Until Galileo Galilei pointed his telescope skyward, the seven stars that change their relative positions in a cyclical pattern were the givens of scientific endeavour. Predicting the movement of these jewels and orbs provided an arena for mathematical virtuosity, a justification for maintaining libraries, a reason for establishing schools of advanced learning, and an excuse for international collaboration. Because the patrons of this apparatus demanded practical results in the way of reliable calendars, astronomers devoted effort toward studying persistent empirical trends, such as the precession of the vernal equinox, the change in the stars behind the sun on the first day of spring.
Patrons demanded a great deal of their star gazers. Astronomers were called upon to pronounce on occasional spectacular events, such as eclipses. Through the twentieth century, astronomers have addressed meteorology – the corruptible, sublunar domain of Aristotelian physics named after the blazing objects in the sky, meteors, that were apparently as ephemeral as the rain. Astronomers were charged with telegraphical signals and radio broadcasts. They measured fundamental physical quantities in gravimetry (the gravitational constant identified by Isaac Newton) and optics (the speed of light, first calculated by Ole Christensen Römer [1644–1710]). Occasionally they chronicled the flight of migratory birds and assembled demographical statistics. They addressed whatever depended on a sharp eye and a head for figures. Until the twentieth century, astronomers were the practical masters of the realm of numbers.
Astronomers differed from casual stargazers in that they required a special place for making observations. Observing in a grand observatory required a team of people. They had to be ready for the right moment and hope that a cloud did not intervene. In practice, this implied a support staff of servants and some form of lodging for the observers. Understanding the data required a library and calculating devices – whether pen and paper, abacus, clay tablet, or sand table. Apprentice observers had to be trained. Instruments had to be maintained. Regular reports about celestial omens and calendars had to be produced. Four thousand years of astronomical practice are continued at today’s enormous, mountain-top research installations.
We have seen that the endowed, residential college, or madrasa, was an innovation of medieval Islam. It is also to Islamic civilization that we owe the invention of the astronomical observatory. This occurred under al-Mamûn, early in the ninth century. A great patron of learning, al-Mamûn financed major astronomical complexes at Damascus and Baghdad. These possessed modifications of the instruments mentioned by Ptolemy, including an armillary sphere of concentric circles for tracking the stars, a marble mural quadrant (a graded quarter-circle mounted on a wall) for observing the height of stars above the horizon, and a five-metre gnomon or stile. The observatories assembled a group of perhaps as many as a dozen talented astronomers, one of whom was Ptolemy’s commentator al-Farghânî (Alfraganus, fl. 850), who constructed tables, or zijes, based on observations. Astrological interest, especially as it related to solar eclipses (for which Ptolemaïc data had to be corrected), was undoubtedly the motor of al-Mamûn’s astronomical patronage.
Knowledge may naturally tend to disaggregate, pooling here and there, channelling along one or another stream, evaporating into the air. The disaggregation is present in Islamic astronomy. During the Abbasid golden age, al-Mamûn’s observatories were distinct from the learned academy at Baghdad, the Dar al-Hikmah or House of Wisdom, which had been founded by Caliph Harun al-Rashid. The academy functioned as a collector and filter of learning from all sources, east and west. Greek and Indian texts, and possibly also Hebrew ones, were recovered and translated into Arabic. Among the most notable academicians was Abu Jafar Muhammad ibn Musa al-Khwarizmi (fl. 830), author of the first Arabic text on algebra (based on both Greek and Indian sources) as well as a work on Indian numerals. Al-Khwarizmi also composed a treatise on Hindu astronomy, recalculated much of Ptolemy’s data for the seven planets, and provided tables for calculating eclipses as well as trigonometrical functions. He certainly knew about the work conducted at al-Mamûn’s observatories, especially on establishing the obliquity of the ecliptic, but he chose not to incorporate the new results.
Al-Mamûn’s observatories did not survive his reign (he died in 833), but they established a precedent for observing nature. Over the next centuries, Islamic observatories extended their programmes to all the planets. The institutions became characterized by grand instruments (sometimes surpassing in size those at European locations up to the eighteenth century) and the staff (more numerous than European staff) to manoeuvre them. Observatories acquired legal status and operated under the eye of a director. The astronomical work and instrumental innovations of the polymath Ibn Sînâ (Avicenna, 980–1037), based on observations taken early in the eleventh century at an observatory financed by the amir of Isfahan at Hamadân, followed the earlier pattern. But the institutional evolution occurred unevenly. Distinguished observers, such as al-Battânî (Albategnius, fl. 880) and Ibn Yûnus (late tenth century), seem not to have availed themselves of a permanent observing facility, even though they were much concerned with astronomical innovation. Ibn Yûnus, for example, invented something akin to the method of transversals.
European commentators have traditionally celebrated Islamic savants as transmitters of Hellenistic learning; less time has been spent detailing Islamic scientific innovation. But there is no doubt that in astronomy, Islamic observations expanded and became more sophisticated. The crucial tasks of an Islamic observatory related only to the sun and the moon. One needed to establish dates of religious observances (for the Muslim lunar calendar) and times of daily prayers, keyed to sunrise and sunset. With the accessibility of Hellenistic texts, precise measurements of the sun led to interest in anomalous motions, such as precession of the equinoxes, and eventually to concern with the five remaining planets. Indeed, programmes to observe the five smaller bodies provided a justification for the permanent endowment of an observatory. It takes about thirty years of watching the sky to document all planetary regularities, and this is the working lifetime of an astronomer. Among observatories with a long-term programme was the one founded by the late eleventh-century Seljuq sultan Jalal al-Dîn Malikshâh at Isfahan; its staff of as many as eight men included al-Khayyami, the mathematician and astronomer known for his poetry as Omar Khayyam (ca.1048–ca.1131). The astronomers at the Malikshâh Observatory were the first to emphasize to their patron that it would take thirty years to record changes in the sky; from their time forward the generational argument became an astronomical watchword.
The slow pace of institutional development reflected uncertainties about using large measuring devices. One principle has dominated astronomy since antiquity: the larger the measuring device, the more accurate the observations. During the Islamic period large azimuthal rings installed on the ground to measure points on the compass were cast in copper (notably one five metres in diameter at the early twelfth-century al-Afdal al-Bataihî Observatory in Cairo), and large mural quadrants were cut into the ground and faced in marble. The moving parts of these instruments were usually made from wood – indeed, wood was preferred to brass for mural quadrants and even sextants up to the eighteenth century. But the wood warped with time and weather, especially as the large instruments were normally open to the elements. Heavy moving parts – the arm on one of Ptolemy’s rulers, for example – had to be suspended in such a way as to minimize creep. One reason for the slow growth of early observatories is that many astronomers, among them Ibn Yûnus, actually favoured small devices – even portable ones – that could be manipulated by one observer.
The peak of Islamic observatory-building took place during the thirteenth century, and its exemplar was the one founded at the city of Marâgha, south of Tabriz in present-day Iran, by Mangû, brother of the Muslim conqueror Hulâgû. Mangû, by all accounts a convinced patron of learning, seems to have first thought about inviting the most distinguished astronomer