is
indeed precisely similar with regard to the moon. The continual pull of
the earth's gravitation takes the place of the string. If the moon were
to go round and round slower than it does, it would tend to fall in
towards the earth; if, on the other hand, it were to go faster, it would
tend to rush away into space.

The same kind of pull which the earth exerts upon the objects at its
surface, or upon its satellite, the moon, exists through space so far as
we know. Every particle of matter in the universe is found in fact to
attract every other particle. The moon, for instance, attracts the earth
also, but the controlling force is on the side of the much greater mass
of the earth. This force of gravity or attraction of gravitation, as it
is also called, is perfectly regular in its action. Its power depends
first of all exactly upon the mass of the body which exerts it. The
gravitational pull of the sun, for instance, reaches out to an enormous
distance, controlling perhaps, in their courses, unseen planets circling
far beyond the orbit of Neptune. Again, the strength with which the
force of gravity acts depends upon distance in a regularly diminishing
proportion. Thus, the nearer an object is to the earth, for instance,
the stronger is the gravitational pull which it gets from it; the
farther off it is, the weaker is this pull. If then the moon were to be
brought nearer to the earth, the gravitational pull of the latter would
become so much stronger that the moon's rate of motion would have also
to increase in due proportion to prevent her from being drawn into the
earth. Last of all, the point in a body from which the attraction of
gravitation acts, is not necessarily the centre of the body, but rather
what is known as its _centre of gravity_, that is to say, the balancing
point of all the matter which the body contains.

It should here be noted that the moon does not actually revolve around
the centre of gravity of the earth. What really happens is that both
orbs revolve around their _common_ centre of gravity, which is a point
within the body of the earth, and situated about three thousand miles
from its centre. In the same manner the planets and the sun revolve
around the centre of gravity of the solar system, which is a point
within the body of the sun.

The neatly poised movements of the planets around the sun, and of the
satellites around their respective planets, will therefore be readily
understood to result