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SCIENCE PRIMERS 



FOR 



ELEMENTARY SCHOOLS. 



UNDEfl THE JOINf EDITORSHIP OF 

PROFESSORS HUXLEY,. ROSCOE, AND 
BALFOUR STEWART. 

PRIMER OF CHEMISTRY. By H. E. Roscoe, 

F.R.S., Professor of Chemistry in Owens College, Man- 
chester. i8mo. Third Edition, is. 

PRIMER OF PHYSICS. By Balfour Stewart, 

F.R.S., Professor of Natural Philosophy in Owens College, 
Manchester. i8mo. Third Edition, is, 

PRIMER OF PHYSICAL GEOGRAPHY. By A. 

GEIKIE, LL.D., F.R.S., Miirchison Professor of Geoloiry 
and Mineralogy in the University of Edinburgh. With 
numerous Illustrations. i8mo. cloth. Second Edilion. is. 

PRIMER OF GEOLOGY. By Professor Geikie, 

F.R.S. With Illustrations. iSmo. Second Edition, is. 

PRIMER OF PHYSIOLOGY. By Michael Foster, 

M.D., F.R.S. With Illustrations. i8mo. is. 

PRIMER OF ASTRONOMY. By J. N. Lockyer, 
F.R.S. 

In Preparation, 

INTRODUCTORY. By Professor Huxley, F. R.f 

PRIMER OF lifllHHHIHM^^i^^' ^'^" 
F.R.S. 



MACA 




SCIENCE PRIMERS, ^^/^^ ^^ 

Professors Huxley, Roscoe, and 

Balfour Stewart. 



ASTRONOMY. 



^M 


HlJ^pl 


|i^ 


' 'at 1m 



S^atnte |Briratrs. 



ASTRONOMY, 



BY 



J. NORMAN LOCKYER, F.R.S., 

Correspondent of the Institute o/ F ranee y 
Author of ^^ Elementary Lesions in Astronomy" &»c. 



WITH ILLUSTRATIONS. 



If oitboit : 
MACMILLAN AND CO. 

1874. 

im^r I^i^hi 0/ Translation and Reproduciion 



is Rcse*^^*^^' 



LONDON : 

K. CLAY, SONS, AND TAYLOR, PRINTERS, 

BREAD STREET HILL. 



PREFACE. 



In writing this little book I have endeavoured 
first to help the reader, by means of simple ex- 
periments, to form true ideas of the motions of 
the heavenly bodies; and then to give a sketch 
of the Earth's place in Nature, and of the use 
made of the heavenly bodies for Geographical 
purposes. 

I have been much aided, by my friend, 
Mr. G. M. Seabroke of the Temple Observatory, 
Rugby, to whom my acknowledgments are due. 

J.N. L. 



CONTENTS. 



PAGE 

Introduciion I 



• 



I. The Earth and its Motions. 

SECT. 

I. — The Earth is round 4 

2. — The Earth is very large 7 

3. — The Earth is not at rest 10 

4. — The Earth spins or rotates like a top ... 13 

5. — The Earth rotates once in a day 15 

6. — The rotation of the Earth is not its only motion 19 

7. — The Earth travels round the bun once in a year 22 

8. — ^The two motions of the Earth are not in the 

same plane 23 

9. — Why the Days and Nights are unetiual ... 26 

10. — The Seasons depend upon the difference in the 

lengths of the Day and Night 33 

II. — Why the movements of the Sun and Stars appear 

different in different parts of the Earth . . 35 

n. Thk Moon and its MotioiSS. ' 

I. — The Moon travels among the Stars .... 40 

2. — The Moon changes her form « *?- 

3. —How the Mocu cawsea 'CcYv^^es* . . . • • 
4. — What the Moon. \s \\ke . . • • - • * 



X CONTENTS. 



III. The Solar System. . 

SECT. PAGE 

I. — How bodies like the Earth, nearer the Sun, would 

appear to us 56 

2. — How bodies like the Earth, further off from the 

Sun, would appear to us 58 

3. — Are there such bodies ? — The Planets ... 6c 

4. — The Interior Planets 62 

5. — The Exterior Planets 66 

6. — Comets, Meteorites, and Falling Slaij ... 77 

IV. The Sun — The Nearest Star. 

I. — The influence of the Sun in the Solar System . 81 

2. — The Heat, Light, Size, and Distance of the Sun 82 

3. — What the Sun is like 83 

4. — Sun-spots 84 

5. — The Sun's Atmosphere 86 

6. — What the Sun is made of 87 

7.— The Sun is the nearest Star. 88 

V. The Stars and Nebula 

I. — The Stars are distant Suns 89 

2. — ^The brightness of the Stars 89 

3. — The Constellations 91 

4. — Apparent movements of the Stars • . • • 93 

5 — Real movements of Stars , • 96 

6.— Multiple Stars 96 

7.— Clusters and Nebula 97 

8, — The nature of Stars and Nebulae • » . • . \QO 



CONTENTS. xk 



VI. How THE Positions of the Heavenly Bodies are 

Determined, and the Use that is Made of 
Them. 

SECT. PAGE 

I. — Recapitulation — Star Maps 102 

2. — Polar Distance 103 

3. — Polar Distance is Hot sufficient . • • . . . 104 
4. — Right Ascension .....••.•. 106 

. 5. — Recapitulation ,.108 

6. —The Latitude of Places on the Earth , . m ^ 108 
7. — The Longitude of Places on the Earth . • .111 

VII. Why the Motions of the PIeavenly Bodies are so 

Regular. 

I. — What Weight is 114 

2. — Gravity Decreased with Distance 117 

3. — How this explains the Moon's path round ttie 

Earth I18 

4. — ^The Attraction of Gravitation .•.,.. 12c 



LIST OF ILLUSTRATIONS. 



PAGE 

Plate I. — Frontispiece. A Lunar Crater • • . 7 o face Title 

„ 2. — The Solar System BePivcen 60 & 6i 

Fig. I. — How ships become visible and disappear at sea . 4 

2. — Explanatory of the above 5 

3. — Diagram showing how, wjien we suppose the 
earth is round, we explain that ships at sea 
appear as they do 6 

4. — Diagram explaining how it is that the higher we 

go the further we can see 7 

5.--Dir.gram showing that the larger the earth is sup- 
posed to be, the further removed from us is the 
place at which the sky appears to touch the 
earth 8 

,, 6.— Explanation of sun-rise and sun-set, and star- 
rise and star-set 11 






>» 



}} 



7. — The same I 12 

8. — A top spinning 14 

9. — The direction of the earth's spin 14 

10. — Experiment to illustrate the spinning of the eavlli, 

as causing day and night 15 

II. — Explanation of the earth's motion round the sun 19 

,, 12.^ — The plane of the ecliptic '^'^ 

,,• ij.— 7Vo planes cutting each oXVvet 2kl t\^\"^ ^^s^^*^ • "^"^ 

,, 14. — Two planes cutting eacAi oVYvex o^:^vc^^^^^ * ' * 



»» 



»» 



)) 



>» 



»f 



xi V LIST OF ILL USTRA TIONS. 

PAGE 

Fig. 15. — Earth with axis of rotation inclined to plane of 

ecliptic 26 

16. — The Earth, as seen from the Sun at the Summer 

Solstice, June 2a (noon, at London) . » . . 29 

17. — llie Earth, ^5 s^ie^ from thft Sun at the Winter 

Solstice, Deoi Z2, (noon ^t London) .... 30 

,, iS,-— The Earthy iis^ seen from the Sun at th© Vernal v 
Equinox, March 22 (noon at London) ... 31 

19. — The Earth, as seen from the Sun at the Autumnal 

Equinox, Sept. J2 (noon at London) ... 32 

20. — Explanation of the Seasons • • 34 



>» 



>» 



»» 



»f 



,, 21. — The Pole Star and the Constellation of the Great 
Bear in four different positions, after intervals 
of six hours, showing how the Great Bear 
appears to travel round the Pole Star . . . 36 

22.— The Moon's motion round the Earth , . • . 43 

23. — Total Eclipse of the Sun ........ 46 

24. — Annular Eclipse of the Sun ....... 47 

25. — Eclipse of the Moon ,48 

26. — Showing the inclination of the Moon's orbit to 

the plane of the ecliptic 50 

27. — Division of the Circle into degrees 51 

28. — Diagram illustrating the motions and appearances 

of a body between us and the Sun . . . . 57 

29. — Diagram illustrating the motion of a body travel- 
ling round the sun outside the orbit of the earth 59 

30. — Venus, showing the markings on its surface . . 64 

31. — Apparent size of Venus, at its least, mean, and 

greatest distance from the Earth ..... 65 

,, 32. — Mars, showing snow cap at the pole, and the 

lands and seas 68 

f> 33' — Man. Vjew of ano»luM part of the p\atie\. . . 69 



>> 



M 



»♦ 



> > 



>> 






♦ > 



»» 



»» 





34. 


»> 


35. 


*> 


36. 


«l* 


37. 


It 


38.- 


»> 


39'- 


}» 


40. 


»» 


41. 


»t 


42. 


»» 


43. 


)> 


44. 


}> 


45- 


>» 


46. 


»» 


47. 


»» 


48. 



LIST OF ILLUSTRA TIONS. xv 

I'Acr. 

-Jupiter, showing the cloud belts . . . • . . 71 

-Diagram explaining the eclipses, occulta! ions, and ^ 

transits of Jupiter's satellites >. 72 

-Saturn and hi^ rings 74 

-General' view of a Comet ....•••. 77 

-Head and Envelopes of a Comet . . • . . 79 

-How the size of the Sunis.determined ... 83 

-A Sun-spot 85 

-Explanation of the a]ipearances presented hy 

Sun-Spots . . • 86 

-The Sun's coronal atmosphere 87 

-Orbit of a Double Star ..••••.. 97 

-The Cluster in Hercules 98 

-The Great Nebula in Orion , , 99 

-How to define the position of anything . , . 104 

-How the positions of stars are stated . • • . 106 

-Diagram showing the fall of the Moon towards 

the Earth ; ... 119 



SCIENCE PRIMERS. 



ASTRONOMY. 



INTRODUCTION. 

1. Everyone who is going to read this book 
knows what a school-room or school-house is. Now 
suppose it had windows that you could not see 
through, and that you never went out of it : then 
you would think, perhaps, that the school-house was all 
the world. But you know better. You know that 
your school-house is only one house out of many, 
perhaps in the same street, or at all events in the 
same parish, whether in the country or the town; 
most of you even will have walked or ridden into the 
Parishes which lie round the one in which you 
live. 

2. If my reader lives in London, he will have done 
more than this, perhaps, for if he has crossed one of 
the bridges over the Thames he will have gone from 
one County to another (a county being a collection 
of parishes as a street is a collection of houses), for 
the River Thames divides the cou\i\.\fc^ oJl ^KSS^SSr-'sk^ 

wand Surrey. 

n^' Just as a county is a coWecUoxi oi ^^^^^^^-^ 



^C3 



SCIENCE PRIMERS. 



the Country of England, or of Scotland, or of 
Ireland, or of Wales, is a collection of counties ; these 
four Countries forming the United Kingdom of Great 
Britain and Ireland. Now wherever you are, whether 
in a town or village school, whether in the United 
Kingdom, America, Australia, or India, before you 
read tlie next paragraph, write down tlie 



School, 

Street, 

Parish, 

County, 

Country, 

Kingdom, 



X in which you are, 



and this will show you that your school-house is only 
a very little speck on the broad lands which together 
form the United Kingdom, or whatever kingdom you 
happen to be in. 

4. Although you may not have gone to France or 
Germany, you have heard of those places. What are 
they ? Well, the United Kingdom, France, Germany, 
Russia, Italy, Turkey, and other countries, form the 
continent of Europe, a continent being a collection 
of countries, as a country is a collection of counties, 
and as a county is a collection of parishes, 

5. You may also have heard of America, Asia, 
Africa, and Australia, as well as of Europe: nay, 
you may even be living in one of these, which, like 
Europe, are Continents. 

6. Now these continents are the largest stretches 
of dry land on ^Ae surface of The Earth, the suriace 

being partly water and partly land. 



ASTRONOMY. 



7. I have next to tell you that the earth, taken 
as a whole, is a body which astronomers call 
a planet : what this is you will learn by and by. 
Before going further, write down as before, the 

School, \ 

Street, 

Parish, 

County, I . ... 

Country, / '" '^^'^^ >'^" ^'^- 

Kingdom, 

Continent, 

Planet, y 

8. Some of you may think that I have made a 
mistake, and am going to write a book on Geography 
instead of Astronomy. I have not made a mistake. 
I want to show you that where Astronomy leaves off 
Geography begins ; that just as the shape, and size, and 
position of your school, which is a litde speck on the 
planet on which we dwell, called the earth, can be 
stated, and just as men by travelling, can find out 
lands on the earth, far away from your school, and tell 
us all about them, so are the shape, size, and position 
of the earth itself, among all the bodies in the skies, 
known, and its relation to them can be made clear to 
you. This is what I have to try to do, and if I 
can manage to do it, then you will understand better 
when you come to read about the surface of the 
earth. 



SCIENCE PSlME/iS. 



I.— THE EARTH AND ITS MOTIONS. 

S I.— THE EARTH IS ROUND. 

9. Now I have said that we are on a planet which 
we call The Earth, but what sort of thing is it? Is 
it flat or curved, square or round? How are we to 
find this out? If you look in any direction, if you 
are in a hilly country, you see hills and valleys ; and 
if you walk over these hills, more hills are generally 
found rising up, which limit the view to a few miles ; 
if you are in a flat country, the trees and shrnbs 
appear to meet the sky in every direction around you. 
We may travel to any place we like, still there is this 
line where the surface of the earth and the sky meet, 
so that for aught we could tell to the contrary in this 




Fio I —How tfae sh ps appear aud diiappev tt MAf 

f^a^, the earth might be a neaily flat suriace of luig^ 
cjctenL 



ASTRONOMY. S 

lo. But let us try where there are no rocks or trees, 
where the surface of the earth is unbroken and smooth ; 
let us try the surface of the sea. Watch the ships 
in the distance just coming into view, and you will find 
that only their roasts are visible ; as they approach, 
more and more of the hull appears, until it is quite 
visible. (Fig. i). Now if you watch a ship going away 
from you the hull will disappear first. 

n. Now what does this mean? Let us make an 
experiment Get a smooth table on which there are 
two flies, let us say, and if the flies are not there, 
pretend that they are; and suppose them to be 
moving about Now it is clear that the flies, as long 
as they keep on the surface of the table, will always ' 
be in full view of each other. They will look smaller 
to each other when they are furthest apart, and larger 




when Dearer each other ; buX. one ■past "A •Cm;Si:1 '^'^ 
not disappear, the othei patta ^i«»»^'\e&. '^"^^^^ **" 



SCIENCE PRIMERS, [§ i 



the case of the ships. Therefore the surface of the 
sea is not flat like the surface of the table. 

12. Another experiment. We will take an orange 
this time, and suppose a fly standing still at the top, 
say at A^ Fig. 2, and another fly at the bottom, at B. 
Now it is clear that the flies cannot see each other, 
because the orange is between them. But suppose B 
moves towards A. When it gets to C, A can just see 
the top of ^'s head over the edge of the orange, 
and C can see the top of ^'s head over the edge. 
No more can be seen yet, because the other parts of 
each fly are still hidden by the orange as the whole 
was before. But when B gets still nearer to A^ each 
fly will be in full sight of the other. 

13. We have then by means of the round orange 
and the moving flies managed to represent exactly 
what happens on the surface of the earth with ships, 
though we could not manage this on the flat table. 

14. Therefore the earth is like a ball or an orange, 
and not flat like a table. 

15. You will now easily understand why we see the 
tops of ships first, and how it is that the higher we 




FiG 3. — Diagram showing how, when we suppose th« earth is round, we 
explain how it is that ships at sea appear as they do. At A the ship is 
invisible, at B its topmasts begin to be seen, and at C it is in full sight 

ascend the further we see. We look over the 
edge of the earth in any case, and the higher 
we are above the surface, the iurtViet a^ay 
/s the edge we look over. 



ASTRONOMY. 



1 6. You must not imagine from this that there is 
an edge that you can fall over ; since the earth is a 




Fig. 4. — Diagram explaining how it is that the higher we go the further 
we can see. To an eye at A the edge is at A' A', to an eye at B the 
edge is at B'B'^ and so on. 

globe, the apparent edge retreats as you advance. 
Think this out for yourselves by help of the orange 
and flies. 



§ II.— THE EARTH IS VERY LARGE. 

17. We have employed an orange to prove that 
the earth is a globe. Some of you may ask, " If the 
earth is round like an orange, is it also small like an 
orange ? " Or again, ** Is it fair to use a smooth orange, 
while on the earth there are high mountains and all 
manner of roughnesses ? because, though I can believe 
that the surface of the earth is part of a curve when 
I look out upon the sea, yet when I see high moun- 
tains and deep valleys, I dor^\ \ixA<eK»\ax\.^^Nss^ ^»s^ 
an irregular surface can b^ s^oVevv oi ^s» \»^ ^^* 
curve/' I must try then to aivs^ex >Oaes^ q^^'s^nxcs^ 



8 SCIENCE PRIMERS, [§ ii. 

1 8. In the first place, it is clear that if you are 
at the same distance above two globes, one large, 
the other small, the edge at which objects begin, or 
cease to be, visible when they are moving to or 
from the eye, will be further ofif in the case of tlie 
larger globe. 




Fig. 5 — Diagram showing that the larger the earth is supposed to be, the 
further removed from us is the place at which the sky appears to touch 
the earth. 



19. Thus, in Fig. 5, if A represent the heii^ht of 
the fly's eye above the orange BB^ the distance 
from A to B would represent the distance of the 
edge over which the other fly would begin to be 
visible, while it would be represented by the distance 
from A to C, if the flies were on a globe as much 
larger than an orange, as the circle indicated by CC 
is larger than the circle indicated by BB, 

20. Now since, when you stand on the sea-shore, 
you can see some miles out to sea, it must be clear to 
you that the earth is very large. This, then, answers 
the first question. It is, in fact, some 8,000 miles in 

diameter: that is to say, a straight line froin svxrfac^ \.o 
surface through the centre would be 8,000 u\\\^^\oxv^^ 



ASTRONOMY, 



21. I want next to make you understand that the 
earth, in spite of its mountains, is really much smoother, 
comparatively, than an orange is. 

Suppose, for instance, that the distance of the 
surface of the earth from the centre is 4,000 miles, 
which is not far from the truth. Then a moun*^ain 
four miles high will only be the one-thousandth pan 
of this distance higher than the general level, and 
such roughnesses would be included in the thickness 
of the paper covering a large school globe. You 
will see at once then that the earth is comparatively 
much smoother than an orangej for if you were to 
magnify an orange up to the size of a school globe, 
it would look very rough indeed. 

22. We see then, (i) it is only when the surface is 
level, as on a great plain or on the sea, that we can 
judge by the eye as to the real form of the earth. 
(2) But even in the most rugged ground the curve is 
there, though we may fail to notice it. (3) The 
curve, is a very gentle one, because you can see the 
vessels at sea for many miles before they sink down 
out of sight. (4) The facts that the curve is so 
gentle, and that the high mountains make so little 
difference, show that the circle of which it forms a 
part is large, and therefore that the earth itself is 
large; and (5) the earth is so big, that even the 
highest mountains are in comparison merely like little 
grains on the surface ; its diameter or distance from 
side to side through its centre is 8,000 miles. 



lo SCIENCE PRIMERS, [§ in. 



§ III.— THE EARTH IS NOT AT REST. 

