Arabian astronomers had, however, noticed that this was not correct. Thus Abul Hassan Ali ben Amadjour early in the tenth century stated that he had often measured the greatest latitude of the moon, and found results greater than that of Hipparchus, but varying considerably and irregularly. Ibn Yunis, who quotes this, adds that he had himself found 5° 3′ or 5° 8′. Other Arabians are, however, said to have found from 4° 45′ to 4° 58′, which does not speak well for the accuracy of their observations.[1] Tycho first began to suspect that the value of Hipparchus was wrong when examining an observation of the comet of 1577. On November 13 he had measured the distance of the comet from the moon, and found 18° 30′, while the observed distances of the comet from stars by computation gave its distance from the moon equal to 18° 9′, allowing for the lunar parallax. At first he attributed the difference to refraction, but in 1587, when the moon attained its greatest latitude about Cancer, so that neither errors in the parallax nor refraction could influence the result much, he found the lunar inclination to be 5° 15′, and thought it might have increased since the days of Ptolemy, just as the obliquity of the ecliptic had diminished.[2] The examination of all his observations showed him, however, later, that the inclination varied between 4° 58′ 30″ and 5° 17′ 30″, while the retrograde motion of the nodes was found not to be uniform, so that the true places of the nodes were sometimes as much as 1° 46′ before or behind the mean ones. This inequality of the nodes had not been detected by the ancients, because it disappears in the syzygies and quadratures, where they alone observed the moon. Tycho explained this and the change of inclination by assuming that the true pole
- ↑ L. A. Sédillot, Matériaux pour servir, &c., t. i. p. 283 et seq. The sons of Musa ben Schaker (about 850) seem to have been the first to find a value differing from that of the ancients. Some Chinese observers found 5° 2′. Copernicus, an Internat. Journal of Astronomy, vol. ii. (1882), p. 128.
- ↑ De mundi aeth. rec. phen., p. 40.
