ब्राह्मस्फुटसिद्धान्त (ब्रह्मगुप्त - शून्य, कुट्टक, बीजगणित एवं सम्पूर्ण २१ अध्याय सान्वय सटीक)

ARABS ON MOTIONS OF FIXED STARS 41
twice that of Mars.¹ 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.²
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.³ 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, Tābit
ben Korra, substituted a physically less objectionable theory.⁴

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 'l Faraj, p. 199, gives
   a similar series from 93 to 15½ 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 philosopher Ibn Sina (Avicenna) defined
   as a spherical, single (not composite) body, emanating directly from God
   and subject to dissolution, 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.