Islamic astronomy

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Ptolemy was one of the last great scientists of antiquity, and in the centuries following his death Greek scientific activity declined in both quality and quantity. When the Muslims conquered the lands surrounding the Mediterranean in the seventh century, they would have found the records of 1000 years of Greek intellectual thought, but they would not have encountered much ongoing scientific inquiry.

After a couple of centuries of rapid expansion, the Islamic civilization settled down and scholarship began to flourish. From the eighth to the fourteenth centuries, most of the advances in astronomy were achieved by scholars in the Middle East, North Africa, and Moorish Spain. This work crossed religious and ethnic boundaries, with contributions from, among others, Arabs, Iranians, and Turks, and from Muslims, Jews, and Christians. Islamic scholarship explored all branches of knowledge and built on not only the traditions of Greek science and philosophy, but also those of Persia, India, central Asia, and to some extent, China. The unifying feature of this endeavour was the Arabic language, which was very flexible so that it was possible for translators to create the

Described in Katz (1998), pp. 212-15, for example.

necessary technical vocabulary. Greek works were translated into Arabic from the late eighth century onwards, including those of Galen, Aristotle, Euclid, Archimedes, and Apollonius. The Almagest was translated several times, an excellent translation being made by Ish. aq ibn H. unain - the son of one of the first leaders of the Academy set up in Baghdad called the 'House of Wisdom' -and this was edited later by Thabit ibn Qurra in the latter half of the ninth century. Ptolemy's Planetary Hypotheses, Tetrabiblos, and substantial parts of the Handy Tables were also translated.

The Islamic civilization contributed a great amount of theory, computation and instrumentation to astronomy, but it did not provide many observations for later use because, by and large, Islamic astronomers followed Ptolemy's procedures for obtaining planetary parameters from a limited number of selected observations. Many astronomers produced works, each known as a zij, a word used originally to mean 'a set of astronomical tables' (of which the Handy Tables was the prototypical example) but which was later used to refer to any astronomical treatise. Many of these works survive to this day.

Perhaps the first zij to appear in Arabic was based on the Siddhanta of Brah-magupta, which was probably translated in the 770s. It is, however, doubtful that the Arabs would have had the necessary expertise to make much use of the tables until they had become familiar with Greek geometrical methods. Other Sanskrit works such as the Aryabhatiya of Aryabhata were translated around this time and would have been the authority on astronomy until the Greek works became available. From these texts the Arabs became aware of Hindu mathematical advances in areas such as trigonometry and numeration.

Arabic mathematics came of age with the work of al-Khwairizmi, an early member of the House of Wisdom. His great influence on the development of mathematics comes from his liisab al-jabar wa-l-muqabala, written in about 825, which contains the beginnings of the subject now known as algebra, al-jabar referring to the operation of taking a subtracted quantity from one side of an equation to the other and al-muqai bala referring to the operation of subtracting the same quantity from both sides of an equation. Al-KhwarizmTis also responsible for introducing the Hindu decimal numeration system, including a symbol for zero, to the Arabic speaking world. He described algorithms (the word being derived from his name) for using these numbers to perform the basic operations of arithmetic. Al-KhwarizmTcompiled a zij containing tables constructed largely from Ptolemy's theories, though also incorporating aspects

The period of translation is described in detail in O'Leary (1948), Chapter XII (see also

Kunitzsch (1974), Toomer (1984), p. 2, for more information on translations of the Almagest).

They have been catalogued by Kennedy (1956b).

of Indian and Persian astronomy. This work set the style for future Islamic astronomy and was influential in Europe in the Middle Ages after it was translated into Latin in the twelfth century by Adelard of Bath.

The Almagest was very difficult to get to grips with, and a number of new manuals of theoretical astronomy appeared, one of the most influential of which was by al-FarghanT(known in Latin as Alfraganus), who was employed by the caliph al-Ma'miin, the ruler in Baghdad between 813 and 833. Al-FarghanT's work usually is known by the title Elements of Astronomy and was a comprehensive summary of Ptolemaic astronomy that was entirely descriptive and non-mathematical. It became very popular as a textbook and was the primary medium by which knowledge of Ptolemy's work spread until the sixteenth century. Al-Farghani recomputed the distances of the planets from the Earth based on the parameters in the Almagest, exactly as Ptolemy had done in the Planetary Hypotheses. He arrived at values similar to Ptolemy's but, conveniently, the philosophically unacceptable gap between Venus and the Sun had disappeared. It was Gerard of Cremona's translation into Latin of al-FarghanT's work in the twelfth century that was the main source of knowledge about Ptolemaic astronomy Dante used when writing his Divine Comedy, and John of Seville's translation a few decades earlier formed the basis of Johannes de Sacrobosco's On the Sphere, which went through more than 200 editions and was used at universities throughout Europe until the early seventeenth century.7

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