THE MOON’S CHANGES.
The Moon in our sky does not always seem to be of the same shape.
Sometimes she is quite round, like a plump laughing child-face, with
eyes and nose and mouth marked in grey shadows. Sometimes a part of
the round face seems to be shaven off on one side. Sometimes she is
a half-round. Sometimes she is a bright crescent, wider or narrower.
Sometimes she is only a slender sickle of light.
Now, how is this? What causes so many changes in the Moon?
You know in what way our Earth shines--as shine she does, if only we
were far enough off to see it. You know that she is bright on that side
alone which faces the Sun. And you know too that, as she spins daily on
her axis, each country in turn comes into the Sun’s rays, is lighted up
for awhile, then passes away into the night-time of darkness which is
on the side of the Earth turned away from the Sun.
And you also know that as the Earth shines so the Moon shines.
The Moon has no radiance of her own. She can only, like a
looking-glass, reflect the Sun’s radiance. In other words, she receives
his light and throws it off again.
As the Earth spins, so the Moon spins, but very much more slowly. It
takes our big Earth only twenty-four hours to whirl once round upon her
axis. It takes the Moon about twenty-eight days to spin once round upon
her axis.
Only that half of the Moon upon which the Sun shines is bright. Half of
her is turned towards the Sun, and this half is bright. Half of her is
turned away from the Sun, and this half is dark.
And we from Earth can see, usually, only the _bright_ side of the Moon,
or just so much of the bright side as happens to be towards us.
Sometimes the whole of the bright side is turned towards us. Sometimes
only a part of it, and sometimes none of it is turned towards us.
Now and then we catch a little glimpse of the dark body of the Moon
when it is not shining in the sunlight. We see a round dark ball held
in the arms of the silver crescent. That is because our own Earth
shines so brightly upon the _dark_ side of the Moon as to light it up
and show it to us. But more often we only perceive the sunlighted side,
and the darker part is quite hidden.
[Illustration: _The New Moon._]
You have noticed the Full Moon, of course, because the Moon then is at
her best and brightest. Once a month we always have a Full Moon. It is
only on one night that the Moon is really quite full; but for two or
three days before and after she is very nearly so.
At Full Moon _the Earth is between the Sun and the Moon_. On one side
of our Earth is the Sun; on the other side is the Moon. The Sun shines
full upon that side of the Moon which is turned towards us; and so
we see the whole of her round bright face. We know that her farther
side is in darkness, because it is turned away from the Sun: in almost
pitch-darkness, for it has not even our Earth to light it up, being
turned away from us also. It has only Stars on the farther side.
I am speaking of when the Sun has set, and is below our horizon, so
that we cannot see him, although his rays travel straight to the Moon.
The Sun having set means only that the solid body of our Earth is
between him and us. It does not interfere with his shining upon the
Moon.
If our Earth at Full Moon were _exactly_ between the Sun and the Moon,
that would interfere with his shining upon her. More commonly, however,
our Earth is not just in the line between, but only very nearly so.
Thus we get the best possible view of the Moon’s round face.
The Moon does not stay on one side of the Earth. She is always
travelling round from one side to the other side of us. At another part
of her monthly journey things are quite different.
A fortnight after Full Moon we have _New Moon_.
At Full Moon the Earth is between Sun and Moon; but at New Moon the
Moon has come right round to the opposite side, and is _between the Sun
and the Earth_. Not usually in the exact line between, so as to hide
the Sun from us, but very nearly so.
Then still the Moon has a bright round face; only we on Earth cannot
see it. For her bright side is, as always, towards the Sun; and her
_dark_ side is towards us.
Just at first we cannot see the New Moon at all, for it is entirely
dark. As she journeys on to one side we get a glimpse of a thin line of
light shaped like a sickle; and this widens every day. It is while we
see the sickle of light that we sometimes catch a glimpse of the dark
side of the Moon, dimly lighted up by Earth-shine.
[Illustration: _The Moon. Second quarter. 10½ days old._]
When the Moon gets half-way back to where she was at Full Moon we are
able to see _half_ of her bright face, and we call that the “First
Quarter.” The other half of the bright face is still turned away from
us, and half of the dark side is still towards us. We do not see the
whole round face till she gains once more the place of Full Moon--after
which, between Full Moon and New Moon, follows the Third Quarter, which
is much the same as the First Quarter.
You know that the rising and the setting of the Moon in our sky,
by night or by day, are not real movements. They are only seeming
movements, caused by our own Earth’s daily spinning on her axis.
But these “Phases” as they are called--these changes in the shape and
brightness of the Moon--are brought about by her own movements, as she
travels round the Earth. She does not actually alter her shape: but
she does actually alter her place in the sky, so that we get different
views of her from week to week.
The four weeks of the Moon’s phases are called a “Lunar Month.”
You can make clear to your mind how the changes come about, by acting
them out with a lamp and a big ball.
There must be no other light in the room.
Stand first with your back to the lamp and the ball in your hand, held
out at arm’s length: so that your head is _nearly between the lamp
and_ _the ball_. Not quite between, so as to shade the ball. Hold the
ball just a little higher than your head: and the lamplight will fall
upon that side of it which is towards your face.