23. The Earth, then, with its surface of land and 
water, is a great globe, so big that supposing there 
were a road all round it from your school, and that 
you were to walk on day and night without rest, at 
the rate of three miles an hour, it would take you 
nearly a year to get to school again. 

24. The earth, too, hangs in space as you some- 
times see a balloon. Now is it at rest? or does it 
move ? Perhaps you will say that it does not move, 
because your school-house is where it always was; 
that the houses or trees near to it are no further 
away or nearer than they were. 

25. But this does not help us : let us take a large 
ball of worsted, or an orange, to represent the earth, 
and stick into it one pin to represent the school- 
house, and other pins to picture to you the trees and 
homes round it. 

26. You will see at once that whether the worsted 
ball or the orange is at rest or in motion, the positions 
of the pins with regard to each other will not change. 

27. How, then, are we to settle the question? By 
looking at something not on the earth. Go out 
on a clear evening, and look in the east (every boy 
and girl should know where the north, south, east, and 
west points are) : you will see the stars rising higher 
and higher above the edge of the earth, that is, the 
line where the earth's surface and the sky meet, which 
we must henceforth call the horizon. Those in 

the west will be gradually disappeanng ^M-st va. .Ke 
sa/ne way; the moon also follows tVieit eKaTa^\^ Ixv 



ASTRONOMY. ii 

the day-time we find that the sun rises in the east 
and sets in the west, in exactly the same manner. 

28. Here there is proof positive that while the 
houses and trees on the earth's surface do not move 
with regard to each other, the sun, stars, and moon, 
which are not on the earth's surface, do move, or 
appear to move, with regard to the earth. 

29. Now let us think about this. What do we mean 
when we say that a star or the sun rises and sets ? 
We mean that it is just passing either up or down over 
the edge of the earth seen from the place where we 
are; the sun or star in fact does, or appears to do, 
just what the ships that we referred to in par. 10 did. 
The ball of worsted or the orange should make this 




quite clear. Put it on the middle of a table, and 
stick a pin into its side, the pin's head to represent 
your eye. Now imagine yourself to be the sun or 
a star, and walk round the table as represented in 
Fig. 6, keeping your eye on a level with the "svci.-, ^s. 
one point the pin will be seen ^msV tvswvij, ^t^va. ■»»«- 
edge of the ball ; you are p\ay\wg, flae ^a.-ft 0I ^ ^^^^, 
san or siar, to your owr\ eye leptesetiVe.i'O'i -"^^ "^ 



12 SCIENCE PRIMERS. [§ HI. 

head; at another point in your journey round the 
table the pin's head will disappear, and at last will be 
hidden by the edge of the ball. Here you are playing 
the part of a setting sun or star, supposing the earth 
to be at rest. 

30. Now sit down and get someone to turn the ball 
of worsted round for you, keeping the pin*s head 
always at the same distance above the table. In this 
case, the motion of the ball, while you are at rest, will 
give rise to the same appearances as those you saw 
when the ball was at rest, and you walked round it. 




Fig. 7.— Diagram explaining Fig, 6 ; with the direction of motion indicated 
a body at A is setting, at B is rising, and at C is overhead. 

31. Hence the appearances connected with the 
rising and setting of the sun and stars, may be due 
either to our earth being at rest and the sun and 
stars travelling round it, or the earth itself turning 
round, while the sun and stars are at rest The 
ancients thought that the earth was at rest, and that 
t/je sun and stars travelled round U. But ^^ Tio>N 
^yjotF that it is the earth which moves. 



ASTRONOMY, 13 



""*"* *• . ^ . ■ j^ - 



§ IV.— THE EARTH SPlNS OR ROTATES LIKE 

A TOP. 

32. You have then to take it as proved that the earth 
moves, and that the seeming movements of the sun, 
moon, and stars, as they travel from east to west, the 
sun by day, and the moon and stars by night, are not 
real movements, but are apparent movements only, 
brought about by the actual movement of the earth. 

33. How then does the round earth move? Let 
us think a little. Have we, any familiar example of 
such apparent movement of objects at rest brought 
about by our own movement ? Yes, certainly we 
have. You will all at once think how, when you 
are sitting in a railway-carriage, all the objects, trees, 
houses and what not, that you can see out of the 
window and are really at rest, appear to fly past you as 
if you were at rest. Further, they appear to sweep 
past you in the direction exactly opposite to the one 
in which you are going. 

34. So far so good. Now will it do to apply this 
reasoning at once to the earth and stars, to imagine 
that the whole earth is really moving rapidly from the 
point that we call West towards the East, and is 
rushing rapidly past the sun and moon and stars? 
and that this is the reason they appear to move from 
East to West ? 

35. You will at once see that it will not do to reason 
thus, because we should thus never see the same sua 
and moon and stars again. 

j5. How then can we expV«x\tv \>ci^^^cX.'s»l '^ ^ ^"^^ 
imagine that the earth spm^ toxx.^x^^'^^ 



SCIENCE PSIMERS. 



does, so that ^vtry morning every boy and girl, 
whether living in England, or America, or Australia, 




Kic 8.— A top ipTnniiiE' 

or India, sees the same sun rise, and every evening 
sees the same sun set. 

37. It is in fact because the earth does turn in this 
way that we have morning and evening at all, and day 




»ad night are the best proofs that X\ve taith does 
reatfyspin as I state that it does. 



ASTRONOMY, 



15 



38. And because the sun seems to rise in the East 
and set in the West, the earth really spins in the 
opposite direction, that is, from West to East. 

39. Now get a common school globe. Set it spin- 
ning as you would a top ; that is, let the axis be 
upright as a top's is. Which way is it to turn? 
With your right hand push the right-hand surface of 
the globe away from you. The globe then represents 
the direction in which the real earth turns. 

§ v.— THE EARTH ROTATES ONCE IN A DAY. 

40. Take an orange, to represent the earth, into a 
dark room, with a lamp to represent the sun; stick 
a knitting needle through the centre of the orange, 
and then upright into a pincushion having also stuck 
a small pin as far as it will go into the orange, so that 




Fig. xa — Experiment to illustrate the spinning of the earth, as causing day 

and night. 

its \iea.d. shall represent aiv obsetvex Qra. "^^^ ^'asxsa. 
Twist the needle round, and so xciaVLe \^^ ort-a.^^ 



1 6 SCIENCE PRIMERS. [§ v. 



round slowly^ in the contrary direction to that in 
which the hands of a watch move, as in Fig. 9. 

41. Examine what happens. First, there will be two 
points on the orange through which the knitting needle 
passes, which do not move, and these are called the 
poles, the one at the top we will call the north 
pole, and the bottom one the south pole, the line 
joining the poles we will call the axis ; this is repre- 
sented by our needle. Draw a circle round the middle 
of the orange, everywhere at the same distance from 
the poles, or just where we should cut the peel if 
we were going to cut a lily or other similar device 
from the fruit: this line we will call the equator. 
Let the pin's head be near this line and opposite the 
lamp representing the sun. One half of the orange 
will, of course, be lighted up by the lamp, representing 
day, and the other half dark, representing night. 

42. Now twist the knitting needle slowly, and you 
will see that the pin's head, instead of being exactly 
in the middle of the half of the orange first lit up by 
the lamp, will, when the orange has turned through a 
quarter of a circle, be just visible at the edge of the 
lighted portion ; a slight turn more, and no light reaches 
it, — the lamp has set. Turn the orange another 
quarter of a circle, and you find the pin's head is 
in the centre of the dark side, with its head turned 
exactly opposite to the lamp ; another quarter's turn, 
and the pin's head is just coming into the lamp- 
light — the lamp is rising; a quarter of a turn 
more, and the orange has turned round once, and the 
lamp is again shining directly overhead as at first 

43, The lamp has therefore apparently passed from 
over the pin's head, set, and riseT\, and cota^ \.o \3ci^ 



ASTRONOMY. 17 



same place again, simply by turning the orange 
round. 

44. So with the earth, it rotates as the orange has 
done, in the same way, round, not a knitting-needle, 
but an imaginary axis, passing through its poles. 

45. Day and night are thus caused, and as the 
sun appears to taice twenty-lour hours to move from 
where it is at any time to the same place again the 
next day, we learn that the earth actually takes twenty- 
four hours to turn once on its axis. (Par. 41.) 

46. It is time now that we made use again of an 
ordinary school-globe. Get one of these and place the 
lamp a few feet from it, on a level with its centre. Let 
the axis of the globe be upright, and make the globe 
turn round. Whether it is allowed to remain at rest 
or is sent spinning round rapidly, the half of it next 
the lamp will be illuminated, and the other half away 
from the lamp will be in shade. When it is at rest, 
the places on one side remain in the light, while 
those on the opposite side remain in the dark. As 
you turn it round, each place in succession is brought 
round to the light, and carried on into the shade 
again. And v/hile the lamp remains unmoved, the 
rotation of the globe brings alternate light and dark- 
ness to each part of its surface. 

47. Now, instead of the little school-globe, imagine 
the earth, and in place of the feeble lamp, the great 
sun, and you will see how ihe rot.ition or spinning 
round of the earth on its axis n)ust bring alternate 
light and darkness to every country. 

48. You must not sui)pose that tl\ex^ \^ -azw^ •^Oo^-^ 

rod passing tli rough the earlVi lo Iepx^^e\^\ o\ixV\\>x^>^'^'^' ^ 

nccdlt and the steel rod o£ tVie sc\\ooV^o\i^> '^'^ "^^"^^ 



i8 SCIENCE PRIMERS. [§ v. 

the axis round which it turns. The axis is only an 
imaginary line, and the two opposite points where it 
reaches the surface, and where the ends of the rod 
would come out were the axis an actual visible thing, 
are still called the North Pole and the South 
Pole, both on the globe and on the earth itself. 

49. The earth spins then round this axis once in 
every twenty-four hours. All this time the sun is 
shining steadily and fixedly in the sky. But only those 
parts of the earth can catch his light which happen at 
any moment to be on the side turned towards hiin. 
There must always be a bright side and a dark side, 
just as there was a bright side and a dark side wheh 
you placed first the orange and then the globe oppo- 
site to the lamp. Now you can easily see that if there 
were no motion in the earth, half of its surtace would 
never see the light at all, while the other half would 
never be in darkness. But since it rotates, every 
part is alternately in sunlight and in darkness. When 
we are catching the sun's light, we have Day ; when 
we are on the dark side, we have Night. 

50. The sun seems to move from east to west. The 
real movement of the earth, is, for a reason which 
has been stated in par. 38, just the reverse of this, 
viz. from west to east. In the morning we are 
carried round into the sunlight, which appears in 
the east. Gradually the sun seems to climb the sky 
until he appears highest at noon, and gradually he 
sinks again to set in the west, as the earth in its 
rotation carries us round once more out of the light. 
At night we trace the movement of the earth by the 
waym which. thQ ^idii^ one by one rise and set, as 

the sun rises and sets in the daytime. 



ASTRONOMY. 



-9 



§ VI.— THE ROTATION OF THE EARTH IS NOT 

ITS ONLY MOTION. 

51. You are now probably convinced of these facts. 
First, that the earth is a globe. 
Secondly, that the earth spins like a top. 





WALL A 




fS 


TABLE 


< 


ORANGE 


i 


Cb "3 

LAMP 1 

'■-©■* 


5 

r- 

r 



L 


WALL C 





Fic. II.— Explanation of the Earth's motion round the Sun. 

And lastly, that without this spinning t\v^\^ cjcss:^^ 
be no day and night, so l\\at tVe x^^w^x '=^xo:-<^'y^^sJ^ 
of day and night is caused b^ v\vv.s ^v^m\\xv%. 



20 SCIENCE PI^IAIEIiS. [§ vi. 

52. Here then we have fauly proved that the earth 
has one motion. Now the question comes, has it 
more than oner* How shall we settle this? Well, 
first of all let us see if this one motion will account 
for all the things we see. 

53. To do this we ftiust again have our globe and 
orange, and imagine them in a room with many pic- 
tures on the walls. You wonder what pictures have 
to do with it? Well, I want the pictures to represent 
the stars in the sky. There are stars all round the 
part of space in which the earth and the sun are, only 
we cannot see them in the daytime, because the sun 
is so bright. So that if you have pictures all round 
the globe and orange they wiil represent the stars. 
Of course there should be pictures on the ceiling 
and floor too, but we will content ourselves by 
imagining them to be there as well. 

54. Now imagine the globe at rest and the 
orange at rest. Do not turn it round even. 
Then, as we have already seen, if we imagine 
the orange tp represent the earth, and the lamp 
to represent the sun, th^t part of the orange turned 
.to the sun, represented by the lamp, will have per- 

i)etual day, and will always see the same \ /^ 

\ o Li 1 J I 

in the same place ; from that part of it turned away 

from the sun the same \ ^ \ will always bti 

visible in the same place. From the parts of the 

I ^^rtlf ^ ( ^^^^ ^^^^ boundary of li^ht and shade 

i/j^ same //"''' ^f^[^ \ wiW be (ot ^Ntt n> 
/ lamp, pictures J 



ASTRONOMY, 2i 



parently near the horizon (par. 27) in the same 
place. 

55. Now stick a pin in the equator (par. 41) of the 
orange up to the head, to represent an observer on the 
earth, turn the orange round to represent the spinning 
or rotation, as we must now call it, of the earth, and 
mark that whenever the observer represented by the 
pin's head is in the middle of the lighted-up half, the 
part exactly opposite is in the middle of the dark 
half, and that half a turn of the orange brings the 
pin's head from the middle of the lighted-up to the 
middle of the dark portion. Now these two positions 
— namely, the middle of the lighted-up half and the 
middle of the dark half — represent nearly enough for 
our present purpose the position with regard to the 
sun which an observer is made to occupy at midday 
and midnight by the earth's rotation. 

56. You will see in a moment, therefore, that if 
neither the sun nor the earth move from their places, 
we shall always see one particular set of stars at mid- 
night, another particular set at sunrise^ and another 
particular set at sunset. 

57. Think this well over and reason it out with the 
pictures, for it is a very important point for you to 
understand clearly* 

58. Now, is it a fact that w^ always do see the 
same stars at midnight? No. Then what are the 
facts ? 

(i). If we view the stars at midnight in summer, 
and again at the same time in winter, we see 
dift'erent stars. Here then is a ^ttax Ocsa.\^^ ">». i^<sv 
months, 
(2). If WG view tlie sUrs toi kv^tv^ xvv^V3.\^ ^vsj.,^ 



22 SCIENCE PRIMERS. fi vii. 



sion at midnight, we find them gradually falling away 
to the west. Here is a slight change in a few days. 

(3). After the lapse of a year the same stars are 
visible at midnight. 

59. Now move the orange round the lamp in 
the same direction as the earth rotates, and 
you will see at once that this explains all the facts. 

60. In Fig. 1 1, I have given a drawing of the lamp, 
orange, table, and room, as you would see them from 
above. First consider the orange at A, Then at mid- 
night the observer on the dark side would see the stars 
opposite to the sun, the pictures on wall A : at B^ at 
midnight he would see the stars opposite the sun, now 
represented by the pictures on wall B ; and therefore 
no longer the same stars as were seen before. So 
on with the positions at C and D, 

61. I must next point out to you that the same effects 
would be produced as those we see and have thus 
accounted for, by supposing the sun to travel round 
the earth in the opposite direction. But we know 
that the earth really travels round the sun, and not 
the sun round the earth. 



§ VII.— THE EARTH TRAVELS ROUND THE 
SUN ONCE IN A YEAR. 

62. The earth then not only rotates on its axis 
once a day, but travels round the sun. In this way we 
have accounted for the fact that as seen at midnight, or 
At the same hour every night from any part of the 
earth, whether England, America, Australia, or India, 
the stars visible are continually chanft\T\g, 'We Yv^n^ 
found also that they change very \\i\\e *m a ^e\^ 



ASTRONOMY. 23 



nights, very much in six months, and that after 
twelve months the same stars again appear in the 
same places. 

62^, Now my reader should again go to his lamp 
and orange, and he will find that precisely as the 
earth spins in a day, so it goes round the sun 
in a year. 

64. For it is clear that if for instance the journey 
only required six months, then in six months the same 
stars would be visible at midnight, and so on for any 
other period you might choose to suggest. Here 
then we have the origin of the year, which is the time 
the earth requires to get back to the same place in its 
path round the sun. 



§ VIII.— THE TWO MOTIONS OF THE EARTH 
ARE NOT IN THE SAME PLANE. 

65. "How does the earth travel round the sun? 
does it jerk, or go up and down, or always smoothly 
and right on, keeping the same level ? " some of you 
may ask. I answer, the earth travels smoothly, and 
always keeps the same level ; as horses do, galloping 
round a very level race-course. To picture this more 
exactly, imagine a very large ocean with the sun and 
earth floating on it up to their middles, then imagine 
the earth to travel thus round the sun once a year 
in a nearly circular path, that is, always keeping 
about the same distance from the sun. 

66, Now get five balls, one larger than the others., 
to represent the sun ; weight t\ve\xv ^o "Owax. ^'e^ ^ccJ*^ 
up to their middles, ai\d t\veiv ^vxX. xJcv^^ *^^ ^ "^^^ ^ 

water as shown in Fig. 12. 



34 SCIENCE PRIMERS. \% tiii. 

67. We have now a representation of the sun, and of 
the earth in four parts of its annual jomney. What 
I want you to understand is that the motion of the 
i;arth is not only smuolh, but that ils motion is in 
the same plane, a plane being a level surface re- 
presented by a sheet of cardboard or the surface ol 
the water in the tub: and next that this plane in 




which the earth moves passes through the centres of 
the sun and earth, as the centres of the balls will be 
on a level with the water if you have weighted them 
properly. Further let me call the plane represented 
by the level surface of the water the Plane of the 
li^cliptic. 

68. Here then is defined the pkne of the earth's 
motion yearly round the sun ; this plane . of the 
ecliptic is the earth's race-course. What is the rela- 
tionship of this to the plane of the earth's daily 
motion round its axis ? 

69, Now it is clear that if the earth's axis is sup- 
pased to be upright mth regard to the p\a.i\e oS ft« 



ASTKO.VOMY. x$ 

ecliptic, or to form a "right angle" with it, the plane 
of the earth's spin will be the same as the plane 
of the earth's motion round the sun. This is the 
state of things represented in Fig. n. 

70. But are these planes the same? Let us sup- 
pose them to be so. Stick a pin into one of ths 
smaller balls, make the ball spin uprightly like a hum- 
ming top, and it nil! represent the earth as it travels 
round the sun, and you will find that on this sup- 




position, the days will always be of the same length, 
because the boundary of light and datltness would 




pass through the two poles, sii VnM. ^^JJ'o. ■^■^^ ^ '^^. 
t:arth's surface would be an eqvioX "aKie «» *^*'^^'^ '*' 



26 SCIENCE PRIMERS. [| IX. 

up, and in the dark, lialf, if the motion of rotation 
were uniform. But the days are not all of the same 
length ; in winter in England they are short, and 
the nights are long; and in summer the days are 
long, and the nights are short ; and, further, while it 
is Christmas here in England and America it is 
summer in Austialia. 

71. So then the planes of the two motions cannot 
be coincident; but we can explain all the facts by 
assuming them to be inclined to each other as shown 
in Fig. 14, so that the earth's axis in its journey round 
the sun is really represented by the little balls in 
Fig. 15, in which they no longer spin upright as in 
Hg. 12, but their axes are inclined. 




% IX.— WHY THE DAYS AND NIGHTS ARE 
UNEQUAL. 

