The House of Wisdom (22 page)

Ignoring traditional religious questions that had preoccupied his predecessors, such as the annual dating of Easter, Walcher instead used his observational data to create a pair of new lunar tables. His new approach was in contravention of the teachings of the church fathers—and the authority of no less than the Venerable Bede.
¹²
Walcher also adopted the modern system, already well established among the Arabs, of recording astronomical data in degrees, minutes, and seconds. This replaced the clumsy and less precise Roman fractions then in common use across Europe.
¹³
Walcher’s tables were far more accurate than those that came before, which were based not on direct observation but on the traditional medieval
computus
. Nonetheless, they proved grossly inadequate. Walcher soon found that his prediction of a full moon for New Year’s Eve, 1107, for example, was off by as much as sixteen hours.
¹⁴

Despite their new empirical basis, Walcher’s tables still suffered from the medieval convention of assigning an equal number of days to each of the twelve months. This made for tidy calculations, but it seriously undercounted the days of the year. An almost contemporaneous revision of the calendar in Persia by the savant Omar Khayyam—known in the East not for the poetry of
The Rubaiyat
but for his supremely elegant mathematics—calculated the length of the solar year to eleven decimal points. Lacking a theoretical understanding of the movement of the planets, Walcher and his colleagues were unable to exploit their newfound precision in scientific measurement. They needed help from the Arab astronomers.
¹⁵

Adelard’s translation of the
zij al-Sindhind
provided one piece of the puzzle, giving the West its first real look at the inner workings of the Arabs’ mathematical astronomy. The geometry of Euclid supplied another, for it allowed the vast magnitudes involved in measuring the celestial bodies to be captured and expressed in terms of “angular distance” relative to the earth or to one another. It also allowed the accurate calculation and mapping of terrestrial and celestial positions, either on a sphere or “projected” onto a two-dimensional map or chart, or onto the faceplate of the astrolabe. With the publication of his original treatise
On the Use of the Astrolabe
, probably around 1149 or 1150, Adelard further revolutionized the way Western man understood the universe around him.
¹⁶
He also made explicit the link between the new technology and the comprehensive Arab scientific edifice that stood behind it. From timekeeping to navigation, the secrets of the physical world could now be fully explored.

To Adelard, the astrolabe was more than just an instrument to aim at the sun or a prominent star and then use to take measurements or tell time; it was a polished bronze symbol for a fresh way to view the world, informed by classical philosophy and the innovations of the Arab scholars of the House of Wisdom. Armed with such a device, man could measure and begin to decipher the regular movements of the stars and the planets. He could explore the laws of nature and gain new insights into how things work. The universe was no longer an ineffable divine masterpiece; it was transformed into a giant laboratory, as well as an object of research to be studied and analyzed like any other. Attributes such as time and distance were no longer vague abstractions but took on real numerical values, opening the door to the rise of empirical science and the creation of organized, modern societies.

On the Use of the Astrolabe
presented the Latin world, for the first time, with the beginnings of a coherent and comprehensive cosmology. Adelard laid to rest Isidore of Seville’s misguided teaching that the earth was flat and “shaped like a wheel” and other expressions of Western sacred geography. At the center of this new worldview sat the sphere—the “perfect solid” of the ancient Greeks and the only one that can rotate on its axis in absolute symmetry, always displacing the same space—and its two-dimensional representation, the circle. “Concerning the universe … and its different parts I will write in Latin what I have learned from the Arabs. You can take it for granted that the universe is not square, or rectangular, but a sphere. What is said of the sphere can be said of the universe,” Adelard informs the future King Henry II.
¹⁷
Adelard dedicates the work to Henry, whom he may have earlier served as personal tutor.

