatory. RGON: Royal Greenwich Observatory Neptune file in CUL. TC: Trinity College Library. Adams, J.C., 1844. Elements of the comet of Faye. Monthly Notices of the Royal Astronomical Society, 6:20. Adams, J.C., 1846. Duplicity of Biela’s comet. Monthly Notices of the Royal Astronomical Society, 7:83. Adams, J.C., 1847. An Explanation of the Observed Irregularities in the Motion of Uranus, on the Hypothesis of Disturbances caused by a more Distant Planet; with a Determination of the Mass, Orbit, and Position of the Disturbing Body (read to the RAS on 13 November 1846). Monthly Notices of the Royal Astronomical Society, 16:427–459. Airy, G.B., 1847. Account of some circumstances historically connected with the discovery of the Planet exterior to Uranus (read 13 November 1846). Memoirs of the Royal Astronomical Society, 16:385–414. Airy, W.A., 1896. Autobiography of Sir George Biddell Airy. Cambridge, Cambridge University Press. Anderson, R.A., 1891. John Hymers. In: Dictionary of National Biography, Sidney Lee (ed.), Vol. 28. “A. R.” [Arthur Ranyard], 1891. Émile Gautier. Nature, 43 (2 April):518-519. Asimov, I., 1979. The Sea-green Planet. In: Quasar, Quasar, Burning Bright. New  York, Avon Books:140–150. Babbage, C., 1830. Reflections on the Decline of Science in England and Some of Its Causes. London, Fellowes. Baum, R and Sheehan, W., 1997. In Search of Planet Vulcan: the ghost in Newton’s clockwork universe. New York and London, Plenum. Bristed, C.A., 1852. Five years in an English university. New York, G.P. Putnam. Brookes, C.J., 1970. On the Prediction of Neptune. Celestial Mechanics, 3:67–80. Chadwick, O., 1960. The Mind of the Oxford Movement. Stanford, Stanford University Press.

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Chapman, A., 1988. Private Research and Public Duty. Journal for the History of Astronomy, xix:121–139. Chapman, A., 1996. Adams, Airy and the Discovery of Neptune in 1846. Astronomy Now (September):42–45. Clark, JW and Hughes, T.M., eds., 1890. The Life and Letters of the Reverend Adams Sedgwick. Cambridge, Cambridge University Press. 2 vols. Clercq, P.D. and Pedersen, K.M., eds., 2010. An Observer of Observatories: the Journal of Thomas Bugge’s Tour of Germany. Aarhus, Aarhus University Press. Crilly, T., 2004. The Cambridge Mathematical Journal and its descendants: the linchpin of a research community in the early and mid–Victorian Age. Historia Mathematica 31:455–497. Deacon, R., 1985. The Cambridge Apostles. London, Farrar, Strauss, & Giroux. Forfar, D., 1996. What became of the Senior Wranglers? Mathematical Spectrum, 29, 1:1–4. Gascoigne, J., 1984. Mathematics and Meritcoracy: the emergence of the Cambridge Mathematical Tripos. Social Studies of Science, 14(4):547-584. Gifford, M., 2005. Missions, Moons & Masterpieces: the Giffords of Oamaru. Palmerston North, Heritage Press. Glaisher, J.W.L., 1896. Memoir. In: The Scientific Papers of John Couch Adams. William Grylls Adams (ed.), Cambridge, Cambridge University Press. Grant, R., 1852. History of Physical Astronomy from the earliest ages to the middle of the nineteenth century. London, Robert Baldwin. Grosser, M., 1962. The Discovery of Neptune. Cambridge, Mass., Harvard University Press. Hall, A.  R., 1969. The Cambridge Philosophical Society: a history 1819-1969. Cambridge, Cambridge University Press. Hanson, N.R., 1962. Leverrier: The Zenith and Nadir of Newtonian Mechanics, Isis, 53, 3:359–378. Harrison, H.M., 1994. Voyager in Time and Space: the life of John Couch Adams, Cambridge Astronomer. Lewes, Sussex, The Book Guild. Herschel, J.F.W., 1846. Address. In: Report of the Meeting of the British Association for the Advancement of Science. London, John Murray, xxvi–xliv. Herschel, J.F.W., 1849, Outlines of Astronomy. London, Longman, Brown, Green, Longmans. Hutchins, R., 2008. British University Observatories, 1772–1939. Aldershot, Ashgate. Johnstone, P., 2016. “A History of Mathematics in Cambridge.” accessed on website https://www. maths.cam.ac.uk/about/history. Kronk, G., 1984. Comets: a descriptive catalog. Hillside, New Jersey, Enslow. Lacroix, S.F., 1802. Traité Élémentaire de calcul differential et de calcul intégral. Paris, Duprat. Lacroix, S.F., 1816. An Elementary Treatise on the Differential and Integral Calculus. Pt. 1, Differential Calculus, tr. By C. Babbage. Pt. 2, Integral calculus, tr. By G. Peacock and J.F.W. Herschel. Cambridge, J. Deighton and Sons. Leedham-Green, E.S., 1996. A Concise History of Cambridge University. Cambridge, Cambridge University Press. Littlewood, J.E., 1957. The Adams-Airy Affair, in: A Mathematician’s Miscellany. London, Methuen & Co. Peacock, G., 1820. Collection of Examples of the Applications of the Differential and Integral Calculus. Cambridge, J. Deighton & Sons. ‘Peregrine’ [pseudonym]. Adams’s Year. The Queen, 11 November 1893. Pym, H.N., ed, 1882. Memories of Old Friends: Caroline Fox of Penjerrick, Cornwall. London, Smith, Elder, & Co., 2nd ed. (2 vols.). Rawlins, D., 1992. The Neptune Conspiracy: British Astronomy’s post-discovery discovery. DIO 2.3 (October):115–142. Sampson, R.A., 1904. A Description of Adams’s Manuscripts on the Perturbations of Uranus. Monthly Notices of the Royal Astronomical Society, 54:143–170. Sedgwick, A., 1834. Discourse on the studies of the University. Cambridge, J. & J.J. Deighton, and London, John W. Parker, 2nd ed.

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Sheen, B., 2013. Neptune: towards understanding how Adams’s upbringing enabled him to predict, then miss the discovery of the century. Antiquarian Astronomer, 7:71–81. Smart, W.M., 1947. John Couch Adams and the discovery of Neptune. Occasional Notes of the Royal Astronomical Society, 2(11). Smith, C. and Wise, M.N., 1989. Energy and Empire: a biographical study of Lord Kelvin. Cambridge, Cambridge University Press. Snyder, Laura J., 2011. The Philosophical Breakfast Club: four remarkable friends who transformed science and changed the world. New York, Broadway Books. Somerville, M., 1874. Personal Recollections of Mary Somerville. Martha Charters Somerville (ed.), London, John Murray. Speakman, C., 1982. Adam Sedgwick: geologist and Dalesman. Groombridge, East Sussex, Broad Oak Press, together with the Geological Society of London and Trinity College, Cambridge. Statutes and Ordinances, 2019. Cambridge University. http:www.admin.cam.ac.uk. Warwick, A., 2003. Masters of Theory: Cambridge and the rise of Mathematical Physics. Chicago, University of Chicago Press. Wilkes, M.V., 1990. Herschel, Peacock, Babbage and the development of the Cambridge curriculum. Notes and Records of the Royal Society of London, 44(2):2205–219. Wilson, D.  B., 1985. The educational matrix: physics education at early-Victorian Cambridge, Edinburgh and Glasgow Universities. In: Wranglers and physicists: studies on Cambridge physics in the nineteenth century. P.M. Harman (ed.), Manchester, Manchester University Press. Woodhouse, R., 1803. The Principles of Analytical Calculation. Cambridge, J. Deighton & Sons. Wordsworth, W., 1850. The Prelude or, Growth of a Poet’s Mind; an Autobiographical Poem.

Chapter 5

Urbain Jean Joseph Le Verrier: Predictions Leading to Discovery James Lequeux

Abstract  The discovery of Neptune was undeniably one of the major scientific events of the 19th century. It is well known that the effort to discover the troubling planet responsible for the abnormal behavior of Uranus was carried out simultaneously in England and France, and that Le Verrier was the first to announce the discovery, John Couch Adams having independently obtained similar results. From these events, interminable controversies followed, in which nationalism played a large role, and even today, they are not totally extinguished (In 1880, Aimable Gaillot (1834–1921), the only pupil of Le Verrier, wrote (Gaillot, 1880:103): “There had been in the past discussions, more passionate than impartial, about the priority of the discovery; today, the question is settled: to each one his due, and the mutual esteem between the two scientists proves that both, at least, were giving justice to each other. But it is to the sole Le Verrier that the discovery of the planet is due.” However, controversies revived from time to time). But at least it is now possible to look at these matters with cooler heads, and with relative neutrality. Recent research, including the translation into English of my biographies of Arago and Le Verrier, facilitate offering this French view of events. This is the subject of the present chapter. First, let us examine the problem that Le Verrier set out to tackle: the problem of the motion of Uranus.

5.1  The Problem of the Motion of Uranus The chance discovery of that planet by William Herschel in 1781 had huge repercussions. As soon as it became evident that Herschel had discovered, not a comet as first thought, but a new planet, records of earlier observations began to turn up, in which it was mistaken for a fixed star, as we saw in Chapter 2. The earliest, by the J. Lequeux () Observatoire de Paris/PSL, 61 avenue de l’Observatoire, F–75014, Paris, France e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_5

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Fig. 5.1  A fragment of Bremiker’s celestial map on which a German hand (Galle’s?) has plotted the position of Neptune predicted by Le Verrier (Neptun bereibnet) and the actual position (Neptun beobachtet). The position predicted by Adams is also indicated

English Astronomer Royal John Flamsteed, dated to 1690; others were made by Flamsteed himself, James Bradley, Tobias Mayer, and Charles Le Monnier. In his celebrated treatise on celestial mechanics, Pierre Simon de Laplace had developed mathematical expressions for the mutual perturbations exerted by the planets as a result of their gravitational attraction. Using these expressions, one could carry out numerical calculations to produce tables of the positions of the planets over time. The responsibility for doing so was claimed by the Bureau des Longitudes, headed by Laplace himself, though the work of actually performing these backbreaking calculations was distributed among several astronomers at the Bureau, including Jean-Baptiste Delambre, Johann-Karl Burckhardt and Alexis Bouvard. Bouvard (see Chap. 2, Fig. 2.5) Laplace’s student, was assigned the most thankless task. In 1821, he began the laborious calculation of tables predicting the movements of the three giant planets: Jupiter, Saturn, and Uranus. The calculation of the tables of Jupiter and Saturn was relatively straightforward. Uranus, however, proved to be highly intractable. Even after taking into account the perturbations exerted by the other planets, Bouvard could not derive a set of orbital elements that would successfully account for the movements of Uranus during the entire period over which it had been observed (Fig. 5.1). As he struggled with the problem, Bouvard attempted various expedients. First, he used only the numerous positions of Uranus measured since its discovery in 1781 (those covering the period 1781–1821). But then he could not satisfy the earlier “ancient” observations going back to that of Flamsteed in 1690. The discrepancy for Flamsteed’s observation reached more than a minute of arc, which seemed too great to ascribe to errors of observation. On the other hand, if Bouvard accepted the observations between 1690 and 1781, the more recent observations failed to fit. Resigned to defeat, Bouvard wrote in the introduction of his Tables of Uranus in 1821 that it would remain the task of future investigators to determine whence arose the difficulty in reconciling these two data sets: whether the failure of the observations before 1781 to fit the tables was due to the inaccuracy of the older observations or whether they might depend on “some foreign and unperceived source of

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disturbance acting upon the planet.” (Bouvard, 1821: XV) Bouvard’s tables were based only on the observations between 1781 and 1821, but soon after their publication, discrepancies began to appear once more. They accumulated over time, until it became impossible to ascribe them to the effects of observational errors alone. Following Alexis Bouvard’s death in 1843, his nephew Eugène Bouvard (1812–1879) was charged by the Bureau des Longitudes to work on new tables of the planets. He submitted his results to the Academy on 1 September 1845, but they were never published. By then he had come to regard the discrepancies between observation and theory as irreconcilible without adding another factor, and personally found “entirely plausible the idea suggested by my Uncle that another planet was perturbing Uranus.” (Comptes rendus hebdomadaires des séances de l’Académie des sciences, CRAS, 1845, 21: 524–525). It seems, then, that Alexis Bouvard himself has been first to speculate that the anomalous motion of Uranus could be occasioned by the gravitational action of a new planète troublante (disturbing planet). Others would later claim to have independently come up with the same idea. Perhaps they did, though by then it was so much in the air that there was little credit attached to doing so. Indeed, the idea spread rapidly through the scientific world, and began to attract the attention of the larger public, not only in France but elsewhere. For instance, in November 1834, an amateur astronomer, the Reverend Thomas J.  Hussey  (1792-c. 1866), rector of Hayes, Kent, wrote to George Biddell Airy, the Plumian professor of astronomy at Cambridge, about the matter. During a previous visit to Paris, Hussey had met Eugène Bouvard, and had become convinced that an exterior planet disturbing Uranus from its course was the likely explanation of the discrepancy between observation and theory. He now proposed to Airy, a first-rate mathematician, that if the latter would work on its approximate position, he, Hussey, would gladly take up the search for it.1 In 1835, the astronomer Benjamin Valz (1787–1867), who the following year was to be appointed director of the Marseilles observatory, proposed to Arago carrying out a search for the planet from its possible perturbing effects on Halley’s comet. Moreover, in 1840, the renowned German astronomer Friedrich Wilhelm Bessel, who from 1830 onwards had conjectured about the possible existence of just such a perturbing mass, gave a public lecture on the topic. He also discussed it with John Herschel, the son of the discoverer of Uranus and a well-­ known astronomer in his own right. François Arago  (1786–1863), who was de facto responsible for the Paris Observatory, evidently hoped that the problem of Uranus would be taken up at his Observatory, but he lacked confidence in Eugène Bouvard, whose measurements at the eclipse expedition of 1842 had been of poor quality. Since there was no one else at the Observatory he deemed capable of tackling such a difficult problem, he turned to another man he knew but who was not a member of his staff: Urbain Le Verrier.

1  These letters are preserved in the archives of the Royal Greenwich Observatory in Cambridge, UK, in Papers of George Airy, general ref. GBR/0180/RGO 6. Important extracts are to be found in Airy G.B. Monthly Notices of the Royal Astronomical Society 7, 1846:121–144.

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5.2  Urbain Le Verrier Urbain Jean-Joseph Le Verrier (Fig. 5.2) was born on 11 March 1811 in Saint-Lô (Manche). His father, Louis-Baptiste Le Verrier (1780–1846), was an estates manager, born in Carentan2. His mother, Marie‐Jeanne‐Joséphine‐Pauline, néé de Baudre Le Verrier, (1785–?) was born in Baudre. Carentan and Baudre are two communities close to Saint-Lô; the grandparents of Le Verrier were also born in the region. The house in which Urbain was born was a bourgeois home, modest enough, which attests to their limited means. First a student at the local College of Saint-Lô, Le Verrier entered as intern at the Collège royal de Caen where he studied mathematics from 1827 to 1830. The latter year, he presented himself at the entrance exam for the École polytechnique, but failed, to everyone’s surprise, for he was a very brilliant student. His father decided to send him to Paris, and sold his house to defray the cost. After one year of study under the direction of the mathematician Charles-Adrien Choquet (1798–1880), Le Verrier obtained the second prize in mathematics in the Concours général (a national competition), and was received in the École polytechnique in 1831, at age 20, which was typical of the period. He graduated in eighth place two years later, in the École des Tabacs, and followed for two additional years courses at this school preparing to tobacco industry, situated at the time at the quai d’Orsay in Paris. His studies consisted largely of chemistry, in which he interested himself to the point where he began researches in a small laboratory that he set up at his home. In 1835 he published a first article on chemistry, and then another one in 1837. His work was of high quality, and would be cited long afterward by chemists. He married in 1837

Fig. 5.2  Portrait of Le Verrier by Charles Daverdoing, 1846 (Credit: Bibliothèque de l’Observatoire de Paris)

2  From the birth certificate of Le Verrier, reproduced in Centenaire de la naissance de U.-J.-J. Le Verrier (hereafter Centenaire), p. 8.

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Lucile Marie Clotilde Choquet (1820–1877), the daughter of his former professor, 17 years old; he at the time was 26. The Le Verriers were to have three children. In 1836, two positions as répétiteur (private tutor or assistant teacher; someone who explains to the students the professor’s lectures) at the École polytechnique became vacant: one in chemistry and the other in astronomy. Le Verrier applied for the former, but it was Henri Victor Regnault (1810–1878) who obtained it (who would go on to become a celebrated chemist). As Le Verrier needed money, and was a good mathematician, he then applied for the other post, which he obtained. With a surprising facility, spurred by an ambition already solidified, he completely changed the orientation of his researches. During this period, astronomy was concentrated on the positions of the stars, and more especially the movements of the planets relative to them, and on celestial mechanics that predicted these movements. Pierre-Simon Laplace, the greatest representative of celestial mechanics in France and in the world, died in 1827, and there was no person of the same caliber to succeed him. Le Verrier, who hardly had any sense of his limitations, was to try, in the words of Jean-Baptiste Dumas (1800–1884) at Le Verrier’s funerals, “Laplace’s inheritance was unclaimed; and he boldly took possession of it.” (CRAS 85, 1877:524–5). It would only take him two years before he presented to the Academy of Science his first investigations into celestial mechanics. Within his lifetime he would write over 200 contributions to the Comptes rendus de l’Académie des sciences, on many varied topics and these give a wonderful reflection of the debates held within the Academy of Science at that time. (Laskar, 2017:504) In his Traité de Mécanique céleste, the culmination of all the work of the 18th century, Laplace used only a relatively limited number of terms for the series describing the mutual gravitational perturbations of planets, and everything seemed to work out well enough: the Solar System seemed stable and, even if the orbital parameters of the planets varied slowly over time, these slow variations were periodic, though with a period that could be as long as several centuries. But it was realised, notably by Siméon Denis Poisson (1781–1840) (Connaissance des temps (CdT) for 1831, additions:23–48) that if higher order terms were introduced, difficulties appeared: certain elements seemed to grow or increase without limit, even though very slowly, a circumstance which brought into question the long-term stability of the Solar System. Moreover, it did not seem impossible that Laplace had omitted certain small terms; and even if Laplace had been convinced of this stability, Le Verrier had some doubts, and was to renew the attack on the problem of the stability of the Solar System by considering as large a number of terms as possible. The first part of Le Verrier’s work was completed in September 1839, as can be seen by the summary of his published memoir, in the 16 September issue of the Comptes rendus des séances de l’Académie des sciences (CRAS 9, 1839:370–372). The memoir itself was printed in 1840 in the Connaissance des temps for 1843 (one may wonder about the difference in these dates, but one must realise that the ephemerides of the Connaissance des temps were of no value for preparing observations unless they were published well before the period to which they applied). In this work, and in an important new report, published in the Connaissance des temps for

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1844, Le Verrier showed that the third-order terms in the perturbation functions, hitherto regarded as negligible, can become very important because the denominators are close to zero; however, for the same reason, they are not easy to take into account. Though hardly affecting calculations of the movements of the giant planets, Jupiter, Saturn, and Uranus, whose orbits are accordingly quite stable, in the case of the inner planets, their movements over long time periods cannot be predicted with any precision. Le Verrier affirmed: “The direct integration of the equations, including the third order terms, is the only way around these difficulties.” This integration, however, was impossible at the time, and Le Verrier thus came to doubt the long-term stability of the orbits of the inner planets. This problem had been taken again much later by Henri Poincaré and his successors, and it is presently well-known that the long-term behavior of the Solar System is indeed chaotic (we are talking here about billions of years); here, Le Verrier had demonstrated a remarkable insight. Le Verrier also worked with great success on the motion of Mercury. For the latter, he discovered some problems so that he was led to revisit his work some years afterward. We know the result of this 1859 work: the anomalous advance of the perihelion of Mercury. This apparently esoteric finding would excite considerable interest, leading to the premature announcement of the discovery of a new planet and culminating in the correct explanation of the anomaly by Albert Einstein in 1915: it provided, in fact, the first observational verification of his General Theory of Relativity. In 1842, Le Verrier occupied himself with the planet Uranus for the first time. Uranus presented marked anomalies: for a number of years, the tables had failed to accurately account for its movement. Le Verrier went not very far at the time as he was not the only one to work on this problem. For example, in Germany and in Denmark, Peter Hansen (1795–1874), who had developed new methods of celestial mechanics, attempted to apply them to Uranus. In France, Liouville charged a young man, Charles Delaunay (1816–1872), to examine the work of Hansen and to check the results, a task which he successfully carried out in 1841 (CRAS 14, 1841:371–373). But Delaunay also found that certain terms in the series expansions, which he thought Hansen had not considered, were not negligible (CRAS 14, 1841:406). Le Verrier did not agree with Delaunay, and a rather technical controversy ensued, which can be followed in the Comptes rendus de l’Académie des sciences (CRAS 14, 1841:487–488, 579–582, 660–663). In the end, Le Verrier proved to be right, but he ungenerously mocked his less experienced colleague. At odds for the first time, the two men would engage in such conflicts often in the future, and came to regard each other as mortal enemies. Between 1840 to 1843, when he was chiefly preoccupied with Uranus and Mercury, Le Verrier also concerned himself with the motion of the minor planet Pallas: the resulting memoir (CRAS 16, 1843: 1435) attracted particularly the attention of the Academy of sciences. Next, in 1844, he worked on the orbits of various comets. This latter work (CRAS 18, 1844:826–827 and 19, 1844:559–560), even though it has been eclipsed by his work on the planets, is of considerable novelty and interest.

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This was the situation when Arago decided to turn to Le Verrier to tackle the problem of the anomaly of Uranus. No wonder that he had great faith in Le Verrier’s mathematical abilities, and so, at his request, Le Verrier abandoned the investigation of comets and devoted himself to Uranus. He recalled, in the first notice of this work to the Academy of Sciences: During the course of this last summer 1845, Mr. Arago made clear to me that the great importance of this question imposed a duty on every astronomer to contribute, to the utmost of his powers, to the clarification of certain points. In response to his plea, I abandoned, therefore, my researches into comets, of which several fragments have already been communicated, in order to occupy myself fully with Uranus. Such is the origin of that work that I have the honor to present to the Academy. (CRAS 21, 1845:1050–1055)

5.3  Le Verrier’s Work on Perturbations of Uranus Le Verrier scrupulously examined all the available observations of Uranus up until 1845, namely those that had recently been made at the Paris Observatory, which Arago put in his hands, and that he found of excellent quality; and also those made at Greenwich which were sent by the director, Airy. He also examined carefully Alexis Bouvard’s calculations (he seems not to have considered those of his nephew Eugène). He discovered that certain terms had been neglected without justification, and he found several errors: he therefore had to redo parts of the calculation. Next, he undertook to determine the location of the perturbing planet. The problem was entirely novel: hitherto, the position of each planet was determined by taking into account the perturbations of the others whose positions were known by observation. In the present case, it was a matter of determining the position of a planet about which one knew nothing except the perturbations that it exerted on the other planets. In mathematics, this is called an inverse problem. It is both difficult and complex, because there are many unknowns to be determined. Also Le Verrier simplified the problem from the very beginning by supposing as known the distance of the planet from the Sun and the inclination of its orbit. He wrote on 1 June 1846: It would be natural to suppose that the new body is situated at twice the distance of Uranus from the Sun, even if the following considerations didn’t make it almost certain. I mean that the sought-after planet could not be close to Uranus (since then its perturbations would have been very evident). However, it is also difficult to place it as far off, say, at three times the distance of Uranus. It would be necessary in effect, under this hypothesis, to attribute to the planet an excessively large mass. But then its great distance both from Saturn and Uranus would mean that it would disturb each of these two planets in comparable degree, and it would not be possible to explain the irregularities of Uranus without at the same time introducing very sensible perturbations of Saturn, of which there exist no trace. We might add that since the orbits of Jupiter, Saturn, and Uranus all have a very small inclination to the ecliptic, it is reasonable to suppose, as a first approximation, that the same must apply to the sought-after planet. (CRAS 22, 1846:907–918)

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By such legerdemain, Le Verrier had reduced the number of unknowns by two: using the Bode’s law (see Chap. 2, Sect. 2.3), he assumed the semi-major axis of the orbit, a quantity that would have been particularly difficult to determine otherwise. He also assumed a null inclination of the orbit. Nevertheless, there remained more than enough other unknowns, in part because the orbital elements of Uranus were themselves poorly determined owing to the lack of any solution fitting all the observations. The hypothesis of a perturbing planet did change this. Of course, it is possible to take the orbit of Uranus as given; then one can work out all the perturbations of the other planets except the new one, and finally establish the discrepancies between the calculated and observed positions so as to show the effect of the perturber: an example is given in Fig. 5.3. Nevertheless, it is impossible in this way to obtain a unique solution to the problem since any number of other orbits remain possible for Uranus. Seeing, this, Le Verrier was obliged to determine simultaneously both the orbital elements of Uranus and those of the new planet. This is a problem with twelve unknowns. However, as we have seen, Le Verrier had already settled on two for the unknown planet, and using the same reasoning he settled the same ones for Uranus: the semi-major axis and the orbital inclination. With this simplification, there remained eight unknowns, to which he added a ninth, the mass of the perturbing planet. We cannot enter here into the details of the solution of the corresponding

Fig. 5.3  An example of the discrepancies between the calculated longitudes of Uranus C and the observed ones, O from 1690 to 1845, from Le Verrier’s paper in CdT for 1849, additions, p. 3–254, table p. 129–136; calculations and drawing by André Danjon (1890–1967) (Danjon, 1946a). Here, the calculated longitudes, which take into account the perturbations by Jupiter and Saturn, are those of Bouvard, the theory of which, corrected by Le Verrier, uses only the observations between 1781 and 1821. This solution is not necessarily the correct one, because one could as well have used the observations before 1781 to calculate the orbit of Uranus (Credit: L’Astronomie, Société Astronomique de France)

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equations.3 The unpublished manuscripts in which Le Verrier details some of these calculations are preserved in the library of the Paris Observatory (Ms 1063(27)). In total they take a thousand pages, two of which are reproduced in Lequeux (2013)! So far sure of himself, Le Verrier affirmed, in his presentation to the Academy of Sciences on 1 June 1846, I prove that all the observations of the planet (Uranus) can be represented with the exactitude they deserve…. I conclude also that one can effectively model the irregularities of Uranus’s movements by the action of a new planet placed at a distance of twice that of Uranus from the Sun; and what is just as important, that one can arrive at the solution in only one way. To say that the problem is susceptible to only one solution, I mean that there are not two regions in the sky in which one can choose to place the planet in a given epoch (such as, for instance, 1 January 1847). Within this unique region, we can limit the object’s position within certain bounds. (CRAS 22, 1846:916)

Then Le Verrier indicated within 10° the possible positions occupied by the perturbing planet for 1 January 1847. The uncertainty was still considerable, and Le Verrier added that he could do no better at the time of his presentation, since the work for which he had just presented an abstract to the Academy “must be considered as a rough draft or outline of a new theory, which was only in the initial stages.” The orbital elements he calculated were provisional, but he hoped to extend his labors to provide more precise results. He concluded: Dating from the year 1758, the illustrious geometer Clairaut declared in his publication to the Academy of Sciences, on the subject of the perturbations of the comet of Halley, that an object which traverses the remotest regions might be subject to totally unknown forces, such as the action of planets too distant to ever be perceived. Let us hope that the stars which Clairaut spoke of will not be all of them invisible; that, if Uranus has been discovered by chance, nevertheless, the new planet will be successfully found from the position I have calculated. (CRAS 22, 1846:918)

Despite Le Verrier’s seeming confidence, several people remained skeptical. Thus Airy wrote on 26 June to Le Verrier to ask for further clarifications, at the same time sending him further Greenwich observations, which Le Verrier was to make use of (Centenaire, 1911:12–13). In a considerably later letter to Le Verrier dated 20 December 1876, Airy offered the following clarifications: The part which depends on calculation of observations is to be divided between Bessel and me. Such work required generally only ordinary powers of judgement. But I believe that both Bessel and I looked, with a doubtful hope, to the possibility that our work would at some time be reconciled with theory. But at that time there was no Le Verrier. I was one of the few persons who might rashly have taken up the enterprise, but my time has always been very closely subscribed.

 Jean-Baptiste Biot attempted to explain Le Verrier’s methods in six papers in Journal des Savants (October 1846, p. 577–596; November 1846, p. 641–664; December 1846, p. 750–768; January 1847, p. 18–35; February 1847, p. 65–86; March 1847, p. 182–187). Arrived at the third paper, he wrote: “As I progress in the task I have undertaken, the difficulty of the subject seems to increase.” In order to understand what Le Verrier did, the best thing is to read his own papers. A more elementary account can be found in Tisserand & Andoyer (1912). 3

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In discriminating amongst the various persons concerned in this great enterprise, we must give a very high position to Bessel. The failing of his first discussion of Bradley’s observations have been well pointed out by you; but they did not in any case greatly affect the results as for planets: and in proceeding downwards along the course of time by uniform scale, they were practically annihilated. But I must say that Bessel, in the construction of his Tabulæ Regiomontanæ, showed himself as the first man who profoundly felt that Astronomy is a science of connection and comparison. And my perception of this point in his character and the character of his work induced me to undertake (using his work as foundation) the reductions which you state to have been so useful to you…. (Bibliothèque de l’Observatoire de Paris, MS 1072 (35))

Meanwhile, in June 1846 Le Verrier thanked Airy for his assistance, and responded to his specific questions. He even proposed to communicate the orbital elements of the perturbing planet, if Airy were at all inclined to search for it. Airy was very impressed by Le Verrier’s confidence (Library of Paris Observatory, Ms 1072). Although his skepticism was completely overcome, he declined Le Verrier’s offer, for reasons that were questioned and challenged after the discovery, became contentious, but are explained in Chap. 7. Despite the novelty of the problem and the great mathematical difficulties, Le Verrier needed only three months to specify the orbital elements of the perturbing planet, guess at its mass, and even provide an order of magnitude estimate of the apparent diameter that it would present in the telescope. In his note of 31 August 1846 (CRAS 23, 1846:428–438), he summarized his method and gave the predicted orbital elements for the new planet, to a degree of precision that would prove, however, to be totally illusory. Table 5.1 compares his values to the true values of the orbital elements: Le Verrier modified the semi-major axis of the orbit slightly from the initial hypothesis in which he had simply (and rather arbitrarily) followed the Bode’s law, and taken it to be twice that of the orbit of Uranus, i.e., 38 astronomical units. The eccentricity adopted by Le Verrier is important, in that it made the values for the distance between Neptune and Uranus during the period in question differ little from the actual ones. Le Verrier predicted that the planet lay in the sky about 5 degrees east of the star delta in the constellation Capricorn. As noted, he indicated also the approximate values of the apparent diameter and brightness of the planet, probably in an attempt to stimulate the ambition of an observer to look for it. On October 5 (i.e., after the discovery), Le Verrier calculated the inclination of the orbit

Table 5.1  Values of Principal elements of the orbit of Neptune Value Semi-major axis of orbit (AU)a Sidereal period of revolution (years) Eccentricity Mass (solar masses) Mass (earth masses)

Le Verrier 36.154 217.387 0.10761 1/9,300 36

Actual value 30.0690 163.723 0.008586 1/19,424 17.14

The astronomical unit (AU) is equal to the semi-major axis of the Earth’s orbit, i.e. 1.486 108 km

a

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of the planet, which as a first approximation he had taken as zero. He found that it must be inclined at least 4° 3′ to the orbit of Uranus (CRAS 23, 1846:657–659).

5.4  Neptune Discovered Hence on 31 August 1846, Le Verrier presented a paper to the Academy of sciences containing the elements of the planet and the place where it ought to be found. He then wrote to several foreign astronomers in an effort to enlist some powerful instrument in the search. Sadly, there were at the time no such instruments at the Paris Observatory itself (the largest telescope was still a 9 ½  inch (23  cm) Lerebours refractor, finished in 1823, but of such mediocre quality that the outer zones of the object glass had to be masked out with a diaphragm). Furthermore, the observatory did not have available any deep maps of this part of the sky. Despite all that Arago and Le Verrier between them had done, the planet would not, and indeed could not, be discovered in Paris. Among the foreign astronomers contacted by Le Verrier was Johann Gottfried Galle of the Berlin observatory. Le Verrier wrote to him on 18 September. The letter reached Berlin on 23 September; that night Galle, after receiving permission from the observatory’s director Johann Franz Encke, and assisted by a graduate student from Berlin, Heinrich Louis d’Arrest (Fig. 5.4), quickly discovered the planet. On 25 September, Galle wrote to Le Verrier (Fig. 5.5) in French: the latter did not know German (Centenaire, p. 19). Here is the translation of the beginning of this letter. The full translation is in Chap. 6, Sect. 6.6.3. Sir, The planet whose position you had indicated really exists. On the very day I received your letter I found an eighth magnitude star, which did not appear in the excellent chart

Fig. 5.4  Left: Johann Gottfried Galle (1812–1910); right: Heinrich Louis d’Arrest (1822–1875) (Credit: Wikimedia Commons)

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Fig. 5.5  The first page of the letter of Galle to Le Verrier, announcing the discovery of Neptune, as reproduced in Danjon, 1946a:269 (Credit: L’Astronomie, Société Astronomique de France)

Hora XXI (drawn up by Dr. Carl Bremiker) from the collection of celestial charts published by the Royal Academy of Berlin…

On 28 September, Encke congratulated Le Verrier again, in impeccable French: Permit me, Monsieur, to congratulate you most sincerely for the brilliant discovery by which you have enriched astronomy. Your name shall henceforth be associated with the most brilliant imaginable demonstration of the correctness of universal gravitation. I believe that as these few words encompass all that a scientist’s ambition could hope for, it would be superfluous to add anything more. (Centenaire, p. 20–22)

He nevertheless did add more, as follows: There was, nevertheless, a great deal of luck in the search. The Academy’s chart of Mr. Bremiker, which has perhaps not even yet arrived in Paris but which I shall send out at once, happens to include, close to its lower edge, precisely the region where you have designated the position of the planet (see Fig. 5.1). Without the fortuitous circumstance of having a

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chart containing all the fixed stars down to the tenth magnitude, I do not believe the planet would have been found. I would add that your position for the planet does not differ from its actual one for noon on September 23 by more than 54 minutes and 7 seconds in longitude; while, if my calculations are correct, the observed retrograde motion is 73.8 seconds (per day), just a bit greater than the 68.7 seconds predicted by your elements. It is possible, therefore, that the planet is not quite as far away as you have supposed, though the difference is actually very small.

Shortly after the announcement of the discovery, the planet was viewed in Paris by Le Verrier himself (thus putting paid to Camille Flammarion’s (1842–1925) doubtful story that Le Verrier, the consummated theorist, never saw it himself), as well as by several other astronomers; by Otto Struve (1819–1905) and his father Wilhelm (1793–1864) at the Pulkova Observatory near Saint Petersburg, by Émile Plantamour (1815–1882) in Geneva, by Carl Ludwig von Littrow (1811–1877), in Vienna, by John Russell Hind and James Challis in England and by Carl Friedrich Gauss in Göttingen, etc. Filled with enthusiasm, Le Verrier wrote on 5 October: This success leads to the hope that, after observing the new planet for 30 or 40 years, it will be possible in turn to use it to discover the orbit of the next one in order of distance from the Sun, and so on. Unfortunately, the more distant objects will be invisible because of their immense distance from the Sun. Nevertheless, in the course of centuries, their orbits will be traced out with great exactitude by means of their secular inequalities. (CRAS 23, 1846:657–659)

Needless to say, his hope was not fulfilled in the way he expected. Other bodies in the Solar System more remote than Neptune, such as Pluto and Eris, have been found (See Chap. 10, Sect. 10.3). However, they are so remote and their masses are so small that the influence they exert on the orbit of Neptune is negligible. The discoveries of these “dwarf planets,” as they are now known, resulted not from mathematical investigations on the kind that led to Neptune’s discovery but from systematic photographic or CCD surveys. A flood of salutations rained down on Le Verrier, but those of his own colleagues meant the most to him4. He became famous overnight, and was the recipient of countless honors: officer of the Legion of Honor (Galle had already been nominated as chevalier four months previously), assistant member of the Bureau of Longitudes, chair of celestial mechanics in the faculty of sciences in Paris, the latter specifically created for the occasion. King Louis-Philippe (1773–1850) named him preceptor of astronomy for his grandson, Philippe d’Orléans (1838–1894). The Royal Society of London awarded him the prestigious Copley Medal, the very same that William Herschel had received for the discovery of Uranus, and inscribed him among its foreign members. Many other learned societies followed suit. The Minister of Public Instruction, Narcisse-Achille de Salvandy (1795–1856), commissioned a bust of him by the celebrated sculptor James Pradier (1790–1852). He presented it as a gift to Madame Le Verrier on 31 December 1846, with instructions to place it

4  Congratulation letters from Johann Franz Encke, Christian Schumacher, Émile Plantamour, Otto Struve, Francesco de Vico, Karl Ludwig von Littrow, Benjamin Valz and George Biddell Airy are published in Centenaire.

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in the College of Saint-Lô, in Le Verrier’s hometown. Miraculously, it survived the general destruction of Saint-Lô during the Battle of Normandy in 1944 that led the playwright Samuel Beckett to call Saint-Lô “the Capital of Ruins.” It is now displayed in the City Hall. Meanwhile, a few discordant notes were being heard. A congratulatory letter from Benjamin Valz to Le Verrier on 30 October refers to “wicked quibbling” and urges Le Verrier not to let it upset him. “I’ve seen,” writes Valz, “that one of the members of the Academy revives the stars of (Gaetano) Cacciatore and (Louis François) Wartmann.” Indeed, in the Comptes rendus de l’Académie des sciences of 12 October, on page 716, appear the following words: “Several academicians have examined whether there might be any truth in identifying Le Verrier’s planet with two other objects observed several years ago by Messrs. Cacciatore and Wartmann.” Wartmann (1793–1864), an amateur astronomer in Geneva, had observed in 1831 an object that, like Neptune, followed a slow retrograde motion, and published an account of it (CRAS 2, 1836:307–311). (Wartmann is often cited by the UFO fans as having observed one in 1831.) The identification of the perturbing planet with Wartmann’s object was proposed by Guglielmo Libri (1803–1869), a member of the Academy of Sciences. Though Libri was a good enough mathematician, he chiefly devoted himself to polemics and to plundering libraries, and was an avowed enemy of Arago. He wanted to minimise the credit for Le Verrier’s discovery by insinuating that the planet had been discovered previously. During the following meeting on 19 October, Arago showed decisively that neither Cacciatore nor Wartmann could possibly have observed the new planet (CRAS 23, 1846:741–754), a conclusion fully corroborated by subsequent research; in particular, although Wartmann’s object was, in 1831, 9° from where Neptune was lurking at the time, the latter’s motion in the heavens was too slow for it to be the same object. The daily press, nevertheless, until the end of 1846, tried to stir up controversy by repeated claims or propositions similar to those of Libri (Centenaire, p.51–52). Remarkably, Wartmann may well have recorded a planet—but it was not Neptune. Since Wartmann had observed a decade after the Uranus-Neptune conjunction, Uranus and Neptune were still quite close together in 1831. Indeed, they were just 9° apart. If we assume that Wartmann made a small error in plotting or reading the position from a map, or perhaps failed to properly apply a correction for precession, the position of Wartmann’s object turns out to coincide almost exactly not with Neptune, but Uranus! (Baum, 2007:271–274). More serious criticisms began to surface as soon as it became apparent that the true orbit of the new planet differed significantly from that predicted by Le Verrier (Fig. 5.6). It is true that Le Verrier had indicated the planet’s elements with deceptive precision, when in fact they were necessarily rather uncertain. In his enthusiasm, he had confidently fixed the planet’s distance within an excessively narrow range, giving 35 to 38 astronomical units (AU) for the semi-major axis of the orbit. After the discovery, the actual value was found to be only 30 AU by the American astronomer Sears Cook Walker (1805–1853) (see Chap. 9, Sect. 9.1).

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Fig. 5.6  The orbits of Uranus and Neptune. The axes mark longitudes as seen from the Sun. The positions of Uranus on its orbit are indicated by dark grey circles for different dates from 1800 to 1846. The empty circles schematize the positions that Uranus would have occupied if not perturbed by Neptune (the separation is very exaggerated on the figure). The orbit of Neptune is plotted with grey circles indicating the positions at the same dates. As Neptune revolves in its orbit more slowly than Uranus, one sees that its attraction had accelerated Uranus before the 1821 conjunction, and retarded it afterward. Le Verrier’s orbit of Neptune is shown, with positions at the same dates (black circles). Seen from the Earth, the position calculated for the date of the discovery is about 1° behind the real one. Adam’s orbit for Neptune is not very different from that of Le Verrier for the considered period, but the direction for the date of discovery is at more than 2° of the real position

Similarly, he put the sidereal period between 207 and 233 years, when in fact it is only 164 years. Moreover, the discovery of Triton, a satellite of Neptune, on 10 October 1846 by the English amateur astronomer William Lassell  (1799–1880), using his 24-inch (60 cm) reflector, led at once to an accurate determination of the planet’s mass. (See Chap. 8, Sect. 8.1). Lassell kept the satellite under observation for several months, during which he found that the period of revolution of Triton around Neptune was just over 6 days. He worked out the orbit of this satellite (CRAS 25, 1847:465–466), and using the distance given by Le Verrier, derived from this the mass of Neptune to be 20 times that of the Earth. Le Verrier had expected the planet to have a mass 36 times that of the Earth. In fact, even Lassell’s value was too high since he used an incorrect value of the distance; we now know that the mass of Neptune is 17.2 times that of the Earth. Clearly, these discrepancies were substantial, and this led the Harvard astronomer Benjamin Peirce (1809–1880) to maintain, in a discussion of Lassell’s observations, that Galle’s discovery had been a matter of sheer luck (Proceedings of the American Academy of Arts and Sciences 1, 1846–48:286–295). But it was soon noted that Peirce himself had made a mistake: in discussing Lassell’s observations, he incorrectly deduced that Triton revolved

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around Neptune in 21 days, in which case the mass of Neptune would have been much too small to have any significant effect on the motion of Uranus. All these criticisms melted away, however, in the face of the immense success of the discovery of Neptune. The solution to these difficulties lies in the fact that the only quantity that can be accurately determined by the study of perturbations is the intensity of the perturbing force. In this case, it was that exerted by Neptune on Uranus near the times of conjunction of the two planets, i.e., when they are closest together, for example in 1821 (Fig.  5.7). In accordance with the law of gravity, the force is proportional to the mass of Neptune and inversely proportional to the square of its distance from Uranus. Figure 5.6 shows that the distance between Uranus and Neptune predicted by Le Verrier for this epoch does not differ greatly from the actual distance: it is, however, a bit too great, a circumstance that is more or less canceled out by the excessive mass he assigned to the planet. Having used a too-large semi-major axis for the orbit, Le Verrier exaggerated its eccentricity, which is in fact nearly zero (circular orbit). Delaunay, an incontestable specialist in celestial mechanics, but not on good terms with Le Verrier, summed up matters in 1868: M. Leverrier (Delaunay wrote his name always in this way) is certainly a talented individual. He has done excellent work in theoretical astronomy, and has bequeathed to science the best tables that we possess concerning the movements of the Sun, and of the planets Mars, Venus, and Mercury. It is remarkable that the work leading to the discovery of Neptune and

Fig. 5.7  Comparison of the perturbing force exerted on Uranus by Neptune at different epochs (full arrows) and by the hypothetical planet of Le Verrier (dashed arrows) (Danjon, 1946b). One sees that the direction of the perturbing force is approximately correct (although the date of conjunction of Neptune with Uranus is too late by 1½ years), but that its intensity is too large (Credit: Ciel et Terre, Société Royale Belge d’Astronomie)

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which served to establish his immense reputation (with the assistance of M. Arago) was nevertheless the worst of all his works: he did not dare introduce them to the Annals of the Observatory, where he has published all his other Memoirs. (Bigourdan, G., 1933:A.30–A.33)

This judgment is severe, but at least in part justified by the exaggerated precision with which Le Verrier believed he was able to determine the elements of the new planet. In defense of Le Verrier, one must say that the problem he solved was entirely new. But he was not the only one to tackle it.

5.5  The Competition Given that the idea of the existence of a planet disturbing the movements of Uranus was very much current, it is not surprising that several astronomers attempted, as Le Verrier did, to predict its position through calculations. Who then were these competitors? One of those interested in this problem was none other than the great Friedrich Wilhelm Bessel. He had spoken as early as 1840 about the idea of a perturbing planet in a public lecture. About 1845, he wrote to Alexander von Humboldt (1769–1859): I believe the moment will come when the solution of the mystery of Uranus will perhaps be furnished by a new planet, whose elements will be ascertained by its action on Uranus and verified by those it exerts upon Saturn. (Letter cited by Felix Tisserand (1845–1896), 1889:375)

Even before 1840, Bessel had gone so far as to assign his student Wilhelm Flemming (1812–1840) the task to collect and reduce the observations of Uranus, so as to compare them with the tables of its motion. Flemming’s premature death in 1840, and the long illness leading to that of Bessel himself in 1846, prevented their success. Otherwise, it is entirely possible that Bessel, a mathematician of genius, would have arrived at the solution, perhaps before Le Verrier. Another competitor did actually succeed: John Couch Adams. Less than two weeks after Le Verrier had announced the discovery of the planet at the Academy of sciences, on 5 October 1846, Le Verrier received a letter from Airy who was just back from a trip in Germany. In this letter dated 14 October, one reads the following sentences: I do not know whether you are aware that collateral researches had been going on in England, and that they had led precisely the same result as yours. I think it probable that I shall be called on to give an account of these. If in this I shall give praise to others I beg that you will not consider it as at all interfering with my acknowledgement of your claims. You are to be recognised, beyond doubt, as the real predictor of the planet’s place. I may add that the English investigations, as I believe, were not quite as extensive as yours. They were known to me earlier than yours. (reproduced in Centenaire 1911, p. 29–30)

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Le Verrier, who knew nothing about any of this, responded sharply to Airy on 16 October: The satisfaction you have given me has been, I confess, disturbed by a letter of Mr. Herschel as communicated to me, which is in very bad taste, and fails to do me justice. What can be his motive? I can’t quite understand him, especially when he descends to insinuations that I find mortifying. Of what use, therefore, is it for Mr. Herschel to cry out, before all England, that I was not good enough to deserve his confidence… Have I been so wrong in the theory of the secular inequalities? Is the theory of Mercury so far wrong…? … But the story is perfectly clear, given that not a single line of serious work had been published (by anyone else) in the course of all my researches. And only now Mr. Herschel belatedly comes around to raise a claim in favor of historical documents! Why would Mr. Adams have kept silent for four months? Why wouldn’t he have spoken from the month of June (onwards) if he had had something to say? Why wait until the object has been seen in the telescope? I could add many other questions as well on this subject. But I will only ask one of Mr. Herschel. How is it that the son of the immortal astronomer who discovered Uranus has the audacity to write that my calculations alone would not have given confidence he showed before the British Association? Why, the day after the discovery of my planet, does he not see that he brought into question his scientific judgment, by placing under an injurious suspicion a labor which in fact had been confirmed in the most spectacular manner? (Centenaire, p. 30–33)

The incident to which Le Verrier refers was that Sir John Herschel had published in a letter in the journal The Athenaeum, on 3 October, which became known on the other side of the Channel right after the discovery of Le Verrier. In it, he stated that he had had confidence in the French astronomer’s calculations only because they had been corroborated by those of Adams. After Le Verrier and Airy exchanged several letters, Airy attempted to set out a full account of the matter. He wrote: I received your letter of the 16th and I am very sorry to find that a letter published by Sir John Herschel has caused you so much pain…. I am certain that Sir John Herschel would be equally sorry, for he is the kindest man, and the most scrupulous in his endeavours to do justice to all persons without giving offence to any, that I ever saw. I am confident that you will find, upon examining closely into the matter, that no real injustice is done to you: and I hope that you will receive this expression the more readily from me, because I have not hesitated to express to others as well as to yourself very strong feeling upon the extraordinary merit of your proceedings in this matter. This I intend shortly to express in a more public manner…. Meanwhile I will state to you a few facts and a few considerations which will enable you to judge of the justice of Sir John Herschel’s expressions. A considerable time ago, probably in the year 1844 or in the beginning of 1845 (I have not had leisure since my return (from Germany) to refer to my papers) I supplied Mr Adams with several places of Uranus, expressly for an investigation into the cause of its disturbance. In October or November 1845 I received from Mr. Adams a notification that the disturbances could be explained by supposing another planet to exist, of which he gave me the elements. Shortly after this I addressed to him the same inquiry which I afterwards addressed to you, namely whether the error of radius vector was explained by the same disturbing planet. I know not whether any accident prevented Mr. Adams from receiving my letter: at any rate, he gave me no answer. Had he answered me, I should have urged him immediately to publish his investigations.

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In June 1846 the Comptes Rendus issue containing your investigations was received by me: I was astonished and delighted to find that the elements were nearly the same and the present apparent place of the disturbing planet nearly the same as those given by Mr Adams’ investigations. On June 29th a meeting of the Board of Visitors of the Royal Observatory was held (at Cambridge) at which Sir J. Herschel and Professor Challis were present. At this meeting there was question about the expediency of distributing subjects of observations among different observatories, and I strongly urged the importance of distribution in some such cases, and I specially stated the probability of now finding the disturbing planet if one observatory could be devoted to the search for it. I gave as my reason the very strong evidence afforded by the agreement of the result of your researches and Mr. Adams’ researches. It was my strong statement upon this that induced Sir John Herschel to express himself at the meeting of the British Association and to write such a letter to The Athenaeum. It was my statement which (followed by some correspondence) induced Professor Challis to search for the planet. Professor Challis commenced his search on July 29, and saw the planet first on August 4, and subsequently on August 12 (without comparing his observations and so failing to realise it was a planet). All the rest of the history is known to you. (Airy to Le Verrier, 19 Oct. 1846, McA 33:5, RGO, in response to Le Verrier to Airy, 16 Oct. 1846, McA 33.4, RGO)

Airy’s strategy was an attempt to justify Herschel’s distrust by insisting on the necessity of verifying the calculations, and by affirming that the English astronomers had waited to make their announcement until Adams’s results were confirmed by those of Le Verrier, rather than the other way around. For his part, Challis wrote on 15 October in The Athenaeum, giving details about his observations and maintaining that he had seen on 12 August in the region of the sky where Adams had indicated that one would find the perturbing planet, an object that was not where it had been on 30 July: I undertook to make the search, and commenced observing on July 29. The observations were directed, in the first instance, to the part of the heavens which theory had pointed out as the most probable place of the planet; in selecting which I was guided by a paper drawn up for me by Mr. Adams… On July 30, I went over a zone 9' broad, in such a manner as to include all stars to the eleventh magnitude. On August 4, I took a broader zone,—and recorded a place of the planet. My next observations were on August 12; when I met with a star of the eighth magnitude in the zone which I had gone over on July 30,—and which did not then contain this star. Of course, this was the planet; −the place of which was, thus, recorded a second time in four days of observing. A comparison of the observations of July 30 and August 12 would … have shown me the planet. I did not make the comparison … partly because I had an impression that a much more extensive search was required to give any probability of discovery—and partly from the press of other occupations. The planet, however, was secured…. The part taken by Mr. Adams in the theoretical search after this planet will, perhaps, be considered to justify the suggesting of a name. With his consent, I mention Oceanus as one which may possibly receive the votes of astronomers. (The Athenaeum, 17 October 1846, p. 1069)

The publication of these letters from England, notwithstanding that they contained Challis’s admission of failure, excited considerable alarm in France. Arago mounted the battlements to defend Le Verrier before the Academy on 19 October, alleging justly that the English had not published anything and that “there exists only one rational and just way to write the history of science: that is, to rely exclusively on

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publications having a definite date; beyond that, everything is confusion and obscurity.” (CRAS 23, 1846:751–754). The cartoonists were let loose on an orgy of nationalism and proceeded to attack Adams and defend Le Verrier. Eventually the furore settled down. English astronomers, including Airy and Herschel, recognised Le Verrier’s priority at least for the time being: later they qualified their position. In particular Airy, in a long presentation given before the Royal Astronomical Society on 13 November 1846, put Adams and Le Verrier on an equal footing, even while showering the latter with praises (Airy, 1846:121–144). It’s this version by Airy that would long seem the definite source among historians, planting the seeds from which stereotypical images of the protagonists grew up. Adams was cast as a shy and callow youth who would go on to be acknowledged as “the greatest English astronomer after Newton.” Challis, for his part, was considered (not entirely without reason) as an incompetent bumbler who recorded the planet without recognising the significance of what he had seen. Airy himself was the narrow bureaucrat, etc. It is only since recovery of the ROG Neptune File in 1998 that historians have been able to reassess events. Hence in Chaps. 4 and 7 Adams, Challis and Airy emerge in an entirely different light. Additionally, newly accessible sources in France and Germany have made possible this volume including full French and German accounts for the first time. In contrast to Le Verrier, Adams, who had nonetheless demonstrated a great ability (his method was similar, though he had used Peter Hansen’s equations instead of those of Laplace), had hesitated to make known his results. Even though he had calculated the orbital elements of the new planet as early as October 1845, he did not give Airy all the information needed to induce the latter to undertake a search for the planet, by not answering his question about the radius vector. Still lacking complete confidence in his calculations, he had rather desperately tried to use the positions of Wartmann’s object to narrow the scope of his investigation. Also he has relied on the information provided by Le Verrier on 1 June 1846 to produce for Challis an ephemeris of sky positions of the perturbing planet to direct the latter’s search (this was the “paper drawn up for me by Mr. Adams,” which Challis alluded to, somewhat misleadingly, in his letter of 15 October 1846). Adams estimated the uncertainty of the positions in this paper as 20° in longitude, which was much greater than that of Le Verrier and would have reinforced Challis’s expectation that he was undertaking a prolonged siege rather than a brief skirmish. Adams’s hesitation, which contrasts with Le Verrier’s assurance, may have been a function of his personality. (Sheehan & Thurber, 2007; see Chap. 4). In any case Airy’s verdict on Le Verrier was to see virtue in “the firmness with which he proclaimed to observing astronomers, ‘Look in the place which I have indicated, and you will see the planet well.’… It is here, if I mistake not, that we see a character far superior to that of the able, or enterprising, or industrious mathematician; it is here that we see the philosopher.” (Airy, 1846:42). Adams, though Airy implicitly defends him, is nevertheless placed, in a different, inferior category. Challis also has some excuse for his inertia for his tardy recognition of the planet’s presence among the stars he was mapping: he did not have available the Hour XXI star-map of the Berlin Academy Airy had suggested to use, since it had not yet been sent out from Berlin. But he

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had—or at least the Cambridge University Observatory had—another chart from the same series, which contained the section of the sky in which the planet was lurking and which would have sufficed for him to locate the perturbing planet. However, the existence of this chart has been unearthed only by recent investigators. Mercifully, Challis himself probably never realised that he had it. Another footnote in the Neptune discovery is the fact that John Herschel himself nearly discovered the planet on 14 July 1830, as he informed Le Verrier on 9 January 1847 (Library of Paris Observatory, Ms 1072,8). He recognized the object he observed was not a star because it showed a small disk, but he supposed it to be a planetary nebula and thought no more of it. Also, as with Uranus, an observation of Neptune turned up that was made a long time before its discovery: Michel Lefrançois de Lalande, the “nephew” of Joseph Jérome de Lalande, observed Neptune on 6 and 8 May 1795 with the large Bird mural quadrant of his observatory at the École militaire. He recorded it as a “star” whose positions differed slightly between two observations. However, believing this to be an observational error, Jérome de Lalande only gave the position of the 8 May in his Histoire céleste. The star is given in the Berlin maps, which were based partly on Lalande’s catalog, but it has gone missing when the American astronomer Sears Cook Walker (1805–1853) returned to this location on 2 February 1847: it had indeed moved a great deal. Walker correctly deduced from this that the missing star might be Neptune, and using Lalande’s position for it in 1795 was able to work out the first high-quality orbit for the planet (The Intelligencer, 5 June 1847, 1, col. 6): the semi-major axis was 30.2 astronomical units, the eccentricity 0.0088 and the period of revolution of 166.4 years, values which are much closer to the modern values than those of Le Verrier given in Table 5.1.

5.6  Janus, Oceanus, Neptune or Le Verrier? Astronomers have habitually named planets and asteroids after the gods and goddesses of the Greeks and Romans. After the discovery of the perturbing planet, it was necessary to agree on a name for it. Normally, the astronomer who makes the discovery offers a proposal, and a learned scientific society votes on its appropriateness. As we have seen, Galle proposed the name Janus, then Adams and Challis suggested Oceanus. Since Le Verrier was considered to be the true discoverer of the planet, as Arago put it so poetically, he had discovered it “at the tip of his pen,” these other suggestions were dark horses at best. Ultimately, it was Le Verrier’s prerogative to name the planet. Indeed, it seems to have been Le Verrier himself who first proposed Neptune, asserting, moreover, to his correspondents that the Bureau des longitudes had already selected this name and introduced a symbol for the planet, a trident (curiously, transcripts of the society show no record of any of this!). To illustrate the confusion, consider this excerpt from Airy’s letter of 14 October 1846 cited earlier:

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There is one thing which somewhat disturbs my mythological ideas, namely the name Neptune, which (it is understood,) you propose to fix upon the planet. There seems to be an interruption of order which is unpleasant. If you would consent to adopt the name Oceanus instead, it would, I think, be better received, as more similar in its character to that of its predecessor, Uranus, and more closely related to the mythological ideas of the Greeks. I beg you to think of this carefully, for experience has shown that a name will not last unless it is well selected. The name of Stella Medicea (given by Galileo to the satellites of Jupiter in honor of the Medicis) has perished, and the adjunct of Ceres Ferdinandea (by Piazzi) has perished, and the name Georgium Sidus (for Uranus, by William Herschel) has perished, although all these were given by their respective discoverers.

In fact, Oceanus, the name proffered by Challis and Adams and now endorsed by Airy, was never seriously considered outside of England. But now Le Verrier, having first proposed Neptune, seemed to have had second thoughts. Unaccountably, he resigned the task of choosing the planet’s name to Arago. Arago, in turn, promptly proposed a different name —“Le Verrier.” In a rather fawning letter of 6 October to Encke (Centenaire, 1911:40), Le Verrier pretended embarrassment at this turn of events: Mr. Schumacher has done me the honor of writing, and asks me to send him a name for the planet. I have asked my illustrious friend, Arago, to take charge of this care. I was a bit taken aback by his decision announced before the assembly of the Academy. I would not be able to explain in what my consternation consists if I did not find at the same time an opportunity of paying tribute to your admirable work on Encke’s comet.5 The obscure name that M. Arago wants to give to the planet would bestow upon me the same honor as that accorded to the illustrious Director of the Berlin Observatory (who was none else than Encke himself), and I do not deserve it.

In effect, one finds this in transcript of the meeting of 5 October of the Academy of Sciences, during which Arago presented the discovery: … Mr. Arago has announced to the Academy that having received from Mr. Le Verrier a very flattering assignment: the right to name the new planet, he has decided to designate it with the name of the person who has shown such erudition in discovering it, and to call it Le Verrier… How is this! One names comets with the names of astronomers who have discovered them, or of those who have computed their orbits, and one refuses the honor to the discoverers of planets!... Is one preoccupied or worried because my resolution would seem to entail other changes? Fine, I don’t hold only to this: Herschel will dethrone Uranus, the name of Olbers will be substituted for that of Juno (a minor planet that Olbers had discovered), etc.; it is never too late to shed the swaddling clothes of old habits. I will commit myself, Mr. Arago concluded, to never call the new planet, by any name other than Planet Le Verrier. I believe that in this way I will give an undeniable proof of my love for science, and will follow the inspirations of a legitimate national sentiment. (CRAS 23, 1846:662)

Not everyone agreed with this proposal. One reads, indeed, in the Revue des deux mondes (October 1846, p. 383)

5  Contrary to planets, comets are designated by the name of their discoverer; actually, it is JeanLouis Pons who discovered the comet in question, but because Encke found previous observations and showed that it was the comet with the shortest known period, his name has been given to it; the same thing had occurred before with Halley’s comet.

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We will only say one word concerning a minor incident that has arisen regarding the discovery by Mr. Le Verrier: what name will be given to the new planet? Despite the judicious observations of M. (Louis Jacques) Thénard and M. (Louis) Poinsot, M. Arago persists in calling this planet by the name of Le Verrier.

Le Verrier was evidently highly satisfied with Arago’s proposal. Moreover, he attempted to secure the situation by trying to arrange that Uranus carried henceforth the name Herschel, a name which in effect had been used occasionally (CdT for 1849, Additions, note on p. 3): In my subsequent publications, I will consider it a strict duty to make disappear completely the name Uranus, and to only refer to the planet using the name HERSCHEL. I sorely regret that my already published writings do not permit me to follow the determination that I shall religiously observe henceforth.

Nevertheless, the name “Le Verrier” encountered more and more fierce opposition, and finally the name Neptune was adopted. How did this happen? It is not entirely clear. A letter of John Herschel to Le Verrier showed that John Herschel did not wish to give the name of his father to Uranus, and therefore, a fortiori, the name of Le Verrier to Neptune. It’s interesting in this regard to page through the volumes of the Astronomische Nachrichten. As said before, one finds no trace of a decision in favor of the name Neptune in the proceedings of the meetings of the Bureau des Longitudes, though it would have been within their purview to make such a decision. Arago, moreover, was there to supervise the matter. He manifested, also, his discontent, by recalling in the Annuaire du Bureau des longitudes for 1847 (p. 371), that he had proposed to call the planet Le Verrier, and that foreigners, leaning on alleged decisions of the Bureau des longitudes, call it nowadays Neptune. Then he complained that he did not find any collaborators to help him write a “detailed history of the new planet,” which is hardly surprising because no one wanted to get mixed up in such a scabrous affair. Le Verrier was evidently furious about the Bureau des longitudes’ decision to adopt the name Neptune, and wrote to Airy on 26 February 1847 (Centenaire, 1911:45): When the planet was discovered, it was proposed by the Bureau of longitudes to call it Neptune. I was not part of the Bureau at the time, and I did not charge it with this decision…. I declared … to M. Arago that the Bureau was a little too hasty, and that I would specifically entrust him with the task of presenting to the Academy of Sciences whatever he judged to be most suitable. Since then I have had no further involvement in the matter.

Airy responded two days afterwards (same reference) that, despite Le Verrier, he would adopt the name Neptune because of the agreement of the “principal astronomers of Northern Europe,” and, of course, his “English friends.” In the end, everything about this muddled naming affair becomes comprehensible as soon as one admits to a certain duplicity on the part of Le Verrier. Still, in retrospect, Arago’s fervid interventions before the Academy are also perplexing. One suspects that there may be something missing from the whole account. One perhaps far-fetched possibility is hinted in a letter written in 1869 by Delaunay to

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the Minister of Public Instruction. After complaining about Le Verrier’s willingness to use blackmail to get his way, Delaunay wrote: In 1846, in the aftermath of the discovery of the planet Neptune, M. Arago, driven by certain hideous circumstances from which it is not appropriate to lift the veil, had placed M. Le Verrier on a pedestal, and made him out to be an extraordinary man, one of the greatest geniuses that France had ever produced. Some months later, M. Arago recognised his enormous mistake, but the harm was already done, and he could do nothing to repair the damage. His final years were darkened by his vision of the dreadful consequences that followed inevitably from this. (Bigourdan, 1933:A.30–A.33)

What were these “hideous circumstances?” It’s a shame that the veil had not been raised! One persistent rumor speculates that M.  Arago had had an affair with Madame Le Verrier. Le Verrier, discovering it, took advantage of the situation by using it to blackmail Arago into supporting a proposition that was clearly indefensible. This, however, is impossible to verify, and seems rather far-fetched. What cannot be denied is that Le Verrier always seemed to feel that Arago never did enough for him, and this attitude would eventually lead to a definitive and fateful rupture. Acknowledgements  I thank Bernard Sheehan for his excellent translation of my book on Le Verrier. I also wish to acknowledge fruitful exchanges with Davor Krajnović during the preparation of our chapters.