72. We can now leave the tub, and come bacli to 
the lamp and omnge, remembering tbat vVc V.n\W.\Tv%- 
needle mast no longer he upright as we a.Wo'wei ix. 



ASTRONOMY. 27 



to be in Fig. 10, and that the plane of the ecliptic is 
represented by the horizontal plane in which lies the 
line joining the centre of the lamp and the centre of 
the orange. 

73. We have before accounted for day and night, 
now let us see if we can explain why they differ in 
length, at different seasons of the year. Place the 
lamp as before on a table in the middle of the room, 
and support the orange at the same height as before, 
inclining the upper end of the knitting-needle 
a little way from the lamp. Let us call the 
upper pole the north pole. 

74. Now turn the orange round, and you will see 
that the light never shines on the part of the orange 
near the north pole, and always shines on a part round 
the south pole, however rapidly you turn the orange ; 
but that, as before, parts near the equator alternately 
become lighted and darkened. Now stick a pin in 
the orange, to represent an observer near the north 
pole, and again twist the orange, and you will see that 
he never gets into the light region ; stick it near the 
south pole, and here he will always see the lamp, so 
that, with the earth in this position with regard to the 
sun, to a person at the north pole it is always night, 
and at the other pole always day. 

75. Again stick the pin in the orange, about half-way 
between the equator and the north pole, and twist the 
orange, and you will see that, as it travels round with 
the orange, it has a much longer journey round on the 
dark side of the orange than it has on the light side. 
At this point, therefore, the n\^\\\. \s» tKs\Ocs.Vsw$?:x "^^iscv 
the day, and you will see t\\aX v\v^ xv^^^ex -3cs>^ ^S?^ 
the pin to the north poAe, tVve ^\voxV^x -w'^Wi^^^'^^^ 



28 SCIENCE PRIMERS. [§ ix. 



of illumination, till it gets so far north as never to be 
illuminated at all. 

76. On the other hand, the hearer yoii place the 
pin to the equator in the northern half of the orange 
the longer it is lighted, or the days become longer 
and the nights shorter, till on the equator the journey 
in the light is just equal to that in the dark, 

77. Exactly the reverse takes place on the south 
side of the equator; the further you place the pin 
towards the south pole, the longer will its journeys in 
the light become, till near the pole it never passes 
into darkness. 

78. Now if you increase the inclination of the knit^ 
ting-needle away from the lamp, you will see that the 
days and nights become more and more unequal at any 
place where you choose to place the pin, except at the 

^ equator, and the less you incline it from the lamp 
the less is the inequality, so that when it is upright, 
day and night are equal all over the orange. Now 
you all know that England is on the north side of 
the equator, about half-way between the equator and 
pole, but somewhat nearer the pole than the equator ; 
and you also know that in winter the days are much 
shorter than the nights, and we at once therefore 
account for this by supposing the axis of the earth to 
be tipped in the same manner and direction as that of 
the orange, so that the orange in the case just men- 
tioned represents the earth in the winter, 

79. It is, however, not always winter with us, and 
following winter comes spring, when the days and 
nights are equal in length on March 22 ; then comes 

summer in thrte months more, wbeiv xVve da>f^ are 
Jonger than the nights ; just the teveise ol ^\\ax Wv 



.-ISTA'OAVJI/y. 29 

pens io winter. In autumn, on Se|)teinber 22, the 
days and nights are again equal. How can we ac- 
count for this? Let us consider, and return to our 
orange; we might try to ex|jlain it, by tipping the 
orange less and less till the axis is upright to re- 
present spring, and then tip it towards the lamp to 




represent summer, for you will see from what has 
been said before, that if the north pole be tati.\«4. 
away from the lamp, the nig\its weXo^^^t ■Ca='-'^ ■^'*- 
tldys; when it is upright tVy ate cQp,aX \ ^■«*- ^"^ 
it in turned towards Uie limp, Viic Aa.'j* "*'^'^ ** 



30 SCIENCE PRIMERS. [g ix. 

than the nights; but the earth's axis does not alter 
in its direction, as we always find chat the axis points 
very nearly to the same star, called the pole-star, at 
all times of the year. 

80, We must therefore try another method. Move 




the orange the contrary way to the hands of a watch, 
round the lamp, still keeping the axis pointing in the 
same direction, or more correctly, keeping the axis 
represented by tlie knitting-needle always parallel to 
itself; let it be moved a quarter of the way round the 
/■amp and rotate the orange, and observe xiie Vetv^^^x 
''dajr and night as before ; you wi\\ ^en ftia.x. V\\« 



ASTRONOMY. 31 

poles are on the boundary which separates the light 
from the dark half, and tiie journey of every part of 
the orange through light and darkness is equal. 
This position corresponds to the conimen cement ot 
spring, March aa. 

8r. Move the orange another quarter of a circle 




round the lamp ; now you see the north pole is tilted 
towards the lamp, and at every place north of the 
equator, or in the northern half, or liemisphere, day- 
is longer than night, corresponding to summer, and 
the reverse at the southern hemisphere, sci ■«& Va.-i^ 
matters just reversed by xhomto^ v^tx^ c.\a.';\%a V-i^- 
way round the lamp. 



iCl SCTEI^'CE rjUME^a. [g IX. 

8z. Another quarter's turn, and day and night are 
again equal, corresponding to autumn, Sept 23 ; 
one more quarter brings the orange to its original 
position, 

83. Just in tlie same way the earih moves roni.d the 
sun in a. year, pas&iug from winter through spring to 




summer, and through autumn to vinfer a;;ain ; the 
positions of the earth in spring and autumn when the 
days and nights are equal, are called the equinoxcE, 
that is, the "equal nights." 
S4. You will also be able to see that during the sum- 
nieria tht: northern hemi.Hphere the sun '« tottrasMa-j 



ASTRONOMY. 33 



visible above the horizon at places surrounding the 
north pole ; for instead of setting in the west, it goes 
apparently round by north to east agaih above the 
horizon ; and in winter it is continually below the 
horizon, never rising at all. In the southern hemi- 
sphere the same thing happens, so at the |>oles there 
is a day of six months succeeded by a night of the 
same length. 

85. I have given four drawings of the earth as seen 
from the sun in Spring, Summer, Autumn, and 
Winter. The centre of each diagram represents the 
point over which the sun is at the different times of 
the year. Imagine the globe to turn once round in 
each of these positions^ and what I have told you will 
be much clearer. 



§ X.—THE SEASONS DE]f>END UPON THE DIF- 
FERENCE IN THE LENGTHS OF THE 
DAY AND NIGHT, 

^6, If you have really understood why the day and 
night are of unequal length you have really understood 
also how it is that, both in England and Australia, 
there is winter and summer, the English summer 
happening at the same time as the Australian winter ; 
why in fact on the earth the seasons change, 
and we have the succession of Spring, Summer, 
Autumn, and Winter, in both the northern and 
the southern hemisphere, (that is, the half of the 
earth north or south of the equator) and at different 
times of the year. 

87. When the days are long aT\d VX\e tsv^V^^^^'^^ 
in either the northern or the so\i\\v«vxk\weav\^^?ciR:^^^^ 



34 SCIENCE PRIMERS. [% x. 

that hemisphere the sun is visible in every twenty- 
four hours for a, longer period than it is absent, 
therefore the heat accumulates. On the other hand, 
when the days are short and the nights are long in 
either hemisphere, the sun is absent for a longer time 
than it is present, so the absence of the heat is more 
felt. 




88. In spring, although the days and nights ate equal 
as in autumn, the powers of nature are renewed by 
tbeir winter's rest, so spring is the time o( buds, while 
auiuma is the time of decay. 



ASTRONOMY. 35 



§ XI.— WHY THE MOVEMENTS OF THE SUN 
AND STARS APPEAR DIFFERENT IN 
DIFFERENT PARTS OF THE EARTH. 

89. I must now endeavour to explain how it is that, 
as seen from different parts of the earth, the motions 
of the heavenly bodies appear to be very different. 

90. Not only at the poles is there a day and a 
night, of six months, and not only at the equatoi" are 
the days and nights always equal, but at the poles the 
stars seem to travel round a point overhead, while at 
the equator the stars which travel overhead seem to 
rise and set almost vertically, and not on a slant as 
they do in England, America, and Australia. 

91. We have already become acquainted with 
risings and settings as seen here, but let us observe 
the stars, not east and west, but in other parts of 
the sky, and see how they move ; you will see that in 
England thte stars iiear the south rise only a little 
east oif the south, get to the highest point above the 
horizon exactly south, and set as far west of south as 
they rose east of it Those that we at first see rising 
in the east, pass over the south much higher above 
the horizon, and set in the west again. The felars 
near the north neither rise nor set, never going below 
the horizon, but moving in circles round a point in 
the heavens, marked by a star called the pole star, 
a star easily found by its being pointed at by the 
pointers of the Great Bear, as shown in the diaf^^^xe^ 
(Fig. 21). 

g2. Now, to illustrate this, taVe a. ^t^2^\ ^c^^^^k^sS^^^ 
hs axis upright, and in order to md\c3A.'i ^^^ \vo^'«-'^ 



SCIENCE PRIMF.RS. 



of any place quite plainly, cut a piece of card about 
the size of a penny and gum the centre of it on the 




globe as near the upper axis or north pole as the 
mounting will permit, or put it on the axis if you can ; 
then a person standing at or near the pole would be 
able to see everything above the card, but not below — 
in fact, the edge of the card represents the horizoii. 
Now spin the globe to represent the motion of the 
earth, and watch what the. appearance of the stars re- 
presented by the pictures on the walls (Art. 55) would 
be to a person standing ?% the pole. You will at once 
.we that the card simply turns round like a wheel, and 
the picturea (hat were above \t at fttst Temswci wi. So 
tlic stars would not rise or set to a. ■geisoti M. ftv« -^Xa, 



ASTRONOMY, 37 



but remain at the same height above the horizon, and 
only apparently move round the points of the com- 
pass ; the pole star being of course overhead, and the 
stars turning in circles round it. If you fix on a picture 
on the walls below the plane (Art. 67) of the piece 
of paper to represent the sun, you will see you cannot 
make it appear to rise or set by turning the globe 
round, it can only be thrown above the horizon by 
tipping down the globe as is done to represent the 
seasons. Now you will recollect that for one half of 
the year the north pole of the earth is tipped towards 
the sun, and during the other half away from the sun, 
so that it can only have day during the summer half 
of the year, and night during all the winter; and if you 
will look at Fig. 20 you will see that during the 
summer the whole of the small circle round the pole 
is lighted, so that there is no night there as the earth 
turns round, and in winter for the same reason there 
is no day, but in spring and autumn half the circle is 
light and half dark, so that every place is brought by 
the turning of the earth into daylight and back into 
night every twenty-four hours. 

^7^, So much then for the view of the heavens at 
the pole. Now let us examine what takes place at the 
equator. To do this, gum the disc of card on the 
equator, and turn the globe. You will see that it 
no longer turns like a wheel, but turns somewhat 
as a penny does when spun on its edge; and on 
turning the globe half-way round, an entirely new set 
of stars appears above the horizon, represented by 
the edge of the card, the two places in the heave'ck«3. 
pointed to by the poles ot' the ^oX^e ^'^ ^^^ \^'^ ^"^ 
the horizon, die north pole-titax *^w^x. oyv x>;x^ ^ofcCicw^^' 



XV 



3S SCIENCE PRIMERS, [§ XL 

part of the horizon, and the south pole just on the 
southern part of the horizon, and the stars which 
rise due east will pass exactly over the paper, and set 
due west as the globe is turned, 

94. If you fix on one picture to represent the sun, 
you will see that the globe can be just turned half- 
way round while the sun, or the picture representing 
it, is above the paper horizon, and half-way round 
while it is below it ; and as the earth turns round once 
every twenty-four hours, the sun will be twelve hours 
above and twelve below the horizon, so the day and 
night at the equator are always of equal length, and 
by tipping the globe to represent the changes of 
seasons you will find ^hat the length of a day or 
night remains unaltered. 

95. Now try for yourself, and place the card in 
other positions on the globe, beginning at the equator 
and going up to the north pole, andwatph the gradual 
change in the apparent movements of the stars in 
rising and setting. 

96. All that has been said refers to the apparent 
motions of the stars as seen on the equator, or to the 
north of it ; so, in order to examine the apparent 
motions of the stars visible in the southern heipisphere, 
you must stick the card at different places south of 
the equator of the globe, and turn the globe and 
observe what takes place. First place ^t between 
the equator and south pole, to represent the posi- 
tion of an observer in Australia, then the equator 
will be north of him instead of south, and his pole 
south instead of north, as in our hemisphere, and if he 

looks towards the north he will see exactly the same 
rising and setting of the stars as \\e vjouXd m ^^ 



ASTRONOMY. 39 



northern hemisphere ; but his right hand will be 
towards the east and his left towards the west, so that 
the stars will rise on his right hand and set on his 
left, traversing the heavens in an exactly opposite 
direction to that they take in the northern hemi- 
sphere. Further, he will see near the northern horizon 
the stars seen in England near the southern horizon, 
the northern stars being altogether invisible to him. 

97. In order to make the apparent movements of 
the stars visible in the southern hemisphere more plain, 
call the upper pole of the globe south, and the lower 
north, and turn the globe contrary to the way in which 
you turned it before ; for the earth appears to revolve 
in a different direction according to the position from 
which it is viewed, like the hands of a watch, for 
they go in one direction if looked at on the face, 
and in the contrary direction if looked at on the 
back, supposing the watch to be transparent; so to 
an observer in the southern hemisphere the earth 
appears to rotate in the opposite direction to that 
as seen from the northern hemisphere, and conse- 
quently, if we make the south pole the uppermost 
we must reverse all the motions including its motion 
round the sun. 

98. When you have done this, bring the true south 
pole of the globe to the top, and then experiment 
with the paper horizon as before. 

99. On the globe you will probably find a " wooden 
horizon," this represents the horizon of the centre of 
the earth, as we have supposed the circumference of 
the card disc to represent the horizon of a ijlacA^ 



40 SCIENCE PRIMERS. [§ i. 



II.--THE MOON AND ITS MOTIONS. 

' § L— THE MQON TRAVELS AMONQ THE 

STARS. 

loo. You have now become acquainted with the 
form of the earth and with its motions, first its spin 
or rotation round its own axis in twenty-four hours, 
and secondly its movement round the sun, which it 
accomplishes in a year. 

loi. We have also seen how these two real move- 
ments of the earth give rise to two apparent motions 
of the sun and stars, the daily movement of rising and 
setting, and the yearly movement by virtue of which, 
month after month, we see diflferent stars in the south 
at the same time in the evening, until, after the expira- 
tion of a year, the grand procession begins afresh. 
The "Physical Geography Primer" will teach you what 
the earth is like — that it is a cool body surrounded 
with an atmosphere set in motion by the sun's heat. 

1 02. Some of my readers will wonder why as yet I 
have said nothing of the moon, which appears to us 
almost as large as the sun, and which sometinles 
throws such a strong light on the earth. 

103. It is now the moon's turn. Look at it some 
fine evening, and notice its position amongst the 
neighbouring stars; it is difficult to see small stars 
near it, so it is best to take an opportunity when it is 
near a large one. Observe it again some hours after- 
wards, or, if need be, on the following evening ; you will 
at once see that it no longer occupies the same position 

among the stars, but that it has moved ataoiv^ \\\exa 



ASTRONOMY. 41 



considerably towards the east. It will be observed 
to rise later and later every day, by three quarters of 
an hour to an hour, as is easily noticed by timing its 
rising for a few successive days. It keeps on losing, 
as it were, on the sun, till, from being seen at sunset, 
it does not rise till just before the sun in the morning. 
After this, the sun apparently passes it, and a icvf 
evenings afterwards it is again seen in the west just 
after sunset, only to lose on the sun and be over- 
taken again every twenty-eight days as before, in the 
same manner as the hour-hand of a clock is overtaken 
and passed by the minute-hand. 

104. We have now made our observations : let us 
see how they can be explained. We must return to 
our orange and lamp, and, in addition, shall require 
a much smaller orange to represent the moon. Now 
keep the orange, representing the earth, still, and 
move the small one representing the moon in a circle 
round it, as the earth moves round the sun. 

105. We have to see if this motion will account 
for our observations. First, let the moon be at E 
(Fig. 22), in a line with the sun, and as in such a 
position it would clearly appear to us to be in the 
sky near the sun, then it will appear to rise and set 
at the same time as the sun does, and on twisting 
the earth round on its knitting-needle, this will at 
once be clear. Next move the moon to T to re- 
present its position a few days later; you will now 
see that the sun will set some time before the moon, 
for to a person at A the sun is just set, but the^ 
moon is above the horizon. Again, mov^ \3cv^ xssa^cs^ 
to F, and you will see it is just sow\)cv ol ^^ c5^^"etN^'«^ 

at A, when the sun has set, so iVv^lI Vt Va.^ \o^^. ^^^"^"^ 



42 SCIENCE PRIMERS, [§ ii. 

six hours on the sun. Move it farther on to G^ and 
it will just be rising >vhen the sun is setting, and will 
be «outh at midnight:, having lost twelve hours on the 
sun, as will be seen supposing ap observer to be at D ; 
move the moon further to H^ then to the observer at A^ 
to whom the sun has just set, the moon will not have 
risen ; having lost eighteen hours on the sun, it will 
rise at mid-night, as wilj be seen by the observer at D, 
To the observer at C, the moon is southing and the 
sun is rising ; move it on further to AT, it will nearly 
have lost a whole revoli^tion on the sun, and will 
rise about twenty-one hours after it, if we reckon from 
the time they both rose together (or three hours be- 
fore it, if we reckon the other way), and in two or 
three more days they will both rise together again. 
Now it is clear from what we have seen that its losing 
on the sun may be accounted for by supposing it to 
travel round the earth in about twenty-eight days. 
And this we know to be the case. 

§ II.— THE MOON CHANGES HER FORM. 

1 06. We have thus explained the moon's own motion 
among the stars, but something else happens to her : 
as she moves round us, she changes her form from a 
crescent to a circle. These changes have become so 
familiar to us, having heard of the changes of the 
moon as far back as we can remember, that we are 
apt to look on them as a matter of course, without 
inquiring into their cause. Let us ask the question, 
" Does the moon really change ? " No, it is al^vays 
there, but a portion is sometimes unillumi- 
nated and invisible to us. 



ASTRONOMY, 43 



T07. Observe the moon some evening ; suppose you 
see it at the "full moon" as it is called, when it appears 
round, like the sun : observe whereabouts it is in the 
sky, and you will find that it is on the opposite side of 
the earth to the sun, and that it cppsequently rises at 
sunset and sets at sunrise, in fact it is in position G 
(Fig. 22) ; now place the ball, representing the moon 
at G on the opposite side of the orange to the sun, 
then the h^lf of the ball, which is white in the diagram, 







LAMP OR SUM 

® 



EARTH 



^ 




% -Cis- 3 O 



C M 



3 3 



T 



^^ » O 






Fig. 2».— The Moon's ^otiop round the Earth. 

will be illuminated by the sun, and the other half, oph 
posite to it, will, of course, be dark, in the same manner 
as we have night when the sun is shining on the other 
side of the earth to us, and if you place your eye 
near the orange, you will see all the bright portion 
and none of the dark side ; it is then fall tcvooi^^ •i>xs.^ 
this appearance is represenled \jy ^^ ^V\\fc w^^ lA 
So that it is now clear that, at iwW xs^oot^. \^^ \s\^^^ 



44 SCIENCE PRIMERS. [§ ii. 

on the opposite side of the earth to the sun, and we 
see therefore the bright side. 

