n6 



A Short History of Astronomy 



[CH. IV. 



revolve round the sun, since the centre of the epicycle 

 did not always lie in the direction of the sun, but might 

 be anywhere in the ecliptic. One peculiarity, however, 

 in the motion of any of the superior planets might easily 

 have suggested their motion round the sun, and was either 

 completely overlooked by Ptolemy or not recognised by 

 him as important. It is possible that it was one of the 

 clues which led Coppernicus to his system. This peculi- 

 arity is that the radius of the epicycle of the planet, 

 / j, is always parallel to the line E s joining the earth 

 and sun, and consequent^ performs a complete re- 

 volution in a year. This 

 connection between the 

 motion of the planet and 

 that of the sun received 

 no explanation from 

 Ptolemy's theory. Now 

 if we draw E j' parallel 

 to j j and equal to it in 

 length, it is easily seen * 

 that the line j' j is equal 

 and parallel to E/, that 

 consequently j describes 

 a circle round j' just as 

 j round E. Hence the 

 motion of the planet can 

 equally well be repre- 

 sented by supposing it to move in an epicycle (represented 

 by the large dotted circle in the figure) of which j' is the 

 centre and j' j the radius, while the centre of the epicycle, 

 remaining always in the direction of the sun, describes 

 a deferent (represented by the small circle round E) of which 

 the earth is the centre. By this method of representation 

 the motion of the superior planet is exactly like that of 

 an inferior planet, except that its epicycle is larger than 

 its deferent ; the same reasoning as before shows that the 

 motion can be represented simply by supposing the centre 

 ]' of the epicycle to be actually the sun. Ptolemy's epicycle 

 and deferent are therefore capable of being replaced, with- 

 put affecting the position of the planet in the sky, by a 

 * Euclid, I. 33. 



FIG. 45. The epicycle of Jupiter. 