References The preceding text comes with a few changes from Chap. 1 and 2 of Lequeux, J., 2013 Le Verrier— Magnificent and Detestable Astronomer, Springer. London. The Comptes rendus hebdomadaires des séances de l’Académie des sciences (CRAS), Annuaire du Bureau des longitudes (Ann. BdL), Connaissance des temps (CdT), Journal des savants, Centenaire de la naissance de U.-J.-J. Le Verrier (Centenaire), Tisserand (1889–1896) and Tisserand & Andoyer (1912), can be consulted freely on the site of the Bibliothèque Nationale de France: https://gallica.bnf.fr. Airy, G. B., 1846 Account of the Observations at the Cambridge Observatory for detecting the Planet exterior to Uranus. Monthly Notices of the Royal Astronomical Society, Volume 7, Issue 9, November 1846. Baum, R., 2007. The Haunted Observatory. Amherst, New York, Prometheus. Bouvard, A., 1821. Tables astronomiques publiées par le Bureau des longitudes de France…, Paris, Bachelier et Uzard, accessible via gallica.bnf.fr Centenaire de la naissance de U.  J.-J.  Le Verrier, 1911, Paris, Gauthier-Villars (Académie des sciences), text by G. Bigourdan. Danjon, A., 1946a, La découverte de Neptune, L’Astronomie 60, p. 255–278 Danjon, A., 1946b, Centenaire de la découverte de Neptune, Ciel et Terre, 62, p. 369–383, 62, p. 369–383. Gaillot, A., 1880, Le Verrier et son œuvre, La Nature 2d semester 1880, p. 102–107. Laskar, J., 2017, Des premiers travaux de Le Verrier à la découverte de Neptune. Comptes Rendus Physique. Volume 18, Issues 9–10 November–December 2017, Pages 504–519.

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Lequeux, J., 2013, Le Verrier, magnificent and detestable astronomer, edited with an introduction by William Sheehan, translation by Bernard Sheehan, Springer, New York. Sheehan W. & Thurber, S., 2007, John Couch Adams’s Asperger syndrome and the British non-­ discovery of Neptune, electronic publication. https://royalsocietypublishing.org/doi/10.1098/ rsnr.2007.0187 Tisserand, F., 1889, Traité de Mécanique céleste, Paris, Gauthier-Villars. Tisserand, F. & Andoyer H., 1912, Leçons de cosmographie, 6th ed., Armand Colin, Paris, p. 279–288.

Chapter 6

“That Star is Not on the Map”: The German Side of the Discovery Davor Krajnović

Abstract  Neptune was telescopically discovered by Johan Gottfried Galle and Heinrich Louis d’Arrest in Berlin on 23 September 1846 based on the prediction by Urbain Jean Joseph Le Verrier. The role German astronomers played in the discovery has often been overshadowed by the controversies that erupted in England and France after the discovery. However, their role was crucial, not only in bringing about the discovery in the first place, but also in resolving some of the post-­discovery controversies that erupted around priority for the prediction and naming of the planet. German astronomers in Central and Eastern Europe possessed some of the best telescopes of the day and had established themselves over several decades as being at the forefront of observational astronomy. They had produced the star charts that in the end proved indispensable for allowing identification of the planet, while a German publication, Astronomische Nachrichten, published by Heinrich Christian Schumacher in Altona, in the vicinity of Hamburg, then part of the Kingdom of Denmark, served as the journal of record during the time period with which these events took place. The general neglect of the German part of the story is most strongly attested by the inaccuracies concerning what actually happened on discovery night, which were long propagated in the English-speaking literature. Notably, for 30 years after the discovery, it was not appreciated that there were two observers, while the role of the sky map used was often exaggerated. This chapter sets forth a more complete picture, and in particular emphasises more than previous accounts the critical role of d’Arrest, arguing that he should be celebrated as a co-discoverer. In addition, evidence presented here, much of it not previously available in the English-speaking literature, shows that the name “Neptune” was eventually accepted throughout the scientific community based on German precedents.

D. Krajnović () Leibniz-Institut für Astrophysik Potsdam (AIP), Potsdam, Germany e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_6

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6.1  Introduction In his autobiography, written long after the discovery of Neptune, Astronomer Royal George Biddell Airy has only this to say about what he calls the “engrossing subject” of the year 1846.: … As I have said (1845) I obtained no answer from Adams to a letter of enquiry. Beginning with June 26th of 1846 I had correspondence of a satisfactory character with Le Verrier, who had taken up the subject of the disturbance of Uranus, and arrived at conclusions not very different from those of Adams. I wrote from Ely on July 9th to Challis, begging him, as in possession of the largest telescope in England, to sweep for the planet, and suggesting a plan. I received information of its recognition by Galle, when I was visiting Hansen at Gotha. For further official history, see my communications to the Royal Astronomical Society, and for private history see the papers of the Royal Observatory. I was abused most savagely both by the English and French. (Airy, 1896:181)

As Airy noted here, he was in Germany when he learned of the discovery of the planet by a German observer at Berlin. He singled out the English and the French as the nationalities that subjected him to savage abuse. Curiously, he did not mention Germans. As we have seen, the international rivalry that erupted after the discovery was between the French, who in Urbain Jean Joseph Le Verrier claimed the only mathematician who published a prediction, and the British, who claimed a prior, although unpublished, prediction by John Couch Adams. The British side also could add James Challis’s seemingly rather secretive search at Cambridge in the summer before the planet was actually found (see Chaps. 4 and 8). Despite having been responsible for the actual discovery, about which there was never any contest, the Germans were rather sidelined, and long rendered almost to the status of a footnote as the dispute raged between the two theoreticians and their respective supporters. Yet, the optical discovery clearly belonged to Germans and them alone. Whereas Adams’s calculations could be rehabilitated for at least partial credit, there seemed nothing creditable in English search, which had ended in nearly total failure, underscored by the embarrassing failing to recognise the planet on two occasions before the definitive news from Berlin arrived. Thus, Airy’s role, whatever it might have been, simply did not put him on a collision course with the Germans. As this chapter will show, Germans turned out to be Airy’s strongest allies. One could hardly guess, from the standard retellings, that there were actually several internationally influential German astronomers at the time, who had a decisive role in the Neptune story. The goal of this chapter is to explore their role, and, in particular, the ways in which they contributed to the discovery. As often is the case in science, a discovery might be attributed to a single person, or perhaps two as occurs here with Neptune, but it almost never happened alone and in isolation. In the case of Neptune, this was particularly true. It should be noted from the outset that by “German astronomy” we cannot refer to a monolithic bloc in the same way we can discuss the British or French astronomy. There was no “Germany”, in the sense of a unified national state, in 1846. There was the “German Confederation” created by the Congress of Vienna in 1815 in the aftermath of the Napoleonic wars as a primarily economic substitute for

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the Holy Roman Empire of the German Nation, which had been dissolved in 1806. The German Confederation consisted of 39 independent states (Fig. 6.1), but did not include all of the German-speaking states and principalities in Europe. In addition to the arbitrary character of a political and economic entity created by fiat, the Confederation was weakened by the rivalry between the Kingdom of Prussia and the Austrian Empire. Although Germans had continued to be divided by politics and religions of their principalities, they nevertheless enjoyed a unified language, a sense of national pride and heritage, and were steeped in a brilliant culture. It is in terms of the latter

Fig. 6.1  Map of German principalities in the first half of nineteenth century. The red line outlines the boundary of the German Confederation established at the Congress of Vienna in 1815, as an economic substitute for the Holy Roman Empire of the German Nation that was abolished by Napoleon in 1806. The development of the political situation after the discovery of Neptune slowly reshaped the confederation, first by creating a customs union without lands under the control of the Austrian Empire, then by addition of Schleswig, and finally with the unification of Germany under Prussian leadership in 1871. (Credit: Wikipedia commons (Author: zieglebrenner))

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sense that we define “German astronomy,” and designate as German astronomers all of those who belonged to this wider German cultural and linguistic circle. Some worked in observatories in the small and large German principalities across Central and Eastern Europe. Though some of those who contributed to the Neptune story were in what today is in modern Germany (Berlin), others were in German states that were powerful at the time, as most notably Prussia (Königsberg, Bresslau), but which are now in different countries and are known by different names (Kaliningrad, Wrocław). Some others were outside German principalities but now belong to modern Germany (Altona near Hamburg). Finally, there are those observatories, which, despite being led by German astronomers, were located then—and remain still— outside of German borders (Dorpat—now Tartu, Pulkovo). As might be expected from their far-flung geographical range and many centres of astronomical activity, German astronomers had a correspondingly widespread influence. Owing to the splintered nature of the German political entities, princely lands, duchies, kingdoms, ecclesiastical territories and free imperial cities resisted centralization of academic efforts and enabled a growth of regional observatories. This is most likely the reason why German lands had more observatories than any other country in the world until 1870, when they were surpassed by the United States. In particular, during the decade of Neptune’s discovery as well as the rest of the nineteenth century, there were more German observatories than in France and Britain put together (Herrmann, 1973:51). If one wanted to search the skies for new planets, German observatories, or observatories led by German astronomers across Europe, offered a natural and a well-connected network, as has been discussed, for instance, by Chapman (2017:18–20).

6.2  Laying the Groundwork As discussed in Chap. 2, the 18th-century discovery of Uranus was an unexpected— if not, in fact, a chance—event. William Herschel was systematically sweeping across fields of stars night after night in March 1781, seeking to make out which of them might be double stars. He noticed one that had a disk, and returning to it again, found that it had moved. Nineteen years later, Giuseppe Piazzi applied exactly the same method and discovered Ceres. He was busy compiling his own star catalogue, but unlike Herschel, Piazzi, working together with his assistant Niccolò Cacciatore, was comparing his observations with stars in the catalogue by Nicolas-Louis de Lacaille, until he found one star that was not present in the catalogued stars of Taurus, and that also moved a bit across the sky the next night. Piazzi’s discovery of Ceres differs from Herschel’s discovery of Uranus, in at least two points. Firstly, while it was also serendipitous, it coincided with a targeted effort to look for planets elsewhere. Secondly, it became a showcase for the predictable power of the celestial mechanics and mathematical methods of the late 18th century. Working at an observatory in the duchy of Saxe-Gotha-Altenburg, astronomer Franz Xaver von Zach was trying to assemble a network of observers, to search the

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sky for a hypothetical planet predicted to occupy the space between Mars and Jupiter. The prediction was based on the Titius-Bode distance sequence of known planets (see Chap. 5). In order to make an efficient search, von Zach realised that one needs good sky charts and tried to organize a network of astronomers who will share a burden of sky mapping. The agreement among von Zach’s collaborators, Johann Hieronymus Schröter, Karl Ludwig Harding and Heinrich Wilhelm Matthias Olbers, was to chart stars down to the 9th magnitude over a region of the sky approximately covering 15o of latitude both south and north of the ecliptic. To do it efficiently a network of 24 astronomers was necessary so the work could be divided in zones of about 8o of longitude. In the end, although von Zach’s “Celestial Police” turned out to be quite successful in discovering minor planets (they found three of the first four asteroids), Piazzi discovered the first minor planet while they were still trying to organize themselves. The Celestial Police did not quite manage to deliver the planned all-­encompassing charts of the zodiacal region. Harding created Atlas novus coelestis (Harding 1822) long after the group’s efforts to search for possible planets had stopped. Harding’s work stands in marked contrast to previous star charts, including the celebrated Atlas coelestis of John Flamsteed (Flamsteed 1729), Atlas coelestis by Johann Gabriel Doppelmayr (Doppelmayr 1742) and Uranographia produced by Johann Elert Bode (Bode, 1801). Whereas Uranographia has been referred to as “the pinnacle of gloriously engraved star-atlases” (Olson and Pasachoff, 2019:42), Harding’s Atlas novus coelestis completely dispensed of pictorial extravagance. More to the point, it is historically significant as being the first sky chart published without drawings of the allegorical constellations, with an unprecedented purpose of being solely created in support of practical work. (Fig. 6.2). These events are not directly related to the discovery of Neptune some 45 years later. In retrospect, however, they indicate that the discovery of Neptune was a story in two parts. The first one was theoretical, belonging to what was then called “mathematical astronomy.” The newly developed mathematical tools and, in particular, the confidence in the Newton’s theory of gravitation, ultimately emboldened the mathematical astronomers to put forward unprecedented predictions. The other part was empirical and consisted in preparing the ground, such as charting the skies to facilitate location of transiting bodies, and the actual observations necessary for any genuine discovery of astronomical objects. In both of these activities the most prominent German astronomer of the generation, Friedrich Wilhelm Bessel, took a central role.

6.3  Bessel’s Verification of Newton’s Theory of Gravitation Bessel was born in Minden, then part of several unconnected principalities belonging to Prussia, but in the vicinity of other major German political entities, Bremen and Hanover. Not a very good Latin scholar, but inclined to natural sciences, he asked his father to allow him to choose a business profession, one that would allow him to perform calculations (Schmeidler, 1984:11). In 1799, he was sent as an

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Fig. 6.2  This star chart from the 1822 Atlas novus coelestis by Harding (1822) shows the region where Neptune was found. The red hexagon shows the location of Neptune at the discovery night. The incompleteness of this map can be judged by comparing it with Carl Bremiker’s Hora XXI (Fig. 6.5). Note how constellations are marked only in a rudimentary way. (Credit: Harding’s Atlas kept in the Library of Leibniz-Institut für Astrophysik Potsdam)

apprentice to Kulenkamp & Sons in Bremen, the trading company for which he worked for seven years. While the company appreciated his talents and even started paying him a decent salary, Bessel quickly recognised that the work in the office, even of an expanding business, could not occupy his mind fully. In order to understand the intricacies of the sea trade, especially determination of the location of a ship, he started studying the art of navigation, which led him to trigonometry and more complex mathematics, and finally to astronomy. Bessel was a self-taught prodigy who happened to be at the right time at the right place. At the start of 1801 Piazzi discovered Ceres and during the same year Johann Carl Friedrich Gauss predicted its location, which helped Olbers and von Zach to find the asteroid. Then on 28 March 1802 Olbers discovered the second minor planet, Pallas. The Celestial Police was in full swing with Harding discovering Juno in 1804 and Vesta in 1807 by Olbers again. The discovery of Pallas was of high importance for Bessel; not only did it captivate his mind, it also put his interests firmly into the domain of astronomy, but he also knew (at least on sight) its discoverer. Heinrich Wilhelm Olbers was a well-known physician in Bremen with a practice below and an observatory on top of his house. In 1804, then 20-year-old Bessel gathered courage and approached Olbers with his calculation of the orbit of Halley’s comet based on the 1607 data by Thomas Harriot (1560–1621) and Nathaniel

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Torporley (1564–1632), unearthed and published by von Zach in 1793  in Astronomisches Jahrbuch of the Berlin Observatory (Hamel, 1984a:16–17). Olbers was impressed by the work of young Bessel, Gauss as well, and while Bessel continued to work for Kulenkamp & Sons, it was only a question of time when he would change his profession. Bessel first moved to a private observatory run by one of the key members of the Celestial Police, Johann Schröter in Lilienthal, rejecting an offer of a secure and a well-paid job at Kulenkamp. Bessel replaced Karl Harding in the position of an assistant. (Harding, following the fame he had achieved as the discoverer of Juno, had accepted a position as professor of practical astronomy at Göttingen.) Schröter’s observatory was well equipped with telescopes of “exquisite beauty,” as Bessel himself declared (Bessel, 1810:4). It also had one of the largest telescopes in the world at the time, with a mirror 48.4 cm in diameter, well suited for Schröter’s passion of observing the surface of the Moon and planets (Hamel, 1984a:22). Nevertheless, in 1810, after 3 years with Schröter, and now aged 26, Bessel moved to Königsberg as the director of a newly founded observatory and became the professor at the Albertus University. Only then did his financial means start equating with what he could have earned as a businessman.1 Bessel’s move from Lilienthal was timely; Schröter’s Lilienthal observatory did not survive the retreat of the French soldiers in 1813, who destroyed or carried off clocks, telescopes, and other astronomical instruments (see Sect. 2.4). In such wartimes, it is remarkable to consider that the government of Prussia, after the defeat by Napoleon in 1807 and effectively being occupied by the French, was thinking of building astronomical observatories. This effort should probably be viewed within the larger reforms within the Prussian state, in which Wilhelm von Humboldt (1767–1835) led an educational reform. Next to establishing the first university in Berlin (today’s Humboldt University), he also convinced the King that the east Prussian capital Königsberg should have a well-equipped observatory led by a prominent astronomer. The choice of Bessel, by that time sought after for p­ ositions in Leipzig, Griefswald and Gotha (Schmeidler, 1984:17), was exceptionally longsighted. Of the German astronomers connected to the Neptune story, Bessel is nowadays by far the best known. His contributions to mathematics, geodesy and astronomy are numerous. To a general scientific audience, he is best known for his estimate of the oblate shape of the Earth, his solutions to certain differential equations, his analyses of instrumental errors and their effects on observations, and his first measure1  When Bessel was moving to Lilienthal, Kulenkamp & Sons offered him a salary of 700 Thalers per year, while for the appointment in Königsberg he got 800 Thalers per year together with a house and heating expenses covered (Schmeidler, 1984:18). This could be compared with the wages (Wulf, 2016:124) of a skilled worker (up to 200 Thalers per year), Alexander von Humboldt’s pension by the King of Prussia (2,500 Thalers per year) and Wilhelm von Humboldt’s salary as the ambassador in Rome (13,400 Thalers per year). When Johan Franz Encke became the director of the Berlin Observatory in 1825, he had a salary of 2,000 Thalers, of which 300 Thalers he earned as the secretary of the Berlin Royal Academy of Sciences, but his compensation did not include the accommodation (Bruhns, 1869:115).

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ment of stellar parallax (61 Bessel 1838), beating to it Wilhelm Struve and Thomas Henderson (1798–1844). He also predicted the existence of an “invisible” companions to stars Sirius and Procyon (Bessel 1844a, b). It is less known, however, that Bessel spent a considerable effort on a theory of gravity directly motivated by the anomalies of the motion of Uranus. This is not surprising as nothing of Bessel’s theory was published, and it only resurfaced in unpublished papers and letters Bessel exchanged with fellow astronomers including Olbers, Heinrich Christian Schumacher, Johann Franz Encke, Wilhelm Struve, and Gauss. It was one of those scientific projects that seem to lead nowhere, and in hindsight looks to be a completely incorrect path. For Bessel, however, this was a theory worthy of investigation, but once he was convinced that it did not work, he completely abandoned it. He was convinced that Newton’s law of gravitation can be used at the vast distances of the space, including other stars, and that there is only one solution to the Uranus problem (Fig. 6.3). Bessel’s hypothesis for the deviations of Uranus from its predicted path was that Newton’s law of gravitation should be modified. He asserted in an unpublished manuscript (Bessel 1823a) that “a planet can act upon two bodies not belonging to itself, at equal distances with unequal accelerating forces, or, according to previous use of language, with unequal masses.” Jürgen Hamel (Hamel, 1984b:280) presented this quotation in 1984 soon after the discovery of Bessel’s unpublished manuscript. Bessel’s idea was that the gravitational action of one body on another depended on the nature of each body. Thus, one can assume that the mass with which Saturn acts on Uranus is different from the mass with which Saturn acts with the Sun (or Jupiter). If this idea were true, then the Fig. 6.3 Friedrich Wilhelm Bessel (22 July 1784 – 8 April 1846). Bessel was intrigued by the Uranus problem for 30 years, observing it and testing Newton’s theory of gravity until he was convinced an external planet must be perturbing Uranus’s orbit. The painting is from 1839 by Christian Albrecht Jensen, who also painted a number of other prominent German astronomers of the time. (Credit: Wikipedia, Public Domain)

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anomalous motion of Uranus might be accounted for on the basis of this modification of Newton’s law. In particular, this issue was recently raised by Alexis Bouvard (1767 – 1843) who showed that one could either satisfy the old (pre-discovery) or new (post-discovery) locations of Uranus with any given orbit, but not both at once (see Sect. 2.2 and 2.4; Bouvard 1821) Olbers, who seems to be the only one among Bessel’s correspondents to show an interest in this theory, called it “Wahl  – Anziehung” or “Selective Attraction” theory (Hamel, 1984b:279). Bessel, however, did not use Bouvard’s data. Instead he asked his students Friedrich Wilhelm Argelanger (1799–1875) and Otto August Rosenberger (1800–1890) to reduce the known observations of Uranus and derive the deviations, respectively. The data stopped with 1821, and the manuscript is not finished; Bessel does not seem to have been able to solve the problem of Uranus motions using the selective attraction hypothesis. Instead he abandoned the project and moved onto other matters. Bessel’s abandonment of selective attraction is evident from his letters, notably on 2 October 1823 he wrote to Encke that he was not satisfied with his results. The derived masses for Saturn (as compared to the mass of the Sun) was 1/2482.2 when it interacted with the Sun, and 1/2427.7 when it interacted with Uranus (Hamel, 1984b:281). The later value was problematic as it was not compatible with the mass required to explain the interaction of Saturn with Jupiter, a problem already (mostly) solved in the eighteenth century (Wilson 1985). Soon after, on 9 October 1823 in a letter to Olbers, Bessel wrote (Hamel, 1984b:282): “My investigations into the motion of Uranus had to be concluded, because they have led to a result which makes further investigations on Jupiter and Saturn necessary.” Bessel’s incursion into doubting the universality of the Newton’s law was not confined only to the motion of Uranus. He performed numerous experiments testing the validity of the law of gravitation, experimenting with specially devised pendulum experiments. He investigated the effects of the increase of the moment of inertia of the pendulum due to air resistance, thus increasing the precision of the measurements. Inspired by the Uranus problem and his selective attraction hypothesis, following Newton’s experiments, he also tested if by using different materials the oscillations of the pendulum would change. He used gold, silver, lead, iron, zinc, brass, marble, quartz, water and meteorite iron and stones (likely the first experiments using meteoritic materials), and concluded that gravity does not depend on the type of material to within a fractional error of 6x10-4 (Hamel, 1984a:48), an improvement of two orders of magnitude on Newton’s result (Bessel, 1831, 1832). Although Bessel’s experiments were performed several years after Bessel abandoned his work on the Uranus problem using the selective attraction hypothesis, that was not the end of Bessel’s fascination with Uranus. As soon as he equipped his new observatory in Königsberg (Fig. 6.4) with decent instruments, Bessel started a systematic observing campaign of the planet. The first campaign was between 1814 and 1818, while the second extended a full 17 years from 1820 to 1837. At least the later part of the second campaign was fuelled by the conviction that Uranus motions could only be explained by an existence of an external planet. Already in 1824, in a letter to Gauss dated 14 June 1824, he mentioned this possibility, although scepti-

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Fig. 6.4  Königsberg Observatory in 1830. Bessel chose the location of the building and oversaw its construction. As the Prussian government delayed funds for the building, Bessel threatened to move to the observatory in Mannheim. The Prussian government backed down and provided necessary funds. Napoleon, passing with his armies through Königsberg to launch the attack on Russia apparently asked: “Does the King of Prussia still have time to think on such things?” (Schmeidler, 1984:20). (Credit: Wikipedia, Public Domain)

cally, as an alternative to his failed selective attraction hypothesis (Hamel, 1984b:282): “I did not believe this; if one does not want to assume unknown, disturbing planets.”

6.4  Bessel’s Unfinished Project It is not clear at what point in time Bessel convinced himself that it must be an external planet causing the perturbations. Peter Andreas Hansen, a Danish astronomer who spent most of his career as the director of the Seeberg Observatory in Gotha, claimed that he was always convinced on the existence of an external planet, contrary to Bessel (Wattenberg 1959:3). Next to observing Uranus continuously, it seems Bessel actively came back to the topic in 1837 when he gave to an assistant Friedrich Wilhelm Flemming the task of re-reducing all available Uranus data, observed at Greenwich, Paris and Königsberg. Flemming’s colossal task of reducing 361 observations (148 from Greenwich, 70 from Paris and 143 from Königsberg) and comparing them with Bouvard’s tables was posthumously published by Schumacher (Schumacher 1850). Bessel must have expected a big breakthrough from Flemming’s work. In a public lecture on 28 February 1840 in Königsberg (Bessel 1848:447), Bessel strongly expressed his opinion that an external planet will be found: “Additional explana-

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tions will aim to assign an orbit and mass for an unknown planet beyond Uranus, which may not be visible because of its exceeding faintness, which is such as to produce disturbances of Uranus, which cannot be put into agreement with existing observations.” He furthermore even mentioned Flemming’s work in a postscript to a letter to Johann Gottlieb Galle dated 30 April 1840 (Wattenberg 1959:2). The fact that Bessel himself wrote to Galle at all is remarkable: Galle at that time was just an assistant to Encke and the letter was written to congratulate Galle on discovery of three comets. A few days later, replying to a letter of Alexander von Humboldt who asked him about news on the “planet beyond Uranus,” on 8 May 1840, Bessel was a bit more cautious, saying I think I have already told You that I have worked extensively on this; it has not progressed further than the certainty that the existing theory, or rather, its application to our knowledge of the Solar System as we know it, is not sufficient to solve the riddle of Uranus. However, in my opinion, therefore, one should not regard it as unsolvable. First, we need to know exactly and completely what is observed of Uranus. One of my young assistants, Flemming, has reduced and compared all observations; and with that I now have the facts in full. (Wattenberg 1959:5)

After bringing Humboldt up to date with the issues with Uranus perturbations, Bessel continued I therefore thought that there would come a time when one would find the solution of the riddle: perhaps in a new planet, whose elements could be recognised from their effects on Uranus and confirmed by them on Saturn. I have been far from saying that the time has come: I will only try to see how far the existing facts can lead. It is this work that has accompanied me for many years, and for which I have pursued so many different views, that their end will excite me, and therefore will be brought about as soon as possible. I have great confidence in Flemming who will continue the same reduction of the observations he has now made for Uranus, for Saturn and Jupiter in Danzig, to which he is called. It is fortunate, in my opinion, that he (for now) has no means of doing observations and is not obliged to lecture. (Wattenberg 1959:5)

One interesting difference between the Bessel-Flemming approach and those of Le Verrier and Adams is that Bessel was not satisfied with knowing the deviations of Uranus only. He was pushing Flemming to investigate those of Saturn and Jupiter as well. No doubt this would have made the calculations much more complex, and as Le Verrier showed, not really necessary. Wilhelm Flemming, however, died soon after his move to Danzig (Gdanks) in 1840 and the actual work stopped. There are two further instances in which Bessel is connected to Neptune in his last years of life. In the aftermath of Neptune’s discovery, John Herschel announced in The Athenaeum (Herschel 1846:1019) his evening discussion with Bessel about the possibility of existence of the external planet, to which Bessel said that the motions of Uranus, as he had satisfied himself by careful examination of the recorded observations, could not be accounted for by the perturbations of the known planets; and that the deviations far exceeded any possible limits of error of observations.

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Herschel concluded that Bessel did not yet try to actually locate the planet in the sky, but also mentioned that Bessel sent him a letter on 14 November 1842 saying: “Uranus is not forgotten.” The evidence is, however, that Bessel returned only once more to the question of Uranus and that was in a letter to Gauss written on 8 November 1843 (Breifwechsel 1880:564–569 [Bessel to Gauss, 8 November 1843]). At some point earlier Gauss asked Bessel for a suggestion of a topic that could be proposed as a prize question by the Royal Academy of Göttingen. Bessel’s answer was “Wenn Sie Uranus zur Preisaufgabe machen…” (If you make Uranus the prize question). In the same letter Bessel went back to describe his failed attempt in the 1820s on the selective attraction hypothesis and recognised that his investigations had stopped after Flemming’s death. Bessel’s conclusion was “Ich halte den Uranus allerdings für einen sehr geeigneten Gegenstand; aber ich fürchte, daß dies viel Aufopferung fordern” (I consider Uranus to be a very suitable object; but I fear that this will require much sacrifice). Gauss took notice of Bessel’s suggestion and in the 1845 edition of the Proceedings of the Royal Academy of Göttingen covering years 1842–1844, the Uranus problem was put forward as a prize topic (Abhandlungen 1845:X). The deadline for the submission was November 1846. In some previous works (starting with von Lindenau, 1848:235) on the discovery of Neptune, the announcement of the prize is mentioned to be 1842, but this earlier date is incorrect. Bessel’s letter is dated on 8 November 1843, and there is a letter dated 4 December 1843 in which Gauss informed Schumacher that he had the newest prize questions (Briefwecshel, 1863a:195 [Gauss to Schumacher, 4 December 1843]). The publication of the prize in early 1845 is also consistent with the fact that Adams became aware of it (Adams, 1846:149), most likely before the Long Vacation in the summer 1845 when he produced the Hyp I solution. Of all German astronomers Bessel worked the most on the problem of Uranus. His interest in the problem spanned most of his career, from 1814 when he started observing the planet, to at least 1843 and the letter to Gauss suggesting it as a prize topic. He approached the problem with an open mind and used it as a test of the theory of gravity. When that did not work, Bessel accepted that there might be yet another planet beyond Uranus. Being an observer par excellence equipped with state-of-the-art instruments, he performed his own observations and set students to the task of analysing this data. Bessel was not the first one to speculate about an external planet, but his approach was meticulous. Perhaps even too much so, as adding the deviations of Saturn’s motions would have made the problem even more complex. Flemming’s death in 1840 was certainly a big setback for the work, but 1840 was an exceptionally hard year for Bessel. He lost his old friend and father figure of Olbers who died in March; then in October he received the news of the death of his son Wilhelm in Berlin. Even if picking up Flemming’s work in the last years of life might have been too much for Bessel, his work on charting the skies left a legacy that eventually resulted in the discovery of Neptune.

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6.5  Charting the Skies Regardless of the way the story of the discovery of Neptune is told, its central point is that an accurate star chart was necessary to find the planet in a timely manner. More detailed accounts like to dwell also on the chart’s presence in Berlin and its absence in Cambridge. The map in question is the Hora XXI (hour 21 in right ascension) of the Akademischen Sternkarten made by Carl Bremiker (1804–1877) for the Royal Academy of Sciences in Berlin (Köninglichen Akademie der Wissenschaften zu Berlin). Figure 6.5 shows the map kept at the library of the Leibniz-Insitut für Astrophysik Potsdam, the successor of the Berlin Observatory. The story of the making of that map, as well as the whole series of star charts, started in 1824 with a letter from Bessel to Johann Elert Bode, then the director of the Berlin Observatory. In a letter dated 18 August 1824, Bessel asked Bode if he thought the Berlin Academy (of which Bessel was a member) would be willing to support a project of charting the skies. Bessel’s point was that only with maps of all stars down to the 9th or 10th magnitude, in the ecliptic region, would it be possible to look for additional planets. Bode answered enthusiastically on 21 September. By 15 October Bessel dispatched his proposal letter to the Berlin Academy (Bessel’s letter to the Academy is reproduced in the catalogue that accompanied the star charts; Cataloge, 1859:III–V). The Academy enthusiastically accepted Bessel’s proposal during a meeting on 4 November 1824 (Hamel, 1989:12). The project was officially launched in 1825, and 34 years elapsed before it was successfully concluded.

6.5.1  Bessel’s Star Chart Idea The purpose of the maps for Bessel was straightforward: to help determine precise positions of comets or planets, and to discover all major planets (“Hauptplaneten”) of the Solar System. Even though no new minor planets had been discovered since Olbers spotted Vesta in 1807, Bessel’s experience with star catalogues, which listed stars that were no longer visible, gave him hope that more planets could be found (Cataloge, 1859:III). Bessel’s idea of star charts as tools for the discovery of comets and planets was not new. He acknowledged in the proposal that the Lilienthaler Astronomische Gesellschaft (part of the Celestial Police centered around Schröter’s observatory), as well as the Astronomical Society of London (the forerunner of the Royal Astronomical Society), had map-making as their mission. The problem was that the existing catalogues or sky atlases, such as those of Flamsteed, Bradley, Piazzi, Lalande, Mayer, and Bode were incomplete both in spatial coverage and brightness. They had regions that were under- and over-crowded with stars, and simply did not extend to stars beyond the 8th or 9th magnitude that were deemed necessary (Bessel 1823b). Furthermore, the actual maps were often more works of art than useful tools for examining the skies.

Fig. 6.5  Hora XXI of the Royal Berlin Academy of Sciences assembled by Carl Bremiker, kept in the Library of Leibniz-Institut für Astrophysik Potsdam. Only part of the map with declinations between 11° and −15° is shown. Horizontal lines next to stars (two or one only) indicate the number of times each star was observed. The handwritten notes show the predicted (square) and observed (circle) locations of Neptune. The notes are attributed to Galle but were added some time after the discovery. The map continued to be used for regular work, as the red sequence of dots testifies. The main advantage of this map compared to Harding’s (Fig. 6.2) is that it was complete to stars of the 9th magnitude, which is particularly noticeable in the vicinity of Neptune’s location on the discovery date.

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Bessel’s main idea was essentially what von Zach and the Celestial Police were supposed to make: a set of charts covering the ecliptic and divided into zones one hour (in right ascension) wide, mapping the positions of all fixed stars brighter than a specified magnitude. In 1822, Harding published the only existing set of maps of a similar kind, but those maps, which served as a template for Bessel’s charts, were still incomplete (compare Figs. 6.2 and 6.5 around the location of Neptune on the discovery night). Bessel’s proposal to the Academy ended with five propositions (Cataloge, 1859:IV–V): 1) The maps should include stars in the zone 15° north and 15° south of the ecliptic, divided in 24 zones of one hour width, including 4 arcminutes (1°) overlaps on both side of the hour. Each map should be divided in a grid of 510 squares, each of 1° width and height. 2) As a reward for making the map, each astronomer should receive a prize (as a medal) worth 20 to 25 Dutch Ducats. 3) The Academy should set up a commission to take charge of the project and the awarding of the prizes. 4) Each map would be cast in copper so that it could be reproduced at will at a later date. 5) A summary of the project [a flyer] would be made available to the public. Bessel understood creating accurate star charts was complex and not very scientifically exciting work suitable to be done in parallel by a number of “Freunde der Wissenshaft” (friends of the science): “This is a business which requires much time, but few resources and even less knowledge, and which is therefore most suitable for lovers of astronomy and for those astronomers whose observatories are not so fully equipped, that every night deprived of science will not cost it a considerable sacrifice.” His proposal was quite clear on who should be employed to make these charts: Considering the number of young astronomers, or older not fully equipped, which is currently available, one must also wish from this side that serious means are taken now to leave such a large and glorious monument for posterity. (Cataloge 1859:V)

Staubermann (2006) describes the process of remaking one of the Academy charts using Bessel’s suggestions and instrumentation available in the first half of the 19th century. His conclusion was: “The first ten or so stars posed a challenge, the first fifty called for skills that needed aptitude, such as the ability to copy the stars from the telescope on paper, but after about eight stars the exercise became dull and boring.” Indeed, among the astronomers who volunteered at the beginning there were already famous names, such as Wilhelm Struve and Alexis Bouvard, but many of the maps were made by Bessel’s “friends of the science”, such as Carl Bremiker, Thomas John Hussey, Karl Ludwig Hencke (1793–1866), observers whom one would nowadays most likely call serious amateur astronomers.

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Bessel thought out the full process of chart making, and in addition to his initial proposal to the Academy he also provided details of how to observe in practice. The framework Bessel set up, next to the detailed description of what each map should look like, consisted also of how astronomers should make their observations and put them onto the map. The grid of 510 squares of 4 minutes of time (1°) width and 1° height should first be filled with stars observed in Königsberg (by Bessel), Paris (Lalande 1801), and Palermo (Piazzi 1814), adjusted to the epoch of 1800. The observations would then commence: each observer would use a standard comet seeker (Bessel suggested a Fraunhofer refractor of 750 mm focal length or a Dollond of 400 mm focal length) together with a cross-wire micrometre. Such a telescope would provide a field of view comparable to (or larger than) one of the squares of the grid and allow for mapping stars of 9th or 10th magnitude. The observer’s task would be to “by eye” (“blosen Augen”) plot fainter stars (those that are not in the catalogues) onto the map. For each fainter star (dimmer than 7th magnitude) there should be an indication if it was observed once or twice. Special care should be taken depicting binary stars, and charts should be verified by comparison with heavens (Cataloge, 1859:IV). Bessel himself observed 32,000 stars down to the 9th magnitude in the same −15° to +15° zone, but he realised that more was needed to improve on previous star charts. The main reason was that three of the four discovered asteroids were fainter than the 8th magnitude so, therefore, 9th or 10th magnitude was a necessary limit. Going much deeper would increase the number of stars considerably, as James Challis experienced in his search for Neptune, and render the project unfeasible. Nevertheless, even down to just 8th magnitude, the number of stars was still large, about 2,000 to 3,000 per zone. It was imperative to find a working compromise between the precision (both in terms of the recorded spatial location and brightness of stars) and the time required to map a zone. Already in 1823 Bessel introduced a term “Zeitersparnis” (Bessel, 1823b:261). This “time saving” idea was his attempt to minimize the time needed for observing stars. What is striking for Bessel, the leader of precision astronomy and error analysis, is that he decided to sacrifice the precision of the measurement to reduce the time needed to make it. At that time, to determine the precise location of a star, astronomers used a meridian circle to observe a star as it crossed the local meridian and at the same moment note precisely the local time. Meridian observations demanded expensive instruments (including good clocks) as well as an assistant to help with measuring the time. In such a way, Bessel, with the help of his student and assistant Argelander, was able to measure the precise position of three stars in a minute (Bessel, 1823b:262). Therefore, for map making, Bessel’s advice was different, tailored for less equipped observations by a single astronomer. Entering fainter stars “by eye” onto a grid of pre-existing stars sped up the process tremendously but required multiple re-observations. Bessel’s student Carl Steinheil (1801-1870) designed an apparatus to help observers in plotting stars by eye (Steinheil 1826). It consisted of a wooden drawing board (see Staubermann, 2006:27 for a replica and its usage) with two brass rulers,

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set up perpendicular to each other and extending the full length of the board. The first ruler covered from −16 to +19 degrees (of declination) with intervals of 15′, while the second covered 68′ of right ascension in intervals of 15″. With such a mapping system, it was possible to position stars with a precision of 1 arc minute in declination and 1 second of time in right ascension. Such precision was much less compared to that at which the stars in Bessel’s, Lalande’s, or Piazzi’s catalogues were given, but it is the limiting accuracy at which a human eye can effectively position the star on a map of such specifications (Staubermann, 2006:28). In addition to plotting the relative positions of new stars, the observer had to record their brightnesses. Bessel’s idea was to divide them into one magnitude bins down to the 8th magnitude and then to note stars of 8th–9th, 9th, and 9th–10th magnitudes, each with a different symbol (see bottom of the map on Fig.  6.5). Even though it is possible for a trained observer to make a finer grading of stellar brightness, in fractions of magnitudes, that was not the aim for the charts. More important was to show how many times a fainter star (that have been located “by eye”) had been observed.

6.5.2  Star Charts of the Royal Academy of Berlin Bessel had the foresight to see how complicated and costly2 such a project would be and therefore asked for the support of the Royal Academy of Berlin. In hindsight he was completely right: the project lasted 34 years (counting from the proposal announcement). Twenty astronomers produced 24 maps in observatories across German lands (Kingdom of Prussia including parts of Poland, Kingdom of Bavaria, Kingdom of Hanover), as well as Denmark, Imperial Russia (Finland and Ukraine), the Habsburg Empire (Bohemia), the Grand Duchy of Tuscany, the Kingdom of Two Sicilies and the British Isles. Fewer than one third of the maps were started and completed by the same person. For the majority of maps, two, three, or even four astronomers worked on them at some point. The total number of people involved was 36. The overseeing committee established by the Academy did not fare much better. All but one of its members changed during the three-plus decades of the project: a total of seven members served on the committee, of whom four died and one moved to a post in a different principality. That constant member was Johann Franz Encke (Fig. 6.6), who was elected to the commission (June 1825), as its chair, even before his arrival to Berlin in October 1825 (Hamel, 1989:12). Encke was a veteran of the Napoleonic wars, in which he served as a Prussian artillery officer, twice stopping his studies in Göttingen under Gauss to join the war activities. After the war he worked at the Seeberg Observatory and became interna-

2  Estimated expenses were 3,000 Thalers, of which 2,000 Thalers would go as prizes to astronomers (Hamel 1989).

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Fig. 6.6  Johan Franz Encke (23 September 1791 – 26 August 1865). A veteran of the Napoleonic wars, Encke became famous internationally for proving that several sightings of comets were consistent with belonging to a single short period comet, the first of its kind to be discovered. Observations of the comet’s predicted return in 1822 from the newly established Parramatta observatory in Australia earned Encke the very first Gold Medal of the Astronomical Society of London (the forerunner of Royal Astronomical Society). Encke was awarded the second Gold Medal in 1830 for his work on the ephemerides at the Berlin Observatory, becoming the first recipient of more than one Gold Medals. (Credit: The picture is based on a photograph and published in an 1869 biography of Encke (Bruhns, 1869))

tionally famous by showing that comets discovered by Pierre Méchain in 1786, Caroline Herschel in 1795 and Jean-Louis Pons in 1805, and again in 1818, were the same object, and therefore establishing the existence of the second known periodic comet (the first being the Halley’s comet). Another important point about Encke’s comet, as it has become known since, is that, is that it has the shortest period (3.3 years) of any comet and its aphelion falls within Jupiter’s orbit (Bruhns, 1869:59–65). The accuracy of Encke’s calculation was tested in 1822, the year he predicted the comet would reappear. Indeed, Carl Ludwig Christian Rümker (1788–1862) spotted it from the Parramatta Observatory in Australia on 1 June 1822 (Bruhns, 1869:64). The influence of Encke’s discovery on the astronomical community of the time is best substantiated by the action of the Astronomical Society of London (which later became the Royal Astronomical Society), which awarded Encke its very first Gold Medal (Rümker was given the silver medal) in 1824. The recognition of the Royal Astronomical Society likely had an effect when Encke was considered for the directorship of the Berlin Observatory, on Bessel’s recommendation (Bessel himself turned down the job as he was satisfied with his position in Königberg; Bruhns

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1869:102). Encke was given the post of the director of the Berlin Observatory (upon retirement of Bode), professorship at the University and a position in the Royal academy. Even though he arrived after the beginning of the star chart project, he was crucial for its organisation and execution. The project was officially announced in No. 88 of the Astronomische Nachrichten The issue (Schumacher, 1826), printed and distributed in December 1825, contains Bessel’s proposal to the Academy together with an example map, also prepared by Bessel. Bessel’s name does not appear anywhere; the reader only knows of Bessel’s involvement from a letter sent by Encke to Schumacher on behalf of the Royal Academy of Berlin. Only later, in No. 93, printed in April 1826, readers learned that the star chart effort was Bessel’s project, as he provided additional instructions on how to make a map (Bessel 1826), followed by a description of the Steinheil’s drawing board apparatus for efficiently plotting stars (Steinheil 1826). Encke’s role in the completion of the project cannot be understated. His team of observers was spread across Europe. The team members, most of whom volunteered and some of whom were directors of local observatories, were often slow in answering his letters, and after years of non-responding would announce their withdrawal from the project. As the years progressed, Encke started relying on his students Carl Bremiker, Jakob Phillipp Wolfers (1803–1878) and Heinrich Luis d’Arrest to complete maps, some of them started years before. The celebrated Hora XXI map was itself such a case (Fig. 6.5). Even before the official announcement of the project in the Astronomische Nachrichten, some volunteers were selecting their preferred hour zone. One of them was Otto August Rosenberger, a former student of Bessel and now a professor in Halle (Saale). He started working on the Hora XXI map in 1826 (see Hamel 1989 for a detailed history of Hora XXI). In 1828, Rosenberger informed Encke that the progress was slow due to weather conditions. In 1831, Encke sent letters to all astronomers still working on maps (only four maps had been completed by then), and Rosenberger replied that he was hoping to keep the deadline of 1833 suggested by Encke. By 1839 there were still no data from Rosenberger. After 14 years of waiting, Encke decided to change the astronomer in charge. He gave this work to Carl Bremiker, his former student who worked as an inspector for the Prussian Minister of Commerce. Bremiker actually took over the work on three maps for which he was supposed to receive 300 Thalers (somewhat more than envisioned by Bessel’s proposal). Rosenberger provided all the data that he had collected so far and Bremiker was able to complete the work within a year (from 1 January to 4 December; Hamel, 1989:17–18). It took another four years for the map to be checked and accepted by the Academy, and finally it was engraved in copper and printed on 9 November 1845. On 10 November, all the copies were ready to be distributed, but the map seemed to have faded from the memory of all, until resurrected by Heinrich Louis d’Arrest on 23 September 1846. To understand this lapse of time, it is necessary to understand that Hora XXI was just one map in a very large and, by then, a very long project. Bessel, Encke, and the commission for the maps originally thought that it would take just a few years to complete the project. The plan was to print out a few hundred copies of each map and send them out in pairs (two different maps). There

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were several predefined recipients, both institutions and individuals. For example, the maps were sent to the Royal Society, the Royal Astronomical Society, Bureau des longitudes, Institut de France, Imperial Academy of Petersburg, Royal Academy in Naples, as well as directly to Gauss, Olbers and Schumacher. Until 1841, maps were distributed in this way, two at a time (Cataloge, 1859:IX). When Bremiker finished Hora XXI, Encke was also expecting Hora XI from Palm Heinrich Ludwig von Boguslawski (1789  – 1851) in Breslau (Wrocław). Indeed, Boguslawski sent it on 12 June 1845. The first printed proof of Hora XI was sent back to Boguslawki in February 1846 for his examination, but he did not return it until 1851, as he was improving it (Galle, 1882a:220). It was not unusual that maps shipped together were finished at different times, but for the first 13 the usual time lag was of the order of a year or two. In the summer of 1846, even though time was slowly slipping away, there was no indication that a successful and timely distribution of Hora XXI and XI would not happen. To a large extent, the events surrounding the discovery of Neptune took over and only in hindsight one could ask the question: Why was Hora XXI not distributed earlier? From Encke’s project management point of view, there was nothing special in Hora XXI, compared to any of the other 10 yet unfinished sky charts in 1846. An interesting parallel can be made with Hora IV, the first of the Berlin maps that made history. On 8 December 1845, a retired postmaster and amateur astronomer, Karl Ludwig Hencke, discovered the asteroid Astrea. He did this exactly in the way Bessel thought the discovery of planets should happen; following observations with a good star chart. The map Hencke used, Hora IV, was made by Karl Friedrich Knorre (1801–1883) in Nikolayev (Mykolaiv, Ukraine). Hora IV was finished in 1834, but was sent out only in 1837. It is not clear what caused the delay in distribution (Cataloge, 1859:X), although it was likely induced by corrections and proofs. The map sat for eight years in drawers of learned societies and observatories across Europe until Hencke used it to discover the first new minor planet since 1807. Even the example of Hora IV did not suggest that Hora XXI was in any way special. Perhaps this is where the irony of the Neptune discovery lies: in addition to the stunning mathematical prediction, there was a lot of luck involved.

6.6  The Discovery in Berlin Neptune was discovered at the Berlin Observatory in the night of 23 September 1846, based on the prediction by Urbain Jean Joseph Le Verrrier. It was found to be 1° 03′ 06.7″ away from Le Verrier’s predicted position. (Gapaillard, 2015:55).3 With hindsight, it was a lucky find that surprised everyone involved—the observers, the observatory director, and the whole world.

3  See the same reference for a discussion why there was a persistent statement in the literature that Neptune was found “within one degree” when the discrepancy is actually larger than a degree.

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There are many scenarios in which Neptune could have been discovered somewhere else at some other time. After all, it was observed several times before 23 September 1846. Galileo Galilei had observed it in 1613, Jerome Lalande published the observations of his nephew Michel Lalande (1766–1839) in 1795 in Paris, and John Lambert observed it as late as on 11 September 1846 in Munich (see Sect. 9.1). However, a series of opportune decisions led straight to the discovery in Berlin. The crucial decision started two decades earlier, with Alexander von Humboldt and Joseph von Fraunhofer.

6.6.1  Humboldt’s Masterpiece: A New Observatory Among other brilliant accomplishments, Fraunhofer, a famous physicist, is known as the father of spectroscopy. He developed the first modern version of this new instrument, the spectroscope, and he was also an optical manufacturer who created some of the best telescopes of his time. Bessel used a Fraunhofer heliometer in Königsberg to determine the parallax of 61 Cygni, just beating Wilhelm von Struve, then at the Dorpat (Tartu) Observatory, who also used a Fraunhofer refractor (which was then largest in the world, having a lens 24 cm aperture and focal length 432 cm). Fraunhofer made another refractor, identical to the one sold to Struve (who was financed by the Tsar of Russia). After Fraunhofer’s death in 1826, that second telescope was offered for sale by Joseph von Utzschneider (1763–1840), the co-owner of the company. Gauss was interested, and he asked Schumacher, then visiting Munich, to find out more about the possibility of purchasing it. Schumacher’s news was not very satisfactory for Gauss: the telescope was being offered to Joseph von Littrow (1781–1840), at the Vienna Observatory, possibly just for bargaining reasons (Knobloch, 2013:49–50), for a price of about 20,000 Thalers (Bruhns, 1869:179). As it became clear that Gauss had decided not to purchase the expensive instrument, Alexander von Humboldt entered the bidding game. His approach was much more subtle, but he also asked Schumacher for help. Schumacher was supposed to convey to Utzschneider that Humboldt would like to buy the instrument and was willing to pay 8500 Thalers. What Humboldt did not want Utzschneider to know was that the money would come from the King of Prussia, with potentially a much “deeper” purse. In the end, Utzschneider accepted the offer, and Schumacher, while unable to help Gauss, played a crucial role in Humboldt’s grand scheme to improve the Berlin Observatory. (Knobloch 2013). Appointing a new director, then, as now, was a good opportunity to renew instruments, refurbish buildings, and in general argue that more funding was necessary to keep up the good work and the prestige of the observatory. The origins of the Berlin observatory go back to the calendar reform of the Prussian state in 1699 (then still a dukedom) and the foundation of a Kurfürstlich-Brandenburgischen Societät der Wissenschaften (a forerunner of the Royal Academy of Berlin of Sciences) proposed by Gottfried Wilhelm Leibniz. The observatory was awarded the calendar

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patent, which provided the income for the observatory and the academy for most of the next century. The first building was constructed in 1711 in the Dorotheenstraße in the academy building tower above a former stable in the central part of the city. Humboldt’s idea was to obtain a first-class telescope and then to ask for a new building suitable for the telescope. Encke, who by 1826, started living opposite the old observatory (as Bode was still alive and occupying the director’s apartments) was not too enthusiastic (Bruhns, 1869:179), but the opportunity arrived with the Fraunhofer telescope (Fig. 6.7). In March 1828 the new refractor reached Berlin stored in the boxes that remained its home for several more years. According to Humboldt, such an expensive and excellent instrument needed a proper observatory building, and he turned to the king again. A new building was approved and Encke selected a location, which was part of a larger park without nearby houses. Already by the mid-1840s, however, houses appeared around the new observatory building; even though their height was restricted, parts of the horizon visible from the observatory dome were obstructed (Bruhns, 1869:185–186).

Fig. 6.7  Fraunhofer telescope of the Berlin Observatory (identical to the one at the Dorpat Observatory) was used for the discovery of Neptune by Galle and d’Arrest. The refractor has a lens of 24 cm aperture and focal length of 432 cm. The telescope is kept in the Deutsches Museum in Munich. (Credit: Deutsches Museum)

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Fig. 6.8  The new building of the Berlin observatory by Carl Daniel Freydanck (oil on canvas, 1838). The architect of the observatory was Karl Friedrich Schinkel who also designed several other buildings in Berlin. The observatory was opened in 1835. The main drive to build the new observatory was to provide an appropriate place (the dome) for the new Fraunhofer telescope. (Credit: Wikipedia, Public Domain)

The location of the observatory in present Berlin was at the crossing of the Enckestraße and Besselstraße, southeast of and not far from Checkpoint Charlie. Today nothing remains of the elegant neo-classical building designed by Karl Friedrich Schinkel (Fig.  6.8). The building of the observatory started in October 1832 and the main house and the dome were finished by December 1835 (Bruhns, 1869:186–187). Encke moved in on 24 April 1835 and set up his older instruments just in time to observe the return of the Halley’s comet. The new Fraunhofer refractor had to wait a bit longer in boxes: the unpacking started on 28 September 1835 and five days later it saw “first light”. Encke’s report about the excellent quality of the new telescope (and the new observatory building) appeared in February 1836 in Astronomische Nachrichten (Encke 1836). Humboldt’s deal of getting a telescope and then building the new observatory was extended to include a paid assistant for the director. (Wattenberg, 1962:34). The paid assistant, who also lived at the observatory, moved in just three days after Encke (Wattenberg, 1963:34). His name was Johann Gottfried Galle (Fig. 6.9).

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Fig. 6.9  Johann Gottfried Galle (9 June 1812 – 10 July 1910). By the time of Neptune’s discovery, Galle was internationally known as a discoverer of three comets in the same year. This photograph of Galle was taken when he was about 45 years old. (Credit: Wattenberg 1963:70)

Galle was born in Pabsthaus, next to the village of Radis in the vicinity of Wittenberg in 1812. His family’s business was producing tar, but there seemed to be a clear indication from an early age that Johann Galle should not continue his father’s business but go to school instead. He finished the gymnasium in Wittenberg in 1830 at the top of his class and matriculated at the Berlin University as a student of mathematics and philosophy. Encke was Galle’s teacher and, in later study years, Galle started going to the old observatory and contributing calculations for Encke’s Berliner Astronomisches Jahrbuch (Wattenberg, 1963:26). His relations with Encke must have been very good, as in winter of 1834 Galle wrote to Encke asking if there were a chance for a paid assistantship in the new observatory that was being built (Wattenberg, 1963:32–33). The positive answer was sufficient for Galle to resign as a teacher in Friedrichs-Wederschen Gymnasium in Berlin, take the post of the assistant and devote his life to astronomy. Over the next decade, the newly installed Fraunhofer refractor proved to be an exceptional instrument. Using the telescope, Encke in 1837 discovered a gap in the A ring of Saturn, while Galle in 1838 discovered the darker C-ring. Galle also found three comets between December 1839 and March 1840, which attracted the attention of the wider public and professional peers. First Galle received the Gold Medal from the Prussian king (14 March 1840). Then, through Humboldt’s Parisian connections (chiefly François Arago), Galle was awarded the Lalande Prize from the Paris Academy of Sciences (received on 2 November 1840 as a gold medal and a

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money prize of 635 Francs), and finally, through Schumacher, Galle received three Gold Comet Medals by the king of Denmark (23 September 1841). He also received a job offer to be an assistant to Johan Heinrich Mädler (1794–1874) at Dorpat (Tartu), which Galle, however, declined (Wattenberg, 1963:43–44). Perhaps the most important benefit of the discovery of the three comets was that in June 1846 Galle was also awarded a stipend by the King of Prussia in the amount of 150 Thalers to cover the costs of the promotion to a Doctor of Philosophy (Wattenberg, 1963:44). Encke suggested the topic: a re-reduction of Ole Rømer’s (1644–1710) observations called Triduum observationum astronomicarum. These observations were all that was left of Rømer’s observation of more than 1,000 stars in 1706 after fire ravaged Copenhagen in 1728. They consisted of observations of 88 stars, the Moon, the Sun and all the known planets, as well as an observation of the lunar eclipse on 21 October 1706. Galle completed the work and submitted his ­thesis on 26 December 1844. On 2 January 1845 he paid the fee (100 Thaler and 9 Silbergroschen) and defended the thesis on 1 March 1845 (Wattenberg, 1963:45–47). The examination committee consisted of Wolfers, a co-worker at the Observatory, R. Jacobs (1807–1877), a professor at the Joachimsthalschen Gymnasium, and G. Michaels (1813–1895), an assistant at Friedrichs-Wederschen Gymansium. The Triduum also contained three observations of Mercury in 1706. Galle was aware of Le Verrier’s work on Mercury (Le Verrier 1843), in which he was collecting all observations of Mercury, current and old, in order to resolve its orbital anomalies (see Sect. 9.5). Galle, therefore, thought his “ancient” observations might be of help and sent Le Verrier a copy of his dissertation.

6.6.2  Letters from Paris It is tempting to say that Galle’s action was a crucial moment for the discovery of Neptune in Berlin. The fact is that Le Verrier wrote a letter on 18 September 1846 to Galle (translation cited from Grosser, 1962:115–116, with the interpolation of orbital elements from p. 111): Paris, 18 September 1846 Sir, – I have read with much interest and attention the reductions of Roemer’s observations which you have been kind enough to send me. The prefect clarity of your e­ xplanations, the complete rigor of the results which you present are on a par with those which we should expect from a most able astronomer. At some future time, Sir, I will ask your permission to review several points which interested me, in particular the observations of Mercury included in your paper. Right now I would like to find a persistent observer, who would be willing to devote some time to an examination of a part of the sky in which there may be a planet to discover. I have been led to this conclusion by the theory of Uranus. An abstract of my research is going to appear in the Astronomische Nachrichten. I will then be able, Sir, to offer my excuses to you in writing, if I have not fulfilled my obligation to thank you for the interesting work which you sent me.

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You will see, Sir, that I demonstrate that it is impossible to satisfy the observations of Uranus without introducing the action of a new Planet, thus far unknown; and, remarkably, there is only one single position in the ecliptic where this perturbing Planet can be located. Here are the elements of the orbit which I assign to this body: Semimajor axis Sideral period Eccentricity Longitude of perihelion Mean longitude, January 1, 1847 Mass True heliocentric longitude, January 1, 1847 Distance from the Sun

36.154 217.387 years 0.10761 284° 45′ 318° 47′ 1/9300 326° 32′ 33.06 a.u.

The actual position of this body shows that we are now, and will be for several months, in a favorable situation for the discovery. Furthermore, the mass of the planet allows us to conclude that its apparent diameter is more than 3″ of arc. This disk is perfectly distinguishable, in a good glass, from the spurious star-diameters caused by various aberrations. I am, Sir, your faithful servant, U.J. Le Verrier Will you convey to Mr. Encke, although I have not had the honour of meeting him, my compliments and deep respect.

This is a remarkable letter, not only because it, in hindsight, initiated the epoch of making observations at Berlin. The letter starts as an acknowledgment, about a year and a half late, full of praise and flattery. However, with the fifth sentence Le Verrier changes the topic completely, stating a desire that the “persistent observer” looks for his prediction on the sky. Why did Le Verrier remember Galle so suddenly and send him a letter? Why did Le Verrier expect that Galle would follow his request? The postscript is equally interesting: Le Verrier was not acquainted with Encke; was this the reason to choose Galle, who had already written to him, instead of directly writing to Encke? The Staatsbibliothek zu Berlin keeps a copy of a letter by Le Verrier to Schumacher dated on 1 October 1846, which started with (in French): “When I sent you my work, I wrote directly to Mr. Struve and Mr. Galle.” (Le Verrier to Schumacher, 1 Oct 1846). Le Verrier was referring to his calculations that he presented at the Academy in Paris on 31 August, but he did not explain why he sent letters to Galle in Berlin and to Otto Wilhelm von Struve (1819–1905) at Pulkovo Observatory. They were junior members of the respective observatories, although Otto Struve had already started taking over the duties of the observatory in the mid-­1840s from his father Friedrich Georg Wilhelm von Struve. Wilhelm Struve took over the directorship of the Pulkovo Observatory close to St. Petersburg and moved there from Dorpat (Tartu) in 1839. Soon after he mostly devoted himself to

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science letting his son run the observatory. Following his father’s death in 1864, Otto Struve became the official director of the Pulkovo Observatory. Le Verrier’s choice of astronomers asked to do the search can be traced to his correspondence with Schumacher. In particular, Le Verrier and Schumacher had already exchanged several letters regarding Le Verrier’s theory. For example, in a letter dated 30 June 1846 Le Verrier (Le Verrier to Schumacher, 30 June 1846) informed Schumacher of his results on Uranus and prediction of the location of an external planet. In that letter, Le Verrier did not send any detailed calculations as he was working on improving them, but promised to send to Schumacher the final calculations as soon as they were done. Le Verrier’s added: “… as the planet in question will arrive at its opposition, in the middle of August, I will ask for your benevolence for the immediate printing of my article.” Schumacher understood Le Verrier’s reason for a speedy publication, and in the letter now surviving only partially printed in the Centenaire de la naissance de U.J.J.  Le Verrier (and not dated; Centenaire, 1911:14) he gave advice as to who would be able to effectively search for the planet. It was clear to Schumacher that one needed a telescope of excellent quality and large aperture, and he suggested Le Verrier to contact the owners of the two biggest telescopes at the time: “M. Struve” and “Lord Rosse.” In 1846, the Pulkovo Observatory had the world’s biggest refractor with a 38 cm aberration-free lens, built in Munich by Merz & Mahler, the successor of Fraunhofer’s optical company (Mathematisch-Feinmechanische Institut), that also finished Fraunhofer’s telescopes for Königsberg and Berlin. Schumacher was, however, not specific about whom to write, the father Wilhelm or the son Otto. The other suggestion referred to what was then the largest reflecting telescope in the world (until the Mount Wilson 100-inch saw first light in 1917). This was the famed Leviathan of Parsonstown, a gigantic telescope with a speculum-metal mirror of 1.8 m diameter, that had been completed in 1845 by William Parsons (1800–1867) (who was better known as Lord Rosse) on his estate at Birr Castle in Ireland. Le Verrier’s letter from 30 June is remarkable for two other aspects. It was written two days after a letter he sent to Airy (Le Verrier to Airy, 28 June 1846) answering Airy’s question regarding the radius vector of Uranus (see Chapter 4.10). Le Verrier considered the radius vector issue important enough to write about it to Schumacher as a proof that his calculation was indeed correct. He declared: “Thus, although I first occupied myself only with the longitudes deduced from the oppositions, the theory I arrived at did not fail to bring to the radius vector of Uranus the corrections required by the observations made in the quadrature.” This letter also entered the German literature about the discovery of Neptune in an indirect way. Schumacher sent it to Gauss as part of another letter (Schumacher to Gauss, 7 July 1846; Briefwechsel, 1863b:176) and asked Gauss’s opinion on the matter. Gauss answered (Gauss to Schumacher, 11 July 1846; Briefwechsel, 1863b:179) by saying: “The weight of his [Le Verrier's] hypothesis cannot be judged until his calculations are submitted for substantive examination.” This statement was sometimes used to show how prominent astronomers of the time did not put too much trust in Le Verrier’s prediction (e.g. Wattenberg 1946,

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Dick 1986), but it should be taken at a face value: Gauss simply had not seen Le Verrier’s actual work. Le Verrier did not follow the advice of Schumacher fully, for a good reason. For example, unlike the Pulkovo equatorial, Lord Rosse’s reflector was a meridian instrument. Of course, there were other large telescopes, for example, in Cambridge. Airy did not answer Le Verrier’s suggestions: “If I could hope that you will have enough confidence in my work to seek this planet in the sky, I would hasten, Sir, to send the exact position to you, as soon as I have obtained it.” (Le Verrier to Airy, 28 June 1846). Regardless, Le Verrier did directly ask O. Struve and Galle (there is no indication Le Verrier was aware that O. Struve was taking care of the Pulkovo Observatory’s day-to-day work). If we are allowed to speculate, perhaps Le Verrier thought junior (and similar age – Le Verrier was born 1811, Galle in 1812 and O. Struve in 1819) astronomers might be more enthusiastic about the search, or he thought that a son (O. Struve) would be able to convince the director-father (W. Struve) to attempt the search. Le Verrier’s choice in asking Galle was, however, very fortunate.

