Not only does the earth attract every
other body, and every other body attract the earth, but each of these
bodies attracts the other; so that in its more complete shape the law of
gravitation announces that "every body in the universe attracts every
other body with a force which varies inversely as the square of the
distance."

It is impossible for us to over-estimate the importance of this law. It
supplies the clue by which we can unravel the complicated movements of
the planets. It has led to marvellous discoveries, in which the law of
gravitation has enabled us to anticipate the telescope, and to feel the
existence of bodies before those bodies have even been seen.

An objection which may be raised at this point must first be dealt with.
It seems to be, indeed, a plausible one. If the earth attracts the moon,
why does not the moon tumble down on the earth? If the earth is
attracted by the sun, why does it not tumble into the sun? If the sun is
attracted by other stars, why do they not rush together with a frightful
collision? It may not unreasonably be urged that if all these bodies in
the heavens are attracting each other, it would seem that they must all
rush together in consequence of that attraction, and thus weld the whole
material universe into a single mighty mass. We know, as a matter of
fact, that these collisions do not often happen, and that there is
extremely little likelihood of their taking place. We see that although
our earth is said to have been attracted by the sun for countless ages,
yet the earth is just as far from the sun as ever it was. Is not this in
conflict with the doctrine of universal gravitation? In the early days
of astronomy such objections would be regarded, and doubtless were
regarded, as well-nigh insuperable; even still we occasionally hear them
raised, and it is therefore the more incumbent on us to explain how it
happens that the solar system has been able to escape from the
catastrophe by which it seems to be threatened.

There can be no doubt that if the moon and the earth had been initially
placed _at rest_, they would have been drawn together by their mutual
attraction. So, too, if the system of planets surrounding the sun had
been left initially _at rest_ they would have dashed into the sun, and
the system would have been annihilated. It is the fact that the planets
are _moving_, and that the moon is _moving_, which has enabled these
bodies successfully to resist the att