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
about 120 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 of 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. Moreover, Ptolemy professes to deduce from a comparison of his observations with those of Hipparchus and others the (erroneous) value 36" for the precession, which Hipparchus had given as the least possible value, and which Ptolemy regards as his final estimate. But an examination of