6.6.3  Party Crashers The letter from Le Verrier arrived in Berlin on 23 September 1846, the 55th birthday of Encke. As his plan was to skip the work that night and enjoy the evening party, Encke gave permission to Galle to conduct the search. Galle’s own account (Galle 1877) was that Encke, who had expressed his “doubt and disproval” about a possible planet as the solution to Uranus problem, understood that Galle had “eine gewisse moralische Verpflichtung zum Nachsehen an der betreffenden Stelle” (a certain moral obligation to look up the position in question), i.e. to search for the planet now that he had received the letter from Le Verrier (Galle, 1877:350). Their conversation was overheard by a third person, Heinrich Louis d’Arrest, who was a student of Encke and a volunteer at the observatory, living in “a room under the roof” of a nearby building. D’Arrest was native of Berlin, born in 1822, a descendant of a family of French Huguenot refugees (Dreyer 1876). He attended a local school and Collège Français in Berlin, and later enrolled at Berlin University. From the start he showed a special interest for astronomy and was called to assist at the Berlin Observatory. By September 1846, d’Arrest had distinguished himself by discovering a comet on 9 July 1844 (one already discovered two nights before in Paris), and then another comet on 28 December 1844, for which he was awarded the Gold Comet Medal from the King of Denmark. His work at the Berlin Observatory was, however, more theoretical as in 1846 he published his tour de force calculation of the orbit of the recently found asteroid Astrea in Astronomische Nachrichten (d’Arrest 1846). As Galle reported in his first description of that famous observing night (Galle 1877), the sky was clear and conditions favourable. Galle’s plan was to look for an object with a clear disk-like appearance as Le Verrier suggested it should be 3″ in

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diameter, distinguishable by the Fraunhofer refractor, even in Berlin’s unsteady atmosphere. d’Arrest was likely given the task of preparing the observations (Galle, 1882b:96), such as determining what position in the sky Le Verrier’s prediction actually placed the planet, but at first neither Galle nor d’Arrest used more than the standard Harding’s star chart that was in the dome (Fig. 6.2). After about an hour of sweeping the estimated zone looking for a disk-like object, Galle was ready to abandon the project. The Fraunhofer refractor in Berlin was at that time one of the world’s largest and best telescopes. Four decades later, Dreyer (1882) listed only five other observatories worldwide that at the time had a superior equatorial telescope: Pulkova, Harvard College, Munich, Cambridge, and Markree Observatory.4 In spite of the Berlin telescope’s large aperture and superb optics, Galle and d’Arrest were not able to find a disk-like object. But they were fully aware that Harding’s map was not complete. Galle (1877:351) reported that even though they were well aware of the existence of the Royal Academy of Berlin maps (in his second retelling of the events, Galle (1882a:219) also mentions that he had corrected some of them), and that they knew how the Hora IV map was used to find Astrea, Galle did not think there was another such chart that they could use. In that moment, however, d’Arrest remembered that there was a chart lying somewhere among Encke’s papers that should cover the part of the sky they were examining. Galle and d’Arrest went to Encke’s hall (Vorzimmer), as Galle knew where the maps were stored, and indeed, they found Hora XXI by Carl Bremiker (Fig. 6.5). Back in the dome they divided the work: Galle was at the Fraunhofer looking for stars and describing their locations on the sky, while d’Arrest checked if the stars were drawn on the map at a nearby desk. At some moment after 22:00 local time5 Galle reported a star that d’Arrest could not find on the map. Did Galle say: “What?” Did d’Arrest say: “Can I have a look?” Did the two young men dance in the dome of the Berlin Observatory next to the sixth or seventh largest telescope in the world? Did they wonder how it was possible for Le Verrier to be so precise in his prediction, that the possible planet was scarcely more than a degree or so away from the predicted position? Maybe all this happened, or maybe nothing at all. What is certain is that Galle and d’Arrest turned from discoverers of a new planet to excited party crashers. The report (Galle, 1877:352) states that Encke, presumably still at his birthday party, was “given all details” and immediately left to join in with the observations. Expressing a bit more emotion Galle

 Dreyer’s 1882 recollection was inaccurate concerning one telescope. Harvard’s 15-inch (38-cm) Merz and Mahler refractor was not mounted until June 1847. In September 1846, the largest telescope in the United States was the 11-inch (28-cm) Merz and Mahler refractor at the Cincinnati Observatory (see Chap. 9). 5  Encke’s report of the discovery (Encke 1846), lists 22:52 for the first observation of Neptune with the precisely determined coordinates. There are three more observations at 23:47, 00:52 and 01:08, with Galle listed for the first three and Encke for the last one. It is however likely that the planet was discovered before these scientifically reported measurements. 4

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(1877:352) reported: “The evening of the 24. Sept had to be awaited with much excitement.” The reason was to determine the actual motion of the planet. Four hours of observations on the night of the 23rd were not able to determine with certainty that the object really moved against a backdrop of stars, although there was an indication. They also recognised the disk-like appearance of the planet, as predicted by Le Verrier, although the measured diameter was somewhat smaller than expected. Once the object was located, both Galle and Encke confirmed that the apparent size of the planet was about 2.7″ or 2.9″, compared to Le Verrier’s estimate of a bit over 3″. One can only imagine how long the day of the 24th was for the three observers, as they waited to confirm the existence of the planet. The night of 24 September finally settled, but the skies were not as clear as on the previous night and the observations had to be soon called off due to clouds. Nevertheless, a motion of 4′ in right ascension along the path of Le Verrier’s prediction was sufficient to confirm that the object was a new planet. Next morning, and during following days, Galle and Encke were busy writing letters to announce their amazing discovery. Galle sent the celebrated letter to Le Verrier dated 25 September, here presented at full length for the first time in a translation by James Lequeux (Galle to Le Verrier 25 September 1846; see also Chap. 5): Berlin, 25 September 1846 Sir, - The planet whose position you had indicated really exists [Galle’s underlining]. On the very day I received your letter I found an eighth magnitude star, which did not appear in the excellent chart Hora XXI (drawn up by Dr. Carl Bremiker) from the collection of celestial charts published by the Royal Academy of Berlin. The observation of the following day clinched the matter: here was indeed the planet we were looking for. We compared it, Mr. Encke and I, with the great refractor of Fraunhofer, with a ninth magnitude star (a) from Bessel Zone 119 21 h 50 m 51 s.00 -13° 30′ 7″.9 and we have found: Mean Berlin time Sept 23. 12h 0m14s.6 Plan. Sept 24. 8h 54m40s.9 Plan.

= (a) + 21′ 21.″.5 in RA = (a) + 1 36.8 in Dec = (a) + 20′ 19″.9 in RA = (a) + 1 15.9 in Dec

Apparent place of the (a) star from Bessel Sept 23 Sept 24

327° 58' 2".5 2".4

−19° 25′ 49″.0 49".0

We have compared twice this star with Piazzi XXI 944, whose position is also found in Taylor and hence in the British Association Catalogue. From these comparisons and the position in the Brit. Ass. Cat., one has for (a): Sept 23 24

327 57 54.5 54.4

−19 25 45.0 45.0

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This determination will be preferable, Bessel being worried about Zone 119. Thus we have for the planet: Mean Berlin time Sept 23 12h 0m 14s.6 Sept 24   8h 54m 40s.9

RA 328° 19′ 16″.0 18    14.3

Dec −13° 24′   8″.2 24  29.7

Also, the diameter seemed near 3 seconds; however, one can only be sure in case of very favorable atmospheric circumstances, and it is mainly the chart that facilitated the search. This planet might be worthy of being called Janus, one of the most ancient deities of the Romans, whose two-sided face would signify its position at the frontier of the Solar System. J. G. Galle

A great irony of science is that while it took four days for a letter to travel from Paris to Berlin, it took 11 days to arrive at Pulkovo. Le Verrier’s letter urging Otto Struve to look for the planet arrived on 29 September. By the time O. Struve was ready to observe two days later (due to bad weather), another letter arrived, this time from Encke in Berlin, announcing the discovery (Dick 1986). In his autobiography, O. Struve wrote that, as soon as he got the letter from Le Verrier and the observing conditions were good, he calculated the predicted position and started searching for the planet based on its disk diameter (O. Struve also pointed out that Pulkovo still didn’t have the Bremiker map). In that moment his father W. Struve arrived with the letter from Encke and the coordinates of the discovered planet. O. Struve answered: “Sag mir nichts nähers, ich will ihn auch unabhängig aufsuchen” (Don’t tell me anything, I want to find it independently). Within quarter of an hour he was able to call his father to the tower to show him the planet. (Dick 1986:257). Not everything was exactly as O. Struve wrote in his autobiography many years after the event. Wolfgang Dick (1986:257), based on Wilhelm Struve’s correspondence, deduced that Encke’s letter must have arrived by 1 October, while the first night suitable for observing in Pulkovo did not occur before 3 October. Nevertheless, Wilhelm Struve wrote that his son was able to recognise the planet by its disk, which is also evident from O. Struve’s letter to Le Verrier published in Centenaire de la naissance de U. J. J. Le Verrier (Centenaire, 1911:23) where O. Struve states that the planet could be recognised even without a map by its clear disk, 2.78 seconds of arc in diameter, consistent with Berlin measurements.

6.7  The Aftermath If Galle and d’Arrest were barely suppressing their excitement breaking up their director’s birthday party, what was going through Le Verrier’s thoughts when he received the letter from Galle four or five days after it was sent from Berlin on 25 September? Did his hand tremble while he was cutting the envelope? He read the opening sentence of Galle’s letter “Le planète, dont vous avez signalè la position, rèellement existe,” which changed his life.

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Le Verrier answered Galle as soon as he received the announcement (Le Verrier to Galle, 1 October 1846): Paris, 1st October1848 Sir, – I thank you heartily for your alacrity to tell me about your observation of 23 and 24 September. Thanks to you, we are definitively in possession of that new world. The pleasure I have felt when seeing that you have encountered it at less than a degree of the position I had given, is a bit darkened by the idea that if I had written you four months earlier we would then have obtained the result we have just reached now. I will communicate your letter next Monday to the Academy of Sciences. Let me hope that we will frequently go on with a correspondence that is starting under such happy auspices. I am with my high consideration, Sir, your devoted servant. U.J. Le Verrier Please be kind to write me rue Saint-Thomas-d’Enfer, n° 5. I do not belong to the Observatory, where you have sent your letter. The Board of Longitudes here has proposed the name of Neptune, with a trident as symbol. The name of Janus would mean that this planet is the last of the Solar System, something that there is no reason to believe.

This letter, preserved among Galle’s papers and kept in the Staatsbibliothek in Berlin, shows Le Verrier’s happiness on the discovery, the evidence of an anxiety (Galle’s announcement was sent to a wrong address) as well as the foretelling of the trouble that will engulf him, as Le Verrier was starting his battle to name the predicted planet. The lives of Neptune’s telescopic discoverers changed as well, albeit in different ways. The attention for the discovery of Neptune was focused on the predictors, Le Verrier and Adams. In spite of this, both Galle and d’Arrest became internationally recognised astronomers with respected careers, as directors of observatories and professors at universities. It is, however, surprising that d’Arrest’s role in the discovery was not widely known until after his death in 1875. The first two encompassing records of Neptune’s telescopic discovery present it in mutually consistent ways, which are, however, notably different from that written by Galle in 1877 and accepted today. John Pringle Nichol (1804–1859) in his book published in 1849 The Planet Neptune: an Exposition and History wrote: Now, it so happened that the map of that precise region where the new planet was expected, had been completed by Dr. Bremiker; and it was printing, or just printed at Berlin—I believe that the Observatory of Berlin had obtained the proof-sheet. The Astronomers of this Institution were thus in a position of power regarding such inquiries, enjoyed by no other Observatory in existence: they had simply to notice Bremiker’s Map and then the Sky—observing if there was a discrepancy between the two pictures, that could be accounted for by the planetary motion of some one star: so that—with their renowned sagacity, and the excellence of their Instruments—an inspection of the Heavens on one clear night might accomplish the resolution of this great problem. And thus it even was; the Planet was discovered actually by M. GALLE, on the very evening of the day on which he received the letter of LEVERRIER indicating its place. (Nichol 1849:88–89)

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A year later, Bernhard von Lindenau (1780–1854) devoted only a single sentence to the discovery in his 30-page essay printed in the Astronomische Nachrichten: As Le Verrier’s communication of his newest location of the presumed planet with the desire of its search arrived on 23 Sept 1846 to Dr. Galle at the Berlin Observatory, its discovery succeeded in that same night, as near the calculated place a star of 8th magnitude was perceived, which was missing on the Dr. Bremiker’s academic star map and the next evening it was recognised as the sought for planet, by the motion in the sense of Le Verrier elements. (von Lindenau 1848:12)

Both accounts mention only the astronomer Galle and highlight the availability of Bremiker’s map. The search is presented as a matter of pointing the telescope, and d’Arrest is not mentioned at all. The events in Berlin did not spark a scandal, but rumours were going around that not all was as it seemed. Understanding the roles of Galle, d’Arrest, and Encke, as well as the pivotal contribution of Bremiker’s star map, is a necessary ingredient of any complete account of the discovery of Neptune.

6.7.1  Johann Gottfried Galle Among two discoverers and three observers in the dome of Berlin Observatory in the night of 23 September, Galle received the most attention. Already on 4 October 1846, Galle was decorated with “Roten Alderorden IV. Klasse” by King Friedrich Wilhelm IV of Prussia. This was soon followed by the news that the French king Louise-Philippe had named him the Knight of the Legion of Honour. The Danish king, who had already awarded three gold medals in 1840 to Galle for discovering comets, sent him another gold medal. Galle’s award season finished on 31 July 1847 when he was decorated with the “Roten Alderorden III. Klass,” but more importantly, on Humboldt’s initiative Galle’s salary increased by 200 Thalers per year in October 1846 (Wattenberg 1963:53–54).6 Collecting the figurative laurel wreaths, Galle withdrew from discussions about Neptune. In the announcement letter to Le Verrier he suggested the name of the planet (Janus), but as Le Verrier refused it as a new name had already been chosen by the Bureau des Longitudes (Neptune), Galle decided he should recognise Le Verrier’s “right of the discoverer”’ to name the planet and focused on his usual work at the observatory. At that time, he was also collaborating with Humboldt, providing information and calculations (Wattenberg 1963:65–69) for Humboldt’s epoch-­ making multi-volume Kosmos. In 1848 came a possibility for leaving Berlin. Galle was a candidate for the directorship of the Königsberg Observatory that had been vacant since Bessel’s death in early 1846. The Neptune fame, however, did not help him at that time, and, on the insistence of Carl Gustav Jacob Jacobi (1804–1851), 6  Another person that got awarded for the discovery of Neptune was Carl Bremiker. On the personal request of Encke, Bremiker was awarded with the Gold, Silver and Bronze Leibniz Medals of the Royal Academy of Berlin: (Bruhns 1869: 122–123).

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the post was given to Bessel’s long-standing assistant, August Ludwig Busch (1804 – 1855; Wattenberg 1963:58). Jacobi’s point was that Galle might be known for discovery of a planet and four comets, but Hencke discovered two planets (Astrea in 1845 and Hebe in 1847) and “Comet discovery is the job of every dilettante who knows the starry sky and has therefore become the business of several ladies” (Knobloch 2013:59). Galle travelled to Frombork (Frauenburg) in 1851, the town where Copernicus had lived and worked, to observe the total solar eclipse of 28 July, and, then moved to Breslau (Wrocław), finally getting his tenure as the director of the observatory there and a professor at the university. Galle married and had a scientifically fruitful 44 years as the director. He investigated the climate and the Earth’s magnetic field and proposed a novel technique to estimate the solar parallax using minor bodies of the Solar System (Galle 1873). He thereafter coordinated a network of observatories to observe the solar parallax by his technique, and in 1875 published their results (Galle 1875). Galle retired aged 83, on the 50th anniversary of his PhD thesis. In 1835, just after moving to the Berlin Observatory at the start of his astronomical career, Galle observed Halley’s comet. He had the good luck to do it again in 1910, just a few months before his death. (Wattenberg 1963:106). Neptune featured little in Galle’s life, except at two periods of the post-discovery career. The pleasant events happened towards the end of his life, when Galle was celebrated as the discoverer. He received accolades in 1896, for the 50th anniversary of the discovery. (Wattenberg 1963:109–110). This was followed with the award of “Stern zum Kronen-Orden 2. Klasse” for his 90th. (Wattenberg 1963:114). Somewhat less pleasant events happened some 30 years after the discovery, when Galle felt the need to come back to the discovery of Neptune with no fewer than three published statements. They form the important evidence for the actual role the Bremiker map played in the discovery, but, crucially, two of the letters also provide authoritative evidence for d’Arrest’s role in the discovery.

6.7.2  The Star Chart The significance of the Hora XXI map in finding the new planet, and its curious, but, as we have seen earlier, understandable location at the Berlin Observatory, induced early historians of the event to conclude that its existence was the reason why Le Verrier wrote to Galle. That supposition, however, can be invalidated easily. Before we do that, it is instructive to see why the star chart was considered so important. On several occasions immediately after the discovery, Encke emphasised the crucial role of the map: in his first report of the discovery to Astronomische Nachrichten (Encke 1846), the congratulatory letter to Le Verrier (Centenaire 1911:20–22), as well as in the address to the Royal Academy of Berlin, also printed in Astronomische Nachrichten (Encke 1847). In the letter to Le Verrier, Encke was very specific (translation from Lequeux 2013:37): “Without the fortuitous circum-

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stance of having a chart containing all the fixed stars down to the tenth magnitude [of this particular area of the sky], I do not believe the planet would have been found.” Encke, in the first official scientific report, implicitly suggested that Le Verrier’s idea of recognizing the planet by its disk did not work, but having a map allowed for a quick discovery: By a letter that arrived here on 23rd Sept. Mr. Le Verrier asked Dr. Galle especially to look after it [the planet], probably led by an assumption, pronounced in his treatise, that the planet will be identified by a disc. The same evening Mr. Galle compared the excellent map, drawn by Mr. Dr. Bremiker (Hora XXI of the academic star charts), with the sky and noted immediately that very close to the position determined by Mr. Le Verrier a star of the 8th magnitude was missing in the map. (Encke 1846:49–50)

Encke was similarly convinced when speaking to the academicians in Berlin that the discovery was due to the existence of the map (Encke 1847:191): “The new planet whose orbit and location were announced by Mr. Le Verrier in Paris in advance, was almost immediately found by Mr. Dr. Galle on the 23rd of September at the local observatory, where the Hora XXI of the academic star charts [...] was compared with the sky…” Here are those two remarkable aspects found in von Lindenau’s, Nichol’s, and other accounts: Encke mentioned only Galle as the discoverer and that the planet was found “almost immediately” by comparison with the star chart. Such impressions certainly fixed the importance of Bremiker’s map, together with comments, chiefly among British astronomers, that its absence in Cambridge hindered Challis’s search (e.g. Nichol, 1849:131). It is, therefore, not surprising that this notion started to spread throughout the literature. The only authentic retelling of the events leading to the discovery was not available until 1877 (Galle, 1877). By then, Galle was the only person closely related to the discovery of Neptune in Berlin still alive. Encke died in 1865 in Berlin, d’Arrest in 1875 in Copenhagen and, on the day of the 31st anniversary of the discovery, Le Verrier died in Paris. In writing his own account of the events, Galle had three goals: to describe the discovery as he saw it happened, particularly now that he was the only surviving member of the observing team, place the due importance to Bremiker’s map, and highlight the contribution of d’Arrest (see Section 6.7.2). He was prompted by the publication of the translation of Hugo Gyldén’s (1841 – 1896) book, written originally in Swedish, Die Grundlehren der Astronomie: When the news of Paris arrived at the Berlin Observatory, the then assistant to the observatory, Mr. d'Arrest, hurried to design a small map of the designated area of the sky to facilitate the search for the planet. As soon as he had pointed the telescope to the relevant celestial region, the observer Dr. Galle arrived, looked into the telescope and  – saw the planet. (Gyldén 1877:248)

Gyldén’s account of the discovery is obviously a somewhat corrupted and much shortened retelling of the story that was by then circulated. Its origin, as it will become evident later, might have been a rumour. It is remarkable in two aspects: it mentions d’Arrest prominently, and it actually does not mention which map d’Arrest used.

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Galle’s 1877 retelling forms the current framework of the story (Wattenberg 1946; Grosser 1962; Standage 2000) as described in Sect. 6.6.3. The English language literature was, however, somewhat slower in accepting Galle’s version. Smart (1946) wrote: “A young student-observer, d’Arrest, suggested that the first thing to do would be to find out if Bremiker’s star-chart (Hora XXI) had been finished.” He rightly put d’Arrest in the center of action, but simplified the events in a similar manner as Gyldén’s book. Other additions are common: for example, Grosser (1962:118) gave a large role to d’Arrest, suggesting even that he was the one who went to inform Encke, although there is no evidence for this. Standage (2000:119) incorrectly considered the Berlin sky charts as volume in an atlas that “had recently become available, … and had yet to be widely distributed.” On the other hand, the French summary of the events (Danjon 1946) was shorter, stating that following the suggestion of a young assistant, dArrest, Galle used the Berlin Academy map. This account is essentially correct, but it leaves out the subsequent role of d’Arrest. Galle’s statement in the Astronomische Nachrichten might have rehabilitated d’Arrest’s role, but it did not stop the speculation about the importance of the map. According to the German translation Populäre Astronomie of the book Popular Astronomy by the American astronomer Simon Newcomb (1835–1909) (Newcomb 1881:392): “Le Verrier wrote to Galle in Berlin and asked him to look for the planet based on the just finished page of the 21h in the academic star chart.” This statement actually was an addition to the text by the editor (Rudolf Engelmann, 1841–1888) of the German edition, and it does not appear in the English original. The British edition from 1878 states only: Leverrier wrote to Dr. Galle, at Berlin, suggesting that he should try to find the planet. It happened that a map of the stars in the region occupied by the planet was just completed, and on pointing the telescope of the Berlin Observatory, Galle soon found an object which had a planetary disk, and was not on the star map. (Newcomb 1878:361)

Here again, d’Arrest is not mentioned, while it is not clear whether the planetary disk had been helpful in the search (contrary to Encke’s reports). The statement in Populäre Astronomie prompted Galle to issue a rebuttal in Astronomische Nachrichten (Galle 1882a:221) stressing: “Von dieser Karte hatte jedoch Le Verrier […] keinerlei Kenntiniss” (Of that map Le Verrier […] had no knowledge). Crucially, Galle and d’Arrest did not prepare for the search with Berlin charts in mind. That was already explicitly stated in Galle’s (1877) account of the observations, after it became clear that searching for the disk alone did not produce results (Galle 1877:351): “…one had to think about the procurement of a star map, for which there were, next to Harding’s Atlas, only the Berlin Academy star charts, still very full of gaps and long awaiting completion.” Galle’s (1882a) letter to Astronomische Nachrichten did not seem to have a big influence on the German publisher Wilhelm Engelmann; the second edition (1892) of the Populäre Astronomie, even though the editor has changed, kept the same sentence (Newcomb 1892:403). Only in the third edition in 1905, the editor Hermann Carl Vogel (1841–1907), removed the explicit statement that Le Verrier asked Galle to use the map (Newcomb 1905:413). It is important to point out that the same publisher, Engelmann, published both Gyldén’s and Newcomb’s books,

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which might explain the propagation of the error. A similar coincidence might be that Vogel was the director of the Astrophysikalischen Observatorium Potsdam between 1882–1907, coinciding with Galle’s move to Potsdam after retirement. Following Galle’s statements, it can be assumed that Hora XXI, while ready and waiting to be distributed, was not included among the standard material for assisting observations at the Berlin Observatory. That must be the reason why Galle and d’Arrest did not even consider starting the search using Hora XXI. Nevertheless, even in Galle’s German obituary, Wilhem Foerster (1832–1921) suggested that Arago, always in a close contact with Alexander von Humboldt, was undoubtedly aware of the availability of that particular map, and suggested that Le Verrier contact Berlin: “… [Arago] undoubtedly had knowledge of the existence of such maps in Berlin and was thus able to take very good reason to point out the participation of Berlin to Le Verrier.” (Foerster, 1911:20). That is not exactly contrary to what Galle was trying to refute in Rudolf Engelmann’s translation of Newcomb’s book. Le Verrier’s letter to Galle did not mention the maps in any way, but that does not mean that Arago or Le Verrier were not aware of the whole map project. The discovery of Astrea in 1845, using the Hora IV map, happened sufficiently early for the general usefulness of the maps for discovery of new sky objects to be widely known. The insinuation that the existence of the map might have been related to the reason Le Verrier wrote to Galle can, however, be contradicted with the following three facts. 1) Le Verrier also wrote to O. Struve, who was not involved in the making of maps in any way. He might have acted on a suggestion by Schumacher to select people with best telescopes. 2) In July and August, when the search for Neptune started in Cambridge, there was no effort to use the Argelander’s Hora XXII map (Fig. 6.10), completed in 1832 and present in Cambridge, which covered the region of Neptune’s orbit for those months. There were several good reasons why Challis did not attempt a search with a map (see Chap. 4), but there are no reasons that Parisian astronomers were more confident in (or thinking of) using Berlin maps than their peers in Cambridge. 3) Le Verrier’s letter, both to Galle and O. Struve, focuses on the fact that the planet should be discerned as a large (more than 3″ in diameter) disk. A good telescope, and both Pulkovo and Berlin had first class telescopes (which was not the case for Paris), in Le Verrier’s estimate should have been able to resolve it. Le Verrier’s letter is that of a convinced theoretician: it suggests that an observer equipped with a good telescope only needs to look in the direction indicated by the orbital elements, and the planet will be recognised by its large disk. In reality, such observations are typically more difficult than that, as Galle’s and d’Arrest’s search testified. (W. Struve, however, claimed his son recognised the planet by its disk-like appearance alone, Dick, 1986:257). Le Verrier, not trained as an observer, might not have fully grasped what it takes to find an unknown object in the sky. As a theoretician he understood the value of a good, aberration-free telescope, and that is most likely the reason he wrote to Berlin and Pulkovo, known for good telescopes. We

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Fig. 6.10  Hora XXII of the Royal Academy of Sciences in Berlin completed by Friedrich Wilhelm August Argelander in 1832, kept in the Library of Leibniz-Institut für Astrophysik Potsdam. In August 1846 Neptune could be located by using this map as it was passing through the rightmost quadrants (between 21h 56m and 22h) at the declination of approximately -13o (See Fig. 6.5 for comparison). A copy of this map existed in both Paris and Cambridge in 1846.

might not know Le Verrier’s reasons, but we can be certain that although Bremiker’s map Hora XXI played a crucial role for the discovery, the search was not prompted by existence of the map.

6.7.3  Heinrich Louis d’Arrest The role of d’Arrest in the discovery of Neptune was known only by a few until John Louis Emil Dreyer (1852 – 1926) wrote a German language obituary of dArrest (Dreyer, 1876). That omission, however, did not hinder d’Arrest’s career. Two years after the discovery, d’Arrest moved to Leipzig to a newly founded observatory under August Ferdinand Möbius (1790–1868). There he got a doctorate in 1850, published a treatise on the known asteroids in 1851, and married a daughter of his director in 1857. In 1851, d’Arrest was invited to move to Washington D.C as an extraordinary professor (Dreyer, 1876, page 5), but Leipzig University retained him by opening a professorship post for him. In the same year, he finished the Hora III of the Berlin star maps that Encke had entrusted to him some time earlier. In 1856 he was invited by the Imperial Russian government on O. Struve’s suggestion to take over the directorship of the Kiev Observatory (Dick, 1986:251). After much deliberation,

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Fig. 6.11  Heinrich Louis d’Arrest (13 August 1822 – 14 June 1875). His role in the discovery of Neptune became generally known only after his death, through popularisation by J. L. E. Dreyer and Galle’s accounts. The photograph is by Bertel Christian Budtz Müller and is kept in Royal Library, Copenhagen. (Credit: Wikipedia, Public Domain)

d’Arrest declined, but by 1857 he decided to move and relocate his family to not-so-­ distant Copenhagen, where he became the director of the observatory and a professor at the university. D’Arrest discovered a periodic comet 6P/d’Arrest (1851) and the asteroid 69 Freia (1862). His later work focused on nebulae and galaxies (of which he published a catalogue and for which he was awarded a Gold Medal of the Royal Astronomical Society) including pioneering work in spectroscopy of nebulae (Fig. 6.11). Dreyer, a native of Copenhagen, started frequenting the observatory even before he enrolled at the university in 1869 (Obituary Notices 1927). In 1870 he got a key to the observatory and started using it regularly, sometimes observing with d’Arrest, his professor of astronomy. d’Arrest seemed to enjoy talking about the discovery night to his students. Dreyer noted that d’Arrest said in one of his lectures “that he had been present at the finding of Neptune and that ‘he might say it would not have been found without him.’” (Dreyer, 1882:64). The first public announcement of d’Arrest’s contribution is a short mention in Dreyer’s obituary of d’Arrest (Dreyer 1876:3), where he wrote: “From 1846 he [d’Arrest] took part in the ongoing work of the Berlin Observatory, and acted as Galle’s assistant when he, on September 23, 1846, found Neptune.” Nothing else was said in the obituary about Neptune’s discovery. Nevertheless, Dreyer’s statement and the description of the discovery night in the German edition of Gyldén’s book, which mentioned the map and accentuated the role of d’Arrest, must have been sufficient to prompt Galle to write his version of events on the discovery night (Galle, 1877). Galle gave a relatively prominent role to d’Arrest, describing him as a student and an assistant (“Gehüfe”) who worked at the Observatory (Galle, 1877:350). To make his work more practical and allow him to

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take part in the observations, he lived in an attic room of an adjoining building on the premises of the observatory. Galle continues that d’Arrest overheard the conversation between Galle and Encke (Galle, 1877:350): “When I communicated the arrival of Le Verrier’s letter, the same [d’Arrest] expressed the wish to be present at the evening of the search for the planet, which, as was understood by itself, was gladly fulfilled.” In Galle’s retelling, d’Arrest was mentioned once more, a few sentences later (Galle, 1877:351): “… it was first d’Arrest who raised the question of whether there should be taken a look at the academic star charts, if perhaps the position in question was already included among them.” This sentence is remarkable as it clearly highlights d’Arrest as the person who got the idea to look for Bremiker’s map, but the phrasing is such that there is no doubt who was in charge in the dome of Berlin observatory on that night. Galle’s version of the events did not fully satisfy Dreyer, but nothing happened for five years. In the meantime, Dreyer started an international journal of astronomy, called Copernicus, and Galle again felt he needed to issue a statement regarding the latest incorrect description of the events, this time in Populäre Astronomie, the German version of Newcomb’s Popular Astronomy (Galle, 1882a), as mentioned earlier. Dreyer did not miss the opportunity to write a short “Historical note concerning the discovery of Neptune” in his journal Copernicus. His motivation is probably best seen in this sentence (Dreyer, 1882:64): “As various conflicting accounts seem to have been circulated from time to time (without any having been printed except the one in Prof. Gyldén’s book) this does not appear to me altogether useless.” Dreyer was not directly saying if the account in the German edition of Gyldén’s book was correct, but here we have for the first time a clear indication that rumours were circulating and that the editors of Gyldén’s book knew them. Dreyer focused on what he heard directly from d’Arrest when they met and observed together: … and [d’Arrest] continued: “We then went back to the dome, where there was a kind of desk, at which I placed myself with the map, while Galle, looking through the refractor, described the configurations of the stars he saw. I followed them on the map one by one, until he said: and then there is a star of the 8th magnitude in such and such a position, whereupon I immediately exclaimed: that star is not on the map!” (Dreyer, 1882:64)

Dreyer’s mission was to establish: That d’Arrest thus not only first thought of looking for a map (without which the search might have proceeded as slowly as the operations at the Cambridge Observatory did), but actually took part in the observation, does not appear to be without historical interest, and it seems only just to that afterwards distinguished astronomer to say that Neptune was found by Galle and him, observing together. (Dreyer 1882:64)

Dreyer’s “Historical Note” was dated February 1882, and Galle saw it as part of the March issue of Copernicus (Galle 1882b:96). He immediately sent a note “Ueber die erste auffindung des planeten Neptun”, dated 14 April 1882 to the same journal, which was published as part of the same volume as Dreyer’s note (Galle 1882b). Galle started with an explanation that he was writing to correct previous

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omissions of both Encke and himself. The first point Galle addressed was that Le Verrier wrote to him based on the “frühere Correspondenz” (previous correspondence). Note: It is important to stress that in 1882, Le Verrier’s letter to Galle on 18 September 1846 was yet to be published. Neither was his connection to Le Verrier prior to the discovery generally known. Le Verrier’s letter was first published, in German translation, in Julius Scheiner’s (1858–1913) article, dealing with a wider issue of the “astronomy of the invisible” and including the discovery of Neptune as an example, in a general audience magazine Himmel und Erde (Scheiner 1893). Notably, Scheiner wrote that: …as with most important discoveries, and this one included, a number of legends and distortions got intertwined with them, so that in almost no popular work there is a correct representation of them; we therefore considered it interesting to give a strictly correct account of the situation here.

The transcript of the French original letter was presented in W.W. Payne’s journal Popular Astronomy (See 1910:475), then in Galle’s obituary in the Monthly Notices of the Royal Astronomical Society (Turner 1911:275), and, with a number of other relevant letters, in the Centenaire de la naissance de U.  J. J.  Le Verrier (Centenaire 1911:14), which had a wide impact. Returning to Galle: Galle’s next point in his 1882 letter to Copernicus was to repeat the part of the 1877 retelling that Encke allowed him to attempt the search on his own. The crucial part of the text (translated from the original German) then followed: In the meantime d’Arrest, hearing this, expressed the implicit wish to be allowed to participate in the observation. Although it was not my intention to relinquish this search and the possibility of an eventual observational success to another observer, such thoughts were remote and it would have seemed unkind to me, to in some way reject the wish of this young zealous astronomer, so I gladly gave my consent to the attendance. The same had, therefore, helped with writing down, looking up at the map, and perhaps a few other tasks, which I have no particular recollection of, which, however, as does not need to be discussed further, could have been carried out by me without a considerable loss of time, and while being personally appreciated, they were objectively irrelevant. On the other hand, I have always considered, as a significant contribution to a faster exploration of the planet, d’Arrest’s quick memory of the academic maps, although here, too, opinions may and may have differed, to what extent it was inevitable and necessary for me to remember the academic maps.

Galle finished his letter repeating his regrets that he did not highlight d’Arrest’s contributions earlier. This 1882 note was the last time Galle wrote of the discovery night, and it is clear that he was aware of the rumours. Perhaps to address those, he explicitly stated that d’Arrest did at least take part in preparations for the observations. It is easy to imagine that Galle, once he accepted d’Arrest as his co-observer, also told him to prepare what needed to be prepared for the night: e.g. convert Le Verrier’s orbital elements to a location on the sky where they had to point the telescope. There is no doubt that Galle did not need d’Arrest to do that, but it is not surprising that a junior member

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of the team had done the (basic) groundwork. Grosser‘s (1962:117) statement that Galle “had computed the geocentric coordinates of the planet from Le Verrier’s elements” is therefore likely not true. It was, most likely, done by d’Arrest. Luck was also involved in the role d’Arrest played in the discovery. It might have happened that both Galle and d’Arrest were looking through the refractor searching for Le Verrier’s disk, and that at some point even d’Arrest had a possibility to see the planet first. Perhaps, as Dick speculated, d’Arrest saved the day by remembering the map, when Galle was about to call off the search for Le Verrier’s disk. Regardless of details and conjectures, d’Arrest’s role is clear: he was part of the team of two, and thus there is no reason not to call him a co-discoverer. Modern retellings of the Neptune discovery (e.g. Grosser 1962:115–118; Standage 2000:118–120) typically mention d’Arrest’s role, although his name seems to be the first to be removed when a shorter version of the discovery is needed (Dunkely 2018:50). Furthermore, d’Arrest doesn’t really feature in a prominent role even of longer accounts. This habit of mentioning d’Arrest in passing, while giving more space to Galle (or Le Verrier and Adams, the true stars of the event), is most plainly noticeable in how the scientific community honours the principal actors of the Neptune story. Adams, Le Verrier and Galle are names of craters on the Moon and Mars. The name d’Arrest was given to a crater on the Moon, and to a crater on Phobos, the satellite of Mars. More to the point, the ring system of Neptune, established by Voyager, is differentiated between rings named after (in order of distance from the planet): Galle, Le Verrier, Lassell, Arago and Adams (de Pater et al. 2018)7. We are left to hope that Neptune ring system is richer by at least one ring, and d’Arrest joins the illustrious company. To summarise, Galle (1877), Dreyer (1882), and Galle (1882a, b) provide the description of the discovery night. They corrected common misconceptions from that time that are sometimes present in later recounts. They provide evidence that the Bremiker map was not the reason Le Verrier contacted Galle, that there were two observers and that Galle was given the responsibly for the search and leading the observations. The second member of the team, d’Arrest, had assisted in preparing the observations and provided crucial input by his timely remembering of the existence of the Hora XXI star chart. The planet was discovered through teamwork: Galle looking at the sky, d’Arrest looking at the map.

6.7.4  Not Everything Is Rosy in Berlin Dick was the first in modern literature to point out the puzzling lack of mention of d’Arrest in immediate reports of the new planet (Dick, 1986). His observation was prompted by the discovery of an autobiographical manuscript of Otto Struve at the 7  For the sake of completeness, the four illustrious names are also given to main belt asteroids, 1996 Adams (1961), 1997 Le Verrier (1963), 2097 Galle (1953) and 9133 d’Arrest (1960), where the number in parentheses is the year of discovery.

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Kharkiv Observatory in Ukraine (Dick 1985). At one point in it, O. Struve reminisced about the time when he was trying to bring in d’Arrest as the director of the Kiev Observatory. Around the same time (1856), O. Struve travelled to Berlin and met with Encke and wrote: d’Arrest gladly agreed [to come to Kiev], and when I came to Berlin with this news, I was surprised by a visit from Encke, who embraced me by saying casually: “You took a heavy weight from my heart by placing d’Arrest in such a favourable position; I have constantly been remorseful that I attributed the actual discovery of Neptune solely to Galle and, if possible, in his favour, while the principal merit of the discovery is due to d’Arrest” (translated from Dick 1986:251).

Struve’s account also confirmed a rumour that was circulating. He wrote in his autobiography: In these words, I can only recognise a confirmation of the general rumour of the time: Galle had no confidence in Le Verrier’s calculations and had made no preparations for finding the planet in the sky, but d’Arrest had done so, calculated the position in advance and prepared everything for the first favourable evening, to make a survey of the area of ​​the sky in question, by comparing it with the first draft of Bremiker’s map. Galle had approached [the telescope], and, because d’Arrest was already a little weary, proposed to temporarily replace him at the refractor. It coincidentally happened that in one of the first objects that Galle had registered in the sky, d’Arrest, who had compared Galle’s information with that map, had to declare that this object was not on the map. With that, the planet was discovered and in that it must be admitted that Galle was the first to have seen the planet as such in the sky. (Dick 1986:253)

This is a fascinating discovery by Wolfgang Dick, as it bears a close resemblance to what the German editor of Gyldén’s book added about the discovery of Neptune. It contains two aspects that Galle tried to refute in his published versions of the event: that d’Arrest was the main observer and that the map was at hand. Struve’s autobiography, however, did not stop here, as it mentioned two other interesting points. It first pointed out that Dreyer’s version of the events from his 1882 “Historical Note” was met with some disapproval. Struve wrote: “Later objections to this conception were made, and [Anders] Donner [1854–1938] protested by claiming the merit of the discovery, based on d’Arrest’s own statements to him, exclusively to Mr. Galle.” (Dick 1986:255). Nevertheless, O. Struve had a chance to ask d’Arrest personally how much of the rumour was true when they met in 1869. D’Arrest answered enigmatically that (Dick 1986:255): “...there was some truth to the rumour, but he wished to admit that Galle deserved the main merit of actual discovery.” Encke’s omission of d’Arrrest’s role created the possibility for all sorts of rumours and gossip. We might speculate if it had even contributed to the departure of d’Arrest from Berlin by 1848, even though Encke and d’Arrest’s relationship did not seem to suffer (e.g. d’Arrest took over the work on one of the star charts). Our chief evidence about the rumour comes from an autobiography written some 40 years after the discovery. One should also consider that O. Struve was “robbed” of the possibility to search for the planet, as Le Verrier’s letter arrived at Pulkovo on 29 September, while the announcement of the discovery from Encke arrived on 1 October, before Struve’s observations could even start due to the weather.

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Nevertheless, the source of the rumours was likely to have been, at least in their initial form, d’Arrest himself. After all, this is how Dreyer found out d’Arrest’s role in the discovery. Worthy of a remark is the fact that d’Arrest lived for 10 years in Leipzig (where he was one of the two main astronomers). The publisher of Gyldén’s and Newcomb’s books, Wilhelm Engelmann (1808 – 1878), and his eldest son and the editor of the book Rudolf Engelmann, that first publicised d’Arrest’s role, had their business also in Leipzig. Encke’s omission might have been accidental, in the spirit of the time, where directors would report discoveries of their assistants, not necessarily mentioning their names. Encke did the same thing with Galle (Encke 1837) when he reported in passing Galle’s discovery of Saturn’s C ring. This omission led to Galle’s publications in 1851 and 1882 (the latter is the same paper where he pointed out the mistake regarding the discovery of Neptune in the German edition of Newcomb’s Popular Astronomy) about his role in the discovery of the ring before George P.  Bond (1825–1865) at the Harvard College Observatory (Galle 1851, 1882a). To this extent one should add that Encke, while a strict director who expected prompt execution of tasks, also allowed to his assistants “a free scope in the choice of theoretical studies, accounting tasks, observations and other practical examinations” (Bruhns 1869:191). Everything was not rosy in Berlin immediately after the discovery, as is evident in an exchange of letters between Schumacher and Le Verrier. The Saatsbibliothek zu Berlin keeps Le Verrier’s letter dated 7 January 1847, in which Le Verrier wrote: I am distressed to think that one might not be more reasonable in Berlin than in Paris, and that Mr. Galle might well have his part of inconvenience, as you were kind enough to write to me in confidence. It seems to me at present that we can no longer hope to receive an answer from Berlin. I suppose that Mr. Encke may have thought that all the merit was in having the Map built and that it was to him that the honour had to be returned to a large extent. There can be some truth in that. But I did not have a voice when all this was done. And my intention, now that I am almost certain of being listened to, was, as soon as Mr. Galle had answered, to ask that the King’s justice be extended to Mr. Encke. Nothing like this is possible as long as these people persist in their strange silence. I regret it a lot. The goodness of the relations between scientists is so nice but unfortunately so rare! So, my dear friend, if you learn something, and if I can make some step agreeable to these people, I count that you will inform me; I am sure that you know that I am above all this childish susceptibility. (Le Verrier to Schumacher. 7 January 1847)

The Schumacher letter to which Le Verrier refers does not seem to have survived, but we can guess that the source of the rumour was Humboldt. In a letter to Schumacher on 10 November 1846 (Bierman 1979:85), Humboldt relayed a “sad, sinister secret” that Encke told him (Humboldt) that prevented the King awarding Galle the “Roten Alderorden IV. Klasse” for the telescopic discovery of Neptune (cited from Knobloch 2013:59). Encke, it should be said, supported Galle receiving a significant pay raise. Possibly finding himself under pressure, Encke had to explain his motives to the finance minister in a letter. Encke’s point was very simple (Knobloch 2013:59): Galle found Neptune because he had the Academy chart. There was a bit of luck involved, and anyone else could have discovered it. That was why he thought an increase in salary was better suited than a royal medal. Encke also pointed out that

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when Hencke discovered Astrea in December 1845, using another of the Academy charts, he was given a handsome increase of his pension, but no medal. Another rumour that circulated among German astronomers was the speculation that Galle was actually not very confident in Le Verrier’s results and did not much care for the observations, as the Struve’s gossip suggests. Nevertheless, as Dick (1986:255–256) pointed out, the picture of an excited young astronomer who, in spite of the views of his director, enthusiastically searched for the planet might not be completely true. This picture entered the story through Galle’s obituary and Grosser’s book, but Galle himself was careful in stating that he had: “a certain moral obligation to look it up at the position in question” (Galle 1877:350). That might have been an expression of the fact that Galle shared Encke’s doubts, as argued by Dick, or perhaps, a very good way to argue for the telescope time with his director. In 1882, Galle repeated this situation in somewhat different words (Galle 1882b:96): “Immediately after the reception I showed Encke the letter, who had previously been very averse to taking this matter into consideration, but now agreed, but left the execution of this request, which I personally had received, to myself.” This is consistent with what Encke wrote in his letter of congratulation to Le Verrier, published in Centenaire: I dare say that without the letter you have kindly addressed to Mr. Galle, the search would not have been made in Berlin. Reading with great interest your excellent Memoir (Comptes rendus, August 31), I thought I could wait until more details were published; I had it about eight days before September 23, and certainly would have started the search right away, if the conviction of the necessity that a planet should exist would have been strong enough […] However your letter specially inviting Mr. Galle to occupy himself with this research, it was not necessary at all to urge him to engage in it. (Centenaire 1911:21)

Encke was quite clear that Le Verrier’s idea to write to Galle, and not to him, was crucial for the observations to take place. This is consistent with Bruhns’s biography of Encke, which stressed that Encke allowed his assistants and students to pursue their own research (e.g. Galle worked on Fraunhofer lines independently of Encke; Bruhns 1869:191), and therefore implies that Galle actually wanted to do the search. In the same letter Encke also writes “Il y a eu beaucoup de bonheur dans cette recherché” (There was, nevertheless, a great deal of luck in the search; Lequeux 2013:37). He referred here to the existence of the Bremiker’s map, but as this ­chapter shows, there were many happy coincidences that resulted in the discovery of Neptune in Berlin.

6.8  German Contribution The English and the French might have abused Airy most savagely, but the Germans turned out to be Airy’s strongest ally in the moment when tensions were at the highest. The international scandal around Neptune’s discovery erupted when John Herschel (1846:1019) announced on 3 October, in a British literary magazine The Athenaeum, that “a young Cambridge mathematician, Mr. Adams” also predicted

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the location of Neptune. At a meeting of the French Academy of Sciences in Paris, Arago declared that Le Verrier gave him his permission to choose the name of the new planet, and he called it “Planète de Le Verrier.” It thus must have been a great surprise when on 15 October Challis described his own search in Cambridge together with the announcement of his pre-discovery observations made of Neptune in August 1846, as well as the new (and much more correct) orbit of the planet calculated by Adams based on Cambridge and Berlin observations (Challis 1846a:1069). Challis and Adams went further and suggested a name for the planet: “Oceanus.” Proposing a name was a bold move by the Cambridge astronomers, perhaps incited by a letter from Arago published in “Our Weekly Gossip” column of The Athenaeum on 10 October. It stated that the discoverer of the planet, Galle …appears disposed to call the new planet Janus, from considerations borrowed from the hypothesis that it may be on the confines of our Solar System. M.  Le Verrier, to whom belongs the right of naming it, does not agree to the significative of the name of Janus, but will consent to any other – Neptune, for instance – which would have the assent of astronomers. (Arago 1846:1046)

Galle did not feel he had any claim to naming the planet, and dropped the issue waiting to see what the final verdict would be. When he published observations of Neptune, for example in early 1847, he called it “Le Verrier’s planet” (Leverrier’schen Planet), a much more neutral form than “Planète Le Verrier”. Other German astronomers were much more involved than Galle, and some made a crucial contribution to establish the name Neptune. Four persons deserve to be mentioned here: Encke, Gauss, Schumacher and Wilhelm Struve. Each of them had an important role in building the case for adopting the name “Neptune,” instead of “Le Verrier,” “Oceanus,” “Minerva,” “Hyperion,” or “Janus,” as they appeared in the press or astronomical literature (see Chap. 5).

6.8.1  Gauss and Schumacher at a “Possenspiel” Heinrich Christian Schumacher’s (Fig. 6.12) mostly indirect contribution should be highlighted first. He was born in 1780 in Bramstedt (today called Bad Bramstedt) in the province of Holstein, then ruled by the King of Denmark, but part of the German Confederation. As a child he was introduced to the future king of Denmark, a meeting which helped secure a number of favours in later life. He finished a gymnasium in Altona and then studied in Kiel, Göttingen and received a doctorate in Dorpat (Tartu). Later he returned to Denmark, became a professor of astronomy at Copenhagen, but mostly lived in Altona where he founded an observatory that worked on the geodetic survey of Denmark. His crucial contribution to astronomy was not with a discovery or scientific work in general, but by establishing, and editing until his death in 1850, the Astronomische Nachrichten (Astronomical News), the most important astronomy journal of the

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Fig. 6.12 Heinrich Christian Schumacher (3 September 1780 – 28 December 1850) was a central figure in astronomical circles in Germany through his correspondence and his editing and publishing of the journal Astronomische Nachrichten. Lithograph by F. Ausborn based on a painting by Christian Albrecht Jensen. (Credit: Wikipedia, Public Domain)

time. This journal, published continuously from 1821 to the present day, was truly international. It was different from other scientific journals of learned societies as it focused only on astronomy (including instrumentation and relevant mathematics), and, crucially, by its content it had a strong international presence. Astronomers across Europe, and later the world, sent letters with reports of their observations or calculations and Schumacher would publish them in the original form and their original language. The impact of the Astronomische Nachrichten (Fig. 6.13) in the mid-1840s can be judged by the articles related to Neptune. The journal contains the report of the discovery (Encke 1846), the summary of Le Verrier’s calculations (Le Verrier 1846a, b, c), Challis’s report on the Cambridge observations and Adams’s new orbit (Challis 1846b, c),8 predating Challis’s address at the Royal Astronomical Society, as well as the reprint of Airy’s Royal Astronomical Society memorandum (Airy 1846b, 1847a). Reports on the observations of Neptune were numerous, particularly in Volume 25 of the journal, coming from observatories across Europe, extending from Russia to the British Isles and Ireland, as well as reports from the U.S.  This paper is somewhat more scientific version of the paper Challis published in The Athenaeum (Challis 1846a), but it does not contain any mention of the Challis’s and Adams’s suggestion of the name “Oceanus.” It seems that Challis initially wanted the suggested name to appear in the Astronomische Nachrichten, but Schumacher told Airy that it was omitted because Challis had requested that part be removed from the printed version (Schumacher to Airy, 11 December 1846). 8

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Fig. 6.13  The first page of Astronomische Nachrichten in 1821 with Schumacher’s mission statement for the new journal and the address to which contributions could be sent. Schumacher assures that the print will be as close as possible to the original, including the original language (if in English, French, or Latin, although some articles also appeared in Italian), and would be distributed as soon as there was enough material for printing. Schumacher was responsible for contributions signed “S.” (Credit: Library of Leibniz-Institut für Astrophysik Potsdam)

Astronomische Nachrichten also included Lassell’s discovery of Triton (Lassell 1847), new calculations of the orbit by American astronomers (e.g. Henry and Walker 1847; Everett 1847; Pierce 1848), and was used as a sign of scientific quality (Hubbell and Smith 1992) in the controversy of the “happy accident” (see Chap. 9). Volume 25 is also a record of astronomers’ attitudes to the new planet’s name. As the editor of such a journal, Schumacher was in the middle of a vast correspondence network of astronomers with whom he exchanged information and gossip. Regarding the Neptune story, we have seen Schumacher’s, perhaps very influential, advice to Le Verrier of where to look for astronomers with good and large telescopes. The naming issue featured prominently among the journal’s correspondence. Arago, for example, asked him in a letter on 6 October (printed 22 October) to publish the official announcement of the planet’s new name “Le Verrier” (Schumacher 1846a). He also exchanged private letters with Le Verrier and Airy about the issue. The correspondence between Schumacher and Airy is particularly interesting for understanding how the name “Neptune” was adopted, and we will come to it later. Schumacher was on very friendly terms with the mathematician Johann Carl Friedrich Gauss (Fig. 6.14), and they exchanged dozens of letters on the topic of Neptune, and specifically its name (Wattenberg 1962). Almost all letters between Schumacher and Gauss in the period after the discovery, until the summer of 1847,

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Fig. 6.14  Johann Carl Friedrich Gauss (30 April 1777 – 23 February 1855), as painted in 1840 by Christian Albrecht Jensen. The leading mathematician of his time, Gauss was likely the only person to refer to Neptune as “Adams-Le Verrier’s” planet. (Credit: Wikipedia, Public Domain)

at least partly dealt with the naming issue (Briefwechsel 1863a, b). For example, when Gauss sent his first observations of Neptune, Schumacher delayed publishing them until he got an answer from Gauss about what he wanted to call the planet in print. In the beginning Gauss opted for the neutral “Leverrier’s planet” (Briefwechsel 1863b: 223–224, Gauss to Schumacher, 31 October 1846), but soon after stated: “Den Namen Le Verrier werde ich schwerlich jemals gebrauchen, weil ich es unschicklich finde” (I will hardly ever use the name Le Verrier because I find it improper; Gauss to Schumcher, 31 October 1846; Briefwechsel 1863b:224). At one point Gauss referred to the planet as “Adams-Leverrier’schen Planet” (Gauss to Schumacher 18 December; Briefwechsel 1863b:265), probably the only instance it was called that. Gauss’s and Schumacher’s exchange of letters about the naming of the new planet focused on two themes of particular importance (Wattenberg 1962). The first one was about the origin of the idea that Arago could name the planet. Schumacher, as a trained lawyer, pronounced that there was no legal problem as Le Verrier was present when Arago had announced that Le Verrier passed the right to him, and Le Verrier did not contradict this (Schumacher to Gauss 24 October 1846; Briefwechsel 1863b:219). Schumacher, however, suspected that this “transfer of right” was actually the work of Le Verrier, who could not the name the planet directly for himself. At first Gauss, could not believe that something like that could have really taken place, and referring to Arago he stated (Gauss to Schumacher, 31 October 1846; Briefwechsel 1863b: 223–224): “Incidentally, what you mentioned in your letter, and had earlier mentioned that L. V. had only advanced it to Ar[ago], seemed incredible to me then; I could not imagine that a man of real merit could have such a childish vanity.”

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His opinion was rather that the fault came from Arago “…who always likes to blatantly push himself forward, wanted to make himself important by such a protection of his protégé, while to him [Le Verrier] it must be extremely unpleasant, and he, as a subordinate of Arago (LV, as far as I know, is a kind of assistant to the Observatory) did not have the courage to contradict his patron.” Later Gauss changed his opinion and thought that Le Verrier himself had chosen the name “Neptune.” He pointed to a newspaper article (Gauss to Schumacher, 12 December 1846 and 18 December; Briefwechsel 1863b:259–260, 265–267) from the Parisian newspaper Galignani Messanger, published on 2 October (No. 9851), but Gauss read it on the sixth. The text, which Gauss cut out and saved, is identical to that published in The Athenaeum under the Gossip column on 3 October and sent by Arago (cited above). To Gauss this was a confirmation that the name “Neptune” was not chosen by the Bureau des longitudes, but by Le Verrier and therefore was appropriate to be used for the planet. The whole situation annoyed Gauss and he wrote to Schumacher (Gauss to Schumacher, 18 December 1846; Briefwechsel 1863b:265–267): “After this explanation was made, I can only consider the entire later comedy like a Possenspiel [a farce]. It means to take astronomers as fools, if you ask them today what name you want to choose, say Neptune, I give my approval in advance and tomorrow No, it should have my name!” The second issue Schumacher and Gauss discussed enthusiastically in letters was what the name should actually be. Gauss first learned from Encke the suggestion of the name Neptune, together with the sign of a trident, appropriate (“schicklich”). Schumacher was not happy with that name, as it associated the planet’s name with the sea. He could not think about Neptune (Schumacher to Gauss, 16 December 1846; Briefwechsel 1863b:263–265): “… without being dragged by dolphins or other sea-beasts in a seashell-wagon on the sea, and that my imagination finds it too difficult to bring the sea to the sky.” The discussion between Schumacher and Gauss was often relaxed, reflecting their good relationship, and the letters were not necessary intended for anyone else to see. Notable was Schumacher’s letter to Gauss where he compared the English search for Neptune to a contemporary detective story (Schumacher to Gauss 24 October 1846; Briefwechsel, 1863b:215 Schumacher 1846d): “If you have the Athenaeum of 17 October in Göttingen, you will see from [Challis] a proof of incomprehensible astronomical carelessness and a most admirable police report on the new planet, a little miniature piece of joke, as probably no one but de Morgan could have written.” After being informed by Thomas John Hussey that Airy is preparing a “pamphlet” about the discovery (Hussey refers to Airy’s (1846a) account of the Neptune discovery), Schumacher wrote to Gauss (Schumacher to Gauss, 20 November 1846; Briefwechsel 1863b:238): “God grant that a political war between England and France will not follow soon.” Gauss’s contribution to the humorous side of the Neptune naming came in a letter written on 10 February 1847, where Gauss related a dinner joke that (Gauss to Schumacher, 10 February 1847; Briefwechsel 1863b:283–284) “…the god who made the old daddy Uranus tumble, could be none other than Bacchus, and for the

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designation there is no more beautiful symbol than a wineglass, which at the same time will satisfy perfectly the ambitions of Le Verrier.” A few months later Gauss wrote (Gauss to Schumacher, no date; Briefwechsel 1863b:330–331): “The naming of the transuranic planet is in all ways an unclean story.” Even more so, he continues “…if it is true that his [Le Verrier’s] wife played the lead role in it.” Elaborating it further, Gauss added: The joke of the Parisians, who call her Madame Thetis, has something mathematical about it: it is the counterpart to the conclusion:



If x = y,



then 1 / 2x = 1 / 2y. Especially since

Le Verrier = Neptune,

so is



the ( marriage ) half of LeVerrier = half of Neptune.



Schumacher did not want to be outplayed by Gauss and responded (Schumacher to Gauss, 26 July 1847; Briefwechsel 1863b:332): “That Madame Thetis leads the regiment, I have also heard, so it is here 1/2x ~ x, or Hesiod’s statement “δτι πλεον ημισυ παντος” [because it’s halfway through] arrives. From the previous inequality would follow 0 > 1/2x, which is only possible in the not so flattering assumption for [Le Verrier] that x is a negative quantity.” To this position of joking at the expense of Le Verrier, Gauss and Schumacher arrived over a course of several months, in which they convinced themselves that Le Verrier was playing a double game both with Arago and the astronomical community. Schumacher, based on correspondence with Le Verrier, wrote (Schumacher to Gauss, 23 March 1847; Briefwechsel 1863b:286–287): “Arago’s great anger at the word “confus” [my quotes] seems to me hard to understand, if one does not want to presume that the idea of the Cession of the Right, to thereby keep the name Le Verrier, had emanated from Le Verrier himself, and that Arago believed Le Verrier wanted through his letter to Encke to shove everything on Arago.”

6.8.2  The German Choice Of all the different ways German astronomers named the planet in 1846 and early 1847 publications, the most common was the neutral description “Le Verrier’s planet.” Privately there were other suggestions; for example, Encke considered “Vulcan” (Holland 1872:299). Humboldt suggested “Erebus” in a letter to

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Schumacher (Wattenberg 1962:8), but in a letter to Galle he called it “Poseidon” (Wattenberg 1963:53), perhaps as a joke. The opinion of German astronomers was summarised by Schumacher in a letter to Airy at the end of 1846. It was first Airy who in a letter to Schumacher (Galle 1846) asked: “I would be glad to know the opinion of yourself and the Germans on the planet’s name. I think that both Germans and English have a right to be heard in this matter.” Airy also noted why he disliked the name “Neptune” and mentioned John Herschel’s suggestion “Minerva,” as well as other English suggestions such as “Oceanus” and “Hyperion.” Schumacher’s reply was split among four different letters Schumacher wrote to Airy in December 1846, but the one of 18 December was most explicit. Schumacher started his explanation of the German opinion: “My German friends find Arago’s arrogance intolerable.“ (Schumacher to Airy, 18 December 1846). He explained that the German astronomers could not agree with Arago’s dismissal of Adams and his contribution: The priority depends upon the publication and neither you, nor we contest Mr Le Verrier’s claim to legal priority, but we agree with you that there is no doubt, that Mr Adams has found the place of the new Planet, before Mr Le Verrier. We think at the same time that Mr Le Verrier’s research were deeper and rested on more solid ground than those of Mr Adams, as far as we know the last. My German friends incline generally for the name of Neptune (which disagrees with me, as with you) because they suppose that this was the name, to which on the proposal of the Board of Longitude Mr Le Verrier gave his consent, before Mr Arago persuaded him to delegate his right to the Secrétaire perpétual, in order to have named the Planet LeVerrier [underscored and written as a one word].

Schumacher’s private letter might have influenced Airy’s thinking, but it was however, the publicly announced choices of Encke and W. Struve that helped fix the name “Neptune.” On 22 October 1846, Encke presented to the Royal Academy of Berlin the news of the discovery of the planet and devoted a large fraction of his paper to the question of the planet’s name (Encke 1847). He also sent the paper to Schumacher in December 1846 and it was duly printed on 11 February 1847, in No. 588 of the Astronomische Nachrichten. Like many other observatory directors who were publishing astronomical almanacs, Encke had to decide what to call the new planet. Aware of the raging controversy, he used the presentation as a way to argue for the name he liked the most. He started with a historical overview, mentioning both the naming practices for Uranus and Ceres. In Encke’s view, Uranus originally was called the “Georgian planet,” as gratitude to King George III, who “enabled [William] Herschel to build his large reflector”. Ceres was originally called “Fedinandea, to pay tribute to the King of Naples, the founder of the Palermo observatory,” Those names did not endure and by the 1840s they were universally called “Uranus” (except, as Encke remarked, in England) and “Ceres.” Encke pointed out that it was Bode who suggested the name “Uranus,” and with the German discoveries of minor planets by Harding, Olbers (two) and the most recent by Hencke, all those bodies were given mythological names. He also made the point that Pallas was sometimes referred to as “Olbersiana,” until Olbers declared himself strongly against it.

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Encke continued to build his case by quoting from the letter Le Verrier sent to Galle on 1 October 1846 (see Chaps. 5 and 7): “The Board of Longitudes here has proposed the name of Neptune, with a trident as symbol. The name of Janus would mean that this planet is the last of the Solar System, something that there is no reason to believe.” As the final point in his case, he quoted a letter from Gauss, “unserer ersten deutschen astronomischen Autorität” (our first German astronomical authority): “I find the name Neptune chosen by Mr. Le Verrier perfectly decent: as a sign one could perhaps choose a trident, if it were not inappropriate to anticipate the author in any way.” Encke, who, like anyone else, did not know if Le Verrier actually chose Neptune (or if it was chosen by the Bureau des Longitudes), used Gauss’s statement to start building the case that Le Verrier was not against that particular name, in contrast to Arago’s proposal. To understand Encke’s conviction we should look at what Sir Henry Holland (1788–1873), a royal physician and avid traveller, wrote in his autobiography about a night he spent at Berlin Observatory, some “ten or twelve days after the discovery”. Holland heard about the discovery while visiting Bremen and hurried to Berlin. As it was a cloudy night they couldn’t observe, but he reports the following scene: Among other things discussed while thus sitting together in a sort of tremulous impatience, was the name to be given to the new planet. Encke told me he had thought of Vulcan; but deemed it right to remit the choice to Leverrier, then supposed the sole indicator of the planet and its place in the heavens—adding that he expected Leverrier’s answer by the first post. Not an hour had elapsed before a knock at the door of the Observatory announced the letter expected. Encke read it aloud; and, coming to the passage where Leverrier proposed the name of Neptune, exclaimed, “So lass den Namen Neptun sein.” It was a midnight scene not easily to be forgotten. A royal baptism, with its long array of titles, would ill compare with this simple naming of the remote and solitary planet thus wonderfully discovered. There is no place, indeed, where the grandeur and wild ambitions of the world are so thoroughly rebuked, and dwarfed into littleness, as in the Astronomical Observatory. (Holland 1872:299)

The letter Holland mentioned was most likely the letter from Le Verrier to Galle, which arrived at Berlin at the beginning of October (consistent with Holland’s visit to the Observatory) rejecting Galle’s name suggestion (Le Verrier to Galle, 1 October 1846). As the final point in his argument for choosing the name “Neptune,” Encke presented an extract from Le Verrier’s letter sent to him on 6 October, a day after Arago announced his choice for the name. We present it here in the French original, as, according to Le Verrier’s later statements, the meaning of the sentence was misunderstood: J’ai prié mon illustre ami Mr. Arago de se charger de ce soin. J’ai été un peu confus de la décision qu’il a prise dans le sein de l’Académie. (I asked my illustrious friend Mr. Arago to take in charge the choice of a name for the planet. I have been somewhat confused by the decision he has taken at the Academy.)

