Career
Despite his limited knowledge of Ptolemy's Almagest, al-Biṭrūjī was the first astronomer after Ptolemy to present a non-Ptolemaic astronomical system as an alternative to Ptolemy's models. Another original aspect of his system was that he proposed a physical cause of celestial motions. The crater Alpetragius on the Moon is named after him.
Almost nothing about his life is known, except that his name probably derives from Los Pedroches (al-Biṭrawsh), a region near Cordoba. He was a disciple of Ibn Tufail (Abubacer) and was a contemporary of Averroes. Al-Bitruji proposed a theory on planetary motion in which he wished to avoid both epicycles and eccentrics, and to account for the phenomena peculiar to the wandering stars, by compounding rotations of homocentric spheres.
This was a modification of the system of planetary motion proposed by his predecessors, Ibn Bajjah (Avempace) and Ibn Tufail (Abubacer). He was unsuccessful in replacing Ptolemy's planetary model, as the numerical predictions of the planetary positions in his configuration were less accurate than those of the Ptolemaic model, because of the difficulty of mapping Ptolemy's epicyclic model onto Aristotle's concentric spheres. It was suggested based on the Latin translations that his system is an update and reformulation of that of Eudoxus of Cnidus combined with the motion of fixed stars developed by al-Zarqālī.
However, it is not known whether the Andalusian cosmologists had access or knowledge of Eudoxus works. One original aspects of al-Biṭrūjī's system is his proposal of a physical cause of celestial motions. He combines the idea of "impetus" (first proposed by John Philoponus) and the concept of shawq ("desire"), of Abū al‐Barakāt al‐Baghdādī, to explain how energy is transferred from a first mover placed in the 9th sphere to other spheres, explaining the other spheres' variable speeds and different motions.
He contradicts the Aristotelian idea that there is a specific kind of dynamics for each world, applying instead the same dynamics to the sublunar and the celestial worlds.
