Ya'qub ibn Tariq

Yaʿqūb ibn Ṭāriq (يعقوب بن طارق; referred to by some sources as Yaʿqūb; died c. 796) was an 8th-century Persian astronomer and mathematician who lived in Baghdad.

Career
Yaʿqūb ibn Ṭāriq was active in Baghdad as an astronomer during the rule of the second Abbasid caliph, al-Manṣūr ((r. 754 – 775)). He seems not to have been aware of Ptolemaic astronomy, and used a Zoroastrian calendar, which consisted of 12 months of 30 days each, with any remaining days being added after the eighth month, Ābān.

Yaʿqūb ibn Ṭāriq's treatise dealt with cosmography (the placement and sizes of the heavenly bodies). The estimations of their sizes and distances in were tabulated in the 11th century by the polymath al-Bīrūnī, in his work on India. According to al-Bīrūnī, Yaʿqūb ibn Ṭāriq gave the radius of the Earth as 1,050, the diameter of the Moon and Mercury as 5,000 (4.8 Earth radii), and the diameter of the other heavenly bodies (Venus, the Sun, Mars, Jupiter, and Saturn) as 20,000  (19.0 Earth radii). 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 planetary epicycles and speeds. His values for the longitudes and apogees of celestial objects originated from a Persian set of astronomical tables, the written by Yazdegerd III, although he used methods originating from the work of Indian astronomers to calculate the lunar phases.

The Christian astrologer Ibn Hibintā mentioned Yaʿqūb, noting that he used the positions of the Sun and the stars to determine the latitude of places.

Works
Works ascribed to Yaʿqūb ibn Ṭāriq include:
 * ("Astronomical Tables in the 'Sindhind' Resolved for each Degree");
 * ("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.
 * ("Rationales");
 * ("Distribution of the of the Sine");
 * ("Elevation along the Arc of the Meridian").

An astrological work, ("The Chapters"), is ascribed to Yaʿqūb ibn Ṭāriq by an unreliable source.

Yaʿqūb ibn Ṭāriq's zij, written in around 770, was based on a Sanskrit work, thought to be similar to the Brāhmasphuṭasiddhānta. It was brought to the court of al-Mansūr, the third caliph of the Fatimid Caliphate, from Sindh, reportedly by the Sindhi astronomer Kankah.