Working in the tradition of al-Khwarizmi and other Arab scholars, who frequently introduce their scientific works as responses to entreaties that they share their learning with friends, students, or patrons, Adelard opens his astrolabe text with just such an appeal. Prince Henry, he tells us, turned to him for “the opinions of the Arabs about the sphere and circles and motions of the stars.” By this time a respected scholar and England’s foremost Arabist, Adelard forgoes the ritual humility of his Muslim mentors to lecture the teenage Henry on the vital importance of a scientific understanding of the natural world. “You say that whoever dwells in a house is not worthy of its shelter if he is ignorant of its material and makeup, quantity and quality, position and peculiarity. Thus if one who was born and raised in the palace of the world should forebear after the age of discretion to know the reason for so marvelous a beauty, he is unworthy of it and, were it possible, ought to be cast out.”
¹⁸

Adelard first presents the basic tenets and concepts of spherical and theoretical astronomy, as well as key points of geography. He uses a globe as a model of the sphere of the earth, before introducing the computational powers of the astrolabe, the subject of the rest of the book.
¹⁹
Available Latin sources offered some of the same material, but the powerful influence of at least two Arab scholars stands out. The first, of course, is al-Khwarizmi, whose
zij al-Sindhind
Adelard translated earlier. The author of
On the Use of the Astrolabe
assumes the reader is familiar with the
zij
, as well as with his own Latin version of Euclid, and the treatise relies heavily on the Arab star tables to complement the calculations carried out with the device itself. Adelard also makes an important change to some of the technical data from his edition of the
zij al-Sindhind
, converting the meridian from that of Cordoba to that of Bath.
²⁰

Using an approach similar to that in his Euclid text, Adelard provides both Arabic names and Latin equivalents for the various parts of the astrolabe. He also integrates its operation completely with al-Khwarizmi’s
zij
handbook, referring regularly to the data available in the star tables and thereby allowing the user to get the most out of the technology.
²¹
The other central Arab voice is that of Maslama al-Majriti, who first transposed the
zij al-Sindhind
to the meridian of Cordoba and replaced its Persian calendar with the Islamic one. At one point, Adelard mentions an
astrolabium doctoris Almirethi
, a reference to an instrument that once belonged to al-Majriti or came from his school of mathematical astronomers.
²²

In laying out the “opinions of the Arabs,” Adelard devotes considerable space to the use of the circle to measure and depict the movements across the sphere of the universe, suggesting that this may still have been a novel idea among educated Western readers.
²³
Such an understanding was vital, for the circle and sphere provide the basic building blocks for the study of the heavens. Here,
On the Use of the Astrolabe
comes into its own, introducing and explicating the common models behind the perceived movement of the heavens. These include the central notion of a concentric universe, nested spheres controlling the general movements of the celestial bodies, as well as the so-called eccentric orbits of the planets—identified since classical times as the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. Each planet is assigned its own sphere, and all of them are grouped around the earth, at the center, Adelard explains, but their circular orbits within the sphere fluctuate between a high and a low point, carving an eccentric path in their regular rotations.
²⁴
And there are more spheres, including that of the fixed stars, as well as other refinements to keep the whole mechanism running like clockwork.
On the Use of the Astrolabe
is less a how-to on the astrolabe than a groundbreaking introduction to astronomy.

This complex theoretical apparatus reflects the heroic efforts by astronomers and philosophers over the centuries to address Plato’s dictum to “save the appearances”—that is, to account for the increasingly precise observations of the scientists without violating the strict guidelines laid down by the Greeks and seemingly confirmed by common sense. In the fourth century
B.C.,
Plato’s creation myth,
Timaeus
, spelled out some of the central requirements: The world as rendered by the Creator must be a perfect whole; it must be unique, allowing the creation of no other; and it must be immune to decay or corruption. “Wherefore, he made the world in the form of a globe, round as from a lathe, having its extremes in every direction equidistant from the center, the most perfect and most like itself of all figures; for he considered that the like is infinitely fairer than the unlike.”
²⁵
For the Greek philosophers, the perfection of the celestial world should also be reflected in the course of the planets, each tracing eternal, perfect circles across the heavens.

Similar arguments were also advanced for the spherical shape of the earth. Common sense and everyday experience seemed to support this: the round image cast on the moon during a lunar eclipse; the observation of a ship’s mast dropping below the horizon as it sailed from shore; or even the appearance or disappearance of the constellations as one moved north or south along the earth. The fact that a falling body, say an apple from a tree, appeared to plummet toward the center of the earth suggested that it must represent the center of the universe as well. There was no gravitational theory at the time to explain this phenomenon; besides, the notion that man inhabited the center of divine creation had obvious and long-lived religious and psychological appeal.
²⁶
Surely a loving, all-powerful God would not exile his supreme creation, man, to some cosmological backwater.

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.