Naṣīr al-Dīn al-Ṭūsī, in full Muḥammad ibn Muḥammad ibn al-Ḥasan al-Ṭūsī, (born Feb. 18, 1201, Ṭūs, Khorāsān [now Iran]—died June 26, 1274, Baghdad, Iraq), outstanding Persian philosopher, scientist, and mathematician.

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Educated first in Ṭūs, where his father was a jurist in the Twelfth Imam school, the main sect of Shīʾite Muslims, al-Ṭūsī finished his education in Neyshābūr, about 75 kilometres (50 miles) to the west. This was no doubt a prudent move as Genghis Khan (d. 1227), having conquered Beijing in 1215, turned his attention to the Islamic world and reached the region around Ṭūs by 1220. In about 1227 the Ismāʿīlīte governor Nāṣir al-Dīn ʿAbd al-Raḥīm offered al-Ṭūsī sanctuary in his mountain fortresses in Khorāsān. Al-Ṭūsī in turn dedicated his most famous work, Akhlāq-i nāṣirī (1232; Nasirean Ethics), to the governor before being invited to stay in the capital at Alamūt, where he espoused the Ismāʿīlīte faith under the new imam, Alauddin Muḥammad (reigned 1227–1255). (This Ismāʿīlīte state began in 1090 with the conquest of Alamūt by Ḥasan-e Ṣabbāḥ and ended with the fall of the city to the Mongols in 1256.) During this period, al-Ṭūsī wrote on Ismāʿīlīte theology (Taṣawwurāt; “Notions”), logic (Asās al-iqtibās; “Foundations of Inference”), and mathematics (Taḥrīr al-Majisṭī; “Commentary on the Almagest”).

With the fall in 1256 of Alamūt to Hülegü Khan (c. 1217–1265), grandson of Genghis Khan, al-Ṭūsī immediately accepted a position with the Mongols as a scientific adviser. (The alacrity with which he went to work for them fueled accusations that his conversion to the Ismāʿīlīte faith was feigned, as well as rumours that he betrayed the city’s defenses.) Al-Ṭūsī married a Mongol and was then put in charge of the ministry of religious bequests. The topic of whether al-Ṭūsī accompanied the Mongol capture of Baghdad in 1258 remains controversial, although he certainly visited nearby Shīʾite centres soon afterward. Profiting from Hülegü’s belief in astrology, al-Ṭūsī obtained support in 1259 to build a fine observatory (completed in 1262) adjacent to Hülegü’s capital in Marāgheh (now in Azerbaijan). More than an observatory, Hülegü obtained a first-rate library and staffed his institution with notable Islamic and Chinese scholars. Funded by an endowment, research continued at the institution for at least 25 years after al-Ṭūsī’s death, and some of its astronomical instruments inspired later designs in Samarkand (now in Uzbekistan).

Al-Ṭūsī was a man of exceptionally wide erudition. He wrote approximately 150 books in Arabic and Persian and edited the definitive Arabic versions of the works of Euclid, Archimedes, Ptolemy, Autolycus, and Theodosius. He also made original contributions to mathematics and astronomy. His Zīj-i Ilkhānī (1271; “Ilkhan Tables”), based on research at the Marāgheh observatory, is a splendidly accurate table of planetary movements. Al-Ṭūsī’s most influential book in the West may have been Tadhkirah fi ʿilm al-hayʿa (“Treasury of astronomy”), which describes a geometric construction, now known as the al-Ṭūsī couple, for producing rectilinear motion from a point on one circle rolling inside another. By means of this construction, al-Ṭūsī succeeded in reforming the Ptolemaic planetary models, producing a system in which all orbits are described by uniform circular motion. Most historians of Islamic astronomy believe that the planetary models developed at Marāgheh found their way to Europe (perhaps via Byzantium) and provided Nicolaus Copernicus (1473–1543) with inspiration for his astronomical models.

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Today al-Ṭūsī’s Tajrīd (“Catharsis”) is a highly esteemed treatise on Shīʾite theology. He made important contributions to many branches of Islamic learning, and under his direction Marāgheh sparked a revival of Islamic mathematics, astronomy, philosophy, and theology. In the East, al-Ṭūsī is an example par excellence of the ḥakīm, or wise man.