188 THE GREEK ASTRONOMY. 



241 revolutions of the moon with regard to the 

 stars, but only 239 revolutions with regard to the 

 anomaly. This difference could be suitably repre- 

 sented by supposing the eccentric, in which the 

 moon moves, to have itself an angular motion, per- 

 petually carrying its apogee in the same direction in 

 which the moon travels; but this supposition being 

 made, it was necessary to determine, not only the 

 eccentricity of the orbit, and place of the apogee at 

 a certain time, but also the rate of motion of the 

 apogee itself, in order to form tables of the moon. 



This task, as we have said, Hipparchus executed ; 

 and in this instance, as in the problem of the reduc- 

 tion of the sun's motion to tables, the data which 

 he found it necessary to employ were very few. He 

 deduced all his conclusions from six eclipses of the 

 moon 3 . Three of these, the records of which were 

 brought from Babylon, where a register of such 

 occurrences was kept, happened in the 366th and 

 367th years from the era of Nabonassar, and ena- 

 bled Hipparchus to determine the eccentricity and 

 apogee of the moon's orbit at that time. The three 

 others were observed at Alexandria, in the 547th 

 year of Nabonassar, which gave him another posi- 

 tion of the orbit at an interval of 180 years; and 

 he thus became acquainted with the motion of the 

 orbit itself, as well as its form 4 . 



3 Ptol. Syn. iv. 10. 



4 Ptolemy uses the hypothesis of an epicycle for the moon's 

 first inequality : but Hipparchus employs an eccentric. 



