Nor did placing the earth at the center of the stars and planets pose any practical difficulties for science. The observed motions of the heavens could generally be accounted for if the sun were seen to orbit a stationary earth in the opposite direction once a year, at a slight angle to the equator, and the “sphere of the fixed stars” were seen to rotate in a little less than twenty-four hours. Accurate calendars, almanacs, and timekeeping were all possible as a result. Even today, the basic principles of navigation and orientation all work perfectly well when based on an earth-centered model.
But there was one troubling issue, known since ancient times as “the problem of the planets,” and the struggle to resolve it was central to the development of mathematical astronomy. Man had long noted that the planets—the word is derived from the Greek for “wanderer”—appear periodically to break their regular orbits, pause, and then reverse direction, before returning to their expected eastward route. Such retrograde motion occurs in Mercury every 116 days, while Mars reverses course just every 780 days. They also wander slightly north and south among the fixed stars, while generally remaining within the zodiac. The cause, of course, lies with the fact that both the individual planets and the earth itself are in constant motion—although very few in the classical and medieval worlds were prepared to consider that the earth was anything but fixed and central to the entire grand scheme. The moon, meanwhile, posed unique problems of its own; its irregular orbits around the earth, varying as much as seven hours from the average, long frustrated astronomers who sought to rely on this highly visible body as an easy way of marking time.27
Once again, Plato set the early tone, demanding “uniform and ordered movements” that would save the appearances. Soon a series of solutions involving interlocking spheres rotating on different axes around the central earth were advanced. The shelf life of this model was relatively brief, at least in scientific terms. Mathematical astronomy moved beyond it after only a century or so, but not before it had helped shape what was arguably the most long-lasting and influential cosmological vision in recorded history, that of Aristotle. His conception of the universe, that the planets are set in a series of rotating shells around the earth, survived more or less intact as a cosmological system until the early seventeenth century.28 As far as the philosophers were concerned, the cosmos was defined by three general principles: It consisted of a series of rotating shells, with the earth at the center; it was shaped like that perfect solid, the sphere; and its bodies moved in perfect circles. There was, however, somewhat less unanimity concerning one other precept of Aristotle’s cosmology: that the world was eternal. This matter would later come to haunt the great monotheist thinkers of Judaism, Christianity, and Islam alike.
With the work of Aristotle, the “problem of the planets” did not go away; it simply shifted from the realm of philosophy and cosmology to the smaller, more exclusive preserve of mathematical astronomy. In what might be viewed as an intellectual arms race, astronomers drew up ever-more-sophisticated mathematical models of planetary motion, only to find new problems cropping up almost immediately owing to new and better celestial observations and measurements. Two new weapons were introduced early on: the epicycle and the deferent. The deferent was defined as a circle that rotated around the earth, while the epicycle, which carried the planet, rotated around a point on the circumference of the deferent. Adjusted correctly, this combination of motions could approximate the periodic retrograde motion of each of the planets—that temporary reversal of direction—as seen from the earth.
Those minor discrepancies that could not be addressed by these techniques were at times resolved by shifting the center of the deferent circle slightly away from the earth. This created the so-called eccentric orbit, an approach that was particularly useful for the seemingly erratic movements of the sun. By shifting the center of the deferent ever so slightly, for example, astronomers could account for the observed fact that the sun spends almost six more days between the spring and autumn equinoxes than it does between autumn and spring.29 One final innovation completed this complex mathematical edifice, the theory of the equant. According to this notion, the uniform motion of the planets, as demanded by Plato and his successors, was at times maintained not around the center of the deferent but around a displaced location. Seen from the earth, the course of the planet would appear unsteady or seem to wobble; seen from the off-center equant point, however, it would maintain uniform speed and distance, as required by the philosophers.
