the other side of the
moon, which goes round us, always presenting the same face to us.

The earth is not the only world going round the sun; she has many
brothers and a sister; some are nearer to the sun than she is, and some
are further away, but all circle round the great central light-giver in
rings lying one outside the other. These worlds are called planets, and
the earth is one of them, and one of the smaller ones, too, nothing so
great and important as we might have imagined.

CHAPTER II

HANGING IN SPACE

If you are holding something in your hand and you let it go, what
happens? It falls to the ground, of course. Now, why should it do so?
You will say: 'How could it do anything else?' But that is only because
you are hampered by custom. Try to shake yourself free, and think, Why
should it go down instead of up or any other way? The first man who was
clever enough to find some sort of an answer to this question was the
great philosopher Sir Isaac Newton, though he was not quite the first to
be puzzled by it. After years of study he discovered that every thing
attracts every other thing in proportion to their masses (which is what
you know as weight) and their distance from each other. In more
scientific language, we should say every _body_ instead of every
_thing_, for the word body does not only mean a living body, but every
lump or mass of matter in the universe. The earth is a body in this
sense, and so is the table or anything else you could name. Now as the
earth is immeasurably heavier than anything that is on it, it pulls
everything toward itself with such force that the little pulls of other
things upon each other are not noticed. The earth draws us all toward
it. It is holding us down to it every minute of the day. If we want to
move we have to exert another force in order to overcome this attraction
of the earth, so we exert our own muscles and lift first one foot and
then the other away from the earth, and the effort we make in doing this
tires us. All the while you are walking or running you are exercising
force to lift your feet away from the ground. The pull of the earth is
called gravitation. Just remember that, while we go on to something else
which is almost as astonishing.

We know that nothing here on earth continues to move for ever;
everything has to be kept going. Anything left to itself has a tendency
to stop. Why is this? This is because here in the world there is
something