ASTRONOMY 165 



of the celestial movements. In the doctrines professed 

 by this school, it is very difficult to separate the ideas 

 of the master from those of his disciples. 



Although Pythagoras affirmed that the earth is 

 motionless, 1 it appears that he must be given the credit 

 of recognizing that it is a sphere, it may be because 

 he considered this figure perfect, or it may be that he 

 had recognized it in the shape of the terrestrial shadow 

 which causes the lunar eclipses. He was the first to 

 distinguish, in the progression of the sun, of the moon 

 and even of the planets, two movements which take 

 place about distinct poles. One of these movements 

 is diurnal, along the plane of the equator ; the other is 

 annual, in an opposite direction to the first, along the 

 plane of the ecliptic. This is all that can be reasonably 

 attributed to Pythagoras. 



One of his disciples, Philolaus, a contemporary of 

 Socrates, developed the conceptions of his master in 

 the following manner. The spherical universe is sur- 

 rounded by a fire which sustains it, and of which a 

 part is also condensed at its centre. The central fire 

 produces the diffused light of day and the outer fire 

 feeds the stars. The space which separates them is 

 divided into three concentric regions. The most distant 

 is the Olympus, or the sphere of the fixed stars. Then 

 comes the Cosmos, in which are found successively, as 

 the central fire is approached, the planets, the sun, and 

 the moon. The sun, moreover, is not self-luminous, 

 it is a transparent mass like glass, which receives the 

 illumination of the fire from above and sends it back 

 to the earth. Lastly, the Uranus forms the sublunar 

 region in which " are found the things subject to genera- 

 tion, the prerogative of that which animates the trans- 

 mutations." (Aetius, Diels, Vor. I, 237, 23.) This 

 radical distinction between the sublunar region and 

 the space which extends from the moon to the confines 



1 13 Duhem, Systhne, I, p. 8. 



