ASTRONOMY. One of the greatest astronomers of Islam, al-Battani (the Albatenius, Albategni, or Albategnius of the Latin West, d. c.929 CE), declares that astronomy is the most noble of the sciences, elevated in dignity, and second only to the science of religious law (Sayili, 1960, p. IS). This praise of the discipline is not merely a practitioner’s claim; embodies a historical truth.
Indeed, astronomy is the only natural science that escaped the censure of the medieval Muslim opponents of secular sciences (`ulum al-awd’il) and found a home in mosques, receiving the blessing of mainstream religious circles; and it is virtually the only Islamic hard science that lasted well into the modern period, continuing vigorously and fruitfully long after the Mongol sack of Baghdad, when much of Islamic scientific activity began to decline. Moreover, because of its traditional link with astrology and its utility in matters such as calendar reform, the determination of the direction of Ka’bah, and the calculation of the times of daily prayers, Islamic astronomy enjoyed throughout its history the enthusiastic and undiminished patronage of rulers and nobles. In the internal perspective of the science, astronomy is owed credit for the birth of trigonometry, a remarkable creation of Islam; due to astronomy too are numerous other important developments in mathematics, particularly in quantitative techniques and geometry, for all these mathematical disciplines were for a long time subservient to the needs of astronomers. Finally, it should be noted that astronomy was a truly international enterprise of Islam, a collaborative effort involving people from all over the Islamic world, including experts from China and India. It is evident, then, that al-Battani’s claim is hardly exaggerated.
Origins to Ptolemaicization. The origins of Islamic astronomy are intricately eclectic. The earliest Arabic treatises on this subject, sets of astronomical tables known as the zij (Pers., zik), were written in the first half of the eighth century CE in Sind and Qandahar. These treatises were based on Sanskrit sources, but they have been found to incorporate some Pahlavi material as well. Such derivations from Indian and Iranian works, which constitute the first phase of Islamic astronomy, introduced to the Arabic world many concepts of Greek mathematical astronomy-concepts that were largely non-Ptolemaic, having already reached India and Iran through circuitous routes and having been modified by local traditions. A further infusion of Indian and Iranian material marks the second phase of Islamic astronomy, but this was also the time when the works of the famous Greek astronomer Ptolemy (second century CE) and the Pahlavi Zik-i Shahrydran (Ar., Zij al-Shah) were translated into Arabic. This activity took place during the reigns of ‛Abbāsid caliphs al-Mansur (r. 754-775 CE) and Harun al-Rash! (r. 786-809), a period that also saw the emergence of a sustained Sindhind Arabic tradition growing out of the translation of a Sanskrit astronomical text, presumably entitled Mahasiddhanta.
During the early ‛Abbāsid period, thus, three astronomical systems were pursued concurrently: the Indian (Sindhind); the Iranian (Zij al-Shah); and the Ptolemaic. These systems were at many points in conflict, and the Islamic astronomical activity of this period is characterized by perpetual efforts to reconcile them. Astronomers soon concluded that the Ptolemaic system was superior to all others known to them. Thus-with al-Battani marking the turning point-by the beginning of the tenth century Islamic astronomy had undergone a complete ptolemaicization: available now was a newer and better Arabic translation of Ptolemy’s Almagest made by the Nestorian Christian Ishaq ibn Hunayn (d. 910/11) and the “pagan” Thabit ibn Qurrah (d. 901); Ptolemy’s Planetary Hypothesis too was now rendered into Arabic by Thabit; and the Sindhind and Shah traditions were finally relegated to history. The story of Islamic astronomy from this point is characterized by what Thomas Kuhn would describe as “puzzle-solving” within a Ptolemaic paradigm.
Theoretical Innovations. Let us now, following the lead of contemporary historians, set up the theoretical problem to which these different systems had offered different solutions. Consider two rotating wheels. A larger wheel (the deferent, al-hdmil) has a stationary center E and a point S on its rim. Let S, namely the point rotating on the circumference of the deferent, be the center of a smaller wheel (the epicycle, al-tadwir). Let P be a point on the rim of the smaller rotating wheel. Then, if P’s rate of rotation is properly adjusted, it will appear to an observer at E, the center of the deferent, as periodically advancing and receding as the wheels spin, now coming forward, now sliding back. If in this arrangement S represents the sun, E the earth, and P a planet, then this ancient geocentric model of wheels upon wheels provides a valid if simplified explanation of the looped paths of the planets as seen from the earth.
