[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 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.]