34 ^ Short History of Astronomy [Ca. II. 



elsewhere, might with advantage have been noted and 

 followed by many of the so-called Aristotelians of the 

 Middle Ages and of the Renaissance.* 

 s 31. After the time of Aristotle the centre of Greek 

 ^/scientific thought moved to Alexandria. Founded by 

 Alexander the Great (who was for a time a pupil of 

 Aristotle) in 332 B.C., Alexandria was the capital of Egypt 

 during the reigns of the successive Ptolemies. These 

 kings, especially the second of them, surnamed Phila- 

 delphos, were patrons of learning ; they founded the 

 famous Museum, which contained a magnificent library 

 as well as an observatory, and Alexandria soon became 

 the home of a distinguished body of mathematicians and 

 astronomers. During the next five centuries the only 

 astronomers of importance, with the great exception of 

 Hipparchus ( 37), were Alexandrines. 



32. Among the earlier members of the Alexandrine 

 school were Aristarchus of Samos, Aristyllus, and Timo- 

 charis, three nearly contemporary astronomers belonging 



FIG. 13. The method of Aristarchus for comparing the distances 

 of the sun and moon. 



to the first half of the 3rd century B.C. The views of 

 Aristarchus on the motion of the earth have already been 

 mentioned (24). A treatise of his On the Magnitudes 

 and Distances of the Sun and Moon is still extant : he there 

 gives an extremely ingenious method for ascertaining the 

 comparative distances of the sun and moon. If, in the 

 figure, E, s, and M denote respectively the centres of the 

 earth, sun, and moon, the moon evidently appears to an 

 observer at E half full when the angle E M s is a right 

 angle. If when this is the case the angular distance 

 between the centres of the sun and moon, i.e. the angle 

 M E s, is measured, two angles of the triangle M E s are 



* See, for example, the account of Galilei's controversies, in 

 chapter vi. 



