g round it. An
ancient Egyptian system perceived this truth; but the Ptolemaic
system imagined them to revolve round the earth like the rest, with
an artificial system of epicycles to prevent their ever getting far
away from the neighbourhood of the sun.

It is easy now to see how the Copernican system explains the main
features of planetary motion, the stations and retrogressions,
quite naturally and without any complexity.

[Illustration: FIG. 11.--Egyptian system.]

Let the outer circle represent the orbit of Jupiter, and the inner
circle the orbit of the earth, which is moving faster than Jupiter
(since Jupiter takes 4332 days to make one revolution); then
remember that the apparent position of Jupiter is referred to the
infinitely distant fixed stars and refer to fig. 12.

Let E_1, E_2, &c., be successive positions of the earth; J_1,
J_2, &c., corresponding positions of Jupiter. Produce the lines
E_1 J_1, E_2 J_2, &c., to an enormously greater circle
outside, and it will be seen that the termination of these lines,
representing apparent positions of Jupiter among the stars,
advances while the earth goes from E_1 to E_3; is almost
stationary from somewhere about E_3 to E_4; and recedes from
E_4 to E_5; so that evidently the recessions of Jupiter are
only apparent, and are due to the orbital motion of the earth. The
apparent complications in the path of Jupiter, shown in Fig. 10,
are seen to be caused simply by the motion of the earth, and to be
thus completely and easily explained.

[Illustration: FIG. 12.--True orbits of Earth and Jupiter.]

The same thing for an inferior planet, say Mercury, is even still
more easily seen (_vide_ figure 13).

The motion of Mercury is direct from M'' to M''', retrograde from
M''' to M'', and stationary at M'' and M'''. It appears to
oscillate, taking 72.5 days for its direct swing, and 43.5 for its
return swing.

[Illustration: FIG. 13.--Orbit of Mercury and Earth.]

On this system no artificiality is required to prevent Mercury's
ever getting far from the sun: the radius of its orbit limits its
real and apparent excursions. Even if the earth were stationary,
the motions of Mercury and Venus would not be _essentially_
modified, but the stations and retrogressions of the superior