The Alexandrian mathematician Ptolemy took on the final job of assembling and refining this celestial machinery. He was also the architect of the equant theory. The system outlined in the Almagest was so successful at explaining and predicting the motions of the sun, the moon, and the planets as seen from the earth that scholars no longer referred to earlier works on the subject, many of which effectively disappeared. Gradually, Arab astronomers and philosophers began to voice unease about the equant and its violation of the principle of perfect, circular motion around a single center, the earth. A number of serious attempts were made to overhaul the Ptolemaic model, but these were based primarily on theoretical, not practical, grounds.
On the Use of the Astrolabe, accompanied by the earlier zij al-Sindhind, whetted the West’s appetite for astronomy and opened the way for the later reception and eventual assimilation of the Ptolemaic system. The Almagest was translated into Latin from the original Greek in Sicily around 1160, but it was only with a version from the Arabic, completed in 1175, that it became known among Western scientists and philosophers.30 Adelard’s original treatise also helped make the bronze astrolabe all the rage in Europe, its use common as late as the seventeenth century. The instrument’s great utility for casting a horoscope and other astrological operations, as well as its suitability as a teaching aid, fueled the relatively rapid diffusion of this new technology. Peter Abelard and Heloise, the most famous star-crossed lovers of the Middle Ages and accomplished scholars in their own right, had already named their love child Astrolabe. Sooner or later, every self-respecting scholar produced an astrolabe text; Chaucer left behind an unfinished essay on the device, dedicated to his nephew.
But On the Use of the Astrolabe contributed to one other important landmark—the early, tentative infiltration of pagan Greek cosmological thought into Western consciousness. Traditionally, early Latin texts on the astrolabe focused narrowly on just three topics: the theory of stereographic projection that displayed the three-dimensional universe on a two-dimensional surface (a map, a chart, or the disk of an astrolabe), the design and construction of the device, and instructions for its use. In his innovative enumeration and description of the concentric spheres of the universe, Adelard introduces for the first time one notable addition: an indiscernible outermost shell beyond the firmament that imparts power and gives form to things below.31 So far, this is but a shadow of Aristotle’s notion of the Unmoved Mover, who endows the celestial machinery with its eternal motion but otherwise takes no notice of man’s affairs, an idea that would take increasing hold in the Christian West, much to the discomfort of traditional theologians and philosophers.
The order from the religious authorities at the University of Paris was enough to chill the blood, if not the actual pursuit of the new learning from the East: “Let the body of Master Amaury be removed from the cemetery and cast into unconsecrated ground, and the same be excommunicated by all the churches of the entire province.” Moreover, the directive of 1210 commanded that the notebooks of a certain David of Dinant were to be handed over to the local bishop and burned forthwith. Another section of the same order gives a good idea of the nature of their offense: “Neither the books of Aristotle on natural philosophy nor their commentaries are to be read at Paris in public or in secret, and this we forbid under penalty of excommunication. He in whose possession the writings of David of Dinant are found after the Nativity shall be considered a heretic.”32
Five years later, new statutes for the University of P
aris, the West’s leading center of theological studies, repeated the ban on the natural philosophy of Aristotle and the teachings of his two disciples, Master Amaury and David of Dinant. Apparently, the first decree had been generally evaded, or even ignored outright, in the faculty of arts—a tactic that would surface repeatedly in the increasingly contentious relationship between theologians and philosophers throughout the thirteenth century. The same order also spelled out more mundane rules of comportment for the teaching masters, including a ban on sartorial excess: “no round cope shoes that are ornamented or with elongated pointed toes.” Masters were, however, allowed to invite friends and associates to university meetings and receptions, “but only a few.”33
Church authorities had good reason to be concerned with the rapid pace of change in Paris and other fledgling centers of Western knowledge. The traditional clerical controls were starting to loosen, as the locus of advanced teaching began to move from the cathedral schools to the universities that were taking shape from among clusters of teachers and students in cities across Europe. Also beginning to die out was the centuries-old monopoly of the church fathers over philosophical and theological teachings. St. Augustine set the ground rules at the dawn of the Middle Ages when he taught that man should begin with faith and proceed from revelation to reason.34 This established theology as the pinnacle of speculative thought and reduced philosophy, and with it natural science, to the role of “handmaiden” to the theologians—an approach that was beginning to come under fire from technological change and the accompanying trend toward more critical thinking. Still, the ban on Aristotle’s natural philosophy—encompassing theories of nature, the origins of the universe, and similar topics—so early in the Arab-inspired awakening poses something of a riddle.