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The printing of this sentence, without Le Verrier’s approval, contributed significantly to the collapsing relations between Le Verrier and Arago in Paris, as Le Verrier reported to Schumacher. From the correspondence between Le Verrier and Schumacher it is evident that Le Verrier was annoyed that Encke printed it. His point was that it was printed out of context and that the word “confus” was translated incorrectly (Le Verrier to Schumacher 16 March 1847). This was what Le Verrier wrote to Schumacher to explain the tense situation in which he found himself in Paris: One was searching for a long time a pretext to attack me. You will probably judge, Monsieur and friend, that this was not easy, since one had to be content with the following pretext. Mr. Encke published incautiously a few lines of a letter I wrote him, the meaning of it he has not understood. I rewrite the passage entirely now. You will see that it was only a question of explaining to Mr. Encke what Mr. Arago has done, and not at all to disavow him [Arago]. You should remember that we say to someone to whom we owe much: Mr., I am confused [“embarrassed” could be a more suited translation of the world “confus” in this context] of your kindness. This is the meaning of the word “confus”. Mr. Schumacher made me the honour to write to me, looking for a name for the planet. I asked my illustrious friend Mr. Arago to take in charge the choice of a name for the planet. I have been somewhat confused [embarrassed] by the decision he has taken at the Academy. I would not have dared to explain you what this decision is if I had not found in it the opportunity to pay a just tribute of admiration for your work on Encke’s comet. The obscure name that Mr Arago wants to give to the Planet would put me on the same glorious footing as the illustrious director of the Berlin Observatory, and I do not deserve this.

The problem stemmed from misunderstanding the French word “confus”. According to Le Verrier, the true meaning of the sentence published by Encke was related to the next sentences in the letter (not used by Encke): Le Verrier was embarrassed, in a flattered way, that his name would be compared to the name of such a famous person as Encke. It is quite clear, however, that Encke would have not wished to print such a flattering sentence, and perhaps Le Verrier was correct to complain that his statement had been taken out of context. Whether or not Le Verrier had a secret agenda when writing to Encke, as suspected by Gauss and Schumacher, and regardless of the real meaning of the word, Encke assumed that Le Verrier himself did not really approve of Arago’s choice. Therefore, Encke concluded: “Under these circumstances I will, supported by the high authorities of the Bureau des Longitudes in Paris, and of Geh. Hofraths Gauss, keep the name Neptune and the sign of the trident for the next few years, until public opinion in Germany is sufficiently consolidated to establish a definite name.” The paper printed in the Astronomische Nachrichten finished with a paragraph that looks like an add-on to the original presentation at the Academy. Encke stated that in “later letters” (that arrived after his presentation), the name “Neptune” was accepted by Sir John Herschel and Wilhelm Struve. The last sentence also claimed that “the first astronomical authorities in Germany, France, England and Russia” adopted the name (Encke 1847:196). The letter from Wilhelm Struve, the director of the Pulkovo Observatory, to which Encke referred, must have been similar to the letter Wilhelm Struve sent to Airy (Struve to Airy, 23 January 1847), which included a copy of the essay published by Struve (Struve 1848) in the Bulletin of the St. Petersburg Imperial

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Academy of Science. Airy re-published it in The Athenaeum on 20 February 1847 (Airy 1847b:199). W. Struve’s essay was titled: “On the denomination of the planet newly discovered beyond the orbit of Uranus”. It presented a list of arguments why the name “Neptune” was appropriate and why Arago’s suggestion of “Le Verrier” was not. W. Struve started from the same problem as Encke, what name to give to the new planet in their yearly publications with ephemerides? A number of arguments were similar, such as the historical parallels with Uranus, including that even John Herschel called it “Uranus,” and the fact that the Bureau des Longitudes proposed “Neptune” and Le Verrier did not raise any objections to it. Struve’s arguments against “Le Verrier” were also similar: all planets should have mythological names and not names of discoverers. More remarkable is his second argument against the name, as it was not only Le Verrier who predicted the location of the planet, but also John Couch Adams, and that had to be taken into account. W. Struve concluded his letter to Airy in a similar way as Encke (Airy, 1847b:199): “Consequently, we will retain the name of Neptune; and will make no change, unless hereafter the general voice shall determine in favour of another name.” These two powerful statements by the most prominent of German astronomers, that appeared at similar time in print, in two scientific journals, and in a literary magazine, read by the educated people across Europe, finally settled the controversy over the name of the new planet. The Neptune naming scandal that erupted almost immediately after the announcement of the discovery bears witness that science is a human endeavour after all. This is also the most amusing part of the story, especially when one considers some of the semi-private opinions voiced by renowned scientists and respected members of society. Gauss’s and Schumacher’s remarks are just some that are available to us, as they have been preserved in their letters (and even published in books celebrating their achievements.) Another infamous, but still entertaining, remark on the topic came from a letter to Airy sent by Captain William Henry Smyth (1788 – 1865) on 5 December 1846, then president of the Royal Astronomical Society: I don’t quite like this proposed change in the nomenclature of the Planets, for mythology is neutral ground. Herschel is a good name enough. Le Verrier somehow or other suggests the idea of a Fabriquant & is therefore not so good. But just think how awkward it would be if the next planet should be discovered by a German: by a Bugge, a Funk, or your hirsuite friend Boguslawski! (Smyth to Airy, 5 December 1846)

Captain Smyth’s letter was written before the publication of Encke’s paper (Encke, 1847), in which Encke did not fail to mention that “Our German custom has established itself at four, you can say, at five new planets, since Herschel is a German by birth ...” Given the development of astronomy within German-speaking countries, and the efforts described in this chapter, it is perhaps not so exceptional to consider that by 1846 of seven new planetary bodies, five were discovered by Germans: Uranus (Herschel in Bath, England), Pallas (Olbers in Bremen), Juno (Harding in Lilienthal),

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Vesta (Olbers in Bremen), and Astrea (Hencke in Driessen). Furthermore, Ceres was re-discovered by a German (Olbers) based by the prediction made by a German (Gauss). Finally, Neptune was discovered in Berlin (Galle and d’Arrest), based on the prediction by Le Verrier. Acknowledgements  This chapter would not be possible without the help from the librarians of the Leibniz-Institut für Astrophysik-Potsdam (AIP), Regina von Berlepsch and Melissa Thies. Thank you for your indefatigable and very resourceful support. A special thanks goes to the director of AIP, Matthias Steinmetz, who introduced me to the intricate details of the Neptune discovery some years ago over a good dinner in Lyon. I would also like to thank Wolfgang Dick for pointing me to the location of Galle’s letters and useful discussions. Sabine Thater helped with translations from German originals, while Damien Le Borgne for initial and James Lequeux for final translations of Le Verrier’s and Galle’s letters from the French. James, thank you for being so responsive. I had several stimulating email exchanges with a number of co-writers of this book, which I thoroughly enjoyed. I would like to thank Brian Sheen for inviting me to join the project, and the editors William Sheehan, Robert W. Smith, Carolyn Kennett and Trudy E. Bell for many inspiring thoughts and patience in waiting for my contribution.

References Abhandlungen der Königlichen Gesellschaft der Wissenschaften in Göttingen, 1845, In der Dieterichschen Buchhandlung, Göttingen. Adams, J. C., 1846, An explanation of the Observed Irregularities in the Motion of Uranus, on the Hypothesis of a Disturbance Caused by a More Distant Planet; with a Determination of the Mass, Orbit and Position of the Disturbing Body, Monthly Notices of the Royal Astronomical Society, 7, 149. Airy, G.B., 1846a, Account of some circumstances historically connected with the discovery of the Planet exterior to Uranus, Monthly Notices of the Royal Astronomical Society, 7, 121. Airy, G.B., 1846b, Account of some circumstances historically connected with the discovery of the Planet exterior to Uranus, Astronomische Nachrichten, 25, 133. Airy, G.B. to H.C.  Schumacher, 9 December 1846, Staatsbibliothek zu Berlin, Handschriftenabteilung, Nachlass Schumacher. Airy, G.B., 1847a, Account of some circumstances historically connected with the discovery of the Planet exterior to Uranus, Astronomische Nachrichten, 25, 149. Airy, G. B., 1847b, Name of the New Planet, The Athenaeum, No.1008, 20 February Airy, G. B., 1896, Autobiography, Cambridge University Press, Cambridge. Arago, F., 1846, A note in Our Weekly Gossip column of The Athenaeum, No. 989, 10 October, 1046 d’Arrest, H.L, 1846, Bestimmung der Elemente der Astraea mit Rüksicht auf die Störungen aus der ganzen Reihe der Beobachtungen, Astronomische Nachrichten (in German), 24, 277. Bessel, F.  W., 1810, ‘Untersuchungen über die scheinbare und wahre Bahn des im Jahre 1807 erschienenen grossen Kometen’ (in German), Königsberg. Bessel, F. W., 1823a, Untersuchung einer merckwürdigen Erscheinung, welche die Bewegung des Uranus gezeigt hat, Staatsbibliothek zu Berlin, Handschriftenabteilung, Ms. Germ quart. 2266. Bessel, F.W., 1823b, ‘Nachricht von einer auf der Königsberger Sternwarte angefangenen allgemeinen Beobachtung des Himmels’, Astronomische Nachrichten (in German), 1, 257. Bessel, F.W., 1826, ‘Einige Bemerkungen über die Entwerfung der vollständigen Himmelskarten’, Astronomische Nachrichten (in German), 4, 437. Bessel, F.W., 1831, ‘Ueber den Einfluss eines widerstehenden Mittels auf die Bewegung eines Pendels’, Astronomische Nachrichten (in German), 9, 221.

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Bessel, F.  W., 1832, ‘Versuche Über die Kraft, mit welcher die Erde Körper von verschiedner Beschaffenhelt anzieht’, Annalen der Physik und Chemie, Berlin: J. Ch. Poggendorff, 25, 401. Bessel, F. W., 1838 ‘Bestimmung der Entfernung des 61sten Sterns des Schwans’ [Determination of the distance to 61 Cygni]. Astronomische in German), 16, 65–96. Bessel, F.  W., 1844a ‘Ueber Veränderlichkeit der eigenen Bewegungen der Fixsterne’ [On Variations of the proper motions of the fixed stars]. Astronomische Nachrichten (in German). 22 (514): 145–160. Bessel, F.  W., 1844b. ‘Ueber Veränderlichkeit der eigenen Bewegungen der Fixsterne. (Fortsetzung)’ [On Variations of the proper motions of the fixed stars (continued)]. Astronomische Nachrichten(in German). 22 (515): 169–184. Bessel, F.W., 1848, Populäre Vorlesungen über wissenschaftliche Gegenstände, Herausgegeben von H. C. Schumacher. Hamburg. Biermann, K. R., 1979, Briefwecshel zwischen Alexander von Humboldt und Heinrich Christian Schumacher, Beitrage zur Alexander-von-Himboldt-Forschung, Vol. 6, Akademie-Verlag, Berlin Bode, J, E., 1801, Uranographia, sive astrorum descriptio viginti tabulis aeneis incisa ex recentissimis et absolutissimis Astronomorum observationibus, Berlin. Bouvard, A., 1821, Tables astronomiques publièes par le bureau des longitudes de France: contenant les tables de Jupiter, de Saturne et d'Uranus, construites d'après la Thèorie de la mècanique celeste, Bacler et Huzard, Paris. Briefwechsel zwischen, Gauss, C. F. und Schumacher, H. C. 1863a, edited by Peters, C. A.F. Esch, Altona 1863, Volume 4. Briefwechsel zwischen, Gauss, C. F. und Schumacher, H. C. 1863b, edited Peters, C. A. F. Esch, Altona 1863, Volume 5. Briefwechsel zwischen Gauss und Bessel, 1880, Veranlassung der Königlich Preussischen Akademie der Wissenschaften, Wilhelm Engelmann, Leipzig. Bruhns, C., 1869, Johann Franz Encke, Sein Leben und Wirken, Leipzig, 115, 122–123, 191. Cataloge zu den 24 Stunden der akademischen Sternkarten für 15° südlicher bis 15° nördlicher Abweichung, 1859, Herausgeben von der Königlichen Akademie der Wissenschaften zu Berlin, Berlin. Centenaire de la naissance de U.  J. J.  La Verrier, 1911, Gauthier-Villars, Paris (Académie des Scienecs, text by Bigourdan, G). Challis, J., 1846a, The New Planet, The Athenaeum, No.909, 17 October,1069. Challis, J., 1846b, ‘Schreiben des Herrn Professors Challis, Directors der Cambridger Sternwarte, an den Herausgeber’ Astronomische Nachrichten, 25, 101. Challis, J., 1846c, ‘Schreiben des Herrn Professors Challis, Directors der Cambridger Sternwarte, an den Herausgeber’ Astronomische Nachrichten, 25, 105. Chapman, A., 2017, The Victorian amateur astronomer, 2nd edition, Gracewing, Leominster Danjon, A., 1946, Le centenaire de la decuverte de Naptune, Ciel et Terre, 62, 369. Dick, W. R., 1985, Dokumente aus der Dorpater Zeit Wilhelm Struves in der Charkower Sternwarte, Tartu Astr. Obs. Teated Nr. 75, 5. Dick, W. R., 1986, Zur Auffindung des Planeten Neptun an der Berliner Sternwarte im September 1846, Die Sterne, 62, 251–257. de Pater, I et al. 2018, The rings of Neptune, Planetary Ring Systems. Properties, Structure, and Evolution, Edited by M.S.  Tiscareno and C.D.  Murray. Cambridge University Press, 2018, p. 112-124. Doppelmayr, J, G., 1742, Atlas coelestis, Sumptibus Heredum Homannianorum, Nurnberg Dreyer, J. L, E., 1876, Heinrich Luis d’Arrest, Vierteljahrsschr. Astron. Ges. 11, 1. Dreyer, J. L. E., 1882, Historical note concerning the discovery of Neptune, Copernicus, 2, 63–64. Dunkley, J., 2018, Our Universe: An Astronomer’s Guide, The Belknap Press of Harvard Univeristy Press, Cambridge. Encke, F. J., 1836, Schreiben des Herrn Professors und Ritters Encke, Directors der Sternwarte in Berlin, an den Herausgeber, Astronomische Nachrichten, 13, 161.

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Encke, F. J., 1837, Schreiben des Herrn Professors und Ritters Encke, Directors der Sternwarte in Berlin, an den Herausgeber, Astronomische Nachrichten, 15, 17. Encke, F. J., 1846, Schreiben des herrn Encke an den Herausbeger, Astronomische Nachrichten, 25, 49. Encke, F. J., 1847, Vortrag, den Herr Prof. Encke am 22sten Octb. d. J. in der Königl. Acad. der Wissensch. über den neuen Planeten gehalten hat, Astronomische Nachrichten, 25, 191, 196. Everett, E, 1847, On the New Planet Neptune, Astronomische Nachrichten, 26, 375 Flamsteed, J., 1729, Atlas coelestis, London. Foerster, W., 1911, Johann Gottfried Galle, Vierteljahrsschr. Astron. Ges., 20, 46, 17. Galle, J. G., to Le Verrier, J. J. U., 25 September 1846, Bibliothèque de l’Observatoire de Paris; also partially published in Centenaire, p. 19, Grosser (1962: 119) and Wattenberg (1963: 50). Galle, J. G., 1851, Einige Messungen der Durchmesser des Saturns und der Ringe desselben 1838 und 1839 auf der Berliner Sternwarte, nebst Bemerkungen über den schon damals wahrgeuommenen dunkeln inneren Ring, Astronomische Nachrichten, 32, 187. Galle, J.G., 1873, Über die Anwendung von Beobachtungen der kleinen Planeten zur Ermittlung des Werthes der Sonnen-Paralaxe, mit besonderer Rücksicht auf die diesjährige Opposition der Phocäa, Astronomische Nachrichten, 80, 1. Galle, J.G., 1875, Über eine Bestimmung der Sonnen-Parallaxe aus correspondirenden Beobachtungen des Planeten Flora auf mehreren Sternwarten der nördlichen und südlichen Halbkugel im Oct. u. Nov. 1873, Maruschke & Berendt, Breslau. Galle, J. G., 1877, Ein Nachtrag zu den in Band 25 und dem Ergäzungshefte von 1849 der Astr. Nachrichten enthalten Berichten über die erste Auffindung des Planeten Neptun, Astronomische Nachrichten 89, 349–352. Galle, J. G., 1882a, Einige nachträgliche ergänzende Bemerkungen in Betreff der Auffindung des Planeten Neptune und der Beobachtungen des dunkeln Saturnsringes in den Jahren 1838 u. 1839, Astronomische Nachrichten, 101, 219, 221. Galle, J. G., 1882b, Über die esrte Auffindung des Planeten Neptune, Copernicus, 2, 96. Gapaillard, 2015, By how much did Le Verrier err on the position of Neptune? Journal for the History of Astronomy, 46, 48, 96. Grosser, M., 1962, The discovery of Neptune, Dover Publication, Inc. New York. Gyldén, H., 1877, Die Grundlehren der Astronomie, Verlag von Wilhelm Engelmann, Leipzig. Hamel, J., 1984a, Friedrich Wilhelm Bessel, BSB B.G. Teubner Verlagsgesellschaft, Leipzig. Hamel, J., 1984b, Bessels Hypothese der spezifichen gravitation und das problem der Uranusbewegung, Die Sterne, 60, 278, 281, 282. Hamel, J., 1989, Bessels Projekt der Berliner Akademischen Sternkarten, Die Sterne, 65, 11. Harding, K. L., 1822, ‘Atlas novus coelestis XXVII Tabulis Continens Stellas Inter Polum Borealem et Trigesimum Gradual Declinationis Observatas’, Göttingen. Henry, J., Walker S. C., 1847, Schreiben des Herrn J. Henry, Secretairs der Smithsonian Institution an den Herausgeber, Astronomische Nachrichten, 26, 65. Herrmann, D. B., 1973, Zur Statistik der Sternwartengründungen im 19. Jahrhundert, Die Sterne, 49, 48. Herschel, J., 1846, Le Verrier’s Planet, The Athenaeum, No 988, 1019 October 3. Holland, H., 1872, Recollections of past life, D. Appleton & Company, New York Hubbell, J. G, and Smith, R. W., 1992, Neptune In America: Negotiating a Discovery, Journal for the History of Astronomy, 22, 51, 261–91, 278. Knobloch, E., 2013, Es wäre mir unmöglich nur ein halbes Jahr so zu leben wie er: Encke, Humboldt und was wir schon immer über die neue Berliner Sternwarte wissen wollten, HiN, XIV, 26, 49–50, 59. Lalande, J., 1801, Histoire Céleste Française, L’Imprimerie de La République, Paris Lassell, W., 1847, Schreiben der Herrn Lassell an der Herausgeber, Astronomische Nachrichten, 25, 197. Lequeux, J., 2013, Le Verrier  – Magnificent and Detestable Astronomer, Springer, New  York. Heidelberg Dordrecht London. Le Verrier, U.J.J., 1843, CRAS 16.

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Le Verrier, U.J.J., to Airy, G.B., 28 June 1846, RGO 6/96a, Fos. 127–129. Le Verrier, U.J.J., to Schumacher, H.C., 30 June 1846, Staatsbibliothek zu Berlin, Handschriftenabteilung, Nachlass Schumacher. Le Verrier U.J.J., to Galle J.G., 1 October 1846, Staatsbibliothek zu Berlin, Handschriftenabteilung, Nachlass Schumacher. Le Verrier, U.  J. J., to H.  C. Schumacher 1 October 1846, Staatsbibliothek zu Berlin, Handschriftenabteilung, Nachlass Schumacher. Le Verrier, U.J, J., 1846a, Recherches sur les mouvements d’Uranus, Astronomische Nachrichten, 25, 53. Le Verrier, U.J.J., 1846b, Recherches sur les mouvements d’Uranus, Astronomische Nachrichten, 25, 65. Le Verrier, U.J.J., 1846c, Recherches sur les mouvements d’Uranus, Astronomische Nachrichten, 25, 85. Le Verrier, U.J.J., to H.  C. Schumacher, 7 January 1847a, Staatsbibliothek zu Berlin, Handschriftenabteilung, Nachlass Schumacher. Le Verrier, U.J.J., to H.  C. Schumacher, 16 March 1847b, Staatsbibliothek zu Berlin, Handschriftenabteilung, Nachlass Schumacher. von Lindenau, B., 1848, Beitrag zur Geschichte der Neptuns-Entdeckung, Ergänzungs-heft zu den Astronomichen Nachrichten, Altona, 12, 235. Newcomb, S., 1878, Popular Astronomy, MacMillan and Co, London. Newcomb, S., 1881, Pöpuläre Astronomie, Deutsche vermehrte Ausgabe bearbeitet durch Rud. Engelmann, Verlag von Wilhelm Engelmann, Leipzig. Newcomb, S., 1892, Newcomb-Engelmanns Pöpuläre Astronomie, Zweite vermehrte Ausgabe herausgegeben von Dr. H.C. Vogel, Verlag von Wilhelm Engelmann, Leipzig. Newcomb, S., 1905, Newcomb-Engelmanns Pöpuläre Astronomie, Zweite vermehrte Ausgabe herausgegeben von Dr. H.C. Vogel, Verlag von Wilhelm Engelmann, Leipzig. Nichol, J. P., 1849, The Planet Neptune: an Exposition and History, James Nichol, H. Bailliere, Edinburgh. Obituary Notices: Fellows, 1927, Monthly Notices of the Royal Astronomical Society, 81, 257. Olson, R.J.M., and Pasachoff, J.M., 2019, Cosmos: the art and science of the universe, Reaktion Books, London. Piazzi, G., 1814, Precipuarum Stellarum Inerrantium Positiones Mediae ineunte Seculo XIX, Panormi Peirce, B., 1848, Über die Störungen des Neptuns, Astronomische Nachrichten, 27, 215. Scheiner, J., 1893, Die Astronomie des Unsichtbaren, Himmel und Erde, Bd 5, 20–28. Schmeidler, F., 1984, Leben und Werk des Königsberger Astronomen Friedrich Wilhem Bessel, 1984, ILMA-Verlag, Kelkheim/T. Schumacher, H.C., 1826, Entwurf zu einer Herausgabe neuer Himmelskarten, Astronomische Nachrichten, 4, 291. Schumacher, H.C., 1846, Name des neuen Planeten, Astronomische Nachrichten, 25, 81. Schumacher, H.C., 1850, Reduction der in Greenwich, Paris und Königsberg gemachten. Beobachtungen des Uranus, von 1781–1837, Astronomische Nachrichten, 30, Nr. 705, 145– 182; Beil. Zu Nr. 708, 193. Schumacher, H.C., to G.B. Airy, 11 December 1846a, RGO 6/96a, Fo. 316r. Schumacher, H.C., to G.B. Airy, 18 December 1846b, RGO 6/96a, Fo. 330–331. See T.J.J., 1910, Leverrier’s letter to Galle and the discovery of Neptune, Popular Astronomy, 18. 475. Smart, W.M., 1946, John Couch Adams and the discovery of Neptune, Nature, 158, 648. Smyth, W.H.. to Airy, G.B., 5 December 1846, RGO, 6/96a, Fo. 295r. Standage, T., 2000, The Neptune File: A story of Astronomical Rivalry and the pioneers of Planet Hunting, Walker & Company, New York. Staubermann, K., 2006, Excercising patience: on the reconstruction of F.W.  Bessel’s early star chart observations, Journal for History of Astronomy, 27, 28, 37, 19.

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Steinheil, C., 1826., ‘Beschreibung eines Apparats, welcher zur Einzeichnung der astronomisch bestimmten Sterne, in die vollständigen Himmelskarten (A. N. Nr. 88) angewandt worden ist’, Astronomische Nachrichten, 4, 443. Struve, W., to G. B. Airy 1847, 23 January (07 February) 1847, RGO 6/96a, Fo. 424. Struve, W., 1848, ‘Sur la denomination de La Planete Nouvellement Decouverte’, Bulletin de La Classe Physico-Mathématique de l’Académie Impériale de Sciences de St-Pétersbourg, Vol. 6–7. p.77 – 80 (dated 17 (29) December 1846; received 11 January 1847) Turner, H.H., 1911., ‘Obituary notice of Johann Gottfried Galle’, Monthly Notices to the Royal Astronomical Society, 71, 275. Wattenberg, D., 1946, Die Entdeckung des Planeten Neptun auf der Berliner Sternwarte vor 100 Jahren, Die Naturwissenschaften, 33, 129. Wattenberg, D., 1959, Fr. W.  Bessel und die Vorgeschichte der Entdeckung des Neptun, Mitt. D. Archenhold-Sternwarte, Nr. 54. Wattenberg, D., 1962, Der Streit um den Namen des Planeten Neptun, Vorträge u. Schriften d. Archnhold-Sternwarte, Nr. 10, Berlin. Wattenberg, D., 1963, Johann Gottfrid Galle 1812–1910: Leben und Werken eines deutschen Astronomen, Johann Ambrosius Barth Verlag, Leipzig. Wilson, C. 1985., ‘The Great Inequality of Jupiter and Saturn: from Kepler to Laplace’, Archive for History of Exact Sciences, Vol. 33, No. 1/3 (1985), 15–290. Wulf, Andrea., 2016, The Invention of nature: The Adventures of Alexander von Humboldt, the Lost Hero of Science, John Murray, London.

Chapter 7

Clashing Interests: The Cambridge Network and International Controversies Robert W. Smith Abstract  James Challis’s search from the Cambridge University Observatory for the postulated planet beyond Uranus erupted in debates and controversies in Britain and beyond once the planet had been telescopically discovered from Berlin. There arose, in fact, an international furore that permanently damaged the Astronomer Royal’s reputation. Why did the discovery of the planet matter so much to so many people? Why was the allocation of credit for the discovery the focus of so many arguments and negotiations? Exploring these key historical questions is revealing about both the nature of the astronomical and, more broadly, the scientific, enterprise in Britain in the mid-­ nineteenth century. In the high-stakes fights over credit for what was widely seen as one of the most remarkable discoveries in nineteenth-century science, the actions of various “men of science” with different degrees of association with, and links to, Cambridge University, were viewed at the time as crucial. However, the controversies also highlighted the fact that different individuals held very different conceptions of what constituted a true “discovery.” Moreover, they also differed on how credit should be allocated for discoveries, on what counted as a publication, and even on how science should be organized and practiced. The Neptune controversies embroiled men of science who often strongly criticised each other in public forums, as well as in august scientific publications, including those of the Royal Astronomical Society and the French Academy of Sciences. Why all the fuss?

R. W. Smith (*) Department of History and Classics, University of Alberta, Edmonton, AB, Canada e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_7

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7.1  A New Interpretation The telescopic discovery of Neptune made by Heinrich d’Arrest and Johann Gottfried Galle at the Berlin Observatory in September 1846 was a direct consequence of the calculations of the French mathematical astronomer Urbain Jean Joseph Le Verrier. “I mourn over the loss to England and to Cambridge of a discovery which ought to be theirs every inch of it,” wrote a disappointed Sir John Herschel in November 1846 “but I have said enough about it to get heartily abused in France, and I don’t want to get hated in England for saying more” (Herschel, 1846a). Positions for the planet had, as we have seen, been predicted by the Cambridge mathematician John Couch Adams well before the actual optical discovery—and well before Le Verrier published his prediction. James Challis had also instituted a search from Cambridge in July 1846. Cambridge, in the opinion of many people, had been scooped. Rather than seeing Le Verrier and the Berlin astronomers as having succeeded, many writers on the discovery of Neptune over the years have preferred to concentrate on the assumed failure to discover the planet from Cambridge. Airy and Challis have been the chief targets of these critics because they, supposedly, did not search energetically enough for the planet. Such judgemental works have done much to damage the reputations of Airy and Challis. An entry on Challis in the well-known Dictionary of Scientific Biography, for example, claims that he “would now be as forgotten as his peculiar ideas had not the events surrounding the discovery of Neptune in 1845 [sic] given him a genuine opportunity for scientific immortality. But he fumbled it” (Eggen, 1971: 186). Airy died in 1892 and there were plans to commemorate him in Westminster Abbey in London, long a site of special significance to the English then later the British state, and the burial place of numerous kings and queens, as well as of Isaac Newton and Charles Darwin. Even 46 years on from the discovery, however, the Neptune controversies undermined such a move. Airy’s successor as Astronomer Royal was William Christie (1845–1922). One of those involved in the Westminster Abbey plans explained to Christie that it was easy to understand how “Cambridge should have been furious about the matter” as “all England was more or less, and the call for someone to hang was met by fixing on Airy and, strange to say, excluding Challis: but I should have thought that people now were calmer. Unfortunately some are not…. I do not suppose that Airy will have very many zealous supporters” (Tennant, 1892). Airy himself complained in his Autobiography that he had been “severely abused by both English and French” (Airy, 1896: 181). After the optical discovery, feelings were running high for a period and the events around the discovery were very stressful for a number of people. John Herschel, for example, lamented that they had given him “more pain and grief than any national event….” (Herschel, 1846b). There have nevertheless been some historical accounts sympathetic to Airy and Challis. In 1988, Allan Chapman, for example, presented a detailed and nuanced defence of Airy’s actions. In Chapman’s interpretation of events, Airy’s activities

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should be seen in relation to his very demanding public duties as Astronomer Royal. The search for the predicted planet, Chapman claimed, was the first major incident to highlight the distinction between a scientist’s private research and what was seen to be the public duty to the state that paid his salary (Chapman, 1988). Attempts to vindicate or criticize Airy and Challis have nevertheless worked to focus the existing literature on a few individuals and quite narrowly on the discovery itself and issues of blame. Such works have thereby often ignored broad but crucial contextual issues, as well as generally substantially overestimating the importance Airy and Challis (and Adams) attached to the postulated planet prior to its optical discovery. An interpretation of the discovery of Neptune that did put it within broad social and political currents was, however, advanced 70 years ago by Anton Pannekoek (1873–1960), a distinguished Dutch astronomer, as well as a major writer on the history of astronomy (Pannekoek, 1953). Pannekoek claimed that Adams’s researches were treated with indifference by the leading British astronomers out of a lack of interest in the prospect of such a discovery and in the exploitation of such a discovery once made. But the predictions of a new planet did stir interest. The optical discovery itself sparked a highly charged and often bitter debate in Britain, one that strongly engaged in particular the interest as well as the interests—personal and ideological—of various individuals, a number associated with, or with close ties to, Cambridge University. Indeed, the place of Cambridge in the political economy of British science is central to understanding the events surrounding Neptune’s discovery. The discovery was of just the kind to appeal most strongly to this group and some like-minded colleagues as a public demonstration of the power of science and so presented them with an opportunity to advance their own ideological interests. For them, Adams and his researches became a means to further those interests. By defending Adams’s claims, as they saw them, this group was also defending the value and worth of the specialised mathematical training provided by Cambridge (and so its importance for British science in general), a training that had been the target of criticism. This chapter will therefore focus on the responses of this group to the prospect of the discovery and to the discovery itself. Airy’s position within the group as well as his other various social connections form only one aspect of the account, but the most significant one in the post-discovery arguments and debates. Airy, I will argue, worked for a Cambridge discovery. After the optical discovery from Berlin, he was closely involved in what I will call the ‘management of the discovery.’ I also emphasize that there was no conspiracy in place in which people actively plotted or told untruths to obscure secret actions.1 But there were times when the main historical actors made private comments they felt unable to express in public. The rich body of primary material on the discovery enables us to see that the public and private comments and statements of some of the key historical actors

1  That Airy and others did deliberately suppress news of Adams’s investigations has been argued by Dennis Rawlins, for example, in (Rawlins, 1984).

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were at times at variance; thus, interpreting the main events of 1846 and 1847 presents considerable challenges. The optical discovery of Neptune in September 1846 also created an enormous rupture. It, among other things, transformed a possibility or a probability into a reality, it changed personal relationships, and it shifted the broader social position of astronomy. The actual discovery, too, may well have helped ‘remake’ memories—so that we need to be aware that in some cases the historical actors might have offered post-discovery rationalizations of earlier, perhaps different, positions. The standing and reputation of the three main figures in this chapter—Airy, Challis and Adams—also need to be kept in mind. Adams was a very junior and, outside of Cambridge, unknown figure; Challis was the Plumian Professor of Astronomy and Experimental Philosophy so in effect salaried to be a senior lecturer in mathematics and its application to astronomy and experimental philosophy (now known as physics) and a University examiner. Airy was responsible for directing the Royal Observatory at Greenwich and was the single most important person in British astronomy, as well as highly influential in British science in general, including the science pursued within the British Empire. Airy had very extensive contacts with astronomers and savants in Europe. Airy, then, was an experienced public and international figure in ways that Challis and Adams were not. First, however, I will turn briefly to examining the notion of “discovery.” This will sharpen our understanding of events as for part of the time the arguments and debates around the new planet centred on what counts as a discovery. I will not be concerned with the standards applied to discoveries in the early twenty-first century. My goal is not to take off the historian’s hat to replace it with a judge’s in order to decide who, really, should get the credit. Rather, my aim is to keep the focus firmly on the notions and standards of discovery in the mid-1840s.

7.2  What Is a Discovery? “The making of discoveries is what science is all about,” David Miller has argued, “the stuff of the scientific career and of scientific reputation” (Miller, 2004:11). Chapter 6 recounts how the optical discovery of Neptune was made from Berlin in September 1846. But what does it mean to talk of “discovery”? Important for this chapter is that during the 1840s, the idea emerged there was a “necessary connection between scientific merit, priority, and print publication” — and this shifted the ways in which priority disputes were settled. There was, however, no consensus on these matters, and disagreements could flare up—and did in the case of Neptune. As Alex Csiszar has emphasized, “Discovery and its relationship to both history and property were hot topics during the early decades of the nineteenth century. Across Europe, savants and philosophers puzzled over how and whether discoveries could be fixed in time and space, and to what extent they could be attributed to particular individuals.” This “mattered because it was taken to be a key step in defining what marked out scientific activity from other creative forms of

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endeavour, and thus in defining who belonged to these communities of practitioners” (Csiszar, 2018:160). Many scientists and philosophers of science have employed what has been called the “individualist-mentalist” model of scientific discovery in which there is a moment at which discovery occurs, at an identifiable location, and which involves an individual or group of individuals. According to this definition, then, J.G. Galle can be readily identified as the optical discoverer of the planet later to be named “Neptune” as he was the first to see the candidate object. But did the discovery occur on the night of 23/24 September 1846 when the candidate object that turned out to be a planet was identified? Or did it happen the following night when there was confirmation of the object’s motion among the background stars, thereby indicating the object itself was not a star? Also, what about the role of Heinrich d’Arrest, who went to the telescope with Galle on the night of 23/24 September armed with Carl Bremiker’s star chart Aquarius XXI, but whose eye was not at the eyepiece of the telescope when what turned out to be the new planet was detected? Chapter 6 detailed how d’Arrest was not acknowledged as a co-discoverer until later in the nineteenth century, but his role was fundamental. Galle and d’Arrest were also prompted to go to the telescope in the first place by Le Verrier’s calculations. When Galle and d’Arrest hunted for the planet, they knew nothing of the computations of Adams. But Adams’s computations in September 1846 indicated a position distant from the position at which the planet was found and also some distance too from Le Verrier’s predicted position, which itself was about 1° from the planet’s actual position. What credit, then, should accrue to Le Verrier—and what, if any, to Adams? And what about James Challis, who as part of his methodical search for a new planet twice spotted Neptune ahead of the find in Berlin—on August 4 and August 12—but did not recognize it as a planet? Did Challis, then, discover Neptune optically before Galle and d’Arrest? Challis appears, at least for a time, to have believed he had. How different is the case of Challis from that of William Herschel and Uranus that was examined in Chapter 1? Herschel detected an object he believed initially was a comet. It was only very slowly that he accepted that the object was indeed a planet, and then only after extended mathematical analyses of Uranus’s orbit by other astronomers. If, we might ask, Challis did not discover Neptune on August 4, 1846, then what allows us to claim that Herschel discovered Uranus on March 13, 1781? What about still earlier “pre-discovery” sightings of Neptune by Lalande and even Galileo? Did the discovery of Neptune even require the planet to be seen? On 10 September 1846, two weeks before the dramatic optical sighting in Berlin, John Herschel, in an address to a meeting of the British Association for the Advancement of Science, proclaimed, “We see it [the postulated planet] as Columbus saw America from the shores of Spain. Its movements have been felt, trembling along the far-reaching line of our analysis, with a certainty hardly inferior to that of ocular demonstration” (Herschel, 1846c). Herschel did not refer directly to the computations of Adams and Le Verrier that inspired his confident declaration, but the implication is clear.

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Adams’s sense of discovery was similar to Herschel’s, to judge anyway from a letter he wrote to James David Forbes (1809–1868) nine years after the events of 1846. Here Adams talked of the initial observation of the planet in Berlin as “recognition” and “verification” of the discovery, implying the actual discovery had been made mathematically. As he put it: …the first actual recognition of the planet by means of the telescope seem[s] to me to have a very indirect bearing on the merit due to either M. Le Verrier or myself. The verification of M. Le Verrier’s prediction was equally a verification of my own…My preliminary calculations [which had assumed circular motion] obtained in 1843 were much more satisfactory than you suppose…the longitude obtained differed only a few degrees (I think no more than ten) from the final result…. (Adams, 1855: 181).

One of the major developments in the writing of the history of science in recent decades has been the general acceptance that discoveries are often complicated and messy processes that are not easy to localize in time and space. The aftermath of a discovery event can be important, perhaps even more significant in some respects, than the discovery event itself (Kragh and Smith, 2003: 142). A crucial point here is that discovery “accounts cannot be only intellectual accounts of how an idea entered a scientist’s mind, they must also include a social history of how the discovery claim became accepted by the scientific community” (Kragh and Smith, 2003: 144). Discoveries, then, get made in both the physical and the social worlds. To put matters bluntly for the case of the discovery of Neptune: Who were to be credited as the discoverers of the new planet, and why did the historical actors make the choices they did? The answers to these questions of credit were at times hotly debated in 1846 and 1847 and meant a complex social history. At the height of these wrangles in late 1846, Herschel was even moved to lament to one correspondent that “Already the word ‘discovery’ begins to break down under the weight of meaning laid upon it” (Herschel, 1846b). The disputes, arguments, negotiations and sometimes bad-tempered exchanges around the discovery were, fundamentally, the product of a lack of a generally agreed set of rules on credit that could be applied rapidly, compounded by disagreements over what should be counted as a publication (Csiszar, 2018: Chapter 4). The arguments about the date of, as well as who should receive the credit for, the discovery of Neptune were disputes about what had been discovered. Reading back into the events of 1846 our contemporary notions of what counts as a discovery and who should be regarded as the discoverer, then, is guaranteed to be misleading. The potential rewards for the discoverer(s) were also high. For men of science, the discovery of Neptune was widely regarded as astonishing. Delivering his 1847 Presidential Address to the British Association for the Advancement of Science, Sir Robert Inglis (1786–1855) proclaimed the year had been marked by a “discovery the most remarkable, perhaps, ever made as the result of pure intellect exercised before observation, and determining without observation the existence and force of a planet; which existence and which force were subsequently verified by observation” (Inglis, 1848: xxx). With these thoughts about the nature of discovery in mind, first let us focus on the search for the new planet that was conducted from the Cambridge University

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Observatory in 1846 and how it was put into place, and how this failed search helped shape the debates and squabbles around the allocation of credit for the discovery of Neptune.

7.3  The Cambridge Search: Clandestine or Pragmatic? Chapter 4 recounts how once Adams had been elected a Fellow of St. John’s College in 1843, he began to focus more on the analysis of Uranus’s motions. In February 1844, Adams had, via Challis, requested from Airy the observations of Uranus made from Greenwich between 1754 and 1830. By September 1845, Adams had, just before leaving Cambridge for a month, presented Challis with the elements defining the orbit for the postulated planet, as well as its predicted position for 30 September. When he returned to Cambridge, Adams learnt Challis had not undertaken a search as he had expected. Adams was deeply disappointed, but he took no other action. Airy of course also asked Challis on 29 September 1845 to tell Adams that “I am very much interested with the subject of his investigations, and that I should be delighted to hear of them by letter from him” (Airy, 1846a: 129). Airy wrote to Adams himself on 5 November 1845 and put to him his famous query about the radius vector: “…I should be very glad to know whether this assumed perturbation [caused by the unseen planet] will explain the error of the radius vector of Uranus. The error is now very considerable” (Airy, 1846a: 130). But after starting to draft a reply, Adams never completed the letter. Adams published nothing on his calculations of a new planet prior to the optical find, nor were there any public announcements of his analyses ahead of the optical discovery. Late in life, Adams told the historian of astronomy Agnes M. Clerke (1842–1907) that this was to keep the work unknown abroad so that “this country” could have “the full credit of the discovery.” Adams’s reminiscence was 40 years after the event; as he gave no supporting documentary evidence, it is difficult to know how much credence to give to this recollection. Moreover, contradicting this memory was Adams’s known intention at the time to deliver a short paper on his researches for Section A (Mathematics and Physical Sciences) at the British Association for the Advancement of Science meeting in Southampton in early September 1846—a paper that he arrived a day too late to deliver (Adams, 1846a, b). Some people, however, were aware of Adams’s researches. How did those people get to be in the know? Airy, we have seen, was helpful in providing information to Adams and had requested more detail from him in the November 1845 letter about the radius vector. But Airy was sceptical of the chances for success for such a search, a position for which there were reasonable scientific grounds. At the end of June 1846, however, Airy’s attitude was transformed when he read an abstract of the first paper in which Le Verrier published a position for a new planet in an issue of the Comptes Rendus, the weekly journal of the French Academy of Sciences. Airy had learned during a visit to Paris in September 1845 that Le Verrier was working on the problem of

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Uranus’s motions, and that he was doing so at the suggestion of François Arago,2 director of the Paris Observatory (Hutchins, 2008: 92) Le Verrier, as described in Chapter 5, was a former student of Arago’s and already an established mathematical astronomer on the observatory staff, but he had not yet produced a full set of elements to specify the planet’s orbit. In June 1846, Airy saw that Le Verrier’s published calculated position was very close to Adams’s. As Airy wrote to William Whewell (Airy, 1846b) and as he reported to a meeting of the Board of Visitors of the Royal Observatory on 29 June, Le Verrier had predicted the new planet’s most probable position in longitude on 1 January 1847 to be 325°. But Adams’s 1 October 1845 note to Airy had proposed a position of 323°34'. (As we will refer to the Board of Visitors meeting again, it is worth noting that the Board met at Greenwich on 6 June for its annual meeting, but there was a second, follow-up, meeting at the Admiralty on 29 June). Adams had reckoned that this position was unlikely to be in error by as much as 10°. For Airy, it was one thing to be sceptical about the results from a single set of calculations, but quite something else to be confronted by the results of two researchers—one a very competent mathematical astronomer and the other was perhaps known to him as a Cambridge Senior Wrangler—who had come to such similar conclusions. As Airy told the Royal Astronomical Society in November 1846: This memoir [of the Comptes Rendus] reached me about the 23rd or 24th of June. I cannot sufficiently express the feeling of delight and satisfaction which I received from it. The place which it assigned to the disturbing planet was the same, to one degree, as that given by Mr Adams’ calculations … But now I felt no doubt of the accuracy of both calculations, as applied to the perturbations in longitude (Airy, 1846a: 132).

Airy further noted he had also written on 26 June to Le Verrier to pose a question about the radius vector (Airy, 1846a: 132). Le Verrier replied promptly on 28 June and addressed Airy’s question. Le Verrier also explained that if Airy had enough confidence in his work to begin a search, then he (Le Verrier) would hasten to send the planet’s exact position as soon as he had completed further calculations (Airy, 1846a: 132). In his November 1846 talk to the Royal Astronomical Society, Airy reported that as he was soon to leave for the continent, that it had been “useless” for him to ask Le Verrier for the more accurate numbers (Airy, 1846a: 135). Airy, however, noted he had received Le Verrier’s letter on 1 July, well before his continental tour began on 10 August. After the discovery, Airy, as we will see later, explained to Le Verrier, “I do not know whether you are aware that collateral researches had been going on in England and that they led to precisely the same results as yours” (Airy, 1846c). No one was more likely than Airy to have told Le Verrier of Adams’s studies, and, given Airy’s invariably prompt response to letters, this remark beggars belief. Le Verrier was therefore unaware of Adams’s researches until October 1846, and so after the optical discovery (Smith, 1989: 404).

 For a full biography of Arago, see (Lequeux, 2016).

2

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As well as corresponding with Le Verrier at the end of June, Airy, as we have noted, also exchanged letters with his long-time and very close friend William Whewell, master of Trinity College, Cambridge. On 23 June 1846, Whewell, who was at the seaside in Lowestoft and working on a new edition of his History of the Inductive Sciences, sought Airy’s views on various subjects, including the motions of Uranus. Airy told him: The discordance [between the calculated and the observed positions] is increasing…. Peoples’ notions have long been turned to the effects of an external planet, and upon this there are two remarkable calculations. One is by Adams of St. John’s (which in manuscript reached me first). The other is by Le Verrier in the Comptes Rendus of 1 June 1846, which and a previous number I strongly recommend you to consult. Both have arrived at the same result, viz. that the present longitude of the said disturber must be somewhere near 325° (Airy, 1846b).

A few days later, on 29 June, Airy told the second meeting of the Board of Visitors of the Royal Observatory of the “extreme probability” of discovering a new planet in a very short time, provided “the powers of one observatory” were directed to a search (Airy, 1846a: 133). Challis, who attended the meeting, confirmed Airy’s account a few months later. John Herschel was another attendee and he was impressed enough with the information that in his Presidential Address to the British Association for the Advancement of Science in early September he, as we noted earlier, announced that the “postulated planet’s movements have been felt, trembling along the far-reaching line of our analysis, with a certainty hardly inferior to that of ocular demonstration” (Herschel, 1846a). Among the Board of Visitors were, in addition to Challis, two highly competent observational astronomers, William Henry Smyth (1788–1865) and John Wrottesley (Lord Wrottesley) (1798–1867). Smyth’s famous A Cycle of Celestial Objects was first published in 1844 and won him the Gold Medal of the Royal Astronomical Society in 1845. Lord Wrottesley possessed a 7¾ inch Dollond refractor, a telescope more powerful than anything at Greenwich and the telescope that would later be used by John Russell Hind in his planet search. Wrottesley also employed an assistant (with whom he easily observed the new planet after the discovery) and had won the RAS Gold Medal for his Catalogue of the Right Ascensions of 1,318 Stars. Wrottesley, like Smyth, did not search for the postulated planet before the telescopic discovery. But as Hutchins has argued, the astronomers who heard Airy’s words at the Board of Visitor’s meeting knew that the problem was the error-bars in the calculations. Each result was plus or minus some ten degrees, that is, up to twenty degrees across the sky, a huge area to cover, and the predicted mean position would change as the variables were altered in each computation. Then because the brightness of the planet was unknown the search must go deep in order not to miss a faint object. Those knowing of the double prediction believed a planet was there, but that a demanding search would be required. (Hutchins, 2008: 98).

Airy was nevertheless confident of the prospects for success. He did not, however, undertake a search at Greenwich. For Morton Grosser, author of an important book on the discovery that was regarded as the standard work for some decades, there were two reasons why there was no Greenwich search. The first was Airy’s

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Fig. 7.1  George Airy c 1850 (Credit: Science Photo Library)

“irrational dread” of  changing the Observatory’s routine. The second, and this Grosser noted Airy had announced publicly (Fig. 7.1), was his belief that, if the planet did exist, it would be visible only in large telescopes. The largest telescope at the Royal Observatory in 1846 was the Sheepshanks equatorial refractor, which had an aperture of only 6.7 inches. It was vastly exceeded in light-gathering power by the Cambridge Observatory’s 11¾ inch Northumberland telescope (… completed in 1839 under the supervision of George Airy) (Grosser, 1962: 107).

Airy indeed had a love of method and order, and his friend Augustus De Morgan “jocularly said that if Airy wiped his pen on a piece of blotting-paper he would duly endorse the blotting-paper with the date and particulars of its use, and file it away amongst his papers” (Maunder, 1900:116). For Allan Chapman, the reason Airy rejected a search from Greenwich was because it was a functional public institution, and that with “his finely tuned sense of duty, one might have hazarded the suggestion that he foresaw angry admirals and questions being asked in the House  [of Commons] about taxpayer’s money being spent on non-utilitarian projects, if such a ‘philosophical discovery’ was made in the Royal Observatory” (Chapman, 1988:128).

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But is this suggestion entirely plausible? Once Neptune had been discovered, government funds were spent on the analysis of its motions. Airy later secured taxpayers’ money for Peter Andreas Hansen, a German astronomer and director of a private observatory at Seeberg near Gotha, for his researches into the motions of Uranus and Neptune. Airy, in his presidential address of 1851 to the British Association for the Advancement of Science, would claim this grant was “equally honourable” to the British government and to Hansen. Although the Schleswig-­ Holstein war (which lasted from 1848 to 1851) had interrupted the payments and so delayed Hansen’s work, “I have reason to believe,” Airy added, “that the theories of Uranus and Neptune are now undergoing careful study” (Airy, 1852:xlii). Hansen’s studies of the motions of Uranus and Neptune were not essential for the sorts of navigational and so utilitarian ends that provided the main rationale for Greenwich’s activities. Neither Hansen’s post-discovery calculations or a planet search in the summer of 1846 met strictly utilitarian ends (Smith, 1989: 403). Hansen’s post-­ discovery calculations did, however, meet another of Airy’s overall goals for the astronomical enterprise, increasing precision in both computations and observations. We need to take account too of Airy’s comment (in his 25 June 1846 letter to Whewell from which we have already quoted) that if “I were a rich man or had an unemployed staff I would immediately take measures for the strict examination of that part of the heavens containing the position of the postulated planet.” The Sheepshanks refractor of 6.7 inches aperture was the largest telescope at the time at the Observatory and so Airy must have assumed it could do the job. This point, on the face of it, undermines Grosser’s argument that Airy assumed to begin with that a large telescope would be needed for the search. Airy later offered an assistant to Challis to aid him in his search so the remark about an “unemployed staff” too is significant (Smith, 1989: 404) (Fig. 7.2). In June 1846, then, Airy had read the abstract of Le Verrier’s June paper in the Comptes Rendus, written to Whewell about the two sets of remarkable calculations, made his very positive remarks to the Board of Visitors, and received Le Verrier’s answer to his query about the radius vector. He was clear there was the likelihood of a successful search, and now drew up plans for such a search. It was not, however, to be conducted from Greenwich; rather it was, Airy hoped, to be pursued from Cambridge. Airy first wrote to Challis about a planet search on 9 July and did so in urgent terms. By this date Airy had apparently changed his position about the size of telescope needed, or, more likely, he had decided he needed to offer strong encouragement to Challis if Challis was ever to begin a search. Airy told Challis: You know that I attach importance to the examination of that part of the heavens in which there is a possible shadow of reason for suspecting the existence of a planet exterior to Uranus. I have thought about the way of making such examination, but I am convinced that (for various reasons of declination, latitude of place, feebleness of light and regularity of superintendence) there is no prospect whatever of this being made with any chance of success, except with the Northumberland telescope. Now, I should be glad to ask you, in the first place, whether you could make such an examination?

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Fig. 7.2  The Observatory, Cambridge (Credit: Views of Cambridge, 1851) Presuming that your answer would be in the negative, I would ask, secondly, whether, supposing an assistant were supplied to you for this purpose, would you superintend the examination? You will readily perceive that all this is in a most unformed state at present, and that I am asking these questions almost at a venture, in the hope of rescuing the matter from a state which is, without the assistance that you and your instruments can give, almost desperate. Therefore I should be glad to have your answer, not only responding simply to my questions, but also entering into any other considerations which you think likely to bear on the matter. The time for the said examination is approaching near (Airy, 1846d).

Airy’s use of “almost desperate” in this letter is telling. He reckoned that the combination of the postulated planet’s “feebleness of light” and need for “regularity of superintendence” meant only an institution with a powerful telescope could mount an effective search, leading him to rule out private observatories and fixing on the Cambridge Observatory and the Northumberland telescope. Yet Airy believed that the Cambridge Observatory was “oppressed” with work—which, as we have seen in Chapter 4, was certainly correct—and this helps explain his offer of an assistant to Challis. Astronomer and historian David Dewhirst pointed out the very small size of the staff for the work that was done at the Observatory (Dewhirst, 1976) and

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Hutchins has expanded Dewhirst’s analysis (Hutchins, 2008). Challis was pursuing a program of meridian observations with mural and transit circles, was employing the recently completed Northumberland telescope—the largest refractor in England—and observing with a smaller equatorial telescope for extra-meridian work. For this labour he had only one senior and one junior assistant to aid him (Smith, 1989: 405). In addition, he had substantial University teaching duties as the Plumian Professor of Astronomy and Experimental Philosophy. For most of 1846 Challis was heavily occupied with the reduction of comet observations too. Challis had also been severely stretched for three months as he had only one assistant between 16 May when the previous First Assistant, Arthur George Berry (1823–1877), left and 14 August when the much less experienced James Breen (1826–1866) joined on trial. Breen therefore arrived, as we shall see, a couple of weeks after Challis began a search (Hutchins, 2008:123–4. Four days after Airy had first written to Challis about a planet search, on 13 July, Airy pressed Challis again even though he had not received a response to his first letter. Airy outlined for Challis the labour likely to be involved in a search and he also emphasized that “the importance of this inquiry exceeds that of any current work, which is of such a nature not to be totally lost by delay.” Airy knew the predicted planet would be at opposition within six weeks and so well positioned to be observed (Airy, 1846a:136) (Fig. 7.3). Airy’s proposed scheme accompanied his 13 July letter to Challis and was laid out in a four-page paper entitled “Suggestions for the Examination of a portion of the Heavens in search of the external planet which is presumed to exist and to produce disturbance in the orbit of Uranus.” “The investigations of Mr Adams and M. Le Verrier,” Airy began, “having made it probable that the place of the supposed planet is not far from 325° longitude, I would propose …” When Challis replied on 18 July, he did not refer to the Board of Visitors’ meeting at the Admiralty at the end of June. His response therefore reads as if he was giving his reaction to a new proposal from Airy rather than one that Airy might have already raised at that meeting. Challis explained: I have determined on sweeping for this hypothetical planet, first because the research requires a powerful telescope, and again I am desirous of employing the Northumberland Telescope in some way that will not require a great amount of calculation,…I feel much obliged by your suggestion as to the method of conducting the search. The table of distribution into zones of declination which you have sent I shall find very useful. I propose to carry the sweep to the extent you recommend. I would employ one of my assistants exclusively in reading off the Hour Circle microscope, the other in writing down, while I [observe] bisect, call the time, and give the approximate declination reading. … We have not the Berlin maps at this Observatory. I should be obliged by being informed how and where they may be obtained. I am not quite ready yet to take Mr James Breen on trial, but hope to do so in a short time (Challis, 1846a).

Airy’s 21 July reply to Challis also reads as if his proposal had not been presented before he laid it out in his 13 July letter to Challis:

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Fig. 7.3  The Northumberland Telescope (Credit: Royal Astronomical Society/Science Photo Library) I am very glad that you seriously think of looking for the possible planet. With an assistant to read the hour circle you will doubtless get on more rapidly. I believe the Berlin maps may be obtained through Nutt, 158 Fleet Street, or any German bookseller. There is only one which applies partially to this inquiry (Airy, 1846e).

“Airy is referring to the Hour 22 Map since he,” Hutchins has argued, “like all other astronomers, was unaware that the Hour 21 Map (which included the full search area) had been printed in Berlin but not yet distributed. Challis located the Hour 22 Map in the Cambridge University Library, but did not borrow it” (Hutchins, 2008:102). Dewhirst has emphasised that Airy had planned a search to fainter magnitudes than was later found to be necessary, and the search to such fainter magnitudes meant using the Northumberland telescope: Airy had designed that instrument for the physical study of individual objects, double stars, comets, etc., for which it was splendid. But its size, awkward observing chair, the difficult arrangements for reading off the Right Ascension and Declination, rotating the dome, etc., made it a singularly unsuitable instrument for the rapid mapping of a large area of sky (Dewhirst, 2006).

Despite these difficulties, Challis began the search on 29 July with the Northumberland telescope. When Airy wrote to Challis on 6 August just before

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leaving on his continental tour, he made the specific offer of a Greenwich computer, James Breen, to help in the search. Airy added that his Chief Assistant Robert Main “is fully in my confidence and understands the position of the whole matter” (Airy, 1846f). Before the investigations of Adams and Le Verrier, it was widely assumed that the planet, if it existed, would be faint. Adams had nevertheless assured Challis that it would not be fainter than a star of the ninth magnitude. Neither Airy nor Challis appear to have considered seriously looking for an object with a disk like appearance. Challis’s observing plan instead aimed to disclose the planet’s movements against the background stars. He would search for shifts of candidate objects amongst the background stars by mapping all the stars in a zone of the sky some 30° long in the direction of the ecliptic and 10° broad that was centered on the planet’s calculated position. He would chart all the stars in the zone down to the eleventh magnitude three times, and then compare their positions at the different times. A “star” that had changed position with respect to the background stars would, if not a comet or minor planet, reveal itself to be a planet (Challis, 1846c). As I have observed elsewhere, The star-shift method was a natural choice for Airy and Challis. Although the postulated planet’s great distance from the Sun implied that it would travel much more slowly through the background stars than a minor planet...Airy’s statement that “regularity of superintendence” was necessary for a successful search is also consistent with the choice of the starshift method, and the area of sky that needed to be inspected suggested that about three hundred hours of observing time would be required (Smith, 1989: 407).

When he spoke at the Royal Astronomical Society in November 1846, Challis recalled that the search had been guided by a paper that Adams had drawn-up for him. A single sheet now in the Cambridge Observatory Archives and known as the July ephemeris appears to be the paper by Adams noted publicly by Challis. The bottom half of the page is a transcription of parts of Le Verrier’s Comptes Rendus paper and gives the mean position of the postulated planet as 325° plus or minus 10°. Adams wrote “M. Verrier recommends exploring for the New Planet the Region of the Ecliptic from 321° to 335° of heliocentric longitude.” The upper half of the sheet contains one of Adams’s own calculations, with a result of “336°4 nearly” (Adams, 1846a). The paper therefore makes clear that by July 1846, Adams was aware that Le Verrier was hunting for a planet and had already published on the topic, and it seems that it was this awareness that, as Sheehan has emphasized (Hutchins, 2008:105), re-energised Adams’s own efforts. The paper, as Hutchins has underlined, [gave] a positional range beginning 20 July (the early date is Adams’s contribution) is clearly based on Le Verrier’s elements, based by him on a circular Bodean orbit and without inclination. It was meant to guide Challis’s telescopic search, but shows that Adams had nothing fresh to add to refine that search. This paper narrowed the search from Le Verrier’s twenty degrees to one of fourteen, but could not have given Challis any confidence that the perturbing planet would be quickly found, and thus accounts for the scepticism he later admitted to. (Hutchins, 2008:105).

Adams’s paper, then, pointed to the likelihood of a lengthy search.

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When Challis learned of the optical discovery he had already recorded the positions of more than 3,000 stars. When he examined these observations he was mortified to find that star Number 49 in the series of 12 August was not in the series made of the same zone of sky on the second night of the search, 30 July, and so must have moved into the zone after 30 July but before August 4. The supposed “star” was indeed Neptune. When he checked further, he found he had also observed the planet on another occasion, 4 August. Challis later admitted that he had “lost the opportunity of announcing the discovery by deferring the discussion of the observations, being much more occupied with reductions of comet observations, and little suspecting that the indications of theory were accurate enough to give a chance of discovery in so short a time” (Challis, 1846b:liii).

7.4  Other Observers? In his urgency, a situation he saw as “almost desperate,” it is a crucial question as to why did Airy not enlist British observers beyond Cambridge? Airy pressed Challis alone to begin a search, and he had insisted that “there is no prospect whatever of [the search] being made with any chance of success, except with the Northumberland telescope” (Airy, 1846d), and the reasons he gave were to do with a search telescope’s latitude, light grasp, and regular operation. As to latitude, the declination of the planet would change little wherever it was observed from in Britain or Ireland. There were also more powerful telescopes available in Britain and Ireland than the 11¾ inch Northumberland refractor, most notably, William Lassell’s newly erected 24-inch reflector in Liverpool (of which more later), and E.  J. Cooper’s 14-inch refractor at Markree Castle, both of which were mounted equatorially (Smith, 1989: 407). Cooper’s telescope was sited in Ireland and 1846 was a year of the great potato famine, but this did not bring observations to a halt. Neither of these telescopes, however, was located at a professional observatory so Airy may have assumed they could not be employed regularly, or at least not regularly enough for the task of completing 300 hours of detailed star mapping in a timely manner. Yet Airy suspected that might be the case for the Northumberland telescope and so he would offer an assistant to Challis. Lord Rosse’s famed 72-inch reflector, the Leviathan of Parsonstown, had also recently gone into operation, but it was essentially fixed in the meridian and so it was not capable of wide shifts around the sky. If hunting for an object with a disc had been considered seriously at this stage, then the Leviathan would in principle have been capable of being used to search for such an object by making sweeps (in the William Herschel manner) by letting different bands of the sky sweep by the telescope as it was pointed to the meridian. But hunting for an object with a disc was taken seriously only after Le Verrier’s suggestion to do so in his August 31 Comptes Rendus paper. Challis’s search was certainly not secret. William Whewell, who was well acquainted with the activities at the Cambridge Observatory, told J.D. Forbes in Edinburgh on 29 September (and so before Whewell was aware of events in Berlin),

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that “Challis is busy looking for a new planet beyond Uranus. Analysis says there must be one. If a new planet comes out of the equations of motion it will be a grand result” (Craik, 2007: 178). The English astronomer John Russell Hind was aware of both Challis’s search and Adams’s researches as Challis had mentioned both to him. Hind too was chasing the planet (Smith, 1989: 407). Hind had been educated privately and at Nottingham grammar school before becoming an assistant in London to a civil engineer. Shortly thereafter he took up a junior position at Greenwich as an assistant in the Magnetic and Meteorological Department of the Royal Observatory, and by 1846 he was the observer at George Bishop’s private observatory in Regent’s Park, London, after succeeding W.R. Dawes in the position in 1844 (Hutchins, 2004). Here Hind employed a 7-inch Dollond refractor in his search, a telescope more powerful than anything available at Greenwich. Hind was in touch with developments on the continent and on 16 September he told Challis that Hervé Faye, an assistant at the Paris Observatory, had written to him about Le Verrier’s planet. Faye explained a brief search for the planet had been conducted at the Paris Observatory, but had now ended (Smith, 1989: 407). Faye had also provided information that might aid in the search, most importantly, Faye reported that he had read Le Verrier’s Comptes Rendus paper of 31 August. Here Le Verrier proposed a search for a planetary disk rather than trying to detect the motion of one of the “stars” close to the planet’s calculated position (Le Verrier, 1846a). Hind, as he related to Challis, had also told Faye that he and Challis were trying to locate the planet (Hind, 1846a) (Fig. 7.4). Hind in turn told Challis that while he would check for suspicious motion in his own search, he would also put Le Verrier’s proposal into effect (Hind, 1846a). By this point, news of Le Verrier’s researches had been widely publicized in Britain, and not just through copies of the Comptes Rendus. A short account of Le Verrier’s 31 August paper had appeared under “Miscellanea” in the weekly journal The Athenaeum for 12 September (Anon, 1846a). However, a position was not given and the report contained no figures or any encouragement to search for a disk. Le Verrier’s June paper had earlier been the subject of reports in both The Athenaeum for 13 June and the Mechanics Magazine for 4 July, though both had been somewhat muddled (Anon, 1846b, c). However, the report in the Mechanics Magazine had been entitled “New Planet Expected.” Challis, despite the encouragement of Hind’s letter of 16 September, chose not look for a disk until he had read for himself Le Verrier’s paper in the 31 August Comptes Rendus, and a copy reached him only on 29 September. William Thomson (later Lord Kelvin) had graduated as Second Wrangler in 1845 and had been a college lecturer at Peterhouse in academic year 1845/6, but left Cambridge in 1846 for the chair of natural philosophy at Glasgow. Shortly before Thomson left Cambridge, Challis complained to Thomson that he had not been able to see either paper of Le Verrier’s in the Comptes Rendus “as the Phil. Society [Cambridge Philosophical Society] had not received the [numbers] containing them.” Writing to George Stokes on October 25, and so over a month after the optical discovery, Thomson lamented

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Fig. 7.4 John Russell Hind (Credit: Illustrated London News, 28 August 1852, from a Daguerreotype by Claudet)

that the events around the new planet “seem rather to indicate that Cambridge is behind the rest of the world in information on scientific subjects” (Thomson, 1846). On the first evening, 29 September, on which Challis searched for a disk he located a suspect that indeed proved to be Neptune (Challis, 1846c:147). The following day, a letter arrived from Hind with the stunning news that “Le Verrier’s planet is discovered,” and “Our searches are now needless” (Hind, 1846b). Challis had hunted for a ‘star’ with a disk six days too late.

