DECLINE OF ALEXANDRIAN SCIENCE 



121 



With the more complicated lunar theory he was naturally less 

 successful. He is believed, however, to have discovered the more 

 important irregularities of the moon's motion, supposing it to 

 have a circular orbit in a plane making an angle of 5 with that of 

 the sun's orbit the ecliptic. The earth is not at the centre, 

 but the latter revolves about the earth in a period of nine years. 



Extending his study of eclipses to the ancient records of the Chal- 

 deans, he made substantial improvements in the theory of both 

 solar and lunar eclipses, and obtained a close approximation for 

 the distance of the moon. He estimated the sun's radius at about 

 twelve times that of the earth, its distance from the earth at about 

 2550 earth-radii, the moon's radius ^nnr that of the earth, its 

 distance about 60 earth-radii. The comparison of these figures 

 with Ptolemy's and with the actual are (in earth-radii) 



Hipparchus realized that he had no adequate method for de- 

 termining these numbers for the sun. 



The generally accepted order of the planets had now become 

 Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, an order 

 adopted very early in Babylonia, and received as a more or less 

 probable hypothesis from this time until that of Copernicus. In 

 attempting to deal with the motions of the other planets as he had 

 done with that of the sun and moon, Hipparchus was soon baffled 

 by lack of adequate data, and set himself steadfastly to supply 

 the need, resigning to more fortunate future astronomers the task 

 of interpretation. 



Eudoxus, more than two centuries earlier, had developed a logical 

 mathematical theory of the planetary motions. The more exact 

 methods and data of Hipparchus brought out the entire inadequacy 

 of existing theory to furnish anything better than a crude approxi- 



