The Sun and Its Family 0451020375, 9780451020376

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(signet)

P2O37/6O0

SIGNET SCIENCE LIBRARY

THE SUN AND

ITS FAMILY

IRVING ADLER ILLUSTRATED BY RUTH ADLER

Digitized by the Internet Archive in

2012

http://archive.org/details/sunfamilyOOadle

Onr

WORLD

and The UNIVERSE FOR EONS THE SIMPLE EVIDENCE OF HIS EYES WAS ALL MAN KNEW OF THE HEAVENS ABOVE HIM.

The Greco-Egyptian Ptolemy

evolved a rational theory of earth, sun, and stars which was accepted for hundreds of years until Copernicus questioned it with a theory of his own. Galileo's mighty telescope proved Ptolemy wrong. And it remained for later scientists Brahe, Kepler, Newton to evolve a lucid explanation of the behavior of the stars and the solar system.

—

—

In fascinating detail Irving Adler recounts the ways in which astronomers learned that the earth spins, that it revolves around the sun, and tells how they measure the earth's size and weight, and analyze its interior. He also explains how astronomers calculate the size, distance, and motion of celestial bodies: the moon, planets, planetoids, comets, meteors.

with informative drawings by Ruth Adler, this is a book that everyone ca?i understand and enjoy, a book that is especially pertinent today when man has begun to apply his knowledge of the heavens to the conquest of space. Illustrated

Other

SIGNET KEY

and

MENTOR

Books

by Irving Adler

How Life Began A readable account about the origin of

of what science has discovered life. Preface by Linus Pauling.

(#Ks369— 35$)

Illustrated.

Magic House of Numbers Mathematical

curiosities, riddles, tricks, and games that teach the basic principles of arithmetic.

(#KD374— 500) The

Stars: Steppingstones into

Space

A

clear explanation of the mysteries of the heavens, with photographs and line drawings.

(#Ks364— 350) The New Mathematics The

—

first book to explain in simple, uncomplicated language the fundamental concepts of the revolutionary developments in modern mathematics. (#MD281— 50C)

—

Seeing the Earth from Space

A

popular explanation of what man is learning about outer space through the launching of satellites, details of the Titov, Gagarin, Grissom and Shepard flights, and a table of the first fifty Russian and American satellites successfully placed in orbit. (#P2050— 60C)

the

SUN and its

family

by IRVING ADLER Illustrated

by Ruth Adler

SJGMEI BOOKS

A SIGXET SCIENCE BOOK Published by

THE NEW AMERICAN LIBRARY

© 1958 by Irving and Ruth Adler All rights reserved. This book, or parts thereof, must not be reproduced in any form without permission. For information address

The John Day Company, 62 West 45th York 36, New York.

Street,

New

First Printing, January, 1962 Published as a

SIGNET SCIENCE BOOK

By Arrangement

with The John

Day Company,

Inc.

HIONBT THADKMAHK IlEfl. T.fl. FAT. OFF. AND FOREIGN COUNTRIES KEill.HTKIIKI) THADKMAHK MAKCA KKU1BTBADA BEOHO EN CHICAGO, C.B.A.

SIGNET SCIENCE BOOKS are published by The New American Library of World Literature, 501 Madison Avenue, New York 22, New York PRINTED IN THE UNITED STATES OP AMERICA

Inc.

Contents i.

ii.

What

Two

We

See in the

Sky

Systems: Ptolemy

vs.

7

Copernicus

in.

The Earth

iv.

The Earth Revolves Around

v.

vi.

vii.

vni.

32

Spins

The Sun and Our Home,

Its

Index

the

Sun

Family

Planets

50 63

the Earth

Our Nearest Neighbor,

The Other

20

88 the

Moon

98 106 125

CHAPTER ONE

What We See in the

Sky

LOOKS CAN BE DECEIVING SCIENTISTS tell us that the earth is round. It is a big ball, they say, about eight thousand miles wide. It is spinning around like a merry-go-round, and is sailing through space at a speed of lS 1/^ miles per second, or 66,600 miles an hour. It revolves around the sun, making a complete round trip every year. The first time we hear these facts they are hard to believe, because they do not seem to match the way things look. If we stand outdoors and look around us, what we see of the earth doesn't look like a ball at all. It looks more like a flat cake of soil and where we see plains, and with bumps and wrinkles where we see hills and mountains. We don't feel the earth moving or spinning. In fact, it seems to be standing perfectly still. And when we watch the sun, as it rises in the east, climbs across the sky, and then sets in the west, it looks as though the sun is

rock, with a smooth surface

7

The Sun and

8

Its

Family

going around the earth, rather than the other around. But things aren't always the way they seem. The earth is round, does spin, and does revolve around the sun. For thousands of years this truth was hidden from people by misleading appearances. But patient study of the earth and sky finally uncovered the truth and produced evidence that proved it. This book tells the story of the discoveries that broke down false beliefs about the earth and sun, and led us to an under-

way

standing of the real nature of the earth as a

mem-

ber of the sun's family of planets.

A PUZZLE

IN

THE SKY

The sky is like a great stage. The stars, the moon and the planets are the actors that move across it. Day after day and year after year they act out the same play. The scenes we sun, the

see in the sky today

were watched by other peo-

As they observed wondered about its meaning. What is the sun, they asked, whose bright light floods the land by day? What is the moon, that lights the path of the traveler at ple thousands of years ago.

the action in the sky, they

night?

What

moon,

like courtiers attending a

are the stars that surround the

queen in her palace? Why are they in the sky, and why do they move as they do? The sky was a puzzle which the people tried to solve. They looked for

what they saw in the sky. understand their attempts to interpret these

clues to the puzzle in

To

What We See in the Sky clues,

we must

9

begin as they did, by looking at

the sky.

THE SKY-SPHERE When we

look at the sky on a clear night, spotted with thousands of tiny lights, twinkling like flickering candles. The sky looks like a great black bowl that is resting upside down, with us inside it. The stars look like spots

we

see

it

on the inside of the bowl. impression the stars make is one of complete confusion. Some stars look brighter than others. Some parts of the sky are crowded with stars, while, in other parts of the sky, the that have been painted

The

stars

first

look widely scattered.

After nights,

we have watched the sky for many and are better acquainted with it, the

We notice

confusion begins to look less confused. some regular features in the sky that recognize easily every rime we look at

we it.

can

The

10

The Sun and

stars

seem to have a

Our

attention

is

Its

Family

fixed arrangement in the sky.

attracted

by groups of

stars that

are arranged in familiar-looking patterns. In the northern part of the sky we see one group of

arranged in the shape of a dipper. Another group, not far from it, looks like a great letter W. In the southern part of the sky, in the summertime, we see a group of stars whose arrangement suggests the outline of a teapot. Another group of stars close to the teapot looks like a giant fishhook. In the middle part of the sky we see a group that looks like a cross. The stars in a group keep their places from night to night, and the groups themselves have a fixed arrangement, too. If the stars were really spots painted on the inside of a bowl, they would be permanently painted spots. Each star has its regular place that it seems to keep forever. For this reason, in ancient times, people called them fixed stars. The groups of stars are called constellations. Thousands of years ago, in all parts of the world, people made up stories about the stars and constellations, and gave them names. The stories usually involved the adventures of their gods. Today we don't believe these stories, but we find it convenient to use the old names. The constellation that looks like a dipper to us looked like a bear to the ancient Romans. So, while we sometimes call it the Big Dipper, we also call it Ursa Major, which means Big Bear in Latin. The constellation that looks like a is called Cassiopeia, the name of a queen who figures in one of the stories told in ancient Greece. The group of stars that looks like a teapot is called stars

W

1

What We See in the Sky Sagittarius, the archer.

The

fishhook

1 is

called

Scorpius, because the ancients thought it looked like a scorpion. The cross in the middle part of the sky has two common names. It is known as the Northern Cross, and is also called Cygnus,

the swan. If

we watch the sky for several hours during we notice another one of its regular

the night,

features.

While the

stars

seem

stellations, the constellations

They move

as

a star that

is

fixed in the con-

move

across the sky.

though the whole bowl of the sky were turning, carrying the stars along with it as it turns. There is a fixed point in the northern sky around which it seems to turn, and all the stars move in circles around this point. This fixed point is called the North Pole of the sky. There is

seems to

so close to this point that it hardly at all as the sky turns. Because

move

us find the position of the North Pole in the sky, we call it Polaris. this star helps

The Sun and

12

Its

Family

Stars that are near the horizon in the eastern

part of the sky rise above the horizon as the night advances. As they climb up the sky, we see other stars behind

them moving up

to take

This observation forces us to change our picture of what the sky looks like. The way in which more stars keep appearing at the horizon suggests that we picture the sky as a sphere rather than a bowl, with the earth located at the center of the sphere. see the upper half of the sphere above us, but the bottom half is hidden by the ground. As the sphere turns, the hidden part is raised up to where we can see it, making its first appearance in the east. Meanwhile stars near the horizon in the west are carried down by the turning sky into the part that is hidden by the ground. After about twelve hours, these stars which set in the west rise again in the east. The sky-sphere seems to make one complete turn every day, so that the stars make daily trips across the sky where we can see them.

their place.

We

3

What We See in the Sky

1

THE SUN AND THE MOON The brightest objects in the sky are the sun and the moon. They also rise in the east and set in the west, only to rise later in the east once more. Like the stars, they repeat their journey across the sky every day. This makes it seem as though the sun and the moon are on the same sky-sphere that carries the stars, and that they move across the sky because the sphere carries them

as it turns.

When

the sun rises, the stars vanish, but the does not. There are many days during each month when we can see the moon in the sky in broad daylight. The moon then looks paler soon realize that the than it does at night. moon looks pale by day only because we see it against the bright background of a daytime sky that is flooded with scattered sunlight. Then we realize, too, why the stars disappear in the daytime. The same scattered sunlight that makes the moon look pale by comparison is also bright enough to cover up the feeble light of the stars completely. The stars are still in the sky in the daytime, only they are hidden by the bright light of the sun. get a chance to verify this fact when there is an eclipse of the sun. During an eclipse, the moon passes in front of the sun and blocks off its light. The sky then becomes dark as if it were night, and the stars appear, showing us that they were there all the time behind the curtain of daylight. can verify it, too, by looking at the sky through a telescope. powerful telescope can pierce the curtain of daylight

moon

We

We

We

A

The Sun and

14

and reveal the

Its

Family

stars in their places in the

sky in

the daytime.

WANDERERS If the sun is being carried across the sky by the turning of the sky-sphere, then we must

think of the sun as having a definite location among the stars on the sphere. can locate this place on any day by seeing which stars are near the sun, at the eastern horizon, and rise with it in the morning. If we watch for these stars again a week later, we find that they rise about

We

a half hour earlier than the sun does.

Two

weeks

they rise an hour earlier. This shows that the sun does not stay in one place on the skysphere. It moves back steadily from west to east, later

so that,

compared

to the

movement

of the stars

four minutes behind skylong sphere. If time, we find that it travels along a circle on the sky-sphere, and completes a round trip in a year. The path it follows on the sky-sphere is called the ecliptic. The moon, too, is a wanderer. In fact, if you merely watch the moon for a few hours, you can across the sky, the sun

falls

The sun is a wanderer on the we observe its wanderings for a

every day.

see

how

it

shifts its position

among

the stars.

The

moon, like the sun, moves across the sky-sphere from west to east, going about thirteen times as fast as the sun.

a slow rival

Like a

on

fast

horse running against

a circular race track,

it

passes

and then overtakes it again from behind. The moon catches up with the sun in this way about every thirty days. Usuthe sun, gets ahead of

it,

What We See in the Sky ally,

when by

the

moon

15

passes the sun, they

merely

on the sky-sphere. Occasionally, the moon moves right across the spot on the skyside

lie

side

sphere occupied the is

moon

lies

by

the sun.

When

this

happens,

between us and the sun, and there

an eclipse of the sun.

The sun and the moon are not alone in their wanderings across the sky-sphere. Thousands of years ago, people had already noticed that there were some "stars" that wandered, too. They didn't look quite like the other stars, either. While

were just pin points of were distinct discs. They follow very peculiar paths on the skylight,

all

the other stars

the wandering "stars"

16

The Sun and

sphere, too.

