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THE LIBRARY
OF
stud
t THE UNIVERSITY
can
OF CALIFORNIA
obsen
faculty
comm
tion. PRESENTED BY
PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID
LOCKYER'S ASTRONOMY.
ELEMENTS OF ASTRONOMY:
Accompanied with numerous Illustrations, a Colored Repre-
sentation of the Solar, Stellar, and Nebular Spectra,
and Celestial Charts of the Northern
and the Southern Hemisphere.
By J. NORMAN LOCKYER.
American edition, revised and specially adapted to the Schooli
of the United States.
izrrtff. 312 pages. Price, $1.75.
The volume is as practical as possible. To aid the student
in identifying the stars and constellations, the fine Celestial
Charts of Arago, which answer all the purposes of a costly Atlas
of the Heavens, are appended to the work this being the only
text-book, as far as the Publishers are aware, that possesses this
great advantage. Directions are given for finding the most in-
teresting objects in the heavens at certain hours on different
evenings throughout the year. Every device is used to make
the study interesting; and the Publishers feel assured that
teachers who once try this book will be unwilling to exchange
It for any other.
D. APPLETON & CO., PUBLISHERS,
549 & 55 1 BROADWAY, NEW You?:.
SCIENCE PRIMERS, edited by
PROFESSORS HUXLEY, ROSCOE, and
BALFOUR STEWART.
ASTRONOMY.
\
A Lunar Crater.
mners.
ASTRONOMY,
BY
J. NORMAN LOCKYER, F.R.S.,
A
Correspondent of the Institute of France,
Author of "Elementary Lessons in Astronomy," &*c.
WITH ILLUSTRATIONS.
NEW YORK:
D. APPLETON AND COMPANY,
549 AND 551 BROADWAY.
1877.
L(o
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. U
M363157
CONTENTS.
INTRODUCTION f 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 Sun 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 unequal . . , 26
10. The Seasons depend upon the difference in the
lengths of the Day and Night 33
1 1. Why the movements of the Sun and Stars appear
different in different parts of the Earth . . 35
II. THE MOON AND ITS MOTIONS.
i. The Moon travels among the Stars .... 40
2. The Moon changes her form 42
3. How the Moon causes Eclipses 45
4. What the Moon is like 53
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 ... 60
4. The Interior Planets 62
5. The Exterior Planets 66
6. Comets, Meteorites, and Falling Stars ... 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 Nebulae 97
8. The nature of Stars and Nebulae loo
CONTENTS. xi
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 not sufficient 104
4. Right Ascension 106
5. Recapitulation ic8
6. The Latitude of Places on the Earth . . . . 108
7. The Longitude of Places on the Earth . . . in
VII. WHY THE MOTIONS OF THE HEAVENLY BODIES ARE so
REGULAR.
I. What Weight is 114
2. Gravity Decreased with Distance . . . . .117
3. How this explains the Moon's path round the
Earth 118
4. The Attraction of Gravitation . . . . . .120
LIST OF ILLUSTRATIONS.
PAGE
Plate I. Frontispiece. A Lunar Crater . . . To face Title
,, 2. The Solar System Between 60 & 61
Fig. i. How ships become visible and disappear at sea . 4
,, 2. Explanatory of the above 5
,, 3. Diagram showing how, when we suppose the
earth is round, we explain that ships at sea
appear as they do 6
M 4. Diagram explaining how it is that the higher we
go the further we can see 7
,, 5. Diagram 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 , S
6. Explanation of sun-rise and sun-set, and star-
rise and star-set 1 1
,, 7. The same 12
,, 8. A top spinning 14
,, 9. The direction of the earth's spin 14
,, ro. Experiment to illustrate the spinning of the earth,
as causing day and night 1 5
,, ii. Explanation of the earth's motion round the sun 19
,, 12. The plane of the ecliptic 24
,, 13. Two planes cutting each other at right angles . 25
,, 14. Two planes cutting each other obliquely ... 25
LIST OF ILLUSTRATIONS.
PAGB
Fig. 15. Earth with axis of rotation inclined to plane of
ecliptic 26
,, 1 6. The Earth, as seen from the Sun at the Summer
Solstice, June 22 (noon at London) .... 29
,, 17. The Earth, as seen from the Sun at the Winter
Solstice, Dec. 22 (noon at London) .... 30
,, 18. The Earth, as seen from the Sun at the Vernal
Equinox, March 22 (noon at London) ... 31
,, 19. The Earth, as seen from the Sun at the Autumnal
Equinox, Sept. 22 (noon at London) ... 32
,, 20. Explanation of the Seasons . 34
,, 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
9J 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
33. Mars, View of another part of the planet . . 69
LIST OF ILLUSTRATIONS.
