Yaʿqūb ibn Ṭāriq (Persian:یعقوب ابن طارق; referred to by some sources asYaʿqūb;^[1] diedc. 796) was aPersian astronomer and mathematician who lived inBaghdad.

Yaʿqūb ibn Ṭāriq was active inBaghdad as anastronomer during the rule of the secondAbbasid caliph,al-Manṣūr (r. 754–775).[2]^[3] He seems not to have been aware ofPtolemaic astronomy,^[2] and used aZoroastrian calendar, which consisted of 12 months of 30 days each, with any remaining days being added after the eighth month,Ābān.^[2]

Yaʿqūb ibn Ṭāriq'streatiseTarkīb al‐aflāk dealt withcosmography (the placement and sizes of the heavenly bodies).^[4] The estimations of their sizes and distances inTarkīb al‐aflāk were tabulated in the 11th century by the polymathal-Bīrūnī, in hiswork on India. According to al-Bīrūnī, Yaʿqūb ibn Ṭāriq gave theradius of the Earth as 1,050farsakhs, the diameter of theMoon andMercury as 5,000farsakhs (4.8 Earth radii), and the diameter of the other heavenly bodies (Venus, the Sun, Mars, Jupiter, and Saturn) as 20,000farsakhs (19.0 Earth radii).^[5] He wrote that each of the planets had six associated spheres, that the Sun possessed two spheres, and the Moon three. He also spoke of planetaryepicycles and speeds.^[6] His values for thelongitudes andapogees of celestial objects originated from aPersian set of astronomical tables, theZīǧ aš-šāh written byYazdegerd III, although he used methods originating from the work ofIndian astronomers to calculate thelunar phases.^[1]

The ChristianastrologerIbn Hibintā mentioned Yaʿqūb, noting that he used the positions of the Sun and the stars to determine thelatitude of places.^[7]

Works ascribed to Yaʿqūb ibn Ṭāriq include:^[4]

• Zīj maḥlūl fī al‐Sindhind li‐daraja daraja ("Astronomical Tables in the 'Sindhind' Resolved for each Degree");
• Tarkīb al‐aflāk ("Arrangement of the Orbs"). Part of this work, the earliest surviving description of the celestial sky by an Islamic astronomer, is preserved by Ibn Hibintā.^[8]
• Kitāb al‐ʿilal ("Rationales");
• Taqṭīʿ kardajāt al‐jayb ("Distribution of theKardajas of theSine");
• Mā irtafaʿa min qaws niṣf al‐nahār ("Elevation along theArc of the Meridian").

An astrological work,Al‐maqālāt ("The Chapters"), is ascribed to Yaʿqūb ibn Ṭāriq by an unreliable source.^[4]

Yaʿqūb ibn Ṭāriq'szij, written in around 770, was based on aSanskrit work,^[4] thought to be similar to theBrāhmasphuṭasiddhānta. It was brought to the court ofal-Mansūr, the thirdcaliph of theFatimid Caliphate, fromSindh,^[9] reportedly by the Sindhi astronomerKankah.^[10]

• Kennedy, Edward Stewart (1956)."A Survey of Islamic Astronomical Tables".Transactions of the American Philosophical Society. New Series.46 (2). Philadelphia, Pennsylvania:American Philosophical Society:123–177.doi:10.2307/1005726.hdl:2027/mdp.39076006359272.ISSN 0065-9746.JSTOR 1005726.
• Pingree, David (1976)."Yaʿqūb ibn Ṭāriq". In Gillispie, Charles Coulston (ed.).Dictionary of Scientific Biography. Vol. 14. New York:Charles Scribner's Sons.ISBN 978-0-684-16962-0.
• Plofker, Kim (2007)."Yaʿqūb ibn Ṭāriq". In Thomas Hockey; et al. (eds.).The Biographical Encyclopedia of Astronomers. New York:Springer.ISBN 978-0-387-31022-0. (PDF version)
• Sezgin, Fuat (2021)."III. Arab Astronomers".Sezgin Online.doi:10.1163/2667-3975_SEZO_COM_603. Retrieved8 May 2023.

• Hogendijk, Jan P. (1988). "New Light on the Lunar Crescent Visibility Table of Yaʿqūb ibn Ṭāriq".Journal of Near Eastern Studies.47 (2):95–104.doi:10.1086/373260.JSTOR 544381.S2CID 162371303.
• Kennedy, E. S. (1968). "The Lunar Visibility Theory of Yaʿqūb Ibn Ṭāriq".Journal of Near Eastern Studies.27 (2):126–132.doi:10.1086/371945.JSTOR 543759.S2CID 162369374.
• Pingree, David (1968). "The Fragments of the Works of Yaʿqūb Ibn Ṭāriq".Journal of Near Eastern Studies.27 (2):97–125.doi:10.1086/371944.JSTOR 543758.S2CID 68584137.
• Steinschneider, Moritz (1870)."Zur Geschichte der Uebersetzungen aus dem Indischen in's Arabische und ihres Einflusses aus die arabische Literatur".Zeitschrift der Deutschen Morgenländischen Gesellschaft.24: 332.
• Suter, Heinrich (1900).Die Mathematiker und Astronomen der Araber und ihre Werke (in German). Leipzig: Teubner. p. 4.OCLC 230703086.

• 796 deaths
• 8th-century Iranian mathematicians
• Astronomers from the Abbasid Caliphate
• People from Baghdad
• Medieval Iranian astrologers
• 8th-century Iranian astronomers
• 8th-century Arabic-language writers
• 8th-century people from the Abbasid Caliphate

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