A Short History of Astronomy 



[CH. II. 



and rapidity of motion of the epicycle. It is moreover 

 evident that with this arrangement the apparent motion 

 of Jupiter will vary considerably, as the two motions that 

 on the epicycle and that of the centre of the epicycle on 

 the deferent are sometimes in the same direction, so as 

 to increase one another's effect, and at other times in 

 opposite directions. Thus, when Jupiter is most distant 

 from the earth, that is at j 3 , the motion is most rapid, ar 

 j l and J 2 the motion as seen from the earth is nearly the 

 same as that of./; while at J 4 the two motions are in 



opposite directions, and the 

 size and motion of the epi- 

 cycle having been chosen in 

 the way indicated above, 

 it is found in fact that the 

 motion of the planet in the 

 epicycle is the greater of the 

 two motions, and that there- 

 fore the planet when in 

 this position appears to be 

 moving from east to west 

 (from left to right in the 

 figure), as is actually the 

 case. As then at ji and 

 J 2 the planet appears to 

 be moving from west to 



east, and at J< in the opposite direction, and sudden 

 changes of motion do not occur in astronomy, there must 

 be a position between jj and J 4 , and another between 

 J 4 and J 2 , at which the planet is just reversing its direction 

 of motion, and therefore appears for the instant at rest. 

 We thus arrive at an explanation of the stationary points 

 (chapter i., 14). An exactly similar scheme explains 

 roughly the motion of Mercury and Venus, except that 

 the centre of the epicycle must always be in the direction 

 of the sun. 



Hipparchus, as we have seen (41), found the current 

 representations of the planetary motions inaccurate, and 

 collected a number of fresh observations. These, with 

 fresh observations of his own, Ptolemy now employed 

 in order to construct an improved planetary system. 



FIG. 34. Jupiter's epicycle 

 and deferent. 