PAGE
Fig. 34. Jupiter, showing the cloud belts 71
,, 35. 'Diagram explaining the eclipses, occultations, and
transits of Jupiter's satellites 72
,, 36. Saturn and his rings 74
,, 37. General view of a Comet 77
,, 38. Head and Envelopes of a Comet 79
,, 39. How the size of the Sun is determined ... 83
,, 40. A Sun-spot * -85
,, 41. Explanation of the appearances presented by
Sun-spots 86
,, 42. The Sun's coronal atmosphere 87
,, 43. Orbit of a Double Star 97
,, 44. The Cluster in Hercules ...,,.,, 98
,, 45. The Great Nebula in Orion 99
., 46. How to define the position of anything . . . 104
47. How the positions of stars are stated . . . .106
48. 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 counties of Middlesex
and Surrey.
3. Just as a county is a collection of parishes, so
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 the next paragraph, write down the
School,
Street,
Parish,
County,
Country,
Kingdom,
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 the surface of The Earth, the surface
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, ) ln whlch ^ are '
Kingdom,
Continent,
Planet, /
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 oft
Geography begins ; that just as the shape, and size, and
position of your school, which is a little 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 PRIMERS.
I. -THE EARTH AND ITS MOTIONS.
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 shrubs
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
FIG. i. How the ships appear and disappear at sea.
way, the earth might be a nearly flat surface of large
extent.
ASTRONOMY.
10. 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 masts 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.
11. 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
FIG. 2. Orange with flies.
when nearer each other ; but one part of the fly will
not disappear, the other parts being left visible, as in
SCIENCE PRIMERS.
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 JB's head over the edge of the orange,
and C can see the top of A'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 the 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 further away
is 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
FlG. 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 don't understand how such
an irregular surface can be spoken of as part of a
curve." I must try then to answer these questions.
SCIENCE PRIMERS.
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 off in the case of the
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 height 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 from surface to
surface through the centre would be 8,ooCk *niles long.
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 mountain
four miles high will only be the one-thousandth part
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 orange, 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.
10 SCIENCE PRIMERS.
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 disappearing just in the
same way ; the moon also follows their example. In
ASTRONOMY. 11
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
FIG. 6. Explanation of sun-rise and sun-set, and star-rise and star-set.
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 pin ; at
one point the pin will be seen just rising from the
edge of the ball ; you are playing the part of a rising
sun or s'tar, to your own eye represented by the pin's
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
the sun and stars travelled round it. But we now
know that it is the earth which moves.
ASTRONOMY. 13
IV. THE EARTH SPINS 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 01
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 sun
and moon and stars again.
36. How then can we explain the facts ? We can
imagine that the earth spins round as a top
SCIENCE PRIMERS.
[iv.
does, so that every morning every boy and girl,
whether living in England, or America, or Australia,
FIG. 8. A top spinning.
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
FIG. 9. The direction of the earth's spin.
and night are the best proofs that the earth does
really spin 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. 10. Experiment to illustrate the spinning of the earth, as causing day
and night.
its head shall represent an observer on the earth.
Twist the needle round, and so make the orange turn
16 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 ly
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 risen, and come to the
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 take twenty-four 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 while 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 the rotation or spinning
round of the earth on its axis must bring alternate
light and darkness to every country.
48. You must not suppose that there is any actual
rod passing through the earth to represent our knitting-
needle and the steel rod of the school-globe, to form
1 8 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 him.
There must always be a bright side and a dark side,
just as there was a bright side and a dark side when
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 surface 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
th'e 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
way in which the stars one by one rise and set, as
the sun rises and sets in the daytime.
ASTRONOMY.
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
TABLE
WALL C
FIG. ii. Explanation of the Earth's motion round the Sun.
And lastly, that without this spinning there could
be no day and night, so that the regular succession
of day and night is caused by this spinning.
20 SCIENCE PRIMERS. [ vi.
52. Here then we have fairly proved that the earth
has one motion. Now the question comes, has it
more than one? 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 must 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 will 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 to represent the earth, and the lamp
to represent the sun, that part of the orange turned
to the sun, represented by the lamp, will have per-
petual day, and will always see the same \ gun \
in the same place ; from that part of it turned away
from the sun the same | ^ r j will always be
visible in the same place. From the parts of the
near the boundarv of ll s^ and shade
***>* features} will be for ever ap-
ASTRONOMY.
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 we 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
different stars. Here then is a great change in six
months.
