Let E1, E2, &c., be successive positions of the earth; J1, J2, &c., corresponding positions of Jupiter. Produce the lines E1 J1, E2 J2, &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 E1 to E3; is almost stationary from somewhere about E3 to E4; and recedes from E4 to E5; 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.
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.
Fig. 13.—Orbit of Mercury and Earth.