7.5  Battles over Credit Hind wrote to the London Times to announce the find to a wide audience in Britain, and the letter was published on 1 October. Hind reported he had received a letter from Dr. Franz Friedrich Ernst Brünnow (1821–1891) at the Royal Observatory in Berlin announcing that “Le Verrier’s planet was found by Mr. Galle on the night of September 23rd” and that this “discovery may be justly considered one of the greatest triumphs of theoretical astronomy” (Hind, 1846c). Hind also wrote to Adams

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(Smith, 1989: 408). He must have still been unaware of the extent of Adams’s investigations for he explained that “Understanding from Prof. Challis that you are occupied about the planet of Le Verrier, I think that you will be gratified to learn that it was discovered by Dr. Galle at Berlin on Sept. 23. What a grand discovery this is, and how glorious a triumph for analysis” (Hind, 1846d). In November 1846, when the storms were blowing hard over allocating credit for the detection of the new planet, Hind wrote to Richard Sheepshanks, one of the leading members of the Royal Astronomical Society. Sheepshanks had attended Trinity College, Cambridge, and had graduated as 10th Wrangler in 1816. Sheepshanks knew Airy extremely well, and he had also attended the 29 June Board of Visitors meeting and so was aware of Adams’s researches before the optical discovery. In Hind’s view, Cambridge men were being “grossly unfair” in advancing the claims of Adams. Certainly “the Cambridge people … do the best for their own.” It “is very evident,” Hind complained to Sheepshanks, that Challis “did not place much dependence on his observations or he would have found the planet in August last. I am sure you must have noticed the inexcusable secrecy observed by all those acquainted with Mr. Adams’s results” (Hind, 1846e). Challis, in Hind’s opinion, deserved no credit at all. Hind, it is clear from his letters to the Times and Adams, was not aware of Adams’s results when he was searching for the planet. Hind was also a Council member of the Royal Astronomical Society and had even corresponded with Challis about their respective searches. It is not surprising he was resentful. In mid-1846, however, the 24-year-old Hind was regarded by Airy as someone who should be taken down “3 or 4 pegs at least” (Airy, 1846g), an opinion that may well have been shared by Airy’s circle (Smith, 1989: 409). Here it is important to emphasize that there was no single, unified British response to the optical discovery in Berlin and the subsequent debates and arguments over the allocation of the credit for the theoretical discovery. Historical accounts that stress a partisan ‘British versus the French’ fight miss the more complicated nature of the controversies that broke out in the autumn of 1846. Some figures pressed the claims of Adams and Cambridge, while others, like Hind, were critical of claims made on behalf of Adams, and a number from both camps took aim at what they saw as the tardiness of the actions of Airy and Challis. One of those eager to secure some credit for Adams was John Herschel. The day he heard of the planet’s discovery (1 October 1846) Herschel wrote to the The Athenaeum to describe how Adams’s calculations had independently led him to a position for the planet close to Le Verrier’s. The letter was published two days later. Herschel quoted from his own pronouncement to the British Association of a few weeks earlier of the certainty of the discovery. He then explained: The remarkable calculations of M. Le Verrier—which have pointed out, as now appears, nearly the true situation of the new planet, by resolving the inverse problem of the perturbations—if uncorroborated by repetition of the numerical calculations by another hand, or by independent investigation from another quarter, would hardly justify so strong an assurance as that conveyed by my expression above alluded to. But it was known to me, at that time, (I will take the liberty to cite the Astronomer Royal as my authority) that a similar investiga-

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tion had been independently entered into, and a conclusion as to the situation of the new planet very nearly coincident with M.  Le Verrier’s arrived at (in entire ignorance of his conclusions), by a young Cambridge mathematician, Mr. Adams….(Herschel, 1846c).

Also writing on 1 October about the optical discovery was Challis. He penned a letter to The Cambridge Chronicle. Beneath the headline of “Discovery of a New Planet Beyond Uranus” he argued that “the discovery of this planet is, perhaps, the greatest triumph of astronomical science that has ever been recorded.” Challis explained that about four months previously he had become aware that the independent calculations of Adams and Le Verrier both pointed to 325° as the most probable longitude of the supposed planet. Further, he had started a search for the planet two months earlier but had been “anticipated in the discovery of the planet” (Challis, 1846d). A third person in England to write quickly about the new planet was James Glaisher, senior assistant at Greenwich. Writing to the Illustrated London News on October 1, he noted Le Verrier’s calculations and the circumstances of the discovery from Berlin. Two days later, he wrote again, echoing Challis that the “discovery is perhaps the greatest triumph of Astronomical Science that has ever been recorded.” This time Glaisher also mentioned Adams, and his letter, written while Airy was still unavailable because he was abroad until 11 October, closely matched Challis’s to the Cambridge Chronicle. Glaisher contended that the “remarkable calculations of Le Verrier and Mr. Adams, pointed out a position of the New Planet which has proved very nearly to be the true one” (Glaisher, 1846). Glaisher’s letter therefore meant that the Royal Observatory had put its authority behind the claim that Le Verrier and Adams’s positions were in very close agreement, “a position publicly taken that would now have to be sustained” (Hutchins, 2008:108). Challis also wrote to Arago a few days later. To Arago, Challis not only applauded Le Verrier’s achievement, but also noted that he had strictly followed Le Verrier’s advice. A reader of this letter could well get the impression that Challis had hunted for a planet employing Le Verrier’s position but had not so informed Le Verrier. Challis’s private letter became public when Arago published it in the 12 October Comptes Rendus (Challis, 1846b). In early October, Challis, as we have seen, was chagrined to realise that in August he had twice observed Neptune, but had missed out on making the optical discovery himself because he had failed to compare his sets of observations in a timely fashion. Now Challis provided these two positions of the planet to Adams who “immediately and deftly used these two early observations in combination with Galle’s discovery observation to calculate the new planet’s correct distance as 30.05 Astronomical Units (compared to Le Verrier’s minimum 35 AU) and also the correct position of its node and orbital inclination” (Hutchins, 2008:110). Challis decided to write again to The Athenaeum to give details of his own search. His letter was published on 17 October: In September,1845, Mr Adams communicated to me values which he had obtained for the heliocentric longitude, eccentricity of orbit, longitude of perihelion, and mass, of an assumed exterior planet … The same results, somewhat corrected, he communicated, in October, to the Astronomer Royal. M. Le Verrier, in an investigation which was published

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in June 1846, assigned very nearly the same heliocentric longitude for the probable position of the planet as Mr Adams had arrived at, but gave no results respecting its mass and the form of its orbit … I undertook to make the search, and commenced observing on 29 July. The observations were directed, in the first instance, to the part of the heavens which theory had pointed out as the most probable place of the planet, in selecting which I was guided by a paper drawn up for me by Mr Adams.

Challis pointed out that he did not have access to Hour XXI of the Berlin star maps so he had relied on comparing observations made on different nights. He had nevertheless observed the planet twice, on 4 and 12 August. But he reported he did not compare his observations on different nights due to his other work and his desire to obtain more observations: Challis was here being misleading because by not comparing observations at this point he was following Airy’s detailed plan, as he well knew. The planet, however, was secured, and two positions of it recorded six weeks earlier here than in any other observatory,  - and in a systematic search expressly undertaken for the purpose. … The part taken by Mr Adams in the theoretical search after this planet will, perhaps, be considered to justify the suggesting of a name. With his consent, I mention Oceanus as one which may receive the votes of astronomers (Challis, 1846e).

Challis again made clear that the Astronomer Royal had been closely involved in the planet search, and Challis had also enlarged the Cambridge claim by including Adams’s most recent and more accurate and complete elements for the new planet, adding his own claim of first sightings which had thus “secured” the planet. For Challis, this meant there was a Cambridge right to participate in its naming. With this letter Challis was, it seems, making a claim to co-discovery, perhaps even discovery itself, as I will discuss further later. When the telescopic discovery was made from Berlin, Airy was still away from Greenwich on his continental tour. On 5 October, he was in Altona to visit Professor Heinrich Schumacher, editor of the Astronomische Nachrichten, and who was a major figure in the events described in the previous chapter. While in Altona, Airy became the first Englishman to read a manuscript of Le Verrier’s that contained the mathematical calculations that led to his predicted position for the new planet (Hutchins, 2008: 108). The manuscript was published shortly after in the Astronomische Nachrichten. A few days after reading Le Verrier’s manuscript Airy was back at Greenwich after a stormy sea crossing from Hamburg, and where he faced a storm of a different kind. He wrote to Challis: Heartily do I wish you had picked up the planet. I mean in the eyes of the public – because in my eyes you have done so. But these misses are sometimes nearly unavoidable. … Before leaving Germany, I had made up my mind that it would be proper for me to write and to present to the RAS [Royal Astronomical Society] a history of the English transactions relating to the planet. 1st Because it is very illustrative of the History of Science. 2nd Because it would do justice to England. 3rd Because it would do justice to individuals. I ought to do it because 4th I know most of the history and have had no concern in the operations theoretical or observing (Airy, 1846h).

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The subsequent “Account of Some Circumstances Historically Connected with the Discovery of the Planet Exterior to Uranus” (Airy, 1846a) that Airy delivered to the Royal Astronomical Society on 13 November, and the publications that sprang from it, have proved to be arguably the most important contemporary works on the discovery of Neptune. With his “Account,” Airy intended to be no mere chronicler and recorder of events. Rather, he sought to seize control of the narrative around the discovery and manage its reception. The “Account” permitted him to defend his own actions in the planet hunt by arguing that he had done his duty and had acted in an entirely reasonable and prompt fashion throughout, pursuing the matter as far as he could do in his position. Here it is worth pointing to an important observation by Chapman on Airy’s “subtle sense of position.” Chapman has emphasized that Airy, as the son of a self-­ made Excise Officer “of considerable rank who was suddenly dismissed from the Service for irregularity at the age of 63—when George was 12—the Astronomer Royal was to display a lifelong concern with reminding his official superiors of his diligence, efficiency and honesty” (Chapman, 1998:15). Airy’s “Account,” then, would be his opportunity to make his case in a public forum that he had done his duty in the matter of the new planet. Airy took the preparation of the “Account” very seriously and he wrote to Adams on the same day he wrote to Challis, 14 October, to set out his plan to deliver a public account of the events to do with the new planet. As Airy put it, the “matter being one of delicacy,” and himself as “in no way concerned,” his role was to be the guardian of the historical record. He sought permission via his 14 October letters to Challis and Adams to quote from their correspondence. Both agreed. After receiving Airy’s letter, Adams told his parents that Airy was going to deliver a description of events so that “we shall not lose all the credit due to the prediction and discovery” (Adams, 1846d). 14 October had been a busy day of correspondence for the Astronomer Royal as he had written to Le Verrier too. While Airy praised the completeness of Le Verrier’s work, he also, for the first time, mentioned Adams’s researches to Le Verrier: I do not know whether you are aware that collateral researches had been going on in England, and that they had led to precisely the same result as yours. I think it probable that I shall be called upon to give an account of these. If in this I shall give praise to others I beg that you will not consider it as at all interfering with my acknowledgement of your claims. You are to be recognized, beyond doubt, as the real predictor of the planet’s place. I may add that the English investigations, as I believe, were not quite so extensive as yours. They were known to me earlier than yours (Airy, 1846c).

Le Verrier fired off a lengthy and angry reply to Airy: Why would Mr Adams have kept silent for four months? Why would he not have spoken from [since] the month of June if he had had good reason to give? Why wait until the object has been seen in the telescope? … Is it enough to have undertaken researches on a subject in order to claim to share the result? In that case Mr Adams and I will find many competitors in France who will take precedence by much in date (Le Verrier, 1846b)

Airy was now in an awkward position. Not only was Le Verrier irate, but Airy had also read Challis’s letter in the 17 October The Athenaeum, a letter which could only

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fuel Le Verrier’s anger when he got to read it. Le Verrier would surely see Challis’s letter and read Airy’s own reply to Le Verrier’s 16 October letter at about the same time. Airy therefore quickly wrote again to Le Verrier to press the point that “I am confident that you will find that no real injustice is done to you….” Adams, Airy noted, had given him the elements of a supposed planet in October or November 1845. Airy had therefore been delighted to read in the June 1846 Comptes Rendus that Le Verrier’s “elements were nearly the same and the present apparent place of the disturbing planet nearly the same as those given by Mr Adams’ investigations.” Airy’s letter to Le Verrier, then, “put his personal authority behind the Cambridge claim to prior prediction, which inevitably came to be translated by the British public as one of co-discovery” (Hutchins, 2008:111). Airy wrote once more to Le Verrier just a few days later to offer him fulsome praise as a philosopher, not just as a mathematician (Airy’s description of Le Verrier as a “philosopher” would, as we will see later, be an important part of his November Account to the Royal Astronomical Society.). Airy also stressed to Le Verrier that “Of one thing, however, I am sure you may make yourself quite certain, – that no person in England will dispute the completeness of your investigations. … With these things we have nothing which we can put in competition” (Airy, 1846i). When Airy’s attempts at diplomacy in his correspondence with Le Verrier were made public, they stirred national feelings among some British critics who reckoned Airy’s praise of Le Verrier had been overblown. But before then, Challis’s letter to The Athenaeum indeed proved to be a time bomb, primed by Airy’s exchanges with Le Verrier, that duly exploded. The site of the blast was a meeting on October 19 of the French Academy of Sciences. Arago was the key speaker. Arago was a commanding figure in the French Academy of Sciences, and the Comptes Rendus, which had been founded in 1836, was seen by Arago’s critics as a tool for him “to wield personal power over French savants” (Csiszar, 2018:170). He had an intense interest in issues of discovery and priority and wrote extensively on these topics. Arago brought a culture of print publicity to the Academy in the 1830s, and in the early 1840s he systematically developed a doctrine of priority. He had also written extensively on the history of science, including an Éloge of James Watt in 1834 for the Académie des Sciences that strongly supported Watt’s claims to priority in the famous dispute over the composition of water. Here Arago set out “in very stark and challenging terms the case for Watt’s priority and accusing [Henry] Cavendish and his friends of underhand dealings” (Miller, 2004:105). Cavendish (1731–1810) had been an undergraduate at Cambridge, but he never held a fellowship or professorship at the University. Nevertheless, he became something on an intellectual hero to Cambridge natural philosophers and mathematicians as a model natural philosopher, and Airy was one of Cavendish’s strongest supporters in the water controversy (Miller, 2004:139). Arago was therefore no stranger to Cambridge professors in addition to being especially well equipped to wage a priority battle. He was an alarming opponent to face in such a contest. The case around Le Verrier’s planet led Arago to take up arms, and he rode into the fight arguing that the crucial issue in priority disputes was publication. He tore into Challis.

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There had been no public mention of Adams’s researches before the discovery, and for Arago, this “circumstance alone suffices to end the debate. There exists only one rational and just way to write the history of science: it is to rely exclusively upon publications bearing a definite date; abandoning that, all is confusion and obscurity.” Further, Challis “so exaggerates the merit of Mr. Adams’s clandestine work, that he assigns to the young Cambridge geometer the right to name the new heavenly body. This claim will not be accepted….Mr. Adams has no right to figure in the history of the discovery of the planet Le Verrier, neither by a detailed citation, nor by the slightest allusion” (Arago, 1846:754). The bad feeling spilled over into the newspapers who saw evidence of a plot to steal Le Verrier’s discovery, and as James Lequeux notes in Chapter 5, the “cartoonists were let loose on an orgy of nationalism and proceeded to attack Adams and defend Le Verrier” (Fig. 7.5). Le Verrier, who had made clear to Airy that he was very unhappy with John Herschel’s letter of 3 October to The Athenaeum, attacked Herschel publicly too. In a letter to The Guardian that was published on 21 October, Le Verrier took strong issue with Herschel. Le Verrier also complained to Airy of Challis’s “two-faced correspondence.” In the face of these attacks, Challis swiftly backed down, and in a letter to The Guardian on 4 November, Challis wrote “I had no intention of putting a claim to discovery, either for Mr Adams or myself. The facts I stated were, as I thought, sufficient to show that no such claim should be made” (Challis, 1846g).

Fig. 7.5  Adams, in a dunce cap, reads Le Verrier’s report about the new planet (Credit: I’llustration, 7 November 1846)

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If events had followed a simple nationalistic and partisan line, we would have expected British opinion to have formed up solidly behind Adams by this point. Many of the Fellows of the Royal Astronomical Society, in fact, had sided, at least initially, with Le Verrier against Adams, in part because of the seeming murkiness surrounding Adams’s investigations (Smith, 1989: 409). In December 1846, for example, the well-placed Sheepshanks lamented to John Herschel that many Society members had taken an anti-Adams line before they had heard both sides of the story. Other were dismayed at the apparently exaggerated claims published in the Cambridge newspapers on Adams’s behalf. The result was “an internal feeling against any Cambridge discovery. Be this as it may, the whole evidence as to Le Verrier was out, understood and believed, before anything was known of Adams” (Sheepshanks, 1846). J.D. Forbes, the Professor of Natural Philosophy at the University of Edinburgh, told William Henry Smyth, one of the Greenwich Board of Visitors who was a prominent figure in astronomical circles and at this time the President of the Royal Astronomical Society, in December 1846, that: it is established that in consequence of Leverrier’s papers alone the planet was discovered in 1846. Everyone must see that there was not a chance of the Planet being found in England in 1846 from Adams’s predictions, even fortified by the announcement of Leverrier’s result at Greenwich on the 29th June….It was the confidence of his [Le Verrier’s] predictions which staggered even the incredulous. (Forbes, 1846).

A few months later, Sheepshanks told Schumacher that: as to national feeling (which, by the way, is too often national injustice) there is absolutely none whatever, so far as I know, among astronomers. In England at present the current runs the other way, and though I much prefer this failing of the two, yet it is provoking too. I assure you that it was with difficulty that one could get a hearing, while pointing out the fact that Mr. Adams had deduced the elements and place of the planet in October 1845…. Thanks to Struve and Biot, etc. our anti-Adamites are calmer, and as there never was any opposition to Le Verrier, we are quite satisfied at present. (Sheepshanks, 1847a).

The explosions in Paris and the angry criticisms of those who thought the Cambridge search had been started too late and been executed too slowly accentuated the importance of Airy’s forthcoming presentation to the Royal Astronomical Society in November 1846. But in preparing his talk, Airy faced a serious matter, as Hutchins has emphasized. After he had returned to Greenwich from his continental tour on 11 October, Airy found a letter waiting for him from Adams dated 2 September (and so three weeks before the optical discovery), which, given its importance, is worth quoting from at length. In his September letter Adams pointed out that (Fig. 7.6): In the investigation, the results of which I communicated to you last October, the mean distance of the supposed disturbing planet is assumed to be twice that of Uranus. Some assumption is necessary in the first instance, and Bode’s law renders it probable that the above distance is not very remote from the truth: but the investigation could scarcely be considered satisfactory while based on anything arbitrary; and I therefore determined to repeat the calculation, making a different hypothesis as to the mean distance. The eccentricity also resulting from my former calculations was far too large to be probable; and I found

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Fig. 7.6  John Couch Adams in 1851, by Samuel Cousins, after Thomas Mogord (Credit: Royal Institution of Great Britain/ Science Photo Library)

that, although the agreement between theory and observation continued very satisfactory down to 1840, the difference in subsequent years was becoming very sensible, and I hoped that these errors, as well as the eccentricity, might be diminished by taking a different mean distance. Not to make too violent a change, I assumed this distance to be less than the former value by 1/30th part of the whole. The result is very satisfactory, and appears to shew that, by still further diminishing the distance, the agreement between the theory and the earlier observations may be rendered complete, and the eccentricity reduced at the same time to a very small quantity. The mass and the elements of the supposed planet, which result from the two hypotheses, are as follows: Hypothesis I. Hypothesis II. a/a' = 0.5 a/a' = 0.515 Mean longitude of Planet, 1st Oct. 1846 325°8' 323°2' … By comparing these errors, it may be inferred that the agreement of theory and observation would be rendered very close by assuming a/a' = 0.57, and the corresponding mean longitude on 1st October, 1846, would be about 315° 20' which I am inclined to think is not far from the truth (Adams, 1846a).

In this letter Adams also asked Airy for more observations of Uranus at opposition in 1844 and 1845 so he could begin his new calculations. He further explained that he was considering delivering a “brief account” of his researches to the meeting of the British Association for the Advancement  of Science that would begin very

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shortly. This, of course, he did not do. If Adams had spoken, however, there would have been a pre-discovery announcement of sorts of his investigations, but it would have been highly problematical. Adams’s preferred position at this time, as expressed in his letter of 2 September, for the mean longitude of the postulated planet was around 315°20', and so far away from the actual planet’s discovery position, about 12°, that it could well have undermined any claims to credit made on Adams’s behalf. But the 2 September 1846 letter to Airy is significant too because Adams laid out for the first time how he had tackled his calculations. He also made clear he had little confidence in his October 1845 result, yet it was the October 1845 result that had enabled a Cambridge claim to co-prediction. “The letter,” Hutchins argues, explained how Hypothesis II, using a radius vector reduced from the larger Bodean one he had used in 1845, led him to the predicted mean longitude of 323°2' for 1 October 1846. Craig B. Waff makes the point that in the letter Adams then discussed at some length the deviations (“errors”) for each of the two hypotheses between theoretical prediction and actual observation of Uranus’s positions for various dates up to 1840. This was the only test he could make for his calculations prior to discovery of an actual planet. It is on the basis of those deviations or errors that he then stated the speculative extrapolation (not based on a third full calculation), that if he reduced the assumed mean distance further, the predicted mean longitude might be around 315°20'. Adams quickly acknowledged to Airy in his letter of 15 October that he had “rather hastily concluded that the change in the mean longitude deduced would be nearly proportional to the change in the assumed mean distance.” If Adams had had time to make the calculation he would swiftly have determined how wrong the speculation was, and saved himself from uncharacteristic error. (Hutchins, 2008:107).

Adams, therefore, soon realised he had blundered, as he told Airy when he replied to Airy’s letter of 14 October. But Airy had to decide what to say in his historical account to the Royal Astronomical Society in November about Adams’s 2 September letter. In the end, Airy included the entire letter, tables and all, and identified it as “very important.” Airy, however, did not call attention to the difference between Adams’s preferred position in that letter and the discovery position, and did not challenge the story, made public at the beginning of October 1846, that Adams’s calculations led to a result about 1° from Le Verrier’s. By simply presenting Adams’s 2 September letter and offering no discussion of it, Airy had made it, as Hutchins puts it, “largely inscrutable.” So, according to Hutchins, “Airy had done all that he reasonably could [for Adams] in a difficult situation. The origin of the supposed ‘one degree’ myth, created by Challis and Glaisher, cannot be laid at Airy’s door; but he did not correct it” (Hutchins, 2008:113).

7.6  Managing the Discovery: Airy’s Matter of “Delicacy” Airy delivered his “Account” to the Royal Astronomical Society on 13 November and the level of interest provoked by the discovery of the planet meant that he spoke to an “unusually numerous attendance” (Anon, 1846c). It was, Airy stated, a “documentary history.” He was somewhat selective, however, about what he said and how documents were presented—including deleting a phrase that would have been highly embarrassing for him.

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In his November “Account,” Airy heaped praise on Le Verrier. Indeed, he, in effect, put Le Verrier in a different class of investigator to Adams. In discussing Le Verrier’s 31 August 1846 paper, Airy wrote that: I cannot attempt to convey to you the impression which was made on me by the author’s undoubting confidence in the general truth of his theory, by the calmness and clearness with which he limited the field of observation, and by the firmness with which he proclaimed to observing astronomers, “Look in the place which I have indicated, and you will see the planet well.” Since Copernicus declared that, when means should be discovered for improving the vision, it would be found that Venus had phases like the Moon, nothing (in my opinion) so bold, and so justifiably bold, has been uttered in astronomical prediction. It is here, if I mistake not, that we see a character far superior to that of the able, or enterprising, or industrious mathematician; it is here that we see the philosopher. The mathematical investigations will doubtless be published in detail; and they will, as mathematical studies, be highly instructive: but no details published after the planet’s discovery can ever have for me the charm which I have found in this abstract which preceded the discovery (Airy, 1846a:142).

The “philosopher” Le Verrier had shown “a character far superior to that of the able, or enterprising, or industrious mathematician,” who, although Airy does not state it explicitly, was presumably to be identified by his audience as Adams. The term “philosopher” carried a particular meaning at this period and was often interchangeable with “man of science.” William Whewell, for example, in first coining the term “scientist” in 1833, had ranked a “scientist” below a “philosopher” (Secord, 2018). Airy, a former Plumian Professor of Astronomy and Experimental Philosophy, clearly saw himself as a philosopher. After Airy had presented his “Account” orally at the Royal Astronomical Society on 13 November, he was followed by Challis and Adams. Both had also addressed the Cambridge Philosophical Society a few days before, on 10 November. Two days after that meeting, on 12 November, Fenton John Anthony Hort (1828–1892), then newly arrived as an undergraduate at St. John’s College and who would go on to a distinguished career as a Professor of Divinity, told his father he had heard both Adams and Challis speak. There was, Hort reported, “some discussion as to the respective honours of Adams and Le Verrier. Adams gave Le Verrier the full credit of the discovery, “but, as a matter of calculation, he claimed for himself the credit of prior independent conjecture. Challis said the same, and merely claimed credit for himself on the score of having laboured most, having taken between 3,000 and 4,000 observations between the end of July and September” (Hort, 1846). Neither Challis or Adams, then, claimed co-discovery, and Adams would again claim only prior independent conjecture a few days later when he addressed the Royal Astronomical Society. Although Adams discussed his approach in the address to the RAS, he published a detailed account of his methodology and calculations only at the end of the year. Another to witness the meeting of the Cambridge Philosophical Society was George Gabriel Stokes, a Fellow of Pembroke College who in 1849 was to become the Lucasian Professor of Mathematics at Cambridge. The day after the meeting, Stokes told William Thomson that

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Prof Challis gave an account of the methods which he employed in hunting for it [the postulated planet], as well as of the dates and nature of the communications received from Adams and Le Verrier. It is most unfortunate that Adams’s results were not published at the time. He gave his result to Airy as long ago as September 1845. As it is, Le Verrier is the name which must be associated with the planet as to its theoretical discovery. Nevertheless, scientific men, who look into the history of the scientific discovery, will perceive from the evidence that I have no doubt Airy will bring forward that the planet was discovered from pure theory independently by two persons, and in fact that in point of time Adams had the priority (Stokes, 1846).

There are some significant differences between Airy’s private and public responses to the prospect of the discovery and the actual discovery itself. On 14 October 1846, for instance, Airy told Challis: “Heartily do I wish that you had picked up the planet, I mean in the eyes of the public, because in my eyes you have done so. But these misses are sometimes nearly unavoidable” (Airy, 1846h). None of these three sentiments appeared in the “Account,” and if they had, “they would surely have led to accusations that he was trying to turn failure into success” (Smith, 1989: 410). Airy’s letter of 9 July 1846 to Challis is noteworthy in this respect too (Smith, 1989: 410). “You know,” Airy had written, “that I attach importance to the examination of that part of the heavens in which there is a possible shadow of reason for suspecting the existence of a planet exterior to Uranus.” But in Airy’s public version of this letter the words “a possible shadow of” was replaced by four stars (Airy, 1846a: 135). The omission meant he had deleted a phrase that implied he had serious doubts the planet search would be successful, a dangerous point to emerge in the feverish state of the Neptune controversies in November 1846. The criticism levelled at Airy over Neptune, we should keep in mind, came on top of earlier controversy: in the summer of 1846 Airy’s directing of the Royal Observatory had, much to his anger, also been vigorously attacked. Sir Robert Inglis, who in 1847 was both President of the British Association for the Advancement of Science and the Tory M.P. for Oxford University, was prompted by the criticisms to use an Order of the House of Commons for the printing in 1847 of a paper by Sir James South that Airy believed was intended to undermine “almost everything that I did in the Observatory” (Airy, 1896:179). Airy’s “Account” is therefore a crucial source, but it must be read in the context, in part, of Airy constructing his own history for his own purposes. In this enterprise, he indeed succeeded, doing much to frame the structure of the narrative of the writings on the discovery of Neptune for over 140 years. If we regard Airy’s “Account” as a crucial but limited source, how are we to interpret his actions? I will argue that while there were solid astronomical reasons for Airy to push hard for a search from Cambridge, there were significant social reasons too. In the summer of 1846, Airy, I suggest, operated on the unspoken assumption that the Cambridge Observatory was not just the best place to pursue the search for important astronomical reasons (a powerful telescope and constant superintendence so regular observations could be made over an extended period), but also was the most appropriate place for the optical discovery to be made (Smith, 1989: 411). If Challis had discovered the planet, then Adams’s theoretical discovery and

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Challis’s optical discovery would have delivered two astonishing successes for Cambridge. In his time as director (1828–1835), Airy had put the Cambridge University Observatory on the map. Challis had pursued the search with the Northumberland telescope which Airy had designed, and Airy had also overseen the telescope’s erection. As we have already discussed, for more than 150  years, Cambridge-trained mathematicians had provided the major part of the senior, as well as the junior, Greenwich staff. The salary structure too meant that junior staff moved freely between the two observatories. Airy reckoned that Cambridge men made the best observers as well. So close were the ties between the two institutions that Dewhirst termed them the Cambridge-Greenwich axis. Airy and Challis knew each other extremely well too. Airy “was essentially a Cambridge man. He had come up poor and friendless: he had gained friends and fame at the University” (Airy, 1896:117). All these factors, then, underline the idea that for Airy, Cambridge was the natural and obvious choice for the search. Hind, as we noted, told Sheepshanks he interpreted Airy’s actions as a defence of Cambridge interests. Airy’s address to the Royal Astronomical Society in November also suggested such an interpretation to others. Walter White (1811–1893), who would become assistant secretary of the Royal Society, confided to his journal that it “appears from the delivery of Mr. Airy’s address that he is somewhat to blame for the loss of the complete realisation of the discovery of the new planet by Adams of Cambridge, and there appears to have been an attempt to make a Cambridge snuggery affair of it, for Challis and the Northumberland equatorial” (White, 1846:72). A puzzle we have already noted in the Neptune episode is why did Airy not reply to Le Verrier’s letter of 28 June 1846 on the topic of the radius vector. There was no need for further exchanges between Airy and Le Verrier, however, if Airy acted on the basis of what can be  called the ‘Cambridge interpretation.’ Airy had already seen the close agreement of Adams’s and Le Verrier’s results, and so Le Verrier’s letter of 28 June meant Airy soon decided that his task now was to convince Challis to begin searching (Smith, 1989: 412). Here it is important to note that there was a very revealing correspondence between Airy and Adam Sedgwick in late 1846 that probably takes us as close to Airy’s private views on various issues around the discovery as we are likely to get. Sedgwick was Woodwardian Professor of Geology at Cambridge, a Fellow of Trinity College, Cambridge, and one of the leading figures in British science in the 1840s. He also knew Airy extremely well, and the two were close friends. On 8 December 1846, a fuming Airy told Sedgwick that: In regard to the publishing of Adams’ priority: I refer you to what I said in my last letter, and only add that if you were not so serious about it, I should think it very ludicrous. I will put your propositions into the form, in which there is not the most trifling exaggeration. Sedgwick’s Theory, & Rules thereon founded 1 . Every Cambridge man is a Baby, and cannot walk out except he has a Nurse to trot him out. 2. Only extra-Cantabs are eligible as Nurses. No Resident, not even a Plumian Professor, is competent to this office.

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3. Simple nomination of an extra-Cantab by a baby imposes on such extra-Cantab., nolente volente [willy-nilly], all the duties and responsibilities of Nurse. 4. The regular duty of Nurse is, to divine the unexpressed wishes of the Baby to walk, and then to take him out. 5. The responsibility of the Nurse is not removed even though the baby take a fit of the pique and refuse to answer questions, or though the baby refuse to clothe himself in what the Nurse considers to be a proper walking dress. I do not enter into any details about Adams’ notion that the examination of the effect of the radius vector was unimportant. It now suffices for my guidance that I thought it important and still think so. Perhaps it might be sufficient for your persuasion, to tell you that Leverrier also thought it important. And here finishes my Cambridge discussion. The next blow will probably be from Paris ….” (Airy, 1846j).

The next day, Sedgewick, who had no doubts about Adams’s mathematical abilities, wrote to agree that Adams’s actions had indeed been “like a very simpleton” (Sedgwick, 1846). A decade later, Adams conceded to J.D. Forbes that “I was not so young in years at the time of making the discovery, as in knowledge of the world” (Adams, 1855). In his letter to The Athenaeum of 15 October 1846, Challis had reported that in “September, 1845, Mr Adams communicated to me values which he had obtained for the heliocentric longitude, eccentricity of orbit, longitude of perihelion, and mass, of an assumed exterior planet … The same results, somewhat corrected, he communicated, in October, to the Astronomer Royal” (Challis, 1846f). He had not discussed why there was such a long gap between receiving these results and the start of his search. But in December 1846, Challis, in a letter to Airy, addressed the question of why Adams had not responded to Airy’s query about the radius vector: I know that he is extremely tardy about writing and that he pleaded guilty to this fault. He experiences also a difficulty, which all young writers feel more or less, in putting into shape and order what he has done, and well done, so as to convey an adequate idea of it to others by writing. … I believe that nothing but procrastination in fulfilling this intention was the reason of his not sending an answer at all. I have always found him more ready to communicate than by writing. It will hardly be believed that before I began my observations I had seen nothing of this in writing respecting the New Planet, except the elements which he gave me in September written on a small piece of paper without date.

Challis explained he could not make this information public. As he put it, the “public would hardly take such a reason as that I have mentioned to be the true reason for his [Adams] not answering your questions, and I fear therefore that a hiatus must remain in history” (Challis, 1846h). In the febrile atmosphere of late 1846, then, Challis, like Airy, felt compelled to keep some views private.

7.7  M  anaging the Discovery: John Herschel’s Despair and Realism Some decades ago, S. F. Cannon identified what she described as a loose convergence of gentlemen of science in what she regarded as the “Cambridge Network,” an early nineteenth-century group whose members shared many attitudes and

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values and who were often Cambridge trained (Cannon, 1978:Chapter 2). As David Miller has written: During the early nineteenth century there came together at Cambridge a remarkable group of mathematicians and natural philosophers. Among the galaxy of “stars” were Charles Babbage, John Herschel, George Peacock, William Whewell, Adam Sedgwick, and George Airy, all of whom were to exert tremendous influence upon the development of British science for almost half a century. The power of the Cambridge network from the 1830s was to depend upon the mutually reinforcing factors of their institutional base, their substantive scientific achievements and the extent to which their claims to define the nature and purposes of science were accepted (Miller, 1983:13).

Cannon was criticised, however, for putting too much emphasis on reading John Herschel’s view of science as the view of science of members of the Cambridge Network more broadly. Miller has also cautioned about the limits of the notion of a Cambridge Network because there were various splits among the putative members as time went on. Indeed, there were other political formations and groupings among the elite of British science that also became prominent, and that, in Miller’s view, “transcended the Cambridge group” (Miller, 2004:140). All this being said, the notion of a Cambridge grouping is nevertheless still useful for the case of Neptune—provided we do not interpret it in terms of a rigid association or a group of people acting on the basis of commonly agreed positions or decisions—because earlier intellectual and social linkages among Cambridge men did play a part in the Neptune story. Airy, Babbage, Herschel, Peacock, and Whewell, in Cannon’s opinion, formed one node of the Cambridge Network (Smith, 1989:413). Of these five men, only Whewell was not at the 29 June Board of Visitors meeting (Board of Visitors, 1846), but Airy had already written to him of Adams’s researches four days earlier. In early July 1846, Airy visited George Thomas Peacock (1791–1858) (by then Dean of Ely). Peacock had been Second Wrangler in 1812 (Herschel was First Wrangler that year) and he, along with Herschel and Babbage, was prominent in the Analytical Society that aimed to reform Cambridge mathematics In 1837, Peacock became the Lowndean Professor of Astronomy and Geometry (the position that Adams would occupy from 1859–92). Peacock and Airy were close friends too, and Peacock accompanied Airy and his family for part of their nine-week continental tour in 1846. Peacock also stayed with Herschel that summer in order to become godfather to one of his daughters. It was largely due to Peacock’s efforts that the Cambridge University Observatory had been refounded, and it was through Airy’s endeavours that it had been remade as a significant scientific centre (Smith, 1989: 413). Airy first wrote to Challis about a planet search on 9 July and so Peacock may have helped to impress upon him the importance of a search (Fig. 7.7). Herschel, however, is, after Airy, the figure most worth examining for the major role he played, mostly out of public view, in the battles and negotiations on what constituted the optical and theoretical discoveries of Neptune and who should

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Fig. 7.7  John Herschel (Credit: Royal Institution of Great Britain/Science Photo Library)

receive credit for them.3 Herschel was one of the leading figures in British, and indeed global, science, and one of the most famous natural philosophers in the world. He also felt deep emotional and intellectual bonds to the University of Cambridge, as is very evident, for example, in the Presidential Address he delivered to the British Association for the Advancement of Science in June 1845. The Association’s annual meeting was held that year in Cambridge, 12 years after the Association held its first meeting at the University. Herschel proclaimed, in what in places resembles a prose poem devoted to the University of Cambridge, that: The development of its material splendour which has taken place in that interval of twelve years, vast and noble as it has been, has more than kept pace with by the triumphs of its intellect, the progress if its system of instruction, and the influence of that progress of the public mind and the state of science in England. When I look at the scene around me—when I see the way in which our Sections [of the Association] are officered in so many instances by Cambridge men, not out of mere compliment to the body which receives us, but for the intrinsic merit of the men, and the pre-eminence which the general voice of society accords them in their several departments—when I think of the large proportion of the muster-roll of science which is filled by Cambridge names, and when, without going into any details, and confining myself to only one branch of public instruction, I look back to the vast and extraordinary development in the state of mathematical cultivation and power in this University, as evidenced both in its examinations and in the published works of its members, now, as compared with what it was in my own time—I am left at no loss to account for those triumphs and that influence to which I have alluded. It has ever been, and I trust it ever will continue to be, the pride and boast of this University to maintain, at a conspicuously high level, that sound and thoughtful and sobering discipline of mind which mathematical studies imply (Herschel, 1845: xxvii).  On Herschel and Neptune, see also (Case, 2019).

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Herschel also emphasized that while mathematical studies were powerful instruments of investigation, they also possessed great moral worth. He complained of an increasing propensity to “crude and hasty overgeneralization” in natural philosophy. “To all such propensities,” he argued, “the steady concentration of thought, and its fixation on the clear and definite which a long and stern mathematical discipline imparts, is the best, and indeed, the only proper antagonist” (Herschel, 1845: xxviii). Herschel became aware of Adams’s analyses at the Board of Visitors meeting on 29 June. Herschel, who had described astronomy as the “most perfect” of the sciences and the law of gravity as “the most universal truth at which human reason has yet arrived” (Herschel, 1831: 60 & 92), seems to have been very confident that theory pointed the way to a new planet. Indeed, on 10 September 1846, in remarks to the British Association for the Advancement of Science, he had announced, we will recall, in effect the theoretical discovery of a new planet although he did not name either Adams or Le Verrier. As soon as he heard of the optical discovery (1 October 1846), he penned a letter to the The Athenaeum pointing out that Adams had independently calculated a position close to Le Verrier’s (Herschel, 1846c). While Herschel wanted to bring his own earlier pronouncement of the certainty of the theoretical discovery to a wider audience, he was also concerned to defend what he saw as the rights of Adams, a Cambridge Senior Wrangler (Smith, 1989: 414). Curiously, despite his confidence a new planet would be discovered, Herschel had not pushed for a search for the postulated planet. In 1846, he exchanged letters with the Liverpool astronomer William Lassell, proud owner of a recently completed 24-inch reflecting telescope, and Lassell presented examples of its prowess to Herschel. Herschel, however, did not raise the topic of a new planet.4 In August, Herschel nevertheless suggested to William Rutter Dawes that he search for the new planet. William Rowan Hamilton (1805–1865), Astronomer Royal of Ireland at Dunsink Observatory and professor of astronomy at Trinity College, Dublin, explained to a correspondent a couple of weeks after the optical discovery: I may mention that, when I was lately [in August 1846] a guest of Sir John Herschel at Collingwood, we took a drive together to visit Mr. Dawes, an amateur of astronomy, who has erected a private observatory, with some excellent instruments, in that neighbourhood, during which visit the conversation turned on the theoretical announcement of the new planet by Le Verrier. It was not generally supposed that the stranger would show himself till about Christmas; but Sir John Herschel recommended Mr. Dawes to begin looking for him at once (Hamilton, 1846).

Dawes, a well-known amateur astronomer, did not follow Herschel’s suggestions. “Hamilton’s story,” I have argued elsewhere, “is nevertheless significant because it discloses that Le Verrier’s announcement was the focus of the discussion (whether 4  A well-known story concerns a supposed lost opportunity for Lassell to have searched for the postulated planet. I have argued against the credibility of this story (Smith, 1983). Richard Baum has argued in favour of the story’s reliability (Baum, 1996). Lassell will be the central figure in Chapter 8.

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Adams’s work was mentioned or not) as well as that it was perhaps not generally grasped that the best time for the planet hunt was the summer of 1846. Given the planet’s predicted position, it certainly was not sensible to wait till Christmas. The date Le Verrier had cited for computational purposes, 1 January 1847, was, it seems, being mistaken even by some eminent astronomers as the date to start a search” (Smith, 1989: 414). Following Herschel’s 1 October letter to The Athenaeum, Le Verrier, who viewed Herschel’s claims on Adams’s behalf as unjustified, attacked Herschel in print. As Herschel confided to his diary: “In bed half day after a sleepless night wrote to Editor of the Guardian in reply to M. Le Verrier’s savage letter. These Frenchmen fly at one like wild cats” (Herschel, 1846d). Despite his strong support of Adams, Herschel was sure that the credit for the discovery properly belonged to Le Verrier, and he now sought to calm matters and move opinion to both recognising Le Verrier as the discoverer while securing some measure of credit for Adams. To Whewell he confessed in November 1846 that he grieved “over the loss to England and to Cambridge of a discovery which ought to be theirs every inch of it” (Herschel, 1846a). But when Sheepshanks advanced the argument that as Challis had detected what proved to be Neptune in August 1846 he should be recognised as the optical discoverer, Herschel pressed on him the point he had made to Whewell: But though Neptune ought to have been born an Englishman and a Cambridge man every inch of him—Dis aliter visum5 now will never make “An English Discovery” of it do what you will (Herschel, 1846b).

In writing to Hamilton, who was certainly not a Cambridge man as he had not trained there or been employed at the University, Herschel did not refer to Neptune being nearly born a “Cambridge man.” Rather, Herschel argued that it is perfectly possible to do justice to Adams’s investigations without calling into question M. Le Verrier’s property in his discovery. The fact is I apprehend that the Frenchmen are only just beginning to be aware what a narrow escape Mr. Neptune had of being born an Englishman. Poor Adams aimed at his bird it appears first, as well as Le Verrier, but his gun hung fire and the bird dropped on the other side of the fence (Herschel, 1846e).

Herschel had a very well thought-out position on discovery and for him, “He who proves, discovers” (Gooding, 1985). As Herschel emphasized to Whewell in late 1846: I do not call finding an individual object merely including it in a crowd of others (without knowing it is there, and rather suspecting, it not to be with an intention of examining then at leisure to ascertain if it be among them or not). Nobody but Sheepshanks will ever say that C[hallis] found it before G[alle]. Until the planet was actually seen and shown to be a planet–there was no discovery– except in so far as a successful physical hypothesis is one…..Till men’s minds are made up to call the “theory” a “fact” there is no discovery. The sight of the actual thing of course forces them to do so at once (Herschel, 1846f). 5  Dis aliter visum appears in Book 2 of the Aeneid, and can be translated as “The Gods decided otherwise.”

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Herschel, who had battled to change the Cambridge curriculum to cover much more French mathematics, believed that French mathematicians very largely deserved the credit for the discovery. They had provided the mathematical tools to calculate planetary perturbations, and Le Verrier’s analyses, because he had begun by carefully re-examining Uranus’s motions, were fuller than Adams’s. Herschel argued in late 1846 that it was “all French,” “de fond en comble [from top to bottom] Clairaut, D’Alembert, Laplace, Lagrange and more recently Poisson and Pontecoulant for the analysis and Bouvard for the tables [of planetary positions] which though not quite correct were yet correct enough to raise the hue and cry. The new planet is as much Laplace’s as it is either Le Verrier’s or Adams’s….Who made one and all of the formulae by which both have grappled the planet but Frenchmen” (Herschel, 1846g).

If Herschel, whose inclination was always to shy away from public controversies, had adopted this line publicly, he would surely have faced the ire of some critics. He perhaps felt he had to tread especially carefully as he had been involved in a public spat at the end of the British Association for the Advancement of Science meeting in Southampton in September 1846. In the final session of the meeting, Herschel was accused of referring to Charles Wheatstone’s (1802–1875) development of the electric telegraph as an “effervescence, an ephemeral thing, and even ‘scum.’” Herschel rose immediately to deny the use of those words (Anon, 1846f). He nevertheless wrote to the London Times the next day to ensure there was no erroneous impression left of his conduct “as if it were the design of myself or any one present to treat contemptuously British science in comparison with foreign…” (Herschel, 1846i). Others were unhappy at the way matters had turned out over the new planet. In November 1846, W.  H. Smyth grumbled to Herschel, “I am still pestered by the popinjays about the new planet, and can hardly persuade them that we are all … truly ‘delighted’,” thereby indicating they were not delighted. It was a pity, Smyth thought, that Adams had not published a brief report on his findings in the Monthly Notices of the Royal Astronomical Society in November 1845, as soon as he had reached firm conclusions (Smyth, 1846). In 1847, Smyth went further and said publicly that had a search been undertaken in October 1845, the new planet’s visible disk would have meant that “both the prediction and the detection would, infallibly, and without competition, have fallen to Cambridge” (Smyth, 1860: 422). Herschel’s belief that Le Verrier deserved the credit for the theoretical discovery was widely shared among British men of science and was displayed when the Royal Society awarded Le Verrier its Copley Medal, then arguably the most prestigious scientific prize in the world. The Society’s Council knew that Adams’s first prediction predated those made by Le Verrier and at the time of the award they had seen only the outlines of Le Verrier’s calculations that had been published in the Comptes Rendus, not detailed papers. This “prompt action of a body not famous for the rapidity of its movements in the distribution of honours,” the Athenaeum noted, “will do much to remove from the minds of French philosophers the suspicion of English scientific jealousy or unfairness,—and to silence the French newspaper writers who so scandalously assumed them” (Anon 1846e:1190).

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Herschel told Whewell, “I did what I could to assist it (though it could not have been otherwise)” (Herschel, 1846a). Adams was awarded the Copley Medal two years later. The Council of the Royal Astronomical Society voted to award the Society’s Gold Medal to Le Verrier, but even though the proposal was supported by a vote of ten to five, the Society’s bylaws required there be a three-to-one majority to award the medal. The proposal’s failure led to controversy, and we shall turn to this dispute again in the next section. An article in the Illustrated London News in January 1847, also underlines that it was widely agreed in Britain that Le Verrier was to be counted as the discoverer. James Glaisher reported in the Illustrated London News that “we perceive our Correspondent ‘P.Q.R.,’ of Cambridge, thinks we have done less than justice [to Adams] in our previous accounts. We beg, however, to remark, that we have only spoken upon published facts.” He emphasized that this was not an effort to detract from the merit due to Challis and Adams, but “we cannot do an act of injustice by withholding the merit of the discovery from M.  Le Verrier, to whom it certainly belongs” (Anon, 1847). P.Q.R. was in fact no less than William Thomson, who had left Cambridge for a position in Glasgow three months earlier, but who was still clearly very well connected in Cambridge, as we saw earlier in the chapter with the exchanges with G.G. Stokes.6

7.8  What Counts as a Publication? For many involved in the Neptune controversies in late 1846 and 1847, including Arago, publication was fundamental to securing priority for a scientific discovery. But this issue became the subject of another heated debate in early 1847. It was sparked by a letter from Charles Babbage to the London Times in March 1847. Babbage had strong interests in astronomy. He was one of the founding members of the Astronomical Society of London in 1820 (later the Royal Astronomical Society), and had been the Lucasian Professor of Mathematics at Cambridge between 1828 and 1839. He is, however, now usually remembered as a pioneer in computing for his designs and attempts to construct a “Difference Engine” and a far more sophisticated “Analytical Engine.” Babbage was also a key figure in the famous debate in early nineteenth century Britain over the decline of science, at least if Britain was compared to France and Germany. He was one of the “Declinists,” contending that the British state had failed to adequately support scientific research or properly reward its practitioners (Babbage, 1830). In his letter to The Times, Babbage raised questions about the dispute over the award of the Royal Astronomical Society’s Gold Medal. Babbage reported that at the Annual General Meeting of the Society a month earlier, the Society’s Council

6  I am most grateful to Carolyn Kennett for pointing out to me the identity of P.Q.R. (Lagemann, 1956: 132).

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noted that although Le Verrier had been proposed for the Society’s Medal, he had not secured the required 3–1 majority to be awarded it. The vote for the award of the Medal had been 10–5, with Airy’s being one of the votes against. Babbage claimed that the reasons given for the omission were “entirely unsatisfactory to many of the members present” (Babbage, 1847a). Babbage had “seized on the [Neptune] controversy” and become “obsessed” by it and had assembled a file of evidence. As Babbage put it in his notes, “Priority of pubn not of discovery the thing the world rewards” (Csiszar, 2018:188). For Babbage, the story of Neptune was a splendid example of the potential value of his Analytical Engine because it could have aided in the calculations, and in February 1847 he wrote to both Adams and Le Verrier to make that point. Babbage sent Adams a paper on the Analytical Engine by Luigi Federico Menabrea (1809–1896) as well as Ada Lovelace’s (1815–1852) accompanying notes, and also told Adams that he could not “help half suspecting that if you had felt confidence in your arithmetical results which the fact has proved they deserved even the most discouraging circumstances would not have prevented you from publishing the results of a theory of which you entertained no doubt” (Babbage, 1847b; Hyman, 1982:212). The next month, in his March 1847 letter to The Times, Babbage argued that Adams did not deserve the Royal Astronomical Society’s Gold Medal for according to what he called the modern law of discovery, discoveries had to be dated from “the time of their first publication to the world.” Babbage also wrote to his friend Arago to inform him of the letter to The Times which, as Csiszar, has summed it up, was “uncompromising and followed Arago precisely,” but “produced a flood of attacks” (Csiszar, 2018:188). The most vigorous and scathing of these was delivered by Richard Sheepshanks who published a brutally worded 16-page pamphlet to rebut Babbage’s arguments (Sheepshanks, 1847b). A few years earlier, Airy had declared Babbage’s engines to be useless and not worthy of support. Airy too now trained his guns on Babbage with a letter in The Athenaeum which was directly preceded by an account of the Special General Meeting of the Royal Astronomical Society where the award of the medal had been discussed. Airy, like Sheepshanks, took strong issue with Babbage’s claim that the “modern law relating to discoveries is, that they take their date from the time of their first publication.” For Airy, there was no such law “and I doubt whether it has ever been plainly enunciated in equivalent terms before this time.” As a counter example to such a law, Airy recounted how David Brewster (1781–1868), to protect his position in the course of researches involving optical experiments at a time when JeanBaptiste Biot was pursuing similar investigations, had an officer of the University of Edinburgh sign and date a page in Brewster’s experimental journal. Given the lack of past instances and “the non-existence of distinct law, I think we must be guided very greatly by the circumstances of the case.” What sort of publication was best fitted to the Neptune case? Airy asked, “Is it best to put the theory in type, and to leave it to produce its own effect by extensive dissemination?—or is it best to communicate it in manuscript to a few persons whose position is likely to enable them to undertake the search for the planet—the

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communication bearing with it, as far as it goes, the force which is peculiarly given by private and personal communication?” Le Verrier had tried the former method and “it failed totally in its object. It does not appear to have induced a single person to look for the planet” (this claim of course was wrong as Hind had undertaken a search.) Adams had adopted the second scheme, and although “not quite complete in its own kind, it was so far complete that to it is principally owing the research (Prof. Challis’s) in which the planet was first seen.” (This judgement of Airy’s was of course disingenuous hindsight as Adams had not prompted Challis’s search. As Airy well knew, it was Airy’s pressure on Challis that prompted Challis to begin a search.) Airy then argued that Le Verrier later employed personal communication and “then the planet was discovered on the Continent.” But that, as Airy’s readers would have known, was months after Le Verrier had published a position in his 1 June 1846 paper (Airy, 1847). For Airy, Babbage was wrong to try to settle the matter in the way that he had. So here, some months after delivering his account to the Royal Astronomical Society in November 1846, Airy directly challenged publication as the fundamental rule in priory disputes, which indirectly gave support to the case for credit for Adams. Here it is worth noting that, in line with the increasing need that French researchers felt to fix dates in order to secure priority, there had been an explosion in the practice of submitting sealed notes—paquets cachetés—to the Academy [of Sciences]. The practice of using sealed notes deposited with some authoritative body ‘pour prendre date’ went back a long way, but it had never been an everyday occurrence: the eighteenth-century Academy received just a few such notes per year. But it picked up dramatically in the late 1830s, and by the 1840s the Academy was accepting about one hundred such notes per year (Csiszar, 2018:167).

While the same system was still available at the Royal Society, it would be ended in 1854, and the only person to have used it recently was the prominent Scottish man of science Sir David Brewster. Brewster in fact delivered an exceptionally robust defence of Adams’s position in May 1847 when he penned a lengthy and detailed article for the North British Review. He, like Babbage, had been a voluble “Declinist” in the debate over the decline of science in Britain. Like Arago in France, Brewster was a deep student of the history of discoveries and had written extensively on the subject. When he founded the Edinburgh Journal of Science in 1824, Brewster had even introduced a section on “Decisions on Disputed Inventions and Discoveries” (Csiszar, 2018:162). Brewster now wrote on the “Researches Respecting the New Planet Neptune,” for which he drew on published papers, articles and reports to recount the history of the discovery and in so doing berate the failings of British scientific institutions, praise Adams and scold Airy. The point about the failings of British scientific institutions was one that Brewster had long hammered away at, and so the Neptune episode was yet another example for Brewster of where French institutions had shown themselves to be superior to British ones. For Brewster too, as well as Babbage, “the orderly settling of priority disputes was a crucial first step in recognizing and rewarding (both morally and financially) active men of science. Brewster’s ‘Decisions on Disputed Inventions

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and Discoveries’ was an attempt to provide for British men of science a service accomplished by the academies in France” (Csiszar, 2018, 167). Brewster also judged the discovery of Neptune as remarkable both for its nature and the controversy to which it had given rise (Brewster, 1847:209). In his opinion, Adams had provided his solution to Challis in Cambridge and Airy at Greenwich and so Adams had the sole and undivided merit of being the first discoverer of this remarkable body. No act on his part, and no subsequent researches on the part of others, can affect this great truth. He, and he alone, first solved the problem, and pointed to the star…As truly might we maintain that the heat and sunshine of to-day have been reduced by the cold and darkness of the morrow, as that the glory of Mr. Adams could be dimmed by the absence of Professor Challis, or the invisibility of Mr. Airy (Brewster, 1847:217).

That Airy, in his letter of 14 October 1846 to Le Verrier, had told Le Verrier that he “without any doubt” should be “considered as the person who has really predicted the position of the planet,” Brewster found galling as Airy had spoken not of “publication,” but “prediction.” For Brewster, no Englishman could read Airy’s letter “without unmingled pain” (Brewster, 1847:226). Brewster also believed that both Challis and Galle had “verified” Adams’s October 1845 prediction, “and thus the prediction of M. Le Verrier, uttered seven months later, on 1 June 1846, had been verified by the same observers.” For Brewster, “mathematical truth had had a double triumph,” and but for “the incredulity and inactivity of his friends, he [Adams] might have pointed to his planet in the heavens, before a fellow-labourer had appeared on the field” (Brewster, 1847:228). Given Arago’s 19 October 1846 remarks on publication in the Comptes Rendus, Brewster thought it would be “fruitless” to engage with him, so “we shall best fight the battle of Mr. Adams with M. Arago’s allies, the English savants and zealous defenders of the claims of Le Verrier” (Brewster, 1847:229). With his concern for appropriate rewards, both moral and financial, for the original researches conducted by men of science, Brewster had thought deeply about, and was an expert in, patent laws, and while he accepted a patentable invention was somewhat different from a scientific discovery, he argued the patent laws of Europe could help frame a “just decision of the case under our consideration” (Brewster, 1847:237). In disputes over the award of patents, Brewster explained, publication was not the same as printing. Moreover, the “principle of law…on which these decisions rest, is, that an invention or discovery, communicated to more than one person, or placed within view of knowledge of the public, even though they have not seen or known it, is published to such an extent, that no future inventor or discoverer can claim any right of a beneficial character” (Brewster, 1847:238). As Adams’s “discovery” was known to some people in Cambridge, as well as given to Airy and Challis “for the very purpose of giving to the public the benefit of his discovery” this meant the credit should obviously go to Adams, not Le Verrier. Brewster ran through various examples from history to illustrate his arguments. He then invited the reader to imagine the assembly of a highly distinguished international jury to judge the Neptune case. The jury was to be composed of historians

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of science—he named Montucla, Bossut, Priestley, Playfair, and Whewell—and “distinguished philosophers”—Huygens, David Gregory, Musschenbroek, Smith, Robison, Hutton and Dr. Young. All, in Brewster’s opinion, had promulgated to the world “their unanimous decision, that priority of discovery, even when that discovery has neither been communicated to friends nor published to the world, supersedes the claims of priority of publication” (Brewster, 1847:241). J.B. Biot was a leading French researcher of optics, magnetism and astronomy. He was also an old antagonist of Brewster’s and by the time Brewster’s article was published, Biot had already responded to Airy’s piece in the 18 March issue of The Athenaeum. Biot’s reply came at the Academy of Sciences in Paris on 22 March and was crucial because it effectively brought about a settlement of the controversies that Brewster’s polemic would fail to break. Biot noted that Adams had predicted the existence of the new planet in October 1845, and so eight months before Le Verrier’s first announcement of the new planet “and he [Adams] alone was in possession of the secret of its celestial position.” Adams’s calculations had deserved to be communicated without loss of time to the scientific world, whose attention and interest they would have greatly excited. Or, if it were wished to make a local property of them, means should at least have been taken to find the planet; a large equatorial [telescope] ought to have been liberally placed at Mr. Adams’s disposition, with a request that he would employ all his nights searching for it. The opportunity was eminently favourable….I do not speak here in accordance with the narrow sentiment of geographical egotism, so improperly called patriotism. Minds devoted to the culture of science have, in my estimation, a common intellectual country, embracing every degree of polar elevation. In this case, I see only a young man of talent, whom the chance of circumstances has, for the time, ill-treated, and whom we must applaud in spite of fate. I shall say to him, therefore: ‘The Laurel which you have been the first to deserve has been merited also by another, who has carried it off before you had the courage to seize it. The discovery belongs to him who proclaimed and published it to all, while you reserved the secret to yourself. This is the common, imprescriptible law, without which no scientific title could be assured. But in your own mind, you are conscious that the new star was known theoretically to yourself before anyone else knew of it. This inward success ought to give you the consciousness of your power, and excite you to direct it to the many other great questions yet remaining to be resolved in the system of the world; and if my years give me the privilege of offering advice, I shall express it in one word—‘PERSEVERE’ (Biot, 1847:371).

When these remarks were published in The Athenaeum, they were prefaced by the writer reminding readers that since the discovery of the planet, The Athenaeum had pressed to “reserve to Mr. Adams his place in the history of this great fact.” But as Adams’s place was now secured in the discovery “amongst European philosophers generally—we may wind up our own share in this matter,” even though The Athenaeum “did not exactly accept its terms.” This statement, in effect, brought the negotiations, disagreements and squabbles over priority for the theoretical discovery of the new planet involving French and British savants, philosophers, and astronomers pretty much to an end. Brewster’s polemic, brilliant though it was in some respects, failed to provoke a renewal of the debates.