(2). If we view the stars for many nights in succes-
22 SCIENCE PRIMERS. [ vn.
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. n, 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.
6 1. 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 changing. We have
found also that they change very little in a few
ASTRONOMY. 23
nights, very much in six months, and that after
twelve months the same stars again appear in the
same places.
63. 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 them so that they sink
up to their middles, and then put them in a tub of
water as shown in Fig. 12.
4
24 SCIENCE PRIMERS. [ vin.
67. We have now a representation of the sun, and of
the earth in four parts of its annual journey. What
I want you to understand is that the motion of the
earth is not only smooth, but that its motion is in
the same plane, a plane being a level surface re-
presented by a sheet of cardboard or the surface of
the water in the tub : and next that this plane in
Fig. 12. The plane of the Ecliptic.
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
Ecliptic.
68. Here then is defined the plane 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-
posed to be upright with regard to the plane of the
ASTRONOMY.
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. 12.
70. But are these planes the same? Let us sup-
pose them to be so. Stick a pin into one of the
smaller balls, make the ball spin uprightly like a hum-
ming top, and it will represent the earth as it travels
round the sun, and you will find that on this sup-
FIG. 13. Two planes cutting- each other at right angles.
position, the days will always be of the same length,
because the boundary of light and darkness would
FIG. 14. Two planes cutting each other obliquely.
pass through the two poles, so that each part of the
earth's surface would be an equal time in the lighted
SCIENCE PRIMERS.
up, and in the dark, half, 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 Australia.
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
Fig. 12, but their axes are inclined.
FIG. 15. Earth with inclined axis of rotation.
5 IX. WHY THE DAYS AND NIGHTS ARE
UNEQUAL.
72. We can now leave the tub, and come back to
the lamp and orange, remembering that the knitting-
needle must no longer be upright as we allowed it
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 night is much longer than
the day, and you will see that the nearer you place
the pin to the north pole, the shorter will be its period
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 nearer you 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 three months more, when the days are
longer than the nights ; just the reverse of what hap-
ASTRONOMY. 29
pens in winter. In autumn, on September 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 explain 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
FIG. 16. The Farth, as seen from the Sun at the Summer Solstice,
June 22 (noon at London).
represent summer, for you will see from what has
been said before, that if the north pole be turned
away from the lamp, the nights are longer than the
days ; when it is upright they are equal ; and when
it is turned towards the lamp, the days are longer
SCIENCE PRIMERS.
[.
than the nights ; but the earth's axis does not alter
in its direction, as we always find that 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
FIG. 17. The Earth, as seen from the Sun at the Winter Solstice,
Dec. 22 (noon at London).
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 the knitting-needle always parallel to
itself; let it be moved a quarter of the way round the
lamp and rotate the orange, and observe the length
of day and night as before ; you will see that the
ASTRONOMY.
poles are on the boundary which separates the light
from the dark half, and the journey of every part of
the orange through light and darkness is equal.
This position corresponds to the commencement of
spring, March 22.
8r. Move the orange another quarter of a circle
FIG. 1 8. The Earth, as seen from the Sun at the Vernal Equinox,
March 22 (noon at London).
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 hemisphere, day
is longer than night, corresponding to summer, and
the reverse at the southern hemisphere, so we have
matters just reversed by moving the orange half-
way round the lamp.
32 SCIENCE PRIMERS. [ ix.
82. Another quarter's turn, and day and night are
again equal, corresponding to autumn, Sept. 22 ;
one more quarter brings the orange to its original
position.
83. Just in the same way the earth moves round the
sun in a year, passing from winter through spring to
FIG. 19. The Earth, as seen from the Sun at the Autumnal Equinox,
Sept. 22 (noon at London).
summer, and through autumn to winter again ; the
positions of the earth in spring and autumn when the
days and nights are equal, are called the equinoxes,
that is, the " equal nights."
84. You will also be able to see that during the sum-
mer in the northern hemisphere the sun is continually
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 again 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 poles 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 DEPEND UPON THE DIF-
FERENCE IN THE LENGTHS OF THE
DAY AND NIGHT.
86. 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 and the nights are short
in either the northern or the southern hemisphere, in
34
SCIENCE PRIMERS.
[xi.
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.
FIG. 2a Explanation of the Seasons.
88. In spring, although the days and nights are equal
as in autumn, the powers of nature are renewed by
their winter's rest, so spring is the time of buds, while
autumn is the time of decay.
ASTRONOMY. 35
XL 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 equator. 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 the stars near the south rise only a little
east of 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 stars
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 diagram
(Fig: 21).
