* 52, ss] Ptolemy and his Successors 73 



Mercury accompany the sun, and may therefore be regarded 

 as on the average performing their revolutions in a y^ar, 

 the test to some extent failed in their case, but Ptolemy 

 again accepted the opinion of the " ancient mathematicians " 

 (i.e. probably the Chaldaeans) that Mercury and Venus lie 

 between the sun and moon, Mercury being the nearer to 

 us. (Cf. chapter i., 15.) 



52. There has been much difference of opinion among 

 astronomers as to the merits of Ptolemy. Throughout the 

 Middle Ages his authority was regarded as almost final on 

 astronomical matters, except where it was outweighed by 

 the even greater authority assigned to Aristotle. Modern 

 criticism has made clear, a fact which indeed he never 

 conceals, that his work is to a large extent based on that 

 of Hipparchus ; and that his observations, if not actually 

 fictitious, were at any rate in most cases poor. On the 

 other hand his work shews clearly that he was an accom- 

 plished and original mathematician.* The most important 

 of his positive contributions to astronomy were the discovery 

 of evection and his planetary theory, but we ought probably 

 to rank above these, important as they are, the services 

 which he rendered by preserving and developing the great 

 ideas of Hipparchus ideas which the other astronomers 

 of the time were probably incapable of appreciating, and 

 which might easily have been lost to us if they had not 

 been embodied in the Almagest. 



53. The history of Greek astronomy practically ceases 

 with Ptolemy. The practice of observation died out so 

 completely that only eight observations are known to have 

 been made during the eight and a half centuries which 

 separate him from Albategnius (chapter HI., 59). The 

 onlv Greek writers after Ptolemy's time are compilers and 

 commentators, such as Theon (fl. A.D. 365), to none of 

 whom original ideas of any importance can be attributed. 

 The murder of his daughter Hypatia (A.D. 415), herself 

 also a writer on astronomy, marks an epoch in the decay 

 of the Alexandrine school ; and the end came in A.D. 640, 

 when Alexandria was captured by the Arabs.f 



* De Morgan classes him as a geometer with Archimedes, Euclid, 

 and Apollonius, the three great geometers of antiquity. 



f The legend that the books in the library served for six months as 



