PllELUDE TO THE EPOCH OF HIPPARCHUS. 139 



detect a kind of intricate regularity in tlieir motions, which might 

 naturally be described as " a dance." The Chaldeans are stated by 

 Diodorus 2 to have observed assiduously the risings and settings of the* 

 planets, from the top of the temple of Belus. By doing this, the}' 

 would find the times in which the forward and backward movements 

 of Saturn, Jupiter, and Mars recur ; and also the time in which they 

 come round to the same part of the heavens. 3 Venus and Mercury 

 never recede far from the sun, and the intervals which elapse while 

 either of them leaves its greatest distance from the sun and returns again 

 to the greatest distance on the same side, would easily be observed. 



Probably the manner in which the motions of the planets were 

 originally reduced to rule was something like the following : In about 

 30 of our years, Saturn goes 29 times through his Anomaly, that is, the 

 succession of varied motions by which he sometimes goes forwards 

 and sometimes backwards among the stars. During this time, he goes 

 once round the heavens, and returns nearly to the same place. This 

 is the cycle of his apparent motions. 



Perhaps the eastern nations contented themselves with thus referring 

 these motions to cycles of time, so as to determine their recurrence. 

 Something of this kind was done at an early period, as we have seen. 



But the Greeks soon attempted to frame to themselves a sensible 

 image of the mechanism by which these complex motions were pro- 

 duced ; nor did they find this difficult. Venus, for instance, who, upon 

 the whole, moves from west to east among the stars, is seen, at certain 

 intervals, to return or move retrograde a short way back from east to 

 west, then to become for a short time stationary, then to turn again 

 and resume her direct motion westward, and so on. Now this can be 

 explained by supposing that she is placed in the rim of a wheel, which 

 is turned edgeways to us, and of which the centre turns round in the 

 heavens from west to east, while the wheel, carrying the planet in its 

 motion, moves round its own centre. In this way the motion of the 

 wheel about its centre, would, in some situations, counterbalance the 

 general motion of the centre, and make the planet retrograde, while, 

 on the whole, the westerly motion would prevail. Just as if we sup- 

 pose that a person, holding a lamp in his hand in the dark, and at a 



ficit ; in quo cursu multa mirabiliter efficiens, turn antecedendo, turn retardando, 

 *um vespertinis temporibus delitescendo, turn niatutinis se rursum aperiendo, nihil 

 immutnt sempiternis sceculorura a?tatibus, quin eadem iisdem tcinporibus efficiat." 

 And so of the other planets. 

 * Del. A. A. i. p. 4. s Plin. //. X ii. p. 204. 



