ASTRONOMY 171 



predecessors thought to account for the appearance of 

 the wandering heavenly bodies by endowing them 

 with a dual revolution, diurnal and non-diurnal, in an 

 opposite direction to each other ; but this conception 

 did not solve the problem. 



The planets situated on the same plane (the ecliptic) 

 as the sun doubtless traverse the same region as the 

 sun, namely, the constellations of the zodiac, but their 

 progression is irregular and shows stationary points 

 followed by a retrograde movement, then an advance, 

 and so on. 



To account for this irregular motion, Eudoxus of 

 Cnidus gave to each wandering heavenly body a 

 mechanism of homocentric spheres touching and 

 enclosing one another and having the earth as their 

 centre. " The heavenly body is situated in the thick- 

 ness of the last of these spheres, the one which is within 

 all the others, and its centre is on the equator of this 

 sphere." * 



The first sphere, that which is exterior to all the 

 rest, turns with a uniform motion from east to west, 

 in twenty-four hours round the axis of the earth shown 

 by the Pole star. In this manner all the planets share 

 in the diurnal rotation which moves the heavenly 

 bodies. The second sphere, resting by means of its 

 axis on the first sphere, is animated by the same uniform 

 movement, but the speed and sense, as well as the 

 direction, of its own movement are different. In 

 fact, this second sphere turns uniformly from west to 

 east around an axis which is normal to the ecliptic. 

 The duration of this revolution is not the same for the 

 various planets ; it is, for example according to Eudoxus, 

 one year for Mercury, eleven years for Jupiter, etc. 



The third sphere, which is interior and contiguous 

 to the second, is affected by the complex movement of 



1 The system of Eudoxus has been reconstituted by Schia- 

 parelli and summarized by 13 Duhem, Systeme I, p. 114. 



