The Sun and Its Family 0080081959, 9780080081953

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Its

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THE SUN and

Its

Family

IRVING ADLER

By

Ruth Adler

Illustidtrd h\

Author and illustrator of The Stars, Magic House of Xumhers, etc.

The

Su)i

(i)i(l

Family

Its

panion volume to The

is

a

com-

Stars: Stepping-

stones Into Space, the latter describing

new book

the stars in general and this

describing the star which together witli

ticular sini,

which we

is

oin- par-

its

faniilv

the solar system.

call

Roth books take an api)roach that them from otlier books on astronomy. Irving Adler does not just describe tlie sun and the planets, how large they are and how far from

distinguishes

ca( h other, etc

.

He

believes that the

reader will understand the solar system niU( M hciUM. hud it iiiu(Ji more intt resting, and retain his information

uuK

h longer,

mastered This

proach.

if

is,

We

in

(

shoit.

i)egin

a juud)le. llie

how mankind

he sees

tlie sid)je(

wa\

the

in

j^hue.

lustorital

a

bv seeing ii

first

inusi

tlie

ap-

sky as

ha\c looked

men. Then we noii( e that most of the stars seem to siav in tlie same |)la{es with resj^ec to each other, while a lew are w.uidereis; and tli.Ji theto primitive

t

.u

(

(.ojilniurd on hack llap)

THE SUN and

f^mrn9ffi€

Its

Family

public jLibrary

Books by Irving Adler

THE SUN AND

ITS

FAMILY

MAN-MADE MOONS

MONKEY BUSINESS: HOAXES IN THE NAME OF SCIENCE

HOW

LIFE BEGAN

MAGIC HOUSE OF NUMBERS THE STARS: STEPPINGSTONES INTO SPACE TOOLS IN YOUR LIFE FIRE IN YOUR LIFE

TIME

IN

YOUR LIFE

THE SECRET OF LIGHT

THE SUN and

Its

Family

By IRVING

ADLER

Illustrated by

Ruth Adler

J A

The John Day Company

New

York

©

1958 BY

IRVING AND RUTH ADLER

All rights reserved. This bcxjk, or parts thereof,

must

not be reproduced in any form without permission.

Pubhshed by The John Day Company, 62 West 45th Street, New York 36, N.Y., and on the same day in Canada by Longmans, Green & Company, Toronto.

Third Impression

Library of Congress Catalogue Card Number: 58-7464

MANUFACTURED

IN

THE UNITED STATES OF AMERICA

Contents

I.

n.

What

Two

We

See in the Sky

Systems: Ptolemy

vs.

9 Copernicus

23

m.

The Earth

IV.

The Earth Revolves Around the Sun

53

The Sun and

67

V.

VI.

vn.

vm.

Our Home,

35

Spins

Its

Family

the Earth

91

Our Nearest Neighbor, The Moon

101

The Other

109

Planets

Index

127

M000JLAMt

WYOMIM0

^

»

THE SUN and

Its

Family

(Sarp^gU public JUibrury

CHAPTER

What We

I

See in the Sky 1.

'_^ Xi- ia/

^

Looks Can Be Deceiving

SCIENTISTS

us that the earth

is

round.

a big ball, they say, about eight thousand miles

It is

wide. is

tell

It is

sailing

spinning around like a merry-go-round, and

through space at a speed of

I8/2

miles per

second, or 66,600 miles an hour. It revolves around the

making a complete round

sun,

time

first

we

trip

every year.

The

hear these facts they are hard to beUeve,

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

and rock, with a looks more smooth surface 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 going around the earth, rather than the other like a flat

way

cake of

soil

around.

But things

aren't always the

way

they seem.

is round, does spin, and does revolve around the For thousands of years this truth was hidden from

earth sun.

The

9

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 storv of the discoveries that broke

down

false beliefs

about the earth and sun, and led us to an understanding of the real nature of the earth as a

member

of

the sun's family of planets.

A

Puzzle in the Sky

The sky

is

like a great stage.

The

stars,

the sun, 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 see in the sky today were watched by other people thousands they observed the action

about

its

meaning.

What

is

bright light floods the land

of years ago.

in the sky,

As

they wondered

the sun, they asked, whose

by day? What

is

the moon,

that lights the path of the traveler at night?

What

are the stars that surround the moon, like courtiers

attending a queen in her palace?

Why

are they in the

and why do thev move as thev do? The skv was a puzzle which the people tried to solve. They looked for clues to the puzzle in what they saw in the sky.

skv,

To understand

we must

their attempts to interpret these clues,

begin as they did, by looking at the sky.

