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

or of accounting for minor details of the motions.¹ The leading
idea is that of the homocentric spheres, each star being attached
to a sphere, and the motive power is the ninth sphere, the sphere
outside that of the fixed stars. The Spanish philosopher ought,
therefore, to have been content with the system of Eudoxus or
its modification by Aristotle (whom he never mentions by name,
but only as "the sage"), but unfortunately he became possessed
with the notion that the prime mover must everywhere produce
only a motion from east to west, and he had, therefore, to reject
the independent motion of the planets from west to east, and
revert to the old Ionian idea that the seven planets merely per-
form the daily revolution with a speed slightly slower than that
of the fixed stars. The true speed of the primum mobile is a
little faster than this ; the eighth sphere performs a revolution in
a slightly longer period (24 hours), and the effect of the prime
mover is gradually weakened more and more, with increasing
distance, until we find the sphere of the Moon, being furthest from
the prime mover, taking nearly twenty-five hours to complete a
revolution. This was the old primitive Ionian idea, but Al
Betrugi (or his teacher) saw that this was not sufficient, as not
only is the pole of the ecliptic different from that of the equator,
which prevents the planets from moving in closed orbits, but the
planets do not even keep at the same distance from the pole of the
ecliptic but have each their motion in latitude, as well as variable
velocity in longitude ; and all this had yet to be accounted for.
The ninth sphere has but one motion, but the eighth has two,
that in longitude (precession) and another which is caused by the
pole of the ecliptic describing a small circle round a mean position,
thereby producing the supposed oscillation or trepidation of the
equinoxes.² Similarly, the pole of each planet describes a small
circle round a mean position (i. e. the pole of the ecliptic), thereby
producing inequalities in longitude and motion in latitude.³
Whenever the actual orbit-pole of a planet is on the parallel
of the mean pole, it is obvious that the planet will perform its
daily revolution with its mean velocity, while the velocity is
increased or lessened when the actual pole is respectively at its
minimum or maximum distance from the pole of the heavens

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1. Fol. 8 b. 2. Fol. 9 b.
2. Fol. 14 b;. sq.