Then you have Full Moon. The lamp is the Sun: your head is our Earth:
the ball is the Moon. You see how the lamp lights up the half of the
ball which is towards yourself.
Next turn round with your face to the lamp, and hold the ball at arm’s
length _between your head and the lamp_, only a little higher or
lower--not quite in the line between so as to hide the lamp from you.
The lamp-light now falls on the other side of the ball; and the dull
unlighted side is towards your face.
This is New Moon. Once again the lamp is the Sun, your head is the
Earth, and the ball is the Moon. You see how the lamp lights that half
of the ball which is turned away from you.
The real New Moon in the sky is invisible. Here you can see the dark
side of the ball because the lamplight creeps round it. Still even here
you will find a difference between the bright and the shaded parts.
Then, if you hold the ball at arm’s length half-way round on one
side of your head, you will see how matters are at the Quarters. The
lamp still shines full on one side of the ball, but only half of the
brighter side is towards you, and half of the darker side. In the
real Moon the shaded quarter would be hidden, and only the bright
quarter would be visible.
[Illustration: _The Moon. Third quarter. 16¾ days old._]
This “quarter” we call “a Half Moon.” It is a quarter of the whole
Moon, taking the Moon all round; but it is a half of the bright side,
which makes our Full Moon.
All that we really know about the Moon’s surface is what we see on one
side of her. The other side is never turned towards us. No man on this
Earth has ever seen it.
A man living on that side of the Moon which we can see might look
at all parts of the Earth in turn. As the Earth spins on her axis
she turns each side towards the Moon, one after another, in only
twenty-four hours. But, although the moon spins, we see only and always
one side of her.
If somebody should make his home on the farther side of the Moon, and
should never come round to the nearer side, he would not have a glimpse
of the Earth. He would see the Sun, because the Sun shines on each part
of the Moon in turn: but no Earth would be on his sky. One side of the
Moon has a magnificent Moon in the Earth, more than a dozen times as
large as our Moon. But the opposite side of the Moon has only starlight
when the sun sets.
The reason for this is that the Moon takes just exactly the same
length of time to spin once round on her axis that she takes to travel
once round the world: twenty-eight days for the one and twenty-eight
days for the other.
Suppose now that you choose to walk round and round a table with a lamp
in the middle. You may do it in three different ways.
First: you may spin fast on your feet, like a teetotum, as you go. Each
spin of your body may perhaps take a second; and passing slowly round
the table may last half-a-minute. As you thus move, each side of your
head in turn is towards the lamp with every spin.
Secondly, you may pass slowly round the table in the same manner; not
spinning at all but keeping your face fixed on one direction--let us
say, towards the fireplace end of the room. Then again, as you move,
each part of your head in turn will be towards the lamp.
Thirdly: you may pass round the table turning very gradually indeed
upon your feet as you go, turning so slowly that a single spin will
last exactly as long as one journey round the table. If you start with
your face towards the lamp you will continue to face it all the while,
and the back of your head will all the while be away from the lamp, in
shadow. In fact, the lamp will never once have a glimpse of the back of
your head.
[Illustration: _The Moon. Last quarter. 23⅓ days old._]
The last of these three is the manner in which the Moon spins on her
axis and travels round our Earth.
Such a slow spin brings about a curious state of things. We on the
Earth have day and night in every twenty-four hours; but the Moon’s day
and night come only once in every twenty-eight days. The day there is a
whole fortnight in length of our Earth-time: and the night is another
whole fortnight.
Fourteen days of blazing sunshine: then fourteen days of pitchy
darkness--except for the brightness of the Stars, and except also, on
one side, for the beautiful radiance of the Earth.
QUESTIONS.
1. What is meant by the Moon’s Phases?
The different shapes of the Moon as we see her in the sky.
2. What kind of shapes?
As Full Moon; as Half Moon; as Crescent Moon.
3. Which part of the Moon shines?
That side which is turned towards the Sun.
4. Do we ever see her dark side?
Sometimes, not far from New Moon, we have a glimpse of it, lighted up
by Earth-shine very dimly.
5. Does the Moon spin on her axis?
Yes; but very slowly, only once in twenty-eight days.
6. How long does it take the Moon to get once round the Earth?
Twenty-eight days; the same length of time as her spin.
7. How long is the Moon’s day?
About one fortnight of Earth-time.
8. How long is the Moon’s night?
About another fortnight.
9. Does the Sun shine on all parts of the Moon?
On all parts in turn, as the Moon slowly spins round.
10. Do we see all parts of the Moon?
We see only one side, because, as the Moon spins, she journeys round
the Earth just so fast as to keep one face always in our direction.
11. What is meant by Full Moon?
At Full Moon the Earth is between Sun and Moon, and we have our best
view of the full round face of the Moon.
12. Is the Earth exactly between?
Not quite, or she would cut off the sunlight from the Moon.
13. What is New Moon?
At New Moon the Moon is between Earth and Sun, so that her bright side
is away from us and we cannot see her at all.
14. Tell me about the First and Third Quarters.
The Moon is then at the side of the Earth, half-way between New and
Full. Half only of her bright side is towards us and we see her as
“Half Moon.”
15. What is a Lunar Month?
A month of four weeks--the time of the Moon’s changes.