92. Now, to illustrate this, take a small globe, make
its axis upright, and in order to indicate the horizon
5
SCIENCE PRIMERS.
[xi.
of any place quite plainly, cut a piece of card about
the size of a penny and gum the centre of it on the
FIG. 2i. 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.
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 horizon.
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. 53) would
be to a person standing at the pole. You will at once
see that the card simply turns round like a wheel, and
the pictures that were above it at first remain so. So
the stars would not rise or set to a person at the pole,
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.
93. 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, gurn 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 heavens
pointed to by the poles 6i the globe will be just on
the horizon, the north pole-star just on the northern
38 SCIENCE PRIMERS. [ xi.
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 that 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, and watch 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 hemisphere,
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 it 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 he would in the
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 place.
40 SCIENCE PRIMERS.
II. THE MOON AND ITS MOTIONS.
L THE MOON TRAVELS AMONG THE
STARS.
100. 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.
1 01. 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 different 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.
102. 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 sometimes
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 ariiongst 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 among them
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 few
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, move the moon
to F, and you will see it is just south of the observer
at A, when the sun has set, so that it has lost about
42 SCIENCE PRIMERS. [ n.
six hours on the sun. Move it further on to G, and
it will just be rising when the sun is setting, and will
be south at midnight, having lost twelve hours on the
sun, as will be seen supposing an 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 will 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 K^ it will nearly
have lost a whole revolution 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 always
there, but a portion is sometimes unillumi-
nated and invisible to us.
ASTRONOMY. 43
107. 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 consequently 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 half of tire ball, which is white in the diagram,
a
N
LAMP OR SUN
? 9
? o
r <i o
a
FIG. 22. The Moon's motion round the Earth.
will be illuminated by the sun, and the other half, op-
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 ot 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 full moon, and
this appearance is represented by the white circle M.
So that it is now clear that at full moon the moon is
44 SCIENCE PRIMERS. [ 11.
on the opposite side of the earth to the sun, and we
see therefore the bright side.
108. 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 Z>, and the moon to be
rising must be at H> Place the ball, therefore, at
H, 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, N, 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 O, 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
your eye close to the orange you will see just a crescent
of the bright half, and a large portion of the dark half.
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.
in. 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.
113. Let us see if we can make matters clear with
the use of our orange and ball.
46 SCIENCE PRIMERS. [ in.
114. Set the lamp on the table, and stick the knitting
needle supporting the orange into a large pin cushion
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. 23), 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 J3, which the darkest part of the shadow
FIG. 23. Total Eclipse of the Sun.
does not reach-, the whole of the sun will not be covered
by 1 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, all places inside
the penumbra will see a partial eclipse only.
ASTRONOMY.
47
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.
1 1 6. All this will be clearer if the orange be re-
moved and the eye placed in its stead. First place
FIG. 24. Annular Eclipse of the Sun.
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 a partial 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 at A and so
see a total eclipse, and move the moon gradually away
from you, and you will see the moon apparently
48 SCIENCE PRIMERS. [ in.
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 as 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
FIG. 25. Eclipse of the Moon.
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).
1 1 8. 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.
119. You will have understood by this time that an
ASTRONOMY. 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.
1 20. 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 round
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 part of the ring between
the lamp and the orange must be tipped up or down.
122. To make this clearer, get a tub of water as
SCIENCE PRIMERS.
before, and float in the middle a ball to represent the
sun, 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 lie on the surface of the water, this surface,
as we have seen before (Art. 67), represents t-he
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
FIG. 26. Shewing the inclination of the Moon's orbit to the plane of the
ecliptic.
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 of the water, and keeping the other
above, as represented in Fig. 26, where the full line
ASTRONOMY. 51
indicates the part above water and the dotted line the
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 ecliptic, 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
she 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 on 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 ho\V
the amount of inclination is fixed in each case.
126. To do this astronomers divide all circles, whe-
ther large 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 360 is four times
QO, so that two lines containing a quarter of a circle
make an angle ot 90 between them. You will see
52 SCIENCE PRIMERS. [ in.
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 right angle, and two lines which make an
FIG. 27. Division of the circle into degrees.
angle or opening of 90 between them are said to be
perpendicular to each other. A complete circle like
this is contains 360 angles of i, 4 angles of 90, and
so on.
127. Now astronomers conceive such a circle with
its centre at the centre of the earth, and they can
than by their observations determine the angles
ASTRONOMY. 53
formed by the planes to which we have referred in
Art 125; and they 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 THE 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 away, 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 water whatever,
and consequently no clouds to shade the surface from
the sun; and what is more, there is no appreciable
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 !
