Jabir ibn Aflah

Jabir Ibn Aflah

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Jabir ibn Aflah

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Abū Muḥammad Jābir ibn Aflaḥ (, ; 1100–1150) was a Muslim astronomer and mathematician from Seville, who was active in 12th century Andalusia. His work Iṣlāḥ al-Majisṭi (Correction of the Almagest) influenced Islamic, Jewish and Christian astronomers.

This work is a commentary and reworking of Ptolemy's Almagest and is the first criticism of it in the Islamic West. He particularly criticized the mathematical basis of the work. For example he replaced the use of Menelaus' theorem with ones based on spherical trigonometry, in what seems to be an attempt to increase the mathematical precision of the work. These theorems had been developed by a group of 10th century Islamic mathematicians who included Abū al-Wafā' Būzjānī and then also by Abu Abd Allah Muhammad ibn Muadh Al-Jayyani who worked in Andalusia during the 11th century. Interestingly Jābir doesn't credit any of these authors and in fact doesn't refer to a single Islamic author in this work.

One substantial change Jābir made to Ptolemy's account is that he placed the orbits of Venus and Mercury, the minor planets, outside that of the Sun, rather than between the Moon and the Sun as had been the case in the original work.

He invented an observational instrument known as the torquetum, a mechanical device to transform between spherical coordinate systems.


Several later Islamic authors were...
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