Zij

Zij

Zīj (Persian: زيج‎) is the generic name applied to Islamic astronomical books that tabulate parameters used for astronomical calculations of the positions of the Sun, Moon, stars, and planets. The name is derived from the Middle Persian term zih or zīg, meaning cord. The term is believed to refer to the arrangement of threads in weaving, which was transferred to the arrangement of rows and columns in tabulated data. In addition to the term zīj, some were called by the name qānūn, derived from the equivalent Greek word, κανών.

Some of the early zījes tabulated data from Indian planetary theory (known as the Sindhind) and from pre-Islamic Sassanid Persian models, but most zījes presented data based on the Ptolemaic model. A small number of the zījes adopted their computations reflecting original observations but most only adopted their tables to reflect the use of a different calendar or geographic longitude as the basis for computations. Since most zījes generally followed earlier theory, their principal contributions reflected improved trigonometrical, computational and observational techniques.

The content of zījes were initially based on that of the Handy Tables (known in Arabic as al-Qānūn) by Egyptian astronomer Ptolemy, the Zij-i Shah compiled in Sassanid Persia, and the Indian Siddhantas by Aryabhata and Brahmagupta. Muslim zijes, however, were more extensive, and typically included materials on chronology, geographical latitudes and longitudes, star tables, trigonometrical functions, functions in spherical astronomy, the equation of time, planetary motions, computation of eclipses, tables for first visibility of the lunar crescent, astronomical and/or astrological computations, and instructions for astronomical calculations using epicyclic geocentric models. Some zījes go beyond this traditional content to explain or prove the theory or report the observations from which the tables were computed.

Over 200 different zījes have been identified that were produced by Islamic astronomers during the period from the eighth to the fifteenth centuries. The greatest centers of production of zījes were Baghdad under the Abassid caliphs in the 9th century, the Maragheh observatory in the 13th century, the Samarkand observatory in the 15th century, and the Istanbul observatory of Taqi al-Din in the 16th century. Nearly 100 more zijes were also produced in India between the 16th and 18th centuries. One of the most famous Indian zijes was the Zij-i Muhammad Shahi, compiled at Jai Singh II of Amber's Yantra Mantra observatories. It is notable for employing the use of telescopic observations. The last known zij treatise was the Zij-i Bahadurkhani, written in 1838 by the Indian astronomer Ghulam Hussain Jaunpuri (1760–1862) and printed in 1855, dedicated to Bahadur Khan. The treatise incorporated the heliocentric system into the Zij tradition.