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7.9  Exploiting the Discovery and Wider Impact It is important to understand the way some figures exploited the discovery. Writing about a century after the events of 1846, Anton Pannekoek, whose writings we met in the introduction to this chapter, argued that the British were not very interested in the benefits to be derived from making such a discovery as that of Neptune or in using it once made. He contended that French savants did employ the discovery of Neptune to proclaim to the public the predictive power and certainty of science, and the potential value of science to the nation. But he saw relatively little such activity in England and argued that this difference was evidence of the problems French science still faced in subduing the residual authority of the church. So, for Pannekoek, in “England where the bourgeoisie already felt its supremacy such a passionate struggle for scientific enlightenment against clerical tradition was hardly noticeable. This surely explains the calm indifference with which Adams’s work was treated and passed over by the leading astronomers” (Pannekoek, 153:131). But Pannekoek did not take into account the strong interest ahead of the optical discovery such as, for example, Herschel’s claim in September 1846 that astronomers saw the postulated planet “as Columbus saw America from the shores of Spain.” After the optical discovery, it also became a resource for those pressing for further support for both astronomy and science more broadly, but even Adams could not be certain of a scientific position, and this perhaps helps to explain why Herschel and others sought to publicize his investigations. As Sheepshanks lamented to correspondent in April 1847, “I think there is a hope that Mr. Adams will continue his astronomical researches. In any other country there could be no doubt of it, but in England there is no carriéere for men of science. The Law or the Church seizes on all talent which is not independently rich or careless about its wealth” (Sheepshanks, 1847c). Those based at Cambridge or with strong Cambridge connections valued some measure of credit for Adams for the theoretical discovery of Neptune as a justification of the worth of a Cambridge training and the standing of Cambridge in the sciences and mathematics. Some valued it enough to try to secure a knighthood for Adams and so emblematic governmental endorsement. The granting of knighthoods as rewards to men of science had been a feature of Sir Robert Peel’s (1788–1850) second period as prime minister (1841–1846), as had the provision of grants for scientific purposes (Smith, 1989:419). The route to a knighthood for Adams and so the prospect of Sir John Couch Adams was opened once Prince Albert, the husband of Queen Victoria, was elected chancellor of Cambridge University in February 1847. By June 1847 Adam Sedgwick, who we saw earlier was Woodwardian Professor of Geology at Cambridge, had become Prince Albert’s personal and official secretary at Cambridge. Sedgwick was a very prominent Cambridge figure and had already spoken at length with Adams in the aftermath of the optical discovery. He told Adams about the possible knighthood. “It would place me,” Adams wrote, “in a satisfactory position in the eyes of the world, as being a public recognition of my

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claims connected with the discovery of the new planet.” He agreed too it “would also, no doubt, tend to promote the cultivation of science in England, & [show] that the Government is disposed to sympathize with & to encourage those who devote themselves to such pursuits” (Adams, 1847). Adams, however, was in a difficult position (Smith, 1989: 420). First, he had no private fortune, and as taking a title would mean a change in his social position, it would rule out several means of earning a living. For example, it would not, he thought, be socially appropriate for a knight to take pupils. “I do not insist upon the obstacle to my taking orders,” Adams wrote, “as I have not decided that I should do so, however this matter may turn out.” His marriage choices would also be very limited because of the “necessity there would be for keeping up appearances, in some degree corresponding with the title.” His family’s situation was a further concern as they held “a comparatively humble position in life. My father is a farmer, and it might appear rather incongruous that his Son should be Sir John” (Adams, 1847) The upshot was that Adams declined the knighthood. Airy was also offered a knighthood. This was the second time it had been offered to him and the timing was surely a scheme by Sedgwick to offer some sort of official backing for Airy in the face of the criticisms levelled at him for his handling of the Neptune episode, as well as to promote the cultivation of science more generally. Airy, like Adams, raised the problem of costs to Sedgwick, and so he again declined the offer (Airy, 1896:187). Despite Adams’s misgivings, when Prince Albert (1819–1861) and Queen Victoria (1819–1901) visited Cambridge the next month, July, the queen offered him a knighthood. Adams again declined. Thus the move to secure him a knighthood sank on the rocks of practical objections (Glaisher, 1896:xxxiii). The discovery of Neptune was a splendid example of just the sort of discovery the Cambridge group liked to display to demonstrate the power of science—and, by implication, the value of men of science. Whewell, his History of the Inductive Sciences, for example, presented the development of the sciences in terms of progress through discoveries (Smith, 1989: 421). These pivotal discoveries were the result of the brilliance of a few superior minds and the transmission of such discoveries was due to research schools. In Simon Schaffer’s opinion, such a model of scientific progress “perfectly fitted the interests of Whewell’s own reform campaigns within Cambridge schemes of disciplinary education” (Schaffer, 1986:411). Whewell also cited universal gravitation as a paradigmatic example of a scientific theory in, for example, his Philosophy of the Inductive Sciences, first published in 1840. He had also earlier proclaimed that astronomy provided “the most consummate productions of labour and skill, and the loftiest and most powerful intellects which appear among men, are gladly and ominously assigned to the task of adding to its completeness” (Whewell, 1833:xiii). Whewell heaped praise on Adams and Le Verrier. Further, if Neptune had not been found until 1847, there would, due to Whewell, even have been a sort of pre-­ discovery publication of Adams’s investigations. In the revised version of his History of the Inductive Sciences that was in press in August 1846, Whewell

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explained that “M. Le Verrier … and, as I am informed by the Astronomer Royal, Mr. Adams of St. John’s College, Cambridge, have both arrived independently at this result” on a position of a planet beyond Uranus (Whewell, 1847:306). By August 1846, this volume was in press, but Whewell added a postscript, dated 7 November 1846, in which he summarized Neptune’s discovery (Smith, 1989:418). And Whewell, as we noted earlier, had also written at the end of September 1846 to J.D. Forbes in Edinburgh and told him that Challis was busy hunting for a planet beyond Uranus. More broadly, the gentlemen of science valued keeping exemplary scientific work before the public. Herschel had contended a person need not be cut off from an interest in astronomy, optics, or dynamics even if they could not follow the technical details. To “one incapable of following out the intricacies of mathematical demonstration, the conviction afforded by verified predictions must stand in the place of that purer and more satisfactory reliance, which a verification of every step in the process of reasoning can alone afford” (Herschel, 1831:20). William Rowan Hamilton provided in 1832 an especially powerful example of this sort of verified prediction. Hamilton had announced a prediction based on the wave theory of light of an effect later to become known as conical refraction in which a crystal causes a narrow beam of light to change into a hollow cone. “Hamilton realized,” his biographer Thomas Hankins has explained, “that if his prediction of conical refraction could be verified experimentally it would be a major triumph. Nothing like it had ever been observed in the long history of experimental optics, and the fact that he had deduced it mathematically without any hints from experiment made his prediction all the more dramatic.” Humphrey Lloyd (1800–1881), professor of natural and experimental philosophy and author of a Treatise on Optics, searched for the effect on Hamilton’s suggestion, and he soon confirmed its existence (Hankins, 1980:91). For Airy, the confirmation was “perhaps the most remarkable prediction that has ever been made.” Whewell reckoned that “in the way of such prophecies, few things have been more remarkable than the prediction of [conical refraction],” and Herschel was extremely impressed (Hankins, 1980:95). Hamilton’s prediction, Hankins has pointed out, would come to be widely compared to the discovery of Neptune. Indeed, what could have been a “better and more persuasive prediction than the existence and place of a major planet?” (Smith, 1989:422). As Herschel contended in October 1846, he could not “conceive anything better calculated to impress the general mind with a respect for the mass of accumulated facts, laws, and methods, as they exist at present, and the reality and efficiency of the forms into which they have been moulded than such a circumstance. We need some reminder of this kind in England, where a want of faith in the higher theories is still to a certain degree our besetting weakness” (Herschel, 1846h).

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7.10  Americans Assert a Happy Accident! J.D. Forbes, the professor of natural philosophy at the University of Edinburgh, wrote letters to recount for his wife and a correspondent some of the events at the recently concluded meeting of the British Association for the Advancement of Science in Oxford in June 1847. The meeting, Forbes told William Thomson, “was very satisfactory…Adams and Leverrier were the great stars and…conducted themselves with great prudence and amity; to which,” he joked, “perhaps the Circumstance that Leverrier speaks no English, and Adams no French, somewhat contributed.” Forbes had dined one evening with “a small but excellent party” that included Airy, Le Verrier and Adams. As the evening was very fine, the group was told Neptune might be seen. “Fancy the interest,” Forbes told his wife, “of seeing the New Planet for the first time in company with its two discoverers!” (Forbes, 1847a, b). Such a harmonious scene had looked far from likely in the autumn of 1846 as controversy swirled around the discovery of the new planet. But even as Le Verrier and Adams observed Neptune together in June 1847, yet another controversy had emerged. This one had been sparked by the American Benjamin Peirce, who had become University Professor of Mathematics and Natural Philosophy at Harvard in 1833. Nine years later he was appointed as the Perkins Professor of Astronomy and Mathematics. He was widely regarded as one of the best, if not the best, American mathematician of his generation. Peirce had at first identified the new planet as “Le Verrier’s planet.” But at a meeting of the American Academy of Arts and Sciences in March 1847 he reported on the extensive researches conducted by Sears Cook Walker on the new planet’s orbit (Kent, 2011). Walker, a staff member of the U.S.  Naval Observatory in Washington D.C., had been assigned this task by the Observatory’s director Matthew F. Maury in November 1846 (Hubbell and Smith, 1992:265). By early February 1847, Walker had calculated his third set of elements, and these differed significantly from those calculated before the optical discovery by both Le Verrier and Adams. When Le Verrier discussed Walker’s results at the Académie des Sciences in March 1847, he judged that “the small eccentricity which appears to result from Mr Walker’s computations would be incompatible with the nature of the perturbations of Herschel [i.e. Uranus]” (Le Verrier, 1847:531). Walker’s orbit also differed significantly from Le Verrier and Adams’s elements calculated prior to the observational discovery (Hubbell and Smith, 1992:291). Peirce, however, went further and claimed THAT PLANET NEPTUNE IS NOT THE PLANET TO WHICH GEOMETRICAL ANALYSIS HAD DIRECTED THE TELESCOPE; that its orbit is not contained within the limits of space which have been explored by geometers searching for the course of the disturbances of Uranus; and that its discovery by Galle must be regarded as a happy accident (Peirce, 1846–8a:65).

Soon after his talk to the American Academy of Arts and Sciences, Peirce, upon the suggestion of the Harvard historian and soon to be the Harvard President Jared Sparks (1789–1866), sought to make his views clear through an abstract of his remarks to the press, a move that we can interpret as showing that Sparks

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recognized Peirce’s remarks were bound to be controversial (Sparks, 1847). Peirce ended his abstract by diplomatically pointing out that “I would not have you suppose that I am disposed to contest in the least the greatness of Le Verrier’s genius. I have studied his writings with infinite delight, and am ready to unite with the whole world in doing homage to him as the first geometer of the age, and as founder of a wholly new department of invisible Astronomy” (Peirce, 1847:86). The debates and controversies surrounding Peirce’s assertion have been treated in detail elsewhere and are examined in Chapter 9 so there is no need to examine them in depth in this chapter. We should nevertheless note that Peirce’s claim centred on the difference between the observed and pre-discovery predictions of the mean distance of Neptune from the Sun, with Neptune’s mean distance from the Sun only 30 times the earth’s mean distance, and so “five hundred millions of miles nearer the sun that it was distinctly stated by geometry than it possibly could be, in order to be capable of producing the effect [on Uranus] which it actually does produce” (Peirce, 1846–8b:338). In the United States, Peirce’s “happy accident” claim was amplified not only among astronomers but also with the general public, who were avidly following developments through both the enormously popular lectures of Cincinnati Observatory founding director Ormsby MacKnight Mitchel (1809–1862), as well as correspondence by Le Verrier and others in his astronomical monthly The Sidereal Messenger (Shoemaker, 1991:145–164). The writings of Peirce and a few others in the United States drew an angry response from Le Verrier. Also, Le Verrier was cross in August 1848 when the French physicist Jacques Babinet argued at the Académie des Sciences in favour of the idea that if the observed planet was not in fact the planet predicted by Le Verrier and Adams, then another planet awaited discovery. Babinet even gave the postulated planet a name: Hyperion. When Babinet’s paper was published it was followed by one by Le Verrier that was extremely critical of Babinet’s idea. The dispute, however, ended when Le Verrier published another paper in the Comptes Rendus for 2 October. The Athenaeum reported that “M. Leverrier is getting annoyed:—and we do not wonder at it…we recommend M. Leverrier to keep calm—and laugh. Let him take a leaf out of our book, and recommend to his opponents if they say there is another planet to find it” (Anon, 1848:1103). Many, perhaps a majority, of astronomers agreed with Le Verrier that the discovery of Neptune had not been a happy accident. The leading German mathematician C. G. J. Jacobi, for example, wrote in 1848: One must wonder how such precise results could have been drawn from such sparse and uncertain quantities; this can only be attributed to the circumspect treatment of these data and to the exemplary application of all aids. Those who attribute the discovery to chance because the agreement is not closer than the nature of the thing permits should indeed give permission for other chance discoveries to be made (Jacobi, 1848:45–6).

Nor had John Herschel’s faith that Neptune represented a stunning triumph of astronomy been shaken. He argued that although the elements of the theoretical and observed planet differed:

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[i]n all this we see nothing of accident, unless it be accidental that an event which must have happened between 1781 and 1953, actually happened in 1822; and that we live in an age when astronomy has reached that perfection, and its cultivators exercise that vigilance which neither permit such an event, nor its scientific importance, to pass unnoticed. The blossom had been watched with interest in its development, and the fruit was gathered in the very moment of maturity (Herschel, 1849:518).

Although Peirce did not retreat from his claim that the observed planet was not the predicted planet, he did concede that Neptune could explain the observed irregularities in the motion of Uranus. For John Herschel, this made, as he wrote late in 1848, the question of Neptune’s identity “defunct”— the theoretical planet and the observed planet were the same (Herschel, 1848).

7.11  Conclusions Cambridge mathematics via the Mathematics Tripos was at the heart of British physical sciences in the middle decades of the nineteenth century. Many British men of science therefore cared deeply about its standing and reputation, either as advocates or critics. For them, the discovery of Neptune went beyond the finding of a new major planet, highly significant though that undoubtedly was for astronomy and science more broadly. The Neptune story was in fact given a particular charge because it was also immersed in wider issues to do with the worth of British scientific institutions, as well as debates on what were the appropriate systems of credit for scientific discoveries, debates that, as we have seen, even reached to fundamental issues to do with what even counted as a scientific discovery and what could be deemed as a publication. Airy and Herschel emerge as the key figures for what might be termed the “management” of the discovery in Britain after the optical find was announced in Berlin, with Airy being the most prominent and the most public. In this role Airy somewhat resembles that of Joseph Hooker (1817–1911) and Charles Lyell (1797–1875) in 1858, who, after Charles Darwin had received a manuscript from Alfred Russel Wallace (1823–1913) that Darwin feared undercut his claims to priority for the discovery of evolution by natural selection, managed the discovery on behalf of Darwin. Darwin was preoccupied by a sick child and in effect handed over matters to Hooker and Lyell, and they used their scientific clout to play a crucial role in bringing the discovery of evolution by natural selection to a scientific audience. As Darwin was yet to publish anything on his ideas of natural selection, and as Hooker and Lyell regarded publication as crucial to protect Darwin’s priority, they arranged for a paper by Darwin and a paper by Wallace to be published jointly in the Proccedings of the Linnean Society. Wallace was then pursuing his work as a collector of natural history specimens in the Malay Archipelago. A letter would take months to reach him and he was out of touch, and so Wallace had no input into the decision making around the publication of his own paper (Browne, 2002:Chapter

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2). The scheme put into effect by Hooker and Lyell has since become sometimes known as “the delicate arrangement.” In late 1846, Airy had made his own “delicate arrangement” to lay out events from his own perspective and to try to explain and defend his own position, as well as calm angry critics in Britain and France. Nonetheless, some in Britain were irritated at what they regarded as the exaggerated claims made on behalf of Adams, while others were cross because the Cambridge search had not been prosecuted energetically enough. In France, Airy had drawn criticism for seeming to claim too much for Adams, despite Adams’s lack of any publication on the postulated planet prior to the optical find. The great historian of nineteenth century astronomy Agnes Clerke decided that the discovery of Neptune had led to a controversy that had “its ignominious side,” but we can now see that it was a controversy on matters generally judged to be important. We have seen too that the notion that Adams lost the honour of being acclaimed the mathematical discoverer of the planet because of scepticism and tardiness on the part of Challis and Airy is seriously misleading. This view, however, became widespread, and Clerke, who knew Adams later in his life, reported that he “bore the disappointment which the dilatory proceedings at Greenwich and Cambridge had inflicted upon him, with quiet heroism. His silence on the subject of what another man would have called his wrongs remained unbroken to the end of his life; and he took every opportunity of testifying his admiration for the genius of Leverrier (Clerke, 1908:82).” Clerke, however, also put her finger on what the discovery meant in Britain in the middle of the nineteenth century for what was widely regarded as the paradigmatic scientific law, Newton’s Law of Universal Gravitation. As she wrote: Personal questions, however, vanish in the magnitude of the event they relate to. By it the last lingering doubts as to the absolute exactness of the Newtonian Law were dissipated. Recondite analytical methods received a confirmation brilliant and intelligible even to the minds of the vulgar, and emerged from the patient solitude of the study to enjoy an hour of clamorous triumph. For ever invisible to the unaided eye of man, a sister-globe to our earth was shown to circulate, in perpetual frozen exile, at thirty times its distance from the sun. Nay, the possibility was made apparent that the limits of our system were not even thus reached, but that yet profounder abysses of space might shelter obedient, though little favoured, members of the solar family, by future astronomers to be recognised through the sympathetic thrillings of Neptune, even as Neptune himself was recognised through the tell-tale deviations of Uranus (Clerke, 1908:82). Acknowledgements  This chapter is founded in part on a large body of primary material—most notably the Herschel Papers, the Whewell Papers, and various collections in the archives of the Royal Astronomical Society and the Royal Greenwich Observatory, now held in the University Library in Cambridge, and a substantial body of Neptune related material collected by the late Craig B. Waff and very generously made available by Trudy E. Bell. In writing this chapter I have benefited greatly from Roger Hutchins’s extensive, insightful and detailed study of British university observatories in the nineteenth century, an important section of which examines the discovery of Neptune. I also received very helpful comments and criticisms on earlier versions of this paper from Trudy E. Bell, Roger Hutchins, Carolyn Kennett and William Sheehan.

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References Adams, J., 1846a. Letter to G. Airy, 2 September. Quoted in Glaisher, 1896, xxvii. Adams, J. 1846b. Letter to G. Airy, 18 November. Quoted in Glaisher, 1896, xxx. Adams, 1846c. Memorandum for J. Challis, July 1846. Filed with papers related to January 1847, Cambridge Observatory Archives. Adams, 1846d. Letter to his parents, 15 October. Adams Papers, St. John’s College, Cambridge. Adams, 1847. Letter to A. Sedgwick, 15 June. Adams Papers, St. John’s College, Cambridge. Adams, J., 1855. Letter to J. Forbes, 28 June. Quoted in Craik., 2007. Adams, W., 1892. The Scientific Papers of John Couch Adams, vol. 1. Cambridge, Cambridge University Press. Airy, G., 1846a: Account of Some Circumstances Historically Connected with the Discovery of the Planet Exterior to Uranus. Monthly Notices of the Royal Astronomical Society, 7, 121–144. Airy, G., 1846b. Letter to W.  Whewell, 25 June. Whewell Papers, Trinity College Library, Cambridge. Airy, G., 1846c. Copy of Letter to U.  Le Verrier, 14 October. MacAlister Collection in Adams Papers, St. John’s College, Cambridge. Airy, G., 1846d. Letter to J. Challis, 9 July. Cambridge Observatory Archives. Airy, G., 1846e. Letter to J. Challis, 21 July. Cambridge Observatory Archives. Airy, G., 1846f. Letter to J. Challis, 6 August. Cambridge Observatory Archives. Airy, G., 1846g. Letter to R.  Sheepshanks, 19 June. Sheepshanks Papers, Royal Astronomical Society Archives. Airy, G., 1846h: Letter to J. Challis, 14 October. Cambridge Observatory Archives. Airy, G., 1846i. Letter to U.  Le Verrier, 21 October. Copy in Neptune File, Royal Greenwich Observatory Archives.  Airy, G., 1846j. Letter to A. Sedgwick, 8 December. Airy Papers, Royal Greenwich Observatory Archives. Airy, 1847. Letter to The Athenaeum. 20 March, 309. Airy, G., 1852. Address by George Biddell Airy…. Report of the Twenty-First Meeting of the British Association for the Advancement of Science; Held at Ipswich in July 1851, xxxix–liii. London, John Murray. Airy, G., 1896. Autobiography of Sir George Biddell Airy, W. Airy., ed.. Cambridge, Cambridge University Press. Anon, 1846a. Miscellanea. The Athenaeum, 12 September, 941. Anon, 1846b. Miscellanea. The Athenaeum, 13 June, 612. Anon, 1846c. New Planet Expected. Mechanics Magazine, 4 July, 21. Anon, 1846d. The New Planet. The Athenaeum, 21 November, 1191. Anon, 1846e. Our Weekly Gossip. The Athenaeum, 21 November, 1190–1191. Anon, 1846f. British Association for the Advancement of Science. The Times, 18 September, 6. Anon, 1847. M. Le Verrier. The Athenaeum, 2 January, 12. Anon, 1848. Neptune-Whether Neptune or Not. The Athenaeum, 4 November, 1102–3. Arago, F., 1846. Examen des remargques critiques et des questionesde priorité que la découverte de M. Le Verrier a soulevées. Comptes Rendus 23, 751–754. Following translation by Grosser, 1962, 132–133. Babbage, C., 1830. Reflections On The Decline Of Science In England, And On Some Of Its Causes. B. Fellowes, London. Babbage, C., 1847a. On the Planet Neptune and the Royal Astronomical Society’s Medal. Letter to the Editor, 15 March. The Times, 3. Babbage, C., 1847b. Letter to J. Adams, 12 February. Cited in Hyman, 1982, 212. Baum, Richard, 1996. William Lassell and ‘the accident of a maid-servant’s carelessness,’ or Why Neptune was not Searched for at Starfield. Journal of the British Astronomical Association, 106, 217–9. Biot, J., 1847. Report on “The New Planet.” The Athenaeum, 3 April, 371.

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Board of Visitors, 1846. Royal Observatory, Board of Visitors, Additional Manuscripts, meeting of 29 June, Royal Greenwich Observatory Archives. Brewster, D., 1847. Researches Respecting the New Planet Neptune. North British Review, 6:207–246. Browne, J., 2002. The Power of Place: Volume II of a Biography. New York, Alfred A. Knopf. Cannon, S., 1978. Science and Culture. New York, Science History Publications. Case, S., 2019. A “Confounded Scrape”: John Herschel, Neptune, and Naming the Satellites of the Outer Solar System. Journal for the History of Astronomy. 50, 306–325. Challis, J., 1846a. Letter to G. Airy, 18 July. Airy Papers, Royal Greenwich Observatory Archives. Challis, 1846b. Professor Challis’s First Report to the Cambridge Observatory Syndicate upon the New Planet, in Adams, W. 1896, xlix–liv. Challis, 1846c. Account of Observations at the Cambridge Observatory for Detecting the Planet Exterior to Uranus. Monthly Notices of the Royal Astronomical Society, 7, 145– 149. Challis, 1846d. Letter dated 1 October, Discovery of a New Planet Beyond Uranus. Cambridge Chronicle and Journal, and Huntingdonshire Gazette, 3 October. Challis, 1846e. Letter to F. Arago, 5 October. Printed in French translation, Comptes Rendus, 12 October, 715. Challis, 1846f. Letter dated 15 October. The Athenaeum, 17 October, 1069. Challis, 1846g: Letter dated 4 November. Copy in Neptune File, Royal Greenwich Observatory Archives. Challis, 1846h: Letter to G.  Airy, 19 December, Neptune File, Royal Greenwich Observatory Archives. Chapman, A., 1988. Public Research and Private Duty: George Biddell Airy and the Search for Neptune. Journal for the History of Astronomy, 19, 121–139. Chapman, A., 1998. The Victorian Amateur Astronomer. Independent Astronomical Research in Britain 1820–1920. Chichester, John Wiley. Clerke, Anges., 1908. A Popular History of Astronomy in the Nineteenth Century. 4th Edition. London, A. and C. Black. Craik, A., 2007. Mr. Hopkins’s Men. Cambridge Reform and British Mathematics in the 19th Nineteenth Century. London, Springer-Verlag. Csiszar, A., 2018. The Scientific Journal: Authorship and the Politics of Knowledge in the Nineteenth Century. Chicago, University of Chicago Press. Dewhirst, D., 1976. The Cambridge-Greenwich Axis. Vistas in Astronomy, 20, 109–111. Dewhirst, D., 2006. Letter to R. Hutchins, 6 October. Quoted in Hutchins, 2008, 100. Eggen, O., 1971. Challis, James. In Gillispie. C. ed., Dictionary of Scientific Biography, vol. 3, pp. 186–187. New York, Scribners. Forbes, J., 1846. Letter to W. Smyth dated 2 December. Cited in Craik, 2007, 179. Forbes, J., 1847a: Letter to B. Studer, 2 July. Cited in Craik, 2007, 182. Forbes, J., 1847b: Letter to Alicia Forbes, June 1847. Cited in Craik, 2007, 182. Glaisher, J., 1846. Le Verrier’s New Planet. Illustrated London News, 10 October, 230. Glaisher, J., 1896. Biographical Notice in. Adams, W., 1892, xv–xlviii. Gooding, D., 1985. ‘He Who Proves, Discovers’: John Herschel, William Pepys, and the Faraday Effect. Notes and Records of the Royal Society, 39, 229–244. Graves, R., 1885. Life of William Rowan Hamilton, vol. II. Dublin & London, Hodges, Figgis, and Co., and Longmans, Green, and Co. Grosser, M., 1962. The Discovery of Neptune. Cambridge, Mass., Harvard University Press. Hamilton, W., 1846. Letter to R. MacDonnell dated 7 October 1846. Copy in Graves, 1885, 529. Hankins, T., 1980. Sir William Rowan Hamilton. Baltimore, The Johns Hopkins University Press. Harrison, H., 1994. Voyager in Space and Time. The Life of John Couch Adams, Cambridge Astronomer. Lewes, The Book Guild. Herschel, J., 1831. A Preliminary Discourse on the Study of Natural Philosophy. Philadelphia, Carey and Lea.

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Herschel, J., 1845. “Address by Sir John Herschel, Bart., F.R.S. etc. etc.” Report of the Fifteenth Meeting of the British Association for the Advancement of Science; Held at Cambridge June 1845. Xxvii–xliv. London: John Murray. Herschel, J., 1846a. Letter to W. Whewell, 6 November 1846. Herschel Papers, Royal Society. Herschel, J., 1846b. Letter to R. Sheepshanks, 17 December. Herschel Papers, Royal Society. Herschel, J., 1846c. Le Verrier’s Planet. Letter dated 1 October, to The Athenaeum, 1019, issue of 3 October. Herschel, J., 1846d. Diary entry for 25 October. Herschel Papers, Royal Society. Herschel, J., 1846e. Letter to W. Hamilton dated 19 November. Copy in Graves, 1885, 528. Herschel, J., 1846f. Letter to W.  Whewell dated 25 December 1846. Herschel Papers, Royal Society. Herschel, J., 1846g. Letter to R. Jones dated late 1846. Herschel Papers, Royal Society. Herschel, J., 1846h. Letter to The Guardian, 25 October. Copy in volume of clippings, Herschel Papers, University of Texas, Vol. V, 648. Herschel, J., 1846i. Letter to The Times dated 21 September, 6. Herschel, J., 1848. The Planet Neptune. Letter dated 11 November to The Athenaeum, 25 November, 1176. Herschel, J., 1849. Outlines of Astronomy. London, Longman, Brown, Green and Longmans. Hind, J., 1846a. Letter to J. Challis, 16 September. Cambridge Observatory Archives. Hind, J., 1846b. Letter to J. Challis, 30 September. Cambridge Observatory Archives. Hind, J., 1846c. Letter to the Editor, 1 October. The Times, 8. Hind, J., 1846d. Letter to J. Adams, 30 September. Adams Papers, St. Johns College, Cambridge. Hind, J., 1846e. Letter to R. Sheepshanks, 12 November. Sheepshanks Papers, Royal Astronomical Society Archives. Hort, F., 1846. Letter to his Father, 12 November. Cited in Harrison, 1994, 34. Hubbell, J., and Smith, R., 1992. Neptune in America: Negotiating a Discovery. Journal for the History of Astronomy, 30, 261–291. Hutchins, R., 2004. Hind, John Russell (1823–95). Oxford Dictionary of National Biography. accessed online 2 December 2019. Hutchins, R., 2008. British University Observatories 1772–1939. Aldershot, Ashgate. Hyman, A., 1982. Charles Babbage. Pioneer of the Computer. Princeton, Princeton University Press. Inglis, H., 1848. Address by Sir Robert Harry Inglis …. . Report of the Seventeenth Meeting of the British Association for the Advancement of Science; Held at Oxford in June 1847. Xxix–xlvi. London, John Murray. Jacobi, C., 1848: Auszug eines Schreibens des Herrn Professors C.G.I. Jacobi, Mitgliedes der Berl. Akad. D.W. 28, 43–6. Kent, D., 2011. The Curious Aftermath of Neptunes Discovery. Physics Today, 64, 46–51. Kragh, H., and Smith, R., 2003. Who Discovered the Expanding Universe. History of Science, 41, 141–162. Lagemann, R., 1956. Pseudonyms of Physicists. The Scientific Monthly, 83, 130–134. Lequeux, J., 2016. François Arago. A 19th Century French Humanist and Pioneer in Astrophysics. Dordrecht, Springer. Le Verrier, U., 1846a. Sur la planete qui produit Les anomalies observees dans le mouvement d’Uranus: Determination de sa masse, de son orbite, et de sa position actuelle. Comptes rendus de l’Académie des Sciences, 23, 428–438. Le Verrier, U., 1846b. Letter to G.  Airy, 16 October. Royal Greenwich Observatory Archives, Cambridge. Le Verrier, U., 1847. Note presentée par M.  Le Verrier. Comptes Rendus de l’Académie des Sciences, 24, 529–531. Maunder, E., 1900. The Royal Observatory…. London, The Religious Tract Society. Miller, D., 1983. Between Hostile Camps: Sir Humphrey Davy’s Presidency of the Royal Society 1820–1827. British Journal for the History of Science, 16, 1–47.

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Miller, D., 2004. Discovering Water. James Watt, Henry Cavendish and the Nineteenth-Century Water Controversy. Aldershot, Ashgate. Pannekoek, A., 1953. The Discovery of Neptune. Centaurus, 3, 126–137. Peirce, B., 1846–8a. Notice of the computations of Mr. Sears C. Walker, who found that a star was missing… Proceedings of the American Academy of Arts and Sciences, 1, 57–65. Peirce, B., 1846–8b. [Report on Peirce’s Investigation of the Action of Neptune on Uranus]. Proceedings of the American Academy of Arts and Sciences, 1, 332–42. Peirce, B., 1847. Extract from a Letter from Prof. Peirce. Sidereal Messenger, 1, 85–6. Rawlins, D., 1984. “The Discovery of Neptune: Essential Revisions to the History,” Bulletin of the American Astronomical Society, 16, 734. Schaffer, S., 1986. Scientific Discoveries and the End of Natural Philosophy. Social Studies of Science, 16, 387–420. Secord, J., 2018. Mary Somerville’s Vision of Science. Physics Today, 71, 46–52. Sedgwick, A., 1846. Letter to G. Airy, 9 December. Airy Papers, Royal Greenwich Observatory Archives. Sheepshanks, R., 1846. Letter to J. Herschel dated 26 December. Herschel Papers, Royal Society. Sheepshanks, R., 1847a. Letter to H.  Schumacher dated 7 April 1847. Quoted in Glaisher, 1896, xxxii. Sheepshanks, R., 1847b. A Reply to Mr. Babbage’s Letter to “The Times” “On the Planet Neptune and the Royal Astronomical Society’s Medal.” London, privately published. Sheepshanks, R., 1847c. Letter to H.  Christian, 7 April. Quoted in Glaisher, Biographical Notice, xxxiii. Shoemaker, P. S., 1991. Stellar Impact: Ormsby MacKnight Mitchel and Astronomy in Antebellum America. Dissertation. Madison, University of Wisconsin. Smith, R., 1983. William Lassell and the Discovery of Neptune. Journal for the History of Astronomy, 14, 30–33. Smith, R., 1989. The Cambridge Network in Action: The Discovery of Neptune. Isis, 80, 395–422. Smyth, W., 1846. Letter to J. Herschel, 2 November. Herschel Papers, Royal Society. Smyth, W., 1860. The Cycle of Celestial Objects Continued at the Hartwell Observatory to 1859; With a Notice of Recent Discoveries, Including Details from the Aedes Hartwellianae. London, J.B. Nichols. Sparks, J., 1847. Letter to B.  Peirce, 19 March, Peirce Papers, Houghton Library, Harvard University. Stokes, G., 1846. Letter to W. Thomson dated 10 November. Copy in Wilson, 1990, 4. Tennant, J., 1892., Letter to W. Christie, 23 January. Quoted in A. J. Meadows, Recent History (1836–1975), Vol. II of Greenwich Observatory: One of Three Volumes by Different Authors Telling the Story of Britain’s Oldest Scientific Institution: The Royal Observatory at Greenwich and Herstmonceux, 1675–1975 (London: Taylor & Francis, 1975), p. 114. Thomson, W., 1846. Letter to G. Stokes, 25 October. Copy in Wilson, 1990, 2. Whewell, W., 1833. Address. Report of the Third Meeting of the British Association for the Advancement of Science; Held at Cambridge, xi–xxxii. London, John Murray. Whewell, W., 1847: History of the Inductive Sciences …, new ed., rev. and continued, Vol. II. London, John W. Parker. White, W., 1846. Diary Entry for 14 November, in White, 1898. White, W., 1898. The Journals of Walter White. London, Chapman and Hall. Wilson, D., 1990. The Correspondence Between Sir George Gabriel Stokes and Sir William Thomson, Baron Kelvin of Largs: Volume 1 1846–1869. Cambridge, Cambridge University Press.

Chapter 8

Neptune Examined: William Lassell, a Satellite, and Neptune’s “Ring” Robert W. Smith and Richard Baum

Abstract  One ‘discovery’ about Neptune proved to be nothing of the sort. William Lassell, one of the foremost British astronomers of the nineteenth century, rose to fame during the months after the telescopic finding by discovering Triton, Neptune’s chief moon—important for calculating the planet’s mass and its influence on Uranus. But another feature also aroused Lassell’s suspicions; the telescopic image of Neptune suggested to his eye the presence of a ring system. This time Lassell, as he would later come to accept, was wrong. Lassell’s notebooks held at the Royal Astronomical Society and other archive materials reveal that Lassell’s ring resulted from an issue or issues with his telescope, possibly astigmatism that originated in flexure of the speculum-metal mirrors of his 24-inch reflector. Indeed, the history of Neptune’s supposed ring system is a well-documented example of instrumental failure that provides rich insights into astronomical practice during the mid-nineteenth century as well as the great interest in observing Neptune following its discovery.

8.1  A Satellite A brewer by trade in the booming port city of Liverpool, self-taught and highly motivated as an astronomer, William Lassell (1799–1880) belonged in the words of John Herschel “to that class of observers who have created their own instrumental means—who have felt their own wants, and supplied them in their own way” (Herschel, 1849: 87). The prominent astrophysicist William Huggins (1824–1910) even went so far as to compare Lassell’s achievements, especially as a telescope maker, with those of the elder Herschel and Lord Rosse (Huggins, 1881: 188). Today, Lassell is remembered for his bold and determined construction of two R. W. Smith (*) Department of History and Classics, University of Alberta, Edmonton, AB, Canada e-mail: [email protected] R. Baum (Deceased) London, UK © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_8

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large-aperture, equatorially mounted reflecting telescopes, a 24-inch (61-cm) completed in 1846 that he took a few years later to Malta, and a 48-inch (122-cm) also erected at Valetta, Malta, in 1861, and for the discoveries of faint satellites of Saturn, Uranus and Neptune, as well as hundreds of nebulae by his assistant Albert Marth (Chapman, 1988). “We must remember that Lassell’s 24-inch was a truly formidable instrument in 1846,” wrote Allan Chapman, an authority on Lassell, “for in addition to the superb steam powered machine figured and polished mirror, better than the Herschel and probably Rosse mirrors, the whole telescope was the first big reflector to be mounted upon a large all-iron equatorial that could be adjusted with the precision of a watch; probably part-made by Nasmyth” (Chapman, 2019). The telescope’s tracking was both very stable and precise, and following an object with the 24-inch was a simpler business than with the altazimuth mounted reflecting telescopes of William Herschel or Lord Rosse (Fig. 8.1). When J. R. Hind announced the discovery of Neptune to British readers in a letter to The Times of 1 October it drew Lassell’s immediate attention (Hind, 1846: 8). He inserted a cutting of Hind’s letter in his current observing diary, determined no doubt to observe the new planet at the earliest opportunity (Lassell, 1846a). That Fig. 8.1  William Lassell, photographed by Henry Joseph Whitlock in the 1860s. Credit: National Portrait Gallery, London.

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same day, 1 October, John Herschel penned a short note to Lassell. Herschel knew that Lassell had recently built a 24-inch reflector, effectively the most powerful telescope in England, and had installed it at Starfield, his aptly named residence near Liverpool. He also knew that Lassell had overcome some teething troubles in the telescope’s mounting, and had recently been observing the satellites of Saturn. It is thus not surprising that after informing Lassell of the find and giving a position, Herschel should have urged Lassell to look for “satellites with all possible expedition!!” (Herschel, 1846). On 2 October, Lassell swung his great telescope to the appointed spot and logged his first observation of what he referred to then as “Le Verrier’s planet.” He found the planet easily, and saw its disk without any difficulty. Yet, as he confided in his diary the next night, what he saw was not a disk alone: “I observed the planet last night the 2nd and suspected a ring… but could not verify it. I showed the planet to all my family and certainly tonight have the impression of a ring….” (Lassell 1846b). Several days elapsed before Lassell again examined Neptune, possibly due to inclement weather or his heavy business commitments. His next observation was made on 10 October. Now he noted: “I see a satellite or a suspicious looking star and a ring. I see the [satellite] and suspect the ring with various powers 316 to 367….” (Lassell, 1846c). We will continue the story of the ring in the next section, but here it is important to note that through further observations in the autumn and early winter of 1846 Lassell grew increasingly confident of the satellite’s existence (Fig. 8.2). By July 1847, he was certain Neptune possessed a satellite. Many years later it was christened ‘Triton,’ a name first proposed by the French astronomer Camille Flammarion. The satellite was an especially significant find because it enabled astronomers to estimate Neptune’s mass. To do this, they required measurements of the size of the semi-major axis of the satellite’s orbit, as well as the period of its revolution around Neptune. Determining the size of the orbit, however, was not straightforward due to the difficulty of placing the wires of the micrometer on the faint satellite. As Cruikshank and Sheehan have pointed out, Harvard mathematician Benjamin Peirce calculated the planet’s mass to be about four-fifths the mass of Uranus, while Friedrich Georg Wilhelm von Struve at Pulkova reckoned it to be about three-fifths. Neptune, then, was decidedly less massive than Uranus, and “much less massive than expected—Le Verrier, from his analysis of the perturbations of the unknown on Uranus, had projected a planet almost 3 times as massive as Uranus. (In fact, as we now know, the early estimates of Neptune’s mass were much too small; according to the latest results, Uranus’s mass is 14.536 times that of the Earth, that of Neptune a little more, 17.148 times that of the Earth” [Cruikshank and Sheehan, 2018: 76]).

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Fig. 8.2  Report of observations by Lassell of Neptune’s satellite and ring from the Monthly Notices of the Royal Astronomical Society in 1847. The drawings of the planet and satellite refer to observations made from August 31 to September 6, 1847. (Public Domain).

8.2  Discovery and Confirmation of a Neptunian Ring Lassell was certain of the satellite’s existence by the summer of 1847. The ring, however, proved to be more challenging, and to follow his changing positions on whether or not there was indeed a ring around the planet we need to return to the autumn of 1846. When he viewed Le Verrier’s planet on 26 October he was “pretty well assured of the close satellite,” but his drawing omits the ring (Lassell, 1846c). Three days later he observed in the company of John Hartnup (1806–1885), Director

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of the Liverpool Observatory, and William Simms (1793–1860), the well-known London instrument maker. Unfortunately, the conditions were too poor to see the planet well. A more significant set of observations followed on 10 November. Once more with company, Lassell aimed the 24-inch at Neptune: Mr. Hartnup, Mr. Ice[?] and Mr. A. King and Mrs. Kearsley observed Le Verrier tonight-­ power 316—Mr. H. and Mr. A.  King drew independent diagrams of the position of the supposed ring which were quite accordant without any previous prejudice or information. There is therefore no doubt whatever that the telescope shews an appendage. Whether it really exists in the planet remains to be confirmed (Lassell, 1846d).

“The sky in a most tremulous and muddy state,” Lassell wrote the day after, but Le Verrier still gave to me the impression of a ring though less distinctly than before—as might be expected from the unfavourable circumstances. This may moreover be said to be a new telescope as both mirrors are ... different. My daughter Maria not having seen the planet with any suspicion of a ring drew a diagram of it tonight having the same aspect as the previous [diagram]. The supposed satellite is in the same situation tonight (Lassell, 1846e).

Further inspections on 29 and 30 November, and 3 and 15 December, did nothing to shake his near conviction that a satellite accompanied Neptune, and his very strong suspicion of a ring. Bad weather and the rapidly diminishing distance of the planet from the Sun brought the 1846 Neptune season for him to an end on 15 December, and it was the following July before Lassell could again inspect the planet. So far we have examined only the notebook record. Lassell had in fact quickly made public what he believed he had detected around Neptune. On 12 October he had staked his claims to the discovery of both the satellite and the ring by writing to The Times (a common method of the period to establish priority). Here, if anything, he seemed more certain of the ring than he had confided to his private record: With respect to the existence of the ring, I am not able absolutely to declare it, but I received so many impressions of it, always in the same form and direction, and with all the different magnifying powers, that I feel a very strong persuasion that nothing but a finer state of atmosphere is necessary to enable me to verify the discovery. Of the existence of the star, having every aspect of a satellite, there is not the shadow of a doubt (Lassell, 1846f: 7).

Another account was sent to the Royal Astronomical Society and reported in the Society’s Monthly Notices for November 1846. The writer noted that Lassell had observed the planet with his 24-inch reflector and had seen “something like a ring crossing its disc,” and that while atmospheric conditions had not been favourable, the streak was seen in the same direction using two different mirrors and by several observers, so its existence “seems very probable. The appearance may possibly be caused by some distortion or flexure in the mirror, but can scarcely be the effect of imagination” (Anon., 1846a: 157) (Fig. 8.3). In the following issue of the Monthly Notices Lassell was reported as writing that that the planet appeared like a fainter version of Saturn as seen through a small telescope with a low power. Further, “several persons saw the supposed ring, and all in the same direction, as shown by independent diagrams” (Anon., 1846b: 167). Lassell’s status as a good observer (although at this point relatively little

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Fig. 8.3  Lassell’s November 1846 report of a ring published in the Astronomische Nachricten. (Public Domain).

experienced with very large telescopes), and the fact that his 24-inch telescope had the greatest light grasp of any in England, imbued these statements with authority, and to other astronomers they surely made convincing reading. When once an object is reported as seen, especially by an authority of stature, expectation often lends a hand to supporting its confirmation. And indeed that support was soon evident. J. R. Hind and James Challis, both of whom of course as we saw in the last chapter had made unsuccessful searches for Neptune in the summer of 1846, made observations that went a considerable way to providing full and unequivocal verifications of Lassell’s findings. Hind had first seen the disk of the planet on 30 September, the very day on which he had received news of the

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discovery. He had then proceeded to observe the planet whenever conditions allowed. Hind, director of George Bishop’s private observatory in London, used a 7-inch (18-cm) Dollond refractor in his examinations. Nevertheless, it was December 1846 before he noticed anything peculiar about the planet’s disk. As he then described it: “The existence of a ring appears to be as yet undecided, though most probable. Le Verrier presents an oblong appearance in Mr. Bishop’s refractor” (Hind, 1847: 207). Given the wide separation of the satellite from Neptune, it seems very unlikely that Hind had observed the unresolved combination of Neptune and Triton. Shortly after the optical discovery was announced from Berlin, James Challis, Director of the Cambridge University Observatory, had begun a regular series of observations of the new planet. At the time, the urgent need was not physical observations but to secure positional data to establish more precisely the planet’s orbit. However, on 12 January 1847, Challis received his first impression of a ring. Viewed through the 11¾-inch (30-cm) Northumberland refractor the image assumed an appearance “such as would be presented by a ring like that of Saturn, situated with its plane very oblique to the direction of vision.” I felt convinced that the observed elongation could not be attributed to atmospheric refraction, or to any irregular action on the pencils of light, because when the object was seen most steadily I distinctly perceived a symmetrical form. My assistant, Mr. Morgan, being requested to pay particular attention to the appearance of the Planet, gave the same direction of the axis of elongation as that in which it appeared to me (Challis, 1847a: 28).

Two days later, Challis saw that the “ring is very apparent with a power of 215, in a field considerably illumined by a lamp-light. Its brightness seems equal to that of the Planet itself” (Challis, 1847a: 310). Given the brightness of the ring and the symmetrical form, it seems very unlikely he could have seen the unresolved combination of Neptune and the faint (magnitude 13.5) Triton. Lassell was of course delighted by the news. Despite Challis’s failure to discover Neptune in a timely fashion, he was widely regarded as a highly competent practical astronomer, so his opinion carried weight. On 19 January, Lassell told Challis that this observation “puts beyond reasonable doubt the reality of mine; especially as even your measured angle of position agrees with my estimation within four degrees” (Lassell, 1847a). Challis also wrote up his observation for the Monthly Notices of the Royal Astronomical Society and Schumacher’s Astronomische Nachrichten, declaring that he had verified Lassell’s suspicions about a ring (Challis, 1847b). In fact, Challis was to tell Lassell that the ring “is not difficult to discern with the Northumberland telescope with powers of 215 and 200, and once on taking a transit with the former power, and with a field pretty well illumined the Ring appearance so forced itself upon me that I was diverted from my transit” (Lassell, 1847b). Challis and Hind were not alone in finding something odd in the telescopic appearance of the new planet. Writing in December 1846, Ormsby MacKnight Mitchel, Director of the Cincinnati Observatory and editor of The Sidereal Messenger, reported that Matthew Fontaine Maury (1806–1873) at the U.S. Naval Observatory suspected he had seen points of light above and below the planet

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(Mitchel, 1846: 60). In January 1847, Lassell told Challis that he had “heard somewhat indefinitely” that the Italian astronomer Francesco de Vico (1805–1848), director of the Collegio Romano Observatory and who had access to a 6.5 inch refractor, thought he had detected two appendages to the planet, perhaps a ring or cluster of satellites (Lassell, 1847a) Later, in October 1847, William Cranch Bond (1789–1858), using the 15-inch (38-cm) Harvard College Observatory refractor to observe Neptune’s satellite and to measure its separation from the planet (the values were around 15″), found the planet elongated when viewed with low powers, but the elongation vanished when high magnifications were used (Bond, 1848). De Vico’s and Maury’s evidence, however, was too vague to be of much use in the debate one way or the other in 1847 (Baum, 1973: 134–9). Nevertheless, Lassell, Challis and Hind were a formidable company, and by early 1847 opinion in Britain had swung in favour of the ring’s existence, and the Annual Report of the Royal Astronomical Society, dated 12 February 1847, announced that the reality of the “ring seems almost certain” (Anon., 1847: 217).

8.3  The Fading Vision The air of certitude, however, dissipated within months. Indeed, the reality of the ring’s existence was being questioned, ironically, by a close friend of Lassell’s, the well-known double-star observer the Reverend W. R. Dawes, who first expressed doubt. Writing to Challis on 7 April 1847, Dawes regretted that he could not (Fig. 8.4) quite comprehend about the ring. Mr. Lassell has not I think distinctly mentioned in any of his published statements in what direction the ring lies. He merely says that it is “about 70° from the parallel of declination”: but whether from north following to south preceding, or from south following to north preceding he does not state. In private letters to myself however he has given me several diagrams from time to time, in which the inclination of the ring has always been drawn from south following to north preceding . . . or in double star phraseology, the angle of position is given [as] 160° and 340°. If this is what Mr. Lassell has observed it is at variance with your observation (Dawes, 1847a).

This letter would prove to be the turning point, and stock in the ring began to fall. Notably, even Challis was soon confiding to Dawes that he doubted the existence of the ring, and that he regarded it likely as an illusion for which atmospheric effects were to blame (Dawes, 1847b). In the meantime, Lassell himself had cleared up the mystery about the ring’s orientation, as Dawes related to Herschel: Lassell informs me that when he drew the position of the ring . . . in his letters to me, the diagram was taken from its appearance in the telescope without respect to the direction of the parallel;-that of the direction being rather referred to. At first he contented himself with noting that it was nearly at right angles to the diurnal motion: but in his later observations he has expressly depicted it [south preceding to north following]. So that he and Challis agree—Hind draws it [south following to north preceding]; but this of course is of little weight, as well as my own seeing no elongation at all (Dawes, 1847b).

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Fig. 8.4  Part of Dawes’s 7 April 1847 letter to Challis. The drawing of the ring was based on information Dawes received from Lassell. Credit: From Richard Baum, The Planets, p. 135.

Lassell’s information did little to reconcile Challis to the ring’s reality, for on 3 May he again wrote that the ring’s existence was not certain. Adopting the same reasoning as Dawes, Challis was concerned that initially Lassell had stated the ring’s axis to be nearly coincident with a right angle to a parallel of declination, but had later placed it 20° from this position, then about 25° distant. It had struck Challis that so “good an observer as Lassell could not be so much mistaken as to position in his first observation, and that what we see is something due to the refraction of the atmosphere and consequently changeable in position. It seems nevertheless to be an appearance peculiar to this planet” (Challis, 1847c). The most urgent need, Challis judged, was for more observations, and he was anxious to get after them. Lassell agreed. His first chance in 1847 to re-observe Neptune under reasonable conditions came on 7 July. He received “just the same impression of a ring very nearly at right angles to the parallel [of declination]...the south end of the ring a little preceding” (Lassell, 1847c). As usual, the state of the atmosphere at Starfield prevented any immediate follow-up. Even so, in August Lassell told Herschel he had not “thrown overboard the ring of Neptune—on the other hand I have the same impression of its existence as last year” (Lassell, 1847d). About this time, Lassell lamented to John Pringle Nichol (1804–59), Regius Professor of Astronomy at Glasgow University, that he had been “invariably foiled” by the Liverpool climate in his battle to observe Neptune. He was very anxious to publish any “facts”

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respecting Neptune, but he was equally determined not to deceive himself or others (Lassell, 1847e). To a correspondent on 8 September, Lassell again complained that “the sky this year has never been such as to give reasonable hope of a position and absolute settlement of the question [of the ring]. For myself I can only say that some glimpses I have had tend to strengthen my conviction of its existence” (Lassell, 1847f). That very same day, Lassell viewed Neptune in the company of Dawes, who was in the middle of one of his frequent visits to Starfield. Now, for the first time, Dawes too saw the ring. In Lassell’s observing diary is a drawing of the planet showing a ring and it is initialed “W. R. D.” for William Rutter Dawes. Alongside is the comment “south preceding or [north following] perhaps by 10°” (Lassell, 1847g). More light is shed on these observations by Dawes’s own observing book. His entry for 8 September reads: “The planet considered by W.R.D. to be elongated. A measure of the direction of the elongation 73° … but considered too large an angle by 10°. Clouded before measures [could] be completed” (Dawes, 1847c). Five days later Dawes measured the position of what he termed a ring or observed elongation as 81°·82, south preceding, and on the next night, 14 September, he valued it at 87°·90 south preceding. In an attempt to check that the elongation was not the product of an instrumental defect, the telescope was rotated 30° on its axis and the measurement repeated. Now the answer was 88°·60 south preceding, a figure consistent with his earlier result and thus, in Lassell’s view, indicative of the ring’s reality. Lassell, his own beliefs fortified by what Dawes had seen, wrote to The Times that he had several times noticed the same appearance as he had in 1846. He had repeatedly measured the projections or supposed ansae (handles) of the ring, finding them to form an angle with the parallel of declination of about 70° south preceeding. However, he admitted the planet’s low altitude and the poor quality of the atmosphere had hitherto prevented him ascertaining the ring’s precise nature and form (Lassell, 1847h:7). Thus Lassell continued to hang fire, though as far as one can judge, he was inclining again toward belief in the ring’s existence. But Challis by now was a skeptic. In 1848, reporting another observation of the ring, he revealed he was now seeking an instrumental rather than an astronomical explanation: . . . in the autumn of 1847 I several times saw the appearance which I suppose to be a Ring of Neptune, both before and after opposition, and when the Planet was not far from the meridian; and the axis of elongation constantly had the same position relatively to a parallel of declination as when noticed at the beginning of the year. In the present year (1848) I have had occasion to take out the object-glass from its cell, and re-adjust the two lenses: so that at the next opposition of the Planet, I shall be able to ascertain whether the appearance noticed is dependent on the relative positions of the lenses, one of which is now turned about the axis of the Telescope into an entirely different position relatively to the other (Challis, 1848: 20).

Lassell, too, attempted to repeat his earlier observations, but the year passed without him reaching a definite conclusion. In part this was no doubt because of distractions. In September Lassell, independently of Bond at Harvard, had discovered Hyperion, a new moon of Saturn (Lassell, 1848). As a result, Saturn was thrust into the

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forefront of attention, and throughout the autumn of 1848, despite its being then in the best position for observation, Neptune was pushed to the side. Lassell seriously renewed his campaign on Neptune the next year, 1849. In August he was observing in the company of James Nasmyth (1807–1890), a brilliant practical engineer and a highly skilled telescope maker, who had been closely involved in the building of Lassell’s 24-inch telescope. In addition to his engineering skills, Nasmyth was a first-rate draughtsman and observational astronomer. On this occasion Lassell “received as strong an impression of the ring as at any previous view” (Lassell, 1849a). Nasmyth too saw a ring and entered his independent drawing into Lassell’s notebook; the inclination of the ring agreed with that indicated by Lassell. A few days later, Lassell spent an unproductive half-hour studying Neptune, then, the following night, 4 September, observed the planet as soon as it became visible. No ring was to be seen. The capricious manifestation had begun to haunt him. No fewer than fifteen other observations of Neptune are entered in Lassell’s notebook for the autumn and winter of 1849. Not one contains a mention of the ring. As most were made in poor conditions this is perhaps not surprising. For example, on 27 November Lassell complained that the sky was “excessively muddy and [the] planet a mass of indistinct confusion—nothing could be done with it” (Lassell, 1849b). Nor did anything concrete emerge from the 1850 observing season. The following year, Lassell referred to the ring in his public report of “Observations of Neptune and his satellites,” but by then he had clearly given it up. On 26 August, he noted only “an evident appendage, such as I used to take for a ring, but more evident on the south side, nearly at right angles with the parallel of motion. The same appearance remained with 366, but scarcely so striking, and it vanished with 614…The sky, however, was very variable, and clouds and haze coming on prevented my obtaining any measures, even of the known satellite” [emphasis added]. Three nights later he “received again an impression of a ring-like appendage, but it is principally on the south side, and is nearly at right angles to a parallel of declination” (Lassell, 1851a, b: 155). On 12 September, Lassell took advantage of a spell of exceedingly fine observing weather (four nights in a row!) to again examine the planet. His drawing hints at an appendage, but no more. Further observations followed, but, from the evidence of his notebooks, none disclosed a ring. The deterioration of Starfield as a site from which to conduct astronomical observations, caused through the encroachment of buildings and by the indifferent seeing generally encountered, persuaded Lassell to seek out a better site for an observatory. Accordingly in the autumn of 1852 he dismantled and transported his telescope to the island of Malta in the Mediterranean. Here he enjoyed observing conditions better than he had ever experienced in England, and his first excited reports gave Nasmyth reason to expect a definite pronouncement about the ring (Nasmyth, 1852). It was not to be. Lassell, as he remarked in a report in the Monthly Notices, found Neptune generally spherical and well defined even with his highest power, although he did record on 4 November 1852 “a decided impression of ellipticity in the direction of the greatest elongation of the satellite,” and, more significantly, “an impression of an extremely flattened ring in the direction of the transverse axis. I think I

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have never seen Neptune so well before” (Lassell, 1853: 38). But he refrained from judging this as evidence for the ring. Instead he confessed “yet I suspect some illusion,” a far cry indeed from the confident claims of 1846 and 1847. It had come and gone like Hamlet’s ghost, of which it could be said, “`Tis here! ‘Tis here! ‘Tis gone!” We are not at the end of the story, however. Lassell’s notebooks reveal how on the very first night of observing in Malta, 5 October 1852, he saw Neptune accompanied by a ring. A little more than a month later “the indication of the supposed ring immediately struck” Lassell. Its inclination was about 65°, south preceding. The next evening, the ring was `remarkably strong’ with a power of 219; the inclination was 80° south preceding, a shift of 15° from the previous night. However, with a power of 1018 “there was the smallest possible suspicion of the ring together with some elliptic form of the planet-both in the direction of the planet’s satellite-query whether not both due to the telescope”. In a further inspection that night Lassell “found the position of the supposed ring was south preceding and north following” (Lassell, 1852a). He also suspected that Uranus too appeared elliptical. These observations had secured his doubts and on 15 December he firmly and decisively rejected the ring. In what proved to be a crucial test for Lassell, he again measured the ring’s position “to ascertain whether it depended at all upon the telescope.” This test meant once more rotating the telescope tube and repeating his measurements. Unlike his earlier results when he had rotated the axis of the telescope tube, his results now differed widely and were 60°·49, 46°.19 and 76°·45: “It is thus evident that the phenomenon keeps a constant angle with the direction of the telescope and not at all with the parallel [of declination], proving that whatever may be the cause it is more intimately related to the telescope than the object” (Lassell, 1852b). It is puzzling why Lassell got such different results with this test compared to earlier ones, but the key point is that he had spent over six years trying to verify beyond doubt the reality of the ring, and now, in the end, he conceded that the most likely cause was his own telescope. Despite the numerous observations he was still to make of Neptune, Lassell never again indicated or even suggested the planet was encircled by a ring system analogous to that of Saturn.

8.4  What Happened? Shortly after his discovery of Uranus, William Herschel believed he had evidence to suggest that the planet was surrounded by a ring. It is now generally accepted he was mistaken (as he himself came to believe), and that the defect was a product of his adoption of the front-view arrangement for his ocular (Baum, 1973: Chapter 5). Such an explanation, even if correct, does not apply to what was allegedly seen about Neptune. It fails to account for the reports of Challis and Hind since both used refractors; nor did Lassell employ the front-view system in his telescope. What was generally regarded as definitive proof of a ring system around Neptune was provided by the Voyager 2 spacecraft in 1989. But given the present state of knowledge, it seems quite impossible that Lassell detected an objective feature as

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the ring system is far too faint for Lassell to have observed it. How, then, do we make sense of the ring observations? It has been proposed that Lassell’s observations could have sprung solely from a predisposition to see a ring (Hetherington, 1979: 27). Certainly it is today recognized that prior beliefs and commitments can have a very deep influence on observations and observational practice, and that directly observed experience may be transformed in the mind of the observer by consciously or unconsciously held preconceptions. But what might have persuaded Lassell to ‘see’ a ring in the first instance? Saturn possesses rings, so a priori it was not unreasonable to suppose that Neptune might have a system. More importantly, Lassell may have confused John Herschel’s urging to search for satellites with one to examine the planet for satellites and rings. We thus discover Lassell writing to Herschel on 12 October 1846 that “I am obliged by your note directing my attention to the possible ring and satellites of Le Verrier’s planet” (Lassell, 1846g). Herschel’s letter pressing Lassell to look for satellites was written on 1 October, and given the speed of the British postal system of the period, it perhaps arrived before Lassell made his first observation of the new planet on 2 October. Nevertheless, it is clear that sometime between the arrival of Herschel’s letter and 12 October, when Lassell replied, Lassell transmuted Herschel’s remarks about satellites to include satellites and rings, and was thereby given additional justification for his suspicion of the ring’s existence. Unfortunately, there is no definite evidence about whether Lassell made this shift before going to the telescope on 2 October. Hence we are in no position to state with certainty how much he was predisposed to ‘see’ a ring. Yet Lassell was aware of the danger of preconceptions. For example, we find him in 1847 writing to a correspondent about examining Neptune in the company of Dawes: “... Mr. Dawes I believe may have a word to say upon the subject [of Neptune’s ring] but I fancy would rather not say it until he has had the chance of better circumstances—indeed we have scarcely spoken upon the subject being desirous that all observations should be quite independent and free from all bias” (Lassell, 1847f). Naturally, awareness of the possible influence of preconception is hardly a guarantee that observations will be unaffected. Furthermore, as Dawes already knew of the ring and of what had been seen previously by Lassell and other users of the 24-inch as well as by Challis and Hind, Lassell’s remarks seem a little naive. It is thus not entirely beyond belief that preconception alone could have caused Lassell to ‘see’ a ring, and that, in the manner of a string of falling dominoes, several other observers fell into line after Lassell’s initial push. There is, however, overwhelming evidence that Lassell’s telescopes suffered sometimes from troublesome optical defects. Indeed, we shall argue that a much more satisfactory explanation of Lassell’s ring observations is to be found in a combination of optical defects and preconceptions. Lassell himself eventually concluded that faults in his telescope most probably were to blame. He was an experienced observer and knew from the first the danger of mistaking telescope defects for a real feature. But his doubts had been allayed because he was able to see the ring using a wide range of oculars, two different specula (each of the same

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dimensions), besides various flats. He was also fond of substituting the flat by a flint-glass prism by Merz. For instance, on 11 November 1846 he changed both the primary speculum and the flat. He had seen the ring the previous night, but even with the `new telescope’ it was still visible (Lassell, 1846h). The ring had also been seen by other users of Lassell’s telescopes. Amongst these was John Hartnup, director of the Liverpool Observatory and a very experienced practical astronomer, in addition to the renowned “eagle-eyed” Dawes. Moreover, had not Hind and Challis reported sightings akin to what the 24-inch had so often shown? Lassell was thus strongly encouraged to believe that something other than optical defects was at work; that, in short, he was observing an objective feature. As we have seen, his confidence did not last, and by 1851 Lassell was writing of that “evident appendage, such as I used to take for a ring.” His mounting doubts were apparent in Malta a year later when the ring pressed itself upon him once again. This time he adopted a more critical stance, exhibiting a scepticism derived of experience, and marking, if our argument is correct, a decided step in his development as an observer. For, while Lassell was by 1846 an experienced observer of many years standing, he, like nearly all of his contemporaries, had had relatively little opportunity to use a telescope the size of the 24-inch, a giant instrument for the time. Certainly his early hopes for and reports of the excellence of his telescope were somewhat exaggerated, and its performance, as is evident from his notebook entries, was more temperamental than one might believe on the basis just of the published papers, the product one must remember of a proud and very enthusiastic owner and builder. So while the 24-inch was a fine telescope, it, like other big reflectors of the period, possessed its idiosyncrasies and flaws (Smith, 1993). We shall now tum to what Lassell regarded as the most annoying of these. In 1850, Lassell reported that despite the use of lever systems he had continued to have problems supporting the primary speculum of the 24-inch without its flexing under its own weight. At one point he thought he had come close to solving the problem after he had slung the speculum in a hoop of thin iron, equal in length to half of its circumference, the ends of the hoop being attached to swivels fixed in each of the two horizontal brackets, and the lower part of the hoop being thus quite at liberty to rise and fall with the plates. This has nearly, if not entirely, removed all perceptible distortion, yet in some positions and under some circumstances vestiges of it are to be perceived (Lassell, 1850: 17).

Neptune was at declination −13° in 1846, and from Liverpool, even when on the meridian, was only 22° above the horizon. This situation changed little throughout the late 1840s and early 1850s. Lassell therefore had to observe the planet with the telescope tube far from the zenith, and from his words above we might reasonably infer, even without the copious evidence of his notebooks, that his examinations of Neptune were affected by his method of mounting the primary speculum. His introduction of a hoop to support his specula did not satisfy him for long and, again in 1850, he devised a new system of gravity operated levers to prevent his 24-inch mirrors from flexing. The innovative apparatus, he judged happily, totally removed the indications of flexure he had formerly seen, and on 14 May 1851 he used a new

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speculum with this lever system attached for the first time so that “I have no doubt from the vision I have of stars and Jupiter that the evil of the bending is entirely removed and therefore the object in view has been completely gained” (Lassell, 1851b). His optimism was to prove ill-founded. In October 1852, for example, we find him observing Saturn but again annoyed by what he termed “crossing,” that is, the slightly-out-of-focus images appearing elongated and at right angles to each other, thereby indicating an astigmatized image. To improve matters he had to resort to tricks learned from experience such as turning “the eyepiece very much up so as to throw the weight of the speculum in a great measure upon the western bracket and afterwards bring [the telescope] down slowly to a horizontal position [as a result of which] the annoyance nearly if not quite disappeared and the vision became remarkably fine” (Lassell, 1852c). It is, in fact, unlikely that any maker of a large speculum metal telescope ever fully mastered the problem of equably supporting the telescope’s mirrors in the 1840s and 1850s despite the use of various systems of levers and hoops. For example, in 1848 Airy described some observations he had made with Lord Rosse’s famous 72-inch (183-cm) reflector, the “Leviathan of Parsonstown” and so gives an extreme example of what he referred to as astigmatism: Upon directing the telescope to an object very near the zenith, it was seen very well-defined; or, at least, with no discoverable fault. It must be remembered that the image of a star never assumes the neat spherical form to which the eye of an observer with a fine refractor is so much accustomed…But when the telescope was directed to a star as low as the equator its image was very defective. The defect, however, followed that simple law which [William Whewell] has described by the word astigmatism. When the eyepiece was thrust in, the image of the star was a well-defined straight line, 20 seconds long, in a certain direction; when the eyepiece was drawn out a certain distance (about half an inch from the former position) the image of the star was a well-defined straight line, 20 seconds long, in a direction at right angles to the former… The position of the astigmatic lines had no distinct relation to the vertical plane; and this circumstance, as well as the magnitude of the astigmatism, proved that it was not produced by a tilt of the mirror (Airy, 1848: 118).

