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Concept
Zij
Summary
A zij (zīj) 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 ("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. Some such books were referred to as qānūn, derived from the equivalent Greek word, .
The Zij-i Sultani, published by the astronomer and sultan Ulugh Beg in 1438/9, was used as a reference zij throughout Islam during the early modern era. Omar Khayyam's Zij-i Malik Shahi was updated throughout the modern era under various sultanates. Zijes were updated by different empires to suit their various interests, such as the simplified version of Zij-i Sultani by the Mughal Empire.
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 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.
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