Stars and Astrolabes
One of the most conspicuous examples of modern astronomy's Islamic heritage is the names of stars. Betelgeuse, Rigel, Vega, Aldebaran and Fomalhaut are among the names that are directly Arabic in origin or are Arabic translations of Ptolemy's Greek descriptions.
In the Almagest Ptolemy had provided a catalogue of more than 1,000 stars. The first critical revision of the catalogue was compiled by 'Abd al-Rahman al-Sufi, a 10th-century Persian astronomer who worked in both Iran and Baghdad. Al-Sufi's Kitab su-war al-kawakib ("Book on the Constellations of Fixed Stars") did not add or subtract stars from the Almagest list, nor did it remeasure their often faulty positions, but it did give improved magnitudes as well as Arabic identifications. The latter were mostly just translations of Ptolemy.
For many years it was assumed that al-Sufi's Arabic had established the stellar nomenclature in the West. It now seems that his 14th- and 15th-century Latin translators went to a Latin version of the Arabic edition of Ptolemy himself for the star descriptions, which they combined with al-Sufi's splendid pictorial representations of the constellations. Meanwhile the Arabic star nomenclature trickled into the West by another route: the making of astrolabes.
The astrolabe was a Greek invention. Essentially it is a two-dimensional model of the sky, an analog computer for solving the problems of spherical astronomy [see "The Astrolabe," by J. D. North; SCIENTIFIC AMERICAN, January, 1974]. A typical astrolabe consists of a series of brass plates nested in a brass matrix known in Arabic as the umm (meaning "womb"). The uppermost plate, called the 'ankabut (meaning "spider") or in Latin the rete, is an open network of two or three dozen pointers indicating the position of specific stars. Under the rete are one or more solid plates, each engraved with a celestial coordinate system appropriate for observations at a particular latitude: circles of equal altitude above the horizon (analogous to terrestrial latitude lines) and circles of equal azimuth around the horizon (analogous to longitude lines). By rotating the rete about a central pin, which represents the north celestial pole, the daily motions of the stars on the celestial sphere can be reproduced.
Although the astrolabe was known in antiquity, the earliest dated instrument that has been preserved comes from the Islamic period [see cover of this issue]. It was made by one Nastulus in 315 of the Hegira era (A.D. 927-28), and it is now one of the treasures of the Kuwait National Museum. Only a handful of 10th-century Arabic astrolabes exist, whereas nearly 40 have survived from the 11th and 12th centuries. Several of these were made in Spain in the mid-11th century and have a distinctly Moorish style.
The earliest extant Arabic treatise on the astrolabe was written in Baghdad by one of Caliph al-Ma'mun's astronomers, 'Ali ibn 'Isa. Later members of the Baghdad school, notably al-Farghani, also wrote on the astrolabe. Al-Farghani's treatise was impressive for the mathematical way he applied the instrument to problems in astrology, astronomy and timekeeping.
Many of these treatises found their way to Spain, where they were translated into Latin in the 12th and 13th centuries. The most popular work, which exists today in about 200 Latin manuscript copies, was long mistakenly attributed to Masha'allah, a Jewish astronomer of the eighth century who participated in the decision to found Baghdad; it probably is a later pastiche from a variety of sources. In about 1390 this treatise was the basis for an essay on the astrolabe by the English poet Geoffrey Chaucer. Indeed, England seems to have been the gateway for the introduction of the astrolabe from Spain into Western Christendom in the late 13th and 14th centuries. It is possible that scientific activity centered at Oxford at the time contributed to the surge of interest in the device. Merton and Oriel colleges of the University of Oxford still own fine 14th-century astrolabes.
On them one finds typical sets of Arabic star names written in Gothic Latin letters. Included on the Merton College astrolabe, for example, are Arabic names that have evolved into standard modern nomenclature: Wega, Altahir, Algeuze, Rigil, Elfeta, Alferaz and Mirac. Thus as a result of the astrolabe tradition of Eastern Islam, transmitted through Spain to England, most navigational stars today have Arabic names, either indigenous ones or Arabic translations of Ptolemy's Greek descriptions.