The Sky -Sphere

When we

look at the sky on a clear night,

we

see

it

spotted with thousands of tiny lights, twinkling like flickering candles.

bowl

that

10

is

The sky

resting upside

looks like a great black

down, with us inside

it.

The

stars look like spots that

have been painted on

the inside of the bowl.

The

make

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 stars look widely scatfirst

impression the stars

is

tered.

After

we have watched the

are better acquainted with to look less confused.

in the sky that

look at in the

sky for it,

many nights, and

the confusion begins

We notice some regular features

we can

recognize easily every time

we

The stars seem to have a fixed arrangement sky. Our attention is attracted by groups of it.

stars that are

arranged in familiar-looking patterns.

In the northern part of the sky

we

see one group of

11

stars

arranged

in the

not far from

shape of a dipper. Another group,

W.

looks like a great letter

it,

southern part of the sky, in the summertime,

In the

we

see

whose arrangement suggests the out-

a group of stars

Another group of

line of a teapot.

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

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 stars in a

constellations.

Thousands

of years ago, in all parts of the world,

made up

about the stars and constellations, and gave them names. The stories usually involved the adventures of their gods. Today we don't people

stories

believe these stories, but

the old names.

The

per to us looked So, while call

The

it

we

find

it

convenient to use

constellation that looks like a dip-

like a

we sometimes

bear to the ancient Romans. call

it

the Big Dipper,

Ursa Major, which means Big Bear

constellation that looks like a

peia, the

name

of a

queen who

stories told in ancient Greece.

looks like a teapot

fishhook

is

is

called

W

is

we

in Latin.

called Cassio-

figures in

The group

one of the

of stars that

called Sagittarius, the archer.

Scorpius,

because

also

the

The

ancients

looked like a scorpion. The cross in the middle part of the sky has two common names. It

thought

12

it

known as the Northern Cross, and Cygnus, the swan. is

If

is

also called

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

night,

While the

stars

constellations

seem

move

fixed in the constellations, the across the sky. They move as

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

There

is

a star that

is

sky. so close to this point that it

hardly

seems to move at all as the sky turns. Because this star helps us find the position of the North Pole in the sky,

we

call it Polaris.

13

Stars that are near the horizon in the eastern part

of the sky rise

above the horizon

as the night advances.

As they cUmb up the sky, we see other stars behind them moving up to take their place. 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

We 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 of the sphere.

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

we can 14

make

see them.

daily trips across the sky

where

The Sun and the Moon The

brightest objects in the sky are the sun

moon. They

also rise in the east

and

and the

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

move

that they ries

them

When

same sky-sphere that as

it

carries the stars,

and

across the sky because the sphere carturns.

the sun

the stars vanish, but the moon many days during each month the moon in the sky in broad day-

rises,

does not. There are

when we can see hght. The moon then

looks paler than

We soon reahze that the moon looks

it

does at night.

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,

daytime.

The same

why

the stars disappear in the

scattered sunlight that

moon look pale by comparison

is

also bright

makes the enough to

cover up the feeble light of the stars completely. stars are still in the

hidden by the bright to verify this fact

The

sky in the daytime, only they are

We get a chance an ecHpse of the sun.

light of the sun.

when

there

is

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. We can verify it, too, by looking at the sky through a telescope. A powerful telescope can pierce the curtain of daylight and reveal the stars in 15

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 the sphere.

We

can locate

this

on place on any day by stars

seeing which stars are near the sun, at the eastern horizon,

and

rise

with

it

for these stars again a

in the

week

morning.

later,

we

If

we watch

find that they

about a half hour earlier than the sun does. Two weeks later they rise an hour earlier. This shows that the sun does not stay in one place on the sky-sphere. It moves back steadily from west to east, so that, compared to the movement of the stars across the sky, the sun falls four minutes behind every day. The sun is a wanderer on the sky-sphere. If we observe its wander-

rise

ings for a long time, circle

we

find that

it

travels along a

on the sky-sphere, and completes a round trip The path it follows on the sky-sphere is

in a year.

called the ecliptic.

The moon,

too,

is

a wanderer. In fact,

if

you merely

few hours, vou can see how it shifts its position among the stars. The moon, like the sun, moves across the sky-sphere from west to cast,

watch the moon

for a

going about thirteen times as

fast as the sun.

Like a

on a circular race track, it passes the sun, gets ahead of it, and then overtakes it again from behind. The moon catches up with the sun in this way about every thirty days. Usually, when the moon passes the sun, they merely he fast horse

16

running against a slow

rival

side by side on the sky-sphere. Occasionally, the moon moves right across the spot on the sky-sphere occupied by the sun. When this happens, the moon lies between us and the sun, and there is an eclipse of the sun.

