THE MOTION OF THE EARTH.
[Sidenote: Why a ball thrown up comes down.]
When a boy throws a ball up into the air, he thinks that it comes down
of itself. He thinks that it comes down merely because the force with
which he sent it up is spent or lost; but this is not so. It is pulled
down. The earth pulls it down. The earth is pulling upon it all the
time as it goes up, and gradually overcomes the force with which he
threw it up.
There is another thing that helps to overcome the force by which the
ball is sent up. It is the resistance of the air. As the ball goes up,
it has to spend a part of its force in pushing the air away to make a
path for itself.
These two things--the pulling of the earth and the resistance of the
air--gradually stop the going up of the ball. If there was no air, and
if the earth would let the ball go, instead of drawing upon it, it
would not come down. It would fly off out of sight; and more than that,
it would never stop till something stopped it. It could never stop of
itself.
This, perhaps, seems strange to you; but look at it. A ball, you know,
has no power. It lies still if you do not touch it. It can not move
itself, and, for the same reason, it can not stop itself. Once set it
agoing, and it would move on forever if it was not stopped by something.
[Sidenote: Matter can not move itself or stop itself.]
This is true of all matter that is not alive. You can set yourself in
motion, and stop yourself, for you are alive; but common dead matter
can do neither. It moves because it is moved, and it stops because it
is stopped by something.
[Sidenote: How fast the earth moves.]
Now the earth is a ball that is always moving. It never stops for an
instant, but is all the time rolling on around and around the sun. God
a long time ago set it agoing, and it never has been still since. It
takes a year for it to go round the sun; and how fast do you think it
goes? About 68,000 miles an hour--that is, over a thousand miles every
minute. This is two thousand times as fast as the cars go when they are
going very fast indeed. What a ride we are taking on this round ball of
earth!
[Sidenote: Why it does not seem to us to move.]
But you will ask why it is that we do not feel any thing of this
motion, or know something about it, just as we do about the motion of
traveling. The reason of this is very easily seen. Just observe how it
is that you know about the motion in traveling. You see trees, houses,
fences, etc., as you pass by them. You feel the air as you go through
it. If the motion is uneven, you feel it. It is by these things that
you know that you are moving along. But as we are carried along on the
earth as it goes around the sun, there are none of these things to let
us know that we are moving. Every thing goes along with us--trees,
houses, fences, and every thing else. We do not go through the air, but
the air goes along with us. Then, too, the motion is very even. The
earth is not jostled and jarred in its course.
Sometimes, when you are riding in the cars, you hardly seem to move at
all, though you may be really going very fast. The reason of this is
plain. First, the motion is very even; then the air that is in the car
goes along with you, though the air that is outside does not; and the
people in the car that you are looking at are going along with you also.
[Sidenote: Illustrations from the motion of cars in traveling.]
But the moment you look out of the car window you know that the cars
are going quite rapidly, because you see that you are going so fast
by the trees and houses. So, too, if the cars come to a place where
the rails are not so even, the irregular motion lets you know that
you are going fast. Sometimes, when you seem to be going along quite
moderately, because the rails are so even and the road is so straight,
all at once you seem to be twitched along with a very sudden, quick
start. It seems to you as if the cars suddenly went a great deal
faster, but it is not so. The cars are really moving no faster than
before. A turn in the road makes it seem so, because it makes the
motion irregular instead of even.
Now, if the motion of the cars were perfectly even, and you did not
look out, you would not know that they were moving at all. Just so it
is with the earth. Its motion is so even that we do not feel that it
moves at all, though it is carrying us two thousand times as fast as
the cars carry us when they are going thirty-four miles in an hour.
It is true that we look away from the earth as we are riding along on
it just as we look out of the cars; but the sun, and moon, and stars
that we see are so far off that we can not tell by looking at them that
the earth is moving. It seems to us to be standing still. For the same
reason, the cars do not seem to be moving if you look at things a great
way off, instead of those that are near by.
[Sidenote: Mistakes about the earth’s motion.]
A great many mistakes have been made about the motion of the earth,
for things are not always as they appear to be. It seems to us as if
the earth did not move at all; while the sun, and moon, and stars seem
to move, because they are not always in the same direction from us. We
look one way for them at one time, and another way at another time. Now
they do move, but not in the way that they appear to us. The sun seems
to rise, and go up and up, and then go down in the west. But this is
not so. This is all owing to a motion of the earth that I have not yet
told you about. As the earth goes round the sun, it also turns every
day around on itself. It is this motion that makes day and night for
us. As the earth thus rolls over, where the sun shines upon it it is
day, and where it does not shine upon it it is night.
[Sidenote: Its two motions illustrated.]
