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.

We may illustrate the matter by supposing the planet to be running
like a railway engine on a track which has been laid in a long
elliptic path. We may suppose that while the planet is coursing
along, the shape of the track is gradually altering. But this
alteration may be so slow, that it does not appreciably affect the
movement of the engine in a single revolution. We can also suppose
that the plane in which the rails have been laid has a slow
oscillation in level, and that the whole orbit is with more or less
uniformity moved slowly about in the plane.

In short periods of time the changes in the shapes and positions of
the planetary orbits, in consequence of their mutual attractions, are
of no great consequence. When, however, we bring thousands of years
into consideration, then the displacements of the planetary orbits
attain considerable dimensions, and have, in fact, produced a