While

Its

Family

the sun and

moon moved

the other wanderers followed paths that made loops. In ancient times people knew of five wanderers besides the sun know them now as the planand the moon.

along simple

circles,

We

ets

Mercury, Venus, Mars,

Jupiter,

and Saturn,

we know that they aren't stars at all. The word planet comes from the Greek word meaning wanderer. The Greeks referred to all

and

the seven wanderers that they knew, including

the sun and moon, as planets.

Today we

use the

Behind the screen

word in a different way, to mean bodies that revolve around the sun. Using the word in this sense, the sun and the moon are not planets, but Mercury, Venus, Mars, Jupiter and Saturn are. also include among the planets the earth itself, and several bodies that the ancient Greeks never saw because they are too faint to be seen

We

What We by

the naked eye.

See in the Sky

The new

17

planets, discovered

was invented, Uranus, Neptune, and Pluto.

after the telescope

are

known

as

A SHADOW PLAY You have probably which people

seen a

shadow

play, in

act out a scene while standing be-

hind a screen. The audience doesn't see the actors, but sees their shadows projected on the screen by a light that is behind them. The use of these

shadows makes teresting

it

illusions.

produce some inFor example, if one actor

possible to

swings a dagger past the body of another, without bringing it near enough to touch him, on the screen it looks as though he is stabbing him in

On

the shadow screen the knife crosswhere the man's back is, because the real knife passes between the man's back and the light which projects both of them onto the

the back.

es the place

screen.

The Sun and

18

What we ways

see

Family

on the sky-sphere

shadow

like a

Its

play.

things that surround us.

is

in

some

We are looking out We see these things

at as

they were projected against a screen behind them. As a result, two objects may seem to be at the same spot on the sky-sphere when they are actually far apart. They will seem to be together on the sky-sphere when one passes between us and the other. Astronomers realized this fact quite early, as a result of hints they got by watching the wanderers traveling over the skysphere. Every night, for example, they saw the moon move across places on the sphere occupied by stars. Each time this happened, the stars disappeared on one side of the moon, and then reappeared later on the other. They drew the conclusion that the moon was closer to us than the stars, and blocked a star from view whenever it passed in front of it. During an eclipse of the sun, they could see the moon moving across the face of the sun. This showed them that the moon is closer to us than the sun is, and that it sometimes passes between us and the sun. The movements we see on the sky -sphere are not the real move?nents of the heavenly bodies. They are only projections, like shadows on a screen. That is why the play we see acted out in the sky is a puzzle. The puzzle is to find out the real positions of the heavenly bodies and the real movements they make, which make them look the way they do on the sky-sphere.

if

The

puzzle breaks up into many separate quesdoes the sky -sphere as a whole, with the things we see on it, seem to revolve around

tions. all

Why

the earth?

Why

are the fixed stars fixed

on the

What We See in the Sky

19

why do the wanderers wander? What are these bodies we see in the sky, and how far are they from the earth? We could also

sky-sphere, and

ask,

"What

they?"

more

made of?" "How heavy are hot are they?" and many, many

are they

"How

questions besides.

Modern astronomy has found the answers to a large number of these questions. It solved the puzzle of the fixed stars by showing that every really a sun that is very far away. The look so tiny because of their great distance from us, and they seem fixed for the same reason. The evidence and the thinking that solved the puzzle of the fixed stars are presented in the book, The Stars: Steppingstones into Space* Astronomy solved the puzzle of the wanderers by showing that the planets are satellites of the sun, each revolving around the sun with a regular rhythm. The discoveries and the thinking that solved this puzzle are described in the chapters star

is

stars

that follow. * By the same author. New York: The John Day Company- 1956; The New American Library, 1958.

CHAPTER TWO

Two Systems: Ptolemy

vs.

Copernicus SPHERES WITHIN SPHERES

THE puzzle of the fixed stars didn't seem much

to be

ancient

of a puzzle to the astronomers in

times.

They

what they saw with

simply accepted their

own

eyes.

as

real

They saw

the fixed stars rising in the east and setting in the

west, as though they were in fixed positions

on

a

big sphere that surrounded the earth and rotated around it. So they advanced the theory that there really was such a sphere, and they called it the sphere of the fixed stars. But the wanderers were a

little

the sky

more troublesome. Their motion across was more complicated, and needed a

more complicated

explanation. In the first place, the astronomers realized that the wanderers were closer to the earth than the fixed stars were, because they could see them passing between the earth and the stars, blocking the light of the stars when they did so. They

could

see, too, that

some wanderers were 20

closer

Two

Systems: Ptolemy

vs.

Copernicus

21

to the earth than others. The moon, for example, closer than the sun, because it sometimes passed in front of the sun and eclipsed its light.

was

Using clues

how

which will be deV, they even made estimates

similar to those

scribed in Chapter

from the earth each of the seven They arranged them in this order, from the nearest to the farthest from the earth: the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn. Each of them was assumed to be on a sphere of its own, with one sphere inside the other, and all of the spheres surroundof

far

wanderers was.

ing the earth. Since the wanderers, like the stars, rose daily in the east, crossed the sky, and set in the west, the astronomers advanced the theory that their spheres, like the sphere of the stars, also rotated around the earth from east to west. To account for the fact that the wanderers seemed to move eastward among the stars, they assumed that the spheres carrying the planets turned more slowly than the sphere carrying the stars, so that the planets kept falling behind. The theory that there were spheres within spheres rotating around the earth left one important set of facts unexplained: Some of the planets sometimes made loops in the sky. To account for the loops it was necessary to change the theory a bit. In the improved theory, the moon and the sun were left on the spheres that were supposed to carry them around the earth, but the other planets were taken off the spheres. Their places were taken by imaginary points, one on each sphere. Each planet itself was supposed to move in a circle around one of these points while the point was carried around the

22

The Sun and

Its

Family

Hie Ptolemaic System

Moon

Mercury

O^Mar* * Jupftsr

Venus

fe

Earth

2 9

Satum

Two by

earth ideas

we

Systems: Ptolemy

vs.

Copernicus

23

the rotating sphere. These were the find in the theory of Claudius Ptolemy,

the greatest astronomer of ancient times, lived in Alexandria, Egypt,

from about

who

a.d. 85

to 165.

Ptolemy's theory of how the planets move is in the diagram on page 22. At the center

shown

of the universe was the earth. The moon and the by its sphere, moved in a circle

sun, each carried

around the

moved

earth.

in a circle

Each of the other wanderers around an imaginary point,

called the fictitious planet, while the point

carried along a circle around the earth.

was

The

path planet was called a de-

followed by the fictitious ferent. The path followed by the real planet

moved around

as it

was called an epicycle. In Ptolemy's theory, Mercury and Venus were inside the sun's circle, and the fictitious planets they moved around were always on the the fictitious planet

Mars, Jupiter and Saturn were outside the sun's circle, and moved so that the line joining each to its fictitious planet was always parallel to the line joining the earth and the sun. As a planet moved around its epicycle, it spent part of the time outside its deferent, and part of the time inside. While it was outside the deferent, it moved in the same direction as the fictitious planet, caught up with it, and passed it. While it was inside the deferent, it moved in the opposite direction, and fell behind. This alternate passing and falling behind accounted for the loops these planets made on the sky-sphere. Although Ptolemy's epicycles served to explain the loops, they failed to explain other irregularities in the movements of the planets. To line joining the earth to the sun.

24

The Sun and

Its

Family

make his system fit the facts more closely, Arab astronomers who lived after Ptolemy patched it up by adding epicycles to the epicycles, just as he had added epicycles to the deferents. Before they were through with these changes, they were using over eighty circles to explain the motion of the seven wanderers. The Ptolemaic system of wheels within wheels became more complicated than the inside of a modern watch. In this complicated form, it was accepted throughout Europe as a true picture of the universe for about hundred years. During all these years, although astronomers felt free to change the details of Ptolemy's system in order to try to improve it, they insisted on holding onto two of his main ideas. They always

fifteen

placed the earth at the center of the universe, assuming that the earth stood still while the stars and planets moved around it. And they took it for granted that the only paths heavenly bodies could follow were circles. They insisted on placing the earth at the center of the universe for two reasons. First, that is the way things looked when you watched the sky. Everybody could "see" that the stars and the wanderers were revolving around the earth. Secondly, the idea fit in with the common belief that God put these bodies in the sky to serve man. Since man was the center of their thoughts, they very naturally made his home, the earth, the center of the universe. They insisted on using only circles as paths for the planets or the fictitious planets, because of the influence of religious ideas. The planets were in heaven, and all things in heaven were supposed

Two

Systems: Ptolemy

The

25

circle

curves, they said, so

path that a

Copernicus

was the most perfect of was the only kind of heavenly body could follow.

to be perfect. all

vs.

it

A NEW CENTER FOR THE UNIVERSE During the sixteenth century, one of these was challenged by a priest named Nicolas Copernicus. Copernicus, who lived from 1473

ideas

to 1543, devoted his

my. was

life

to the study of astrono-

seemed to him that the Ptolemaic system far too complicated, and that there was a

It

much

simpler explanation for the apparent motion of the stars and planets. He said it was not necessary to believe that all the heavenly bodies revolve around the earth. Their daily motion across the sky could be explained just as well by assuming that the earth was spinning like a top, instead of being at rest. If the earth is spinning from west to east, that would make it look as though everything around the earth is moving the other way, from east to west. Copernicus also found that he could explain the loops in the paths of the planets across the sky-sphere by assuming that the sun, rather than the earth,

was the center of the

universe.

He

offered the

theory that the earth, together with Mercury, Venus, Mars, Jupiter, and Saturn, moved in circles around the sun. Under this theory, only the moon revolved around the earth. Because he held onto the old idea of circles as the only possible paths, he had to use epicycles, too. But his system used only thirty-four circles while Ptolemy's used over eighty.

The Sun and

26

Its

Family

HINTS FROM THE TELESCOPE At the time when Copernicus proposed his theory, there was no proof that it was more correct than Ptolemy's. The only thing that was in favor was that it was simpler. But after the was invented, new facts were discovered that gave hints that Copernicus' theory

its

telescope

might be

true.

These

facts

were uncovered by

the Italian scientist, Galileo Galilei,

from 1564

to 1642. Galileo

was the

who

lived

first

to use

a telescope for the exploration of the sky.

When

he looked at the sun through his telescope, he saw some dark spots on its face. The dark spots moved in a way that showed that the sun was spinning like a top. Here was a hint that Copernicus was on the right track. If the sun was spinning, then it was possible that the earth might be spinning, too.

When

Galileo looked at Jupiter through his

telescope, he

saw what looked

like

faint stars

near it. Watching them from day to day, he could see that they were moons of Jupiter, revolving around Jupiter the way our moon revolves around the earth. Here was another hint that Copernicus could be right. The motion of the moons of Jupiter showed that not all heavenly bodies revolved around the earth. If there could be bodies revolving around Jupiter, then it was possible that there might be bodies revolving around the sun. When he looked at the moon

through the telescope, he saw that the moon had mountains. If the moon had mountains as the

Two

Systems: Ptolemy

earth did, and

if

vs.

Copernicus

27

it showed were no more perfect

the sun had spots, then

that the heavenly bodies

than the earth. Here was a third hint that Copernicus might be right. If the heavenly bodies were not perfect, then perhaps they were no different from the earth. Then the earth, in spite of all its blemishes, could be a planet, like Mars or Jupiter, revolving around the sun. But these were only hints. They showed that Copernicus might be right, but they did not prove that he was.

THE PHASES OF VENUS Galileo made another important when he turned his telescope on

Venus.

discovery the

planet

He

phases, as

found that Venus went through the moon does. In fact, he saw it just

then in the gibbous phase, when more than half of its disc was covered with light. This was clear proof that Ftolemy's theory was wrong. To understand why, let us first see why the moon goes through its phases. The moon does not produce light of its own, as the sun does. It shines only by reflecting sunlight that strikes its surface. At any moment, only half of the surface of the moon receives any sunlight, because only half of the surface is turned toward the sun. The other half of the moon is dark because it faces away from the sun and lies in the

moon's

own

gram below. As

shadow,

the

moon

as

shown

in the dia-

revolves around the

earth, sometimes we see only the sunlit half, sometimes we see only the dark half, and some-

times

we

see a part of each.

