During the Sassanid Empire, Astronomy was studied by the Persians and Babylonians in Ctesiphon and at the Academy of Gundishapur. Most of the astronomical texts during the Sassanid period were written in the Middle Persian language. The Zij al-Shah, a collection of astronomical tables compiled in Persia and Mesopotamia over two centuries, was the most famous astronomical text from the Sassanid period, and was later translated into Arabic.
Zīj (Persian: زيج) is the generic name applied to Persian and 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, and later presented data based on the Egyptian 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 Siddhanta by Aryabhata and Brahmagupta. Later Muslim zijes became more extensive, and typically included materials on chronology, geographical latitude and longitude, 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 went beyond this traditional content to explain or prove the theory or report the observations from which the tables were computed.
- ↑ Kennedy, Islamic Astronomical Tables, pp. 1-2
- ↑ Kennedy, Islamic Astronomical Tables, p. 51
- ↑ Benno van Dalen, PARAMS (Database of parameter values occurring in Islamic astronomical sources), "General background of the parameter database"
- ↑ Kennedy, Islamic Astronomical Tables, pp. 17-23
- ↑ Kennedy, Islamic Astronomical Tables, p. 1