THE GREEK ASTRONOMY. 



of t'he oblique motion of the moon into two separate motions, by 

 Eudoxus, was not the simplest way of conceiving it ; and Calippus 

 imagined the connection of these spheres in some way which made it 

 necessary yearly to double their number ; in this manner his system 

 had no less than 55 spheres. 



Such was the progress which the Idea of the hypothesis of epicycles 

 had made in men's minds, previously to the establishment of the the- 

 ory by Hipparchus. There had also been a preparation for this step, 

 on the other side, by the collection of Facts. "We know that observa- 

 tions of the Eclipses of the Moon were made by the Chaldeans 367 

 B. c. at Babylon, and were known to the Greeks ; for Hipparchus and 

 Ptolemy founded their Theory of the Moon on these observations. 

 Perhaps we cannot consider, as equally certain, the story that, at the 

 time of Alexander's conquest, the Chaldeans possessed a series of ob- 

 servations, which went back 1903 years, and which Aristotle caused 

 Callisthenes to brino- to him in Greece. All the Greek observations 



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which are of any value, begin with the school of Alexandria. Aris- 

 tyllus and Timocharis appear, by the citations of Hipparchus, to have 

 observed the Places of Stars and Planets, and the Times of the Sol- 

 stices, 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 Equi- 

 noxes. 



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 besran 331 B. c. These are the dates used 



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by Hipparchus and Ptolemy. 



