's depiction of the original
round city of Baghdad (1883), where Yaʿqūb ibn Ṭāriq was active during his career Yaʿqūb ibn Ṭāriq was active in
Baghdad as an
astronomer during the rule of the second
Abbasid caliph,
al-Manṣūr (). 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==