THE ALEXANDRIAN PERIOD 87 



from this that the vernal equinox is displaced along the 

 ecliptic, and thus he discovered the precession of the 

 equinoxes. 1 He propounded the problem, which bears 

 his name, concerning the irregular movement of the 

 sun, and he solved it by means of an eccentric move- 

 ment which he calculated. He also discussed the 

 irregularities of the moon and attempted to determine 

 its parallax, and he thus succeeded in accurately pre- 

 dicting eclipses, which justifies the admiration of 

 Pliny (Nat. Hist., I, p. 143, 14). As Bigourdan re- 

 marks, " With this extraordinary man there suddenly 

 appears a perfected astronomy, far superior to that of 

 the preceding age ; the theories of the sun and moon 

 are formulated, and those of the planets outlined ; the 

 great desideratum of ancient astronomy, the prediction 

 of eclipses, is now a problem solved. For the first 

 time, the positions of a great number of stars scattered 

 in the sky were known, and by the discovery of the 

 precession their co-ordinates for any period could be 

 calculated." 2 Hipparchus considered that geography 

 as a science must be based on precise astronomical 

 data, and he severely reproached Eratosthenes for not 

 having satisfied this condition. But taking into 

 account the difficulties of the work, these reproaches 

 are unjust. Moreover they had the effect of retarding 

 the scientific development of geography, which from 

 that time became merely descriptive and ethno- 

 graphical until its mathematical and astronomical 

 aspects were once more studied by Strabo. The 

 latter, however, looked upon exact science as only an 

 occasional help to geography, the main work being to 

 describe the countries known and inhabited in the time 

 of Augustus, and not to make a study of the dimensions 

 of the earth. Strabo certainly acquitted himself mar- 

 vellously of his self-appointed task, particularly as 



1 Doublet, Histoive de I'Astronomie, p. 106. 



2 2 Bigourdan, Astronomie, p. 279. 