The sun and

the

moon are not alone in their wander-

Thousands of years ago, had people already noticed that there were some "stars" that wandered, too. They didn't look quite like the other stars, either. While all the other stars were just pin points of hght, the wandering "stars" were distinct discs. They followed very pecuhar paths on the sky-sphere, too. While the sun and moon moved along ings across the sky-sphere.

the other wanderers followed paths that

simple

circles,

made

loops. In ancient times people

knew

of five

wanderers besides the sun and the moon. We know them now as the planets Mercury, Venus, Mars, Jupiter,

at

and Saturn, and we know that they

aren't stars

all.

17

The word planet comes from the Greek word meanThe Greeks referred to all the seven

ing wanderer.

wanderers that they knew, including the sun and

moon,

as planets.

Today we use

the

word

in a different

Behind th« screen

way, the

to

mean bodies

word

that revolve around the sun. Using

in this sense, the

sun and the

moon

planets, but Mercury, Venus, Mars, Jupiter

urn

are.

We

also include

among

the planets

are not

and tlie

Sat-

earth

and several bodies that the ancient Greeks never saw because they are too faint to be seen by the naked eye. The new planets, discovered after the telescope was invented, are known as Uranus, Neptune, and Pluto. itself,

18

A Shadow

Play

You have probably seen a shadow play, in which people act out a scene while standing behind 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 it possible to produce some interesting illusions. For example, if one actor 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

the back.

On

the

shadow screen the

place where the man's back

is,

because the real knife

passes between the man's back projects both of

them onto the

knife crosses the

and the

light

which

screen.

19

What we a

shadow

round

us.

see on the sky-sphere

is

in

some wavs

like

play. We are looking out at things that surWe see these things as they were projected if

against a screen behind them. As a result, two objects

may seem to be at the same spot on the skv-sphere when they are actually far apart. Thev will seem to be together on the sky-sphere when one passes betv^^een us

and the

other.

Astronomers realized

quite early, as a result of hints they got

this fact

bv watching

the wanderers traveling over the skv-sphere. Every

saw the moon move across 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 tliem that the moon is closer to us than the sun is, and that it sometimes passes between us and the sun. The movements ice see on the sky-sphere are not the real movements 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 night, for example, they

places on the sphere occupied

the sky-sphere.

The puzzle breaks up

Why things

into

many

separate questions.

does the sky-sphere as a whole, with

we

20

see on

it,

seem

to revolve

around

tlie

all

the

earth?

Why

are the fixed stars fixed

on the sky-sphere, and

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 ask, "What are they made of?" "How heavy are they?" "How hot are they?" and many, many more 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 star is really a sun that is very far away. The stars look so tiny because and they seem fixed the same reason. The evidence and the thinking

of their great distance from us, for

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 show-

ing that the planets are satellites of the sun, each revolving around the sun with a regular rhythm. discoveries

and the thinking that solved

this

The

puzzle

are described in the chapters that follow. •

By

the

same

author.

New

York:

The John Day Company, 1956.

21

CHAPTER

Two

II

Systems:

Ptolemy

vs.

Copernicus

Spheres Within Spheres

THE be much

puzzle of the fixed stars didn't seem to

of a puzzle to the astronomers in ancient

They simply accepted as real what they saw with their own eyes. They saw the fixed stars rising times.

and

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 Httle more troublesome. Their motion across the sky was more comphcated, and needed in the east

setting in the west, as

a more complicated explanation.

In the

first

place, the astronomers reaUzed 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 Hght of the stars

when they did

so.

They could

see, too, that

some wanderers were closer to the earth than others. The moon, for example, was closer than the sun, because it sometimes passed in front of the sun and 23

its light. Using clues similar to those which be described in Chapter V, they even made estimates of how far from the earth each of the seven wanderers was. 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 surrounding the earth. Since the wanderers,

eclipsed

will

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 also rotated

around the earth from east

stars,

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

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 earth by the rotating sphere. These were the ideas we find in the theory of Claudius

24

that there

The Ptolemdic System

/

Ptolemy, the greatest astronomer of ancient times,

who

lived in Alexandria, Egypt, from about a.d. 85 to 165.

Ptolemy's theory of

how

the planets

move

is

shown

diagram on page 25. At the center of the uniwas the earth. The moon and the sun, each carried by its sphere, moved in a circle around the earth. Each of the other wanderers moved in a circle around in the

verse

an imaginary point, called the

fictitious planet,

while

the point was carried along a circle around the earth.