Here is an excellent description of the sorts of challenges Lassell also faced, although his were on a smaller scale. In addition, speculum metal mirrors were subject to distortion because of the imperfect temperature equilibrium between themselves and their surroundings. From the outset Lassell had realized that a distortion or flexure in his mirrors might produce a feature akin to a ring. Yet it is not clear if he made many tests for such effects. Dawes’s observing diary for 14 September 1847 refers to two sets of measurements of Neptune’s elongation. In between the two sets of readings the 24-inch tube was turned 30° on its axis (as we noted above). There is no similar record in Lassell’s notebook. Such comparisons may have been made on numerous occasions but went unnoted. On the other hand, the fact that Lassell detailed in full the decisive test of 15 December 1852 perhaps indicates he did not normally make such checks. Anyway, the 30° spread of the measurements of the inclination on that night finally convinced him the ring was “more intimately related to the telescope than the object.” Further, if the feature was due to the telescope, even if it was not

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the product of astigmatism, then we have a ready explanation of why others who observed with the 24-inch likewise saw the ring.1 If image defects help to explain the observations of Lassell, how are we to account for those of Challis and Hind, two observers of considerable repute? A crucial point is that they both noticed some peculiarity in the image of Neptune only after they were aware of Lassell’s ring observations, although they had both examined the planet on several occasions very soon after its discovery. Nor should we forget that Challis and Hind had been beaten by Galle and d’Arrest in the hunt for Neptune in 1846. They knew too that Lassell’s telescope was more powerful than their own and that his observations had been endorsed by other users of the 24-inch. It is therefore likely that Challis and Hind were disposed to see Neptune as elliptical or oblong, and in Challis’s case to ‘see’ a ring, although even here image defects might well have played some part. Lassell and the other observers who saw the ring with the 24-inch were also subject to preconceptions. If our argument about the effects of astigmatism is correct, Lassell and his colleagues perhaps  interpreted elongated images as a ring almost edge-on. For them to have made such an identification must have been due in part to the prior knowledge that Saturn has rings, and, in the case of Lassell, his belief that John Herschel had asked him to look for rings and satellites.2

8.5  Conclusions There was a significant group of astronomers in Britain, both professionals as well as many amateurs, by the 1840s. Lassell was prominent among this group. A provincial brewer by trade, he very much desired recognition for his researches and telescope building, particularly from the influential members of the Royal Astronomical Society and Royal Society. Proud of his achievements, he was ever anxious to demonstrate the excellence of the 24-inch telescope that he had built through his own initiative, money, skills and energy. In his public reports of the 24-inch’s performance he did not emphasize faults--at least, faults of such a magnitude as to produce a ring-like appearance. Lassell made but brief and tangential public reference to his fears that optical illusions had given rise to the supposed ring. As so often happens in science, however, once the ring became suspect it was generally ignored, and only a few contemporary writers mentioned its likely spurious origin. One who did was W. H. Smyth (1788–65). In 1859, he noted that the “case of a ring must probably remain undecided for a few years, until the planet occupies a better situation in the ecliptic, by rising in declination to a good-working altitude: but as we happen to know

 For a short modern discussion of astigmatism in optical systems see (Greivenkamp, 2004).  For an interesting, and somewhat similar, case study of instrumental failure, see (Dobbins and Sheehan, 2004). 1 2

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personally that its existence is now doubted by Mr. Lassell himself, ‘tis as well to say so” (Smyth, 1860: 417). Perhaps the most important conclusions to emerge from the story of Neptune’s ring, however, concern the attraction of the new planet as a site of possible discoveries, as well as the effects of preconception and how these are currently regarded. It has lately become common to use such effects to explain why scientists perceive phenomena in a manner that appears inexplicable to others. We have suggested that both Challis and Hind were strongly influenced by the reports of Lassell’s observations. Further, it is tempting to argue that Lassell himself was a “victim” of preconceptions. But we have indicated that it is highly likely that mirror flexure played a significant role throughout, and a study of the telescope used by Lassell has thus been shown to be just as necessary as the inquiry into the conceptual framework within which Lassell and his contemporaries sought to interpret their observations. We therefore wish to emphasize that a full account of Lassell’s observations requires an analysis of both his instruments and his expectations, and that, in general, it is essential to pay full attention to the instrument employed by an astronomer in cases where preconception is, or might be, invoked to help explain a set of observations or experimental results. As a coda to the story of Lassell’s ring, it is amusing to note that new, tentative observations of ring material around Neptune were achieved in 1984 by Patrice Bouchet and colleagues at the La Silla Observatory in Chile. These proved to be the three densest of five ‘arcs’ within the otherwise thin outer ring when Voyager 2 imaged the rings in 1989. There are five rings, all dusty, rocky and dark, two very narrow and well defined, the others more diffuse and relatively wide. In a late tribute to the astronomers best connected to Neptune, the rings were named, from the planet outwards, Galle, Le Verrier, Lassell, Arago, and the outer ring with arcs for Adams. With a nice touch, within the Adams ring the arcs were named Liberté, Égalité, Fraternité 1 and 2, and Courage. Acknowledgements  This paper is based on Robert W.  Smith and Richard Baum’s, “William Lassell and the Ring of Neptune: A Case Study in Instrumental Failure,” Journal for the History of Astronomy of Astronomy, 15(1984), 1–17. The late Richard Baum opened up the historical subject of Neptune’s ring with a chapter on this topic in his The Planets: Some Myths and Realities, which is referenced below. In preparing the current paper, Robert Smith gratefully acknowledges the helpful comments and criticisms of Trudy E. Bell, Roger Hutchins, Carolyn Kennett, and William Sheehan.

References Airy, G., 1848. Substance of the lecture delivered by the Astronomer Royal on the large reflecting telescopes of the Earl of Rosse and Mr. Lassell, at the last November meeting. Monthly Notices of the Royal Astronomical Society, 9, 110–21. Anon., 1846a. Observations of Le Verrier’s Planet. Monthly Notices of the Royal Astronomical Society, 7, 154–7.

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Anon., 1846b. Observations of Le Verrier’s Planet. Monthly Notices of the Royal Astronomical Society, 7, 165–8. Anon., 1847. Report of the Council to the twenty-seventh Annual General Meeting. Monthly notices of the Royal Astronomical Society, 7, 193–231. Baum, R., 1973. The Planets: Some Myths and Realities. Newton Abbot, David and Charles. Bond, W., 1848. Cambridge, U.S. (Professor W.C.  Bond). Monthly Notices of the Royal Astronomical Society, 8, 9. Challis, J., 1847a. Second Report of Proceedings in the Cambridge Observatory Relating to the New Planet (Neptune). Astronomical Observations Made at the Observatory of Cambridge, 15, 27–30. Challis, J., 1847b. Schreiben des Herrn Professor Challis, Direktor der Cambridge Stemwarte, an den Herausgeber; “Observations of Le Verrier’s planet taken at Cambridge Observatory”. Astronomische Nachricten, 25, 229–32. Challis, J., 1847c. Draft of letter, dated 3 May. Cambridge Observatory Archives. Challis, J., 1848. Astronomical observations made at the observatory of Cambridge, 15, Appendix 1. Chapman, A., 1988. William Lassell (1799–1800). Practitioner, Patron and ‘Grand Amateur’ of Victorian Astronomy. Vistas in Astronomy, 32, 341–370. Chapman, A., 2019. Personal communication, 23 December. Cruikshank, D., and Sheehan, W., 2018. Discovering Pluto: Exploration at the Edge of the Solar System, Tucson, University of Arizona Press. Dawes, W., 1847a. Letter to J. Challis, dated 7 April. Cambridge Observatory Archives. Dawes, W., 1847b. Letter to J. Herschel, dated 21 April. Herschel Papers, Royal Society Archives. Dawes, W., 1847c. Entry in Observing Journal for 8 September. Dawes Papers, Royal Astronomical Society Archives. Dobbins, T., and Sheehan, W., 2004. The Story of Jupiter’s Egg Moons. Sky and Telescope¸January, 114–120. Greivenkamp, John, 2004. Field Guide to Geometrical Optics. Bellingham, Washington, SPIE. Herschel, J. 1846. Letter to W. Lassell, dated 1 October. Herschel Papers, Royal Society Archives. Herschel, J., 1849. Address by the President (Sir J.  F. W.  Herschel, Bart.) on presenting the Honorary Medal of the Society to William Lassell., Esq. of Liverpool. Monthly Notices of the Royal Astronomical Society, 9, 87–92. Hetherington, N., 1979. Neptune’s supposed ring, Journal of the British Astronomical Association, 90, 20–29. Hind, J., 1846. Discovery of Le Verrier’s planet. The Times, 1 October. Hind, J., 1847. Schreiben des Herrn J. R. Hind an den Herausgeber. Astronomische Nachrichten, 25, 205–8. Huggins, W., 1881. Obituary of William Lassell. Monthly Notices of the Royal Astronomical Society, 41, 188–91, Lassell, W., 1846a. Cutting inserted between notebook entries for October 2 and 3. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1846b. Notebook entry for 3 October. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1846c. Notebook entries for 10 and 26 October. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1846d. Notebook entry for 10 November. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1846e. Notebook entry for 11 November. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1846f. Letter to the editor. The Times, 14 October. Lassell, W., 1846g. Letter to J.  Herschel, dated 12 October. Herschel Papers, Royal Society Archives. Lassell, W., 1846h. Notebook entry for 11 November. Lassell Papers, Royal Astronomical Society. Lassell, W., 1847a. Letter to J. Challis, dated 19 January. Cambridge Observatory Archives.

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Lassell, W., 1847b. Letter W.  Lassell to R.  Sheepshanks, dated 9 February 1847. Sheepshanks Papers, Royal Astronomical Society Archives. Lassell, W., 1847c. Notebook entry for 7 July 1847. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1847d. Letter W.  Lassell, to J.  Herschel, dated 14 August 1847. Herschel Papers, Royal Society Archives. Lassell, W., 1847e. Copy of letter W. Lassell to J. Nichol, dated 17 August. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1847f. Letter W. Lassell to R. Sheepshanks, dated 8 September. Sheepshanks Papers, Royal Astronomical Society Archives. Lassell, W., 1847g. Notebook entry for 8 September 1847. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1847h. The Planet Neptune and his Satellite. Letter to the editor, The Times, 24 September. Lassell, W., 1848. Discovery of a New Satellite of Saturn. Monthly Notices of the Royal Astronomical Society, 8, 195–7. Lassell, W., 1849a. Notebook Entry for 21 August. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1849b. Notebook Entry for 27 November. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1850: Description of a Machine for Polishing Specula, etc., with Directions for its Use; together with Remarks upon the Art of Casting and Grinding Specula, and a Description of a Twenty-foot Newtonian Telescope equatorially mounted at Starfield, near Liverpool. Memoirs of the Royal Astronomical Society. 18, 1–20. Lassell, W., 1851a. Observations of Neptune and his satellite. Monthly Notices of the Royal Astronomical Society, 12. Lassell, W., 1851b. Notebook entry for 14 May. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1852a. Notebook Entry for 11 November. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1852b. Notebook Entry for 15 December. Lassell Papers, Royal Astronomical Society Archives. Lassell, W. 1852c. Notebook Entry for 29 October. Lassell Papers, Royal Astronomical Society Archives. Lassell, W., 1853. Extract of a letter from Mr. Lassell. Monthly Notices of the Royal Astronomical Society, 13, 36–39. Mitchel, O., 1846. The New Planet. Its Name, and Discovery. Sidereal Messenger. 1, 59–60. Nasmyth, J. 1852. Letter to the Royal Astronomical Society, dated 5 December. Royal Astronomical Society Letters, Royal Astronomical Society Archives. Smith, R., 1993. Raw Power: Nineteenth Century Speculum Metal Reflecting Telescopes. In N. Hetherington (ed.), Cosmology: Historical, Literary, Philosophical, Religious, and Scientific Perspectives, 289–300. New York, Routledge. Smyth, W., 1860. The cycle of celestial objects continued at the Hartwell Observatory to 1859. With a notice of recent discoveries, including details from Aedes Hartwellianae. Leighton, Son and Hodge, London.

Chapter 9

Neptune’s Orbit: Reassessing Celestial Mechanics William Sheehan and Kenneth Young

Abstract  The almost miraculous discovery of 1846 and at first bitter priority dispute ended in March 1847, with the piece by Jean-Baptiste Biot that was reprinted in The Athenaeum, followed by the orchestrated, highly public display of men of men of science getting along amicably at the Oxford meeting BAAS meeting in June, where Le Verrier and Adams met. Meanwhile, Adams had swiftly refined Neptune’s orbit, and Lassell’s prompt discovery of Triton had enabled Neptune’s mass to be determined. However, in that decade, American astronomy was coming of age, and for lack of large telescopes was led by elite mathematicians who now fastened on Neptune to prove their credentials. Benjamin Peirce of Harvard, with claims that Neptune had been found by “happy accident”, re-ignited controversy by challenging whether a theoretical discovery had been made at all. This, and the potential for further discoveries, sustained a century’s international focus and effort to revise celestial mechanics. In the case of Neptune, the predictions of Adams and Le Verrier would be partially affirmed, though it would also become clear that their success was owing to very special conditions. Meanwhile, Le Verrier attempted to use the same methods of perturbation theory to explain an unexplained advance of the perihelion of Mercury on the basis of an intra-mercurial planet (Vulcan). After briefly covering Le Verrier’s personal obsession with Vulcan, we explain how this and the analyses of other practitioners of celestial mechanics illuminate the struggle to transcend Bode’s law by pushing celestial mechanics in new directions in the pre-computer era. The advent of photography for survey work, epitomized by Percival Lowell’s (1855–1916) dedication to the search for “Planet X” early in the 20th century, fueled the belief that more distant planets awaited discovery. Despite the discovery of Pluto being hailed as a late-ripening fruit of Lowell’s obsessive search, we now know it as only the first of a number of objects of dwarf-­ W. Sheehan () Independent scholar, Flagstaff, AZ, USA K. Young Chinese University of Hong Kong, Hong Kong, China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_9

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planet class populating the outer Solar System (the region of the Kuiper belt). As related in Chap. 10, the methods of celestial mechanics have been largely ­abandoned for empirical methods of searching for planets, leading to greatly extended knowledge of the region beyond Neptune—an unexpected coda to 1846.

9.1  Neptune’s Discovery Revisited: A Happy Accident? The priority disputes had hardly died down when a new challenge bubbled up. Much had been made of the supposed fact (actually, not correct, as we have seen earlier) that Adams’s and Le Verrier’s predictions had agreed within “one degree”: on the face of it, nothing less than astounding, but as we have also seen, the claim of such close correspondence was actually in error. The only prediction falling within “one degree” of the actual position of the planet had been Le Verrier’s second prediction of 31 August 1846. (See: Table 9.1). The supposed near-agreement of independent calculations of such complexity “to within a degree”—actually founded, as noted in Chap. 7, in misconstrual by Airy, Challis, and Airy’s assistant James Glaisher (1809–1903)—made the mathematicians’ astuteness seem uncanny. By the same token, the fact that the planet was found in a search lasting no more than an hour, by Galle and d’Arrest, reinforced that impression. Thus the basis of Arago’s ringing declaration that Le Verrier had discovered a planet with the “tip of his pen.” (Lequeux, 2015:50). Table 9.1  Errors in predicted positions of Neptune Mean helio long. Epoch Orbital period (years) Mean helio. long. at discovery Perihelion Anomaly Eccentricity Eqn. of center True helio. long. Error

Adams I

Adams II

323° 34′

323° 2′

Le Verrier I 325°

318° 47′

1 Oct. 1845 237.6 325° 2′

1 Oct. 1846 227.4 322° 59′

1 Jan. 1847 237.6 324° 35′

1 Jan. 1847 217.4 318° 19′

315° 55′ 9° 7′ 0.161 3° 38′ 328° 41′ 1° 44′

299° 11′ 23° 48′ 0.121 6° 27′ 329° 27′ 2° 30′

0 0

284° 45′ 33° 34′ 0.108 7° 39′ 325° 58′ −0° 58′

324° 35′ −2° 21′

Le Verrier II

In order to compare accuracies, the predictions—Adams’s “Hyp. I” of Oct. 1845 and “Hyp. II” of Sept. 1846 and Le Verrier’s of 1 June and 31 August 1846—have here been converted to true heliocentric longitude for the discovery date of 23 September 1846 (Biot, 1847; Herschel, 1861, art. 769; Rawlins, 1992). The true heliocentric longitude of Neptune on that date was 326° 57′, and there was a 1° difference between the heliocentric longitude, found from computations, and the geocentric longitude required for finding it with a telescope. The difference between the two would have disappeared at opposition, which had occurred in August 1846.

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Agnes M. Clerke, the great Irish historian of 19th century astronomy, wrote in similar vein (Clerke, 1908:102), “Recondite analytical methods received a confirmation brilliant and intelligible even to the minds of the vulgar, and emerged from the patient solitude of the study to enjoy an hour of clamorous triumph. Forever invisible to the unaided eye of man, a sister-globe to our earth was shown to circulate, in perpetual frozen exile, at thirty times the distance from the Sun.” Even as late as the early 1960s, the historian and philosopher of science Norwood Russell Hanson (1924–1967) was still referring to the discovery of Neptune as the “Zenith of Newtonian Mechanics”, with the term “Zenith” necessarily implying a falling off to an ensuing “Nadir.” (Hanson, 1962:359) (The “Nadir” was Le Verrier’s unsuccessful search for the intramercurial planet Vulcan, of which we shall say more below.) The growing discomfort felt by some astronomers over the supposed accuracy of the calculations was based in large part on the fact that the orbits for the planet that Le Verrier and Adams had calculated actually bore very little resemblance to the actual orbit of Neptune. Adams, using Challis’s hitherto unpublished observations of August 1846 to extend the baseline to a point somewhat earlier than the postdiscovery observations allowed, published the first calculated orbit for Neptune within three weeks of the discovery. He found that the mean distance was much less than expected, at just 30 AU. Meanwhile, Challis and other astronomers continued to observe the planet. By January 1847, when the planet disappeared into the glare of the Sun, it had still been observed only through a very short arc of its orbit. Meanwhile, astronomers became interested in the possibility that older records might be found for Neptune as had been the case of Uranus following William Herschel’s discovery. Among the possibilities were Cacciatore’s and Wartmann’s objects (see Chap. 2) which had aroused so much interest beforehand (and had even been consulted by Adams when drawing up his ephemeris for Challis in the summer of 1846). They were re-examined, in the hope that one or the other or both might prove to be the planet, by an astronomer at the US Naval Observatory in Washington, D.C., Sears Cook Walker, who at this point enters as a major figure in the story. (Hubbell and Smith, 1992) (Fig. 9.1). Walker was born into a farming family located near Boston, and early showed remarkable intellectual precocity and a prodigious memory. At Harvard College, he showed excellent aptitude in languages, and after a stint as school teacher and a job as an actuary for an insurance firm, Walker took charge of the observatory erected for the Central High School in Philadelphia, whose 6-inch refractor from the Merz and Mahler firm, installed in 1840, was one of the largest telescopes in the United States between 1840 and 1843. During this period, Walker had become wealthy enough not to have to worry about money; however, poor investments led to a state of destitution by 1845, and incentivized him to take up a position at the U.S. Naval Observatory in early 1846. (Bell, 2007). Walker read Le Verrier’s memoir on Uranus of June 1846 as soon as it arrived in the trans-Atlantic mails. (At the time, it took a packet ship at least two weeks to cross from Liverpool to New York, as steam ships did not come into widespread use

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Fig. 9.1  Sears Cook Walker. An engraving from Popular Science Monthly, 1894. (Public Domain)

for another decade.) As soon as he read it, he went to the Superintendent, Lieutenant Matthew Fontaine Maury, and suggested to him, that a search be made for [Le Verrier’s] theoretical planet. Unfortunately, the Naval Observatory, like its counterpart at Greenwich, was overloaded with the routine work of positional astronomy, and Walker’s search could not be fitted into the schedule until two months later. (Grosser, 1962:110)

But by then, the planet had been found. The U.S. Naval Observatory had a 9.6-inch Merz and Mahler refractor virtually identical to the instruments at Dorpat and Berlin. At the time it was mounted in mid-1844, it was briefly the largest telescope in the United States (until exceeded by the 11-inch Merz and Mahler refractor at the Cincinnati Observatory in April 1845). (Dick, 2003:66; Nourse, 1871:32). Had Maury acted with greater haste, Walker might well have made the discovery, but by the time his search was fitted into the schedule, the planet had already been found in Berlin. Possibly as a consolation prize, Maury assigned Walker to calculate the planet’s orbit, and Walker approached the task with considerable initiative. Working on a suggestion from Yale University librarian and amateur astronomer Edward C. Herrick (1811–1862) that Wartmann’s star of 1831 might have been Neptune, Walker began by attempting to fit the two objects to one orbit—without success. At this point Benjamin Peirce, professor of mathematics at Harvard and later director of the US Coast Survey, also acting on Herrick’s information, and carrying out the same calculation as Walker, came to the same conclusion. In the process, he found,

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as he announced to the American Academy of Arts and Sciences in Boston on 12 November 1846 (the day before Airy spoke to the meeting of the Royal Astronomical Society), that Wartmann’s planet could not have been Neptune, as the four observations of it could not be fitted to Neptune’s orbit. However, owing to the fact that it would have been only eight or ten degrees from the position of Neptune, Wartmann’s attempt to recover his supposed planet in 1832 might, Peirce observed, “easily have resulted in the discovery of the planet by which the name of Le Verrier is now immortalized.” (Hubbell and Smith, 1992:266). Another accidental discovery, like Herschel’s of Uranus, would hardly, however, have been as momentous as the way things actually played out, and would certainly have not been poetic justice, since Wartmann had in fact merely made careless errors in determining the position of his “star”. Once these were corrected, it proved that the “star” was none other than Uranus, lurking in the neighborhood at the time, after passing conjunction with Neptune in 1821–22. Cacciatore’s object, however, has never been accounted for. (See Baum, 2007:269–280; Cunningham, 2017:112–114). Possibly it was an asteroid, or even a Near-Earth asteroid, in which case it would only have been visible for a few days at most (Cunningham, C., personal communication; 19 July 2019). However, after following Herrick’s lead to this dead end, Walker proceeded to carry out a new calculation of Neptune, based on the short arc of observed positions available at the time. The elements specified by Le Verrier and Adams had predicted a highly eccentric orbit; Walker showed (in agreement with Gauss) that the actual orbit was near enough to circular for him to compute a first approximate set of ephemerides on the assumption that it was exactly circular, with a mean radius of 29.93 AU and a period of 163.83 (tropical) years. Next, he compared the actual positions of the planet with ephemerides computed on the basis of this orbit, and—though finding the eccentricity still too small to yield a specific result—by January 1847 had arrived at a new set of elements, in which the mean radius had been revised to 30.20 AU and the period to 165.97 years. (Hubbell and Smith, 1992:267). This was a much more accurate orbit than had been calculated up to that time, and rendered redundant Adams’s and Gauss’s earlier efforts. It also meant that Neptune was so remote that it would not return to the same point in the sky where it had been discovered until the almost inconceivably far off era of 2010–2011. (James and Sheehan, 2011) (Fig. 9.2). Walker had now been seized with the problem, and wanted to do better. The observed arc was slowly lengthening, but still much too short to lead to a fully satisfactory result. But suppose the planet had been accidentally recorded by some cataloger of the stars and set down as a star, as Uranus had been? Walker perused a number of star catalogs before deciding that the only one that offered hope of success was Joseph Jérôme Lalande’s celebrated Histoire céleste française, which contained positions for 50,000 stars. With ephemerides calculated from his orbit, Walker found two nights on which Lalande might have come across Neptune. (Actually, since the titular author had not actually observed any of the stars listed in the catalog ascribed to him, the actual observer would have been his “nephew”, Michel Lalande.) Those two nights were 8 May and 10 May 1795. Walker

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Fig. 9.2  Neptune returns to the same field of stars as on the date of its discovery, one Neptunian year (164.8 terrestrial years) later. Imaged by Michael Conley and William Sheehan with a 25-cm Ritchey-Chretien on the night of 27 October, 2011. Neptune is at the upper left. Compare the stars in this field with those in the figure above, which shows a small section of the Berlin star chart with Neptune’s position on 23 September 1846 indicated by the arrow (Credit: Michael Conley and William Sheehan)

discovered that Michel Lalande had indeed recorded the planet twice, and even had noted a slight discrepancy in the positions. He had therefore indicated the position as uncertain. When the same area was re-examined by Walker or a colleague, Lalande’s star was found to have vanished. (Hubbell and Smith, 1992:267). This was the breakthrough, and subsequently, other pre-discovery observations began to turn up—though none as useful in extending the observed arc of Neptune’s orbit. The Scottish-born astronomer John Lamont (1805–1879) (also known as Johann von Lamont), who received his mathematical training at the Scottish Benedictine monastery at Ratisbon and spent most of his career at the Royal Observatory of Bogenhausen in Munich, was found to have recorded it on 25 October 1845 and again on 7 and 11 September 1846, just days before it was reported from Berlin. Even Sir John Herschel had passed over it once. As he confided to Friedrich Wilhelm Struve of the Pulkova Observatory, near St. Petersburg, Russia, “The discovery of the new Planet is indeed in every way a most spirit-stirring event… I had myself a narrow escape of it. On the night of July 14th 1830, I swept over the zone…. But it is better as it is. I should be sorry if it had been detected by an accident or merely by its aspect. As it is, it is a noble triumph of science.” Struve had evidently suggested that there might still be other planets yet to be discovered, and Herschel agreed with him: “I perfectly agree with you that there are still planets beyond.” (J.F.W. Herschel to F.W. Struve, 27 Dec. 1847; RS.) There would be still other pre-discovery sightings, including some discovered as recently as 1980. These were of particular interest, consisting of several positions of Neptune by Galileo Galilei when he was tracking the newly discovered satellites of Jupiter in December 1612 and January 1613, and recorded Neptune as a star in the same field. (Drake and Kowal, 1980). Those observations were, of course, discovered too late to have any bearing on the story we are telling.

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Table 9.2  Orbital elements for Neptune computed by Walker compared to the pre-discovery orbits calculated by Le Verrier (31 Aug. 1846) and Adams (Hyp II) Mean distance (AU) Eccentricity Inclination Long. ascending node Long. perihelion Period (years) Mass (Sun = 1) Helio. Long. 1 Jan. 1847

Walker 30.25 0.00884 1° 54′ 54″ 131°17′ 38 0° 12′ 25″ 166.381 0.00006 328° 7′ 57″

Le Verrier 36.15 0.10761

Adams 37.25 0.12062

284° 45′ 217.387 0.000173 326° 32′

299° 11′ 227.3 0.00010 329° 57′

Michel Lalande’s two positions were the important ones here, as they extended by over half a century the observed arc of the planet’s orbit and allowed Walker to compute a much better orbit than any hitherto available. His elements are compared to the theoretical elements given by Le Verrier and Adams (i.e., Le Verrier’s 31 August 1846 calculation and Adams’s Hyp II), in the Table  9.2 below (Fig. 9.3). The first point to be noticed is that Neptune did not fit well into the Bode’s law progression—it lay much closer to the Sun than expected. Moreover, the orbit of the actual planet bore hardly any resemblance to those of Le Verrier and Adams, leading Peirce—already gaining a reputation as America’s foremost mathematician—to a startling conclusion. Born in Salem, Massachusetts in 1809, Peirce’s first mentor was the self-taught mathematician and insurance actuary Nathaniel Bowditch, whose son had been in the same grammar school as Peirce, and whose celebrated English translation of Laplace’s Mécanique céleste Peirce read in proof as an undergraduate at Harvard. Except for two years, Peirce remained at Harvard for his entire career, from 1833 as University professor of mathematics and natural history and then, from 1842 until his death in 1880, as first Perkins professor of astronomy and mathematics. He also served as director of the U.S. Coast Survey in Washington, D.C. from 1867 to 1874. (Suzuki, 2007) (Fig. 9.4). At first Peirce had simply assumed that the planet discovered in Berlin was “Le Verrier’s Planet” and paid the French astronomer the compliment of calling him “the first geometer of the age, and … founder of a wholly new department of Invisible Astronomy.” (Mitchel, 1847:86). “This striking phrase,” declare historians John G. Hubbell and Robert W. Smith (1992:272), underscores an important aspect of the discovery of Neptune. The problem solved by Le Verrier and Adams, the so-­called ‘inverse perturbation problem’, was unprecedented in the history of astronomy. Indeed, it was the discovery of Neptune that was to entrench this new approach within astronomical practice. Thus, …there was no generally accepted body of theory and practice by which to gauge the novel researches of Le Verrier and Adams, so judgements on their researches implied judgements on results and methods.

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Fig. 9.3  Orbits calculated by Adams and Le Verrier compared to the actual orbits of Uranus and Neptune. Le Verrier: semi-major axis 36.2 AU, perihelion 285°, eccentricity 0.11. Adams: 37.2 AU, 299°, 0.12. Neptune: 30.1 AU, 0°, 0.0. Uranus 19.1 AU, 170°, 0.05. Note that the hypothetical positions line up very nearly with the actual positions of Neptune during these years (Public domain)

In an address to the American Academy of Arts and Sciences in Boston on 16 March 1847, however, Peirce revealed a stunning change of mind. He underscored the marked discrepancy between Walker’s orbit and the theoretical ones of Le Verrier and Adams. From the mean distance alone, 30 AU, and the resulting period of 166 years, he averred that Neptune “is not the planet to which geometrical analysis had directed the telescope; that its orbit is not contained within the limits of space which have been explored by geometers searching for the course of the disturbances of Uranus, and that its discovery by Galle must be regarded as a happy accident.” (Peirce, 1847a:65). Recalling that Le Verrier had concluded that the planet’s mean distance must be between 35 and 37.9 AU, corresponding to a period of from 207 to 233 years; and that its mean longitude on 1 January 1800 must be between 243° and 252°, he pointed out that either one or the other of these propositions could be reconciled with the observations of Neptune since its discovery, but not both together. In addition, Peirce found that since Le Verrier’s larger value of the period (233 years) was not far from being three times that of Uranus—in other words, a 3:1

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Fig. 9.4  Benjamin Peirce. This photograph, in the Lowell Observatory archives, probably belonged to Peirce’s student, Percival Lowell. (Credit: Lowell Observatory)

resonance—there would in fact have been three solutions, of which the celebrated geometers had found only one. Peirce called attention here to the so-called Laplacian librations, commensurabilities in the periods of the planets similar to those Laplace had analysed in the case of the Great Inequality of Jupiter and Saturn. Thus, he pointed out, Le Verrier’s lower limit—35 AU—was just below the distance of 35.3 AU where the planet’s period would have been 2½ times that of Uranus (i.e., 210 years). However, at this resonance position, the effects of the mutual perturbations would become, in Peirce’s words, “peculiar and complicated”. This, however, seemed, as he saw it, likely to invalidate the continuous law of inference by which Le Verrier had carried out his investigation. Even more significant, said Peirce, was the commensurability at the distance of 30.4 AU, where Neptune’s period would be 168 years, exactly double the year of Uranus. “The influence of a mass revolving in this time would give rise to very singular and marked irregularities in the motions of this planet…,” Peirce believed. (Italics added.) (What happens at resonance are long-term secular effects, which would however have almost no effect on the analysis of a short segment of the orbit.) Peirce concluded, “It is highly probable that the case of Neptune and Uranus is not merely that of near approach to the ratio of 2 to 1 in their times of revolution, but that this ratio is exactly preserved by those planets.” (Peirce, 1847a:67). He even thought that when sufficient observations had become available, Neptune’s period would indeed prove to be not only almost but precisely double that of Uranus. In a second paper published in May, Peirce expanded on his earlier finding that there were actually three solutions to the problem that Le Verrier and Adams had attempted to solve. In addition to the solutions they had published, Peirce found

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others one hundred twenty degrees ahead and one hundred twenty degrees behind, the position of Neptune, so that, he declared, “If the above geometers had fallen upon either of these solutions, Neptune would not have been discovered.” (Peirce, 1847b:144). Ironically, no sooner had the priority dispute about the discovery of Neptune begun to subside than Peirce’s “happy accident” thesis set off another firestorm of controversy. Neptune was born in a climate of disputation. In calling into question the validity of the methods used by Le Verrier and Adams, Peirce was threatening to take the gloss off the greatest scientific discovery of the age.

9.2  Counterattack Peirce’s argument immediately put him at odds with most astronomers in England and in Europe. If, after all, the theoretical underpinnings of Le Verrier’s and Adams’s predictions had indeed been such quicksand as Peirce was now claiming, how could the two mathematicians have come as close as they did—Le Verrier within only 1°, Adams between 2° and 3°, of the planet’s actual place? Happy accident indeed! Le Verrier himself made vigorous, if not entirely persuasive, attempts to refute Peirce’s arguments. (Adams, characteristically, remained silent at the time.) Sir John Herschel would later believe he had found a way out of the impasse. Peirce, he said (Herschel, 1861, art. 776: 438 note), had begun with a “total misconception of the nature of the problem.” The exact (as opposed to approximate) elements of the planet were of little importance to the prediction of the planet’s position. Rather, it was the fact that Uranus and Neptune had been in conjunction late in 1821 which had produced (said Sir John) a maximum perturbation from which Le Verrier and Adams had been able to predict Neptune’s longitude—not least because this maximum perturbation had assured the failure of Bouvard’s Tables (discussed in Chapters 2, 4 and 5) which in the light of hindsight had appeared at the worst possible time (Figs. 9.5 and 9.6). The details of Sir John’s analysis are given here (Herschel, 1861, art. 775). They are rather involved and technical; the reader who is not interested in the details but only in the general flow is invited to skim here (as well as in some of the later technical sections). Herschel began by resolving—in the usual way of vector analysis— the force of the exterior planet into radial and tangential components, then proceeded to analyse their mutual effects. Near the time of conjunction of the two planets, he found the radial force to be nearly twice as powerful as the tangential. But the change of longitude in the vicinity of the conjunction is almost entirely due to the action of the tangential force. (Note, when he refers to a “sudden and powerful acceleration” below, he means sudden and powerful in relative, i.e., in astronomical terms.) He says, The effect of the tangential force on the mean motion takes place through the medium of the change it produces on the axis, and is transient: the reversed action after conjunction (supposing the orbits circular), exactly destroying all the previous effect, and leaving the mean

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Fig. 9.5  Diagrams showing the pull of Uranus on Neptune, as envisaged by Sir John Herschel. From: Rev. Theodore E.  R. Phillips (1868–1942) and Dr. William H. Steavenson (1894–1975) (eds.), Splendour in the Heavens. London, Hutchinson & Co., 1923, vol. 1, p. 381 (Credit: Author’s Collection) motion on the whole unaffected. In the passage through the conjunction, then, the tangential force produces a sudden and powerful acceleration, succeeded by an equally powerful and equally sudden retardation, which done, its action is completed, and no trace remains in the subsequent motion of the planet that it ever existed…. But with the normal force the case is far otherwise. Its immediate effect on the angular motion is nil. It is not till it has acted long enough to produce a perceptible change in the distance of the disturbed planet from the sun that the angular velocity begins to be sensibly affected, and it is not till its whole outward action has been exerted … that its maximum effect in lifting the disturbed planet away from the sun has been produced, and the full amount of diminution in angular velocity it is capable of causing is developed (Herschel, 1861, art. 775: 436).

The argument given here is an interesting one, though from a modern perspective it is deficient in one essential respect. Sir John’s idea is to link the residuals, the discrepancies between the observed and calculated longitudes of Uranus after known perturbing effects have been subtracted out—i.e., effectively, the “extra” displacement—to the force. Here, curiously, the inverse problem is easier than the forward one, provided one has both the angular and radial residuals: one then just has to differentiate twice, since force or acceleration are the second derivative of the

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Fig. 9.6  Sir John Herschel. From: Duyckinik, E.A., A Portrait Gallery of Eminent Men and Women in Europe and America. New York, John, Wilson & Company, 1873 (Credit: Wikipedia Commons)

position vector, and in practice this can be done by taking differences among three nearby observations. Sir John seems to be working from the forward problem, which is more difficult: given the force at time t, one cannot use that alone to determine the displacement at the same time. Also, when there is an extra displacement, the gravitational force due to the Sun also changes, as does the coupling between radial and tangential motion in accordance with Kepler’s second law. These effects are independent of Neptune’s mass. What Sir John’s analysis fails to capture is the need to write down all the coupled effects together. Using a graphical method which he had first devised to tackle binary star orbits (Hankins, 2006), Sir John graphed the residuals, Δϕ, over time, and noted that around the time of conjunction, the tangential force would produce a sudden and powerful acceleration, succeeded by as sudden and powerful a deceleration. The radial force, on the other hand, would show its effect in “the rapid descent of the curve subsequent to the conjunction, indicating the commencement of a series of undulations in its future course of an elliptic character, consequent on the altered eccentricity and perihelion which will be maintained till within about 20 years from the next conjunction.” (Herschel, 1861, art. 775). Given that the interval between

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conjunctions of Uranus and Neptune is about 171 years, after the 1821–22 conjunction, the next one would occur in 1992–93. Not only had Sir John analysed in detail the effects of the tangential force and the radius vector—the latter such an issue for Airy—he had gone on to state a further conclusion: the inflection in the rate of increase of longitude around the time of the conjunction, as revealed by his graphical method, could have been used to locate the planet without any need to conjure up the complicated inverse-perturbation method of Le Verrier and Adams. (Here he erred, but in so doing he set up an enticing illusion that would long captivate successive searchers of trans-Neptunian space, by presenting a seeming shortcut to the laborious methods of those celebrated geometers.) He admitted that the small value of the distance found by Walker and Peirce was unexpected but that with hindsight it might have been foreseen, since, as he had now clearly shown, it is not at all necessary to know with any precision the elements—the axis, eccentricity, or mass—of the unknown planet in order to determine its location. But it is necessary to make an assumption about the ratio of the mean distances. Both Adams and Le Verrier assumed a mean distance, a1, based on the Bode’s law (i.e., twice a, the mean distance of Uranus; ironically, Neptune itself would prove to be the first exception to the Bode’s law progression, the true ratio being not 2.00 but only 1.58). All the observations from 1780 to 1840 were used, and on an equal footing, with the theory purporting to say where the planet was to be found over this whole stretch. However, by assuming a wrong a1, they could be right at 1840 only by being wrong at 1780. Take, for example, Adams’s Hyp. II of September 1846; a1 = 1.94a, and the period is 227 years. If the orbit had been circular, and the angular velocity uniform, he would then have been wrong by 30° for 1780, even though close to the mark in 1840. But despite starting with the wrong a1, the method “responds gallantly”, as Cambridge mathematician J.E. Littlewood put it (1953:126), by making the eccentricity large (1/8) and by assigning perihelion to the place of the Uranus-Neptune conjunction. One can clearly understand why Adams, right up to September 1846, was so interested in trying smaller and smaller values of the mean distance, in order to escape the large eccentricity. The combination of excessive mean distance and high eccentricity makes the effective distance from the Sun more like 1.7a, which reduces the error (in 1780) from 30° to 18°. Thus, the mean distance and eccentricity can be adjusted at will. So can the mass, and because they overestimated the mean distance, both Adams and Le Verrier also made the mass much too large. Adams’s mass was three times, and Le Verrier’s twice that of the actual Neptune, which was, of course, accurately known following Lassell’s discovery of the satellite, Triton. (See Chap. 7) (Fig. 9.7). Actually, the value that Adams and Le Verrier were seeking—the longitude of Neptune during the decades on either side of the conjunction of Uranus in 1822— does not depend very much on the ratio of the mean distances. This means that there exist many possible orbits that could have approximated the gravitational force vector of Neptune on Uranus between 1800 and 1850, and given the position of the hypothetical planet close enough to have allowed its detection. Despite the fact that

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Fig. 9.7  Triton can just be glimpsed, at about 5 o’clock, in this image showing Neptune, above left, juxtaposed with Mars, bright and overexposed at the right. Fast-moving Mars overtook slow-­ moving Neptune and its satellite among the stars of Aquarius on 7 December 2018. Image by Mike Conley with a C-11 Schmidt-Cassegrain (Credit: Mike Conley)

their orbits were so far from correct, the mix of a mean distance, eccentricity, and mass meant that the actual conjunction of Uranus at the end of 1822 occurred close to the predicted times according to Le Verrier (beginning of 1823) and Adams (end of 1824). Thus, despite the poor fit of the orbits of the theoretical planets, in all cases, the attraction of the theoretical planet on Uranus was nearly the same as Neptune’s during the whole interval from 1800 to 1850 m Thus, there was not one orbit that would suffice for the detection; there was a whole family of orbits that did so. Moreover, the elements do not even have to be particularly close to those of Neptune for this to be the case. During the period in question, mean distance between 30 and 40 AU would suffice to give a position accurate enough to lead to the discovery (Brookes, 1970:79), while it turns out that the other elements, such as the eccentricity, longitude of perihelion (giving the orientation of the axis of the orbit), etc., were not accessible using the methods of perturbation theory adopted by Le Verrier and Adams. The timing of the two geometers was certainly impeccable in the sense that they carried out their computations at just that particular epoch (before and after the 1822 conjunction) when errors in the assumed orbital semi-major axis and eccentricity of the unknown perturber had the least possible spoiling effect on their predictions of its position on the geocentric sky. Were it not for that, their predicted positions would have been much worse—out by 10 or 15 degrees, or even more, making the predictions all but irrelevant to a practical search. With such large uncertainties, and

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even equipped with a good map as Galle and d’Arrest (but not Challis) possessed, any searchers would likely have been frustrated. The discovery would then perhaps have been made by some watcher of the skies intent on other things, as Herschel had been in scrutinising double stars, as Piazzi or Cacciatore had been in revising star catalogs, or as Hencke or Hind in hunting up asteroids. In that case the whole story would have been divested of the glamour of the nearly pinpoint theoretical prediction, of its having been discovered “with the tip of a pen.” Something along these lines had in fact been realised shortly after the discovery, at a time when everyone was dazzled by the sheer brilliance of the mathematical prediction, by Elias Loomis (1811–1889). A professor of natural philosophy and mathematics in the University of the City of New-York (predecessor to New York University), Loomis is best remembered today as the author of numerous textbooks on astronomy and mathematics. (Hoffleit, 2007:vol. 1, 706–708). He is notable here for being one of the few who took up the cudgel in defense of Challis, insisting that the pinpoint accuracy of the theoretical position had been completely serendipitous, and underlining the fact that even Le Verrier had suggested that though the most probable longitude was 325°, the planet might well lie anywhere from 321 to 335°, and that “if the planet should not be discovered within these limits, then we must extend our search beyond them.” (Loomis, 1851:58–59). “Professor Challis”, said Loomis, “proceeded like a sagacious as well as grave general. He contemplated a long siege—yet his plan rendered ultimate success almost certain. Dr. Galle took the citadel by storm—yet he had no reason to expect so easy a conquest. His success must have astonished himself as much as it did the world.” Indeed, he continued, … Let us then be candid, and claim for astronomy no more than is reasonably due. When in 1846 Le Verrier announced the existence of a planet hitherto unseen, when he assigned its exact position in the heavens, and declared that it shone like a star of the eighth magnitude, and with a perceptible disc, not an astronomer in France, and scarce an astronomer in Europe, had sufficient faith in the prediction to prompt him to point his telescope to the heavens. But when it was announced that the planet had been seen at Berlin; that it was found within one degree of the computed place; that it was indeed a star of the eighth magnitude, and had a sensible disc, then the enthusiasm not merely of the public generally, but of astronomers also, was even more wonderful than their former apathy.

In short, whereas Challis’s search was ill-starred, though through no fault of his own, Galle and d’Arrest had been incredibly lucky. The latter had been rather like a first-time fisherman told exactly where to fish who on casually casting out their line hauls in a whale. Loomis was exactly right. The discovery of Neptune was very much a “happy accident”—just not so much for the reasons that Peirce had thought it.

9.3  Setting the Record Straight: Adams Responds In the decades after Peirce put forth the “happy accident” thesis, it was generally agreed that the theoretical predictions had been valid, but only in the rather specialised circumstances obtaining around the Uranus-Neptune conjunction. The clearest

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statement of this was given by Adams himself, in an 1876 note published in Joseph Liouville’s Journal de Mathématiques. Even at this late date, Adams was still mulling over Peirce’s attack. He wrote the following letter. It is important, and so despite its length, we quote it here in full. I would now like to make a few remarks in reply to the objections of Professor Peirce, against the legitimacy of the following procedure, more by Le Verrier than by me, for the solution of our problem. Peirce claims that the period of our hypothetical planet differs so much from that of Neptune, that one indicates several intermediate periods which would be exactly commensurable with that of Uranus and that there would be a continuous solution in the perturbations of Uranus caused by two hypothetical planets of which one would have a greater period and the other a smaller period, than the commensurable period that is under question. Further the period of Neptune itself is almost exactly double that of Uranus, and this gives rise to some very considerable reciprocal perturbations of a character quite different to those which would be caused by our hypothetical planets. A few words, in my opinion, would be enough to smooth out this difficulty. It is true that, if one wishes to represent the perturbations of Uranus as being caused by a superior planet during two or more synodic periods, that this could only happen by adopting a period which is approximately true for the planet causing the perturbations; but the case is different when as here, we only have to represent the perturbations produced during a fraction of a synodic period. In this case, if we were to take for the unknown quantities, not the corrections applicable to the mean elements of the orbit of Uranus, but those which would be applicable to elements accepted, for example, for the epoch of 1810, then all the relative considerations to an approximate commensurability in the two periods, would become answered, and the perturbations for the required limited interval could be approximately represented, provided that the perturbing force of the real planet and of the presumed planet were approximately the same in strength and direction during the time when these forces act with the greatest intensity, that is to say when the planets are not very far from their conjunction. Sir John Herschel has shown in his “Outlines of Astronomy” that these conditions are fulfilled satisfactorily by the hypothetical planets of M Le Verrier and myself, when their action is compared to that of Neptune. One shouldn’t attach any value to the strong eccentricity nor to the longitude of the apsis of the orbit of the presumed planet, if it’s not enough to furnish the means to come close to the actual distance and the angular movement of the perturbing body, in the interval where the action made itself most felt. Therefore, from the circumstances that the perihelion of the presumed planet is removed from the first calculation, not far from the line of conjunction, one should, reasonably, conclude that it has given the second calculation, that the hypothesis of the one with least value from the mean distance leads to a value weaker than the eccentricity. One must also remark that the big changes in the values of δe and eδω … result from the transition from my first to my second hypothesis and are changes in the values of the mean elements in the orbit of Uranus, which are greatly affected by the inequality of the mean longitude …whose period does not differ much from that of Uranus, particularly in the case of the first hypothesis…. The observation by Flamsteed in 1690 is too long ago for it to be well expressed by the formulae, but the results accord quite well with those of recent observations. My second hypothesis has given a greater error than the first. It is therefore probable that Le Verrier had too much confidence in the possibility of applying these formulae to the old observations, and so M Le Verrier was led to fix a lower limit to the mean distance of the perturbing planet; which does not agree with the mean distance of Neptune, as has been observed. (Adams, 1896; vol. 1, 63–65, translated by W.S.)

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Neptune, then, as Adams realised, was a special case: the only thing needed to infer the heliocentric longitude, since the perturbations considered occurred around the time of the Uranus-Neptune conjunction, was a short arc of the orbit, and this was what was available. But the other elements of the orbit, such as the period and eccentricity, were not unambiguously determined in such a case. In order to get at them, one would need a much longer baseline, in which the perturbations would be observed over several orbital periods of the perturber, for then its orbital period could be deduced from periodicities in the observed perturbations. The problem then becomes more determinate, since, as Adams wrote, a period (and hence a mean distance) very close to the correct one would be needed in order to represent the perturbations of Uranus over this longer span. (There still remains the indeterminacy of the perturber’s eccentricity but that is much less serious in a planet-planet problem.) Parenthetically, it seems that George Biddell Airy realised this as far back as 1837, and his understanding is reflected in his much-mocked but basically correct comment to the Rev. Thomas J. Hussey, “It is a puzzling subject, but I give it as my opinion, without hesitation, that it is not yet in such a state as to give the smallest hope of making out the nature of any external action on the planet…. I am sure it could not be done till the nature of the irregularity was well determined from several successive revolutions.” (Airy, 1847:123). Airy did not anticipate at the time that by exploiting the special circumstances which obtained for the short arc around the Uranus-Neptune conjunction the planet’s geocentric longitude could be worked out without the longer (more than two synodic period) baseline. (There are many other perspectives on the history of the discovery, including: that of Benjamin Apthorp Gould, Jr. [1824–1896] [1850] which gave a very particular US view. Antonie Pannekoek [1961:359–363] discussed the European response to Peirce’s “happy accident” conclusion and, citing the Leiden astronomer Frederik Kaiser [1808–1872] contrasts the view of European astronomers, who [pp. 362–363] “had seen the discovery of Neptune chiefly as a means to impress the lay world with the perfection of their science.” He wrote that in North America “they did not proclaim the miraculous character of the discovery, but they worked all the harder to make it subservient to the benefit of science.” In the United States, Pannekoek concluded (p. 363), “there was no need for such a fight for social progress against antiquated social systems and powers as in Europe where, on the contrary, militant science was an important factor of a new culture.” However, for a counter-argument to Pannekoek’s Marxist perspective, see (Hubbell and Smith, [1992]). A modern perspective on the perturbations of Uranus is presented in Appendix 9.1.)

9.4  A New Test of Celestial Mechanics: Vulcan The American historian and philosopher of science Norwood Russell Hanson has written (Hanson, 1962:359), “By pressing Newton’s theory to the limit of its capacities to explain and predict, Le Verrier revealed Uranus’s aberrations as intelligible;

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he also predicted the existence of the then-unseen planet Neptune…. In history few have approached Le Verrier’s achievement as a human resolution of an intricate natural problem.” The discovery of Neptune marked the “Zenith of Newtonian mechanics”, in Hanson’s arresting phrase. Soon after the discovery of Neptune, Le Verrier returned to Mercury, in whose motion he had already suspected difficulties even before he had been assigned to sort out the deviations of Uranus by Arago. As far back as 1841, Arago had suggested that Le Verrier apply his unique analytical skills and almost superhuman endurance for long calculations to work out the theory of Mercury’s motions. This planet had always been rather irksome, partly because of its proximity to the Sun, which made exact determinations of its position difficult except during its transits when it actually was projected against the solar disk, partly because of the large eccentricity (0.205) of its orbit. Le Verrier began his investigation by compiling the data for the twelve best-observed transits between 1697 and 1832. He found that previous mathematical astronomers who had attempted to calculate the times of transits from tables of the planet’s motion had been quite widely in error: predictions for the 1708 transit were a day in error, the tables of Philippe de la Hire (1640–1718) and Halley had missed that of 1753 by many hours, while even J.J. Lalande’s tables had failed by 53 minutes in 1786—with the result that all but one of the astronomers of Paris had already abandoned their telescopes before the transit got underway. (Jean Baptiste Joseph Delambre was the one exception.) (Baum and Sheehan, 1997). In his earlier attempt, Le Verrier had struggled with the intractable problem of Mercury’s motion for three years, until 1843, when he published a paper, “Détermination nouvelle de l’orbite de Mercure et de ses perturbations”. (Le Verrier, 1843). Two years later he followed up with a book (Le Verrier, 1845), in which he gave a prediction for the transit of 8 May 1845. It was to be partially visible from France and entirely from North America. The transit was eagerly awaited by the celebrated American astronomer Ormsby Macknight Mitchel, who was equipped with the just-commissioned 11-inch Merz equatorial of the Cincinnati Observatory, then the largest telescope in the United States. Mitchel later described his thorough preparations for the transit (based on Le Verrier’s tables) in one of his popular lectures: The Observatory of which I have had the honor of the direction, so far as the building, and the mounting of a single instrument were concerned, was completed in 1845. In May of that year it was announced that Mercury would again cross the disc of the sun. It was the first observation I ever attempted to make. I had computed with all the delicacy in my power the exact moment when the dark planet would touch the brilliant rim of the sun. I had gone yet further, and computed the exact point on the tremendous circumference of the sun where the contact would take place…. The eventful day arrived, and the sun rose bright and glorious. Not a cloud stained the deep blue of the heavens. As the hours rolled by, and the time approached, there I was, with feelings such as you can not conceive, understand, or comprehend. My assistants were around me, ready with their chronometers to mark the moment of contact. I hoped and believed that our tables and computations were so accurate that five minutes of time would be a sufficient limit, and five minutes before the appointed time I took my place at the great telescope. There I waited and waited, until it

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seemed as if an age had gone. I called out, “Surely the time is passed—what of the time?” “Only a single minute!” Second by second, only a minute had rolled away. It seemed as though hours had been sweeping slowly by. Again I took my watch and waited, until again it seemed as though an age had passed. “Surely,” said I, “the time is gone.” “No—another minute yet.” At last I caught the black disc of the planet just impinging upon the bright rim of the sun—in the limits of a minute? No; but within sixteen seconds of the computed time! (Applause.) (Mitchel, 1859:49–50)

Though Mitchel thought the agreement uncanny, Le Verrier himself was so distressed with a gap of sixteen seconds that he immediately stopped tables of Mercury which were even then being typeset by the Bureau des Longitudes. He was eager to recompute everything by taking in a detail he had so far ignored, the tables of the Sun’s motion (which, of course, meant re-examining the motion of the Earth). It was just then that Arago fatefully reassigned him to the problem of Uranus, and he did not finish the revisions of the tables of the Sun’s motion—and finally get back to Mercury—until 1858 (Fig. 9.8).

Fig. 9.8  The 11-inch Merz refractor of the Cincinnati Observatory, used by Ormsby Macknight Mitchel to observe the 8 May 1845 transit of Mercury. A little over a year later, on 28 October 1846, Mitchel and an assistant used this telescope to observe Neptune. (Mitchel, 1887:175). From: Sidereal Messenger, vol. 1, no. 1, 1846 (Credit: Trudy E. Bell)

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Since most of the Mercury data Le Verrier relied on depended on observations of its transits across the Sun, which can occur only in November and May when Mercury passes near the ascending and descending nodes of its orbit respectively, in revising his tables of Mercury’s motion he now had to tackle the theory of the Sun’s motion. He found that theory and observation agreed within the margins of observational error for the November transits, but not for the May transits. Working back from the 1845 transit, he noted a progressive error from one transit to the next, which reached 12 arc seconds for that of 1753. The errors could be attributed neither to errors of the theory of the Sun nor to errors of observation. They could be corrected only by modifying the time-dependent elements of the orbit, and though most of the effect could be accounted for by Newtonian gravitation (owing to the influence of the other planets), a small amount could not. Famously, Le Verrier found— as he announced in 1859—that there was a small but unmistakable discrepancy in the rate at which Mercury’s perihelion was precessing, of 38″ of arc per century. (Le Verrier, 1859). (The value of this excessive rate of precession was subsequently recalculated by the American astronomer Simon Newcomb, and revised to 43.0″.). This was a small number, but there could be no doubt that it was genuine, and Le Verrier set out to account for it. Pressing into service the same paradigm that had succeeded so brilliantly in accounting for the case of Uranus, the French mathematical astronomer posited the existence of some additional mass inside Mercury’s orbit. It didn’t need to be a planet. It could also have been a ring of asteroidal debris. But whatever it was, it needed to be present in sufficient quantity to account for the excess motion of Mercury’s perihelion without affecting the planet’s nodes or the motions of Venus and the Earth. For a “reasonable” orbital radius, say half that of Mercury, it would have to be about as massive as Mercury in order to cause a rate of advance of Mercury’s perihelion of the right order. (Lo, Young, and Lee, 2015). In contrast to the situation in 1845–46, when it had proved so difficult to find an observational astronomer to look for a planet, Le Verrier’s inference proved greatly stimulating to the imaginations of astronomers, and set off an exciting search directed for a planet or planets in the innermost, rather than the outermost, precincts of the Solar System. (See: Baum and Sheehan, 1997; Sheehan and Misch, 2016). It did no harm that Le Verrier had even given the putative planet name, Vulcan, which made it seem more real than if it had simply been the consequence of a ream of mathematical symbols. Flushed with optimism, astronomers hoped that Vulcan might be spotted during a transit across the disc of the Sun, or as a star-like object near the Sun during a total eclipse. There were brief moments in which Vulcan’s stock seemed to rise, as in the strange case of Doctor Edmond Modeste Lescarbault (1814–1894), an amateur astronomer who reported that he had seen a planet in transit across the Sun on 26 March 1859; he even managed to convince Le Verrier, who remained convinced of Vulcan’s existence until his death in September 1877. At the North American eclipse of July 1878 (Sheehan, 2016a), there were several sightings of strange objects near the Sun; but neither Lescarbault’s planet nor those seen at the eclipse held up to closer scrutiny, and enthusiasm faded. After thirty years of searching for the

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intra-­mercurial planet, Camille Flammarion (1894:349) summed up the negative record: “There has not passed a single day, so to say, without the Sun being examined, drawn, or photographed in Italy, England, Portugal, Spain, America, France, and elsewhere; that the supposed planet would have passed more than a hundred times across the Sun, and that, nevertheless, it has never been seen. It is either well hidden, or it does not exist. Mercury was the god of thieves; his companion steals away like an anonymous assassin.” We now know that Vulcan does not exist, and never did. Neither is there an asteroidal ring massive enough to account for the anomalous precession of Mercury’s perihelion. There is, however, a dust ring associated with Mercury’s orbit, found recently as 2019 by astronomers studying images from NASA’s Solar and Terrestrial Relations Observatory (STEREO) (Stenborg, Stauffer, and Howard, 2018), but it is a mere will-o’-wisp, and has nothing to do with the anomalous precession discovered by Le Verrier. The latter cannot, in fact, be solved using the Newtonian paradigm. Though Le Verrier did not live to realise it, the triumph of Neptune was not repeated in the case of Vulcan. As is well known, the anomalous precession of Mercury’s perihelion was precisely accounted for by Albert Einstein in November 1915, on the basis of the new gravitational theory set forth in his General Theory of Relativity. Indeed, it provided Einstein with the first observational confirmation of his theory. (Will, 1986). Parenthetically, it may be useful to note that Einstein’s prediction was one without free parameters, whereas the Vulcan hypothesis (which failed) or the Neptune hypothesis for Uranus (which succeeded) tried to explain the residuals on the basis of the several parameters of the orbital elements and the mass of the perturber.

9.5  Other Planets Beyond Neptune? As the elusive intra-mercurial planet continued to haunt astronomers’ dreams, minor planets proliferated in the zone between Mars and Jupiter. By the time Le Verrier died, 174 had been identified. After the photographic discovery of no. 323, named Brucia by its German discoverer, Max Wolf (1863–1932) (after his benefactress, the American heiress Catherine Wolfe Bruce (1816–1900)), their trails began turning up in such great quantities on astronomers’ long-exposure photographic plates as to invite scorn. Most astronomers didn’t even bother to follow them up; they were “vermin of the skies:” The quest for new planets did not yet peter out in seeming dead-ends in the inner Solar System, however. Even after Neptune had been found, Uranus did not quite seem to be following the orbit predicted for it, and astronomers began to wonder whether the explanation might lie in the existence of a planet—or planets—even farther out. (In principle, Neptune’s motion should also have been affected by such a planet, but it had been under observation for too short a time to be useful in this regard.)

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Le Verrier had always thought it likely that another planet or planets might exist. Shortly after Neptune’s discovery, he had written to the Swiss astronomer Jean-­ Alfred Gautier, “This success allows us to hope that after thirty or forty years of observing the new planet, we will be able to use it in turn for the discovery of the one that follows it in order of distance from the Sun.” (Baum, 2007:41). Indeed, as far back as 1848, Jacques Babinet (1794–1872), a physicist, member of the Paris Academy of Sciences, popular lecturer on scientific topics, and secretary of the library of the Paris Observatory took up the argument of Peirce that in the calculations leading to the discovery of Neptune “enormous errors” had been made, and that “no one admits any longer the identity of Le Verrier’s planet with that which disturbed Uranus.” (Babinet, 1851:53). Instead he suggested that Adams’s and Le Verrier had discovered nothing at all, and that there were two perturbing planets—Neptune itself, and another planet farther on, which he named Hypèrion. The latter’s mass, he suggested rather naively, was simply the difference between that of Neptune and the planet predicted by Le Verrier! Since Le Verrier was a rather rebarbative personality and had many enemies, none more vehement than Babinet himself, the latter may well have been motivated partly out of sheer spite. In any case, his suggestion was dismissed by all competent mathematical astronomers, including not only by Le Verrier himself but Sir John Herschel, Wilhelm Struve, and the German mathematician Carl Jacobi. Hypèrion proved stillborn, and was soon forgotten (Fig. 9.9).

Fig. 9.9  Jacques Babinet (Public Domain)

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The possibility of a trans-Neptunian planet continued to stimulate the imaginations of astronomers, and now and then a possible discovery was mooted. In 1850 James Ferguson, a U.S. Naval Observatory astronomer remembered as the first to discover an asteroid (Eurphrosyne) on American soil, was tracking the asteroid Hygeia (or Hygea as it was then known) through the starry thickets of Sagittarius, using a 9th magnitude star as his comparison star. (Baum, 2007:41–65). The following year John Russell Hind found that the comparison star had apparently disappeared, and on looking closely at Ferguson’s records, surmised that it might be a trans-Neptunian planet, at a distance of about 137 AU and with a period of 1,600 years. Ferguson himself was charged with attempting to recover it but did not succeed—nor did anyone else. A quarter century later, Christian Heinrich F. Peters (1813–1890), director of the Hamilton College Observatory, finally explained what had happened. Ferguson had apparently mislabeled one of the wires of the micrometer he was using and thus had recorded what was an ordinary star—designated No. 36878 by Lalande—in the wrong position. Like Wartmann’s star, it was a figment produced by observational errors. (Baum, 2007:54–68). A more rigorous investigation of possible denizens of trans-Neptunian space occurred as a byproduct of the mathematical researches of the Canadian-American astronomer Simon Newcomb (1835–1909), whose heroic feats of calculation using perturbation theory were second only to those of Le Verrier himself. In 1866, he published tables of the motions of Neptune, in which he briefly glanced at the possibility that his detailed analysis might discover indications of an unknown planet beyond it. Newcomb wrote (1866:2) The near approach to commensurability of the mean motions of Uranus and Neptune renders the general theory of the mutual action of [these planets] extremely complex… Twice the mean motion of the latter exceeds that of the former by only 329" [of arc] according to Walker, or 304" according to my first revision of his results. The terms in the perturbations which contain this very small quantity as a divisor will, therefore, be very large. Considered as perturbations of the elements, this period will be more than 4000 years. We have an analogous instance in the 900-year equation of Jupiter and Saturn. But in the latter case the perturbations of the mean motion are of the third order with respect to the eccentricities and inclinations, while in Uranus and Neptune they are of the first order. From this circumstance it happens that notwithstanding the smaller masses of the disturbing planets, the perturbation of the mean motion is as great in the case of the planets in question as in that of Jupiter and Saturn, and that of the other elements enormously greater. In fact, the perihelion of Neptune oscillates through a space of eight degrees in consequence of the terms in question. Such a perturbation as this, four degrees on each side of the mean, is, I think, found nowhere else in the solar system.…

In the end, Newcomb could discern no evidence for the existence of a trans-­ Neptunian planet from his analysis of Neptune’s motion. To the contrary, he found that even the existence of Uranus itself could hardly be inferred from the motion of Neptune, and concluded that unless a trans-Neptunian planet was in conjunction with Neptune at the time he wrote this (as Neptune had been with Uranus around the time Adams and Le Verrier had carried out their investigations) or else was much more massive than Uranus, it would not disclose itself.

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Fig. 9.10 Simon Newcomb (Credit: Library of Congress)

Newcomb’s tables of Uranus appeared in 1873 (Newcomb, 1873). With his usual thoroughness, he had carefully re-evaluated the reductions of the old observations, and found systematic errors in some of them—notably, the Greenwich observations for 1831 (reduced under Airy) seemed to be affected with an error of some 2" of arc compared to those the year previous and the year after. New reductions were carried out where needed, though he realised that there was no way of going back and repeating a specific observation, and so the possible errors, especially those involving the “ancient” observations could not be eliminated entirely. The only thing to be done was to give them less weight. He also introduced a small correction for Neptune’s mass. At the end of these vast labors, Newcomb announced (p.  70) that once again he had found no indications of any additional planets (Fig. 9.10).

