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An Islamic Response to Greek Astronomy: Kitab Tad dil Hay at al-Aflak of Sadr al-Sharia.

Medieval Muslim astronomers criticized the models of planetary motion invented by Ptolemy (fl. 150 A.D.) on the grounds that they violated the principle of uniform circularity. This held that the position of any celestial body must be the product of a combination of uniform circular motions. In the thirteenth century, Mu ayyad al-Din al- Urdi (d. 1266), Nasir al-Din al-Tusi (d. 1274), and Qutb al-Din al-Shirazi (d. 1311), all three working at the Maragha observatory in northwestern Iran, undertook to reform the system. With varying success, they attempted to substitute for the Ptolemaic mechanisms models which were uniformly circular. Their efforts were ably continued by Ibn al-Shatir of Damascus (d. c. 1375).

Recently another work has been discovered, completed in 1347 at Bukhara, far east of Maragha, which contains additional planetary models. The book, the title of which is translated as "The Adjustment of the Configuration of the Celestial Spheres," is the subject of this review. Its author, one Sadr al-Shari a, was not a professional astronomer, but of a family famed for its writings on canon law and theology. This work, however, is an introduction to astronomy and cosmology, written with the intent of supplying the reader with the best information on these subjects current in his day. Sadr bases his opus primarily on two similar expositions, the Tadhkira of Tusi and the Tuhfa of Shirazi, but he boldly undertakes to correct such errors as his predecessors have made, substituting for them his own valid solutions. His Ta dil is organized in eighteen chapters (fusul) which, for ease of reference, the editor has further subdivided into numbered paragraphs.

Chapter 1 gives definitions of the geometric and physical concepts needed in the sequel. In the second chapter are "proofs" that the universe is spherical, as is the earth, at the center of the universe. Chapter 3 describes the two rotations of the encompassing sphere of the fixed stars, and defines the equatorial, ecliptic, and horizon coordinate systems. Chapters 4, 5, 6, and 7 deal with the mechanisms which produce the motions of the sun, the moon, the superior, and the inferior planets, respectively.

The solar model of the Almagest is accepted without change. In the case of the moon, much space is expended in showing that the lunar models of the Tadhkira and the Tuhfa fail to solve the problem posed by the Ptolemaic prosneusis, whereas Sadr's description of his own model takes up only fourteen lines of text. As a result, it is impossible to say exactly what the mechanism was. What is indubitable, however, is that he proposed an epicyclet to be added to the linkage, an expedient which was successfully applied by Ibn al-Shatir, and by Copernicus after him.

The objection to the Ptolemaic mechanism for the longitudinal motion of the superior planets (and Venus) was that the effect of the equant point was not obtained by uniform circularity. The single configuration which Sadr describes indeed eliminates the difficulty, by using an isosceles trapezoid. But the latter had been introduced long before, by Urdi, and this is not acknowledged in the Ta dil.

Of the two inferior planets, Mercury, because of its unusual orbit, requires a special solution to the problem of the equant. Sadr chose to ignore Shirazi's valid solution, applying instead the Urdi configuration referred to above, combined with a spherical Tusi couple.

Chapter 8 is easily the longest in the book. This is not surprising, its subject being planetary latitudes, and its content extremely difficult. For each variety of planet, Sadr presents and criticizes the solutions given in the Tadhkira and the Tuhfa and then describes his own. Neither the editor nor the reviewer is competent at this stage to assess the validity of the solutions.

The remaining chapters are all on topics which are standard for this category of writing: planetary sectors, retrogradations, parallax, eclipses, planetary visibility conditions, bounds of the seven "climates," daylight lengths, ascensions and transits, and the equation of time. By and large the information and insights provided are available in other published works, e.g., the Tadhkira.

An exception, however, is the concluding chapter 18, on determining the local meridian and the qibla, the direction of Muslim prayer. The careful manner with which Sadr describes the construction of the level instruments for laying out the Indian circle strongly indicate that he was a practiced observer as well as a theoretician.

Preparation of the publication entailed difficulties involving widely differing disciplines. To begin with, the Ta dil is the third portion of a three-part compendium by Sadr, being preceded by treatises on logic and theology. Moreover, it is in two parts, a text, accompanied by a commentary, where, in this case, the commentary was written by the original author. The decision having been taken to produce at this time the text only, it became necessary to separate it from the commentary, no mean task.

Of the fourteen extant copies of the text, six were made available in microfilm. There ensued the familiar labor of comparing variants to establish the best reading. But Sadr was so heavily dependent upon the Tadhkira and the Tuhfa that it was as important to control the models described in them as those of Sadr himself. And of these two, only the first has been published. So the preparatory work was at least doubled.

The subject matter is highly technical. If it were being composed nowadays it would automatically be expressed in easily understood algebraic symbols. But the original author, himself uncertain, and operating intuitively, was restricted to verbal statements. He was forced to apply and attempt to express such concepts as, say, angular acceleration, before they had received rigorous definition. The modern editor must invert the procedure, and somehow divine what the author had in mind, if anything. The difficulty extends to the translation - shall it be literal, and make no sense? or standard English, the equivalent of what one hopes the author was trying to write? With this edition, the skeptical reader who has some Arabic has instant recourse to the text passage facing the translation.

In the Ta dil, unfortunately, there are practically no actual computations, and very few numerical tables. In a treatise consisting largely of tables, when in doubt about a verbal rule, the editor may attempt to recompute the corresponding table; if he succeeds, the underlying function is clear, and the matter is settled.

On the other hand, the Ta dil does have figures, some twenty-six in the text. These are supplemented in the commentary by seventy-six excellent drawings by the editor. The figures are well-nigh essential for understanding the extremely complicated planetary configurations, conceived of as moving, three-dimensional, physical entities, not geometrical abstractions. Their elucidation has been addressed most admirably by Professor Dallal.

As a result, the volume provides firm evidence that astronomy, far from being in decline, continued to flourish in fourteenth century Central Asia. And at least one practitioner, by his original contributions, maintained the lead achieved by his Muslim predecessors.

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Author:Kennedy, E.S.
Publication:The Journal of the American Oriental Society
Article Type:Book Review
Date:Apr 1, 1997
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