Europe’s churchmen had long venerated the name of Aristotle in association with their beloved technique of logical argument, known as dialectic. In this way, they mirrored the earliest Abbasid encounter with his teachings, which were first mined for their logical systems to use in religious debate against nonMuslims. What philosophical instruction existed in twelfth-century Europe rarely moved beyond these methods of argument to encompass metaphysics or natural science. When it did so, it was often confronted with fragmentary texts and only half-baked understanding. The discipline itself was practiced primarily to enhance the mental acuity of students and to prepare them for the more serious study of theology; it was not designed to impart information, such as a coherent philosophical vision of the cosmos. In general, philosophical speculation, particularly cosmology, had been largely displaced for almost twelve centuries by the all-encompassing worldview of the church, which offered its own explanation of man’s origins, his place in the universe, and his ultimate destiny.35
True, scattered Latin translations, primarily from the Arabic, of Aristole’s major works of natural philosophy had already begun to appear some decades earlier in Spain and Italy, but it is hardly possible to speak in any meaningful way of an organized body of Aristotelian thought available in Latin. Members of the Paris faculty of theology, the driving force behind the bans of 1210 and 1215, would almost certainly have been hard-pressed to name any of Aristotle’s offending texts, or to identify any of the vital Muslim commentaries needed to understand them.36 No less an authority than Roger Bacon, who lectured at Paris, dates the real arrival there of the authentic natural philosophy of Aristotle to around 1230, one hundred years after many of the basic ideas and concepts of Greek and Arab natural science had already entered into circulation.37 Surviving lecture notes from 1245 suggest that Roger himself was among the first to teach this natural philosophy at the University of Paris, although such works were already being debated at Oxford.38
So what precisely did the church have in mind as early as 1210 when it ordered a ban on Aristotle’s natural science and suppressed the teachings of two of his more enthusiastic disciples, David of Dinant and Master Amaury? If Aristotle’s thought was either unknown at the time or, at best, only poorly understood, then what was the threat to Christian orthodoxy? And whence did it come?
Here, as with the introduction of the Arabic Euclid and the star tables of al-Khwarizmi, the key lies with Adelard of Bath, whose hunt for the studia Arabum led him to the foremost classical authority on the subject of astrology, the ninth-century Persian scholar Abu Mashar al-Balkhi, commonly known among the Latins as Albumazar. Evidence from surviving manuscripts reveals that Adelard may have acquired Albumazar’s The Abbreviation of the Introduction to Astrology while in Antioch, along with his copy of Thabit ibn Qurra’s work on talismans. His Latin translations of the two texts appear together with a partial collection of astrological aphorisms, a clue that all three may have been completed around the same time and place.39
The work itself, essentially an astrological handbook, is not particularly remarkable. It is, rather, Albumazar’s stripped-down and simplified version of his own encyclopedic The Introduction to Astrology, written in Baghdad in 848. The idea of the abridged text, the author tells us, was to bring this complex subject “closer to comprehension.”40 Missing is much of the rich philosophical explication and scientific detail that later made the larger work as popular in the West as it once was in the East. Still, it firmly established the importance of Arab astrological learning and incited a profound hunger for more among Latin scholars that lasted at least until the astronomical discoveries of Galileo and others in the seventeenth century.41
Astrology has long been in disrepute, but it was once seen as an important and legitimate field of study that promised a foretaste of human events from the movements of the stars and planets. This view rested on the widespread acceptance of the universal “law” whereby the entire natural world—the affairs of man; the life cycles of the animals and plants: phenomena such as earthquakes, floods, and weather—was directed by the movements of the heavens. This provided a coherent theory of nature that linked man and the cosmos into one single, satisfying whole. At its core lay the classical notion, celebrated in the title of Adelard’s On the Same and the Different, that the eternal, perfect, and unchanging “superior” celestial bodies, the realm of the Same, governed the ever-changing, corruptible “inferior” region of man and the earth, that of the Different.
