88J 



Stationary Points 



119 



or from west to east. Hence between M, and M its motion 

 has changed from direct to retrograde, and therefore at 

 some intermediate point, say ;//, (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 

 ;;/., between M and M a (about Sept. 13 in fig. 7). 



In the case of a superior planet, say Jupiter, the argument 



FIG. 47. The stationary points of Mercury. 



is nearly the same. When in opposition at j (as on 

 Mar. 26 in fig. 6), Jupiter moves more slowly than the 

 earih, and in the same direction, and therefore appears to 

 be moving in the opposite direction to the earth, i.e. as seen 

 from E (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 j, or j (in which the earth appears 

 to the observer on Jupiter to be at its greatest distance 