The path followed by the fictitious planet was called The path followed by the real planet as it moved around the fictitious planet was called an epia deferent.

cycle. In Ptolemy's theory,

inside the sun's circle,

Mercury and Venus were

and the

fictitious planets

moved around were always on

they

the line joining the

earth to the sun. Mars, Jupiter and Saturn were out-

and moved so that the line joining each to its fictitious planet was always parallel to the hne 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 tlie loops these planets made on the sky-sphere. Although Ptolemy's epicycles served to explain the side the sun's circle,

loops, they failed to explain other irregularities in the

movements of the planets. To make his system fit the facts more closely, Arab astronomers who lived after 26

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 mo-

The Ptolemaic system of wheels within wheels became more comphcated than the inside of a modern watch. In this comphcated form, it was accepted throughout Europe as a true tion of the seven wanderers.

picture of the universe for about fifteen hundred years.

During

these years, although astronomers felt

all

free to change the details of Ptolemy's system in order to try to

of his

improve

main

it,

they insisted on holding onto two

They always placed the

ideas.

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

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 for

fit in with the common 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 earth. Secondly, the idea

behef that

the universe.

They

insisted

paths for the planets or the

on using only

circles as

fictitious planets,

of the influence of religious ideas.

because

The

planets were were supposed to be perfect. The circle was the most perfect of all curves, they said, so it was the only kind of path that a in heaven,

and

all

things in heaven

27

heavenly body could follow.

A New

Center for the Universe

During the sixteenth century, one of these ideas was challenged by a priest named Nicolas Copernicus. Copernicus, who lived from 1473 to 1543, devoted his life to the study of astronomy. It seemed to him that the Ptolemaic system was far too complicated, and that there was a 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

at rest. If the earth

is

like a top, instead of

being

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

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. Hints

From

in circles

the Telescope

At the time when Copernicus proposed

28

his theory,

was no proof that it was more correct than The only thing that was in its favor was that it was simpler. But after the telescope was invented, new facts were discovered that gave hints that Copernicus' theory might be true. These facts were uncovered by the Italian scientist, Galileo Galilei, who lived from 1564 to 1642. Galileo was the first to use a there

Ptolemy's.

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

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. that

When

Galileo looked at Jupiter through his tele-

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 scope, he

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

he saw that the moon had mountains. If as the earth did, and if the sun had spots, then it showed that the heavenly bodies were no more perfect 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 telescope,

the

moon had mountains

29

from the

different

earth.

Then the

earth, in spite of all

blemishes, could be a planet, like Mars or Jupiter, revolving around the sun. But these were only hints. its

They showed

that Copernicus

might be

right,

but they

another important discovery

when he

did not prove that he was.

The Phases Galileo

Venus

of

made

turned his telescope on the planet Venus.

moon

does.

then in the gibbous phase,

when

Venus went through phases,

that

In fact, he saw

it

just

He found

as the

more than half of its disc was covered with light. This was clear proof that Ptolemy'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

by 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 shadow, as shown in the diagram below. As the moon revolves around the earth, sometimes we see only the sunlit half, sometimes we see only the dark half, and sometimes we see a part of each. The way in which the moon's phases change is shown in the diagram on sun does.

It

strikes

surface.

it

reflecting sunlight that

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

page it

its

shines only

31.

at the top

30

and bottom

of the diagram.

When

the

moon

between the earth and the sun, the sunht half of the moon faces away from the earth, and we see only the dark half of the moon. Then, as the moon moves to the side, part of the sunht half turns into view. At first we see only a crescent moon. By the time the lies

Light

from the sun

Phases of the

Moon

31

moon

has gone one-fourth of the

way around its moon

the crescent has growTi until half of the

we

orbit,

circle

Then it enters the gibbous phase, when more than half of what we see is covered by light. When the moon reaches the position where the moon and the sun are on opposite sides of the see

is

covered by

light.

earth, the sunlit half of the earth,

moon

faces toward the

and we see a full moon. Then the dark half beview again, and we see less and less

gins to swing into

of the sunlit part until, finally,

when

the

moon

is

be-

tween the earth and the sun again, we see none of the sunlit half at

The that it

it,

all.

fact that \'enus passes too, shines

moves

in

such a

through phases shows

only by reflected sunlight, and that

way

that varying fractions of

its

sun-

lit half face toward the earth. Let us see how this would happen if Ptolemy's theory were correct. The diagram on page 33 shows \^enus moving as Ptolemy thought it did. The fictitious planet around which it

turns, according to Ptolemy, lies

the earth and the sun.

moving

in a circle

Venus

around

on the line that joins supposed to be point. Four possible the diagram. At the

itself is

this

Venus are shown in bottom of the diagram we see what Venus would look like in these positions. In each position, the half of Venus that faces toward the sun is flooded with sunlight. When \'enus 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 sun. We would see the largest amount of reflected sunlight positions of

32

when Venus reached

the side positions

shown

in the

diagram. But in these positions, more than half of the

sunht side

still

faces

away from the

earth, so that only

a crescent would be seen from the earth. theory were correct,

we would

half of the sunlit side of Venus.