The way

in

which

The Sun and

28

Its

Family

o o Mght i

—

from I

the

H

H

Phases of the

Moon

sur)

Two

Systems: Ptolemy

the moon's phases change

is

vs.

Copernicus

shown

29

in the dia-

The moon is shown in several positions that it may have as it moves in its orbit. What we see when it is in each of these positions

gram on page

is

shown by

the diagram.

28.

the circles at the top and bottom of When the moon lies between the

earth and the sun, the sunlit half of the faces

away from

dark half of the moon. Then,

moon

we

see only the

as the

moon moves

the earth, and

to the side, part of the sunlit half turns into view.

At

first

time the

around

we see only a crescent moon. By the moon has gone one-fourth of the way orbit, the

its

half of the light.

Then

more than light.

the

moon it

crescent has

we

circle

see

is

grown

enters the gibbous phase,

half of

what we

see

is

until

covered by

when

covered by

When the moon reaches the position where

moon and

the sun are

on opposite

sides of

the earth, the sunlit half of the moon faces toward the earth, and we see a full moon. Then the dark half begins to swing into view again,

and

we

finally,

see less

when

the sun again,

The

and

the

less

moon

of the sunlit part until, between the earth and

is

we see none

fact that

Venus

of the sunlit half at

all.

passes through phases

shows that it, too, shines only by reflected sunlight, and that it moves in such a way that varying fractions of its sunlit half face toward the earth. Let us see how this would happen if Ptolemy's theory were correct. The diagram on page 30 shows Venus moving as Ptolemy thought it did. The fictitious planet around which it turns, according to Ptolemy, lies on the line that joins the earth and the sun. Venus itself is supposed to be moving in a circle around this point. Four

The Sun and

30

Its

Family

Venus are shown in the At the bottom of the diagram we see what Venus would look like in these positions. In each position, the half of Venus that faces possible positions of

diagram.

toward the sun is flooded with sunlight. When Venus is between the earth and the sun, we would see no part of this sunlit side at all. The illuminated part would begin to turn into view only as Venus moved away from the line to the

a,c

Venus as seen from the earth according to Ptolemy

sun.

We would see the largest amount of reflect-

ed sunlight tions

shown

when Venus

reached the side posi-

in the diagram.

But

in these positions,

more than half of the sunlit side still faces away from the earth, so that only a crescent would be seen from the earth. If Ptolemy's theory were correct, ive would never see as much as half of the sunlit side of Venus. But Galileo saw more than half of the sunlit side, because he saw Venus

Two

Systems: Ptolemy

in the gibbous phase. This

vs.

Copernicus

31

proved that Ptolemy

was wrong.

A COMPROMISE SYSTEM The phases of Venus showed that Ptolemy was wrong, but they did not prove that Copernicus was right. Although Copernicus' theory permitted Venus to go through the correct phases, as we can see them in the telescope, it was not the only theory that did. The phases of Venus are explained just as well by the theory of Tycho Brahe, Dutch astronomer who lived from 1546 to 1601. Tycho's theory was a compromise between Ptolemy's theory and Copernicus'. He yielded to Copernicus to the extent of saying that the planets revolve around the sun. But he stuck by Ptolemy in his belief that the sun, accompanied by its planets, revolved

around the earth. He insisted that the earth stands and is the center of the universe. But Tycho's compromise didn't last long, because further study of the earth and the sky produced positive evidence that the earth does move. have definite proofs now that the earth does spin on its axis, and that the earth does revolve around the sun. These proofs, described in the still

We

next two chapters, finally established Copernicus' theory as a true picture of the solar system.

CHAPTER THREE

The Earth Spins A

SPIN

TO PROVE we

need

DETECTOR that the earth really spins,

a spin detector.

a spin detector

works, that

it

if

An

object can serve as

we know, from

would behave

in

one

way it way if the

the

earth were not spinning, and would behave in another way if the earth were spinning. Then we can find out whether or not the earth spins

by merely watching

way

it

behaves.

One

this object to see which of the things that can serve

is a swinging pendulum. pendulum, tie a weight to one end of a string, and then tie the other end to a chandelier or anything else that can support it while

as a spin detector

To make

a

To set the penweight to one side, and then let it go. Experiments with the pendulum show that if a swinging pendulum is not disturbed, it keeps swinging back and forth in the sa?ne plane. This property of the pendulum makes it useful as a spin detector. it

hangs

down

over the ground.

dulum swinging,

pull the

32

The Earth

To

Spins

33

how

a pendulum serves as a spin deout with a phonograph turntable. First set the pendulum swinging over the turntable while the turntable is at rest. The plane in which the pendulum swings crosses the turntable along a line. can make this line visible by drawing over it with a piece of chalk. As long as the turntable is at rest, the pendulum keeps swinging back and forth over the chalk see

tector, try

it

We

But the behavior of the pendulum looks Then, while the pendulum keeps swinging back and forth in the same plane, the chalk line, carried by the spinning turntable, turns under the pendulum. The pendulum no longer crosses over the same line with each swing. If there were some tiny creature living on the turntable, it would look to him as though the plane of the pendulum were turning. If the turntable turned in the same direction line.

different if the turntable spins.

as the

hands of a clock, or clockwise,

it

would

The Sun and

34

look to

this creature as

pendulum were turning terclockwise.

The

Its

Family

though the plane of the the other way, or coun-

length of time that

it

took for

pendulum to seem to make one complete turn would actually be the length of time that it took for the turntable to make one complete turn. If the creature on the turntable were curious and intelligent, he would realize that the swinging pendulum could tell him when the turntable was spinning and when it was at rest. If he saw the pendulum cross over the same line with each swing, then he would know that the turntable was at rest. But if the plane of the pendulum seemed to turn, then it would show that the turntable was spinning. the plane of the

We

can use the pendulum in the same

way

as

However, the earth turntable. It is phonograph is not flat, like a round, like a ball. This makes the effect of its spinning a little more complicated, as we shall see. a spin detector for the earth.

The Earth

IF

THE EARTH

If the earth

able to

tell

Spins

is

that

SPINS

really spinning,

it is

by

35

we

should be

the effect that the spin-

ning has on the motion of a pendulum. But this effect is not the same at all places on the earth's surface, on account of the fact that the earth is shaped like a ball.

The Sun and

36

When there are that

lie

Its

Family

a ball spins like a top around an axis,

two

on the surface of the

points

on the

axis.

On

the earth

we

ball

call these

North Pole and the South Pole. shows up in different ways at the two poles. As seen from a point above the North Pole, the spinning earth is like a turning phonograph turntable, spinning counterclockwise around the North Pole as center. Because of this fact, the plane of a swinging pendulum at the North Pole turns clockwise. Since the

two

points the

The

spin of the earth

earth makes one complete turn in a day, a pendulum there, if it doesn't stop swinging too soon,

makes a

full

turn in twenty-four hours.

Down

The Earth

Spins

South Pole, an observer

at the

37

sees the spinning

from the other side. As a result, he sees the ground spinning in the opposite direction. At the South Pole the ground spins clockwise, so there the plane of a swinging pendulum turns counterclockwise, and makes a complete turn in earth

twenty-four hours, too.

Halfway between South Pole

we

down

would

see

still

the

Here

the

the spinning of the

another effect.

at the earth it

North Pole and

the circle around the earth that

call the equator.

earth has

ing

is

An

observer look-

from above the equator

rolling like a barrel.

The

rolling

mo-

ground forward, but does not turn like a turntable. Since the ground

tion carries the

make

it

does not turn there, the plane of a swinging pendulum held at the equator does not turn at all. At places between the poles and the equator, the spinning of the earth has an intermediate effect. Here the plane of a swinging pendulum turns, but more slowly than it does at the poles, so that it makes less than a full turn in twentyfour hours. In the Northern Hemisphere it turns clockwise, just as it does at the North Pole. In the Southern Hemisphere, the pendulum turns counterclockwise.

FOUCAULT'S PENDULUM The

spinning of the earth was

first

detected

by

swinging pendulum in the year 1851. The French scientist Foucault made a pendulum of a heavy iron ball about a foot wide, attached to a a

The Sun and

38

Its

Family

wire that was over 200 feet long. He hung the pendulum from the dome of the Pantheon building in Paris.

To

be sure that the swinging of the

pendulum was not influenced by any

accidental

swinging with extra special care. First he pulled the ball to one side, and tied it with a string. He left it tied for many hours, to be sure that it came to rest. Then he released the ball by burning the string, and the pendulum began to swing. The space over which the pendulum swung was surrounded by a rail, and a thin layer of sand was placed on the top of the rail. At each swing, the ball of the pendulum passed over the rail, and a pin in the bottom of the ball made a line in the sand. But with each pushes,

he

new swing

set

it

the pin

made

going over the old ones.

a

new line instead of The successive lines

appeared side by side going clockwise, showing that the plane of the that direction.

was enough

The

pendulum was turning in which it was turning

rate at

to carry

it

through

a

complete turn been

in 32 hours. Since 1851 this experiment has

repeated many times at many different places. Each time the plane of the pendulum has turned

The Earth

39

Spins

could be predicted from its posibetween the equator and one of the poles. These experiments serve to prove two things at the same time. Besides showing that the earth at a rate that

tion

really spins,

spinning

they also show that the earth

is

a

ball.

A FALLING BODY A

body that falls from a great height above the earth can also serve as a spin detector. If the earth were not spinning, a freely falling body would move straight down along the line that joins

its

starting place to the center of the earth.

its

Then

it

would land

starting place.

at the point directly

But because the earth

beneath is

spin-

ning from west to east, a falling body does not land at the point under its starting position. Inlittle further east. To happens, look at the diagram page 40. The letter A marks a point on the ground in the northern hemisphere. The letter B marks a point high up in the air above the point A. As the earth spins from west to east, the ground at and the air at B both travel eastward along cir-

stead,

see

it

why

lands at a point a this

A

cles.

These

Because is, it

B

is

circles are

further

shown

from the

in the diagram.

earth's axis than

travels along a larger circle.

Both

A

and

A B

each along its own circle, in twenty-four hours, the time that it takes the earth to make one complete rotation. Since B covers a greater distance than A does this time, it means that B is traveling east faster than A. Now if a body is held in place at B, it travels

make

a complete

round

trip,

40

The Sun and

Family

Its

eastward with the speed of the air at B. Then, when the body is dropped, its motion is made up of two parts. tends to carry

ground.

downward motion

It

has a

it

directly to the point

It also

A

that

on the

has an eastward motion that

it

C5 B

/

/

V

got from its starting place at B. A also is moving eastward, so it is as though the point A and the falling body were running a race to the east. But

The Earth as the falling carries

body approaches

down with

it

41

Spins

the ground,

it

the higher eastward speed

of the upper level. This higher speed makes it possible for the falling body to get ahead of the

ground and win the it

race. Instead of landing at

lands a bit to the east of A.

The

A

fact that this

actually happens serves as another proof that the

earth rotates.

THE PATH OF AN ARTILLERY SHELL There is further proof that the earth rotates in the effect that the rotation has on the path of a from a big gun. Suppose an artillery Northern Hemisphere is aimed north and fires a shell. If the earth were not spinning, the shell would travel northward, in the direction in which the gun is aimed. But because the earth is spinning, the shell veers toward the east from its course. The reason why this happens is shell fired

gun

in the

similar to the reason for the eastward drift of a

body. The spinning of the earth carries every point on the ground around in a circle. The diagram above shows that the size of this falling

The Sun and

42 circle

depends on

North

how

Its

Family

close the point

is

to the

Pole. Points near the pole travel along a

smaller circle than points nearer to the equator,

they move eastward more slowly. fired from a gun that is aimed north, the shell has the eastward motion of the place that it starts from. But as the shell travels north, it travels over ground that is moving eastward more slowly. The shell gets ahead of the ground in the race to the east, and so it veers to

and

as a result

When

a shell

is

the right.

In the Southern Hemisphere the earth's rotation has the opposite effect. There, as a shell travels north, it passes over ground that is moving eastward faster than the shell does. So here the shell falls behind in the race to the east, and veers to the left. The earth's rotation has this effect no matter which way the gun is aimed. When artillery guns are fired, this influence of the earth's rotation has to be taken into account.