1 08. After the full, the moon rises, as we have seen 
before, later and later after sunset, and we will suppose 
you observe it a week after the " full." It will rise, as 
you will find, about midnight. Rather late, you say, 
to sit up, but the day of astronomers is other people's 
night. The moon now is no longer apparently round, 
only half of it is visible. Return to the diagram : in 
what position is the moon if it rises at midnight? It 
is midnight to an observer at D, and the moon to be 
rising must be at H, Place the ball, therefore, at 
ZT, and the eye at D ; now the part, white in the 
diagram, is the bright half illuminated by the sun ; but 
in this position the whole of it is not visible, but only 
half of it and half of the dark portion, you will there- 
fore see that we ought to have the appearance of half 
moon, iVJ in this case, which we do in reality. 

109. Let us continue our observations. If it is too 
late to sit up after midnight, try and get up before 
sunrise and you will see that, as the moon is appa- 
rently overtaken by the sun, it will get more and more 
crescent shaped, and when at K it appears as at 0^ till 
it is lost in the sun's rays and comes to position E. 
How ought it to appear now ? Place the ball between 
the eye and the lamp, and you will see the whole of 
the dark half and none of the bright portion. It is 
** new moon ; " look at it a few days after, when it will 
be visible just after sunset. It will appear in a thin 
crescent, and will be in the position marked T in the 
diagram. Place the ball in this position, and by placing 

j'oureye close to the orange you will see ^ust a crescent 
^lAe bright half, and a large portion of the daxVLYv^i, 



ASTRONOMY, 45 



1 10. As the moon appears to get further and further 
from the sun, and to set later and later, more and 
more of the bright half will be seen, till we get to 
half moon in position F, It is now south at sunset. 
Place the ball in this position, and your eye close to 
the orange, and you will see the observation is ac- 
counted for. Another week more and the moon again 
becomes full, and opposite the sun. 

111. All these observations may be thoroughly 
mastered by standing at a distance from the lamp, or 
gas-light, which should be the only one in the room, 
and moving an orange, or ball, round your head, 
when all the changes of the moon will be rendered 
clear to you. The moon, therefore, revolves 
round the earth in the same manner as the 
earth goes round the sun, passing from full 
moon to full moon in about twenty-nine and 
a half days. 

§ III.— HOW THE MOON CAUSES ECLIPSES. 

112. From what we have seen, you might think that 
the moon ought to pass between us and the sun every 
month, and produce what is called a total eclipse 
of the sun ; but, for reasons of which we shall 
presently speak, it sometimes passes a little above 
the sun, and at others a little below, when there is 
no eclipse at all, or it passes over a part only of the 
sun, and so only covers a portion of the sun's disc 
from our view, producing what is called a partial 
eclipse. 

1 1 J. l^t us see if we catv m^V^fc Taa.\.\Rx^ evR»s.^«>S!cw 

the use of our orange and baW 



46 SC/ENCE PK/MEKS. B in. 

114. Set the lamp on the table, and stick tbe knitting 
needle supporting the orange into a large pincushion 
at some distance from it; then take the small ball 
representing the moon and suspend it by a string, so 
that you can move it round the earth (Fig. 33), without 
the fingers casting a shadow on it. Now bring the 
moon between the sun and earth, holding it near the 
earth as at C (Fig. 23), so that the shadow of the moon 
falls on the earth: wherever this shadow falls on the 
earth there will the sun be invisible, and there will be 
a total eclipse at that place. At other places on 
the earth, as at H, which the darkest part of the shadow 




does not reach, the whole of the sun will not be covered 
by the moon. Here, then, we shall have only a partial 
eclipse, and the further you go from this region the 
more of the sun will be visible, so that round the 
total shadow is another kind of half shade, called the 
penumbra, and, as we have seen, ai\ v^atcs ui^ide 
ftti penumbra will see a paiLial eclipse on\y. 



■ ASTXONOMY. 



115. Now move the moon further away from the 
earth, to say D (Fig, 24), and you will see that the 
shadow of the moon is not sufficiently long to reach 
the earth, so there can be no total eclipse, the moon 
being so far away that its disc is not sufficiently large 
to cover the sun completely, so there remains the 
outside edge of the sun visible ; this sort of eclipse 
is called an annular eclipse. 

116. All this will be clearer if the orange be re- 
moved and the eye placed in its stead. First place 




your eye where the shadow was (Fig, 24), that is, in the 
umbra of the moon, and you will see a total eclipse. 
Then move the eye a little lower, still keeping the 
moon in the same place, and you will see a crescent 
of the sun, in fact apartial eclipse, and the further you 
move your eye from it, the more of the sun you 
will be able to see. Now place the eye aX A ^».^ ■*«> 
see a total eclipse, and move i.\ve mocfu %\aS>>MSi^ ■awa.-i ^ 
from you, and you will see t^^e wiooa ai^Niai*^^^^ 



48 SCIENCE PRIMERS. f§ ill. 

-■-- - — — --- 

getting smaller, so that at D (Fig. 24), it is no longer 
large enough to cover the sun, and you see the 
bright edge of the sun round the moon ; in fact, an 
annular eclipse. 

117. Besides eclipses of the sun, there are eclipses 
of the moon, occasioned by the moon passing 
through the shadow of the earth. You will readily 
understand how these happen by placing the lamp 
and orange a? before : on passing the ball, repre- 
senting the moon, round on the opposite side of the 
earth to the sun, it will go into and through the 
shadow of the earth, and will be darkened, not, as 




FlGi 25. — Eclipse of the Mooa* 

in the case of an eclipse of the sun, by an opaque 
body coming between us and the sun, but by its 
being shaded by our earth (Fig. 25). 

118. To an observer on the moon during a total 
eclipse of the sun, the earth would appear to have 
a black spot on it, moving rapidly across it; and 
surrounding the spot would be a circle of half shade, 
the penumbra, in which a partial eclipse is seen from 
the earth ; but in the case of a total eclipse of the 
moon, the shadow of the earth entirely envelopes the 
moon. 

^19' You will have understood by V\\vs \!\m^ \)wa.\. «cl 



ASTROXOMY, 49 



eclipse of the sun can only take place at new 
moon, and an eclipse of the moon can only 
take place at full moon. The reason being that 
when the moon is between us and the sun, that is, 
when an eclipse of the sun can happen, the moon's dark 
side will necessarily be turned towards us ; and when 
the moon is on the other side — on the opposite side 
of us to the sun, that is, when an eclipse of the moon 
can happen, it must have its bright side towards us. 

120. We have spoken (Art 112) of the moon 
passing sometimes above, and at other times below 
the line joining the earth and sun, and, as you will 
see by referring to the orange and ball, an eclipse of 
the sun and another of the moon must happen every 
month if the moon did not so pass: 

121. Let us see how we can account for the fact 
that the moon does thus pass sometimes above and 
sometimes below the sun, thus preventing monthly 
eclipses. - We have found that the moon revolves roun^ 
the earth in nearly a circle (with the earth at the centre) 
called its orbit or path. Let us represent this orbit 
by a piece of wire, bent in a circle round the orange, 
and let the moon be represented by a large bead or a 
small ball strung on it. Hold the ring of wire so that 
the earth (orange) is in the centre, and move the moon 
on the wire round it, and you will find that if the ring 
is held horizontally the moon will pass between the 
earth and sun, represented as usual by the lamp, at 
every revolution. Now this we have observed is not 
the case with the real moon, and in order to make the 
bead pass above or below, the pattoi vVv^ \vcv%'^^x^^^^ 
the lamp and the orange imist \>^ \!\v^^^ ^a-^? ^"^ ^^^^^-^ 

122. To make this cleaier, ?,e\. ^ x>^^ ^'^ ^"^"^^^ '^ 



50 SCIENCE PRIMERS. • ' B 111. 

before, and float in the middle a ball to represent the 
SUD, so that half is above water and half below. Float 
another small ball near the side of the tub to repre- 
sent the earth, then the earth can be floated round 
the sun, to represent its annual path. Now, as its 
orbit will He on the surface of the water, this surface, 
as we have seen before (Art 67), represents the 
plane of the ecliptic. 

123. But we have already suspected that the 
moon's orbit is inclined to this plane, so that at 
certain times no eclipse takes place ; and if we take the 




wire ring as before, to represent the moon's orbit, and 

place it round the earth, dipping one half of the ring 

below the surface 01 the water, and keeping the other 

ibove, ajs i-<fpresented in Fig. 26, wheie ftvelM\\\Kv« 



ASTRONOMY^ 5 1 



indicates the part above water and the dotted line th'^ 
part below, we represent the inclination of the moon's 
orbit to the plane of the ecliptic, and the line joining 
the points where the orbit cuts this plane is called 
the line of nodes, and B and D are the nodes. 

124. This will render it clear that eclipses, suppos- 
ing the orbit of the moon to be inclined to the plane 
of the echptic, could only happen when the moon is 
at the part of its orbit near a node when she comes 
in a line with the earth and sun, for only then does 
slie in her revolution pass between the sun and the 
earth. At the other parts of the orbit there can be 
no eclipse, because the bead pn the ring would at its 
nearest approach to an eclipse be below or above the 
water, and not on its surface in a line with sun and 
earth. And as eclipses do not happen every month 
we know that the moon's orbit is inclined as we have 
supposed it to be. 

125. We have seen before that the plane of the 
earth's motion round its axis is inclined to the plane 
of the ecliptic, and we now find that the plane of the 
moon's motion round the earth is inclined to the same 
plane. We' should now endeavour to understand how 
the amount of inclination is fixed in each case. 

126. To do this astronomers divide all circles, whe- 
ther krge or small, into 360 degrees (written 360°), 
(see Fig. 27), and if we draw two lines from the 
centre of a circle to the circumference the number of 
degrees intercepted between the points where they cut 
the circumference is the measure of the angle between 
the two lines at the centre. Now t^^o Vs* ^c^xix •ixwNR.'s. 
Qo, so that two lines contavxvvtv^ ^ C3^^.\\.ex o^ "^ ^^^1^ 

make an an«le of qo*" beiweetv \\\exxv. Xo^ ^^ ^"^ 



S3 SCIENCE PRIMEHS. [S IK. 

that the size of the circle is of no consequence, for if 
you draw a number of circles, one inside the other, 
all having the same point for their centre, and from 
the centre draw two lines intercepting a quarter or 
90° of the outer circle, then you will see that it inter- 
cepts also a quarter of each of the others. Each 90" 
is called a li.^ht an^le, and two lines which make an 



_i 

j /^ 


1 / / / / Vd , 

nJ f/y -^-'^ 



angle or opening of 90° between thera are said to be 

perpendicular to each other. A complete circle like 

this is contains 360 angles of 1°, 4 angles of 90°, and 

sooa 

13^. Now astronomers conceive such a circle with 

J:s centre at the centre of the earth, itid they can 

*Vn by their observations determxtvc ft^ft xni^i 



ASTRONOMY. 53 



formed by the planes to which we have referred in 
Art. 125; and thiSy have thus found that the angle 
made by the plane of the ecliptic, and the plane of 
the earth's motion of rotation is 23°, or thereabouts ; 
and the angle made by the plane of the ecliptic and 
the plane of the moon's motion round the earth, is 
a little over 5°; 

§ IV.— WHAT ¥hE moon IS LIKE. 

128. I have already referred to the teachings of 
Physical Geography with regard to the Earth. The 
moon is near enough to us, being only some quarter 
of a million of miles awayj to enable us to learn much 
about its surface. 

129. If the moon be looked at with the unaided 
eye its surface appears mottled, some portions being 
darker than others; and those darker places were 
thought by the ancients to be seas, and, although they 
have since been found to be dry land, they still retain 
the name of seas : so we have " Sea of serenity,^ 
"Sea of storms," and the like, as you will see on 
looking at a map of the Moon, for we have a map of 
the Moon as we have a map of the Earth. If you 
employ a telescope to aid the eye — and a small one 
will answer the purpose, — the surface is seen to be 
almost completely covered with mountains, hills, and • 
valleys, but not altogether mountains and valleys as 
we have them here, covered with verdure, but all 
dry and barren. There are no lakes or rivers, and, 
as far as is yet known, there is no ^^.\!«. ^Nxra^RN^x^ 
and consequently no clouds to ^Yva.^'e, X^cve. ^>\V«^^^^'^^'^ 
ihc sun ; and what is more, \\v<iTe \s tvq ra.^^^jrt^'^^''^ 



54 SCIENCE PRIMERS, [§ iv. 

atmosphere. Hence there is probably no life on the 
moon. Nearly the whole surface is covered with ex- 
tinct volcanoes of enormous extent, and, unlike those 
you read of on the earth. 

130. You will see from these facts about the moon 
how the conditions of the planet on which we dwell 
may not apply to the other bodies in the skies. Fancy 
a world without water, and therefore without ice, 
cloud, rain, and snow, without rivers and streams, 
therefore without vegetation to support animal life : 
a world without twilight or any gradations between 
the fiercest sunshine and the blackest night; a world 
also without sound, for as sound is carried by the air 
the highest mountain on the airless moon might be 
riven by an earthquake inaudibly ! 

13 1. You will recognize, too, that the moon must 
resemble the earth in this : it does not shine by 
its own light. The bright part of the moon is that 
on which the sunlight falls ; where this light does not 
fall the moon is invisible : hence moonlight is sunlight 
second-hand, and the moon does not give us light of 
its own. 

132. The diameter (Art. 22) of the moon is about 
2,000 miles; and, bulk for bulk, its materials are 
lighter than those of which the earth is built up. 
This is expressed by saying that the density of the 
moon is §, that of the earth being i. 

133. Now this requires a little explanation. You 
know that some things are very dense and heavy, 
others are very light ; lead for instance is very dense 
and heavy, cork is very light. Now you know 

what an inch is, and a square mcb, aivd a cubic 
lacA, Suppose that you took a cuVvc \\\e\v ol\^^^> 



ASTRONOMY. 55 



and a cubic inch of cork, then, by weighing them 
both, you would be able to tell exactly how much the 
lead was heavier than the cork. Calling the weight 
or density of the cork i, the weight or density of the 
lead would be so and so. And of course if you took 
instead of a cubic inch, a cubic yard or a cubic mile, 
the lead would weigh exactly the same number of 
times more than the cork; 

134. Astronomers have found out the weight of the 
earth, and of the moon, and thiey also know how 
many cubic miles (or cubic inches) each contains. 
They can therefore easily find whethet" a cubic inch or 
mile of the materials of which the moon is built up 
weighs less or more than a cubic inch or mile of the 
materials of which the earth is built up; in other 
words, whether the earth is less or more dense than 
the moon. And they have found that a cubic inch of 
the earth's materials weighs i| times as much as 
a similar quantity of the moon's materials^ hence they 
say that the moon is only § as dense as the earth; 

135. More commonly the weight or density of 
a cubic inch of water is taken as i, then we say that 
the density of the earth is 5 4, and that of the moon 
5I times greater than that of water. Thus then we 
have in the case of each celestial body : 

a. Its volume expressed in cubic miles or cubic 
inches determined from its diameter. 

b. Its weight or mass, that is to say how many 
tons it weighs, this is determined from its action on 
other bodies. 

c. Its density, that is how much a. cxsfc^si \sss^ ^^s^ 
cubic mile weighs ; this is fovxti^ \>^ ^\n \^yc^^S^.^ '«^'*^^ 
or weight by its volume. 



56 SCIENCE PRIMERS. [§ i. 

136. The same side of the moon is always turned 
towards us, for as the moon goes round the earth it 
slowly turns on its own axis, and makes one revolution 
in exactly the same time as it takes it to get round 
us, just in the same way as you would do if you were 
to take hold of a pole stuck in the ground, with your 
hands, and go round it, always keeping your face 
turned towards the pole. You would then see, by 
looking at adjacent objects, that you turned round 
once every time yoii went round the pole, and you 
will probably become giddy, thereby giving conclusive 
evidence of your rotation. 

137. It follows from this fact that the moon only 
turns round once on its own axis during each re- 
volution round the earth, and that the lunar days are 
about 29 of our days. We are lighted by the sun for 
about 12 hours, or the half of 24 hours ; each portion 
of the moon is lighted for about 14 days, or the half 01 
29 days, so you can imagine how intensely heated the 
surface must become during the lunar day, and how 
cold the opposite side must get during the 14 days' 
night. 



III.— THE SOLAR SYSTEM. 

L—HOW BODIES LIKE THE EARTH, NEARER 
THE SUN, WOULD APPEAR TO US. 

138, So fax as we have gone the earth oh which we 
divell, the large sun and moon, and l\\e tvxv^ ^x^x^, ^^ 
^/je only bodies with which we have deaVl, 



ASTRONOMY. 57 



139. Let us see what we should observe in the 
heavens if there were other bodies, not shining by 
their own light — other earths like ours, revolving round 
the sun as we do. How would they appear to us ? 
And first let us take the case of a body travelling 
round the sun but at a less distance from him than we 
are. Let us think. Take the lamp to represent the 
sun, the orange for the earth, and the ball used for the 
moon to represent the other earth ; then all we have 
to do in order to represent the appearance of the new 
world in its journey round the sun, is to move the balb 
round the lampj and see how it appears from the 
orange in its different position Si First place it in the 





A 



Fig. 28. — Diagram illustrating: the motions and appearances of a body 

between us and the sun. 



position represented by A, Fig. 28, between the lamp 
and the orange — then it will appear in the same line 
with the sun, and accompany the sun in its path 
across the sky, at which time of course it will be in- 
visible on account of the superior brightness of the sun, 
but it will set and rise with it ; now move it to B — 
it will then appear on the right side of the sun, and 
will rise before daylight and set before the sun, so that it 
would only be seen before sunrise, changing its place, 
— " wandering" among the stars from da^ to <ia.^ VJs^fc 
word planet means a '*wandeIeT"\X.o\i^^^^^.<5^a^.X^^'^'^ 

staxs by the day. Move it to poivtiotv C— SX.^'^'^^'^^'^^^ 



5S SCIENCE PRIMERS, [§ ii. 

and set with the sun, and will be lost in the sun's rays 
as at A. Again move it to D — it is then on the left 
side of the sun and will rise after daylight, and set after 
sunset, so that it will be seen only in the evening. A 
little consideration will make it plain that this body will 
go through the same changes as the moon, and again 
that we can never see it at midnight. But there will be 
an important difference. As we go round the sun, keep- 
ing always about the same distance from the sun, the 
sun always seems to be about the same size ; and as 
the moon goes round the earth, keeping about the 
same distance from it, the moon always seems to be 
about the same size. Mind, I do not say the same form. 
But the new earth about which we are now think- 
ing goes round the sun; so it will sometimes be 
between us and the sun and sometimes on the opposite 
side of the sun, so that its distance from us will vary ; 
therefore, its apparent size will vary. 

140. Hence, if we were to examine this new earth 
with atelescope, we should see it vary in size and also 
in shape like the moon, and if its atmosphere were 
clear, we should see its seas and continents, and so 
by their motion we should be able to ascertain how 
fast it turned round on its axis — whether its day was 
longer or shorter than ours. 



§ II.— HOW BODIES LIKE THE EARTH, FUR- 
THER OFF FROM THE SUN, WOULD 
APPEAR TO US. 

14 T. In order to represent the appearance of an 
earth outside us, we have only to move the ball in 
^ c/rcle round the sun, outside the eatth's oi\>\X, \ifc\. 



ASmOAOMV. 



