Page:A short history of astronomy(1898).djvu/165

§ 88] or from west to east. Hence between 1 and its motion has changed from direct to retrograde, and therefore at some intermediate point, say m1 (about Aug. 23 in fig. 7), Mercury appears for the moment to be stationary, and similarly it appears to be stationary again when at some point m2 between and 2 (about Sept. 13 in fig. 7).

In the case of a superior planet, say Jupiter, the argument is nearly the same. When in opposition at (as on Mar. 26 in fig. 6), Jupiter moves more slowly than the earth, and in the same direction, and therefore appears to be moving in the opposite direction to the earth, i.e. as seen from  (fig. 48), from left to right, or from east to west, that is in the retrograde direction. But when Jupiter is in either of the positions 1 or (in which the earth appears to the observer on Jupiter to be at its greatest distance