THE DIRECTION OF THE WIND The

earth's rotation reveals itself, too, in the

on the direction of the wind. It wind in the same way that it influences an artillery shell. Just as a shell is a moving body of metal aimed from the gun to a target, effect that

it

has

influences the

The Earth

Spins

43

a wind is a moving body of air aimed from a highpressure region to a low-pressure region. If the earth were not spinning, the high-pressure region would push the wind directly to the low-pressure

But since the earth is spinning, it compels to veer from its course. In the Northern Hemisphere, the wind veers to the. right. So, in the Northern Hemisphere, when you face in the direction in which the wind blows, the lowpressure region is on your left. In the Southern Hemisphere it would be on your right. This fact is well known to weathermen, and is additional proof that the earth is spinning on its axis. region.

the

wind

CYCLONES AND HURRICANES The wind region from

rushes in toward a low-pressure directions. If the earth were not

all

rotating, lines showing the direction of the wind near a low-pressure center would look like the spokes of a wheel, as shown in diagram 1. But since the earth is spinning, the winds veer away from the low-pressure center, as shown in diagram 2. In the Northern Hemisphere, where

The Sun and

44

Its

Family

they veer to the right, the result is that the winds around counterclockwise. Weathermen call such a spiraling wind system a cyclcme. In the Southern Hemisphere, where the winds veer to the left, the cyclone turns clockwise. The center

spiral

+.

low

.«•

of a hurricane

is

a low-pressure region, so the

winds of a hurricane spiral in toward it, carrying the storm clouds on their backs. When a hurricane is viewed on a radar screen, the spiral arrangement of the clouds can be seen distinctly. The spiral shape of cyclones and hurricanes is yet another proof that the earth really spins.

THE GYROCOMPASS A

rather interesting spin detector is made out of something that has a spinning motion of its own, and is known as a gyrocompass. The gyrocompass contains a spinning wheel, or gyroscope, and takes advantage of the peculiar way in which

'

The Earth

Spins

45

any attempt to tilt its axis. on which it operates can be demonstrated by using a bicycle wheel mounted on an axle in such a way that the wheel can spin

a

gyroscope

The

resists

principle

freely while the axle

if,

j^H

f'

:

grasped firmly in both

is

"i^&k;

'

>

Ik

W\ IP^»II % ;

V

'

^8W»S^

/

IS Picture of a hurricane

hands.

you

If,

try to

on a radar screen

while the wheel tilt

1

is

spinning rapidly,

one end of the axle down, you find

The Sun and

46 that the

The

Its

Family

wheel stubbornly resists being tilted. on an especially interesting

resistance takes

form on a

you repeat the experiment while sitting whose seat is free to turn, like the

if

stool

counter seats in an ice-cream parlor. Then, while

tilting,

ing

the seat of the stool begins to turn, carry-

you and

the wheel

is

the spinning wheel around with

spinning rapidly,

if

you

try to

it.

tilt

The Earth

Spins

47

one end of the axle down, instead of the wheel you try to tilt the other end of the axle down, the seat starts turning the other way. Any attempt to tilt the axis of a spinning gyroscope causes a If

rotation at right angles to the direction of the tilting force.

In the gyrocompass, a wheel that

its axis is

horizontal,

in a horizontal plane.

is

mounted

so

and can swing around

An

the wheel spinning steadily

electric

on

motor keeps

its axis. If

the axis

points east and west, the rotation of the earth tends to tilt it by tilting the eastern end down

and the western end up. The result is that the swings around in the horizontal plane, just as the seat of the stool did in the experiment with

axis

The Sun and

48

wheel.

the bicycle

Family

Its

keeps swinging around and south. If it swings

It

until the axis points north

beyond

that position, the effect

is

reversed.

The

before is now tilted up by the earth's rotation, so this time the axis swings around the other way. Because of this corrective action, the axis finally settles down in the northsouth position. By its peculiar behavior the gyrocompass not only proves that the earth rotates. It also points out which way is north, and so really serves as a compass, as its name suggests. In fact, it is a better compass than the magnetic compass, because it points out the true position of the North Pole, while the magnetic compass only points to the magnetic north pole, which is not in the same place. It is better, too, because it can be used inside a metal ship, where a magnetic compass is useless because the surrounding metal shields if from the earth's magne-

end that was

tilted

down

tic field.

A

SPIN

WE DO NOT FEEL

Our own bodies are spin detectors, too. We know this from what happens when we ride on a merry-go-round.

around,

we

from

center.

is

its

the pull

feel

we

As

the merry-go-round spins

ourselves

The feel.

faster

being pulled away turns, the stronger

it

This pull

is

known

as the

centrifugal force (force pulling away from the center) that comes into existence whenever a

moving body this force,

in

is

we

we can feel we are riding

spins or turns. Because

can

tell

when

a train

rounding a curve. While the

train

is

going

The Earth around the curve

we

49

Spins

feel ourselves

forced to

lean over toward the outside of the curve.

The

proofs described in this chapter show that is turning like a merry-go-round, so there must be a centrifugal force tending to pull us away from the earth's axis, the center around which it turns. In spite of this fact, we do not feel any such pull. Although our bodies are usually good spin detectors, they do not help us detect the spin of the earth. do not feel the centrifugal force because it is covered up by an even stronger force acting in the opposite direction. This stronger force is our own weight. Our weight is a force that tends to pull us toward the center of the earth. The centrifugal force caused by the earth's rotation opposes our weight, but is not strong enough to wipe it out. So we still feel ourselves pulled toward the ground rather than being flung away from it. This is why the spin of the earth is one spin that we do not feel. the earth

We

CHAPTER FOUR

The Earth Revolves

Around the Sun NOT A

RIPPLE IN

TO PROVE around the sun, or, is

some device

we

YOUR COFFEE

that

the

earth

revolves

have to find a motion detect-

that will

show whether

traveling through space or

is

the earth

staying in one

we have to decide where to look for motion detector. Can we find it, perhaps, in some effect that the earth's motion has on us, or the things we do? The answer is that we cannot, and we can find out why by referring to our experience with railroad trains. When a train starts moving and picks up speed, we can feel it because place. First this

we and

are

we

feel

forward. When because then

it,

when speed,

When the train slows down because then we are thrown the train rounds a curve, we feel

thrown back.

stops,

it,

we

are

thrown

to the side.

But

the train travels in a straight line at a steady

we

railroad

don't feel

company on

boast that

its

trains

its

motion

at all. In fact, a

the Atlantic coast used to

gave you such a smooth ride 50

The Earth Revolves Around was "Not

the

Sun

51

Ripple in Your Coffee" dining car. Experience can shows that we cannot feel steady motion. only feel a change in the motion, like a change in speed or a change in direction. The earth's motion in its orbit around the sun goes on at a steady pace of about 18 54 miles per second. Because it is steady, we do not feel it. It is true that the motion keeps changing direction, because the orbit is curved. But because the orbit is very large, the change in direction is so slow that we do not feel that either. The earth's motion around the sun cannot be detected by any effect it has on us or anything else that is on the earth. To find our motion detector we have to look that there

when you

ate

in

a

the

We

away from

the earth.

THE SUN DOESN'T HELP Since

we

are trying to prove that the earth

moves around the

sun,

we may

be tempted to

search for the evidence in what we see when we look at the sun. But this will not work, either. Again, our experience with railroad trains will explain why. Everybody who has traveled on a

probably had an experience like this: and there is another train alongside yours, on another track. While you wait impatiently for the train to start again,

train has

Your

train stops at a station,

you watch train.

Then,

again.

Your

the faces of the people in the other at last,

train

is

you

see that

you

are

moving

slowly slipping past the other

train. You pass car after car, finally pass the end of the train, and then discover with a shock that

The Sun and

52

your

train

is

Its

not moving at

Family

all. It is still

standing

was moving, in the opposite direction. You were misled because the effect is the same whether your train moves or the other train moves. In both cases you

at the station. It

merely see the

was the other

trains passing

You

train that

each other and then

which you look away from the

pulling apart.

can't tell

ing until station or the ground.

Our motion around

the sun

is

train

is

mov-

trains to the

similar in this

two Looking at the sun will not tell us whether the earth is moving around the sun or the sun is moving around the earth. To find proof that the earth is moving around the sun we have to look away from both bodies.

respect to the motion past each other of trains.

THE RAIN OF STARLIGHT We shall find

the motion detector that

looking for by looking at the light,

which

falls

on the earth

stars.

we

are

In the star-

like a gentle rain,

there are several proofs that the earth revolves

around the sun. In fact, one of the proofs is found in the fact that the rays of starlight falling on the moving earth behave like raindrops falling past a

moving

train.

Suppose you are riding in a train on a rainy but windless day, and you are watching the rain through the car window. Because there is no wind, the raindrops fall straight down to the ground. When the train is at rest at a station, you can see this fact in the way the raindrops move across the

window

pane.

Each raindrop,

falling

The Earth Revolves Around straight

down,

the

Sun

53

traces out a vertical line against

window pane. But the rain seems to shift when the train moves. Then, instead of falling vertically, the raindrops streak past the window in slanting lines, as if they were being blown by the

a wind. Let us see why this happens. While the raindrops are falling

your window, the

train

is

down

past

moving forward

past

the raindrops. The forward motion of the train makes the raindrops move backward across the window pane, away from the front end of the train. This backward motion combines with

downward motion

of the drops to give them window pane. This has the effect of shifting the direction that the raindrops seem to be coming from. While the raindrops are really coming from directly overhead, the

a slanting path along the

The Sun and

54

Its

Family

the motion of the train makes it look as though they are coming from a place ahead of this over-

head

position.

The motion

of the earth in

light of the stars in the

its

orbit affects the

same way that the motion

of a train affects the paths of the falling raindrops. It shifts the direction from which the starlight seems to come. This shift in the direction of the light is known as the aberration of light.

The

always in the direction in which the moving. But when the earth revolves around the sun, it keeps changing the direction in which it moves. As a result, the stars seem to shift around on the sky-sphere. shift

is

earth itself

is

same plane in which the seem to move back and forth in a straight line. Stars whose light comes to us at right angles to this plane seem to make little circles on the sky-sphere. Stars between these two extreme positions trace out little ovals on the sky-sphere. The amount of the shift is not very great. Whether the stars seem to move in Stars that are in the earth's orbit lies

straight lines or circles or ovals, the greatest shift

41 seconds of arc on the skytiny this is can be seen from the fact that 60 seconds (of arc) make one minute, 60 minutes make one degree, and 360 degrees make a full great circle girdling the sky. However, although the shift is so small, it can be detected by modern telescopes and can be measured. This shift in the position of the stars on the

in each case

sphere.

is

How

sky-sphere is a motion detector for the earth. It proves that the earth is moving around the sun. It even tells us the rhythm of that motion, be-

The Earth Revolves Around

the

Sun

55

cause the rhythm of the stars, as they shift around on the sky-sphere, matches the rhythm with star which the earth revolves around the sun.

A

that

moves around

a

little

oval on the sky-sphere

make a round trip around the oval. This shows that the earth takes a year to make its own round trip around the sun. saw in Chapter I that the sun is a wanderer on the sky-sphere. It moves along the circle known as the ecliptic, and makes a complete trip around in a year. Now we can understand why it seems to wander in the sky, and why it repeats its wanderings every year. The sky-sphere is like takes a year to

We

which we

see the sun, like

a stage against

which the audi-

a big curtain against

the backdrop

on

ence sees the actors in a theater. When the sun seems to move across this curtain it is because the earth itself is moving. As the earth moves around in its orbit, the sun seems to move the other way, just as the trees

we

pass

when we

ride

by

in a

seem to move toward the rear of the train. When the sun takes a year for a round trip along

train

the ecliptic,

it is

takes a year for

only another sign that the earth round trip around the sun.

its

THE SHIFTING COLORS IN STARLIGHT Starlight is a mixture of colors. The colors can be separated by passing the light of a star through a wedge-shaped piece of glass known as a prism. When the starlight emerges from the prism, the colors lie side by side, arranged the way they are in a rainbow, from violet to red. This spreadout arrangement of the colors in the light of a

The Sun and

56

Its

Family

is called its spectrum. Some of the colors are bright in the spectrum. Other colors are weak, and in contrast to the brighter colors that sur-

star

round them, they look

like dark lines crossing the spectrum. The dark lines form a pattern, and each star has its own pattern. These dark-line patterns have turned out to be a gold mine of information about the stars. The book called The Stars: Steppingstones Into Space explains how astronomers find hidden in these

from which they figure out how how far away it is, and even how much it weighs. What interests us here is the fact

patterns clues

hot

a star

is,

that these patterns also contain another proof

that the earth revolves is

around the sun. The proof

based on the fact that the dark lines in the

spectra of

many

stars

in the spectrum, in a

keep shifting their position

movement

called the

Dop-

pler shift. Part of the time they shift toward the violet end, and then they shift back toward the red. They repeat the shifting in the same way

The Earth Revolves Around

the

Sun

57

every year. Let us see how this proves that the is revolving around the sun. Light is a vibration that travels through space.

earth

The

vibration in light of

any color has

a definite

rhythm, and the color depends on the rhythm.