131. 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 f , 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 inch, and a cubic
inch. Suppose that you took a cubic inch of lead,
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 they also know how
many cubic miles (or cubic inches) each contains.
They can therefore easily find whether 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 ij times as much as
a similar quantity of the moon's materials, hence they
say that the moon is only J 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^, and that of the moon
3 1 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 cubic inch or
cubic mile weighs \ this is found by dividing its mass
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 you 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 of
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.
I. HOW BODIES LIKE THE EARTH, NEARER
THE SUN, WOULD APPEAR TO US.
138. So far as we have gone the earth on which we
dwell, the large sun and moon, and the tiny stars, are
the only bodies with which we have dealt.
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 ball
round the lamp, and see how it appears from the
orange in its different positions. First place it in the
4
A
FIG. 28. Diagram illustrating the motions and appearances of a tody
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 day to day (the
word planet means a "wanderer"), to be put out like the
stars by the day. Move it to position C it will then rise
58 SCIENCE PRIMERS. [ n.
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 a telescope, 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.
141. In order to represent the appearance of an
earth outside us, we have only to move the ball in
a circle round the sun, outside the earth's orbit. Let
ASTRONOMY.
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,,
FIG. 29. Diagram illustrating the motion of a body travelling round the suni
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 bright
side is visible to the earth, although at the two.
positions corresponding to A and B, Fig. 29, it will;
7
60 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, 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. There are such bodies as we have just been
considering, both interior ones and exterior ones,
and they are 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 after the ancient deities ; the
two interior ones, Mercury and Venus, and the exterior
ones, Mars, Jupiter, Saturn, Uranus, and Neptune j
the three first being smaller than our earth, and the
remainder a great deal larger.
144. Mercury and Venus are known to 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 far away as to be
seen long after sunset or before sunrise, and is called
the Evening or Morning star, accordingly.
145. The exterior planets, as we found such bodies
ASTRONOMY. 61
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 impossible 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-fifths of a mile ; Uranus,
a full-sized cherry, or small plum, upon the circum-
62 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 give
you any idea of this distance. I can only state tlm
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 II., 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 be floating on the surface of water, part of Mercury's
orbit would be slightly below the surface and part
over. From the diagram you will see that Mercury
ASTRONOMY. 63
will always appear to us near the sun. When it is
on our left of the sun it apparently follows the sun on
its daily course, and sets just after it ; when on the
other side 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 be taken 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. Little is known of 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 the earth.
VENUS. '
153. Next to Mercury 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, according to its position
in its orbit round the sun, in the same manner as
Mercury, only its orbit being outside that of Mercury
6 4
SCIENCE PRIMERS.
[iv.
it can get further away from 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 have shown in speaking of the 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
sime change of
phases as Mer-
cury does, and
FIG. 30. Venus, showing the markings on its of COUrSC lor
the same reason.
Very little is known of the surface' of Venus : certain
dark markings, however, are seen frequently with first-
ASTRONOMY.
rate instruments on the surface, which may possibly
be breaks in clouds, through which the planet itself is
seen. The density of Venus is about the 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 will alter. Let us inquire into this
a little 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
FIG. 31. Apparent size of Venup. at its least, mean, and greatest distance
from the Earth.
miles from the sun) ; but when she is on the other side
of the sun she will be 157 millions away from us
66 SCIENCE PRIMERS. [ v.
(that is, 91 millions from us to the sun and 66 millions
from the sun 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 Mercury, 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 again 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.
159. The next member of our system is Mars.
Mars revolves in an orbit having a mean or average
distance of 139 millions of miles from the sun. It
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.
1 60. Mars requires 686 days to complete its annual
revolution round the sun, making its year nearly
double the length of ours. Since its orbit lies outside
ours this planet never can pass between us and the sun,
mid 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 J, 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.
1 6 1. 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 similar to ours.
162. When looked at with the eye alone, Mars
appears of a reddish tint, by which it can be easily
63 SCIENCE PRIMERS. [ v.
recognized, but when seen through a telescope the
redness in a measure disappears, and the planet
appears to have a bright surface, on which are darker
portions, the former 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
FIG. 32. Mars, showing snow cap at the pole, and the lands and seas.
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 corresponding to those
on our earth. The drawing will give some idea of
the appearance of Mars as seen in a large telescope,
ASTRONOMY.
69
one of the 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
FIG. 33. Mars. View of another part of the planet.
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, and from their smallnessare
worth little notice. Their orbits are more inclined to
70 SCIENCE PRIMERS. [ v.
the plane of the ecliptic than 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
to a loth magnitude star in brilliancy, and their
surface may possibly be not much larger than the area
of 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 unmistakable 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-
meter is found to be about ten times the diameter of
our earth, so that the flattening of the poles and the
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 of
our planet at the equator, or 20,000 miles per hour.
