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 aspect 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 ("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. His alternative system spread through most of Europe during the 13th century, with debates and refutations of his ideas continued up to the 16th century.
Copernicus cited his system in the
De revolutionibus while discussing theories of the order of the inferior planets. ==Works==