From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, pp. 1250-1251 Courtesy of Springer.

Kim Plofker

Yaʿqūb ibn Ṭāriq

FlourishedBaghdad (Iraq), 8th to 9th century

Yaʿqūb ibn Ṭāriq is known as a contemporary and collaborator of the 8th‐century scholars in Baghdad (particularly Fazārī) who developed from Greek, Indian, and Iranian sources the basic structure of Arabic astronomy. Works ascribed by later authors to Yaʿqūb include the Zīj maḥlūl fī al‐Sindhind li‐daraja daraja (Astronomical tables in the Sindhind resolved for each degree), Tarkīb al‐aflāk (Arrangement of the orbs), and Kitāb al‐ʿilal (Rationales [of astronomical procedures]). He is also said to have written a Taqṭīʿ kardajāt al‐jayb (Distribution of the kardajas of the sine [sine values]), and Mā irtafaʿa min qaws niṣf al‐nahār (Elevation along the arc of the meridian), which may be related to or incorporated within one of his more general works. An otherwise unknown astrological work entitled Al‐maqālāt (Chapters) is also attributed to Yaʿqūb by one (unreliable) source. None of the above works is now extant, and only the first three are known in any detail from later writings.

Yaʿqūb's zīj (handbook with astronomical tables), like that of Fazārī, was apparently based on the Sanskrit original of the Zīj al‐Sindhind, translated by them in Baghdad in the 770s. (A highly embroidered 12th‐century account of Yaʿqūb's involvement in this translation is given by Abraham ibnʿEzra.) Also like Fazārī's, the surviving fragments of Yaʿqūb's zīj are a heterogeneous mix from different traditions. For example, the mean motion parameters are Indian, as is the rule for visibility of the lunar crescent; the calendar is Persian; and the Indian sunrise epoch for the civil day appears to have been converted to the Greek‐inspired noon epoch by the simple expedient of moving the prime meridian 90° (or 1/4th day) eastward from the usual location of Arin (Ujjain).

The Tarkīb al‐aflāk was an early work on the topic that became known as hayʾa or cosmography (i.e., the arrangement, sizes, and distances of the celestial orbs). Yaʿqūb's work apparently stated the orbital radii and sizes of the planets, as well as rules for determining accumulated time according to techniques in Sanskrit treatises. Bīrūnī in the 11th century mentioned the Tarkīb as the only Arabic source using the Indian cosmographic tradition (although at least some of the same values were known from other zījes); if his descriptions of some of Yaʿqūb's rules are accurate, Yaʿqūb did not always fully understand or correctly interpret the Indian procedures.

It is also from Bīrūnī that we derive our knowledge of the Kitāb al‐ʿilal, an early representative of the genre of “rationales” or “causes” treatises that undertook to provide mathematical explanations of the computational rules in zījes. All of Bīrūnī's references to this work are contained in his al‐Ẓilāl (On shadows), so they are limited to trigonometric procedures using gnomon shadows in calculations of time and location. By this time, evidently, Yaʿqūb's works were valued primarily for the information they provided about early influences from the Indian tradition, many of which were replaced in later Islamic astronomy by predominantly Ptolemaic techniques.

Selected References

Hogendijk, Jan P. (1988). “New Light on the Lunar Visibility Table of Yaʿqub ibn Ṭāriq.” Journal of Near Eastern Studies 47: 95–104.

Kennedy, E. S. (1968). “The Lunar Visibility Theory of Yaʿqūb ibn Ṭāriq.” Journal of Near Eastern Studies 27: 126–132.

Pingree, David (1968). “The Fragments of the Works of Yaʿqūb ibn Ṭāriq.” Journal of Near Eastern Studies 27: 97–125.

——— (1976). “Yaʿqūb ibn Ṭāriq.” In Dictionary of Scientific Biography, edited by Charles Coulston Gillispie. Vol. 14, p. 546. New York: Charles Scribner's Sons.

Sezgin, Fuat (1978). Geschichte des arabischen Schrifttums. Vol. 6, Astronomie, pp. 124–127. Leiden: E. J. Brill.