The

vibrations are faster, for example, in violet

light than

they are in red is given by

the vibration tells

the

number of

a second.

The

The rhythm of frequency, which

light. its

vibrations that take place in

vibrations travel to us through

space in the form of waves. The frequency of the vibration in a ray of light is also the number of waves that reach us in a second. In a spectrum, the colors are arranged according to their frequency. The low-frequency colors are near the red end of the spectrum, and the higher-frequency colors are near the violet. In the pattern of lines in a star's spectrum, each line represents a definite frequency, and this frequency can be measured. When the lines shift toward the violet end of the spectrum, it means that the frequency has increased. When they shift back toward the red end again, it means that the frequency has decreased.

This change in the frequency is the clue that shows that the earth is moving. When a ray of light of a particular color comes to us from a star, its waves are spaced out one behind the other, like lines of soldiers marching in a parade. If the earth were standing still with respect to the star, the waves would arrive at the earth with an unchanging frequency, a definite number coming in each second. But since the earth moves, this frequency keeps changing. When the earth moves toward the star, it is rushing toward the waves

The Sun and

58

Its

Family

and therefore meets more of them in

a second. of each line of light in the spectrum is increased, and so all the lines shift closer to the violet end. When the earth moves away

The frequency

from the star, it is running away from the approaching waves. This slows down the rate at which the waves reach the earth. The frequency of each line of light in the spectrum is decreased, and all the lines shift toward the red end of the spectrum.

The way

the lines in the spectra of the stars

back and forth shows that the earth is alternately approaching them and then moving away from them. As it moves around in its orbit, it is headed now toward one star, and now toward another, and so the shifting of the lines in the spectrum passes from one star to another. It is passed around in a circle, like a bean bag in a game, showing that the earth follows a curved orbit rather than moving in a straight line. shift

The

fact that the shifting of the lines

in the

same way

after a year

earth completes a round trip in

and retraces

its

steps over

is

its

is

repeated

a sign that the

orbit in a year,

and over

again.

THE PARALLAX OF STARS The

light of the stars contains a third

proof

around the sun. This proof is based on the fact that when one object is viewed from several different positions, it is that the earth revolves

you you you move

seen in a different direction each time. If look at a tree that is directly in front of you,

look straight ahead to see

it.

Then,

if

The Earth Revolves Around several yards to the left,

right to see the tree. If

Sun

59

you have to look you move to the

to the

the

right,

then you have to look to the left to see the tree. In fact, as you move past the tree, because of your change in position it looks as though the tree is moving the other way. This apparent shift in the position of something you look at when you move is called parallax. Not all the things you look at are affected equally by parallax. When you walk down a

road past a group of trees, the trees seem to shift backward, but some of them shift more slowly

The nearest trees seem to shift the and the farther away a tree is, the more slowly it seems to move. The parallax of an object that you pass depends on how far away it is. than others. fastest,

Some

of the nearer stars show a parallax shift from the motion of the earth in its

that results

The Sun and

60 orbit.

Its

The diagram on page — shows why

shift takes place. If the star is

Family

shown

observed in the month of January,

this

diagram

in the

it is

seen in

a definite direction. Six months later, the earth is on the other side of the sun. If the star is observ-

ed now from the earth's new position, the star is seen in a different direction. The shift in the

earth's position as

it

moves along

its

orbit

is

re-

flected in an apparent shift of the star's position

on the sky-sphere. Because the earth moves around along an oval path, the oval on the sky-sphere.

The

star describes

an

parallax shift of a star's position has a by the

certain resemblance to the shift caused

aberration of light. But there difference

pends on

is

an important

between them. The parallax

how

away

The

shift de-

aberration shift does not. All the stars in the same region of the sky-sphere show the same kind of aberrafar

a star

is.

The Earth Revolves Around tion shift, but they

of parallax

shift.

parallax shift,

the

Sun

61

all show the same kind the nearest ones show a

do not

Only

and the nearer the

star,

the greater

the shift. This difference between the influence of parallax and the aberration of light makes it possible to distinguish one from the other.

The stars

are

all

so far

away

that their parallax

very small. In fact, when the nearest star is observed from opposite sides of the earth's orbit, its parallax shift is less than one second of arc. This is so small that it cannot be observed with the naked eye. Parallax shifts can be detected only through measurements made on photographs of the sky that are taken with the help of

shifts are

a telescope.

The parallax of about seven hundred stars has been measured. The measurements are additional evidence that the earth is moving around the sun.

They stars

also serve as clues to the distance of these

from the

earth.

The Sun and

62

Its

Family

COPERNICUS WAS RIGHT Galileo proved that Ptolemy was wrong, but he didn't have enough evidence to prove conclusively that Copernicus was right. Now we have the evidence, in the aberration of light, the Doppler shift, and the parallax of the stars. The earth is not the center of the universe, as the ancient astronomers thought. It is only another planet revolving around the sun. This makes it necessary to reject Tycho Brahe's compromise

system

as

well as the older system of Ptolemy. It is a curious fact conclusion by pur-

Tycho Brahe was wrong. that he was led to his wrong

suing an idea that was right. Tycho understood the principle of parallax, so he reasoned that if the earth is really moving around the sun, its motion should be reflected in a parallax shift of the stars. His own observations of the stars were the most accurate that anybody had ever made, and he saw no evidence of parallax. So he concluded that the earth did not move. He did not realize that the stars are so far

parallax

was too small

away

that their

to be detected

instruments. His reasoning

by

his

was good, but the

evidence he based it on was faulty. When better instruments were used by later astronomers, they found the parallax that Tycho was unable to see.

CHAPTER FIVE

The Sun and

Its

Family

THE EARTH IS PUT IN ITS PLACE IN PTOLEMY'S theory of the universe, the earth occupied the place of honor. It was the center around which all other heavenly bodies were supposed to revolve. Copernicus theory ,

pushed the earth out of this special position. At the same time, it gave the sun a special position as the body around which the earth and the other planets revolve.

member

The

earth

became

of the sun's family of

just another

satellites,

the solar

system.

NEW QUESTIONS As long

as the earth

center of the universe,

was thought to be the it was the center of

reference for the questions astronomers tried to answer. When they studied the motions of the

The Sun and

64

Its

Family

they asked, "How long does it take each planet to make a round trip around the earth?" When they tried to make a map of the universe, they asked, "How far is each planet from the earth?" But now that the sun was seen as the center of the solar system, the sun became the center of reference for these questions. Now the astronomers asked, "How long does it take each planet to make a round trip around the sun?" and, "How far is each planet from the sun?" To answer these new questions, the astronomers re-examined all the facts they had about the apparent motions of the planets on the sky-sphere. They found the answers to these questions, and the answers led them to a better understanding of why the planets move the way they do. planets,

TRIPS The

AROUND THE SUN

planets revolve around the sun.

make

How long

round trip? This question would be easy to answer if we were on the sun. We would begin by noticing where the planet is in the sky. In practice, this does

it

take each planet to

a

would mean noticing what star the planet is next to on the sky-sphere. Then we would see

how

long

it

this star after

takes for the planet to return to moves around its path on the sky-

it

sphere. This length of time, that could be observed directly from the sun, is known as the

But we are not on the cannot observe the planet's sidereal period directly. When we look at the planet

planet's sidereal period.

sun, so

we

The Sun and from the

earth,

Its

Family

65

what we

the fact that the earth

is

see is complicated by moving, too. have to

We

take this fact into account, in order to calculate the planet's sidereal period from what we can see.

THE EARTH AND THE PLANETS RUN A RACE The

space around the sun is like a giant race earth and the other planets are running a race around this track. The earth makes a complete round trip in a year, so the earth's sidereal period is one year, or 365 J4 days. Two of the planets, Mercury and Venus, keep gaining on the earth in the race. This shows that they circle the sun in less time than the earth does, so their sidereal periods are less than one year. The other planets keep falling behind the earth in the race. This shows that their sidereal periods are longer than a year. Let us examine more closely the earth's race with Mercury. There are times when the earth track.

The

and Mercury are

side

by

side,

running neck and

neck. This happens when the earth and Mercury are on the same straight line from the sun. Immediately after this happens, Mercury starts pulling ahead of the earth. Its gain becomes larger and larger until it catches up with the earth again from behind. When this happens, it has gained one complete lap in its race against the earth. can measure the length of time it takes for Mercury to gain a lap, and we call it Mercury's synodic period. In the same way, we

We

The Sun and

66

Its

Family

can measure Venus' synodic period, the length of time it takes Venus to gain a lap in its race with the earth. The other planets also have a synodic period. In their cases, since they fall behind the earth, a synodic period is the length of time it takes for the planet to lose a lap in its race with the earth. The synodic period of a planet is a clue from which we can calculate what its sidereal period

is.

To show how is

the sidereal period of a planet

calculated, let us use

Mercury

Mercury

as

an example.

has a synodic period of about four

months. This means that Mercury gains a

full

Part of orbit covered by earth in

one month

lap in four months, so in one month's time

gains one-fourth of a lap.

when Mercury and

Now suppose we

the earth are side

by

it

start

side in

and then see how far Mercury moves in a month. The earth moves one-twelfth of a lap in a month, because there are twelve months in a year. But while the earth moves forward their race,

one-twelfth of a lap, Mercury gets one-fourth of a lap ahead of it. That means that in one month, Mercury moves one-twelfth of a lap plus one-fourth of a lap. This adds up to four-twelfths or one-third of a lap. But if Mercury covers one-

The Sun and

Its

Family

third of a lap in a month, then

months to cover real period, trip it

is

takes three

So Mercury's sidetakes for a complete

a full lap.

the time

around the sun, calculated

it

67

is

it

about three months.

more accurately

When

in days,

it

is

found to be 88 days long. The sidereal periods of the other planets can be found by the same reasoning. Venus takes 225 days in its trip around the sun. Mars takes 687 days. Jupiter takes almost 12 years, Saturn takes 29 years, Uranus takes 84 years, Neptune takes 165 years, and Pluto takes about 250 years.

MAKING A MAP OF THE SOLAR SYSTEM According to Copernicus' picture of the solar system, the earth moves around the sun in a circle.

A

circle

is

a plane curve; that

entirely in a flat surface of a plane.

As

is,

it lies

the earth

moves around the sun, it looks to us as though the sun moves the other way in the same plane.

The sun seems

to move across the sky-sphere along the circle called the ecliptic. So the ecliptic shows us the position in space of the plane of the earth's orbit. The planets, as they wander over the sky-sphere, are always very close to the ecliptic. This shows that their orbits are almost in the same plane as the earth's orbit. So, if we assume that the sun, the earth, and the other planets all lie in one plane, we are not making a very serious mistake. But if they all He in one plane, it is possible to make a reasonably accurate map of the solar system on a single sheet of

paper.

