DECLINE OF ALEXANDRIAN SCIENCE 119 



sible positions of the planet in its epicycle. The distance of P 

 from E obviously varies; the apparent motion of P being com- 

 pounded of a forward motion of C and a backward motion at PI 

 is slower, at PS faster, than the average. By suitable adjust- 

 ment of the dimensions and velocities there may be retrogression 

 for a certain length of arc near PI, bounded by stationary points 

 where the two motions seem to an observer at E to neutralize each 

 other. 



How far this complicated scheme really departed from the 

 original postulate of uniform circular motion is sufficiently in- 

 dicated by Proclus' remark, "The astronomers who have pre- 

 supposed uniformity of motions of the celestial bodies were ig- 

 norant that the essence of these movements is, on the contrary, 

 irregularity." While in point of fact the theory of epicycles and 

 that of excentric circles have much in common, the former gradu- 

 ally displaced the latter on account of its greater simplicity. Had 

 Aristarchus worked out the earlier system in full detail, the history 

 of astronomy might have been considerably modified. 



At the Museum of Alexandria a school of observers of whom 

 Aristillus and Timocharis were notable members instituted sys- 

 tematic astronomical observations with graduated instruments 

 and made a small star catalogue. Thus was laid a foundation 

 for the brilliant discoveries of Hipparchus and Ptolemy, while 

 astronomy, which had in the work of Eudoxus assumed the 

 character of true science, though with a too slender observational 

 basis, now became an exact science, gradually shedding its encum- 

 brances of speculation and vague generalization. 



HIPPARCHUS. STAR CATALOGUE. The next great astronomer 

 and much the greatest of antiquity is Hipparchus, probably a 

 native of Bithynia, but long resident at Rhodes, a city which 

 rivalled Alexandria itself in its intellectual activity. All his works 

 but one are lost, but his great successor and disciple, Ptolemy, has 

 based his famous Almagest on the work of Hipparchus and it is 

 possible to determine in a general way how much is to be credited 

 to each. Having at his disposal the primitive star catalogue of 

 Aristillus and Timocharis, Hipparchus was prof oundly impressed 



