Glen M. Cooper

Ṣadr al‐Sharīʿa al‐Thānī: ʿUbaydallāh ibn Masʿūd al‐Maḥbūbī al‐Bukhārī al‐Ḥanafī

Died  Bukhara, (Uzbekistan), 1346/1347

Ṣadr al‐Sharīʿa (al‐thānī, i. e., “the Second”) was a theoretical astronomer and religious scholar who created original and sophisticated astronomical theories of time and place, and under circumstances that have long been considered devoid of original scientific research. Ṣadr was famous for his commentaries on Islamic jurisprudence (sharīʿa, hence his nickname Ṣadr al‐Sharīʿa, “preeminent [scholar] of the sharīʿa”). He was called “the Second,” after his great‐great‐grandfather, Ṣadr al‐Sharīʿa al‐Awwal (“the First”). Ṣadr also wrote on Arabic grammar, kalām (theology), rhetoric, legal contracts, and ḥadīth (prophetic traditions).

Ṣadr's astronomical writings are found in the third volume of his three‐volume encyclopedia of the sciences, the Taʿdīl al‐ʿulūm (The adjustment of the sciences). The first two volumes dealt with logic and kalām. The third volume was called Kitāb Taʿdīl hayʾat al‐aflāk (The adjustment of the configuration of the celestial spheres).

Ṣadr al‐Sharīʿa represents one of several theorists who worked within the astronomical tradition of theoretical astronomy (hayʾa). This tradition had its roots within the early Islamic period, especially with Ibn al‐Haytham,  but it began to flourish among the group of astronomers who were assembled at the Marāgha Observatory in northwestern Iran by the polymath Naṣīr al‐Dīn al‐Ṭūsī. One of the major issues that was of concern to these theorists was the irregular motion produced in several of Ptolemy's models, such as that brought about by the equant, and they sought to substitute models that would adhere to the physical principle of uniformity of motion in the heavens. Ṣadr frequently cites two works from this tradition  – Ṭūṣī's al‐Tadhkira fīʿilm al‐hayʾa (Memoir on astronomy), and al‐Tuḥfa al‐shāhiyya (The imperial gift) of Quṭb al‐Dīn al‐Shīrāzī. He does this in order to correct their work, and to present solutions to problems they missed.

In the Kitāb Taʿdīl hayʾat al‐aflāk, Ṣadr critically reviews the planetary models of his predecessors, especially Ptolemy, and points out their weaknesses. He then describes his own models that are meant to rectify them. The most significant problems Ṣadr addresses are: the lunar prosneusis point, the equant; planetary latitude theory, and the motion of Mercury.

In the case of the Moon, Ptolemy proposed that one orb rotate uniformly around the center of the Universe while maintaining a constant distance around another point, the deferent center; Ṣadr objects to this since it produces irregular motion in the celestial realm. Furthermore, rather than measure the motion in anomaly from the visible apogee of the lunar epicycle, Ptolemy measured it from the mean epicyclic apogee aligned with a point, the prosneusis, introduced into the model solely for this purpose. In offering a physically consistent model, Ṣadr employed both a rectilinear and a curvilinear “Ṭūsī couple.” Both of these devices combined circular motions in such a way as to produce a compound motion that oscillates along a line. In the rectilinear case, a smaller circle, internally tangent with a larger circle, rotates in such a manner as to produce linear motion; and in the curvilinear case, concentric spheres are made to rotate in such a way as to produce an approximate curvilinear motion along the surface of the epicycle sphere.

In the case of the upper planets (Mars, Jupiter, and Saturn), for which Ptolemy was compelled to introduce the equant point, Ṣadr followed Muʾayyad al‐Dīn al‐ʿUrḍī and Shīrāzī, without acknowledgment, and employed an epicyclet (an epicycle on an epicycle).

The Ptolemaic theory of planetary latitude and the revisions to it made by Islamic successors attempted to provide models for the planets' deviations from the ecliptic and involved complex, nonuniform spherical motions. Ṣadr summarized the work of his three predecessors and offered his own observations. As of this date, however, this problem has been insufficiently studied, so the significance of Ṣadr's work on the theory of planetary latitude remains obscure.

