Thus if in the course of a year _either_ the sun passes successively through the positions A, B, C, D (fig. 38), the earth remaining at rest at E, _or_ if the sun is at rest and the earth passes successively through the positions _a_, _b_, _c_, _d_, at the corresponding times, the sun remaining at rest at S, exactly the same effect is produced on the eye, provided that the lines _a_ S, _b_ S, _c_ S, _d_ S are, as in the figure, equal in length and parallel in direction to E A, E B, E C, E D respectively. The same being true of intermediate points, exactly the same apparent effect is produced whether the sun describe the circle A B C D, or the earth describe at the same rate the equal circle _a b c d_. It will be noticed further that, although the corresponding motions in the two cases are at the same times in _opposite_ directions (as at A and _a_), yet each circle as a whole is described, as indicated by the arrowheads, in the _same_ direction (contrary to that of the motion of the hands of a clock, in the figures given). It follows in the same sort of way that an apparent motion (as of a planet) may be explained as due partially to the motion of the object, partially to that of the observer. [Illustration: FIG. 38.—The relative motion of the sun and moon.] Coppernicus gives the familiar illustration of the passenger in a boat who sees the land apparently moving away from him, by quoting and explaining Virgil’s line:— “Provehimur portu, terræque urbesque recedunt.” [Illustration: FIG. 39.—The daily rotation of the earth.]