59 



us begin by holding the ball on the opposite side of 
the sun to the earth — then it will be lost in the 
sun's rays, and on moving it further round in the 
contrary direction to the hands of a clock, it will be 
seen on the left side of the sun, and will therefore 
set after it just as the interior earth did ; but as you 
move it on after it has made a quarter of a revolution, 
it appears to recede further and further from the sun, 




Fio. 29. — ^Diagram illustrating the motion of a body travelling round the bun 

outside the orbit of the earth. 



instead of again approaching it, and passing between 
the earth and sun ; and eventually it comes to the 
opposite side of the earth to the sun and rises at 
sunset, and is visible in the south at midnight, which 
as we have seen was impossible in the case of a 
body between the sun and the earth. 

142. You will also notice that nearly all the V«\s^ 

side is visible to the eatl\\, a\\}cvo>\^ "ax '^^ "^^^ 

positions corresponding to A atv^ B,^\%- •^'^'» >x^^ 



6o SCIENCE PRIMERS, [§ in. 

show a portion of its dark side, so that an exterior 
earth would not go through all the changes that an 
interior one would do. While, therefore, the interior 
earth would appear to swing from side to side of 
the sun J only the exterior one would take a sweep 
round outside our earth. Such a body will vary its 
size, but not to so great an extent as an interior one. 



§ III— ARE THERE SUCH BODIES?— THE 

PLANETS. 

143. Ther^ are such bodies as We have just been 
considering, both interior ones and exterior ones, 
and they dre all called Planets, and the earth is 
called a planet simply because it, like them, would 
appear to wander among the stars to astronomers on 
the Other planets, if such there be. The principal 
planets are eight in number, including our earth. 
They have been named iafter the ancient deities ; the 
two interior ones, Mercury and Venus, and the exterior 
ones. Mars, Jupiter, Saturn, Uranus, and Neptune ; 
the three first being smaller than our earth j and the 
remainder a great deal larger; 

144. Mercury and Venus are known td be interior 
planets, that is, planets between us and the sun, 
because they appear to swing, as we have found such 
bodies should do, on either side of the sun. Mercury 
very seldom leaves the sun sufficiently to rise so early 
before the sun, or set so late after him, as to be 
visible. Venus, however, gets so fat away as to be 
seen long after sunset or before sunrise, and is called 

tAe Evening or Morning stat, accoidViigVy. 
^4S' The exterior planets, as we iound s\3ic\v\>o^vi.'& 



ASTROyOMY. 6 1 



should do, make a complete tour of the heavens. All 
these movements are, however, rather more com- 
plicated than we have found with the orange and ball, 
for the earth is not fixed, but going round the sun 
quicker than the exterior, and slower than the interior 
planets; and, in order to represent the true apparent 
motions you must move the orange round the sun at 
a rate depending upon which planet you wish to re- 
present by the ball. 

146. The sun and planets revolving round him 
form what is called the solar system ; in fact, 
everything over which the sun has continued in- 
fluence is a member of this system. 

147. Thus besides the planets there are other 
members of the system, namely, comets and falling 
stars, which will be mentioned again more fully here- 
after : all these bodies form a sort of family having 
the sun for their head, and on Plate II. will be seen 
a view of this system as it would appear when 
looked at from above; but it is impossibly thus to 
give an idea of the true scale of the system. In order 
to do this, take a globe a little over two feet in dia- 
meter to represent the sun : Mercury would now be 
proportionately represented by a grain of mustard- seed, 
revolving in a circle 164 feet in diameter; Venus 
a pea, in a circle of 284 feet in diameter; the 
earth also a pea, at a distance of 430 feet; Mars, a 
rather large pin*s head, in a circle of 654 feet ; the 
smaller planets by grains of sand, in orbits of from 
1,000 to 1,200 feet; Jupiter, a moderate sized orange, 
in a circle nearly half a mile across ; Saturn, a small 
orange, in a circle of four-fvfths ol %, wXfc \ ^3T«:«s«.^ 

a full' sized cherry, or sr(\a\\ pVxjim, x^^oxi ^^ cLvt^>^^«^r 



6:2 SCIENCE PRIMERS. [§ iv. 

ference of a circle more than a mile and a half; and 
Neptune, a good-sized plum, in a circle about two 
miles and a half in diameter. 

148. I have already told you that the earth's 
distance from the sun, represented in Art 147 by 
430 feet, is really 91 millions of miles. I cannot giv^ 
you any idea of this distance. I can only state that 
if a train going at the rate of thirty miles an hour were 
to leave the earth on the first of January, 1875, it 
would only reach the sun in the middle of the year 
2213. 

149. Beginning with this rough idea we will now 
consider the interior planets— those, namely, which 
are nearer the sun than the earth. 



§ IV.— THE INTERIOR PLANETS. 

Mercury. 

150. Mercury, the nearest planet to the sun, revolves 
round him at a distance of about 35 millions of miles ; 
the earth's distance from the sun being 91 millions, 
it has a diameter about one- third of that of the earth. 
It can be seen at certain times just after sunset, and at 
others just before sunrise, as it never quits the neigh 
bourhood of the sun. It is eighty-four days in travers- 
ing its orbit, so that its year is less than a quarter of 
ours. Its orbit is represented in Plate XL, and, like 
the moon's, is slightly inclined to the plane of the 
ecliptic ; that is to say, if the earth's orbit is supposed 
to he floating on the surface of water, part of Mercury's 
orbit would be slightly below the suviac^ axvd ^art 
^Ker. Jrrom the diagram you will se^ XV\^X ^V^xcm\>j 



ASTEOXOMV. • 63 



will always appear to us near the sun. When it is 
on oiir left of the sun it apparently follows the sun on 
its daily course, and sets just after it ; when on the 
otTierside it precedes the sun, and therefore sets before 
it, and so is only seen in the morning, when it rises 
just before the sun. 

151. If Mercury be watched with a telescope it is 
found to go through the same changes as our moon, 
and for the same reason. You will understand this 
from Fig. 28, where the ball may betaken to represent 
Mercury in its different positions as it revolves in its 
orbit. When it is between us and the sun (or in 
what is called inferior conjunction) we do not 
see it as its dark side is turned towards us, and as 
it moves round we see more and more of the bright 
side, till when it is opposite to us, or in what is called 
superior conjunction, we see the whole of the 
bright side. 

152. Liltle is known pf Mercury itself; we know 
not whether it has a land and water surface like the 
earth or is waterless like the moon, whether it is 
enveloped in a dense cloudy atmosphere which pro- 
tects the inhabitants, if such there be, from the 
intense heat of the sun, or not. We only know that 
its density (Art. 133) is greater than that of die earth. 

Venus. 

153. Next to Mercur}' comes Venus, at about 66 
millions of miles from the sun, with a diameter nearly as 
large as the earth. It can generally be seen either just 
after sunset or before sunrise, accotdm%\o>&.'3»^^<^^>5o^'^"^ 

in its orbit, round the sut\, \tv t\v^ s»^xs\^ \ssax>:^^'^ "^"^ 
Mercury, only its orbit be^\^g o\iX.s\d^ \>^^x. oS. V^'^^^^^ 



64 SCIENCE PRIMERS. [3 iv. 

it can get further away f.'ora the sun's apparent place 
among the stars, consequently we can examine it better. 
It is the brightest of the planets, and when visible 
cannot be mistaken. It takes 224 days to perform its 
annual revolution, and 23 hours and a quarter for its 
rotation on its axis, which determines the length of 
its day. 

154. We liave shown in speaking of tlie earth that 
the inclination of its axis produces the seasons, and 
that the pole of the earth, instead of being upright or 
perpendicular to the ecliptic, is inclined 23° (Art. 71). 
In the case of 
Venus there is 
affirmed to be an 
inclination of 50°, 
or about half-way 
between upright 
and horizontal ; 
the consequence 
is that the seasons 
there change to 
a much greater 
extent than ours 
do. 

154. Venus also 
goes through the 
s4me change of 
phases as Mer- 
ciuy does, and 

flc. 30,— Venus, ibo^iDgtlieiluiliuieioiiiM of COUTSe lor 

"^■"' the same reason. 

Very little is known of the surface of Venus : certain 

<iari markings, however, arc seen tretvieTi&v VvCn fea- 




ASTRONOMY. 65 

rate instruments on the surface, which may possibly 
be breaks in clouds, through which the planet itself is 
seen. l"he density of Venus is about Uie same as 
that of the Earth. 

155. If you will think a little you will see that in the 
case of Venus the apparent size as seen from the earth 
should greatly change, as the nearer she is to us the 
larger would she be if we could see her completely; so 
that, although like the moon she has phases, unlike 
the moon her size wil! alter. Let us inquire into this 
a httle closer. When Venus is nearly between us 
and the sun — when, therefore, we can only see a fine 
crescent^ — she will be but some 25 millions of miles 
away from us (because we are 91 and she 66 millions of 




miles from the sun) : but when she is ot\ vVit <i*\*^ ^\fc.t 
of the sun she will be 157 n\\\\\oT\% a.'s^-j ^\owv vi-":-. 



66 SCIENCE PRIMERS. [§ v. 



(that is, 91 millions from us to the sun and 66 millions 
from the snn to Venus on the other side), so that her 
size will vary in the proportion of 157 to 25, or say 6 
to I ; so that the crescent of Venus will appear to form 
part of a circle 6 times larger than that presented by 
Venus when she is full to us. These changes are 
shown in Fig. 31. 

156. Venus and Mercur}% at times when they are 
on the earth's side of the sun, are visible as black 
spots on the sun's disc. This is called a transit of 
Mercury or Venus; that is, the passage of the 
planet exactly between us and the sun, so that it is 
seen on the sun's disc. 

157. A transit of an interior planet, like an eclipse 
of the sun by the moon, can only happen when the 
planet passes the sun at the time it is near one of 
its nodes, that is when it passes from one side of the 
plane of the ecliptic to the other. A transit, in other 
words, can only happen on the coincidence of the 
earth and planet both being in a line with each other 
at either node. A transit of Venus happens in 1874., 
and agam in 1882, and not again for 105^ years. 



158. Next to Venus comes the Earth, the planet 
on which we dwell, and which has already been 
described. We therefore pass on to the exterior 
planets. 

§ v.— THE EXTERIOR PLANETS. 

1S9. The Dext member of our system is Mars. 
Jllars revolves in an orbit having a mea.w or average 
distance of ijg millions of miles frora tXv^ svwi. \v 



ASTRONOMY. 67 



revolves on its own axis in 24 hours and a half, 
making its days half an hour longer than ours. Its 
diameter is about one half that of our earth. 

160. Mars requires 686 days to complete its annual- 
revolution round the sun, making its year nearly 
double the length of ours. Since its orbit Hes outside 
ours this planet never can pass between us and the sun, 
and consequently it does not show the same phases as 
Venus or Mercury ; it however at two positions in its 
orbit becomes what is called gibbous, losing appa- 
rently its brightness to a small extent on one side, as 
will be seen in Fig. 29, where the two positions, 
when the earth is at Ey are marked A and B, and 
at these two points a small part of the dark side will 
be turned towards us, presenting an appearance like 
the moon two or three days before or after full. 

161. When Mars is on the opposite side of us to 
the sun at M, it is said to be in opposition ; it is then 
at its nearest point to us (its distance being 139 — 91 
=: 48 millions of miles) and fully illuminated ; so then 
this is the time to examine the planet. Its orbit is, 
however, very eccentric or oval, consequently it is 
much nearer the earth's orbit in one direction than 
in others ; and when an opposition happens, as is the 
case when Mars and the Earth are in this position of 
their orbits closest together, we have a most favour- 
able opposition, at which time Mars is only about half 
the distance it is from us at the most unfavourable 
one. The inclination of its axis is nearly the same 
as that of the earth, being about 29°, so that the 
Martial seasons must be very similaY t.o o\m%. 

162. When looked at wit\\ X.\\^ e^^ 2\w\^, Vw^'^ 
appears of a reddish tint, by w\uc\\ '\x. e^Tv\i^ ^-j^s^.^ 

^ 1. 



6S SC/EXCE PRIMERS. \% v. 

recogniied, but wlien seen through a telescope the 
redness in a measure disappears, and the planet 
a|>p€ars to have a bright surface, on which are darker 
portions, the Tormer being the lands, and the latter 
the seas. Mars is the most remarkable among the 
planets in this, that it appears to us as the earth 
would appear to its inhabitants. Around the poles 
the surface appears white, and on watching the spots 




from time to time each is seen to grow small as 
summer is approached in that hemisphere while the 
opposite one gets larger in winter, so we suppose 
these to be the polar snows corresponiling to those 
nn our earth- The drawing will give some idea of 
'Aff appearance of Mais as seen in a\ai%e V-tXc^^-^, 



ASTRONOMY. C9 

one of tlie main features being that instead of there 
being about four times more water than land as on 
our earth, there is on Mars about four times moie 
land than water. 

The Asteroids. 

163. Beyond Mars we come to the Asteroids, or 
minor planets, a number of small bodies not varying 




greatly in distance from the sun, and revolving in 
orbits outside that of Mars. Vesta, Juno, Ceres, and 
Pallas are the principal ones, but they are only some 
few hundred miles in diameter, and are barely visible 
to the naked eye, if at all. andhQm.'il&eii ^.■wiiSw.e.^'^-a:^^ 
worth littie notice. Theii oib\Vs axe. lao'^t w<&^^^ ^ 



70 SCIENCE PRIMERS. R v. 



the plane of the ecliptic tlian those of the larger 
planets, but we have no knowledge of the inclination 
of the poles of these small planets to their orbits. 
Their number is large, about 130 ; and we say about, 
for several are discovered every year, and the names 
of nearly all the deities must have been used for 
them. The greater number of these are only equal 
10 a loth magnitude star in brilliancy, and their 
surface may possibly be not much larger than the area 
ox a good Scotch estate. 

Jupiter. 

164. Outside the orbits of the numerous asteroids is 
the largest planet of our system, Jupiter, a body that has 
no doubt been pointed out to you some time or other. 
When above the horizon, it is unmistaka'ble by its 
excessive brightness, being only surpassed by Venus, 
which can generally be recognized from it by its 
proximity to the sun. Jupiter revolves in an orbit at 
a distance of 476 millions of miles from the sun, com- 
pleting his year in 4,333 days. ■< 

165. When observed with a telescope of moderate 
power, Jupiter appears of an oval shape, very much 
flattened at the poles, and crossed by several dark 
belts, as represented in the figure ; large black spots and 
other markings of which we shall say more presently, 
are also frequently seen on the surface, and from 
the motion of those markings, the time of rotation on 
its axis has been ascertained to be about 10 hours, 

that is less than half one of our days, and its dia- 
nieter is found to be about ten tivnes tVve dx^LTOfcter of 
Iter earth, so that the flattening of tV\e poVe^ axv^ V^cv^ 



ASTRONOMY. 



71 



protuberance of the equator must necessarily greatly 
exceed that of our earth, for the velocity that the 
equator moves at must be twenty times the velocity ot 
our planet at the equator, or 20,000 miles per hour. 

166. We have mentioned the belts and other mark- 
ings on its surface; it is probable that Jupiter is covered 
with clouds, giving rise to its bright appearance, and 
that the dark belts are openings in the clouds through 




Fig. 34.— Jupiter, showing the cloud belts. 

which we see the .darker surface of the planet, or 
more probably of lower beds of clouds beneath. 
The number and size of the belts are continually 
changing, and bridges of cloud are constantly being 
thrown over the dark spaces, clearly showing that vt 
is not the surface of the plaii^X. v^e ^^^>\s^ ^'^ "^ 
very cloudy atmosphere. 



72 SCIENCE PRIMERS. [§ v. 



167. So far as we have gone the planets have been 
unlike the earth in one respect, they have no moons. 
Jupiter, however, has four satellites or moons revolving 
round hin* and going through the same changes as 
bur own. They are all nearly of the same size, about 
2,000 miles in diameter, but at different distances, 
and consequently they take very different times to 
revolve round their primary, Jupiter, the first taking 
less than 2 days, the second 3^ days, the third 7 





Fig. 35. — Diagram explaining the eclipses, occultations, and transits of 

Jupiter's satellites. 

days 3 hours, the fourth i6f days. They all re- 
volve in orbits very slightly inclined to the plane of 
Jupiter's orbit, and consequently whenever they pass 
between the sun and Jupiter there is an eclipse of the 
sun visible on some part or other of the planet's 
surface ; only the fourth has an orbit sufficiently 
jnclined to enable it to pass above or below the line 
Joining the sun and Jupiter, this prevewts it from 
causing an eclipse at every revolution. ¥ot xVv^ ?.o.m^ 



ASTRONOMY. 73 



reason of course the moons also are eclipsed at every 
revolution by the planet's shadow. 

1 68. When viewed with a telescope the moons 
appear to oscillate on either side of Jupiter (just as 
the interior planets appear to us to oscillate on either 
side of the sun), and in their passage from one side 
to the other they generally pass over the disc of the 
planet; there is then what is called a ** transit" of 
the moon over the disc. We also see the shadow of 
the moon traversing the disc whenever we are so far 
from the line joining the sun and Jupiter, that the moon 
does not cover the shadow. The moons in passing 
round on the other side at times suddenly disappear, 
or are eclipsed, when they pass into the shadow 
of the planet, but we may be in such a position that 
Jupiter's shadow lies on the opposite side of the planet 
to that behind which the moon passes ; the satellite 
then goes behind the disc uneclipsed, and is said to be 
"occulted." The diagram will make this clearer; 
when the earth is at the point E of its orbit, the moon 
N appears in transit, while the M is occulted and O 
eclipsed, and from this point of view every satellite 
must be occulted before it is eclipsed ; but when the 
earth is at i^the moon M\^ no longer occulted, and 
will pass into the shadow and become eclipsed 
without an occultation, and from this point P will 
be in transit and O also eclipsed, but as soon as it 
leaves the shadow it will be behind the planet, and 
will reappear from an occultation. 

169. The inclination of Jupiter's axis is very small, 
only a little over 4*, so that there can be tio ^.^^^^^<^- 
able change in the Jovian seasons. MxJawvj^'^^'^^T-^; 
or, more correctly speaking, ttie \o\\xai^> ^'^ ^^^^^"^^^ 



>s. 



74 SCIENCE PRIMERS. [S v. 

more than 1,300 times that of the earth, — that is, 1,300 
globes of the size of our earth, if made into one 
world, would only be of the size of Jupiter, — still its 
weight is only 300 times the weight of the earth, so 
that the materials composing Jupiter are of a much 
lighter kind than those composing the earth ; thus 
representing the density of the earth by i, Jupiter's 
density is less than \. 

Saturn. 

170. We next come to Satum, a truly grand sight In 
a telescope, Satum having, besides eight moons, an 
bright ring surrounding the globe. This 




/jJanet revolves in an orbit at about 872 millions of miles 
from the sun, taking 10,759 days, or neaily thirty of our 
years, to complete its year, and havinga diarocXei Twot 



ASTRONOMY. 75 



times greater than that of the earth. From observa- 
tions of spots and belts on the surface (sbraewhat 
similar to those on Jupiter) the time of its diurnal 
revolution has been fixed at about loj hours, a little 
longer than that of Jupiter, and it is probable that 
Saturn has much the same constitution as that planet, 
as it appears to us to be covered with an extensive 
cloudy atmosphere producing belts as on Jupiter; it 
is also made up of very much lighter materials than 
our earth is, materials of only half the density of those 
composing Jupiter. Saturn's axis is inclined at an 
angle of about 26^°, so there are seasons there as on 
our earth. 

171. Now as to the rings, what are they ? Their 
general appearance is that of three rings lying outside 
each other in succession as shown in the diagram, 
Fig* 36, the diameter of the outer ring being about 
1 66,000 miles. The two outer ones are the brightest, 
the inner pr crape ring being only just visible in 
a large telescope, the ball of the planet being seen 
through it In spite of their enormous breadth, the 
thickness of the rings is only about 138 miles, and 
when edgeways to us, as is the case in certain positions, 
when Saturn moves in its orbit, they are barely visible 
in the best telescopes. It is thought that the rings 
represent a vast assemblage of small satellites or 
moons revolving round Saturn. 

