Though the moon is going round and round the earth incessantly, yet it
always manages to avoid affording us a view of what is on the other
side. Our satellite always directs the same face towards the earth, and
we may reasonably conjecture that the other side is covered, like the
side we know, with rings and other traces of former volcanoes. In this
respect the moon is quite a peculiar object. The other great celestial
bodies, such as the sun or Jupiter, turn round on their axes, and show
us now one side and then the other, with complete impartiality. The way
in which the moon revolves may be illustrated by taking your watch
and chain, and as you hold the chain at the centre making the watch
revolve in a circular path. At every point of its path the ring of the
watch is, of course, pointed to the centre where the chain is held. If
you imagine your eye placed at the centre, to represent the earth, the
movements of the watch would exemplify the way the moon turns round it.
One more point I must explain about the moon before we close this
lecture. There is nothing more familiar than the fact that a heavy body
will fall to the ground. Indeed, it hardly matters what the material
of the body may be, for you see I have a small iron ball in one hand
and I hold a cork in the other. I drop them at the same moment, and
they reach the ground together. Perhaps you would have expected that
the cork would have lagged behind the iron. I try the experiment
again and again, and you can see no difference in the times of their
falling, though I do not say this would be true if they were dropped
from the top of the Monument. In general we may say that bodies let
drop will fall sixteen feet in the first second. Even a bit of paper
and a penny piece will fall through the same height in the same time
if you can get over the difficulty of the resistance of the air. This
is easily managed. Cut a small piece of tissue paper which will lie
flat on the top of the penny, and hold the penny horizontal with the
paper uppermost. Though there is nothing to fasten the paper to the
penny, you will find that they fall together. If we could conduct the
experiment of dropping the penny and the bit of paper in a vacuum,
then, whether the paper was laid on the penny or placed in any other
way, the two objects would reach the table at the same moment if
released at the same moment at equal heights.
Wherever we go we find that bodies will always tend to fall in towards
the centre of the earth; thus in New Zealand, at the opposite side of
our globe from where we are now standing, bodies will fall up towards
us, and this law of falling is obeyed at the top of a mountain as it is
down here. No matter how high may be the ascent made in a balloon, a
body released will fall towards the earth’s centre. Of course, we can
only ascend some five or six miles high, even in the most buoyant of
balloons; but we know that the attraction by which bodies are pulled
downwards towards the earth extends far beyond this limit. If we could
go ten, twenty, or fifty miles up, we should still find that the earth
tried to pull us down. Nor, even if you could imagine an ascent made to
the height of 1000 miles, would gravitation have ceased. A cork or an
iron ball, or any other object dropped from the height of 1000 miles,
would assuredly tumble down on the ground below.
Suppose that by some device we were able to soar aloft to a height
of 4000 miles. I name that elevation because we should then be as
high above the earth as the centre of the earth is below our feet. We
should have doubled our distance from the centre of the earth, and the
intensity of the gravitation would have decreased to one-quarter of
what it is at the surface. A body which at the earth’s surface falls
sixteen feet in a second would there fall only four feet in a second,
and the apparent weight of any body would be so much reduced that it
would seem to weigh only a quarter of what it weighs down here. Thus,
the higher and higher we go, the less and less does gravity become; but
it does not cease, even at a distance of millions of miles. Therefore
you might say that as gravity tries to pull everything down, wherever
it may be, why does it not pull down the moon? This is a difficulty
which we must carefully consider. Supposing that the earth and the moon
were simply held apart, both being at rest, and that then the moon
were to be let go, it would no doubt drop down directly on the earth.
The movement of the moon would, however, be very different if, instead
of being merely let fall, it was thrown sideways. The effect of the
earth’s pull upon the moon would then be shown in keeping the moon
revolving around us instead of allowing it to fly away altogether, as
it would have done had the earth not been there to attract it.
[Illustration: FIG. 44.--An Illustration to explain the Movement of the
Moon]
We can explain this by an illustration. On the top of a mountain I have
placed a big cannon (Fig. 44). We fire off the cannon, and the bullet
flies away in a curved path, with a gradual descent until it falls to
the ground. I have made the mountain look hundreds of times larger than
any mountain could possibly be; and now I want you to imagine a cannon
far stronger and gunpowder more potent than any powder or cannon that
has ever yet been manufactured. Fire off a bullet with a still greater
charge than the last time, and now the path is a much longer one, but
still the bullet curves down so as ultimately to fall on the earth. But
make now one final shot with a charge sufficiently powerful, and away
flies the bullet, following this time the curvature of the earth, for
the earth’s attraction has the effect of bending the path of the bullet
from a straight line into this circular form. By the time the bullet
has travelled a quarter of the way round, it is no nearer to the earth
than it was at first, nor has it parted with any of its original speed.
Thus, notwithstanding its long journey, the bullet has practically just
as much energy as when it first left the muzzle of the cannon. Away it
will fly round another quarter of the earth, and still in the same
condition it will accomplish the third and the fourth quarters, thus
returning to the point from which it started. If we have cleared the
cannon out of the way, the bullet will fly again over the mountain top
without having lost any of its speed by its voyage round the earth.
Therefore it will be in a condition to start again, and thus to revolve
around the earth permanently. If, then, from the top of a mountain
240,000 miles high a great bullet 2000 miles in diameter had once been
projected with the proper velocity, that bullet would continue forever
to circle round and round the earth, and even though the mountain and
the cannon disappeared, the motion would be preserved indefinitely.
This illustration will, at all events, show how a continuous revolution
of the moon round the earth can exist, notwithstanding that the earth
is constantly pulling our satellite down towards its surface.