Nasir al-Din al-Tusi

Muḥammad ibn Muḥammad ibn al-Ḥasan al-Ṭūsī (1201–1274; AH 598–673), commonly known as Naṣīr al-Dīn al-Ṭūsī (Arabic: نصیر الدین الطوسی; Persian: نصیر الدین طوسی) or simply (al-)Tusi, was a Persian polymath, architect, philosopher, physician, scientist, and theologian, as well as a prominent Shi'ite scholar. He wrote extensively on mathematics, engineering, prose, mysticism, philosophy, theology, and the sciences, and is widely regarded as one of the greatest scientists of medieval Islam.

Born in Ṭūs, from which his name derives, al-Tusi later worked under an Ismāʿīlī ruler at Alamūt, the centre of the Nizārī Ismāʿīlīs, and produced Ismāʿīlī works that acquired considerable authority, although his own personal religious allegiance during this period remains uncertain. When Hūlāgū Khān and the Mongols captured Alamūt in 1256, al-Tusi entered Hūlāgū's service as an astrologer and was instrumental in saving numerous libraries from destruction.

Al-Tusi made significant contributions across several scientific disciplines. In astronomy, he compiled highly accurate tables of planetary motion, developed an improved planetary model, criticized aspects of Ptolemaic astronomy, and compiled the astronomical tables known as the Zīj al-Khānī. He also introduced the mathematical device later known as the "Ṭūsī couple". His work extended to logic, mathematics, biology, chemistry, and especially trigonometry, which he is often credited with establishing as an independent mathematical discipline rather than merely an instrument of astronomy. The Muslim historian Ibn Khaldun regarded him as the greatest of the later non-Arab scholars. His astronomical work has also been discussed as a possible influence on Copernicus and the development of Copernican heliocentrism.

A prolific author on subjects ranging across the sciences, philosophy, theology, ethics, and mysticism, al-Tusi's most enduring works include al-Akhlāq al-Nāṣiriyya ("The Nasirean Ethics", 1235) and Tajrīd al-Iʿtiqādāt ("The Definition of the Articles of Faith"). His broad intellectual output, together with his mathematical and astronomical innovations, secured his reputation as one of the major polymaths of the medieval Islamic world.