If

Ptolemy's

never see as

much

as

But Gahleo saw more

than half of the sunht side, because he saw Venus in the gibbous phase. This proved that Ptolemy was

wrong.

"ibboos phase

WHAT GALILEO SAW

a,c

Venus as seen from the earth according to Ptolemy

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 33

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'. the telescope,

He

it

yielded to Copernicus to the extent of saying that

the planets revolve around the sun. But he stuck

Ptolemy

in his belief that the sun,

by

accompanied by

around the earth. He insisted that 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. We 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 next two chapters, its

planets, revolved

the earth stands

still

finally established

Copernicus' theory as a true pic-

ture of the solar system.

34

CHAPTER

III

The Earth Spins A

Spin Detector

TO PROVE need a spin

detector.

that the earth really spins,

An

we

object can serve as a spin

detector if we know, from the way it works, that it would behave in one way if 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 this object to see which way it behaves. One of the things that can serve as a spin detector

To make string,

is

a swinging 'pendulum.

a pendulum,

and then

tie

tie

a weight to one end of a

the other end to a chandeher or

anything else that can support over the ground.

To

set the

it

while

it

hangs

down

pendulum swinging,

the weight to one side, and then let

pull

Experiments with the pendulum show that if a swinging pendulum is not disturbed, it keeps sunnging back and forth in it

the same plane. This property of the it

go.

pendulum makes

useful as a spin detector.

To try

it

see

how

a

pendulum

serves as a spin detector,

out with a phonograph turntable. First set the

pendulum swinging over the turntable while the turntable is at rest. The plane in which the pendulum 35

swings crosses the turntable along a this line visible

chalk.

As long

by drawing over

as the turntable

is

line. it

We can make

with a piece of

at rest, the

pendulum

keeps swinging back and forth over the chalk

line.

But the behavior of the pendulum looks different if the turntable spins. 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 as the hands of a clock, or clockwise, it would look to this creature as though the plane of the pendulum were turning the other way, or counterclockwise.

36

The length

of time that

seem

to

took for the plane of the pendulum to

it

make one complete

length of time that

it

turn would actually be the

took for the turntable to

make

one complete turn. If the creature on the turntable were curious and intelligent, he would reahze 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 Hne 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.

We can use the pendulum in the same way as a spin detector for the earth. However, the earth

is

not

flat,

phonograph turntable. It is round, like a ball. This makes the effect of its spinning a little more com-'

like a

plicated, as

we

shall see.

37

If the

Earth Spins

is really spinning, we should be able to by the effect that the spinning 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.

If

tell

the earth

that

When

it is

a ball spins like a top around an axis, there

on the axis. On the earth we call these two points the North Pole and the South Pole. The spin of the earth shows up in different ways at the two poles. As seen from a are two points on the surface

38

of the ball that

lie

point above the North Pole, the spinning earth

is

like

a turning phonograph turntable, spinning counterclockwise around the North Pole as center. Because of

plane of a swinging pendulum at the North Pole turns clockwise. Since the earth makes one complete turn in a day, a pendulum there, if it doesn't stop swinging too soon, makes a full turn in twentyfour hours. Down at the South Pole, an observer sees the spinning earth 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 this fact, the

the plane of a swinging wise,

pendulum

and makes a complete turn

turns counterclock-

in twenty-four hours,

too.

Halfway between the North Pole and the South Pole

is

the circle around the earth that

we

call the

39

equator.

Here the spinning

other effect.

An

of the earth has

observer looking

down

still

an-

at the earth

from above the equator would see it rolling like a barThe rolling motion carries the ground forward, but

rel.

does not

make

it

turn like a turntable. Since the ground

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

Here the plane of a swinging pendulum turns, but more slowly than it does at the poles, so that it makes less spinning of the earth has an intermediate

than a

full

effect.

turn in twenty-four 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

swinging pendulum

in

was

first

detected by a

the year 1851.

The French

Foucault made a pendulum of a heavy iron about a foot wide, attached to a wire that was over 200 feet long. He hung the pendulum from the scientist

ball

dome

of the

in Paris. To be sure pendulum was not influenced

Pantheon building

that the swinging of the

by any accidental pushes, he

set

it

swinging with extra

special care. First he pulled the ball to one side, tied

it

with a

string.

He

left

it

tied for

many

and

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 40

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 new swing the pin made a new line instead of going over the old ones. The successive lines appeared placed on the top of the

rail.

