| From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, pp. 1002-1003 |
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Ṣadr al‐Sharīʿa al‐Thānī:
ʿUbaydallāh ibn Masʿūd al‐Maḥbūbī
al‐Bukhārī al‐Ḥanafī
Glen M. Cooper
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 fī ʿilm 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.)