| From: Thomas Hockey et al. (eds.). The Biographical Encyclopedia of Astronomers, Springer Reference. New York: Springer, 2007, pp. 1250-1251 |
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Courtesy of 
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Yaʿqūb
ibn Ṭāriq
Kim Plofker
Flourished Baghdad (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
ibn ʿEzra.) 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.