YAʿQūB IBN ṬāRIQ

(fl. Baghdad, second half of eighth century),

astronomy

Ya ʿqūb ibn Tāriq was the astronomer most closely connected with al–Fāzarīin introducing the Zīj al–Sindhind to Islamic scientists; he seems, in fact, to have collaborated personally with the Indian astronomer who came to Baghdad with an embassy from Sind in 771 or 773. The most important of his works in this connection were Zīj mahlūl fī al–Sindhind li daraja daraja “Astronomical Tables in the Sindhind Resolved for Every Degree” Tarkīib al–aflāak “Composition of the Spheres” and Kitāb al–ʿilal (Boos of Causes”).

Evidently the most prominent feature of the Zīj was that the interval between the entries in the columns of arguments for the tables was one degree. Its basic parameters were very similar to those of the Zīj al–Sindhind al–kabīr of al–Fazārī, except that Ya’qub completely accepted the equations of the center from the Zij al–Shah while mixing in some equations of the anomaly from the Zij al–Arkand (the ardharatrika system; see essay in Supplement).

In the Tarkīb al–aflāck Yaʿqub also drew upon the Zīj al–Sindhind and the Zīj al– Arkand, as well as on his conversations with the Indian astronomer. The subjects covered in this work, insofar as they can be determined, were the geocentric distances of the planetary orbits, geography, the computation of the ahargana and perhaps the geometric models of planetary motion. The Kitāb alʿilal is known only from citations by al–Bīrūunī in his work On Shadows the extant fragments deal exclusively with rules for employing the gnomon. Like al–Fazāarī, Ya ʿqūb is inconsistent, adopting whatever formula comes to hand without regard for its relation to the other formulas in his books. Also like al–Fazārī, he is significant primarily for his role in the transmission of Indian science to Islam.

BIBLIOGRAPHY

The fragments by Ya’qub ibn Tariq are collected and discussed in D. Pingree, “The Fragments of the Works of Yaqub ibn Tariq,” in Journal of Near Eastern Studies27 (1968), 97–125; and in E. S. Kennedy, “The Lunar Visibility Theory of Ya’qub ibn Tariq,” ibid 126–132

David Pingree