86] 



Motion of the Planets 



the epicycle being always in the direction of the sun, the 

 ratio of the sizes of the epicycle and deferent being fixed, 

 but the actual dimensions being practically arbitrary. 

 Ptolemy preferred on the whole to regard the epicycles of 

 both these planets as lying between the earth and the sun. 

 The idea of making the sun a centre of motion having once 

 been accepted, it was an obvious simplification to make 

 the centre of the epicycle not merely lie in the direction 

 of the sun, but actually be the sun. In fact, if the planet 



FIG. 43. The orbits of Venus and of the earth. 



in question revolved round the sun at the proper distance 

 and at the proper rate, the same appearances would be 

 produced as by Ptolemy's epicycle and deferent, the path 

 of the planet round the sun replacing the epicycle, and the 

 apparent path of the sun round the earth (or the path of 

 the earth round the sun) replacing the deferent. 



In discussing the time of revolution of a planet a dis- 

 tinction has to be made, as in the case of the moon (chap- 

 ter ii., 40), between the synodic and sidereal periods of 

 revolution. Venus, for example, is seen as an evening star 



8 



