چکیده
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In the Almagest, Ptolemy finds that the apogee of Mercury moves progressively at a speed
equal to his value for the rate of precession, namely one degree per century, in the tropical
reference system of the ecliptic coordinates. He generalizes this to the other planets, so that the
motions of the apogees of all five planets are assumed to be equal, while the solar apsidal line is
taken to be fixed. In medieval Islamic astronomy, one change in this general proposition took
place because of the discovery of the motion of the solar apogee in the ninth century, which
gave rise to lengthy discussions on the speed of its motion. Initially B¯ır ¯un¯ı and later Ibn al-
Zarq¯alluh assigned a proper motion to it, although at different rates. Nevertheless, appealing to
the Ptolemaic generalization and interpreting it as a methodological axiom, the dominant idea
became to extend it in order to include the motion of the solar apogee as well. Another change
occurred after correctly making a distinction between the motion of the apogees and the rate
of precession. Some Western Islamic astronomers generalized Ibn al-Zarq¯alluh’s proper motion
of the solar apogee to the apogees of the planets. Analogously, Ibn al-Sh¯at.ir maintained that
the motion of the apogees is faster than precession. Nevertheless, the Ptolemaic generalization
in the case of the equality of the motions of the apogees remained untouchable, despite the
notable development of planetary astronomy, in both theoretical and observational aspects, in
the late Islamic period.
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