PRELUDE TO THE EPOCH OF HIPPARCHUS. 181 



Hipparchus and Ptolemy founded their theory of 

 the moon on these observations. Perhaps we can- 

 not consider, as equally certain, the story that, at 

 the time of Alexander's conquest, the Chaldeans 

 possessed a series of observations, which went back 

 1903 years, and which Aristotle caused Callisthenes 

 to bring to him in Greece. All the Greek observa- 

 tions which are of any value, begin with the school 

 of Alexandria. Aristyllus and Timocharis appear, 

 by the citations of Hipparchus, to have observed 

 the places of stars and planets, and the times of the 

 solstices, at various periods from B.C. 295 to B.C. 

 269. Without their observations, indeed, it would 

 not have been easy for Hipparchus to establish 

 either the theory of the sun or the precession of the 

 equinoxes. 



In order that observations at distant intervals 

 may be compared with each other, they must be 

 referred to some common era. The Chaldeans 

 dated by the era of Nabonassar, which commenced 

 749 B.C. The Greek observations were referred to 

 the Calippic periods of 76 years, of which the first 

 began 331 B.C. These are the dates used by Hip- 

 parchus and Ptolemy. 



