From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, pp. 1153-1155

Courtesy of

ūsī: Abū Jaʿfar Muammad ibn Muammad ibn al‐asan Naīr al‐Dīn al‐ūsī

F. Jamil Ragep

Bornūs (northeast Iran), 17 February 1201

DiedBaghdad (Iraq), 25 June 1274

Naīr al‐Dīn al‐ūsī's major scientific writings in astronomy, in which he worked to reform Ptolemaic theoretical astronomy, had an enormous influence upon late medieval Islamic astronomy as well as the work of early‐modern European astronomers, including Nicholas Copernicus. ūsī wrote over 150 works, in Arabic and Persian, that dealt with the ancient mathematical sciences, the Greek philosophical tradition, and the religious sciences (law [fiqh], dialectical theology [kalām], and Sufism). He thereby acquired the honorific titles of khwāja (distinguished scholar and teacher), ustādh al‐bashar (teacher of mankind), and al‐muʿallim al‐thālith (the third teacher, the first two being Aristotle and Fārābī). In addition, ūsī was the director of the first major astronomical observatory, which was located in Marāgha (Iran).

ūsī was born into a family of Imāmī (Twelver) Shīʿa. His education began first at home; both ūsī's father and his uncles were scholars who encouraged him to pursue al‐ʿulūm al‐sharʿiyya (the Islamic religious sciences) as well as the ʿulūm al‐awāʾil (the rational sciences of the ancients). He studied the branches of philosophy (ikma) and especially mathematics in ūs, but eventually traveled to Nīshāpūr (after 1213) in order to continue his education in the ancient sciences, medicine, and philosophy with several noted scholars; among the things he studied were the works of Ibn Sīnā, who became an important formative influence. ūsī then traveled to Iraq where his studies included legal theory; in Mosul (sometime between 1223 and 1232), one of his teachers was Kamāl al‐Dīn ibn Yūnus (died: 1242), a legal scholar who was also renowned for his expertise in astronomy and mathematics.

In the early 1230s, after completing his education, ūsī found patrons at the Ismāʿīlī courts in eastern Iran; he eventually relocated to Alamūt, the Ismāʿīlī capital, and witnessed its fall to the Mongols in 1256. ūsī then served under the Mongols as an advisor to Īlkhānid ruler Hūlāgū Khan, becoming court astrologer as well as minister of religious endowments (awqāf). One major outcome was that ūsī oversaw the construction of an astronomical observatory and its instruments in Marāgha, the Mongol headquarters in Azerbaijan, and he became its first director. The Marāgha Observatory also comprised a library and school. It was one of the most ambitious scientific institutions established up to that time and may be considered the first full‐scale observatory. It attracted many famous and talented scientists and students from the Islamic world and even from as far away as China. The observatory lasted only about 50 years, but its intellectual legacy would have repercussions from China to Europe for centuries to come. Indeed, it is said that Ulugh Beg's childhood memory of visiting the remnants of the Marāgha Observatory as a youth contributed to his decision to build the Samarqand Observatory. Mughal observatories in India, such as those built by Jai Singh in the 18th century, clearly show the influence of these earlier observatories, and it has been suggested that Tycho Brahe might have been influenced by them as well. In 1274 ūsī left Marāgha with a group of his students for Baghdad.

ūsī's writings are both synthetic and original. His recensions (taārīr) of Greek and early Islamic scientific works, which included his original commentaries, became the standard in a variety of disciplines. These works included Euclid's Elements, Ptolemy's Almagest, and the so called mutawassiāt (the “Intermediate Books” that were to be studied between Euclid's Elements and Ptolemy's Almagest) with treatises by Euclid, Theodosius, Hypsicles, Autolycus, Aristarchus, Archimedes, Menelaus, Thābit ibn Qurra, and the Banū Mūsā. In mathematics, ūsī published a sophisticated “proof” of Euclid's parallels postulate that was important for the development of non‐Euclidian geometry, and he treated trigonometry as a discipline independent of astronomy, which was in many ways similar to what was accomplished later in Europe by Johann Müller (Regiomontanus). Other important and influential works include books on logic, ethics, and a famous commentary on a philosophical work of Ibn Sīnā.

In astronomy, ūsī wrote several treatises on practical astronomy (taqwīm), instruments, astrology, and cosmography/theoretical astronomy (ʿilm al‐hayʾa). He also compiled a major astronomical handbook (in Persian) entitled Zīj‐i Īlkhānī for his Mongol patrons in Marāgha. Virtually all these works were the subject of commentaries and supercommentaries, and many of his Persian works were translated into Arabic. They were influential for centuries, some still being used into the 20th century.

