Zij

A zīj (Persian: زيج‎‎) is an Islamic astronomical book that tabulates parameters used for astronomical calculations of the positions of the Sun, Moon, stars, and planets.

The name zij 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, .

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. 23 Some zījes go beyond this traditional content to explain or prove the theory or report the observations from which the tables were computed.

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