In practice, however, this mechanism needs adjustments to bring it in accord with the observed planetary motions. Indeed, Ptolemy managed to make a drastic improvement in the correspondence between theory and observation by introducing into the arrangement a geometric device known as the equant (mu’addal al-mash). What Ptolemy did was to shift the earth a small distance from the center of the deferent E to, say, Eg thereby making the deferent eccentric with respect to the earth. Furthermore, from E he displaced the center of the uniform motion of S to a rigorously calculated point O. Thus the motion of S was uniform with respect neither to E nor to Eg, but with respect to an imaginary point O, and this O was Ptolemy’s fateful equant.
Ibn al-Haytham (Latin, Alhazen, d. 1039), the scientific giant of Islam, wrote an attack on Ptolemy’s planetary theory: if Ptolemy’s system was not merely an abstract geometrical model but represented the real configuration of the heavens-as Ptolemy had claimed it did-then it violated the accepted classical principle of uniform velocity for all celestial bodies, a principle the Greek astronomer had himself espoused. Indeed, in his Planetary Hypothesis Ptolemy had conceived of the observed motions of the planets as produced by the combined motions of corporeal spherical shells in which the planets were embedded. The idea of an eccentric celestial shell was unacceptable to Ibn al-Haytham, as it was to many astronomers who shared his views.
The subsequent history of Islamic mathematical astronomy is a chronicle of attempts to modify the Ptolemaic system so that it would accord more accurately with observations while at the same time preserving the principle of uniform circular motion. It was more than two centuries after Ibn al-Haytham that Nasir al-Din alTusi, the head of the celebrated Maraghah observatory built by Hulegu in 1259, inaugurated outstandingly successful efforts along these lines. Tusi appears to have been the first to recognize that if one circle CI with a diameter D rolls inside another circle C2 with a diameter 2D, then any point on the circumference of C1 describes the diameter of CZ. In modern terminology this device can be considered a linkage of two equal and constantlength vectors with constant angular velocity (one moving twice as fast as the other); this is the famous “Tusi couple.” By means of this device the observed phenomena were explained by Maraghah astronomers solely in terms of a combination of uniform circular motions. The apex of these Maraghah techniques is embodied in the work of Qutb al-Din al-Shirazi (d. 1311), who, eliminating the Ptolemaic equant, constructed a highly accurate geometrical model for Mercury, by far the most irregular planet visible to the naked eye. In the middle of the fourteenth century the astronomer Ibn al-Shatir, a muwagqit (timekeeper) at a mosque in Damascus, further refined the Tusi innovations and managed to develop for the Moon and Mercury new models that were far superior in accuracy to those of Ptolemy.
Historians have pointed out that the mathematical devices created by the Maraghah, scientists and the planetary models constructed by the muwagqit reappear two centuries later in the work of Copernicus. In particular, Copernicus’s models of the Moon and Mercury have been found to be identical with those of Ibn al-Shatir; moreover, both astronomers employ the Tusi couple, and both eliminate the equant in essentially the same manner. Here the possibility of historical transmission has not been ruled out.
Observational Astronomy. A characteristic feature of the Islamic astronomical tradition is the separation of theoretical exercises from observational activity. Thus observational astronomy took its own independent course, guided by the Ptolemaic concept of testing (mihnah or Nibdr), which requires constantly renewed corrections of the observational data collected by preceding generations. Thus from the early ‘Abbasid period, astronomical observation remained an intensely pursued activity in Islam, with numerous observatories built over the centuries throughout the Islamic world from Baghdad to Samarra and Damascus, and from Egypt to Persia and Central Asia. Lunar and solar eclipses, meridian transits of the sun, transits of fixed stars, planetary positions and conjunctions-these were all part of the observational repertoire of Islamic astronomy.