172. The moons of Saturn, eight in number, are not 
of such interest as those of, Jupiter. Their distance 
from us precludes us generally from observing their 
eclipses and occultations ; their orbits ako -ax^ Vax'^^ 
inclined to the orbit of Salwnx, ^xi^ corasfco^'^^^^:^ 

edipses are rare. 



76 SCIENCE, PRIMERS. [§ v. 



Uranus. 

173. We next come to Uranus, of which little is 
known, its distance — 1,753 millions of miles from the 
sun, being so immense ; it takes 30,686 of our days to 
complete its annual revolution, and it is known to 
have four moons. Its diameter is four times greater 
than that of our earth, and its density is about \ 
that of the earth. 

Neptune. 

174. Then comes Neptune, the most distant planet 
oi our system at present known, at 2,746 millions of 
miles from the sun, and taking 60,1 26 days to go round 
the sun. Its diameter is over four times greater than 
that of our earth, and its density is slightly less than 
that of Uranus. 

175. Its discovery is interesting as showing how the 
position, mass, and other attributes of a planet can 
be calculated by their effect on other bodies at a dis- 
tance before the planet has actually been seen. It 
had been noticed for a long time that Uranus moved 
at one part of its orbit slower, and at another, faster, 
than its proper rate, and from these observations 
the position, mass, period, &c. of the planet were de- 
termined before it had ever been seen, and it was found 
very close indeed to its calculated place. Neptune 
has only one moon at present discovered. 



ASTRONOMY. 



176. Besides the planets, there are Other members 
of our system, of a difi'erent kind. We may say that 
the planets are the members of the solar household ; 
the bodies we are about to consider are visitors. 

177. Those who have seen a comet will not require 
to be reminded of the strange appearance of those 
bodies, and those who 
have not seen one will 
get some idea of what 
this class of bodies is 
like from the diagram. 
Comets vary so much in 
form and size and bright- 
ness, that no two are 
precisely alike : some- 
times they resemble a 
small planet or star with 
a bright point called 
the nucleus, an im- 
mense tail stretching for 1 
millions of miles behind^ 1 
at other times they ap- 
pear with a nucleus with 
mist extending equally 

round it; in fact, then- •■■"■ jj— "="=■"■>•=">■■ av-ui!'"- 
shapes arc almost as various as those of the clouds. 
The greater number of comets are invisible to the 
naked eye. 

178. The majority of comets Ofta\, i::Q\v.t; '«*-'^ '=>"™ 
system from outside, are atttactedVowaii^'Or-*!*"*^-'^?* 




78 SCIENCE PRIMERS. [§ vi. 

by it, and then continue on away from our system 
again; while there are others that belong to om 
system, and revolve round the sun as the planets do, 
only instead of having nearly circular orbits, their 
paths are very eccentric, so that the comets approach 
near the sun at one time, and then recede to im.mense 
distances away. There are several such comets whose 
orbits are known, and these are called after their dis- 
coverers ; such as Encke's comet, which revolves 
round the sun once every five years, and Halley's, 
that has a period of about seventy-four years. 

179. The orbits of comets have very various, and 
some of them very great, inclinations, not like the 
orbits of planets, which all lie nearly in the same 
plane, the plane of the ecliptic; the majority go 
round the sun the contrary way to planets, and are 
said to have a retrograde motion. 

180. Their weight is excessively small, while their 
volume or bulk is immense — that of Donati, figured 
in tlie diagram, having a tail millions of miles long, 
through which faint stars, which a thin cloud or puff of 
smoke would obscure, were visible. As a comet ap- 
proaches the sun,, envelopes or jets are formed. 

181. Now, before I say anything more about these 
strange things, I must remind you that perhaps when 
you have been looking at tlie sky, you may have 
noticed a bright point, like a star, shoot rapidly 
across the heavens, leaving a bright streak for a 
second or two behind it. Several may generally b 
seen every fine night with a little attention. These 
are called meteors or falling stars, or, if they 

aciually fall, as some do, to the earth, tneteorites. 
TAcy vary greatly in apparent size ax\d Wx^Xxv^^'s 



ASTRONOMY. 79 

the smaller being most prevalent; the larger, called 
meteors, are rare, and sometimes appear as large 
and almost as bright as Jupiter or the Moon, and 
traverse the sl<y for some seconds, leaving a luminous 
trail behind thein. 

182. Now of course, as some of these bodies fall to 
the earth, the chemist can exanine them and find out 
what they are made of, as I 
he has found out what the I 
earth is made of Some I 
are especially metallic in \ 
their nature, others espe I 
cially stony As they rush I 
into our atmosphere they I 
are heated so hot that I 
the) burn, and the small I 
ones are consumed beiore 
they can reach the earth 
the larger, on the other 
hand, are not entirely con 
sumed, though melted on 
the surface and consider | 
ably reduced m size 
number of these that ha* 
escaped destruction are ( 
be seen in the Bntish '" comeL"""'"'" 

Museum some reaching the weight of three tons 

183 troni constant observation it has been found 
that on difterenU nights liie majority of shooting stars 
appear to come from certain jwris of the sk>, and 
on certain nights in the year many more fail ifcasi «^ 
others. There are, for instance, X\\e: ^eSv^^««^v tiSci 
of November 13 and August 10, t\\o^e. cS. ■^'^''*^^^**^^ 




8o SCIENCE F RIMERS, [§ vr. 

coming from the constellation Leo, and consequently 
called the Leonides, and those of August from Perseus, 
and called the Perseids. 

184. We now know that these meteors travel round 
the sun as the planets do, and the strange thing is 
that when we come to examine the shape, size, and 
position of their orbits, they are found to be the same 
as those of some of the comets; so that since some me- 
teorites and comets have the same path or orbit, it has 
been suggested that comets are clouds of meteorites. 
This hint of a connection between comets and meteor- 
ites is one of the greatest discoveries of late years in 
the science of astronomy ; and the observations on the 
beautiful comet visible in 1874 have shown that 
possibly- the heat and light of a comet may be due 
to the clashing together in space of these very bodies 
which, when they fall into our air, give rise to the 
appearance oi falling stars, for we know that comets 
are not very hot, that they do partly consist of solid 
particles or masses, and that the vapour given off is 
that of a substance known to exist in meteorites. 

185. Comets, from their sudden and curious appear- 
ance, were looked on with great awe by the ancients, 
and all kinds of calamities were attributed to them. We 
learn, for instance, that about the year 975 the Ethio- 
pians and Egyptians felt the dire effects of the comet 
to which Typhon, who reigned then, gave his name. 
It appeared all on fire, and was twisted in the form of 
a spiral, and had a hideous aspect It was not so 
much a star as a knot of fire. We thus see how 
science replaces the terror felt in past ages by an 

admiration of the wonders of the vmiverse in which 
•t'e drvelL 



ASTRONOMY, 8i 



IV.-^THE SUN-^THE NEAREST STAR. 

§ I.— THE INFLUENCE OF THE SUN IN THE 

SOLAR SYSTEM. 

1 86.' In what has gone before I have tried to show 
you what the Earth is — (I do not mean what it is 
made of; that you will learn in the Chemistry Primer : 
or what it is like — how its surface is one of land and 
sea, or how it is surrounded by an atmosphere — that 
you will learn in the Physical Geography Primer) — and 
we have found that it is a cool body travelling round 
the sun, and because it is cool it has no light of its 
own, its light being, as a matter of fact, borrowed 
from the sun. 

187. Next, I have shown you that it is one of several 
similar bodies travelling round the sun, which bodies, 
called planets, are cool like the earth, and as such 
they give out no light of their own. 

188. We have also seen that the length of the earth's 
year, and of the years of the other planets, depends 
upon the time each planet takes to go round the sun ; 
and further, that the length of the earth's day, and of 
the days of the other planets, depends upon the rate 
at which each planet spins round, and so brings each 
part of its surface into the sunlight. 

189. Further, we have seen how the inclination of 
the axis of the earth, and of that of each planet, de- 
termines the seasons, the change of which is chiefly 
due to the difference, at any one period of the year, 
between the time during which each part oC ^^^^sj^v. 
is exposed to the sun and t\\e Ivrci^ ^wtvxv^ ^\i\s^ ^^ 
is withdrawn from the sun's Vn^wetic^^ 



82 SCIENCE PRIMERS, [§ li. 

190. So that you see the sun has to do with every- 
thing. What, then, is this Sun, which occupies the 
central position round which all the planets travel, 
and which is so important to them that their very life 
as it were depends upon its rays ? » 



§ II.— THE HEAT, LIGHT, SIZE AND DISTANCE 

OF THE SUN. 

191. First, I have to tell you that you may regard the 
sun as a globe of the fiercest fire : the heat of the sun 
is so enormous that it is useless for me to attempt to 
give you any idea of it. Remember, I have already 
told you that the other planets, like the earth, are 
cool bodies ; that is, bodies on the surface of which 
various substances can exist in the solid state : hence 
we talk of the "solid earth." But on the sun nothing 
is solid, everything exists in the shape of white hot 
vapour. 

192. Next, I have to tell you that in consequence 
of this tremendous heat, the sun shines by its own 
light. Remember, I have told you that the planets 
and their moons (including of course our moon) do 
not. 

193. And lastly, I have to tell you that the sun is a 
globe of such enormous dimensions, that it is 500 
times larger than all the planets put together. If you 
were to take nearly \\ millions of Earths, and knead 
them into a ball, yorf would then have a globe about 
as large 2ls the sun. 

iP4, I have already told you thai iVve d\?»l^xvce of 
t/ie sun from us is about 91 ti\\\\\o\\s o^ rcv\\e?». '\q 



ASTRONOMY. 83 



go into the mode of measureraent would lead us too 
far into mathematics for my present purpose ; but it 
may be stated here that knowing its distance and 
apparent size, we can proceed to find its diameter in 
this way. Let us draw imaginary lines from either 
side of the sun to the eye, as AB and AC^ Fig. 39, 




-A_ P D B 

Fig. 39. — How the size of the Sun is determined 

CB representing the diameter of the sun, we find that 
the inclination of the two lines to each other is such 
that all lines drawn from one line to the other, as 
DE or FG^ are equal in length to ^\j of their dis- 
tance from A^ so also B C'\% y J^ part of the distance 
A B, which we know is 91 milHons of miles; divid- 
ing this by 107 we get 850,467, which is the distance 
from B to C, or the diameter of the sun in miles. 

§ III.— WHAT THE SUN IS LIKE. 

195. There are not many observations that can be 
made on the sun without the aid of a telescope and 
dark glasses, and its intense heat and light render it 
dangerous to look at it without special precautions.^ If 
you smoke a piece of glass over a candle, and look at 
the sun through it, it will appear to be a round bright 
object, because each part of it shines by its own light : 
unlike the moon, it is always round. This bright 
part is called the photosphere. In telesc-c^N^^'s^ 

' llie youn^ reader must not attem^Jt to \ooV aX. x^t sv^xv ^5cvTO\x^'a-«»a^ ^J^'^- 
5co;;c, for he or she may be bWndied vu \!tvG ^vxjcxcs^V' 



84 SCIENCE PRIMERS. f§ rv. 

black ^ots are frequently seen on its surface, and 
these, inaeed, are sometimes of sufficient size to be 
visible without the telescope. 

196. In the neighbourhood of the spots brighter 
portions than the general surface are seen : these 
are called faculae, and probably are immense banks 
of brighter vapours several thousands of miles long. 
If the spots and faculae be watched from time to time 
they will be found to be constantly changing their 
shape. 



§ IV. - SUN-SPOTS. 

197. Although the sun is so far away from us, 
in consequence of its immense size and the violence 
of the forces at work, these spots are fine objects in 
the telescope. I give a drawing of one (Fig. 40) so 
large that several Earths might have been hurled 
into it. 

198. If these spots be observed and their positions 
carefully noted, and again observed one or two days 
afterwards, they will be found to have changed their 
position towards the west, and they will be seen to be 
gradually moving from the east side of the sun's disc 
to the west, where they will gradually disappear. 

199. Now, since all of these have the same motion 
iu the same direction, it is evident that the surface 
of the sun is moving and carrying the spots with it, 
and if a well-marked spot be observed when passing 
off the disc to the west, it will be found about 1 2 days 

^/ler to appear again on the east side and get to the 
position nJjcre it was first observed m abouV ^^ ^3l>j^^ 



ASTRONOMY. 



85 



having in that time gone right across the disc and 
round the back. 

200. The surface of the sun has therefore moved 
round in 25 days, or in reality the whole sun itself 
is turning round on its axis at this rate, carrying spots 
and faculje with it. 

201. Let us now see what kind of thing a spot is. 
If ft pretty regular one is observed near the middle of 




the disc It appears round , if it be again observed a few 
daysafier, near the edge, it will appear no longer of the 
same shape, the darkest middle part having apparently 
moved to the left while the half shade round it has 
vanished. I^et us see what we can leaxw ^\aKi 'Ctos>. 
Take an ordinary saucer, and 'Vva.vmft ■yi'aK!*.«t\e.&. ^«-^ 
parf o{ it on which the cup getvexaVi-j ?,\mv&^- ^^^'^ 



86 



SCIENCE PRIMERS. 



[§v. 



Straight at it — you will see the black part equally sur- 
rounded by the sloping sides, as at A; now twist the 
saucer till it is seen more edgewise, and you will see 
the edge on the left hand quite disappear, while the 
right side is nearly flat in front of the eye, and it will 
have the appearance of C. 

202. Now, if a cavity like the saucer were cut on a 
large globe, it would go through just the same changes 
that we find the saucer and the spot do, so we may 
conclude that the spots on the sun are hollows in the 






Fig. 41. — Explanati 11 of the appearances presented by dun-spots. 

bright substance of the sun ; but it is found from 
other evidence that these hollows are not empty, but 
filled with gases stopping the light given out below. 



§ v.— THE SUN'S ATMOSPHERE. 

203. The round sun that we see is not all there 
is of the sun, but only the denser part of it ; the less 
dense and luminous vapours extend for hundreds of 
thousands of miles beyond the visible sun;, but 
generally we cannot see them any more than we 
can the stars; still, in Eclipses, when, as we have 
seen, the light of the sun is cut off by the moon, 
we can see them, as we can see the stars (Art. 
-'^^J' The luminous vapours then appeal o^ exc\u\^\\.^ 



ASTRONOMY. 87 

colours, red being most common. These vapours, 
however, get brighter nearer the sun, and form an 
envelope round him, called the Chromosphere, and 
these can be observed by a special method. It is 




then seen that the lighter vapoursof the real sun 
are shot up into its outer atmosphere, called the 
coronal atmosphere, taking fantastic shapes called 
prominences, aiid these prominences rapidly change. 

§ VI.— WHAT THE SUN IS MADE OF, 

204. By analysing the light of the sun by means 
of a spectroscope, an instrument that splits light up 
into its component colours, in the same manner 3£ 
you have seen light sp\U up '\tvXo sNi fea wJv.'wot*. *:^ 
the rainbow by the g\ass tVtovs o^ On«i*-^'*-'^'*"> "^ 



8S SCIENCE PRIMERS, [§ i. 



has been found that a great number of our metals 
exist in the sun, not of course in their metallic state, 
but in a state of vapour, the heat there being so 
intense that the metals evaporate as water with us 
does into steam. There are first of all, among the 
elements that we know here, the gas hydrogen, and 
then vapours 'of magnesium, calcium, sodium, iron, 
manganese, nickel, barium, strontium, and very many 
more metals, besides probably two other gases, not 
yet found on the earth. 

205. Since, as we have seen, the sun is so largely 
composed of gases^ you will not. be surprised that its 
density is much less than that of the earth ; indeed, it 
is less than a quarter of that of our planet 

* 

§ VII.— The sun is the Nearest star. 

t 

206. I have been careful to dwell at some length on 
what is called the physical constitution of the sun, 
not merely because in it we have an example of 
a class of bodies very unlike the planets, as we have 
seen, but because we now know that the sun is a 
star; bigger and brighter than the other stars, not 
because it is unlike them, but simply because it is so 
near to us. 

207. We can now, then, define the solar system to 
consist in the main of a number of cool bodies revolv- 
ing round a hot one. As we can take^the earth 
as a type of the planets, so we can take the 
sun as a type of the twinkling stars that 

people the depths of space ; and it is not too 
much to believe that every star is sutrowrvded \y^ \\& 
^mily of planets in the same way as tV\e s\\t\ \s. 



ASTRONOMY. 89 



v.— THE STARS. 

§ I.— THE STARS ARE DISTANT SUNS. 

208. From the sun — the nearest star — that gives us 
heat and light, we must now turn to the more distant 
ones. After what has been stated you will not be sur- 
prised at my turning from a large body like the sun, 
the beams of which are so hot, to those tiny specks 
of light distributed in the heavens, the heating power 
of which is imperceptible, since those little twinkling 
bodies are suns, giving out light and heat like our 
sun, only they are at such incredible distances from 
us, — the distance of some of the nearest stars is more 
than 500,000 times the distance of our sun,— that 
their size becomes inappreciable : we have, never- 
theless, reason for believing that many of them are 
several hundred times larger than our sun. 

§ II.— THE BRIGHTNESS OF THE STARS. 

209. When we look at the stars at night, one of the 
first things we notice is that they are of different 
brightnesses. Is it that some are smaller than others, 
or are the brightest the nearest to us? It -is difficult 
to say exactly, for in some cases the bright stars are 
nearest to us, and in others there are small ones as 
near, so that both size and distance come^nto play. 

210. Stars are classed in magnitudes according to 
their order of brightness, the brightest being said to 
be of the first magnitude, the next of the second 
magnitude, down to the fifteenth and svx.t^^^x^^Jcvs"'*^^^^ 
require the most powerful te\e?»co\)^ \.o n\^^ •C5c\je«s.. 
The faintest star visible on a daxV x^^^- ^s. 0I ^^^^ 



90 SCIENCE PRIMERS. [§ iii. 

the sixth magnitude. After what has been said you 
must not think that magnitude means real size, as a 
large star may be iax away, and so be classed so far as 
brightness goes with a smaller one nearer to us. 

211. There are about 3,000 stars from the first to the 
sixth magnitude visible at once to the naked eye, and 
there are over 20,000,000 visible in large telescopes. 

212. You may have also noticed, on a clear dark night, 
a zone, or band of faint light, stretching from the hori- 
zon on one side, nearly over our heads to the horizon 
on the other. This is called the milky way. It is 
composed of an almost infinite number of small stars, 
apparently so close together as to form a luminous 
mass ; and of the 20,000,000 telescopic stars, probably 
18,000,000 are in the milky way. A view of this 
gives us some little idea of the immensity of our 
universe, if we consider that it is not the real close- 
ness of the stars that we observe, but only their 
apparent closeness, placed, as they probably are, one 
almost behind the other so as to be in nearly the same 
line of sight, and at a distance from each other perhaps 
as great as that from our sun to the nearest star. 

213. If you suppose a wood in which all the trees 
are the same distance apart, and you place yourself in 
the wood near one side of it, the trees will appear 
nearest together on the other. So is it with the stars 
in the milky way ; there is the greatest number of stars 
in the line of sight. 

214. The colours of the stars are various, some being 
white, others orange, red, green, and blue. For in- 
stance, Sirius is white, Arcturus yellow, Betelgeuse 

red, but these colours are more noticeable with a tfele- 
scqpe than with the eye alone. 



