ARABS ON MOTIONS OF FIXED STARS 41 twice that of Mars. As to the nature of the stars, they seem generally to have been assumed self-luminous, being condensed parts of the sphere, though Abraham ben Chija says that the eighth sphere does not shine with a uniform light, but has denser spots, which are illuminated by the Sun and appear to us as the fixed stars.* To account for the apparent slow motion of the stars para lleled to the ecliptic, from west to east, whereby their longitudes increase while their latitudes remain unaltered, it became neces- sary to inroduce a ninth sphere (primum mobile), turning in twenty-four hours and communicating this motion to the eighth sphere, while the latter moved extremely slowly round its own axis forming an angle of 23° 35' with that of the ninth. But the simple phenomenon of precession was by many Arabian astro- nomers complicated by being assumed variable. It may be mentioned that according to Theon and Proklus it had been assumed by some astronomers apparently before the time of Ptolemy, that the precessional motion of the stars was not pro- gressive, but was confined to an oscillation along an arc of 8%, along which the equinoctial points moved backwards and forwards on the ecliptic, always at the same rate of 1° in 80 years. The absurdity of the sudden change of direction must have become obvious as soon as astronomy began to be cultivated among the Arabs, for we find that one of the earliest astronomers, Tabit ben Korra, substituted a physically less objectionable theory.* 1. Al Fargani (p. 85, Golius) gives the cubic contents of the six spheres as 107, 90, 72, 54, 36, 18 times that of the Earth. Abu 'I Faraj, p. 199, gives a similar series from 93 to 154 for the average star of each class. Shems ed-din of Damascus in his Cosmography (p. 3) merely says that the smallest fixed star is much larger than the Earth, 2. According to Suter, p. 77. a writer called Ibn Zura wrote a treatise "On the cause of the light of the stars, though they and the spheres consist of one single substance." 3. The outermost sphere is by the philo'opher Ibn Sina (Avicenna) defined as a spherical, single (not composite) body, emanating directly from Gop and subject to dissoluticn, endowed innately with circular motion as an expression of its praise of the Creator (Mehren in Oversigt. K. Danske Vid. Selskab, 1883, p. 70). 4. The treatise "On the motion of the 8th sphere" has never been printed; an abstract is given in Delambre's Hist, de l'astr. du Moyen Age, p. 73. Compare a quotation by Ibn Junis, Caussin, Notices et Extraits. VII. p. 116.