1 66. 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 it
is not the surface of the planet we see, but only a
very cloudy atmosphere.
8
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 him, and going through the same changes as
our 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 i6j 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
inclined to enable it to pass above or below the line
joining the sun and Jupiter, this prevents it from
causing an eclipse at every resolution. For the same
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
chengoes 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
/^appears in transit, while the Mis 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 /^the moon J/is 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 no appreci-
able change in the Jovian seasons. Although the size,
or, more correctly speaking, the volume, of Jupiter is
74
SCIENCE PRIMERS.
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 Saturn, a truly grand sight in
a telescope, Saturn having, besides eight moons, an
immense bright ring surrounding the globe. This
FIG. 36. Saturn and his rings.
planet revolves in an orbit at about 872 millions of miles
from the sun, taking 10,759 days, or nearly thirty of our
years, to complete its year, and having a diameter nine
ASTRONOMV. 75
times greater than that of the earth. From observa-
tions of spots and belts on the surface (somewhat
similar to those on Jupiter) the time of its diurnal
revolution has been fixed at about 10^ 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 265, 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
166,000 miles. The two outer ones are the brightest,
the inner or 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
iti 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 also are largely
inclined to the orbit of Saturn, and consequently
eclipses are rare.
76 SCIENCE PRIMERS.
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
of our system at present known, at 2,746 millions ot
miles from the sun, and taking 60,126 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.
77
VI. COMETS, METEORITES, AND FALLING
STARS.
176. Besides the planets, there are other members
of our system, of a different 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 a.nd 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
millions of miles behind;
at other times they ap-
pear with a nucleus with
mist extending equally
round it ; in fact, their FlG ' 37-General view of a Comet.
shapes are almost as various as those of the clouds.
The greater number of comets are invisible to the
naked eye.
178. The majority of comets that come into our
system from outside, are attracted towards the sun, pass
73 SCIENCE PRIMERS. [ vi.
by it, and then continue on away from our system
again; while there are others that belong to our
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 immense
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.
1 80. Their weight is excessively small, while their
volume or bulk is immense that of Donati, figured
in the 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.
1 8 1. Now, before I say anything more about these
strange things, I must remind you that perhaps when
you have been looking at the 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 be
seen every fine night with a little attention. These
are called meteors or falling stars, or, if they
actually fall, as some do, to the earth, meteorites.
They vary greatly in apparent size and brightness
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 sky for some seconds, leaving a luminous
trail behind them.
182. Now of course, as some of these bodies fall to
the earth, the chemist can examine them and find out
what they are made of, as
he has found out what the
earth is made of. Some
are especially metallic in
their nature, others espe-
cially stony. As they rush
into our atmosphere they
are heated so hot that
they burn, and the small
ones are consumed before
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 in size. A
number of these that have
escaped destruction are to
be seen in the British Comet.
Museum, some reaching the weight of three tons.
183. From constant observation it has been found
that on differents nights the majority of shooting stars
appear to come irom certain parts of the sky, and
on certain nights in the year many more fall than on
others. There are, for instance, the well-knoAvn falls
of November 13 and August 10, those of November
FIG. 38. Head and envelopes of
8o SCIENCE PRIMERS. [ vi.
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 thai
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 of 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 universe in which
we dwell
ASTRONOMY. 81
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 itj 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.
1 88. 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 of a planet
is exposed to the sun and the time during which it
is withdrawn from the sun's influence.
82 SCIENCE PRIMERS. [ n.
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, you would then have a globe about
as large as the sun.
194. I have already told you that the distance of
the sun from us is about 91 millions of miles. To
ASTRONOMY. 83
go into the mode of measurement 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,
C
FIG. 39. How the size of ths 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 T ^ T of their dis-
tance from A, so also B C is T T part of the distance
A B, which we know is 91 millions 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. 1 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 telescopes
* The young reader must not attempt to look at the sun through a small tele-
scope, for he or she may be blinded in the attempt.
9
84 SCIENCE PRIMERS. [ iv.
black spots are frequently seen on its surface, and
these, indeed, 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 faculse, and probably are immense banks
of brighter vapours several thousands of miles long.
If the spots and faculse 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
in 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 12 days
after to appear again on the east side and get to the
osition where it was first observed in about 25 days,
ASTRONOMY.
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 faculse with it.
20 1. Let us now see what kind of thing a spot is.
