Nur ad-Din al-Bitruji
|Era||Islamic Golden Age|
|Notable ideas||First non‐Ptolemaic astronomical system; physical cause of celestial motions|
|Criticism & Awards|
Nur ad-Din al-Bitruji (also spelled Nur al-Din Ibn Ishaq Al-Betrugi and Abu Ishâk ibn al-Bitrogi; another spelling is al Bidrudschi) (known in the West by the Latinized name of Alpetragius) (died ca. 1204 AD) was an astronomer and a Qadi from Al-Andalus. Despite his limited knowledge of Ptolomy's Almagest, al-Biṭrūjī was the first astronomer after Ptolomy 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.
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.
Planetary model 
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.
Al-Bitruji wrote Kitāb al-Hayʾah (The book of theoretical astronomy/cosmology, Arabic, كتاب الهيئة), which presented criticism of Ptolomy's Almagest from a physical point of view. It was well known in Europe between the 13th and the 16th centuries, and was regarded as a valid alternative to Ptolemy's Almagest in scholastic circles.
- Samsó 2007.
- Bernard R. Goldstein (March 1972). "Theory and Observation in Medieval Astronomy", Isis 63 (1), p. 39-47 .
- Ptolemaic Astronomy, Islamic Planetary Theory, and Copernicus's Debt to the Maragha School, Science and Its Times, Thomson Gale.(inaccessible document)
- Pederson, Olaf. (1978) Science in the Middle Ages. ed. by David Lindberg. Chicago: Chicago University Press. p. 321
- Samsó, Julio (2007). "Biṭrūjī: Nūr al‐Dīn Abū Isḥāq [Abū Jaʿfar] Ibrāhīm ibn Yūsuf al‐Biṭrūjī". In Thomas Hockey et al. The Biographical Encyclopedia of Astronomers. New York: Springer. pp. 133–4. ISBN 978-0-387-31022-0. (PDF version)
Further reading 
- Helaine Selin, Encyclopaedia of the history of science, technology, and medicine in non western cultures, p. 160