Article ID: CBB915899938

Holding or Breaking with Ptolemy's Generalization: Considerations about the Motion of the Planetary Apsidal Lines in Medieval Islamic Astronomy (2017)

unapi

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 Bīrūnī and later Ibn al-Zarqālluh 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-Zarqālluh's proper motion of the solar apogee to the apogees of the planets. Analogously, Ibn al-Shāṭir 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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Authors & Contributors
Niazi, Kaveh
Ragep, Sally P.
Ragep, F. Jamil
Gamini, Amir Mohammad
Violet Moller
Ghalandari, Hanif
Concepts
Arab/Islamic world, civilization and culture
Astronomy
Hay'a (Heavenly Configurations)
Cosmology
Arabic language
Mathematics
Time Periods
Medieval
13th century
Ancient
9th century
14th century
12th century
Places
Europe
Salerno (Italy)
Córdoba (Spain)
Hellenistic world
Toledo (Spain)
Baghdad (Iraq)
Institutions
Ikhwān al-Ṣafā'
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