If a pretty regular one is observed near the middle of
FJG. 40. A Sun-spot.
the disc it appears round ; if it be again observed a few
days after, 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. Let us see what we can learn from this.
Take an ordinary saucer, and having blackened the
part of it on which the cup generally stands, look
86
SCIENCE PRIMERS.
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. Explanation of the appeara ices presented by Sun-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.
114). The luminous vapours then appear of exquisite
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
FIG. 42. The Sun's coronal atmosphere.
then seen that the lighter vapours of the real sun
are shot up into its outer atmosphere, called the
coronal atmosphere, taking fantastic shapes called
prominences, and 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 as
you have seen light split up into all the colours of
the rainbow by the glass drops on chandeliers, it
88 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.
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 surrounded by its
family of planets in the same way as the sun is.
ASTRONOMY. 89
V. THE STARS.
^ I.-THE STARS ARE DISTANT SUNS.
208. From the sun the nearest star that gives u
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 into 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 sixteenth, which
require the most powerful telescope to view them.
The faintest star visible on a dark night is of about
90 SCIENCE PRIMERS. [ in.
the sixth magnitude. After what has been said you
must not think that magnitude means real size, as a
large star may be far 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 tele-
scope 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, Li-
bra, Scorpio, Sagittarius, Capricornus, Aquarius, and Pis-
ces, 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."
2i 6. The constellations visible in the Northern Hemi-
sphere above the zodiacal constellations, are called
the northern constellations, they are as follows :
Ursa 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 Lady's Chair).
Perseus. Perseus.
Auriga. The Waggoner.
92
SCIENCE PRIMERS.
[iv.
Serpenta rius
Serpens.
Sagitta.
Aquila.
Delphinus.
Equuleus.
Pegasus.
Andromeda.
Triangulum.
Camelopardalis.
Canes Venatici.
Vulpecula et Anser.
Cor Carol i.
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.
Cam's Minor.
Argo Navis.
Hydra.
Crater.
Corvus.
Centaurus.
Lupus.
Ara.
Corona Australis.
Pis cis Australis.
Monoceros.
Columba Noachi.
Crux Australis.
The Whale.
Orion.
The River Eridanub.
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.
2 1 8. In order to learn the positions of the various
constellations and stars you will want a star-map or
planisphere; and will also require some friend to point
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 (/?), and then, when all the letters are used, they
are numbered i, 2, 3 ; so we can refer to a star as
Alpha (a) Lyrae, the brightest star in the constellation
of the Lyre, or (/3) Cygni, the second brightest in the
Swan, 6 1 Cygni, and so on, so that every star can be
named. In addition to these names the principal
stars have other names, thus (a) Lyrae is also called
Vega, a Canis Majoris is called Sirius, a Bootis, Arc-
turus, 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 moving, and not the ships.
Just so, as we have seen the earth turns round, and
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 would 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.
222. 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 stais
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-
pended on the position of the earth's axis they were
found to be continually changing. Here, then, is
another apparent change in the positions of the slars,
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 out, though,
like the ships, some close up in the direction in
which the majority open out and vice versa; but by
10
96 SCIENCE PRIMERS. [ vi.
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
tht others, then we can safely say it has a real 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, but
instead of the revolution being performed in a year.
ASTRONOMY. 97
the shortest known time of revolution or period of a
double star is thirty-six years. Up to the present
time some 800 of these systems have been discovered.
FIG. 43. Orbit of a Double Star.
230. The distances of the stars from us is so
immense that if they had planets revolving round
them these would be invisible with our most power-
ful instruments. But it is probable that each star is
the centre of a planetary system : in the case of close
double stars, therefore, the planets of one star must be
so near the other as to receive a considerable amount
of light from it ; in fact, the planets would have two
suns, and, in some cases, suns giving light of different
colours.
VII. CLUSTERS AND NEBULA.
231. Besides the scattered stars of which we have
been talking, there are a number of white patches in
the sky like little pieces of the Milky Way, a few of
which are visible to the naked eye. When these are
looked at with a telescope, some of them are seen to
be very closely packed clusters of small stars ; in some
SCIENCE PRIMERS:
[ VII.
the separate stars are seen with telescopes of low
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 stars, and those which still appear
FIG. 44. The Cluster in Hercules.
of a cloudlike structure when the most powerful
telescopes are brought to bear upon them, are called
nebulse.
232. We may therefore divide these objects into
three classes : (i) the clusters, in which the separate
stars are easily seen gradually merging into (2) the
resolvable nebulae ; and (3) the irresolvable
ASTRONOMY.
99
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 in this they are unlike the
clusters.
ioo SCIENCE PRIMERS. [ vm.
