Nur ad-Din al-Bitruji

Nur ad-Din al-Bitruji
Born	12th century Morocco
Died	c. 1204 Al-Andalus (South of Spain)
Residence	Caliphate
Academic background
Influences	Avempace, Ibn Tufail
Academic work
Era	Islamic Golden Age
Main interests	Astronomy
Notable ideas	First non‐Ptolemaic astronomical system; physical cause of celestial motions
Influenced	Copernicus

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) (born in Morocco, died in South of Spain c. 1204) was an astronomer and a Qadi from Al-Andalus. Al-Biṭrūjī was the first astronomer to present a non-Ptolemaic astronomical system, with the planets borne by geocentric spheres, 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.

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