Such astrology served for centuries as a perfectly legitimate scientific theory: It appeared to account successfully for the observable world; it addressed the central questions of the day; and it proved fertile ground for further research and inquiry. Despite some misgivings among Muslim, Christian, and Jewish theologians that astrology imperiled man’s freedom to choose good over evil and threatened individual accountability, its basic tenets were largely accepted without serious challenge. The philosopher Albertus Magnus found nothing amiss in reconciling astrology’s fundamental principle with the biblical account of the sixth day of Creation, when the earth “brought forth” the living creatures. “Since the power to produce animals is not in the earth, but, according to astronomers, is in the heavens,” Albertus concludes in his Summa theologiae, the earth must have provided the material principle for the animals, while the active part resided with the heavens.42 It took another four hundred years, and Isaac Newton’s universal law of gravitation, for the distinction among the learned between the heavens, on the one hand, and man’s earthly home, on the other, to begin to blur and finally dissolve. Even then, it persisted in the fields of biology and medicine until Darwin’s theory of evolution, published in 1859, finished it off forever.43
Given the central importance of astrology among the Arabs, it is little wonder that Adelard turned his own hand to what the West came to know as “the judgments of the stars”—as distinct from astronomy proper, which addressed the regular movements and positions of the heavens. “Here begins the smaller introduction to the science of the stars of Jafar the Astrologer, taken out of the Arabic by Adelard of Bath,” reads the opening line of his Latin translation of Albumazar, the first complete Arabic astrology manual to appear in the West.44 Adela
rd then goes on to introduce his readers to the fundamental import of astrology and its essential link to the mastery of the other sciences: “Whoever, seeking the higher science of philosophy with constant study, investigates the admirable effects of the celestial [beings] on the sensible universe—in that the likenesses of forms on high appear by certain natural motion over this lower world, and portend the foreknowledge of future things—can hardly achieve this without the knowledge of the degrees of the circle and the signs.”45
The appearance of Adelard’s Abbreviation of the Introduction to Astrology, completed around 1120, helped ensconce Albumazar in the West as the supreme authority on all aspects of astrological science.46 Within two decades, translators working in Spain completed two different Latin versions of the full, unabridged astrology textbook, more than six times longer than Adelard’s edition. These restored the philosophical and scientific underpinnings excised in the abbreviated text. This provided the West with its first real gateway to the natural science of the Arabic Aristotle. Albumazar had set out to justify and defend astrology in terms of general scientific understanding. In the world of ninth-century Baghdad in which he lived and worked, that meant linking it to the traditions of Greek physical science and metaphysics known as falsafa. And that meant, first and foremost, Aristotle—at least Aristotle as the Arabs understood him. The result is an eclectic work of Arab, Alexandrian, Persian, and Hindu astrological ideas, resting on a relatively solid footing of classical Greek scientific thinking.47
The Introduction to Astrology provided the budding natural philosophers of the Latin-speaking world with a compelling and comprehensive vision of the cosmos in which the machinery of the universe was governed by laws of motion and causality. Another popular work by Albumazar, also translated into Latin, showed how these same laws could be applied to the course of human history. In the preface to this second work, the Arab astrologer makes explicit the linkage between the heavens and events on earth, identified here in purely Aristotelian terms: “This is the book on the sum of the indications of the celestial bodies on terrestrial events occurring in the world of generation and corruption … It is called the Book of Religions and Dynasties.”48
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