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 closely examined,
seem to be surrounded by a kind of fog, and these
we know are not true stars. Such bodies are called
nebulous stars.
234. 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
NEBULAE.
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 which 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 if mark, I only
say as if such stars were colder than the sun.
236. Although the nebulae appear to be very dif-
ferent from stars, it is possible that there is a very
close connection between them, for it has been
ASTRONOMY.
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 nebulas 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.
102 SCIENCE PRIMERS. [ 11.
VI. HOW THE POSITIONS OF 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 mdtion of the earth. We have referred
to the nature qf 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 night,
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 might be able
to make a sketch-map of the positions of the stars,
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 distance 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
o 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 are divided into 360, Art. 126) we can deter-
mine those stars which have 10 polar distance, then
20, 30, and so on, till we come to 90, which of
io 4
SCIENCE PRIMERS.
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.
III. 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
C 6 C
-
-
M
i
FIG. 46. How to define the position ol anything.
the position of a star. Now, to begin with, how
would you fix the position of a dot on a piece of
paper? Let us see. Take a sheet of paper A BCD,
Fig. 46, and stick a pin in or make a dot E on
it. Now let us see how we can state its position :
divide the side AD into, say, 10 equal parts, and
AB into, say, the same number; then on joining EG
ASTRONOMY. 105
and EF, you will see that E is 4^ divisions from
the line AB measured along AD, 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 draw 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 4^ 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 is 2j 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 straight lines 10 apart
between it and each pole.
io6
SCIENCE PRIMERS.
IV.
IV. RIGHT ASCENSION.
249. Evidently therefore, to make our statement of
a star's position complete, we want another line at
FIG. 47. How the positions of stars are stated.
right 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.
250. Now the equator, and the row of pins
which represents it, can only be in one place on
the orange, that is half-way between the two poles.
ASTRONOMY. 107
But you may make the second circle wherever you
choose, 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 i 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 astronomerr
has to do is first to regulate his clock so. that the
stars shall appear to travel round the earth in exactly
24 hours ; to let it show o h o m 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 the brightest star
(o.) in the Bull is 4 h 28 ; of the brightest star in the
Virgin, 13" i8 m , and so on.
11
lo8 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.
VI. THE LATITUDE OF 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
along a certain road, or in a certain direction, say
S.VV. of your house. This answers very well for short
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 earth 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 if that 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 which 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 have stated, must have been
made before the positions of any places were laid
down. Thus, in maps, you will find the distance of
no SCIENCE PRIMERS. [ vii.
London from the equator shown as 5i| N., be-
cause the star y Draconis, with a north declination
of 5 1 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 68J miles north it is i above
his horizon, his latitude is said then to be i, 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 number 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 the
same latitude as Pekin and New York.
ASTRONOMY. m
VIL-THE LONGITUDE 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
does 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, how 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 or west of the
circle which passes through the imaginary first point
of Aries. It is a question of time.
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 that this pin will come between the lamp and
the knitting-needle after the row has passed ; that
is, there will be a difference in the times at which
the row of pins and the solitary pin pass the lamp,
since all the watches are set to Greenwich time.
Let us suppose that at the row of pins the Greenwich
time is i h ; 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 b , 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 the row of pins
will be shown to the east of Greenwich.
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 ones care-
fully-adjusted chronometers had to be carried from
one station to the other, to convey the correct time ;
but now, when telegraph wires are laid from one place
ii 4 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
then I must attempt to explain to you how it is that
we are enabled to foretell the movements.
ASTRONOMY. 115
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 answer 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 of anything
only means the force with which the earth attracts it,
and causes it to gravitate towards itself.
Ii6 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 by gravity; but
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 full 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 the
earth attracts it. I have now to add that this force
is not the same for bodies at different distances from
the earth.
283. 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 is
not attracted with sufficient force to overcome the
friction of its rolling on the table, and the magnet
1 1 8 SCIENCE PRIMERS. [ 1 1 1 .
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 rind 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
be at M; then if gravity ceased to act, the moon would
continue on in the same straight line that it was moving
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 M\
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 M' to B through
which the earth draws the moon in one second, which
is a little under one-eighteenth of an inch.
1 IG. 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 ; and
the attraction there should be sixty times sixty, or
3,600 times less at the moon's distance; but the force
of gravity at the surface of the earth is such that
19
120 SCIENCE PRIMERS. [ iv.
bodies fall sixteen feet a second, so at the distance
of the moon they should fall y-faf 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
which we dwell, but to make use of the motions of
the heavenly bodies for purposes of daily life.
THE END.
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