sion of the equinoxes is
seen to be dependent on, and caused by, a slow conical movement of the
earth's axis.

The prolongation of each end of the earth's axis into the sky, or the
celestial north and south poles, will thus slowly trace out an
approximate circle among the stars; and the course of the north pole
during historic time is exhibited in the annexed diagram.

It is now situated near one of the stars of the Lesser Bear, which we
therefore call the Pole star; but not always was it so, nor will it be
so in the future. The position of the north pole 4000 years ago is shown
in the figure; and a revolution will be completed in something like
26,000 years.[3]

[Illustration: FIG. 15.--Slow movement of the north pole in a circle
among the stars. (Copied from Sir R. Ball.)]

This perception of the conical motion of the earth's axis was a
beautiful generalization of Copernik's, whereby a multitude of facts
were grouped into a single phenomenon. Of course he did not explain the
motion of the axis itself. He stated the fact that it so moved, and I do
not suppose it ever struck him to seek for an explanation.

An explanation was given later, and that a most complete one; but the
idea even of seeking for it is a brilliant and striking one: the
achievement of the explanation by a single individual in the way it
actually was accomplished is one of the most astounding things in the
history of science; and were it not that the same individual
accomplished a dozen other things, equally and some still more
extraordinary, we should rank that man as one of the greatest
astronomers that ever lived.

As it is, he is Sir Isaac Newton.

We are to remember, then, as the life-work of Copernicus, that he placed
the sun in its true place as the centre of the solar system, instead of
the earth; that he greatly simplified the theory of planetary motion by
this step, and also by the simpler epicyclic chain which now sufficed,
and which he worked out mathematically; that he exhibited the precession
of the equinoxes (discovered by Hipparchus) as due to a conical motion
of the earth's axis; and that, by means of his simpler theory and more
exact planetary tables, he reduced to some sort of order the confused
chaos of the Ptolemaic system, whose accumulation of complexity and of
outstanding errors threatened to render astronomy impossible by the mere
burden of its detail.

There are many imperfections in his system, it is true; but his