$ * 9 ] Aristotle : the Phases of the Moon 3 1 



through the familiar phases of crescent, half full, gibbous, 

 full moon, and gibbous, half full, crescent again.* 



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DIRECTION 



OF THE SUN 



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FIG. 10. The phases of the moon. 



Aristotle then argues that as one heavenly body is 

 spherical, the others must be so also, and supports this 

 conclusion by another argument, equally inconclusive to 



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 FIG. II. The phases of the me on. 



us, that a spherical form is appropriate to bodies moving as 

 the heavenly bodies appear to do. 



29. His proofs that the earth is spherical are more in- 

 teresting. After discussing and rejecting various other 

 suggested forms, he points out that an eclipse of the moon 

 is caused by the shadow of the earth cast by the sun, and 



* I have introduced here the familiar explanation of the phases of 

 the moon, and the argument based on it for the spherical shape of 

 the moon, because, although probably known before Aristotle, there 

 is, as far as I know, no clear and definite statement of the matter in 

 any earlier writer, and after his time it becomes an accepted part of 

 Greek elementary astronomy. It may be noticed that the explanation 

 is unaffected either by the question of the rotation of the earth or 

 by that of its motion round the sun. 