'""^^^^m^mm^^-

by side going clockwise, showing that the plane of the pendulum was turning in that dii'ection. The rate at which it was turning was enough to carry it side

through a complete turn in 32 hours. Since 1851 experiment has been repeated many times at many ferent places.

Each time the plane

of the

between the equator and one

dif-

pendulum has

turned at a rate that could be predicted from tion

this

its

of the poles.

posi-

These

experiments serve to prove two things at the same time. Besides showing that the

show

that the earth

is

eaHh

really spins, they also

a spinning ball.

41

A

Falling

Body

A body that falls from a great height above can also serve

as a spin detector. If the earth

spinning, a freely falling

down

along the line that joins

its

directly beneath

starting place.

its

it

spinning from west to

not land at the point under stead,

why

it

lands at a point a

this

42

happens,

look

would land

at the point

But because the

east, a falling

body does

starting position. In-

its

little

at

straight

starting place to the

Then

is

were not

body would move

center of the earth.

earth

the earth

further east.

the

diagram

To

see

above.

The letter

em

A marks

a point on the ground in the north-

B marks a point high up above the point A. As the earth spins from west to east, the ground at A and the air at B both travel eastward along circles. These circles are shown in the diagram. Because B is further from the earth's axis than A is, it travels along a larger circle. Both A and B make a complete round trip, each along its own hemisphere. The letter

in the air

circle, in twenty-foin* hours,

earth to

make one complete

greater distance than

B

is

of

two

tends to carry It also

covers a

it

eastward with the speed of the the body is dropped, its motion is

travels

it

Then, when

air at B.

made up

B

means that a body is held

in this time,

Now if

takes the

it

rotation. Since

A does

traveling east faster than A.

in place at B,

it

parts. It has a

downward motion

directly to the point

A

has an eastward motion that

starting place at B.

as

the time that

though the point

ning a race to the

that

on the ground. got from its

it

A also is moving eastward, so it is A and the falling body were runeast.

proaches the ground,

it

But

carries

as the falling

down with

it

body apthe 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 race. Instead of landing at A it

lands a bit to the east of A.

happens serves

The Path There

is

The

fact that this actually

as another proof that the earth rotates.

of an Artillery Shell

further proof that the earth rotates in the

effect that the rotation has

from a big gun. Suppose an

on the path of a artillery

gun

shell fired

in the

North-

43

em

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 similar to the reason for the eastward drift

The spinning

of a falling body.

of the earth carries

every point on the ground around in a

gram above shows

circle.

The

dia-

that the size of this circle de-

pends on how close the point

is

to the

North Pole.

Points near the pole travel along a smaller circle than points nearer to the equator, and as a result they

When

move

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 eastward more slowly.

race to the east, and so

44

it

a shell

veers to

is

fired

tlie right.

In the Southern Hemisphere the earth's rotation has the opposite

eJBFect.

There, as a shell travels north,

passes over ground that

is

moving eastward

faster than

behind

in the race

the shell does. So here the shell to the east,

and veers

artillery

falls

The earth's rotation way the gun is aimed.

to the left.

has this effect no matter which

When

guns are

fired, this influence of

earth's rotation has to

be taken

The Direction

Wind

The that

it

of the

it

the

into account.

earth's rotation reveals itself, too, in the effect

has on the direction of the wind.

It influences

same way that it influences an artiUery shell. Just as a shell is a moving body of metal aimed from the gun to a target, a wind is a moving body of air aimed from a high-pressure region to a low-pressure region. If the earth were not spinning, the high-pressure region would push the wind directly to the lowthe

wind

in the

45

pressure region. But since the earth

is

spinning,

it

compels the wind to veer from its course. In the Northem Hemisphere, the wind veers to the right. So, in the Northern Hemisphere, when you face in the direction in which the wind blows, the low-pressure 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.

Cyclones and Hurricanes

The wind rushes in toward a low-pressure region from all directions. If the earth were not rotating, lines showing the direction of the wind near a low-pressure center would look like the spokes of a wheel, as shown 46

in

diagram

veer

1.

But since the earth

diagram

2.

is

spinning, the winds

shown in In the Northern Hemisphere, where they

away from

the low-pressure center, as

low

veer to the right, the result

is

that the winds spiral

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

A

^

r^

on

is

made

out of

47

Picture of a hurricane on a radar screen

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 pecuhar way in which a gyroscope resists any attempt to tilt its axis. The principle 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 freely while the axle

hands.

If,

is

while the wheel

is

grasped firmly in both spinning rapidly, you

try to tilt one end of the axle down, you find that the wheel stubbornly resists being tilted. The resistance takes on an especially interesting form if you repeat the experiment while sitting on a stool whose seat is

49

free to turn, like the counter seats in an ice-cream

Then, while the wheel is spinning rapidly, if you try to tilt one end of the axle down, instead of the wheel tilting, the seat of the stool begins to turn,

parlor.

carrying you and the spinning wheel around with If

you

try to

tilt

it.