Refining Ptolemy
It would be wrong to conclude from the preponderance of Arabic star names that Islamic astronomers made exhaustive studies of the sky. On the contrary, their observations were quite limited. For instance, the spectacular supernova (stellar explosion) of 1054, which produced the Crab Nebula, went virtually unrecorded in Islamic texts even though it was widely noted in China. Modern astronomers struck by this glaring gap often do not realize that Islamic astronomers failed to document most specific astronomical phenomena. They had little incentive to do so. Their astrology, unlike that of the Chinese, depended not so much on unusual heavenly omens as on planetary positions, and these were quite well described by the Ptolemaic procedures.
The planetary models that Ptolemy devised in the second century A.D. had the sun, the moon and the planets moving around the earth. A simple circular orbit, however, could not account for the fact that a planet periodically seems to reverse its direction of motion across the sky. (According to the modern heliocentric viewpoint, this apparent retrograde motion occurs when the earth is passing or being passed by another planet on its way around the sun.) Hence Ptolemy had each planet moving on an epicycle, a rotating circle whose center moved about the earth on a larger circle called the deferent. The epicycle, together with other geometric devices invented by Ptolemy, gave a fairly good first approximation to the apparent motion of the planets. As a great theoretician, Ptolemy must have been fairly confident of the particular geometry of his models, since he never described how he settled on it.
On the other hand, the idea of applying mathematics to a specific numerical description of the physical world was something rather novel for the Hellenistic Greeks, quite different from the pure mathematics of Euclid and Apollonius. In this part of his program Ptolemy must have realized that improved values for the numerical parameters of his models were both desirable and inevitable, and so he gave careful instructions on how to establish the parameters from a limited number of selected observations. The Islamic astronomers learned this lesson all too well. They limited their observations, or at least the few they chose to record, primarily to measurements that could be used for rederiving key parameters. These included the orientation and eccentricity of the solar orbit and the inclination of the ecliptic plane.
An impressive example of an Islamic astronomer working strictly within a Ptolemaic framework but establishing new values for Ptolemy's parameters was Muhammad al-Battani, a younger contemporary of Thabit ibn Qurra. Al-Battani's Zij ("Astronomical Tables") is still admired as one of the most important astronomical works between the time of Ptolemy and that of Copernicus. Among other things, al-Battani was able to establish the position of the solar orbit (equivalent in modern terms to finding the position of the earth's orbit) with better success than Ptolemy had achieved.
Because al-Battani does not describe his observations in detail, it is not clear whether he adopted an observational strategy different from that of Ptolemy. In any case his results were good, and centuries later his parameters for the solar orbit were widely known in Europe. His Zij first made its way to Spain. There it was translated into Latin early in the 12th century and into Castilian a little more than 100 years later. The fact that only a single Arabic manuscript copy survives (in the Escorial Library near Madrid) suggests that al-Battani's astronomy was not as highly regarded in Islam as it was in Europe, where the advent of printing ensured its survival and in particular made it available to Copernicus and his contemporaries. In De revolutionibus orbium coelestium ("On the Revolutions of the Heavenly Spheres") the Polish astronomer mentions his ninth-century Muslim predecessor no fewer than 23 times.
In contrast, one of the greatest astronomers of medieval Islam, 'Ali ibn 'Abd al-Rahman ibn Yunus, remained completely unknown to European astronomers of the Renaissance. Working in Cairo a century after al-Battani, Ibn Yunus wrote a major astronomical handbook called the Hakimi Zij. Unlike other Arabic astronomers, he prefaced his Zij with a series of more than 100 observations, mostly of eclipses and planetary conjunctions. Although Ibn Yunus' handbook was widely used in Islam, and his timekeeping tables survived in use in Cairo into the 19th century, his work became known in the West less than 200 years ago.
Throughout the entire Islamic period astronomers stayed securely within the geocentric framework. For this one should not criticize them too harshly. Until Galileo's telescopic observations of the phases of Venus in 1610, no observational evidence could be brought against the Ptolemaic system. Even Galileo's observations could not distinguish between the geo-heliocentric system of Tycho Brahe (in which the other planets revolved about the sun but the sun revolved about the earth) and the purely heliocentric system of Copernicus [see "The Galileo Affair," by Owen Gingerich; SCIENTIFIC AMERICAN, August, 1982]. Furthermore, although Islamic astronomers followed Ptolemy's injunction to test his results, they did not limit themselves simply to improving his parameters. The technical details of his models were not immune from criticism. These attacks, however, were invariably launched on philosophical rather than on observational grounds.
by Owen Gingerich.
Scientific American, April 1986 v254 p74(10) COPYRIGHT Scientific American Inc.
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