68 A Short History of Astronomy [CH. n. 



Ptolemy obtained the two directions required by means 

 of observations of the moon, and hence found that c M 

 was 59 times c B, or that the distance of the moon was 

 equal to 59 times the radius of the earth. He then uses 

 Hipparchus's eclipse method to deduce the distance of the 

 sun from that of the moon thus ascertained, and finds 

 the distance of the sun to be 1,210 times the radius of 

 the earth. This number, which is substantially the same 

 as that obtained by Hipparchus ( 41), is, however, only 



M 



FIG. 33. Parallax. 



' ?" ; 



about ^ of the true number, as indicated by modern 

 work (chapter xiii^ 284). 



The sixth book is devoted to eclipses, and contains no 

 substantial additions to the work cf Hipparchus. 



50. The seventh and eighth books contain a catalogue of 

 stars, and a discussion of precession ( 42). The catalogue, 

 which contains 1,028 stars (three of which are duplicates), 

 appears to be nearly identical with that of Hipparchus. 

 It contains none of the stars which were visible to Ptolemy 

 at Alexandria, but not to Hipparchus at Rhodes. More- 

 over, Ptolemy professes to deduce from a comparison of 

 "his observations with those of Hipparchus and others the 

 (erroneous) value 56" for the precession, which Hipparchus 

 had given as the least possible value, and which Ptolemy 

 regards as his final estimate. But an examination of 