ūsī's work in practical astronomy, as well as his Zīj‐i Īlkhānī, were not particularly original or innovative. This was not the case with his work in planetary theory. There he sought to rid the Ptolemaic system of its inconsistencies, in particular its violations of the fundamental principle of uniform circular motion in the heavens. ūsī set forth an astronomical device (now known as the ūsī‐couple) that consisted of two circles, the smaller of which was internally tangent to the other that was twice as large. The smaller rotated twice as fast as the larger and in the opposite direction. ūsī was able to prove that a given point on the smaller sphere would oscillate along a straight line. By incorporating this device into his lunar and planetary models, ūsī reproduced Ptolemaic accuracy while preserving uniform circular motion. A second version of this couple could produce (approximately) oscillation on a great circle arc, allowing ūsī to deal with irregularities in Ptolemy's latitude theories and lunar model.

These models were first found in ūsī's Persian treatise all‐i mushkilāt‐i Muʿīniyya (Solution of the difficulties in the Muʿīniyya), written for his Ismāʿīlī patrons, and were further developed and incorporated years later in his famous Arabic work al‐Tadhkira fī ʿilm al‐hayʾa (Memoir on astronomy), composed during his years with the Mongols. ūsī's devices are of major significance for several reasons. First, they produced models that adhered to both physical and mathematical requirements; the two versions of the ūsī couple, from the perspective of mathematical astronomy, allowed for a separation of the effect of distance of the planet from its speed (which had been tied together in the Ptolemaic models). ūsī was thus able, for example, to circumvent Ptolemy's reliance on a circular motion to produce a rectilinear, latitudinal effect. Second, ūsī's new models greatly encouraged and influenced the work of Islamic astronomers, such as his student Qub al‐Dīn al‐Shīrāzī and Ibn al‐Shāir (14th century) as well as the work of early‐modern European astronomers such as Copernicus. The ūsī couple also appears in Sanskrit and Byzantine texts.

ūsī also influenced his astronomical and cosmological successors with his discussion of the Earth's motion. Although he remained committed to a geocentric universe, ūsī criticized Ptolemy's reliance on observational proofs to demonstrate the Earth's stasis, noting that such proofs were not decisive. Recent research has revealed a striking similarity between ūsī's arguments and those of Copernicus.

ūsī was committed to pursing knowledge in all its forms, and he tried to reconcile the intellectual traditions of late Greek Antiquity with his Islamic faith. As was the case with many Islamic scientists, he held that the certitude of the exact mathematical sciences, especially astronomy and pure mathematics, was a means toward understanding God's creation.

Selected References

Al‐Ṭūsī, Naṣīr al‐Dīn (1998). Contemplation and Action (Risālah‐i Sayr wa sulūk), edited and translated by S. J. Badakhchani. London: I. B. Tauris.

Kusuba, Takanori and David Pingree (2002). Arabic Astronomy in Sanskrit: Al‐Birjandī on Tadhkira II, Chapter 11 and Its Sanskrit Translation. Leiden: E. J. Brill. (For the translation of a part of a commentary on Tūsī's Tadhkira into Sanskrit.)

Ragep, F. Jamil (1987). “The Two Versions of the Tūsī Couple.” In From Deferent to Equant: A Volume of Studies in the History of Science in the Ancient and Medieval Near East in Honor of E. S. Kennedy, edited by David A. King and George Saliba, pp. 329–356. Annals of the New York Academy of Sciences, Vol. 500. New York: New York Academy of Sciences.

——— (1993). Naṣīr al‐Dīn al‐Ṭūsī's Memoir on Astronomy (al‐Tadhkira fī ʿilm al‐hayʾa). 2 Vols. New York: Springer‐Verlag.

——— (2000). “The Persian Context of the Tūsī Couple.” In Naṣīr al‐Dīn al‐Ṭūsī: Philosophe et savant du XIIIe siècle, edited by N. Pourjavady and Ž. Vesel, pp. 113–130. Tehran: Institut français de recherche en Iran/Presses universitaires d'Iran.

——— (2000). “Al‐Ṭūsī, Naṣīr al‐Dīn: As scientist.” In Encyclopaedia of Islam. 2nd ed. Vol. 10, pp. 750–752. Leiden: E. J. Brill.

——— (2001). “Ṭūsī and Copernicus: The Earth's Motion in Context.” Science in Context 14: 145–163.

——— (2004). “Copernicus and His Islamic Predecessors: Some Historical Remarks.” Filozofski vestnik 25: 125–142.

Ridawī, M. M. (1976). Aḥwāl wa‐āthār… Naṣīr al‐Dīn. Tehran: Farhang Iran.

Rosenfeld, B. A. and Ekmeleddin Ihsanoğlu (2003). Mathematicians, Astronomers, and Other Scholars of Islamic Civilization and Their Works (7th–19th c.). Istanbul: IRCICA, pp. 211–219.

Sayılı A. (1960). The Observatory in Islam. Ankara: Turkish Historical Society. (On the Marāgha Observatory, see pp. 187–223.)