9.6  The Quest Continues Though Newcomb’s memoirs on the motions of Uranus and Neptune did nothing to encourage belief in the existence of a trans-Neptunian planet, the dream of being covered in the glorious mantel of planet-discovery proved irresistible to some astronomers. One of these was Newcomb’s own associate at the U.S.  Naval Observatory, David Peck Todd (1855–1939). Rather than follow the complicated perturbation theory that Newcomb had used to calculate his tables, Todd took a short cut, adopting a graphical method like that Sir John Herschel had used to

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elucidate the circumstances of Neptune’s discovery (a graphical method that Todd claimed to have discovered independently). (Hoyt, 1980:75–76). Instead of using Newcomb’s residuals, Todd used those published by Le Verrier in the same year, 1873. Having graphed these, Todd looked for any unexplained inflection points similar to those Sir John had found for Uranus around the time of the 1822 conjunction with Neptune—and seemed to find one indicating the existence of a planet with a period of 375 years and a mean distance of 52 AU.  He estimated a longitude of the planet for epoch 1877.84 of 170°, plus or minus 10°, putting it in the constellation Gemini. He guessed its magnitude as approximately 13+. Despite the uncertainties, he thought it worthwhile to point a telescope to the indicated region of the sky and spent thirty nights between 3 November 1877 and 6 March 1878 sweeping the stars of Gemini with the U.S.  Naval Observatory’s 26-inch (66-cm) refractor—without success. Realising the tenuousness of the grounds of his search, he kept it secret for three years, and revealed it only when another planet seeker, George Forbes (1849–1936), professor of natural philosophy at Anderson’s University, Glasgow, announced on equally speculative grounds his own pair of putative planets, lurking at 100 and 300 AU, and having periods of about 1,000 and 5,000 years, respectively. (Hoyt, 1980:76–77) (Fig. 9.11).

Fig. 9.11  David Peck Todd, with the planetary camera used for the Lowell Observatory Expedition to the Andes to photograph Mars in 1907 (Credit: Lowell Observatory)

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Meanwhile, yet another method of trying to constrain the whereabouts of one or several trans-Neptunian planets was introduced, in almost casual fashion, by Camille Flammarion, in his best-selling book Astronomie populaire. Flammarion noted that a number of comets belonged to families with aphelia near those of the major planets presumed to have perturbed and captured them. Jupiter, Saturn, Uranus and Neptune all had such families, and beyond Neptune, Flammarion claimed to find similar indications. Might there not be a large planet at a distance of 48 AU, he asked? (Flammarion, 1894:471) (Fig. 9.12). Eventually, these faint and somewhat fleeting intimations of another world encouraged the more forceful and persistent efforts of Harvard College Observatory astronomer William H. Pickering (1858–1938) and Flagstaff astronomer Percival Lowell (1855–1916). Pickering used the short-cut graphical method, and in 1908 presented to a meeting of the American Academy of Arts and Sciences the results of an investigation which had revealed the existence of a possible trans-Neptunian

Fig. 9.12  Jupiter’s family of comets (Credit: Percival Lowell, The Solar System. Boston, Houghton and Mifflin, 1903)

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planet, “Planet O” (so-called because O was the next letter in the alphabet after N, for Neptune). Planet O closely resembled Todd’s putative planet, though this is hardly surprising, since Pickering too relied on Le Verrier’s residuals. The mean distance was 51.9 AU, the period 373.5 years, the mass twice that of the Earth. Its position put it in the constellation in Gemini. He carried out a secret, unsuccessful search for it before publishing his result. Later, in 1919, he managed to persuade Milton Humason (1891–1972) of the Mt. Wilson Observatory to search for it (the image of Pluto actually registered on the plates, but this was not recognised at the time). Undiscouraged, he began publishing with almost metronomic regularity new predictions for planets, which he called “P”, “Q”, and “R”, with elements notable for the complete lack of rigour with which they were derived, being based rather casually on graphical assessments of residuals or statistical analyses of comet aphelia. The sheer number of his predictions and the qualitative nature of his methods did not inspire confidence, though after the discovery of Pluto, Pickering argued, completely unconvincingly, that it was actually none other than his planet “P”. (Cruikshank and Sheehan, 2018:127) (Fig. 9.13).

Fig. 9.13  William Henry Pickering (Credit: Wikipedia Commons)

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Lowell, like Pickering a native of Boston who in 1894 founded (with Pickering’s assistance) an observatory at Flagstaff, Arizona Territory, to study Mars, was to become the most energetic and persistent searcher for a trans-Neptunian planet, which he called Planet X. Indeed, in the last decade of Lowell’s life, the planet search even more than Mars became his principal preoccupation. (Hoyt, 1980; Cruikshank and Sheehan, 2018). Convinced by the comet aphelia data to which Flammarion had called attention, Lowell, as early as 1905, hired a series of assistants from Indiana University to systematically photograph the “invariable plane”— the mean orbital plane of the known planets—where any unknown planet orbiting the Sun would most likely be lurking. This was, in fact, the first systematic photographic search for a major planet of the Solar System in history. Assistants John Charles Duncan (1882–1967), Earl Charles Slipher (1883–1964), and Kenneth Powers Williams (1887–1958)—of whom only Slipher, was to remain as a permanent staff member of the observatory after the temporary stint of planet-searching had ended—exposed hundreds of plates over the next three years with a 5-inch (13-­ cm) photographic refractor. Their routine was to photograph a given region of the sky twice at an interval of two weeks and then ship the plates to Lowell’s Boston office, his base between Flagstaff observing runs. There Lowell examined the plates himself, laying one on top of the other on his lap and examining the star fields with a hand magnifying lens (Fig. 9.14). Fig. 9.14  Percival Lowell, examining negatives on the porch of the Baronial Mansion the year he began the photographic search for Planet X (Credit: Lowell Observatory)

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This was slow, tedious work, and eventually Lowell decided that there was little chance of finding Planet X without a calculated position of the kind Le Verrier had furnished to Galle. In addition, he began to feel under some pressure to pick up the pace, as he had been in the audience at the meeting at which Pickering had announced his Planet O prediction. He briefly tried the graphical method, but found it unsatisfactory, and by the summer of 1910 he committed himself to adapting the same analytical method based on inverse perturbation theory that Le Verrier had used in the search for Neptune. (This and the following sections are based on: Hoyt, 1980; Cruikshank and Sheehan, 2018). As had been the case with both Adams and Le Verrier, the calculations depended critically on the residuals, representing the difference between the theoretically calculated and observed positions and in general subject to uncertainties which can come in either from the theoretical side, due to uncertainties in the orbital elements and mass used in the theory, or on the observational side. At first, Lowell hired a computer, William T. Carrigan (1865–1922), to recompute the residuals. However, he soon realised that this had already been done by Le Verrier himself, who in preparing his 1873 tables had published new reductions of all the observations up to that time, including of all of Flamsteed’s and Le Monnier’s observations. Subsequently, he also examined the work done by Le Verrier’s protégé, Jean Baptiste Aimable Gaillot (1834–1921) of the Bureau des Longitudes. Gaillot had completed Le Verrier’s planetary theories after the great man’s death in 1877, and finally published new tables of Uranus (as well as Neptune) in 1909, in which he adopted different masses for the planets from those Le Verrier (and Newcomb) had used. In preparing his equations of condition, Lowell adopted the residuals for Uranus published by Gaillot, and found that they were reduced from the 133″ in Adams’s and Le Verrier’s time to at most 4.5″ at any point in Uranus’s path. This was the maximum; most of the residuals were 2″ or less. Though this might seem small enough to be accounted for by observational errors, Lowell, at least, did not think so. Noting that the residuals based on Le Verrier’s theory differed significantly from those based on Gaillot’s, Lowell declared, “We cannot, therefore, assert that the present theory is not responsible; but we can say from the doctrine of probabilities that the present outstanding irregularities in the motion of Uranus are probably not due to errors of observation.” (Lowell, 1915:34–35). With this statement, the die was cast. Lowell proceeded from what appears to us to be but the faintest signal of a planet to set up equations of condition in which least-mean-­squares of groups of residuals were set equal to the various perturbation equations found in the works of Pontécoulant, Le Verrier, and François Félix Tisserand (1845–1896), whose four-­volume work was the most comprehensive treatment of classical celestial mechanics ever published). (Tisserand, 1889–1896). In retrospect, it appears obvious that Lowell had embarked upon a chimerical quest. The residuals, taken from Gaillot, were almost vanishingly small, and the perturbation methods used by Adams and Le Verrier, which had worked for them only owing to fortuitous circumstances, could be applied only through what was

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almost an act of faith to the present case. Lowell was forced to do as Adams and Le Verrier had done and assume a value for the distance of the perturbing planet in order even to begin. But he was on even more uncertain ground than his illustrious predecessors had been, since Bode’s law, which had so brilliantly succeeded up to and including Uranus, had failed decisively in the case of Neptune. Lowell, acting on a hunch, decided that instead of taking the Bode’s law distance, would take the somewhat smaller distance of 47.5 AU. He then proceeded with a series of iterative calculations similar to (but much more laborious than) Le Verrier’s, scanning values for the longitude of his planet that would minimise the residuals of Uranus. So, the massive calculation got underway in Lowell’s State Street office in Boston. Too vast for Lowell to carry through alone and unaided, he enlisted several human computers (as they used to be called), of whom the chief was Elizabeth Langdon Williams (1879–1981), the first woman to graduate from the Massachusetts Institute of Technology and a good enough mathematician to occasionally find— and tactfully point out—math errors in her boss’s calculations. He also acquired such state-of-the-art calculating instruments as “Thacher’s Cylindrical Slide Rule”, billed as the “ultimate cylindrical slide rule”, and the “Millionaire Calculator”, the first commercially successful motorised mechanical calculator advertised as the “only calculating machine on the market … that requires but one turn of the crank … for each multiplier or quotient.” It was fast, but big and clunky, and occupied an entire desk. Despite this inconvenience, it was a step up from doing multiplication and long division by hand. (Cruikshank and Sheehan, 2018:114). In setting out to surmount this Everest of calculations, Lowell and his assistants were proceeding along a route that was far more difficult than that Adams and Le Verrier had trodden, because of the smallness of the residuals. The technical details of the investigation need not concern us. Suffice to say, the uncertainties were great, and Lowell’s calculations favoured now one and now another position. His first result put the planet in Libra, but over the next several years, the favoured position moved from place to place. Finally he settled on eastern Taurus, in the heart of the Milky Way’s myriad stars (Fig. 9.15). As soon as he arrived at a position, Lowell telegraphed it to his staff in Flagstaff, and waited impatiently for results. At first, staff astronomer Carl Otto Lampland (1873–1951) was charged with searching the star fields near the indicated positions, using the observatory’s 24-inch (61-cm) refractor. Given the small field of the refractor, this was, however, hopeless, and before long Lowell got hold of a more suitable instrument, a 9-inch (23-cm) photographic refractor borrowed from Pennsylvania’s Sproul Observatory. (Hoyt, 1980; Cruikshank and Sheehan, 2018). Other efficiencies were also introduced. Instead of superimposing one plate over the other and examining them with a hand magnifying lens as in the early days, Lampland proposed to Lowell the use of a viewing apparatus specifically designed for such the purpose—the blink comparator, invented by physicist Carl Pulfrich (1858–1927) at Zeiss in 1904, which used an electromagnet to flip a small mirror back and forth to redirect the light path successively from a small area of one plate to the corresponding area on a second matched plate. On one of his regular trips to

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Fig. 9.15 Elizabeth Williams, lead computer for the Planet X search (Credit: Lowell Observatory)

Europe, Lowell had visited Zeiss, and examined such an instrument but was not impressed. However, by 1911 his thinking had changed, and a blink comparator had been acquired. With this apparatus, the image of any planet registered in exposures taken several days apart would betray itself immediately by appearing to jump back and forth, making the search much more efficient. It was by such means that Clyde Tombaugh (1906–1997) long afterward would discover Pluto (Fig. 9.16). Meanwhile, the search ground on, from year to year. Lowell’s wife, Constance (whom he had married when he had reached the rather scandalously advanced age of 53 in 1908; she was 45), would recall to Lampland what life was like during this period of feverish activity (and in addition to searching for Planet X, Lowell kept up his heavy observing routines regarding Mars when it made its biennial close approaches to the Earth (Fig. 9.17): As you know, it is not easy for the observing astronomer to lead a strictly regular life in that the hours at the telescope often make it necessary to use, for the much needed rest, part of the daily hours usually given to work. His intense occupation with his research problems, however, was broken with great regularity for short intervals before lunch and dinner. These times of recreation were given to walks on the mesa or work in the garden. When night

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Fig. 9.16  Thomas Gill, at the eyepiece of the 9-inch (23-cm) Sproul photographic refractor, 1916 (Credit: Lowell Observatory)

Fig. 9.17  The fireplace of the den in the Baronial Mansion on Mars Hill, where Percival Lowell would unwind with books such as Tisserand’s Mécanique celeste and mystery novels, of which 200 were found in his possession following his death in 1916 (Credit: Lowell Observatory)

came, if he was not occupied at the telescope, he was generally to be found in his den. It was not always possible for him to lay aside his research problems at this time of day, but he did have some wholesome views on the necessity of recreation and a necessary amount of leisure to prevent a person from falling into the habit of the “grind.” To those who came to his den the picture of some difficult technical work near his chair, such as Tisserand’s

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Mécanique céleste, will be recalled, though he might at the time be occupied with reading of a lighter character. And occasionally during the evening he might be seen consulting certain difficult parts upon which he was pondering. (Lowell, 1935:153–154)

Long afterward it would be discovered that in 1915 Pluto had been captured on two plates taken by assistant Thomas B. Gill, but its image was excessively faint, lying near the limit of the search and camouflaged in the thickets of the Milky Way. But by then, despite his “wholesome views”, Lowell was succumbing to the “grind”; worn down by the long calculations and discouraged by the negative results, he was on the verge of giving up. In January 1915, he presented the results of his mathematical investigation in an address to the American Academy of Arts and Sciences. It created hardly a stir. The Academy even declined to publish the paper. Nine months later, it appeared—published at Lowell’s expense—as a Memoir on a Trans-­ Neptunian Planet. (Lowell, 1915). It is a tour-de-force, and ends on a somewhat un-Lowellian note of skepticism about the methods used. (Clearly, the inverse-­ perturbation method had lost some of its lustre since the glorious days of 1846.) He wrote: Owing to the inexactitude of our data … we cannot regard our results with the complacency of completeness we should like…. The fine definiteness of positioning of an unknown by the bold analysis of Le Verrier or Adams appears in the light of subsequent research to be only possible under certain circumstances. Analytics thought to promise the precision of a rifle and finds it must rely on the promiscuity of a shot gun after all, though the fault lies not more in the weapon than in the uncertain bases on which it rests. (Lowell, 1915:104).

Despite the evident uncertainties, Lowell offered two heliocentric longitudes for Planet X, 180 degrees apart, of which one was in eastern Taurus near the border with Gemini. The most likely elements were: mean distance from the Sun 43 AU, orbit eccentric and inclined by perhaps 10 degrees to the ecliptic, mass seven times that of the Earth. After publication of the Memoir, Lowell said no more about Planet X. Nominally, the photographic search with the 9-inch (23-cm) Sproul refractor continued until July 1916, when the log book contains a final entry: “Lunch.” This would seem to suggest a project only temporarily suspended. Lowell was just then preparing to embark on a rigorous lecture tour of American universities in the Northwest and Pacific coast, in order to pitch his embattled Martian theories to university students, and when he returned in October, he threw himself into an ambitious program of observing the fifth satellite of Jupiter. Nervous and high-strung at the best of times, he had pushed his limits farther than he knew, and died suddenly of a massive stroke in November 1916. With his death, the search for Planet X had apparently died with him (Sheehan, 2016b). After a hiatus of thirteen years, in which the observatory was consumed with a lawsuit by his widow who wished to break Lowell’s will, and whose settlement in 1925 left the observatory famously broke, the search for Planet X was finally resumed at the Lowell Observatory with a new dedicated telescope for the purpose. This was the 13-inch (33-cm) astrograph acquired by funds from Lowell’s brother (and president of Harvard University), Abbott Lawrence Lowell (1856–1943).

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A young self-taught amateur astronomer from rural Kansas with an enormous capacity to endure boredom, Clyde Tombaugh, was hired to expose the plates and to examine them on the blink comparator. Finally, after a year of searching, in February 1930, Tombaugh discovered a fourteenth magnitude speck jumping back and forth on his plates. It was located only six degrees from where one of Lowell’s last-­ calculated positions (precessed forward), had put his planet. Divulging by its slow motion that it was a denizen of trans-Neptunian space, it was hailed as the ninth planet, Pluto. Whether it should be considered a planet continues to evoke strong reactions, for and against. (Officially, it has been classified as a “dwarf planet” since 2006, when the International Astronomical Union adopted a new definition of a planet.) But was it Lowell’s Planet X? (For more detailed accounts of the search for Planet X and the discovery of Pluto see, for instance: Cruikshank and Sheehan, 2018; Tombaugh and Moore, 1980; Hoyt, 1980) (Figs. 9.18 and 9.19). Pluto proved to be, on closer acquaintance, a rather disappointing object: it did not show a disk in the telescope, and was clearly much too small to have sufficient mass to perturb the sunward giants Uranus and Neptune to any discernible degree (unless its mass was for some reason much greater than it appeared; for instance, if it consisted of degenerate matter, like a white dwarf star, which hardly seemed likely). But then it could not be Planet X after all. Its mass has now been shown to be only 0.024 times that of the Earth, not the six Earth masses Lowell had predicted,

Fig. 9.18  The 13-inch (33-cm) Abbott Lawrence Lowell astrograph at Lowell Observatory, used to search for Planet X beginning in January 1929 (Credit: Lowell Observatory)

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Fig. 9.19 Clyde Tombaugh at the eyepiece of the Lawrence astrograph, 1930 (Credit: Lowell Observatory)

and there is no longer any doubt that Tombaugh’s discovery owed nothing to Lowell’s calculations and everything to the thoroughness of Tombaugh’s search— and to the sheer serendipity of an object happening to be bright enough to be within reach of the means utilised. We now know that Tombaugh was far ahead of his time in discovering the premier object of the Kuiper belt, some sixty years before the next one, QB1 (now known as Albion), turned up in 1992. Immediately after Pluto’s discovery, the esteemed mathematical astronomer Ernest William Brown (1866–1938) of Yale University re-examined Lowell’s calculations (Brown, 1930). He discovered that the values that Lowell had obtained for the distance, mass, and eccentricity of his Planet X all depended on three groups of observations that had been made before 1783, observations that had appreciable errors. The long chain of reasoning by means of which Lowell hoped to find the planet had, according to Brown, rested ultimately on a very weak first link, for Lowell had given no probable errors for his results, nor had he weighted the residuals he had used. However, in Brown’s view (1930:368) this had fatally doomed the quest, since “the weighting of the material in a problem of this nature is of fundamental importance, because the question of the validity of the hypothesis depends almost entirely on the probable error of the unknowns.” In retrospect, Brown was entirely correct. The raw measurements by 18th and 19th century astronomers using meridian circles on which Lowell had erected the towering edifice of his calculations had provided a weak foundation. As with the marginal perceptual data on which he based his theories about Mars, so with his search for Planet X, the “signal”, such as it was, had always been scarcely distinguishable from the noise. In both cases this led to what we would now describe as

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Fig. 9.20 Ernest W. Brown (Credit: Wikipedia Commons)

“Type I errors” (also known as “false positives”). (For more on Type I and Type II errors, see: Sheskin, D.J., 2004:59–60.) (Fig. 9.20). In addition, celestial mechanics itself had undergone significant conceptual changes since the days of Adams and Le Verrier, especially following the publication of Henri Poincaré’s (1854–1912) Les Méthodes Nouvelles de la Mécanique Céleste (Poincaré, 1892). (Percival Lowell seems not to have been aware of this important work, and still followed the formulations given to perturbation theory of Pontécoulant, Le Verrier, and especially Tisserand.) The great American master of celestial mechanics, Forest Ray Moulton of the University of Chicago, said in 1914 (Moulton, 1914:432–433), “Poincaré applied to the problem all the resources of modern mathematics with unrivaled genius. He brought into the investigation such a wealth of ideas, and he devised methods of such immense power that the subject in its theoretical aspects has been entirely revolutionised in his hands.” (For details about Poincaré’s contributions—especially his introduction of the concept of the “phase space”—see Appendix 9.2). The important point here is that, with the introduction of the concept of the phase space, celestial mechanics has been recast as a statistical, probabilistic affair. The reign of the clockwork universe and of Newtonian determinism is over forever.

9.7  Searches Far and Wide Brown’s paper was rather technical, and it is hard to say how widely read it was. It certainly did not prove to be the last word in the quest for planets. There were still suspicions that Pluto might be a mere interloper, and that the icy planet of Lowell’s dreams still remained hidden among the stars, awaiting detection. Various attempts to rework his calculations were undertaken, most notably by Vladimir Kourganoff (1912–2006) for his 1941 doctoral thesis at the University of Paris, who showed, as

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Gibson Reaves (1923–2005) of the University of Southern California concluded (1994:175) “that Lowell’s analysis was rigorous and properly carried out.” However, this perspective rather misses the point. The weak point, as Brown had pointed out, remained the residuals, and there was no way to go back and redo Flamsteed’s or Le Monnier’s meridian circle observations, the uncertain basis on which Lowell’s calculations had ultimately rested. Even Reaves had to admit that Lowell’s assumption that the outstanding residuals in the motion of Uranus were caused by his transneptunian Planet was incorrect. However, he added: “A question still unanswered today is what caused those residuals? Was it simply observational error? Was it Percival Lowell’s method of smoothing those observations? Or was it an error in the calculations of general perturbations in the theory of Uranus?” (Reaves, 1994:176). In general, in the years after the discovery of Pluto, planet searchers paid little attention to mathematical predictions. Instead they proceeded on the assumption that any predictions were likely to be too uncertain to be helpful, and that any planets remaining would have to be uncovered by the same kind of grinding brute-force search that had led to the discovery of Pluto. Such a search was in fact set in motion shortly after the discovery of Pluto—and by none other than Clyde Tombaugh himself. Apart from a hiatus of several years when he went to the University of Kansas to complete his undergraduate degree, he continued to press on doggedly until 1943, when he went on leave from Lowell to teach to teach courses in navigation to Navy personnel at Arizona State College (now Northern Arizona University). Much to his chagrin, after the War ended, Tombaugh was never hired back by Lowell. Nevertheless, he landed on his feet, and went on to a distinguished career designing optical instruments to follow rocket launches at the White Sands rocket proving ground in New Mexico, and then as a planetary astronomer at New Mexico State University, where his proteges included Bradford A. Smith (1931–2018), later Principal Investigator of the Voyager Imaging Team. In all, Tombaugh covered seventy percent of the sky visible from Lowell Observatory, including areas far from the ecliptic where planets with highly inclined orbits might be lurking. All of his plates were exposed with the 13-inch astrograph used to discover Pluto, and reached stars down to the 16th and 17th magnitudes, and in all he estimated that some 90-million-star images had been diligently scrutinised in the observatory’s blink comparator. Though many objects, including asteroids, variable stars, galaxies and even a cluster of galaxies, had turned up incidental to the search, there were no new planets, and Tombaugh affirmed that there were no planets awaiting detection, at least down to the limiting magnitude covered by the search. Perhaps a deeper search would succeed. Between 1977 and 1984, Charles T. Kowal (1940–2011) used the 48-inch (122-cm) Schmidt telescope at Palomar, a telescope with 14 times the light-gathering power of the telescope Tombaugh had used, to search an area 15 degrees on either side of the ecliptic for distant, slow-­ moving Solar System objects. Kowal’s search reached down to magnitude +20, and revealed a number of comets and asteroids—including, out beyond Jupiter, the curious object Chiron, which proved to be the first centaur. (Centaurs are objects with unstable orbits orbiting between Jupiter and Neptune.) He did not, however, find

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what he was hoping to find—the tenth planet of the Solar System. (Pluto at the time was still classified as the ninth planet.) Despite these negative results, there was one data-point that seemed potentially highly significant. As was recognised only in 1980, when Kowal himself (with Stillman Drake) had unearthed pre-discovery positions of Neptune from the dawn of the telescopic era, made by Galileo in January 1613, the asterisks whereby Galileo had recorded Neptune’s position relative to Jupiter and its satellites appeared to be a full minute of arc away from what it should have been at the time. Taking it at face value, such a large discrepancy fell well outside what was permitted by the modern ephemerides of Neptune, and if real, could only be explained by assuming that Neptune had been perturbed by a massive outer planet. (Admittedly, Galileo had produced nothing more than a rather rough diagram, and had not bothered with scale, so the evidence was hardly beyond reproach.) In 1988, Kowal attempted to use both the then most up-to-date residuals of Uranus and Neptune including the early Galileo positions to calculate an orbit for this planet. Unfortunately, but hardly surprisingly, the two predictions failed to agree, and no final conclusion could be drawn. (Littman, 1990:215). If there was a tenth planet, it remained singularly churlish and uncommunicative of its existence. Nevertheless, astronomers did not give up on finding it. Rather, they sought it with ever resourceful methods. Acknowledgements  Guy Bertrand, Trudy E.  Bell, Mike Conley, Roger Hutchins, Carolyn Kennett, Davor Krainović, Jacques Laskar, H.M.Lai, Benjamin Y.P.  Lee, Lucien K.H.  Lo, C.C. Lam, Robert W. Smith, Christopher Taylor, Trudy E. Bell and the late Craig Waff for comments and criticisms. Also thanks to Lauren Amundson of Lowell Observatory for her particular help in obtaining illustrations.

Appendix 9.1 The Perturbation of Uranus by Neptune: A Modern Perspective In the 19th century, mathematical astronomers were able to see this, at best, in a glass darkly. In recent years, investigators have returned to the problem, with new methods of analysis. The starting point must be, now as then, the deviation, first discerned by Bouvard, in heliocentric longitude from a Kepler orbit even after known perturbations of other planets had been subtracted out. The values of this discrepancy, Δϕ, have been summarised in the Table 9.3 below. These were, of course, ascribed by Adams and Le Verrier to the unknown planet, and served as the basis of their attempts to work out the orbital elements of that unknown. In 1990, H.M. Lai, C.C. Lam, and K. Young of the Chinese University of Hong Kong published a new analysis using a simple model, in which the unperturbed orbits of Uranus and Neptune were regarded as circular and co-planar, in order to

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Table 9.3  Discrepancy between observed and calculated heliocentric longitude of Uranus (Δϕ) after known causes of perturbation have been subtracted out, 1690–1840. Note that the observations for 1690–1780 correspond to the “ancient” observations, those from 1781 (Herschel’s discovery) to the “modern” ones (After Lodge, 1960) Year 1712 1715 1750 1753 1756 1764 1769 1771 1780

Δϕ (″ of arc) +92.7 +73.8 −47.6 −39.5 −45.7 −34.9 −19.3 −2.3 +3.46

Year 1783 1786 1789 1801 1810 1822 1825 1828 1831 1834 1837 1840

Δϕ (″ of arc) +8.45 +12.36 +19.02 +22.21 +23.16 +20.97 +18.16 +10.82 −3.98 −20.80 −42.66 −64.64

explain the main features of Δϕ as given above. (Lai, Lam, and Young, 1990). Those features included: 1 ) A typical magnitude of Δϕ is 50 to 100 secs of arc. 2) An approximate periodicity of about 100 years (e.g., measured between peaks in 1705 and 1815). 3) An apparent secular variation, which may be described as a linearly decreasing term. 4) Phase such that Δϕ rises through zero in the year 1777 and descends back through zero in 1830–31. Lai et  al. carried out two calculations using their simple model: (1) the forward problem of calculating the deviations in the position of Uranus and (2) the inverse problem of using the observed deviations to infer the elements of Neptune (Fig. 9.21). The authors noted that Adams and Le Verrier, by assuming a Bode’s law orbital radius and hence a period for the unknown planet, had needed only to find the phase of Neptune in its orbit and the mass in the inverse problem. This had been a reasonable approach at the time, though of course since Bode’s law was later completely discredited—indeed discredited by the discovery of Neptune itself—a more general approach to the problem would have involved allowing the orbital radius of Neptune to vary as well. Adams, indeed, had realised this, and as noted earlier, was proceeding along this line late in the course of his calculations. The most important results of the paper of Lai et al. are as follows: 1) The deviations in Uranus’s longitude caused by perturbation by Neptune are actually an order of magnitude larger than those found by Adams and Le Verrier. Since, historically, in the construction of a planetary theory, the biggest terms in

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Fig. 9.21  Upper: Comparison of the forward perturbation problem with the data. Lower: The solutions of the forward perturbation problem over a longer time period (Credit: Kenneth Young)

the perturbation are those with long periods, or terms having nearly the period of the disturbed body (which happens because of the near-resonance in this case) these can in large part be hidden by throwing them upon the mean motion, the epoch, the eccentricity and the place of the perihelion of the disturbed body. In the case of Uranus, this was mostly accomplished by an incorrect choice of the semi-major axis and eccentricity, especially the latter, assumed for the ­unperturbed orbit of Uranus. The near-resonance was such that a component of the perturbing force is in very nearly 2:1 resonance with the unperturbed motion of Uranus. 2) There is another problem with the historical calculation. Because the motion is responding to the second harmonic of force, the inverse problem suffers from a possible ambiguity in phase (where phase refers to the particular point in time in the cycle). One solution was the one found by Adams and Le Verrier, the other is displaced 180 degrees opposite. Thus there are actually two solutions for the

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heliocentric longitude of Neptune, diametrically opposite each other, which are not too different in terms of the quality to the fit to the data. The fact that Adams and Le Verrier found the correct solution (the one actually occupied by the planet at the time) would seem to be rather fortuitous. 3) The interesting features described under (1) and (2) are consequences of the fact that Uranus and Neptune form a nearly resonating system, as a result of the fact that, as noted above, a component of the perturbing force is in very nearly 2:1 resonance with the unperturbed motion of Uranus. Since the resonance is not quite exact (the period of Neptune differs from twice that of Uranus by 2 percent), this generates the well-known phenomenon of “beats” over a long-period rhythm. The choice of (an incorrect) mean motion that absorbs most of the perturbation means that, during the historical period, the two sinusoidal terms are nearly out of phase and come close to canceling out; thus, the reported deviations, i.e., the difference from the (incorrect) mean motion actually reached a minimum during this period, not a maximum as assumed by Herschel and others after the discovery. Their mostly incorrect understanding of perturbation theory made the deviations symmetric around the 1822 Uranus-Neptune conjunction. Nevertheless, Le Verrier’s and Adams’s calculations were valid within the limitations of the theory they used, insofar as they did succeed in finding the local minimum in the inverse problem—a not inconsiderable achievement, since, according to the authors, “even a search with many fewer parameters [than those actually involved] would have been a major computation endeavor” in the days before electronic calculators. Also, whereas the true frequency as determined by the modern knowledge of the Neptune period leads to a driving force slightly under resonance, the fitted frequency [Δϕ] corresponds to a driving force slightly above resonance. The fact that the perturbation was close to resonance, either slightly above or slightly below, sufficed for Adams and Le Verrier to determine the heliocentric longitude of Neptune during their era, not far from the time when Uranus and Neptune shared the same heliocentric longitude (their conjunction in 1822).

Appendix 9.2 Note on Celestial Mechanics After Poincaré: Phase Space and Chaos The discipline of celestial mechanics has undergone significant conceptual changes since the days of Adams and Le Verrier, especially following the publication of Henri Poincaré’s Les Méthodes Nouvelles de la Mécanique Céleste (Poincaré, 1892). The classical searchers for the disturber of Uranus would hardly recognise their own field.

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Among the most important innovations was recasting the formulae of celestial mechanics in terms of the Hamiltonian function (introduced in 1835 by the Anglo-­ Irish mathematician and astronomer Sir William Rowan Hamilton [1805–1865]), which shifted the focus from velocities to momenta of particles, and allowed the differential equations for position and momentum to be derived from the total energy of the system. This innovation eventually led to the concept of the phase space—an abstract space of a large number of dimensions in which one dimension for each of the coordinates describes a physical system comprising a certain number of particles (e.g., planets, as in the three-body problem). In this way, numerical data was transformed into a contoured map of all allowed possibilities “It was in these investigations,” notes mathematics writer Ivars Peterson (1993:160), “that Poincaré first caught a glimpse of, and to some degree appreciated, what we now know as dynamical chaos.” Dynamical chaos entered in precisely because, as Poincaré realised (pp. 185–186), “small differences in initial conditions could lead to great disparities at future times,” making it “impossible to predict with any degree of assurance the changes an orbit might undergo at some point in the distant future.” Admittedly, the deviations remained quite small over time spans of a few hundred years, such as those that had been considered by Lagrange and Laplace in their celebrated investigations of the “stability” of the Solar System, but it was clear that an orbit might undergo drastic changes in the distant future. It would not be until the advent of digital computers that astronomers such as Jack Wisdom, in the 1970s, began to systematically explore the implications for the evolution of orbits of Poincaré’s phase space in detail, and when they did so they identified that planetary positions and velocities could be plotted out into regions of regular, predictable behavior and regions of chaotic, unpredictable behavior, where the zones of chaos were found around resonance positions. (Wisdom, 1987).

References Adams, J.C., 1896. Scientific Papers of John Couch Adams, ed. Glaisher, J. Cambridge, Cambridge University Press, two vols. Airy, G.B., 1847. Account of Some Circumstances Historically Connected with the Discovery of the Planet Exterior to Uranus. Monthly Notices of the Royal Astronomical Society, vii, 9:121–144. Babinet, J., 1851. Letter of Lieutenant Maury to Hon. William A. Graham, Secretary of the Navy, 3 September 1851. The Astronomical Journal, 2:53. Baum, R. and Sheehan, W., 1997. In Search of Planet Vulcan: the ghost in Newton’s clockwork universe. New York, Plenum. Baum, R., 2007. The Haunted Observatory: curiosities from the astronomer’s cabinet. Amherst, Prometheus. Bell, T.E. 2007. Sears Cook Walker. In: Biographical Encyclopedia of Astronomers. New York, Springer, Vol. 2, 1190–1191. Biot, J.R., 1847. Sur la planète nouvellement découverte par M. Le Verrier. Journal des savants, 83. Brookes, C.J., 1970. On the Prediction of Neptune. Celestial Mechanics, 3, 67–80.

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Brown, E.W., 1930. On the Predictions of Trans-Neptunian Planets from the Perturbations of Uranus. Proceedings of the National Academy of Sciences U.S.A., 16(5):364–371. Clerke, A.M., 1908. A Popular History of Astronomy during the Nineteenth Century. London, A. & C. Black, 4th ed. Cruikshank, D.P. and Sheehan, W., 2018. Discovering Pluto: exploration at the edge of the Solar System. Tucson, University of Arizona Press. Cunningham, C.J., 2017. Studies of Pallas in the Early Nineteenth Century. Heidelberg and New York, Springer. Dick, S.J., 2003. Sky and Ocean Joined: the U.S.  Naval Observatory 1830–2000. Cambridge, Cambridge University Press. Drake, S. and Kowal, C.T., 1980. Galileo’s sighting of Neptune. Scientific American, 243, 74–79. Flammarion, C., 1894. Popular Astronomy: a general description of the heavens. London, Chatto and Windus. Gould, B.A., Jr., 1850. Report on the History of the Discovery of Neptune Washington City, Smithsonian Institution. Grosser, M., 1962. The Discovery of Neptune. Cambridge, Mass., Harvard University Press. Hankins, T., 2006. A Large and Graceful Sinuosity: John Herschel’s Graphical Method. Isis, 97:605–633. Hanson, N.R., 1962. Leverrier: The Zenith and Nadir of Newtonian Mechanics. Isis 53(3):359–378. Herschel, J.F.W., 1861. Outlines of Astronomy. Philadelphia, Blanchard & Lea, 4th ed. Hoffleit, D., 2007. Elias Loomis. In: Biographical Encyclopedia of Astronomers. Hockey, T., Trimble, V. and Williams, T.R. (eds.). New York, Springer. Hoyt, W.G., 1980. Planets X and Pluto. Tucson, University of Arizona Press. Hubbell, J.G. and Smith, R.W., 1992. Neptune in America: negotiating a discovery. Journal for the History of Astronomy, xxxiii:261–288. James, C.R. and Sheehan, W., 2011. Neptune Comes Full Circle. Sky & Telescope (March):28–34. Lai, H.M., Lam, C.C., and Young, K., 1990. Perturbation of Uranus by Neptune: a modern perspective. American Journal of Physics, 58(10):946–953. Lequeux, J., 2015. Le Verrier: magnificent and detestable astronomer. New York, Springer. Le Verrier, U.J.J., 1843. Détermination nouvelle de l’orbite de Mercure et de ses perturbations. Comptes rendus de l’Academie des Sciences, 16:1054–1065. Le Verrier, U.J.J., 1845. Théorie du Mouvement de Mercure. Paris, Bachielier, Imprimeur-Libraire du Bureau des Longitudes. Le Verrier, U.J.J., 1859. Théorie et Tables du movement de Mercure. Chapter XV, Recherches Astronomique,. Annales de l’Observatorie Imperial de Paris, 5:1–196. Littlewood, J.E., 1953. A Mathematician’s Miscellany. London, Methuen. Littman, M., 1990. Planets Beyond: Discovering the Outer Solar System. New  York, John Wiley & Sons. Lo, L.K.H., Young, K., and Lee, B.Y.P., 2015. Vulcan, unpublished manuscript. Lodge, O., 1960. Pioneers of Science. New York, Dover. Loomis, E., 1851. The Discovery of the Planet Neptune. In: The Recent Progress of Astronomy, Especially in the United States. New York, Harper & Bros. Lowell, A.L., 1935. Biography of Percival Lowell. New York, Macmillan. Lowell, P., 1915. Memoir on a Trans-Neptunian Planet. Lynn, Mass., Thos. P. Nichols & Son. Mitchel, F.A., 1887. Ormsby MacKnight Mitchel: Astronomer and General. Boston and New York, Houghton, Mifflin and Co. Mitchel, O.M., 1847. Le Verrier’s Planet, Sidereal messenger, i:85–86. Mitchel, O.M., 1859. Lecture on the Great Unfinished Problems of the Universe, delivered at the Academy of Music, New York, 29 January 1859. In: The Pulpit and the Rostrum: Sermons, Orations, Popular Lectures, &c. New York, E.D. Barker. Moulton, F. R., 1914. An Introduction to Celestial Mechanics. New York, Macmillan, 2nd ed. Newcomb, S., 1866. An Investigation of the Orbit of Neptune, with general tables of its motion. Washington, D.C., Smithsonian Contributions to Knowledge.

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Newcomb, S., 1873. An Investigation of the Orbit of Uranus, with general tables of its motion. Washington, D.C., Smithsonian Contributions to Knowledge. Nourse, J.E., 1871. Memoir of the Founding and Progress of the United States Naval Observatory. Astronomical and Meteorological Observations made during the year 1871, at the U.S. Naval Observatory. Washington, D.C., Government Printing Office. Pannekoek, A., 1961. A History of Astronomy. London, George Allen & Unwin. Peirce, B., 1847a. [untitled paper], Proceedings of the American Academy of Arts and Sciences, 1 (16 March 1847):65–68. Peirce, B., 1847b. [untitled paper], Proceedings of the American Academy of Arts and Sciences, 1 (4 May 1847):144–149. Peterson, I., 1993. Newton’s Clock: Chaos in the Solar System. New  York: W.H.  Freeman and Company. Poincaré, H., 1892. Les Méthodes Nouvelles de la Mécanique Céleste Paris, Gauthier-Villars. Rawlins, D., 1992. Dio, ii/3, 142. Reaves, G., 1994. The Search for Planet X. In: Putnam, W.L., et al., The Explorers of Mars Hill: a century of history at Lowell Observatory. Kennebunkport, Maine, Phoenix. Sheehan, W., 2016b. Percival Lowell’s Last Year. Journal of the Royal Astronomical Society of Canada, 110 (6):224–235. Sheehan, W. and Misch, T., 2016. A special centennial: Mercury, Vulcan, and an early triumph for General Relativity. The Antiquarian Astronomer, 10:2–12. Sheehan, W., 2016a. The Great American Eclipse of the 19th Century. Sky & Telescope (August): 36–40. Sheskin, D.J., 2004. Handbook of Parametric and Nonparametric Statistical Procedures. Boca Raton, Florida, CRC Press. Sternborg, G. Stauffer, J.R., and Howard, R.A. 2018. Evidence for a Circumsolar Dust Ring near Mercury’s Orbit. Astrophysical Journal, 868, 74. Suzuki, J., 2007. Benjamin Peirce. In: In: Biographical Encyclopedia of Astronomers. New York, Springer, Vol. 2, 883–885. Tisserand, F., 1889–1896. Traité de Mécanique Céleste. Paris, Gauthier-Villars, four volumes (vol 1:1889, vol. 2:1891, vol. 3: 1894, vol. 4: 1896). Tombaugh, C.W. and Moore, P., 1980. Out of the Darkness: the planet Pluto. Harrisburg, Pennsylvania, Stackpole Books. Will, C.M., 1986. Was Einstein Right? Putting General relativity to the Test. New  York, Basic Books. Wisdom, J., 1987. Urey prize lecture: chaotic dynamics in the Solar System. Icarus, 72:241–275.

Chapter 10

Neptune Visited and the Outer Solar System Revolutionised, 1989–2019 William Sheehan

Abstract Chapter 9 sketched the labours of elite mathematicians across a century and a half from 1846 to about 1990, discarding Bode’s Law and seeking to refine celestial mechanics in order to narrow the search area and discover further planets in the Outer Solar System. All, including the long-held Laplacian Theory for formation of the Solar System, would be swept aside, as Solar System studies based on Isaac Newton’s most exacting science have been rejuvenated by supercomputers. The Voyager 2 spacecraft’s stunning flyby reconnaissance of the Neptune system in August 1989 revealed astonishing detail of the planet itself and of its large satellite Triton, a planet-sized body now believed to have been captured from the Kuiper belt, of which Pluto and its moon are also members. The flyby explained away the small residuals of Gaillot’s tables of Uranus (1909) which inspired Percival Lowell’s quest for Planet X and tempted later investigators. Meanwhile, resonances such as the near-2:1 resonance between Uranus and Neptune have proved to be key to understanding the present arrangement of planets in the Solar System. Also, the discovery since 1992 of many more bodies populating the Kuiper belt—a region previously regarded as empty and devoid of interest—has utterly changed our ideas about the Outer Solar System and its evolution. Also, discoveries far beyond the Solar System have helped to increase knowledge of our own. Since the first exoplanet system (Pegasi 51 b/Dimidium) was recognised in 1995, thousands of others been discovered, many of them multiplanet systems and most very unlike our own Solar System. The sheer dissimilarity has challenged computer modellers to explain how such different arrangements arise, and in turn have enriched our understanding of the past evolution of our own Solar System leading to its present arrangement. The latter is not God-ordained, or a clockwork system as some of the 18th century followers of Newton (though not Newton himself) imagined. The Outer Solar System in particular has proved to be an important byproduct of this evolution, and attests to past migrations of the giant planets in the early history of the Solar System. Their interactions with planetesmials in the circum-solar disk sculpted the Kuiper belt, and scattered a vast W. Sheehan () Independent Scholar, Flagstaff, AZ, USA © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 W. Sheehan et al. (eds.), Neptune: From Grand Discovery to a World Revealed, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-54218-4_10

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number of asteroids and potential comets into the far outer Solar System where they form the spherical Oort cloud halo round our system. Intense scrutiny including by the new generation of large 10-metre telescopes has led to the discovery of numerous dynamically and physically distinct categories of Kuiper belt objects (e.g., “cold” KBOs, “hot” KBOs, etc.) The understanding of how these categories came to be has helped shape a general picture of the Solar System’s evolution; though details are disputed, there is overall agreement around a concept known as the “Nice Model”, in which Jupiter was the first planet to form, initially migrated inwards through the inner Solar System like a wrecking ball, possibly destroying a first-generation of inner planets. However, the formation of Saturn, and a resonance with Jupiter, caused a reversal of Jupiter’s inward motion and subsequent migration outward to its current position (Jupiter’s Grand Tack), while also pushing the embryonic ice giants—Uranus and Neptune—outward. There, Neptune—the “star not on the map”, in Heinrich d’Arrest’s arresting phrase—is the most massive and dominant body shaping its distant icy realm. At present, it is the outermost of the “major planets”, but it may not be the last. Astronomers have found subtle hints in the way the orbits of some distant Kuiper belt objects are arranged, and surmised the existence of a possible “Planet Nine.” Completing this book’s underlying theme, we close with a look forward to a future in which celestial mechanics, much evolved and altered, but also much more powerful than it was in the hands even of Adams and Le Verrier, continues to play an indispensable role in our attempts to understand the motions of the bodies of the Solar System. That history has yet to be written….

10.1  Grand Tour In Chap. 9, we discussed the residuals of Uranus, which continued to long cast their spell on astronomers hoping to use them to reprise the great accomplishment of Le Verrier and Adams in finding another planet. Those searches were ultimately unsuccessful, and the residuals on which they were based always rather suspect. The residuals not only of Uranus but also of Neptune would finally be exorcised once and for all by the first—and so far, only—spacecraft to travel to Neptune, Voyager 2, which would arrive by gravity assists by Jupiter, Saturn, and Uranus. In planning to fly a spacecraft to within a few thousand kilometres of each of these planets’ cloud tops and moons, it was necessary to know with great precision the positions of those planets at given times. This meant that the influence of any unknown perturber had to be rigorously worked out and eliminated. Jet Propulsion Laboratory dynamicist E. Myles Standish Jr. and his colleagues were charged with this task. They began by discarding all pre-1910 positions for Uranus and Neptune. This alone provided accurate positions of the planets for the encounters. In addition, after the Voyager 2 flyby of Neptune in August 1989, they used the spacecraft’s newly determined mass of the planet, which had been revised downward by 0.5%— an amount comparable to the mass of Mars—to recalculate the gravitational effect

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on Uranus. The residuals were reduced to zero. (Standish, 1993). Theory and ­observation were in precise agreement. It was now possible to return to Percival Lowell’s calculations (Chap. 9) and to answer, once and for all, the questions that had loomed so large in the aftermath of Pluto’s discovery: “What caused those residuals? Was it simply observational error? Was it Percival Lowell’s method of smoothing those observations? Was it an error in the calculations of general perturbations in the theory Uranus?” (Reaves 1994:176). The answer, it turns out, was all three. There was no longer, to recall Airy’s phrase to Challis in July 1846, “a possible shadow of reason for suspecting the existence of a planet” based on Uranus’s residuals (Fig. 10.1). Thanks to Standish and the other dynamicists, Jupiter and Saturn-bound Voyager 1 and Jupiter-Saturn-Uranus-and Neptune-bound Voyager 2 could be aimed with complete confidence at their targets to a high degree of accuracy. From Saturn, Voyager 2 would attempt a very close, hazardous approach to Uranus, in which it would withstand exposure to the planet’s magnetic field, pass close to the inner moon Miranda, then swing behind the planet and Uranus’s slender rings before being swung, by gravity assist, on to Neptune. At Neptune, it would have to survive a similarly harrowing trajectory. The accuracy needed was almost inconceivable, and the calculations worked out brilliantly. As science writer Mark Littman wrote (1990:144) in going from Uranus to Neptune Voyager 2 would have to “hit a point in space at a point in time with phenomenal accuracy. To miss ‘corridor center’ at Uranus by a mile would create an error of 4,000 miles at Neptune…. The accuracy required… was a feat comparable to a golfer sinking a thousand-mile putt.” Remarkably, the thousand-mile putt was successful. On 25 August 1989, Voyager 2 skimmed within 4,905 kilometres of the planet’s upper cloud deck at a point not far from the north pole. As called for by the flight plan, it was then swung by Neptune’s gravitational pull toward Triton and passed within only 40,000 kilometres of the satellite’s surface six hours later. (Smith, 1989; Cruikshank, 1995; Pyne, 2010; Bell, 2015). It is worth pausing for a moment to consider the remarkable transformation of astronomy (and society itself) in the last 180 years that made it possible were quite as extraordinary as the discovery and transformation of the understanding of Neptune itself. Instead of solitary watchers of the night sky, seated at their telescopes and huddled over star maps, or mathematicians working through long trains of equations using pencil and paper, astronomers now travel to mountain-top observatories, and use gigantic telescopes fitted with sensitive devices like CCDs, IR detectors, and spectrographs to display data on computer screens for later analysis. Some observations are made with the Hubble Space Telescope, orbiting the Earth in space. As for the spacecraft itself, it arrived at its destination using rockets, which in the nineteenth-century were only somewhat more than conveyances for fireworks but, thanks to their development as weapons of mass destruction during World War II, using exotic fuels like hydrazine, have become means of transporting complex spacecraft weighing to other planets. (Voyager 2, which made it to Jupiter, Saturn, Uranus, Neptune and out of the Solar System, was almost the weight and size of a

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Fig. 10.1  The residuals of the observations used by Le Verrier for his Neptune calculations based on a comparison with calculated positions given by Miriade, the ephemeris generator of the IMCEE (Institut de mécanique céleste et de calcul des éphémérides). Note that most of the “ancient” observations are reasonably good, though Le Monnier’s of 1768–69, reduced by Bouvard in 1821, are the worst—they appear as a nearly vertical line, with positions ranging from −10 to +15 arc seconds in longitude from the Miriade-computed positions. Clearly there are no systematic trends, and errors in the “ancient” observations, on which Lowell mainly depended for the residuals used to find his Planet X, produced an illusory signal of an unknown planet (Calculated by and credit: Guy Bertrand, Paris Observatory)

subcompact car.) The means of communication, radio waves, had not yet been discovered until the end of the nineteenth century, or developed as a practical means of communication until the early twentieth. Instead of the handful of astronomers and human computers that were involved in the search and discovery of Neptune in the

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nineteenth century, who were able to pursue the search in many cases on a very part time basis, Voyager needed the ample budget of a large nation-state (and then one prodded by intense competition from a rival superpower), and even then was canceled once and then reconfigured because of the cost. Its development and operation required a dedicated team of hundreds of engineers and planetary scientists. Its investigations were carried out by highly specialised teams in imaging science, infrared radiation, ultraviolet spectroscopy, photopolarimetry, planetary radioastronomy, magnetic fields, plasma particles, plasma waves, low energy charged particles, cosmic ray particles, and radio science. Needless to say, not one of these subfields of science even existed in 1846 (Figs. 10.2 and 10.3). What they accomplished was certainly on a grand scale. As the most celebrated planetary scientist of his era, Carl Sagan (1934–1996), has said, “In all the history of mankind, there will be only one generation that will be the first to explore the Solar System, one generation for which, in childhood, the planets are distant and indistinct disks moving through the night sky, and for which, in old age, the planets are places, diverse new worlds in the course of exploration.” (Sagan, 1973:69). By the time Voyager 2 made its flyby of Neptune in August 1989, Sagan’s own generation had largely realised this dream.

Fig. 10.2  Heidi Hammel, at the time Dale Cruikshank’s graduate student at the University of Hawaii, fills the dewar holding a CCD camera with liquid N2. The CCD camera cooled by the liquid N2 to decrease electronic noise sits at the bottom of the tube at the Cassegrain focus of the 88-inch telescope at Mauna Kea. This was an observing run in 1985 on Comet Halley, which was approaching perihelion but still very far out at the time. Hammel went on to become a leading researcher on the outer Solar System, including Neptune. Courtesy: Dale P. Cruikshank.

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Fig. 10.3  Pioneering outer Solar System researchers Dale P. Cruikshank, left and David Morrison, standing next to the University of Hawaii’s 2.2-m telescope at Mauna Kea, 1982. Courtesy: Dale P. Cruikshank.

Members of the Imaging science team (led by Bradford A. Smith [1931–2018]) recognised in Neptune’s methane-blue clouds, including a tremendous anticyclonic storm system, known as the “Great Dark Spot” (or GDS). The GDS was the ­outstanding feature of the whole disk, with a width equal to that of the entire Earth and inhabiting nearly the same latitude, 20 degrees south, as the famous Great Red Spot of Jupiter, also an anticyclonic storm. (Sheehan, 1992:180) (Figs. 10.4, 10.5 and 10.6). The planet’s rotation period had long been an enigma to ground-based observers, but radio bursts detected by Voyager 2 indicated core of Neptune was rotating in 16.11 hours. Relative to the core, the large-scale atmospheric features in the equatorial region have a sweeping westward motion of 1,200 km/hr, with smaller features having twice the speed. Thus, Neptune is the windiest planet in the Solar System. Voyager’s instruments also detected Neptune’s magnetic field for the first time. Rather surprisingly, in light of the fact that Neptune’s axis of rotation has a rather normal tilt in contrast to Uranus, the two planets are very similar magnetically, with Uranus’s magnetic field tilted by some 60° to its axis of rotation, Neptune’s by 47°. Thus, the magnetic fields of both planets wobble wildly as they spin. This is thought to be a result of the fact that both are “oblique rotators”, generating their magnetic fields through convection of electrically conducting material within a thin near-­ surface shell rather than in a molten core as on the Earth.

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Fig. 10.4  Voyager Imaging Team Leader and Clyde Tombaugh protégé Bradford A. Smith, left, and Carolyn A. Porco, at the Voyager 2 Neptune encounter, August 1989. Porco later became the Imaging Team Leader of the Cassini mission to Saturn. Courtesy: Carolyn A. Porco. Fig. 10.5  Voyager 2 image of Neptune, August 1989, showing the Great Dark Spot and surrounding bright cirrus clouds (Credit: NASA/JPL)

Among Voyager’s many other discoveries, it showed that Neptune has a set of complete and some partial rings. The outermost ring, known as the “Adams ring”, lies at a distance of 62,900 km from Neptune’s centre, and contains three bright ring arcs, which are all concentrated in the same segment of the ring and joined to wispy

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Fig. 10.6  Vertical relief in the atmosphere of a giant planet. Cirrus clouds project shadows onto the blue methane cloud deck below (Credit: NASA/JPL)

material that forms a continuous circle around the planet. The ring arcs were shown to be clumps of material produced through resonance interactions with Galatea, one of six tiny moons discovered by Voyager. There is another ring (the “Le Verrier ring”, at a distance of 53,200 km from the planet, as well as two more diffuse rings (“Lassell ring” and “Galle ring”), while a sheet of dust extends all the way down to the planet’s cloud tops. (None of these, of course, has anything to do with the ring suspected by William Lassell and others, as described in Chap. 8.) (Fig. 10.7). Mission planners had been keen to swing the spacecraft by Neptune in such a way that it would encounter Neptune’s large satellite Triton, not quite the diameter of the Earth’s Moon at 2,707 km. Its orbit had always seemed markedly abnormal; though nearly circular, with a semimajor axis of 354,800 km, it is inclined by some 23 degrees to Neptune’s orbital plane, and travels in a retrograde sense around its orbit. This kind of behavior is not unusual among the most minute satellites of the Solar System, and suggestive of capture, but Triton is the only large satellite that behaves this way. Also, its composition is unusual given its great distance from the Sun; while most bodies at such great distances are mostly ice, Triton contains as much rock as Io and Europa, and therefore must have formed in a different part of the Solar System before being snared by Neptune. The dynamics of capture are rather complicated, since in order to be captured, a passing body has to lose sufficient energy to be slowed to a speed less than that required for its escape. One way to do this would be through collision of Triton with another moon, though more recently it has been proposed that Triton was originally a member of a binary system, and on encountering Neptune, one component was expelled, the other becoming bound to Neptune (Agnor and Hamilton, 2006). There seems little doubt—especially after the New Horizons flyby of the Pluto/Charon system in 2015—that Triton originated in the Kuiper belt. If so, it is the largest Kuiper belt object known, ahead of Pluto and Eris. (See below) (Fig. 10.8).

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Fig. 10.7  The faint rings of Neptune appear in this two-image montage from Voyager 2. The two main narrow rings are “Le Verrier” (inner) and “Adams” (outer). The faint diffuse ring closer in to the planet is “Galle” and that between Le Verrier and Adams is “Lassell”. The Adams ring contains clumps of material, of which the three brightest have been named “Liberty”, “Equality”, and “Fraternity” (Credit: NASA/JPL)

Fig. 10.8  Triton from Voyager 2 (Credit: NASA/ JPL)

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When first captured by Neptune, Triton must have followed a highly eccentric orbit, but gradually this shape was tamed into a near circle by tidal gravitational forces. In the process, its interior melted, producing volcanism on a large scale that completely remodeled the surface. At the present time, almost all the nitrogen gas on Triton is frozen onto the surface in the form of an almost transparent glasslike jacket that covers it to a depth of 1 to 1.5 metres. Dust consisting of dark organic materials is embedded in this jacket of frozen atmosphere, and when directly exposed to midsummer sunlight, these particles absorb heat, and once the ­temperature rises above the boiling point of nitrogen, a combination of nitrogen gas and dark organic material is released at explosive speeds of 100 metres/sec. Voyager captured two geysers in the process of erupting, both near the current subsolar point, thereby giving the lie to the expectation, before Voyager, that Triton must be a world in the suspended animation of deep freeze. (Sheehan, 1992:184). Clearly, the geysers must be erupting continually, and Triton has gone from a mere frozen dot in the telescope to a possible abode of extremophile life-forms. What a strange world! One can only wonder what William Lassell would have thought of it.

10.2  Resonances, the Nice Model, and Neptune’s Migrations With the Voyager flyby, our understanding of the Outer Solar System took a gigantic leap forward. Recall that in the 18th century, before William Herschel discovered Uranus, Saturn still claimed the outermost boundary as it had since immemorial times. The Outer Solar System was entirely unknown. The totality of knowledge— or rather of ignorance—of that vast domain could be summed up in John Theophilus Desaguliers’s quaint 18th century picture (Fig. 1.14) in which six planets orbit the Sun, with a few periodic comets thrown in for completeness. Uranus had not yet been revealed, while Neptune was not even a twinkle in an astronomer’s eye. The zone between Mars and Jupiter was entirely empty. Compared to what we’re used to, the Solar System of the 18th century appears startlingly sparse of contents—as clean and unencumbered of unnecessary details as a poem by Alexander Pope (1688–1744), and seeming to rhyme in couplets: Mercury and Venus inside the Earth’s orbit, the Earth and Moon together with Mars, the large planets farther on, Jupiter and Saturn, with their own little orreries of moons. The details began to be filled in—the asteroids between Mars and Jupiter, Uranus, Neptune, and Pluto in the outer Solar System—and gradually the Solar System became less boring. At least the inner Solar System out to about Jupiter (Fig. 10.9). But the outer Solar System, even as late as Voyager, still remained unknown. Compared to the busyness inward toward the Sun, it was an empty—almost preternaturally empty—place. Where was everybody?

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Fig. 10.9  The population of small bodies in the Solar System, including the Main Asteroid belt and the Hildas, Trojans and Greeks (Credit: Wikipedia Commons)

10.3  Discovering the Kuiper Belt It was that preternatural emptiness that prompted astronomer David Jewett, then MIT, and Jane Luu, a graduate student at MIT and the University of California, Berkeley, to begin a systematic search for objects beyond Pluto. Their first attempt, in 1987, used photographic plates with telescopes at Kitt Peak in Arizona and Cerro Tololo Inter-American Observatory in Chile. Since they were using the same technology—photographic emulsions—that the previous searches by Clyde Tombaugh and Charles Kowal (1940–2011). had done, it was not entirely surprising that they failed. As often in astronomy, success awaited new technology. That new technology was becoming available when, in 1988, Jewitt moved to the Institute of Astronomy in Hawaii. There, he and Luu began using a charged coupled device (CCD) instead of a photographic plate. The CCD had been invented at AT&T’s Bell Labs in the late 1960s, and in simple terms, consists of an integrated circuit etched onto a silicon surface; when photons fall on this surface, they generate charge that can be read as pixels by the electronics and turned into a digital image. Since its invention, the CCD had been evolving, and though the early CCD detector arrays had been very small, Jewitt himself was the first to realise that by the late 1980s they were becoming large enough and sensitive enough to use for deep Solar-­ System survey work, and in 1988 he and Luu began using a CCD camera mounted on the 2.2-m telescope on Mauna Kea to lift their game to a new level. (Smith and Tatarewicz, 1994).

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For five years, they failed to find what they sought. But as CCD detector arrays continued to increase in size, until by 1992 they had an array of 1,024 × 1,024 pixels. Having such large and sensitive detectors greatly expedited the search, and in August 1992 they succeeded. They located an object which, at magnitude 22.8, was more than three thousand times fainter than Pluto, and from its rate of motion was clearly trans-Neptunian. Provisionally, until an accurate orbit could be worked out, it was given the designation QB1. When the orbit was calculated, it proved to be at a mean distance of 43 AU, or 2 billion kilometres beyond Neptune, with an orbital period of 289 years, and thus resembling some of the putative planets that Todd, Pickering and others had proposed. It was the first object found that belonged to the Kuiper belt (sometimes also referred to as the Kuiper-Edgeworth belt), a zone of icy bodies posited, on rather speculative grounds, by amateur astronomer Kenneth Edgeworth (1880–1972) in 1943 and Gerard Peter Kuiper (1905–1973) in 1950 to exist outside the orbit of Neptune based on analysis of the paths of short-period comets (those with periods of less than 200  years) and low orbital inclinations. (Cruikshank and Sheehan, 2018:404–406). Both Edgeworth’s and Kuiper’s papers had been highly speculative, however, and the real motivation to look carefully for the Kuiper belt in the late 1980s came from a paper by Martin Duncan at Lick Observatory and Thomas Quinn and Scott Tremaine at the Canadian Institute for Theoretical Astrophysics at the University of Toronto. (Duncan, Quinn and Tremaine, 1988.) Duncan et al. put together the first N-body simulation of the dynamics of short-­ period comets and demonstrated that comets disappear (get injected into the inner Solar System) at a rate so high that unless there is a reservoir of icy bodies beyond Neptune, this population ought to have been depleted long ago. Also, and importantly, their calculations showed that this reservoir could not be spherical, and thus it could not be the Oort cloud, a spherical cloud of predominantly icy bodies (planetesimals) originally theorised by the Estonian astronomer Ernst J. Öpik (1893–1985) in 1932 and then by the Dutch astronomer Jan Oort (1900–1992) in 1950 (van der Kruit, 2019; Cruikshank and Sheehan, 2018) to account for the observed distribution of long-period comet orbits. Far from being either spherical or belt-like, the inclination dispersion of the Kuiper belt is actually on the order of 15°. (Brown, 2001). Jewitt and Luu initially proposed to name QB1 “Smiley” (after George Smiley, the fictional character created by John le Carré (1931–2020); unfortunately, there was already an asteroid named Smiley for an American astronomer of the name). For a long time it continued to be known simply as QB1, but it has now been officially named Albion, after the primeval man of William Blake’s (1757–1827) complex mythology. This was the first of the Kuiper Belt Objects (or KBOs) recognised, though it was subsequently realised that Pluto and Charon are also KBOs (though rather outsized examples). (A word on the terminology: KBO is often used synonymously with trans-Neptunian object (TNO). However, this is not quite right. TNO is a more generic term: all objects with semi-major axes greater than that of Neptune (30.1 AU), collectively, are TNOs. These include objects within the Kuiper belt, scattered disc, and Oort cloud. Hereafter we will generally stick with the term KBO.)

10  Neptune Visited and the Outer Solar System Revolutionised, 1989–2019

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Fig. 10.10  The outer Solar System fills in. A plot of known Kuiper belt objects (KBOs) in the Solar System beyond Neptune (Credit: Wikipedia Commons)

In 1993, Jewitt and Luu found five more KBOs, and a further dozen turned up in 1994. As of October 2020, according to the Wikipedia list, there are 678 numbered KBOs and 2,786 unnumbered KBOs known. The outer Solar System is most emphatically no longer empty! (See: en.wikipedia.org/wiki/List_of_trans-Neptunian_objects and en.wikipedia.org/wiki/List_of_unnumbered trans-Neptunian_ objects.) It has been estimated that more than 100,000 KBOs over 100 km may exist (Stern, 2012). Based on observations of their physical and orbital characteristics, astronomers have subdivided KBOs into several categories. There are cold classical KBOs (where cold here does not apply to their temperatures—they are all cold!—but to the orderly and unperturbed orbits in which they moved: typically with low inclinations (