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

is mounted so that its and can swing around in a horizontal plane. An electric motor keeps the wheel spinning steadily on its axis. If the axis points east and west, the rotation of the earth tends to tilt it by tilting the eastem end down and the western end up. The result is

In the gyrocompass, a wheel

axis

is

horizontal,

50

that the axis swings around in the horizontal plane, just as the seat of the stool did in the

experiment with

the bicycle wheel. It keeps swinging around imtil the

down

it swings beyond that The end that was tilted up by the earth's rotation,

north and south.

axis points

position, the effect

before

is

is

now

If

reversed. tilted

so this time the axis swings around the other way.

Because of tles

down

this corrective action,

the axis finally set-

in the north-south position.

By

its

pecuHar

behavior the gyrocompass not only proves that the earth rotates. It also points out

and In

which way

so really serves as a compass, as

fact, it is

its

name

is

north,

suggests.

a better compass than the magnetic com-

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 it from the earth's magnetic field. pass,

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-goround. As the merry-go-round spins around, ourselves being pulled it

turns, the stronger

known

away from is

the pull

as the centrifugal force

its

we

center. feel.

we

The

feel

faster

This pull

(force pulling

is

away

from the center) that comes into existence whenever a moving body spins or turns. Because we can feel this force, we can tell when a train we are riding in is 51

rounding a curve. While the train is going around the curve we feel ourselves forced to lean over toward the outside of the curve.

The

proofs described in this chapter

show

that the

earth 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. We 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 ro-

is not strong enough 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

tation opposes our weight, but to

feel.

52

CHAPTER

IV

The Earth Revolves

Around Not a Ripple

in

Sun

Your Coffee

TO PROVE we have

the

that the earth revolves around

motion detector, some device that will show whether the earth is traveling through space or is staying in one place. First we have to decide where to look for this motion detector. Can

the sun,

we

find

tion has

that

we

it,

to find a

perhaps, in some effect that the earth's

on

us, or the things

cannot,

and we can

mo-

we do? The answer is why by referring

find out

to our experience with railroad trains.

When

a train

moving and picks up speed, we can feel it because we are thrown back. When the train slows down and stops, we feel it, because then we are thrown forstarts

ward.

When

cause then

the train rounds a curve,

we

are thrown to the side.

we

feel

it,

train travels in a straight line at a steady speed,

don't feel

its

motion

at

all.

be-

But when the

we

In fact, a railroad com-

pany on the Atlantic coast used to boast that its trains gave you such a smooth ride that there was "Not a Ripple in Your Coffee" when you ate in the dining car. Experience shows that we cannot feel steady mo53

We

can only feel a change in the motion, like a change in speed or a change in direction. The earth's

tion.

motion in its orbit around the sun goes on at a steady pace of about 18% 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

The sun cannot be detected by not feel that either.

motion around the it has on us or anything else that is on the earth. To find our motion detector we have to look away from the earth.

earth's

any

effect

The Sun Doesn't Help Since

we

are trying to prove that the earth

around the sun, we may be tempted

moves

to search for the

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 train has probably had an experience like this: Your train stops at a station, and there is another train alongside yours, on another track. While you wait impatiently for the train to start again, you watch the faces of the people in the other train. Then, at last, you see that you are moving again. Your train is 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 your train is not moving at all. It is still standing at the station. It was the other train that was moving, in the opposite direction. You were misled because the effect is the same whether evidence

54

in

your train moves or the other train moves. In both cases you merely see the trains passing each other

and then pulling apart. You can't tell which train is moving until you look away from the trains to the station or the ground.

Our motion around

the sun

is

similar in this respect

motion past each other of two trains. 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 the

To

is moving around the sun from look away both bodies.

find proof that the earth

we have

to

The Rain

We

shall find the

ing for falls

of Starlight

by looking

on the earth

motion detector that

we

are look-

at the stars. In the starhght,

which

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

starhght falling on the moving earth behave like rain-

drops falhng past a moving

Suppose you are riding

train.

in a train

on a rainy but

windless day, and you are watching the rain through the car window. Because there

drops is

fall straight

down

at rest at a station,

the raindrops

move

is

no wind, the

to the ground.

you can see across the

When

this fact in

window

rain-

the train

the

pane.

way

Each

raindrop, falling straight down, traces out a vertical line against the shift

when

window

pane. But the rain seems to

the train moves. Then, instead of falling

vertically, the raindrops streak past the

window 55

in

slanting lines, as

Let us see

why

if

they were being blown by a wind.

this

happens.

