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 arrow-heads, 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.
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."
78. The application of the same ideas to an apparent rotation round the observer, as in the case of the apparent daily motion of the celestial sphere, is a little more difficult. It must be remembered that the eye has no means of
