planets, Mars, Jupiter, &c., would wholly cease.

The complexity of the old mode of regarding apparent motion may be
illustrated by the case of a traveller in a railway train unaware
of his own motion. It is as though trees, hedges, distant objects,
were all flying past him and contorting themselves as you may see
the furrows of a ploughed field do when travelling, while you
yourself seem stationary amidst it all. How great a simplicity
would be introduced by the hypothesis that, after all, these things
might be stationary and one's self moving.

[Illustration: FIG. 14.--Copernican system as frequently represented.
But the cometary orbit is a much later addition, and no attempt is made
to show the relative distances of the planets.]

Now you are not to suppose that the system of Copernicus swept away the
entire doctrine of epicycles; that doctrine can hardly be said to be
swept away even now. As a description of a planet's motion it is not
incorrect, though it is geometrically cumbrous. If you describe the
motion of a railway train by stating that every point on the rim of each
wheel describes a cycloid with reference to the earth, and a circle with
reference to the train, and that the motion of the train is compounded
of these cycloidal and circular motions, you will not be saying what is
false, only what is cumbrous.

The Ptolemaic system demanded large epicycles, depending on the motion
of the earth, these are what Copernicus overthrew; but to express the
minuter details of the motion smaller epicycles remained, and grew more
and more complex as observations increased in accuracy, until a greater
man than either Copernicus or Ptolemy, viz. Kepler, replaced them all by
a simple ellipse.

One point I must not omit from this brief notice of the work of
Copernicus. Hipparchus had, by most sagacious interpretation of certain
observations of his, discovered a remarkable phenomenon called the
precession of the equinoxes. It was a discovery of the first magnitude,
and such as would raise to great fame the man who should have made it in
any period of the world's history, even the present. It is scarcely
expressible in popular language, and without some technical terms; but I
can try.

The plane of the earth's orbit produced into the sky gives the apparent
path of the sun throughout a year. This path is known as the ecliptic,
because eclipses only happen when the moon is in it. The