ASTRONOMY. 91 



§ III.— THE CONSTELLATIONS. 

215. The stars have been grouped, as long as history 
carries us back, into constellations, each one of 
which received some fanciful name according to the 
being or object the stars composing it were thought 
to represent. The sun in his course passes over the 
zodiacal constellations, visible of course both in 
the Northern and Southern Hemispheres of the Earth. 
These are Aries, Taurus, Gemini, Cancer, Leo, Virgo, 
Sagittarius, Capricorn us, Aquarius, and Pisces, the Latin 
names for the Constellations, the order of which you 
will remember from the following rhyme : — 

** The Ram, the Bull, the Heavenly Twins, 
And next the Crab, the Lion shines, 

The Virgin and the Scales, 
The Scorpion, Archer, and She Goat, 
The Man that holds the watering-pot. 

The Fish with glittering scales." 

216. The constellations visible in the Northern Hemi- 
sphere above the zodiacal constellations, are called 
the northern constellations, they are as follows : 



Ui-sa Major. 


The Great Bear (The Plough). 


Ursa Minor, 


The Little Bear. 


Draco. 


The Dragon. 


Cepheus. 


Cepheus. 


Bootes. 


Bootes. 


Corona Borealis, 


The Northern Crown. 


Hercules, 


Hercules. 


Lyra, 


The Lyre. 


Cygnus, 


The Swan. 


Cassiopea. 


Cassiopea (The Lad"<j'?. Ocv^^V 


Perseus, 


Perseus. 


Auriga, 


The V^2Lg§piaet. 



1 



92 



SCIENCE PRIMERS, 



[§iv. 



Serpentxirius, 

Serpefis, 

Sagitta. 

Aquila. 

Delphinus, 

Equuletts, 

Pegasus. 

Andromeda. 

Triangulum. 

Camelopardalis, 

Canes VenaticL 

Vulpecula et Anser. 

Cor Carolu 



The Serpent-Bearer. 
The Serpent. 
The Arrow, 
The Eagle. 
The Dolphin. 
The Little Horse. 
The Winged Horse. 
Andromeda. 
The Triangle. 
The Cameleopard. 
The Hunting Dogs. 
The Fox and the Goose. 
Charles' Heart. 



217. The constellations visible in the Southern 
Hemisphere above the zodiacal ones, called the 
southern constellations, are : 



Cetus. 

Orion. 

Eridanus, 

Lepus. 

Can is Major. 

Cantj Minor. 

Argo Navis. 

Hydra. 

Crater. 

Corvus. 

Centauries, 

Lupus. 

Ara. 

Corona Australia. 

Piscis Ausiralis. 

Monoceros. 

Columba Noachi. 

Crux Australis. 



The Whale. 

Orion. 

The River Eridanus. 

The Hare. 

The Great Dog. 

The Little Dog. 

The Ship Argo. 

The Snake. 

The Cup. 

The Crow. 

The Centaur. 

The Wolf. 

The Altar. 

The Southern Crown. 

The Southern Fish. 

The Unicorn. 

Noah's Dove. 

The Southern Cross. 



21S, In order to learn the positions of the various 
constellatioDs and siox^ you will waul a slai-iaav ot 
Planisphere, and wiil also require some itieixd lo i^oVxvx. 



ASTRONOMY, 93 



out to you some of the chief constellations to begin 
with. I have indicated a few of these by Roman 
letters in the preceding lists. 

219. The stars in each constellation are known by 
the prefix of some letter of the Greek alphabet, the 
brightest being called Alpha (a), the second brightest 
Beta ()8), and then, when all the letters are used, they 
are numbered i, 2, 3 ; so we can refer to a star as 
Alpha (a) Lyrse, the brightest star in the constellation 
of the Lyre, or (/3) Cygni, the second brightest in the 
Swan, 61 Cygni, and so on, so that every star can be 
named. In addition to these names the principal 
stars have other names, thus (o) Lyrge is also called 
Vega, a Canis Majoris is called Sirius, a Bootis, Arc- 
Hirus, and so on. 



§ IV.— APPARENT MOVEMENTS OF THE 

STARS. 

220. We saw in speaking of the earth that it was 
only a moving observatory, and that therefore we must 
distinguish the real motion of external bodies from 
that of the body on which we dwell. We may now 
return to this subject. Let us compare the earth to 
a boat at sea ; imagine yourself in the boat ; then 
if it be suddenly turned round, all the ships in sight 
will, if you are ignorant of your motion, appear to go 
round you in the opposite direction ; but it would be 
highly improbable that all the ships in sight should 
do so at the same rate, keeping their relative posi- 
tions to each other, so that you would at once find 
out that your boat was movitv^^ ^.xA t^oX "^^ ^wssg**. 
Just so, as we have seeu lYve e^x\)cv wa^'^ x^ikXccA'* "«^^ 



94 SCIENCE PRIMERS, [§ v. 

not the stars round us, so the daily motion of the stars 
is only apparent. 

221. Now, let the boat be rowed round a ship. The 
relative positions of the ship and the distant craft 
change, the ship appears to move round you, passing 
between you and the other ships in succession. The 
same appearance woald be produced were the boat to 
remain still, and the distant ships to move round it, 
but you would at once detect that it was your own 
motion. Just so with our annual revolution round the 
sun, the sun apparently passes over the stars in 
succession, the stars which are in a line with the sun 
in summer being opposite to him in winter. 

22 2. In the early days of Astronomy these two ap- 
parent motions of the stars were the only ones known, 
and in order to ascertain whether the stars were really 
fixed maps of them were made, to be compared with 
the stars in the course of a few years, and from the 
comparisons made in this way no alteration of position 
was detected, so the ancients concluded that the stars 
were fixed ; hence the term " fixed star," but this we 
shall see was an error caused by the inaccuracy of 
the maps. ' 

223. When in after years a better method of fixing 
the positions of stars was invented, it was soon 
found that the positions of the stars were not always 
the same, and that this was occasioned by the 
poles of the earth changing the direction in which 
they pointed, just as a spinning top, before falling, 
whobbles; and so of course, as the positions de- 
j[>ended on the position of the earth^s axis they were 
found to be continually changing. Hete, tVven, is 
tuother aj^^are/if change in the posiUons oi v\\^ ^^ax^. 



ASTRONOMY. 95 



and this apparent motion gives rise to what is called 
the precession of the equinoxes. 

224. Now that astronomers are aware of this and 
other motions, they expect to find a continual change 
in the position of stars, which they can calculate 
beforehand, but if the positions of stars are found 
after a lapse of years not to correspond with the 
calculated ones, after allowing for all the known ap- 
parent motions, there must be some motion of the 
earth or stars which was not taken into account. 
But, before we go further, we will return to our boat 
and ship. 

225. Let the ship, and the boat you are in, ad- 
vance in any direction, what apparent changes will 
be produced in the ships on either side of you ? 
They will appear to move in the opposite direction ; 
those you approach will appear to get further apart, 
and those behind you will appear to close together, but 
the ships may all be moving as well as you, some in one 
direction, and some in another, so they all may not 
appear to move regularly according to our supposition ; 
but if there is a large number visible, you would expect 
to find more apparently moving according to our 
supposition than contrary to it, their apparent motions 
being counterbalanced in some cases by their real 
motions, and in others the two motions would be 
added to each other, so that you could judge of your 
own motion. 

226. This is exactly the case ; it is found that 
in one direction the stars have a tendency to close 
up, and in the opposite one to open. o\sX^ >&nssv^'!^^ 
like the ships, some c\ose up m \!cv^ ^Yt^ciCx^^ ^» 
which the majority open out axvd vict Xivr%^ \ ^^^ 



96 SCIENCE PRIMERS. [§ vl 

. : E 

observing the motion of a large number of stars we 
are able to find that the sun, and with it of course 
all the planets, are steadily progressing towards a point 
in the constellation Hercules. 

§ v.— REAL MOVEMENTS OF STARS. 

227. If you saw any ship moving among the others 
whose motion was not accounted for on the supposition 
of any motion of your boat, you would at once pre- 
sume that that ship had a real motion of its own. In 
like manner, when a star is found to move amongst 
the others^ then we can safely say it has a feal motion 
of its own; and by careful observation for a long 
series of years it has been discovered that a very large 
number of stars have what is called a proper motion. 
Arcturus, for instance, is going at about three times 
the rate that our earth does in its orbit round the 
sun, over fifty-four miles a second. From mechanical 
reasons it is probable that all the stars are in 
motion, 

§ VI.— MULTIPLE STARS. 

228. Not only have we such a proper motion along 
a path, but some stars go round each other. 
These take the name of double and multiple stars 
according as there are two or more moving round 
each other, as shown in Fig. 43. 

229. They are what is called physically connected 
with each other, being so close that one revolves 

round the other, just as we revolve round the sun, bul 
Instead of the revolution being pei^oxm^A vcv ^. ^^^ 



ASTRONOMY. 



the shottest known time of revolution or period of a 
double star is thirty-six years. Up to ttie present 
time some 800 of these systems have been discovered. 




Fic 43.— OrUt of > Double S»r. 

«30. Tlie distances of the stars from iis is so 
immeBae that if they had planets revolving round 
them dwae would be invisible with our most power- 
fal instmineiits. But it is probable that eM:h star is 
the centie of a planetary system : in the case fA close 
doobk itats, tiierefore, the planets of one star most l»e 
so near die other as to recdvc a consideiabic amount 
x& tight fenn It ; in fsct, the planets wobM 1iav« two 
suns, «nd, it) some cases, suns giving li^ of different 
coUnav 



8 VII.— CLUSTERS AND NEBULA. 

ajr. Besides the scattered stars of which we have 
been talkb^ there are a number of white patches in 
the sky like Ittfle pieces of the Milky Way, a few of 
which are visible to the naked eye. WWtv •Caeat ^^^ 
looked at n-iih a telescope, sowne oi "Ctve-vft. mc ^i*^ '^ 
be very closely packed chisVeTa o^ smsiX s'>.'M?.-,'«^ ^ 



9S SCIENCE PRIMERS. [8 vii. 

the separate stars are seen with telescopes of Tow 
power, while others require the highest telescopic 
means. Those in which the stars are easily seen; are 
called clusters, while those requiring high powers 
to see the separate siars, and those which still appear 




of a cloudlike stnictore when the most powerful 
telescopes are brought to bear upon them, are called 
nebulae. 

233. We may therefore divide these objects into 

three classes: (i) the clusters, in which the separate 

stars are easily seen gradually merging into (i) the 

j-esplvable nebulec ; and (3) the wresoVjaViVe 



ASTRONOMY. 



nebulae. The spectroscope has shown some of these 
latter to be of a nature different from stars or a col- 




lection of stars, and so vn lVi\s "fce^ wa ^!5^iS*sl ■^*- 
clusters. 



loo SCIENCE PRIMERS, [§ viii. 

233. Nor is this all : not only have we cloudlike 
masses which may be broken up into stars, and 
cloudlike masses which we know cannot consist of 
true stars, but some stars, when dosely examiiied, 
seem to be surrounded by a kind of fog, and these 
we know are not true stars. Such bodies are called 
nebulous stars. 

334. Both the star clusters and nebulae may from 
a different point of view be divided into two other 
classes : those which are very irregular in shape, like 
the Cluster and Nebula shown in Figs. 44 and 45, 
and those again which approach more to a globular 
form. 



§ VIII.— THE NATURE OF STARS AND 

NEBULA. 

235. I have before told you that the stars are dis- 
tant suns, but you are not to suppose that all of them 
are exactly like the sun ; indeed, we have evidence that 
they are not Among those whidi are very bright, 
some seem to have more simple atmospheres than the 
sun ; that is, they do not contain all the elements 
stated in Art 204 ; and among those stars which are 
dimmer, and especially among those the light of 
which is reddish, the atmospheres seem to differ in 
character fiom that of the sun, as ^^mark, I only 
say as (/^such stars were colder than the sun. 

236. Although the nebulae appear to be very dif- 
ferent from stars, it is possible tY\al lYvex^ as a. 'J^vy 

close connection between them, for \l V«s >c>^«tv 



ASTRONOMY. loi 



thought that stars are formed by the coming together 
of the materials of which the nebulae are composed, 
and that the planets are formed in the process. 
Whether nebulae are masses of glowing gas, or clouds 
of stones clashing together, and thus giving rise to a 
luminous appearance, we do not know but the latter 
view is the more probable one. 

237. The idea to which I have referred, which 
connects nebulae with stars and planets, supposes 
that a nebula in its first stage is continually getting 
smaller and rounder, and that when it has done so 
perhaps sufficiently to give rise to the appearance of 
a nebulous star, getting hotter all the time, it leaves 
behind it, round its equator, as it still contracts^ rings 
of vapour, something like the rings of Saturn (Art. 170) 
which eventually break and form a globular mass of 
vapour, which at last forms a planet All the time the 
centre is getting more dense and hot, and at last, the 
rate of contraction still diminishing, it shines out like 
a real sun, and thus goes on giving light and heat 
to those masses, now become cool and habitable, to 
which it originally gave birth. It thus shines, first, 
as a bright star, which afterwards becomes dim, and 
perhaps red, before the state of extinction is reached 
to which it must surely arrive; for, do not forget, 
that any one mass of matter must in time cease to 
give out light and heat, whether that mass of matter 
be a coal in a fire or a star in the heaven. 



IC2 SC/ENCE PRIMERS, \% ii. 



VI.~HOW THE POSITIONS OE THE HEA- 
VENLY BODIES ARE DETERMINED, 
AND THE USE THAT IS MADE OF 
THEM. 

§ I.— RECAPITULATION.— STAR MAPS. 

238. I must now approach a different branch of 
my subject. We have gone through the real motions 
of the earth, moon, and planets, and more recently of 
the stars, and the apparent motions brought about 
by the real motion of the earth. We have referred 
to the nature of nebulae, suns, and planets, and have 
thus got an idea of the Earth's true place in Nature — 
how it is a cool body going round a cooling star, both 
planet and star having probably resulted from the 
condensation and consequent heating of a nebula. 

239. I have also given you an idea of the starry 
heavens; how the stars — so-called fixed — ^have all 
been grouped into constellations, and lettered or 
numbered in the order of their brightness ; and how 
the sun by day, and the moon and planets by nighty 
are perpetually changing their places among the stars 
with the most perfect order and regularity. 

240. I have now to ask your attention to the starry 
vault, considering the stars merely as things the posi- 
tions of which have to be mapped ; and I want to 
show you, first, how positions are determined, and 
then what use we make of them. 

241, If you were clever enough, you Tt\\^\vlbe able 
to make a 5i:etch-map of the positions oi X5^^ sXacc^v 



ASTRONOMY. 103 



but for astronomical purposes the positions of the stars 
must be known with much greater accuracy than 
could be attained by such a rough attempt, and even if 
such maps were perfectly accurate it would be very 
troublesome to have to refer to a star as being south 
of, or below, a well-known star, and to the left, 
or west, of another; another method of fixing their 
places for reference has therefore been adopted. 

§ II.— POLAR DISTANCE. 

242. We imagine the equator and poles of our globe 
extended outwards to the stars, just as their shadows 
would be cast by a light at the centre of the earth 
on the imaginary hollow globe on which the stars 
appear fixed (called the celestial sphere). The 
shadow of the earth's equator thus becomes the 
celestial equator, and we measure north and south 
to it in degrees from the shadows of the poles, calling 
this <listance polar distance. 

243. In this way we can say which star or which 
part of the sky is exactly at the pole, because it will 
have no motion. Get your orange and stick a pin in 
it at each pole ; if you turn the orange round, the pin 
will still point to the same place. This, then, will be 
0° polar distance. Now, with a telescope furnished 
with circles, we can find this spot in the heavens, and 
turning the telescope 10'' from this spot (which we 
can easily do by means of the small circle fixed to 
it, because you have already seen that all circles big 
or little ate divided into 360**, A.x\.. \i^n^ ^^ ^'a::^^ ^^^s=^- 
mine ihost stars which hacve yo** ^c\'ax eCx^xaco^^^'^^ '^^ 
20% 2^0% and so on, tiW we covtve \.^ ^^ -» ^^^ 



104 



SCIENCE PRIMERS. 



[§ Hi. 



course marks the position of the Celestial Equator — 
that is, the line in the heavens which lies exactly 
half-way between the north and south poles, as the 
terrestrial equator does on the earth. 

§111.— POLAR DISTANCE IS NOT SUFFICIENT. 

244. In this way, then, we can determine the polar 
distance of all the stars ; but you will see at once that 
a multitude of stars may have the same polar dis- 
tance, for we can stick a whole row of pins in the 
orange, so that all shall be the same distance from 
the pole of the orange marked by another pin, 

245. It is necessary, then, to distinguish these apart 
somehow. Do not forget that the question is to fix 



kO_ 



c 5 

r- 



H 






M 



B 



Fig. 46. — How to define the position of anything. 



the position of a star. Now, to begin with, how 
^ould you fix the position of a dot on a piece of 
paper? Let us see. Take a sheet of paper ABCD^ 
Fig. 46, and stick a pin in or make a dot E ow 
it. Now let us see how we can state its position : 
divide the side AD into, say, 10 eqvia\ paiXs^ acoA 
^^jnto, say, the same number j tli«n on \o\t\\Tv\^ E G 



ASTRONOMY. 105 



and EF^ you will see that E is 4^ divisions from 
the line AB measured along ADy and is 2^ divisions 
from AD measured along AB, so we can fix the 
position to this point E at once with reference to the 
edges of the paper. So also if you were asked to place 
a dot at 7 divisions from AB and 6 from AD, you 
would drav/ a line HI from the seventh division on 
AD and another KL from the sixth division on AB^ 
then the point M where they cross will be the place 
required. 

246. Now mark well that it is not enough to say 
that E is 44 divisions from AB, because there might 
have been a whole line of pins or dots at that dis- 
tance from AB, and that it is not enough to say 
that E vs 2^ divisions from AD, because in like 
manner there might have been a whole line of pins 
or dots at that distance. 

247. Mark well also that the moment we have two 
sets of measures at right angles (you have not for- 
gotten, I hope, what that means) to each other, we 
can state the position of a pin or dot on our piece of 
paper with the greatest accuracy. 

248. So it is with the stars. I have already made 
you acquainted with one set of measures, that which 
begins at the poles and measures the distances of the 
stars from the poles, or, what comes to the same thing, 
the distance from the equator, because when we know 
the number of degrees a star is from the pole, the dif- 
ference between that number and 90® will give us the 
distance from the equator, as of course the equator is 
90° from each pole. In the next diagram, Fig. 47 ^ I 
have drawn the equator and sUix^X. \\xv^^ vc? -^^-wx 
between it and each pole. 



SCIENCB PRIMERS. 



e IV.— RIGHT ASCENSION. 



249. Eridently therefore, to make our statement o\ 
a star's position complete, we want another line at 




riglit angles to these. Now get your orange, and 
stick a row of pins in it all round to mark the equa- 
tor A B Fig, 47. Next, stick another row of pins in 
at right angles to the first row CD. This second row 
will take the shape of a second circle of pins, passing 
over the poles of the orange, and cutting the equator 
in two opposite points. 

a^o. Now the equator, and the row of pins 
which represents it, can on\y be to cme \i\acft <m 
the omnge, that is half-way between t\\« t«o v***- 



ASTRONOMY, 107 



But you may make the second circle wherever you 
ciioose, and in fact you may suppose an infinite 
number of such circles, all of them at right angles 
to the equator, all cutting it in two opposite points, 
all passing through the poles ; of course we can 
imagine them 1° or 10°, or any other number of de- 
grees apart; if we imagine them to be 15° apart, then 
as the heavens appear to revolve round the earth in 
24 hours, one of these circles will pass over a place 
on the earth every hour, because 15° X 24 = 360"^. 

