But it is impossible for
the orbit of each planet not to be affected in some degree by the
attraction of the other planet. The general law of nature asserts
that every body in space attracts every other body. So long as there
is only a single planet, it is the single attraction between the sun
and that planet which is the sole controlling principle of the
movement, and in consequence of it the ellipse is described. But
when a second planet is introduced, each of the two bodies is not
only subject to the attraction of the sun, but each one of the
planets attracts the other. It is true that this mutual attraction
is but small, but, nevertheless, it produces some effect. It
"disturbs," as the astronomer says, the elliptic orbit which would
otherwise have been pursued. Hence it follows that in the actual
planetary system where there are several planets disturbing each
other, it is not true to say that the orbits are absolutely elliptic.
At the same time in any single revolution a planet may for most
practical purposes be said to be actually moving in an ellipse. As,
however, time goes on, the ellipse gradually varies. It alters its
shape, it alters its plane, and it alters its position in that
plane. If, therefore, we want to study the movements of the planets,
when great intervals of time are concerned, it is necessary to have
the means of learning the nature of the movement of the orbit in
consequence of the disturbances it has experienced.
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