While the raindrops are falling down past your window, the train is 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 com-

downward motion of the drops to give slanting path along the window pane. This has

bines with the

them a the

eflFect

of shifting the direction that the raindrops

seem to be coming from. While the raindrops are really coming from directly overhead, the motion of the train makes it look as though they are coming from a place ahead of this overhead position.

The motion

of the earth in

of the stars in the

56

same way

its

orbit affects the light

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

Hght is known as the aberalways in the direction in which the earth itself is 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. Stars that are in the shift in the direction of the

ration of light.

The

shift is

same plane in which the earth's orbit lies 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 httle circles on the sky-sphere. Stars between these two extreme positions trace out little ovals on the skysphere. The amount of the shift is not very great. Whether the stars seem to move in straight lines or circles or ovals, the greatest shift in

seconds of arc on the sky-sphere.

each case

How tiny

is

this is

41 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.

How-

can be detected by modern telescopes and can be measured. ever, although the shift

is

so small,

it

This shift in the position of the stars on the sky-

sphere

is

a motion detector for the earth. It proves

that the earth

us the

rhythm

is

moving around the

sun. It even tells

of that motion, because the

rhythm of

the stars, as they shift around on the sky-sphere,

matches the rhythm with which the earth revolves around the sun. A star that moves around a little oval on the sky-sphere takes a year to make a round trip around the oval. This shows that the earth takes a year

57

to

make

We

its

saw

own round

in

Chapter

I

around the sun. that the sun is a wanderer on trip

moves along the circle known as the echptic, 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 a big curtain against which we see the sun, like the backdrop on a stage against which the sky-sphere.

It

the audience 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 train seem to move toward the rear of the train. When the sun takes

a year for a round trip along the ecliptic,

it

another sign that the earth takes a year for

its

trip

The

is

only

round

around the sun. Shifting Colors in Starlight

Starlight

is

a mixture of colors.

The

colors can

be

separated by passing the hght 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 spread-out arrangement of the colors in the light of a star

is

called

its

spectrum.

colors are bright in the spectrum.

Some

of the

Other colors are

weak, and in contrast to the brighter colors that sur-

round them, they look spectrum.

has

its

58

The dark

own

pattern.

lines

like

dark lines crossing the

form a pattern, and each

star

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 patterns clues

orange yefJow

from which they figure out how hot a star is, how far away it is, and even how much it weighs. What interests us

here

is

the fact that these patterns also contain

another proof that the earth revolves around the sun.

The proof

based on the fact that the dark lines in many stars keep shifting their position in the spectrum, in a movement called the Doppler 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 every year. Let us see how this proves that the earth is revolving around the is

the spectra of

sun.

Light

is

a vibration that travels through space.

59

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

light.

The rhythm

of the vibration

is

given by

its

number of vibrations that The 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

frequency, which

tells

the

take place in a second.

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

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 tliat the frequency has decreased. This change in the frequency is the clue that shows line represents a definite frequency,

that the earth

is

moving.

When

a ray of light of a par-

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 and therefore meets more of them in a second. The frequency of each line of light in the spectrum is increased, and so all the lines shift closer to the violet ticular color

60

end.

When

the earth moves

away 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 Hue of Hght 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 shift

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.

that the shifting of the lines

way

after a year

is

is

repeated in

The fact the same

a sign that the earth completes a

round trip in its orbit in a year, and retraces over and over again.

The

its

steps

Parallax of Stars

The

light of the stars contains a third proof that the

earth revolves around the sun. This proof

based on the fact that when one object is viewed from several different positions, it is seen in a different direction each time. If you look at a tree that is directly in front of you, you look straight ahead to see

is

Then, if you move several yards to the left, you have to look to the right to see the tree. If you move to the right, then you have to look to the left to see the tree. In fact, as

you move past the

tree,

it.

because of your change in

61

position

it

looks as though the tree

is

moving the other

way. This apparent shift in the position of something you look at things

When

when you move you look

is

Not all the equally by parallax.

called parallax.

at are aflFected

you walk down a road past a group of trees, the trees seem to shift backward, but some of them shift more slowly than others. The nearest trees seem to shift the fastest, 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. Some of the nearer stars show a parallax shift that results from the motion of the earth in its orbit. The diagram on page 64 shows why this shift takes place. If the star shown in the diagram is observed in the 62

month

of January,

months If

later,

the star

is

tion, the star

it is

seen in a definite direction. Six

on the other side of the sun. observed now from the earth's new posithe earth

is

is

seen in a different direction.

in the earth's position as

•

it

moves along

The

its

shift

orbit

is

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