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

• Author: Kennedy, E.S.

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...