Among the observatories that at Maraghah stands out. Indeed, it is regarded as the first observatory in the full sense of the word. It employed a staff of about twenty astronomers, including one from China; it was supported by a library; and it had a workshop for storing, constructing, and repairing astronomical instruments. These instruments included a mural quadrant and an armillary astrolabe, as well as solistical and equinoctal armillaries; also included in the holdings was a new instrument constructed by the Damascene al-`Urdi, which had two quadrants for simultaneous measurement of the horizon coordinates of two stars. Historians have noticed striking similarities between al-`Urdi’s observational devices and those of the Danish astronomer Tycho Brahe (d. 16o1), even though the results of the latter are unprecedentedly precise.
Long after the Copernican Revolution, Islamic observational astronomy continued in the geocentric Ptolemaic tradition. In the 1570s a major observatory was built in Istanbul. Then, in imitation of the Samarkand observatory founded by Ulugh Beg in 1420, the Indian Maharaja of Amber (1693-1743) built as many as five different observatories-at Jayapura, Ujjayini, Delhi, Mathura, and Varanasi-with the purpose of harmonizing Indian astronomy with the Islamic Ptolemaic tradition. Nonetheless, the alter Islamic observatories were not altogether fruitless exercises, for they contributed to European astronomy many of their observational techniques instruments, and organizational features. Even though Islamic astronomy did not take the daring philosophical step of breaking out of the geocentric Ptolemaic system, it has to its credit numerous impressive achievements: it gave to the world of science the astronomical observatory; it created trigonometry; at Maraghah it developed new instruments and powerful mathematical techniques; and it perpetually improved and corrected astronomical parameters. By consensus of historians, Islamic astronomers were the best of their age.
BIBLIOGRAPHY
David Pingree’s “`Ilm al-Hay’a,” in the Encyclopaedia of Islam, new
ed., vol. 3, pp. 1135-1138 (Leiden, 1960-), is a lucid and comprehensive survey of Islamic astronomical tradition. The early history of the field is covered in Pingree’s highly scholarly essay, “The Greek Influence on Early Islamic Mathematical Astronomy,” Journal of the American Oriental Society 93 (1973) 32-43, which includes an extensive survey of literature. A very useful account of an early astronomer is Pingree’s “Masha’allah,” in the Dictionary of Scientific Biography, edited by Charles Coulston Gillispie, vol. 9, pp. 159-162 (New York, 1970-). E. S. Kennedy provides much technical information in readable articles such as “The Arabic Heritage in the Exact Sciences,” AlAbhdth 23 (1970): 327-344; “The Exact Sciences,” in The Cambridge History of Iran, vol. 4, The Period from the Arab Invasion to the Saljuqs, edited by Richard N. Frye, pp. 378-395 (Cambridge, 1975); and “The Exact Sciences in Iran under the Saljuqs and Mongols,” in The Cambridge History of Iran, vol. 5, The Saljuq and Mongol Periods, edited by J. A. Boyle, pp. 659-679 (Cambridge, 1968). For the question of the transmission of Islamic astronomical theories to the West, see Kennedy’s classic paper, “Late Mediaeval Planetary Theory,” Isis 57 (1966): 365-378. A fuller account of the history of trigonometry may be found in Kennedy, “The History of Trigonometry,” Yearbook of the National Council of Teachers of Mathematics 31 (1969). Aydin Sayili’s The Observatory in Islam (Ankara, 1960) is a comprehensive social and intellectual history of the subject. A. I. Sabra’s “The Andalusian Revolt against Ptolemaic Astronomy,” in Transformation and Tradition in the Sciences: Essays in Honor of I. Bernard Cohen, edited by Everett Mendelsohn, pp. 133-153 (Cambridge, 1984), is an important work on the attempts of Spanish Muslim astronomers to improve upon the Ptolemaic system. A brief but rigorous account of Islamic astronomy is Sabra’s “The Scientific Enterprise,” in The World of Islam, edited by Bernard Lewis, pp. 181-199 (London, 1976). D. A. King’s “The Astronomy of the Mamluks,” Isis 74 (1983) 531-555, is a rich and very useful work on the state of the subject during the period under consideration.
S. NOMANUL HAQ