The case of Mercury involved several equant‐like problems and thus was particularly complicated. Ṣadr employed two geometrical tools invented by his predecessors – the “ʿUrḍī lemma” and the spherical “Ṭūsī couple” to arrive at his solution. Late medieval Islamic astronomy has as yet been insufficiently studied to assess fully the possible influence of Ṣadr on subsequent astronomers, such as Khafrī and others.

Ṣadr's work is also significant in that it provides a counterexample to two long‐standing paradigms of Islamic intellectual history. First, Ṣadr, who was a prominent religious scholar, contradicts the conclusions of traditional Orientalist scholarship, according to which the Islamic religious establishment was virtually completely opposed to science, and this opposition was supposedly a major factor in the decline of science in Islam. Second, Ṣadr stands as a major counterexample to the prevalent view of Islamic historiography whereby Islamic culture enjoyed a brilliant flourishing from the 9th century until the 11th century, but then suffered unmitigated decline in large part due to the critiques of rational science and philosophy by such religious scholars as Ghāzālī (died: 1111). Ṣadr clearly represents a very high level of mathematical and scientific sophistication within a tradition that falls well within the period of supposed decline.

Selected References

Al‐Shīrāzī, Quṭb al‐Dīn. al‐Tuḥfa al‐shāhiyya (The imperial gift). (There is as yet, unfortunately, no published edition or translation of this important treatise.)

Dallal, Ahmad S. (ed.) (1995). An Islamic Response to Greek Astronomy: Kitāb Taʿdīl hayʾat al‐aflāk of Ṣadr al‐Sharīʿa. Leiden: E. J. Brill. (Edition of the Kitāb Taʿdīl hayʾat al‐aflāk together with extensive notes and diagrams. This book is an extraction of the main portion of Sadr's text from his own commentary, a somewhat dubious methodology. The commentary portion has not yet been published. If it is ever published, it will cast greater light on how Sadr understood his own work. This edition was the primary source for the present article.)

Ragep, F. J. (1993). Naṣīr al‐Dīn al‐Ṭūsī's Memoir on Astronomy (al‐Tadhkira fī ʿilm al‐hayʾa). 2 Vols. New York: Springer‐Verlag. (Perhaps the most significant study to emerge thus far in the historiography of astronomy in Islam, in which al‐Ṭūsī's treatise was pivotal.)

Saliba, George (1979). “The Original Source of Quṭb al‐Dīn al‐Shīrāzī's Planetary Model.” Journal for the History of Arabic Science 3: 3–18. (Describes the motivation behind the “ʿUrdī lemma”.)

——— (1987). “The Role of the Almagest Commentaries in Medieval Arabic Astronomy: A Preliminary Survey of Ṭūsī's Redaction of Ptolemy's Almagest.” Archives internationales d'histoire des sciences 37: 3–20. (Contains a brief survey of Ptolemaic latitude theory and Tūsī's attempts to rectify it.)

——— (1987). “Theory and Observation in Islamic Astronomy: The Work of Ibn al‐Shāṭir of Damascus.” Journal for the History of Astronomy 18: 35–43. (Contains a description of the “ʿUrdī lemma.”)

——— (1993). “Al‐Qushjī's Reform of the Ptolemaic Model for Mercury.” Arabic Sciences and Philosophy 3: 161–203. (Description of the innovative work of a late Islamic astronomer.)

——— (1994). “A Sixteenth‐Century Arabic Critique of Ptolemaic Astronomy: The Work of Shams al‐Dīn al‐Khafrī.” Journal for the History of Astronomy 25: 15–38. (Survey of the work of another late Islamic astronomer.)

——— (1996). “Arabic Planetary Theories after the Eleventh Century AD.” In Encyclopedia of the History of Arabic Science, edited by Roshdi Rashed, pp. 58–127. London: Routledge. (Excellent survey of the development of planetary models during the so called period of decline of Islamic science. Plentiful and useful diagrams help to illustrate the complexities of this intricate subject.)

Saliba, George and E. S. Kennedy (1991). “The Spherical Case of the Ṭūsī Couple.” Arabic Sciences and Philosophy 1: 285–291. (Presents diagrams helpful in visualizing this three‐dimensional device.)