The Sun and

68

The

Its

Family

sheet of paper represents the plane in orbits of all the planets lie. start

We

which the

map by

putting a small circle on the paper to represent the sun. Then we draw a larger circle around the sun to represent the orbit of the earth. The radius of this circle represents the the

from the earth to the sun. After the map completed, we shall be able to use this distance as a unit for measuring other distances on the map. To show direction in space on the map, we pick a fixed point on the circle to represent the position of the earth on some definite day of the year, say the first day of spring. Then the line from this point toward the sun points out a definite direction in space, and all other directions in the plane can be shown by the angle they make with this line. This fixed point also makes it possible to show on the map where the earth is at any time of the year. The earth is at this point on the first day of spring. Then, for every day that passes, it moves about one degree around the circle, so that after one year, which is about 360 days, it will have traveled 360 degrees, or a full circle. In the diagram on page 69, the position of the earth is shown at three-month intervals starting with the first day of spring. let us see how another planet would be distance

is

Now

on this map. On March 21, the position of the earth is shown by the point labeled E on the map. Suppose we look at the planet Mars on that day. By noticing what stars it is near, we tell the direction it is in, and we draw the line

located

EG

to

show

this direction.

Then we know

that

the point that represents the position of Mars

The Sun and

Its

Family

on the map should be somewhere on The main problem we have to solve is out just

We

how

69 this line.

to figure

on the line this point is. problem with the help of

far out

can solve

this

we

already have about Mars. of 687 days, so Mars will be back at that same point after 687 days have passed, or on February 6th of the second year after our observation. But by that time, the earth will be in a different position, shown by the point F in the diagram. can look at Mars again on that day, and notice the direction it is in. Then, on the map, we draw showing the direction in which the line the information

We know that Mars has a sidereal period

We

FH

Now

seen. we have two lines that point out the position of Mars. The place where they cross on the map shows us where Mars actually

Mars was

^^°^ is.

If

many times, we when it is in different positions orbit. Then if we draw the curve through positions, we have a map of the orbit it-

we

repeat this process

can locate Mars in

its

these

The Sun and

70

Its

Family

By using the same procedure with the other planets, we can locate their orbits on the map, too. map obtained in this way, showing the orbits of Mercury, Venus, the earth, Mars, and Jupiter, is drawn on this page. self.

A

A MAP WITHOUT A SCALE A

no scale of miles, so we from the map how many miles a line represents. But we can use the map for

map

cannot

on

it

like this has

tell

JupCter

M Now he put a large lead ball with mass L directly under the lower pan on the right, and very close to it. By the law of gravitation, the mass L pulled on the mass b, but it was too far from a to affect

by

it.

a

The

balance was upset again.

To

restore

it

once more, another small mass, d, had to put with a and c. Then it meant that the pull of L on b was just balanced by the pull of the earth on d,

94

The Sun and

Its

Family

Our Home,

the Earth

95

The mass

of the earth could then be calculated comparison of b and d, and a comparison of the distance from b to the center of L, with the distance of d from the center of the earth. The result showed that the mass of the earth is about 6,600 billion billion tons. With this mass

from

a

spread out as it is in a sphere that is 8,000 miles wide, an average sample of the material in the earth is about 5 /2 times as heavy as water. l

INSIDE

THE EARTH

Rocks found at the surface of the earth are, on the average, about 2 /2 times as heavy as water. But the earth as a whole is about 5 /2 times as heavy as water. This shows that the mass of the earth must be more concentrated or denser on the inside than it is on the surface. We get more exact information about how this mass l

l

is spread out by using special devices that enable us to "see" the inside of the earth, much as an X-ray machine helps a doctor see the inside of a person's body. The doctor sends X-rays through

the body, and then catches

them on a photographic film to make an X-ray picture. By interpreting the shadows that he sees on the picture, he gets information about the parts of the body that the X-rays passed through. In the

same way,

an earth scientist uses rays that travel through the earth. However, these rays are not sent out by the scientist. They are sent out by the earth itself.

The

rays the earth scientist uses are the vibrations by earthquakes. Every time an earthquake takes place, these vibrations travel through the

caused

96

The Sun and

inside of the earth

Its

Family

and come to the surface again

after passing through.

They

travel at different

speeds through different parts of the interior. As a result, they follow curved paths through the earth. When they reach the surface again, they are detected by a special instrument called a seismograph. The strength of the vibration that reaches different places and the length of time it took to get there provide clues to the nature of the earth's interior.

Earthquake vibrations travel more slowly

in

denser rocks than they do in lighter ones. This fact might lead us to expect the vibrations to travel more slowly when they pass through the denser material that lies under the surface of the earth. Actually, they travel faster through the

denser material. This shows that there is another below the surface that tends to speed the vibrations up, and more than quality to the material

makes up for the slowing-down effect of greater density. This quality is increased stiffness, or rigidity. From the way the vibrations speed up the deeper they go, it appears that this stiffness keeps increasing down to a depth of about two thousand miles. Then the increase in rigidity suddenly stops. Below this level the vibrations begin to slow down as though they were passing through a liquid. The estimated density of this liquid material shows that it is probably made up of molten iron and nickel. The way in which earthquake vibrations bend near the surface of the earth shows that there is a sudden change in the quality of the rock not far from the surface, too. From this, and other information, scientists have concluded that the

Our Home, inside of the earth ers.

The

is

the Earth

made up

liquid center

is

97

of three main lay-

called the core,

and

is

about 4,000 miles wide, or half the width of the earth. It is surrounded by the mantle, which reaches up to about 25 miles below the surface. The outer layer, on top of which we live, is

crust

CORE

called the crust.

The

crust supports the oceans,

surrounded by a thick blanket of air that reaches above the surface for several hundred and

is

miles.

CHAPTER SEVEN

Our Nearest Neighbor, the

Moon

THE DISTANCE TO THE MOON OF ALL the

moon

In fact

is

it is

the heavenly bodies that

the one that close

enough

is

we

see,

closest to the earth.

to have a fairly large

at the moon while another observer, a quarter of the way around the earth, views it at the same time, they see the moon in directions that differ by about one degree, or sixty minutes. This parallax is quite large

parallax. If

compared

one observer looks

to the parallax of the sun,

which

is

only about 9 seconds. In fact, since there are sixty seconds in a minute, and sixty minutes in a degree, it means that the moon's parallax is about 400 times as large as the parallax of the sun. But the bigger the parallax, the smaller the distance. So the distance to the moon must be 400 times smaller than the distance to the sun. If we divide 400 into the sun's distance, which is 93,000,000 miles, we get as a rough estimate that the moon's distance is about 230,000 miles. Because the orbit 98

Our Nearest Neighbor, of the

the

Moon

99

moon around the earth is an ellipse, the moon is not always the same. It

distance to the

from about 220,000 miles up to about 250,000 miles, and the average distance is almost 240,000 miles.

varies

THE The moon,

SIZE

MOON

OF THE

looks about the In fact, often, at the time of a total eclipse of the sun, when the moon passes directly in front of the sun, the disc of the moon seems to fit perfectly over the disc of the sun.

same

But

if

then

as seen in the sky,

size as the sun.

moon and the sun look the same size, can calculate the actual size of the moon

the

we

by comparing

moon

it

to the actual size of the sun.

400 times as can look the same

Since the sun

is

far

away

as the

only if it is it 400 times as wide. The sun is 864,000 miles wide. Dividing by 400, we find that the moon is about 2,160 miles wide, or about one-fourth as wide as is,

size

the earth.

WEIGHING THE MOON

We

weigh the moon by measuring the effect on the earth. The earth's mass and the moon's mass attract each other, just as the sun's mass and the earth's mass attract each of

its

other.

gravitational pull

The

result

elliptical orbit

is

that the

around the

travels in an elliptical orbit

the pull of the

moon on

moon

travels in

an

earth, just as the earth

around the sun. But makes the

the earth also

The Sun and

100

earth move. Calculations

happens

Its

Family

show

that

what

that both bodies revolve

is

between them, and that

really

around

a

point is closer to the heavier body. In fact, the masses of the earth and the moon can be compared by comparing the distances from this point to the

point that

lies

center of each. But

where

first it is

this

necessary to find out

point is. The point around which the earth turns because of the pull of the moon turns out to be this

inside the earth.

The

center of the earth swings

point in a "small" orbit, and this motion is enough to cause a slight shift back and forth in the apparent position in the sky of the

around

this

closer bodies like Mars, Venus, or Eros.

the size of the shift,

it is

From

calculated that the point

around which the earth revolves in this way is about 3,000 miles from the center of the earth. This point is about 80 times as far from the center of the moon as it is from the center of the earth. So the earth must be about 80 times as heavy as the

moon.

GRAVITY ON THE MOON

How

a body is on the earth depends strongly the earth pulls it toward the ground. This, in turn, depends on the mass of the earth, and on the distance from the surface to the center of the earth. In the same way, how

on

heavy

how

heavy a body would be, or how much it would weigh on the moon, depends on the mass of the moon, and on the distance from the surface of the moon to its center. If the moon were as big as the earth, and differed from it only in mass,

Our Nearest Neighbor, then since

its

earth, its pull

mass

is

the

Moon

101

one-eightieth that of the one-eightieth as strong.

would be

moon is actually smaller than that, not quite that small, because the closer the object pulled, the more strongly it is pulled. The actual pull on the surface of the moon turns out to be about one-sixth of the pull of gravity on the earth. man who weighs 150 pounds on the earth would weigh only 25 pounds on the moon. Because objects are so light on the moon, a little push up there would go a long way. The same effort we use for taking normal But since the

the pull

is

A

we walk on the earth would make us advance with giant steps on the moon. For years men have told fictional stories about taking a steps as

trip to the fact,

moon.

ever becomes a very popular in fairy fact at the same time. The

If this fiction

another fiction that

become

tales will

men who they walk

a

land on the as if

is

moon

will discover that

they were wearing seven-league

boots.

A LAND WITHOUT

AIR

Because the pull of gravity on the moon is so weak, the moon has no atmosphere. It is possible that it may have had a blanket of air in the past. But if it did, it lost that atmosphere a long time ago. The gas in an atmosphere is made up of tiny invisible particles that are moving very rapidly.

On the

is strong enough moving particles from moving away from the earth. However, the weak pull of gravity on the moon would not have been strong

earth, the pull of gravity

to prevent these

The Sun and

102

Its

Family

enough to hold them back. They would have wandered off into empty space. We can see that there is no air on the moon

when we

look at

through a telescope.

it

If there

we would see some haze, and perhaps even clouds. The edges of shadows would be blurred. When the moon passed before a star, its

were

air,

atmosphere would dim the

body of there

the

moon

no haze on Shadows are

is

cloud.

abruptly

when

All these facts at

the

the

before the completely. But

star's light

eclipsed

it

moon, and no sign of a and stars are cut off

sharp,

moon

show

passes in front of them.

that the

moon

has no air

all.

Because the

moon

has

no

air, it

cannot have

surface water either. If there ever were any there,

would have evaporated into the air, and then would have wandered away with the rest of the it

air.

So the moon has no

The

entire surface

is

rivers, lakes, or oceans.

stone dry. Photographs of

moon, taken with powerful telescopes, show only mountains of naked stone towering over the

bare plains.

Our Nearest Neighbor,

ECLIPSES OF Sometimes the

moon

the earth and the sun.

the

Moon

103

THE SUN between

passes directly

When

happens, the on the surface of the earth, this

moon's shadow falls and an eclipse of the sun takes place. How the eclipse looks from any place on the earth depends on what part of the moon's shadow passes over it. The moon's shadow is made up of two parts. There is a cone-shaped region in the center from which all sunlight is cut off. This part is called the umbra. The rest of the shadow surrounding the umbra receives light from only part of the sun. This part of the shadow is called the penumbra. If a place on the earth is crossed by the umbra as well as the penumbra, a total eclipse seen to move covers it completely. Daylight gives way to the darkness of night, and the stars come out. But the moon moves steadily on, and little by little the sun is uncovered again. Places that are crossed only by the penumbra see only a partial eclipse, in which only part of the sun is covered and then uncovered again. Places that are not reached by any part of the shadow don't see the eclipse at all. is

seen there. First a black disc

across the face of the sun, until

ECLIPSES OF

An the

eclipse of the

moon

of sunlight

THE MOON

moon

itself

enters the earth's is

is

it

gradually cut

takes place

shadow.

off,

Its

so that

it

when

supply reflects

The Sun and

104 less is

and

less

Its

Family

back to the earth. When the moon umbra, it receives no sunlight at

in the earth's

all.

