nt of astronomy, to which we have as yet made
little reference.

The supreme controlling power in the solar system is the attraction of
the sun. Each planet of the system experiences that attraction, and, in
virtue thereof, is constrained to revolve around the sun in an elliptic
path. The efficiency of a body as an attractive agent is directly
proportional to its mass, and as the mass of the sun is more than a
thousand times as great as that of Jupiter, which, itself, exceeds that
of all the other planets collectively, the attraction of the sun is
necessarily the chief determining force of the movements in our system.
The law of gravitation, however, does not merely say that the sun
attracts each planet. Gravitation is a doctrine much more general, for
it asserts that every body in the universe attracts every other body. In
obedience to this law, each planet must be attracted, not only by the
sun, but by innumerable bodies, and the movement of the planet must be
the joint effect of all such attractions. As for the influence of the
stars on our solar system, it may be at once set aside as inappreciable.
The stars are no doubt enormous bodies, in many cases possibly
transcending the sun in magnitude, but the law of gravitation tells us
that the intensity of the attraction decreases as the square of the
distance increases. Most of the stars are a million times as remote as
the sun, and consequently their attraction is so slight as to be
absolutely inappreciable in the discussion of this question. The only
attractions we need consider are those which arise from the action of
one body of the system upon another. Let us take, for instance, the two
largest planets of our system, Jupiter and Saturn. Each of these globes
revolves mainly in consequence of the sun's attraction, but every planet
also attracts every other, and the consequence is that each one is
slightly drawn away from the position it would have otherwise occupied.
In the language of astronomy, we would say that the path of Jupiter is
perturbed by the attraction of Saturn; and, conversely, that the path of
Saturn is perturbed by the attraction of Jupiter.

For many years these irregularities of the planetary motions presented
problems with which astronomers were not able to cope. Gradually,
however, one difficulty after another has been vanquished, and though
there are no doubt some small irregularities still outstanding which
have not been completely explained, y