251. But we have not yet got over our difficulties. 
All these circles are alike ; we must therefore choose 
one to measure from, to represent the equator, as it 
were. You will perhaps think that the first will be 
made to pass through the brightest star. This is not 
so ; one of the two points of the celestial equator 
which lies exactly in the plane of the ecliptic (Art. 67) 
is chosen. This point is called the first point of 
Aries. 

252. This being determined on, all the astronomer 
has to do is first to regulate his clock so that the 
stars shall appear to travel round the earth in exactly 
24 hoiurs ; to let it show o^ o"* o", when this imaginary 
circle, which passes through the first point of Aries, 
passes what is called the meridian, that is a fixed 
imaginary circle passing from north to south overhead, 
and to note the time when each star also passes it. 
As each star, whatever be its polar distance, passes 
this line, the clock, if it goes correctly, will show its 
distance in time from the first point of Aries. Thus 
we say that the Right Ascension of tha \ycv^\&^ ^Svas. 
(a) in the Bull is 4^ 28"* •, oi lYi^ W\^\\.^^\. ^^--^^^ '^ ^^ 

Virgin, i^^ iS"^, and so on. 



Io8 SCIENCE PRIMERS. [§ vi. 



§ v.— RECAPITULATION. 

253. If you have understood this you will know 
that the place of a star is stated or defined : — 

First — By its distance in degrees from the pole. 
This is called its polar distance ; from which (as 
stated in Art. 249) we can easily determine its dis- 
tance from the equator, called its declination. 

And Secondly — By its distance in time from the 
great circle which passes through the first point of 
Aries. This is called its Right Ascension. 

254. The positions of all stars have thus been 
determined, and further, we can calculate what 
position among the stars the sun, moon, or 
any of the planets will occupy at any instant 
of time. 

255. This is one of the most useful results of As- 
tronomical Science, for it enables us to map the sur- 
face of the earth, and also enables the traveller in the 
trackless waste, or the mariner out of sight of land, 
to find out exactly where he is on that surface. 



i YL^THB LATITUDE OP PLACES ON THE 

EARTH. 

256. Let us see then how we can fix the position of 

any place on the earth. If you were asked to tell 

anyone where a neighbouring town or village was, 

you would probably say so many miles away, and 

itJoDg a, certain road, or in a certain direction, say 

S W. of your hou^^ This aj^^w^TS vwj 7?^\itav ^oix 



ASTRONOMY. 109 



distances, but it would never do to refer all places to 
this distance and direction from your house, or from 
any other one place. If the earUi were flat we could 
use the method referred to in Art. 246, but as the earth 
is not flat, we do this ; we measure from the equator 
towards the pole in either hemisphere, and if you 
refer to a globe you will see that there is a number 
of circles drawn at equal distance apart between 
the poles and the equator. These circles are called 
parallels of latitude. 

257. Remember, that the positions of the heavenly 
bodies have been determined with reference to the 
earth's pole and by means of its rotation. Now, if you 
will think a little, you will see that if there were a star 
known to be of o** north polar distance, that star would 
be exactly over your head if you were at the north 
pole, and therefore you would know you were 
at the pole ifthat star appeared fixed exactly 
over your head. If there were a star known^to-be.. 
of 90° polar distance, that star would be exactly over 
your head if you were at the equator ; and therefore 
you would know that you were at the equator 
if that particular star passed over your head. 

258. Similarly, for any place north or south of the 
equator, we can determine the distance in degrees of 
that place from the equator, by observing whidi star, 
or other heavenly body the declination (Art 253) of 
which is known, passes overhead. And this is the 
meaning of the equator, and of the circles parallel to 
it, you see in maps and globes. An observation, the 
principle of which I luu^e stated, must have been 
made before the positions of ^xv^ ^^.c^esi n*^^ N-^^. 
down. Thus, in maps, yovi m\\ ivtv^ ^^ esssNasx^ci^ ^^ 



no SCIENCE PRIMERS. [§ vii. 

- ' ^ " I "" - ■» ■ .1 ■ ■ I ■—■■■■.■ I ■ —-..■■ ■■ . ■■ ■ ^ ■ I ■ , ■■ ■ — 

London from the equator shown as 51^** N., be- 
cause the star y Draconis, with a north declination 
of 51 J°, passes exactly over London. 

259. This distance from the terrestrial equator Is 
called latitude, the distance from the celestial equa- 
tor being called declination (it is a pity that the same 
word is not used for both), and we have of course N, 
and S. latitude, as we have N. and S. declination. 

260. The latitude of a place can also be determined 
by the apparent altitude of the pole star above the 
horizon, just in the same way as the rotundity of the 
earth is determined. The observer at the equator sees 
the north polar star on his horizon, its altitude is then 
o**, but if he goes about (iZ\ miles north it is 1° above 
his horizon, his latitude is said then to be t°, and so 
on, gradually increasing up to 90° at the poles. So if 
we at any place, or time, measure the altitude of the 
pole star, we at once get our latitude and can then fix 
our position on a map or globe. 

261. We have imagined such a pole star for these 
observations for the sake of simplicitv, but in reality 
there is no star absolutely at the pole, what is called 
the pole star being about ij° from it, so that allow- 
ance has to be made for this. 

262. It will be clear to you that, for the same reason 
that a large ntimber of pins on your orange can be at 
the same distance from the pole of the orange, and a 
large number of stars may have the same polar dis- 
tance, so a large number of places on the earth may 
have the same latitude. Thus, Naples has nearly tha 

same latitude as Pekin and New York. 



ASTRONOMY. m 



§ VII.- THE LONGlTtTDE OF PLACES ON THE 

EARTH. 

263. To determine finally, then, the position of a 
place on the earth's surface, we want something else 
which shall do for the earth what right ascension 
docs for the heavens. This something else is called 
longitude. 

264. To accomplish this, geographers imitate astro- 
nomers ; they imagine a circle belting the earth, cutting 
the Terrestrial Equator, at right angles, at two oppo- 
site points, and passing through the poles of the earth; 
and they measure from this circle. 

265. You will naturally ask where this is. It really 
does not matter where this start-point is taken; so, 
as a matter of fact, each principal nation of the 
world uses a different one, taking that which passes 
along the spider line which marks the centre of one 
of the chief instruments in the Central Observatory. 
In England, for instance, we reckon from the circle 
which passes through the Greenwich Transit Instru- 
ment. In America they reckon in the same way from 
Washington Observatory; in France from the Paris 
Observatory, and so on. 

266. The next question is, ?iow do they measure ? 
The position of a place on the earth, east or west of 
the circle which passes through the real Greenwich, 
is determined in exactly the same manner as the 
position of a star is determined east ot ^^^'sJ^ ^^ ^^c^ 
circle which passes throug\i xVie vca^Tv^T^ fe^x. \««^ 

of Aiim^ It is. a questioa ot txTcwe* 



112 SCIENCE PRIMERS, [§ vii. 

267. To prove this, let us again use the orange 
and knitting-needle. Represent the circle passing 
through the poles and Greenwich by a row of pins. 
Let each pin represent an observer with a watch 
showing the time of the Greenwich clock, and let 
one of them represent the observer at Greenwich; 
let a candle or lamp represent a star, and rotate the 
orange from west to east, as shown in Fig. 9, to repre- 
sent the motion of the earth. The line of pins will 
all come between the candle and the knitting-needle 
at once. Therefore, all the watches of our imaginary- 
observers will note the passage of the imaginary star 
at the same moment. 

268. So that all places exactly north or south of 
Greenwich will have the same start-point of time as 
Greenwich itself; in other words, they will have the 
same longitude. 

269. Now take out the pin representing Greenwich, 
and put it to the west of the row of pins. As the 
orange must still be moved from west to east, it is 
clear th^t this pin will come between the lamp and 
the knitting-needle after the row has passed; that 
is, there will be a ctiflf)&i««ce in the times at which 
the row of pins and the solitary pin- pass the lamp, 
since all the watches are set to Greenwidr tiipe. 
Let us suppose that at the row of pins the Greenwich 
lime is i** ; then it is clear, that as the pin representing 
Greenwich passed under the lamp afterwards, the clock 
at Greenwich itself indicated some time after I**, let us 
say it was 2**. Then there is a time difference of one 

hour between the two places, and all the places of 
the same longitude represented by t\\e low ol ^\xi'& 
»'/// be shown to the east of GreeixwVcYv, 



ASTRONOMY, 113 



270. Now let the lamp represent the sun. The sun 
brings local time to a place, because it is 12 o'clock 
(near enough for our present purpose) at a place when 
the sun is south or crosses the meridian at midday. 
If therefore I have this local time and Greenwich 
time as well, I can tell first whether I am east or 
west of Greenwich, and then how far east or west. If 
when with me it is 10 a.m. it is 12 (noon) at Greenwich, 
then I am situated to the west of Greenwich, and the 
earth must turn for two hours before I am brought 
under the sun; if it is 2 p.m. with me when it is 12 
(noon) at Greenwich, then I am to the east of Green- 
wich, as I passed under the sun two hours ago. Such 
a difference of time of 12 hours = 180°; of 6 hours 
= 90° east or west ; of 3 hours, 45° east or west, and 
so on; so that it is immaterial whether we reckon 
longitude in degrees or hours, for since there are 360 
degrees or 24 hours into which the equator is divided, 
each hour corresponds to 15°. We also express the 
longitude of a place by its distance east of Green- 
wich in hours, so instead of calling a place twenty- 
three hours west it is called one hour east. 

271. In practice a difficulty arises in finding out at 
a distance from Greenwich the exact time at Green- 
wich. A great number of ways have been tried, in 
order to let it be known at one observing station what 
time it is at the other. Rockets have been sent up, 
guns fired, fires lit, and all kinds of signals made at 
fixed times for this purpose ; but these, of course, 
only answer for short distances, so for long one^ crix^- 
fully-adjusted chronometers \\a.d \,o \>^ c-arcv^^ Sxo^sv 
one station to the other, to coxwey X)tv^ corc^^^ ^ ^^ 

but DOW, when telegraph wires axe \av^ ixowv ^tv.^^ 



1 14 SCIENCE PRIMERS. [§ i. 

to another, as from England to America, it is easy to 
let either station know what time it is at the other. 
For ships at sea chronometers answer well for a short 
time, but they are liable to variation. 

272. There are certain , astronomical phenomena 
whose instant of occurrence can be foretold, and 
which occur so far away from the earth that they are 
visible over a great part of its surface at the same 
moment of time ; these are published in the Nautical 
Almanacs, such as the eclipses of Jupiter's moons, and 
the position of our own moon. Suppose that an 
eclipse of one of Jupiters moons is to take place at 

1 P.M. Greenwich time, and it is observed at a place 
at 2 P.M. of their local time, /.<?., two hours after the 
sun had passed the meridian, then manifestly the 
clock at Greenwich is at i p.m. while theirs is at 

2 P.M., and the difference of local time is one hour, 
and the place is one hour or 1 5° east of Greenwich. 
If, however, the eclipse was observed at 12 noon, 
then the place must be one hour west of Greenwich. 



VII.— WHY THE MOTIONS OF THE HEA- 
VENLY BODIES ARE SO REGULAR. 

§ I.— WHAT WEIGHT IS. 

273. We have just seen that the stars are so useful 

to man because we can exactly calculate in what part 

of the heavens they will be at any future time. Now 

of course if their motion or our motion were irregular, 

this could not be done. Before I complete my task 

thea I must attempt to explain to you \vov« Sx *\^ >3BaS, 

«^e are enabled to foretell the movemeTiXs* 



ASTRONOMY. ii; 



274* This brings us to the more mechanical part of 
Astronomy, the laws of the motions of the heavenly 
bodies. The ancients believed the earth to be at 
rest and the sun and planets to revolve round it. 
This idea, however, gave way for the correct one 
which has been stated, and then came the question, 
Why do they so revolve ? It was first supposed that 
the planets were carried round in a vortex or whirl- 
pool of some kind ; and it was afterwards shown that 
the planets revolve round the sun and the moons 
round their primaries, not exactly in circles, but what 
are called ellipses, having the sun not quite in the 
centre. Newton showed that on mechanical princi- 
ples they ought to do so, and I must try to show you 
why. 

275. You have doubtless often seen a ball or stone 
. thrown up in the air and fall again to the earth. 

Did you ever ask yourself the question, why does 
it fall? Probably not; but if you were asked you 
would probably answer, " Because all things that are 
heavy fall to the earth ; " and so you would get out of 
the difficulty, but only to get into another. Why are 
things heavy ? is the next question. The ansAver is, 
that all substances attract each other in the 
same manner as a magnet attracts iron ; so 
one stone attracts another stone, but with very small 
force, and the earth being an immense mass of 
different substances attracts all things on it with such 
a force that the attraction of one stone on another 
is inappreciable in comparison. 

276. The weight or gravity therefore. q>1 -Kxx^'-^iwxw^ 
only means the force with wlcvvcV^ Vh^ ^-axXics. ^xxx-^cxs. >x^ 

md causes it to gravitate to\vaids \\s^'t% 



n6 SCIENCE PRIMERS. [§ ii. 

277. Now the attractive power of bodies is in pro- 
portion to the amount of matter they contain. For 
instance, if the earth were doubled in size, still being 
made of the same materials, it would attract every- 
thing on it with double the force it now does, and 
consequently everything would weigh double its pre- 
sent weight — so that then our legs would have to 
carry as much weight as if there were a person on 
our back continually. Also if we double the quantity 
of matter attracted by the earth, the force with which 
it is attracted, or its weight, is also doubled. For 
instance, a pint of water weighs one and a quarter 
pounds, two pints therefore weigh two and a half 
pounds. 

278. I have before (Art. 135) made use of the words 
quantity of matter or mass. A pint of lead 
contains a greater quantity of matter or has a greater 
mass than a pint of water, and the word mass is 
practically only another word for weight so long as we 
are on the earth ; but a pound weight here would 
weigh over two pounds at Jupiter, although the quan- 
tity of matter or mass is unchanged. So in dealing 
with the weights of bodies under different attractions 
we must use a word expressing a constant quantity of 
matter. 

279. If our earth were doubled in size, a pound 
weight would still balance another pound weight 
in the scales, for both would have their weights 
increased really to two pounds ; so we must use some 
other means to determine any alteration of the force 
of gravity. 

280. A spring can be arranged so as to answer the 
purpose, as it is not altered in any way \iv gKaN\V} sXiMt 



ASTRONOMY. 117 



the most accurate method is to ascertain the 
distance through which a body falls to the earth in a 
certain time, usually one second^ since the greater the 
attraction the quicker will be the fall \ at the surface 
of the earth a body will fall, in a vacuum or space 
without air to resist it, 16 feet in one second, and at 
the end of that second its velocity is 32 feet a second, 
— that is, if the force of gravity ceased at the end 
of the second it would go on through 32 feet in the 
next second. 

281. The force of gravity at the earth's surface is 
therefore represented by 32. On the surface of Jupiter 
the force of gravity is 2|- times that of our earth 
and is represented by 78, meaning that in one second 
a body allowed freely to fall would attain a velocity 
of 78 feet a second. 

§ II.— GRAVITY DECREASES WITH DISTANCE. 

282. I have already told you that the weight of 
anything on the earth means the force with which che 
earth attracts it. I have now to add that this force 
is not the same for bodies at different distances from 
the earth. 

2^3. Those of you who have had a magnet in your 
hands have probably noticed that pieces of iron are 
attracted the more strongly the nearer they are to the 
magnet ; this is easily seen by laying a needle on the 
table and sliding a magnet towards it, when you will 
see that at a distance of a few inches the needle v^ 
not attracted with sufficient iote^ \.o os^^^qvsnr. "Csn^ 
iricixon of its rolling on tV.e tabX^, ^xA ^^ xsN^^i?^'^^ 



Ii8 SCIENCE PRIMERS, [§ in. 

has to be moved nearer to it until the force is suf- 
ficient to overcome the resistance, when the needle 
rushes to the magnet. 

284. It is just the same with gravitation, the further 
a body is away from the earth the less it is attracted; 
and Newton found that the force of gravity at double 
the distance was not half, but half of a half, or one 
quarter; at three times not a third, but a third of a 
third, or one ninth, and so on ; so if the distance be 
increased to eight times, we have to multiply eight by 
itself, or what is called square it, making 64, and 
placing I over it, making -^-^ showing that the attraction 
at eight times the distance is only one sixty-fourth of 
what it was originally. 



§ III.— HOW THIS EXPLAINS THE MOON'S 
PATH ROUND THE EARTH. 

285. Newton tested this by the motion of the moon 
in the following manner : The moon, as we have 
already found, revolves round the earth ; but we have 
not seen yet why it should do so. Now, however, we 
are prepared to find that it is held in its nearly circular 
orbit by the attraction of the earth acting on it as a 
sling does on the stone, preventing it from flying off, 
as it would do if the string of gravity were cut, just as 
the stone flies away in a straight line when the sling 
is released. 

286. Let us consider this with the help of the 
diagram, where E represents the earth and MB A the 
orbit of the moon ; and let us suppose the moon to 

beatA/y then if gravity ceased to act,t\i^Tcvocrji\io\3\!iL 
conf:mue on in the same straight line l\val \\. v^^i^mowvcv^ 



ASTRONOMY. . 



119 



in at the time gravity ceased to act, and would go on 
towards N; and in one second it would get, say to J/*, 
but by the action of gravity we find the moon actually 
at B, showing that the earth's attraction has had the 
effect of drawing it from M' to B, and since we know 
the dimension of the moon's orbit, it is only a matter 
of calculation to find the distance from j\I' to B through 
which the earth draws the moon in one second, which 
is a little under one-eighteenth of an inch. 




A 



Fig. 48.— The fall of the Moon towards the Earth. 

287. Let us see if this fact falls in with Newton's 
idea. What distance ought a body to fall, or be at- 
tracted through in one second, at the distance of the 
moon ? The moon is 240,000 miles from the earth 
roughly, and the surface of the earth is 4,000 miles 
from its centre, at which point we can consider the 
whole attraction concentrated, and 4,000 into 240,000 
goes sixty times, so that the moon is just sixty times 
further from the earth's centre than the surface is \ -assAw 
the attraction there should be sv^X.-^ Xxecv^'s* i>»^^, ^'^ 
3,600 times less at the moon's d\s\.axvc^s\iNi^-'^^^^f^ 
of gravity at the surface of t\ie eaxxX-v 'v3» ^^^^ 



120 SCIENCE PRIMERS. [§ iv. 

bodies fall sixteen feet a second, so at the distance 
of the moon they should fall ^-^jj of sixteen feet, 
or one-eighteenth of an inch, which as we have seen 
is the observed amount 

§ IV.— THE ATTRACTION OF GRAVITATION. 

288. In this way Newton discovered that the very 
same force that draws a stone to the earth, called 
the attraction of gravitation , keeps the moon in her 
path round the earth. Nor did the discovery end 
here, he showed that the earth and all the other planets 
were thus kept in their orbits round the sun ; and that 
the same law of gravitation holds good with the most 
distant star. All the apparently irregular motions 
of the heavenly bodies have been reduced to law and 
order by Newton, who showed that all the motions were 
really regular, and therefore could be calculated before- 
hand. He thus enabled mankind not only to admire 
the divine beauty and harmony of the universe in 
tvhich we dwell, but to make use of the motions of 
the heavenly bodies for purposes of daily life. 



THE END. 



L^PtticMf X, C/ajf, Sonif nnd Tay\or\ Prini^tt,