It is known that the earth revolves in an orbit which,
though nearly circular, is strictly an ellipse. If the earth were
the only planet revolving around the sun then that ellipse would
remain unaltered from age to age. The earth is, however, only one of
a large number of planets which circulate around the great luminary,
and are guided and controlled by his supreme attracting power. These
planets mutually attract each other, and in consequence of their
mutual attractions the orbits of the planets are disturbed from the
simple elliptic form which they would otherwise possess. The
movement of the earth, for instance, is not, strictly speaking,
performed in an elliptical orbit. We may, however, regard it as
revolving in an ellipse provided we admit that the ellipse is itself
in slow motion.
It is a remarkable characteristic of the disturbing effects of the
planets that the ellipse in which the earth is at any moment moving
always retains the same length; that is to say, its longest diameter
is invariable. In all other respects the ellipse is continually
changing. It alters its position, it changes its plane, and, most
important of all, it changes its eccentricity. Thus, from age to age
the shape of the track which the earth describes may at one time be
growing more nearly a circle, or at another time may be departing
more widely from a circle.
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