But

we

still

see

it

then, faintly illuminated

by

a copper-colored light reflected from the earth. Every year there are at least two eclipses, but

many

there are sometimes as

as seven. If there

are only two, they are both eclipses of the sun. If there are seven, five or

of the sun, and of the moon.

four of them are eclipses

two or

three of

THE

TIDES

them

are eclipses

The nearer a mass is to the moon, the more strongly it is pulled by the mass of the moon. This fact is responsible for the tides that rise and fall at the seashore twice a day. The reason for the tides is shown in the diagram below. The ocean at is nearer to the moon than the solid sphere of the earth, so it is pulled more strongly toward the moon. As a result, the ocean and the solid earth are pulled apart, and a great bulge in the ocean is formed there. The ocean at B is

A

farther is

from the moon than the solid earth, so it more weakly toward the moon than the

pulled

is. Again, the effect is that the ocean and the solid earth are pulled apart to form a bulge in the ocean. These two bulges tend to stay in line with the moon. But the earth is spinning all the time, so the bulges move around the earth in the opposite direction. When a bulge reaches a place at the

rest of the earth

seashore, that place has high tide. Since there are

two

of these bulges, high tide occurs twice in the

Our Nearest Neighbor,

time that

it

takes for the

moon

the

to

Moon

make

105

a

round

This interval is about twenty-five hours, so consecutive high tides are separated by about 12^ hours. trip across the sky-sphere.

CHAPTER EIGHT

The Other Planets EVENING "STARS"

TWO

Mercury and Ve-

of the planets,

nus, are nearer to the sun than the earth

is.

Be-

cause they are so close to the sun, they never wander far from the sun's position on the skysphere. As a result, they are seen among the stars that are near the sun either at dawn or at sunset.

When they lie west of the sun they this

rise in the east just

on the sky-sphere, before the sun does. For

reason the ancients called them momi?ig stars. they lie east of the sun on the sky-sphere,

When

they set in the west just after the sun does. When they were in this position, the ancients called them evening stars. It took a long time before people realized that the morning stars and the evening stars were the same. know now, of course, that they aren't stars at all, but we still find it convenient to call them evening stars anyhow. Of the two, the one you are most likely to see is Venus. Mercury is so close to the sun that

We

106

The Other it is difficult

however,

is

Planets

to see with the

107

naked eye. Venus, sky that is

a brilliant object in the

easily recognized. It

is

brighter than the brightest

Only the moon and the sun are brighter than Venus. Like the other planets, Venus and Mercury shine only by reflected sunlight. As each planet revolves around the sun, only the side that is turned toward the sun is flooded with sunlight, and shines. The other side, facing away from the sun, is dark. Since these two planets move within

we see in the

of stars

sky.

when part or all away from us. As a

the earth's orbit, there are times

of the shining side result, these

two

is

turned

planets

go through

phases, like

the phases of the moon. The diagram below shows how the phases depend on the position of the planet with respect to the sun, as seen from the earth.

Phases of Venus and Mercury

The with

when

apparent

its

of each planet also changes planet is in the full phase on the other side of the sun. But then

phases.

it is

size

The

The Sun and

108 it is

farthest

from the

er than usual.

when

The

Its

earth,

planet

Family

and so

is

it

looks small-

in a crescent phase

nearer to the earth, so then it looks in which the apparent size changes with the phase is shown in the diagram below, based on photographs of Venus taken larger.

it is

The way

through a telescope.

o SMALL, HOT, The

AND QUICK Mer-

chief characteristics of the planet

cury are that it is small, hot, and quick. We measure its size by calculating how big it must be to look as small as it does from where it is. We measure its temperature by using a sensitive instrument that detects the heat rays that come from the planet. calculate its speed from the size of its orbit, and the length of time that it

We

takes for a

round

trip

around

it.

Mercury

has a

diameter of only 3,100 miles, so it is less than half as wide as the earth. Its temperature on the bright side is about 660 degrees Fahrenheit. This is hot enough to melt lead. The high temperature

is

not surprising

when we remember

that

it

The Other is

the planet that

is

109

Planets

closest to the sun. Its distance

from the sun averages about 36 million miles. It makes a round trip in its orbit in 88 days. Its speed has to be about 30 miles per second for it to make the trip in such a short time. planet's mass can be measured by noticing how much it pulls on other planets. The pull

A

by disturbing the motion of the other planets, so that they do not follow exactly elliptical orbits. The disturbances are called perturbations. The perturbations caused by Mercury reveals itself

show

that

its

mass

is

only about one twenty-

fourth that of the earth. Because of its small mass and high temperature, Mercury couldn't possibly have an atmosphere. The high temperature would make the particles in any gas on its surface move very rapidly, and the small mass would not pull on them strongly enough to prevent them from escaping.

THE CLOUDY PLANET Venus is almost the same size as the earth. Its diameter is about 7,800 miles, and its mass is about four-fifths that of the earth. Its average distance from the sun is about 61 million miles. It moves at a speed of about 22 miles per second, and completes a trip around its orbit in 225 days. never see the ground on Venus because it is completely surrounded by clouds. The presence of clouds proves that it has an atmosphere. The light reflected from the clouds shows that

We

there

is

practically

no oxygen

in

it.

However,

it

does contain large amounts of carbon dioxide.

110

The Sun and

Its

Family

If men from the earth were ever to travel in a spaceship to Venus, they would have to wear oxygen masks when they landed in order to stay alive.

THE RED PLANET Of

the planets that

the nearest one

lie

outside the earth's orbit,

Mars, the red planet. It is about 142 million miles from the sun, and completes a trip around its orbit in 687 days. Its diameter is 4,215 miles, or a little more than half that of the earth, but its mass is only one-tenth that of the is

earth. Its surface

is

spotted, and the

movement

of the spots shows that Mars is spinning on its axis. It makes a complete turn in slightly more than 24 hours, so a day on Mars has about the

same length as on the earth. Clouds and haze can be seen on Mars, so it definitely has an atmosphere. Mars has two small moons that revolve around it in regular orbits.

LIFE

ON MARS

Certain interesting markings on the surface of Mars have led to speculation that there may be life on Mars. Since Mars is spinning on an axis, has a north and south pole, like the earth. There is a distinct white spot at each pole which

it

grows during Mars' wintertime, and shrinks in the Martian summer. It is likely that these spots are layers of snow, which accumulates in cold

The Other weather, and then melts

warm. In

Planets

when

111

the days

grow

1877, the Italian astronomer Schiapar-

elli reported that he saw lines on the surface of Mars extending from these polar spots toward the equator. He called them canali, and the word

usually translated as canals. Other astronomers have seen them, too, but they do not all agree on what they look like. Some say that the canals are straight, as though laid out and dug according to a plan. Others say that they are short irregular lines. It is impossible to check on these descriptions by means of a photograph of Mars, because is

the lines are too fine to show up clearly on a photograph. Some of those who say they are straight have concluded that they are, in fact, canals,

dug by

intelligent beings to

gation water from the melting

conduct

snow

irri-

to the drier

regions of the planet. But this is only a wild guess supported by no evidence apart from the existence of the much-disputed canals. It is likely,

Some

however, that there

is life

on Mars.

of the markings on Mars are colored, and

the colors change with the seasons. for example,

which

One

region,

green in the springtime, turns yellow in the fall. These seasonal changes suggest that there may be some plants growing on Mars. From evidence found in the spectrum of the light from these colored markings, it seems possible that the plants may be lichens, a combination of algae and fungus. More complicated Martian inhabitants are found in science-fiction stories, but there is no evidence for them in anything we see through our most powerful telescopes.

is

The Sun and

112

Its

Family

BIG YEAR, SMALL There

is

quite a

jump from Mars

from the

sun. It

is

to the next

about 483 million miles about 90,000 miles wide, and

planet, Jupiter. Jupiter

is

DAY

is

317 times as heavy as the earth.

It

makes

a

complete trip around its orbit in about 12 years, and spins around once on its axis in about 10 hours. So, while a year on Jupiter is twelve times as long as a year on the earth, a day there is less than half as long as a day on the earth. Although Jupiter is so heavy, its mass is spread out in such a big space that its density is only 1 Vs as great as that of water. It must have a deep atmosphere to bring its average density down so low. Analysis of the spectrum of light reflected by Jupiter shows that its atmosphere contains large amounts of methane (marsh gas) and ammonia. Because Jupiter is so far away from the sun, we are not surprised to find that it is very cold. Its temperature is about 220 degrees below zero, Fahrenheit. Jupiter has twelve moons. While eight of them move in the same direction around the planet, the other four,

which

are farthest

from

it,

move

the other way. Galileo saw four of the moons, and seeing them helped to convince him that Ptolemy's theory, which had all heavenly bodies turning around the earth, was wrong, while Copernicus' theory was right. Galileo could not see the other satellites of Jupiter because his telescope wasn't powerful enough. The eclipses of

The Other

Planets

113

the moons of Jupiter when they pass behind the planet give us a way of measuring the distance from the earth to the sun. This method was described

on page

80.

THE PLANET WITH RINGS Of all the planets that can be seen with the naked eye, Saturn is the farthest from the sun. Its average distance from the sun is about 886 mil-

The Sun and

114

Its

Family

and it takes 29 J4 years to complete a around its orbit. Saturn is about 70,000 miles wide, and its mass is about 95 times as great as that of the earth. This mass is spread out so thinly lion miles, trip

that Saturn's average density

is

less

than that

of water. It has a deep atmosphere containing methane and ammonia, as the atmosphere of Jupiter does. Saturn, being further from the sun

than Jupiter, is even colder. Its temperature is about 245 degrees below zero, Fahrenheit. As seen with the naked eye, Saturn looks like a disc, as the other planets do. But a telescope it enough to show a strange feature no other planet has. Saturn is surrounded by

magnifies that

three fairly flat rings, one within the other. The innermost ring (not shown in the drawing) is faint and was discovered last. The rings are made up of swarms of small rocks racing around the planet like cars on a crowded highway. The outermost ring has an exterior diameter of 171,000 miles, and is 10,000 miles wide. The middle ring, which is the brightest, has an outer diameter of 145,000 miles, and is 16,000 miles wide. The inside ring is about 12,000 miles wide. Its inner edge is about 7,000 miles from Saturn's equator.

In addition to the swarms of small bodies that move around it in the rings, Saturn also has nine moons. Eight of them move in one direction around it, and the ninth moves the other way.

NEW

PLANETS

The only planets that were known in ancient times were those that are visible to the naked eye.

The Other

Planets

115

to the telescope, we know of three They have been named Uranus, Neptune,

Today, thanks others.

and Pluto. of them, Uranus, was discovered by the German-born British astronomer William Herschel. Herschel found it in 1781 while he was scanning the sky with his telescope to make a map of the sky-sphere. At first he thought it was a comet. But later, as he followed its movements on the sky-sphere, he got enough

The

first

accident

by

information about

it

to calculate

its

orbit

and

show that it is a planet. The discovery of Neptune and Pluto was no Each of them was discovered was found in fact. Uranus, as orbit, was found to be disturbed

accident, however. in theory before

it

moved in its by forces other than

it

its

the pull of the sun. Part of perturbations could be accounted for by the

known planets, but another part remained unexplained. French astronomer, Leverrier, decided that there might be another planet outside Uranus's orbit that was causing the disturbance. He set to work to calinfluence of the other

A

where

this other planet might be. Then, wrote to the observatory in Berlin that if they pointed their telescope toward a particular point on the ecliptic in the constellation Aquarius, they would find a new star there. Galle, an assistant in the observatory, did so, and found the planet. An English astronomer named Adams, working independently of Leverrier, also figured out where the planet should be.

culate

in 1846, he

The ninth planet, Pluto, was discovered in much the same way. The American astronomer, Lowell, predicted where

its

orbit

would

be.