The earth, then, has two motions. First, it goes round the sun. This,
as I have told you, takes a year; but in every twenty-four hours it
turns over also. This is its second motion. It performs this 365 times
while it is doing the first motion once.
These two motions can be made plain to you with a candle and some round
thing, as an orange. Let the candle represent the sun. Carry the orange
around it in a circle, and this will represent the earth going round
the sun. Now, by turning the orange so that the candle will shine upon
one part of it, and then upon another, and so on all around it, you
will see how the second motion of the earth is done, and how night and
day are made. Any thing that you do not quite understand about this
your teacher will explain to you.
[Sidenote: Leap-year explained.]
The earth, I have told you, turns around on itself 365 times in a
year. But there is one thing about this that I must mention to you. It
takes about six hours over the 365 days for the earth to go round the
sun. Now what is done with this six hours in reckoning the year? It
is managed in this way. It is a quarter part of twenty-four hours, or
a day, and so, to make the reckoning come right, a day is added every
fourth year. It is added to the month of February. Every fourth year
this month has twenty-nine days instead of twenty-eight, and the year
is called leap year.
[Sidenote: Idea of a boy.]
[Sidenote: Galileo.]
Astronomers have discovered a great many things about the shape and the
motions of the earth. Before these were understood, people supposed
that the earth was still, and was flat instead of round, and that
the sun really rose in the east and set in the west; and it seems
so to every body now that has not learned what the astronomers have
discovered. A bright little boy said to a lady who was teaching him
about the earth, You don’t mean to say that the world is round? I know
that it isn’t. I can see that it is flat with my own eyes. She assured
him that the earth was round, but he could not believe it, and replied,
Well, I shall ask my father, for gentlemen commonly know more about
such things than ladies do. You will think it strange when I tell
you that, a little more than two hundred years ago, people generally
believed as this little boy did, and that they put a learned man, named
Galileo, into prison because he said that the earth was round, and that
it went around the sun.
[Sidenote: Why we see only a part of the moon most of the time.]
[Illustration]
You will want to know something about the motion of the moon. As the
earth goes round the sun, so the moon moves around the earth. It takes
a little less than a month for it to get round the earth, and it goes
around it about thirteen times a year. As I have told you in another
chapter, the silvery light which the moon sheds upon us is the light of
the sun reflected by the moon. Why it is that only a part of the moon
shines upon us much of the time, I will explain to you. When there is a
new moon, as it is termed, the moon is in such a position that we can
see only a little of that part of it which the sun shines upon. But
when the moon is at the full, it is in such a position that we see all
of it that is lighted up by the sun. So when the moon quarters, as it
is expressed, we see but a half of the lighted portion, and so on. All
this is made plain by this figure. S is the sun, E is the earth, and
_a_, _b_, _c_, &c., the moon in different positions. When the moon is
at _a_ we can not see any of it, because it is between the earth and
the sun. The sun shines upon the half of the moon that is toward it,
and this half is now all away from our sight. As it leaves _a_ we see
a little of it, and a little more every night; and when it gets to _b_
we see a quarter of the part which the sun shines upon. Then, when it
comes to _c_, we see half of it. When it is at _d_ we see rather more
than half: it is then called gibbous. When it is at _e_ we can see the
whole of the lighted-up part, and so the moon is full. Then at _f_ it
is gibbous again, and at _g_ half moon.
[Sidenote: Eclipse of the moon explained.]
[Illustration]
And now you will want to know how an eclipse of the moon happens. This
I can make plain to you by this figure. A B is the sun, C D the earth,
which is smaller than the sun, and M the moon, which is much smaller
than the earth. Now, as the sun shines upon the earth, there is a dark
shadow beyond the earth, as represented. When the moon, therefore,
happens to pass through this shadow, it is in the dark, and no one on
the earth can see it till it comes out from the shadow. While it is in
the shadow there is an eclipse, as it is termed.
_Questions._--What two things gradually stop the going up of a ball in
the air? Could the ball stop of itself? Why can you set yourself going
and stop yourself? How is it with dead matter? What is said about the
earth? How fast does it move? How do you know about the motion in
traveling? Why is it that sometimes, when the cars are going quite
fast, you scarcely seem to be moving at all? How is it if you look
out? How is it if the cars come to a place where the rails are uneven,
or where there is a turn in the road? Give the comparison about
looking out of the cars and looking away from the earth. Tell about
the mistakes that have been made about the motion of the earth. How
is it that day and night are made? Tell about the two motions of the
earth. Describe how you would make these plain with a candle and an
orange. Why is a day added to every fourth year, making it leap year?
What did people suppose about the earth and sun before astronomers
found out so much about them? Give the anecdote of the little boy.
Tell about Galileo. Tell about the motion of the moon. Tell about the
new moon and the full moon. Tell about the eclipse of the moon.