After

The Sun and

116

Its

Family

his death, it was found in 1930 on a photograph taken with a thirteen-inch telescope at the Lowell Observatory in Arizona. Uranus is about 1,785 million miles away from the sun, and takes 84 years to travel around its orbit. It is about 33,000 miles wide, and weighs about 15 times as much as the earth. It has an atmosphere of methane gas. Four moons have been found revolving around it. Neptune is 2,800

away from the sun, and makes a complete trip in its orbit in 165 years. It is 31,000 miles wide, and weighs about 17 times as much as the earth. It has two moons. Pluto is about 3,675 million miles away from the sun, and takes 249 years for a round trip around the sun. Its diameter is about 3,500 miles.

million miles

THE GAP BETWEEN MARS AND JUPITER The

four planets nearest to the sun, namely,

Mercury, Venus, Earth and Mars, seem to be spaced away from the sun in a regular manner. Then there is a sudden big jump in distance to

Then the spacing of the other planets continues in the same regular manner up to Uranus. The big gap between Mars and Jupiter suggested that there ought to be another planet somewhere between them. Astronomers have searched for such a planet, but instead of finding one good-sized one, they have found thousands Jupiter.

The space between Mars and Jupiter swarming with small bodies each following its own orbit around the sun. Because they are really

of tiny ones. is

The Other

117

Planets

midget planets, they are called planetoids. They are also sometimes called asteroids because when they are first seen they look almost like stars. They show that they are not stars, however, by wandering over the sky-sphere, while true stars are "fixed."

The first planetoid that was discovered is was found in 1801. The next year

Ceres. It

was discovered. Two years seen, and three years after covered. Vesta

is

later,

that,

called Pallas

Juno was Vesta was

first

dis-

the only planetoid that can ever

be seen by the naked eye. Ceres is 480 miles wide, Pallas is 304 miles wide, Juno is 120 miles wide, and Vesta is 240 miles wide. In 1898 Eros was discovered. Eros, which is only 15 miles wide, is especially interesting because of its close approach to the earth. Its close approach makes it

useful for finding the scale of distances for the

map of the solar system (see page 78). The presence of thousands of midget where

it

would seem

planets

to be natural to find only

one large one presents a puzzle to the astronTwo theories have been offered to explain their existence. One theory is that there was a single planet there once, but it exploded, and omers.

the planetoids are the pieces scattered plosion.

The

other theory

were formed from

is

that

all

by

the ex-

the planets

swarms of small bodies form one big mass, but bodies, for some unknown

similar

that gathered together to that these particular

reason, did not

come

We

shall have to than we now know about how planets are formed before we can decide which theory is correct.

learn

much more

together.

The Sun and

118

SATELLITES Among

Its

Family

WITH A TAIL

the most interesting

solar system are the comets.

members of

They have

the

orbits

and narrow, so they come in toward from a great distance, swing around the sun, and then shoot off again. It is possible that some of them are only temporary visitors that wandered in from outer space, were turned aside by the sun's gravitational pull, and will break away from its influence when they are far away from the sun again. that are long

the sun

The Other

Planets

119

Comets like these come once and never return. However, there are some that have regular orbits that bring them back where we can see them over and over again. Most comets are so faint with a telescope. Some, however, are very bright objects that dominate the sky when they come. Unlike the other bodies we see in the sky, they have long glowing tails that trail along behind them. A few comets have been so bright that they were visible in broad daylight. A comet seems to be made up of small parthat they can be seen only

ticles loosely

held together.

When

it

approaches some of

the sun, the heat of the sun evaporates

the material of the comet and makes the resulting mixture of dust and gas glow. Small particles, streaming out of the head of the comet, are light enough to be pushed by the sun's rays, so they stream away from the comet to form its glowing tail. The tail always points away from the sun, and sometimes extends for millions of miles. The material in the

tail is

passed through the

so thin that

tail

when

the earth

of a comet in 1861, no

on the earth was noticeable. The most famous of all comets is known as alley s Comet. Halley, studying the orbit of the comet of 1682, decided that it was probably the same comet that had been seen in 1607, and effect

H

many

7

other times before, separated

of about 75 years.

He

by

predicted that

intervals it

would

come

again in 1759, and sure enough, it did. It returned in 1835, and then again in 1910, when the earth just grazed the edge of shall see it again in 1986.

its

tail.

We

120

The Sun and

Its

Family

SHOOTING STARS When the earth passed through the gaseous tail of a comet, there was no noticeable effect. But there is an effect when the earth crosses the path of particles larger than gas molecules or dust. The particles enter the earth's atmosphere at high speed, rubbing against the air as they do so. The friction makes them hot enough to glow, and they begin to burn. Most of them burn up completely in the air, and we see them as "shooting stars" that streak across the sky. Occasionally, one is so big that it doesn't burn up, but crashes to the ground. These small bodies that enter the air from interplanetary space are known as meteors. One that reaches the ground is called a meteorite. Some of them weigh many tons. giant meteorite fell in Siberia in 1908, made big holes in the ground, and blew down the trees of the surrounding forest. In the Arizona desert in the United States there is a great crater 4,000 feet wide and 1,000 feet deep made by a giant meteorite that fell a few thousand years ago. Fragments of the meteorite have been found in the surrounding soil. Some of the meteors that move through interplanetary space travel in large swarms along regular orbits. The earth crosses some of these orbits, passing through at the same time each year. When this happens there is a shower of shooting stars all coming from the same part of the sky.

A

of the orbits of these meteor swarms coincide with the orbits of comets, suggesting that

Some

The Other

Planets

121

they may be the remains of a comet that has broken up.

OTHER

FAMILIES LIKE

THE

SUN'S

The

planets, planetoids, comets, and meteors part of one family. Since they all revolve around the sun, it is the sun's family. The fact

are

all

that the sun

is

a star raises the question

whether

there are other stars that have the same type of family. Every star is a sun. Do any other suns

have planets revolving around them? The answer to this question depends on the answer to an-

How

other question: did the planets come to exist in the first place? Until recently, scientists favored the theory that the planets were formed as a result of a near passing collision of the sun and another star. star, this theory said, came so close to the sun that its gravitational pull tore a big cigar-shaped chunk of gas out of the sun. This mass of gas broke up into pieces, and each piece began to condense. The condensing pieces formed the planets and their moons, the planetoids, the comets and the meteors. If this theory is correct, other stars would have planets too only if they also had a near collision with a passing star. But the stars are so far apart that near collisions of

A

would be very

rare accidents. So this theory with it the conclusion that very few stars could have planetary systems. Today most astronomers reject the nearcollision theory, because it doesn't fit in with an important fact about the stars. Most of the stars

stars

carries

The Sun and

122

Its

Family

sky are double stars. In a double star system, two stars are close to each other, and each revolves around the other, just as the planets revolve around the sun. Some stars are even triple, consisting of three stars, each moving under the influence of the pull of the other two. This shows that systems of bodies swinging around each other are very common in the universe. Double and triple star systems swinging around each other are so much like a planetary system that it is likely that both types of system were born in the same way. might be tempted to say that a double star, like a planetary system, could be the result of a near collision of stars. passing star, we might say, came so close to another star that it tore it in half, and the two pieces now revolve around each other as a double star. But since there are so many double and triple stars in the sky, it would mean that near collisions of stars must have taken place very often. know this cannot be true, because the stars are so far apart. So the near-collision theory has to be rein the

We

A

We

jected.

The theory stars,

that is favored today the sun, and the planets were

is

all

that the

formed

about the same time, from whirling clouds of dust and gas. The attraction of the dust and gas particles for each other made the cloud break up into pieces, like islands surrounded by a sea of at

Each piece contracted, growing hotter as did so. Pieces that were close enough to pull

space. it

on each other strongly, whirled around each other as they contracted. The largest pieces became hot enough to glow. These are now stars. The smaller pieces, not being hot enough to

The Other

Planets

123

glow, shine only by the reflected light of one of their glowing neighbors. Where two bodies whirling around each other are both hot enough to glow, the system is a double star. Where only one of them glows, and the other bodies whirling around it do not, the glowing body is a sun, conclusion from and the others are its planets. this theory would be that our sun is not the only star that has planets. Since we see so many double

A

and triple stars, it is likely that there are many planetary systems, too. can see the double stars because each star glows with its own strong

We

We

cannot see the planets of other stars, because they shine only by reflected light, and reflected light is too feeble to reach us from

light.

great distances.

This conclusion shows

how

far

we

have gone

in our ideas about the earth's place in the universe

was proved to be wrong. Ptolemy thought of the earth as the center of the universe. We see it today as merely a small planet revolving around a star which is only one

since Ptolemy's theory

of millions of stars that have planetary systems of their own.

I

Index iberration of light, 54, 60-1, 62,

Eclipse, 13, 15, 18, 21, 78, 88,

103-4

75,80 Adams, 115

Ecliptic, 14-5, 55,

Artillery shell, path of, 41-2

Epicycle, 23-4, 25

Asteroids, 117

Equator, 37, 90 Eratosthenes, 92, 93 Eros, 78, 117 Evening star, 106

Astronomical

unit, 71-3,

75

Brahe, Tycho, 31, 62, 71

67

Falling body, 39-41

Centrifugal force, 48-9 Ceres, 117 Comets, 118-9

29 Fixed stars, 10, 19, 20 Foucault pendulum, 38-9 Frequency, 57 Fictitious planet, 23,

Constellations, 10

Copernican system, 25-7, 31, 67 Copernicus, 25-6, 27, 31, 62, 67, 73, 74, 112 Cyclone, 43-4

Galileo, 26, 31, 62, 79, 86, 88,

112 Galle, 115 Gravitation, 74, 82, 83, 93

Deferent, 22-3

Gyrocompass, 44-8

Doppler shift, 56-7 Double stars, 122

Halley's Comet, 119 Herschel, William, 115 Hurricane, 44-5

Earth, 16, 88-97, 116 Core, 97 Interior, 95-7 Mass, 93-5

von

Rotation, 26, 32-49 Revolution, 50-2 Shape, 88-91 Size, 92-3

Jolly,

93

Juno, 117 Jupiter, 16, 21-3, 25-7, 67, 70-1, 73,79-80, 112-3, 116

Earthquakes, 95-6

Kepler's laws, 71-5

125

Index Latitude, 90-91 Leverrier, 115

Ptolemy, 23

Lowell, 115

Romer, 79-80

Mars, 16-7, 22-3, 25, 27, 67-71, 110, 116 Mercury, 16, 22-3, 25, 65-7, 71,

Saturn,

106-9 Meteors, 120-1 Moon, 98-105 Distance, 98-9 Lack of air, 101-2

Seismograph, 96 Shooting stars, 120

16,

21-3,

25,

67,

71,

113-4 Schiaparelli, 111

Mass, 99

Sidereal period, 64-7, 69 Sky-sphere, 9-19, 20-21, 54, 55, 60, 64, 67 Solar system, 66-87, 121-3

Phases, 27-29

Spectrum, 56-58, 84

Size,

Moons

The

99 of Jupiter, 26, 79-80

Morning Neptune,

star,

106

17, 67, 71,

Newton, 74-5 North pole, 11-2,

Stars: Stepping-stones Into Space, 19, 56 Sun, 63-87 Diameter, 80-82

Distance, 75

115-6

36-7, 51, 90

Origin of Solar System,

Mass, 82-3, 85 Spots, 26, 86 Temperature, 84 Synodic period, 65-7

121-3

Pallas, 117

Telescope, 26 Tides, 104-5

Parallax, 58-61, 76-8, 98

Pendulum, 33-9 Penumbra, 102-3 Phases of Venus, 27-31, 107-8, 110 Planetoid, 78, 117 Planets,

14-7,21,23,25,27,65,

Umbra,

102-3

Uranus,

16, 67, 71, 115-6

Venus,

16,

73-4,

21-3,

106,

27-31,

65-7,

109-11

Vesta, 117

67-9, 74-5,106-23

Pluto, 17, 67, 71, 115-6 Ptolemaic system, 22-7, 29-30, 62, 63, 123

Wanderers, 26-7 Wind, 42-4

126

14-7,

18-19,

23-4,

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STORM ON THE SUN Reaching 50,000 miles in width, the sun's gigantic storms appear as spots to the telescope's searching eye. These disturbances cause destructive magnetic storms on Earth an echo of a fury some 92 million miles away. This is just one of the mysteries penetrated by astronomers and explained in lucid, concise language by Irving Adler. His fascinating book offers a lively and informative survey of the heavenssun, stars, and solar system— together with a history of astronomical discoveries from Ptolemy to the present day.

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