Medieval Cosmology

In the centuries surrounding the decline and fall of the Roman Empire the level of understanding of astronomy and cosmology in Western Europe was very low, hardly rising above the literal interpretation of a few biblical pronouncements. The Earth was believed to be flat and situated at the bottom of the universe. Above the heavens were the upper waters mentioned in the

Figure 4.2: Al-Shatir's model of the motion of the Moon.

book of Genesis. Although some of the views of the Greek philosophers were known, they were rejected as inconsistent with biblical authority.

The growth of Christian institutions encouraged the preservation of at least a rudimentary level of Greek and Roman science. The Spanish bishop Isidore of Seville (560-636) composed an encyclopedia that included some basic cosmological and astronomical facts from Latin sources. An early eighth-century English monk, the Venerable Bede (673-735), carried out a detailed study of books brought from Rome to two monasteries in northeast England. Drawing from the writings of the Roman author Pliny, Bede taught that the Earth is a sphere, a fact confirmed by the experience of travelers from the observed variation in the altitude of the noon Sun as one traveled south. Bede also called attention to the sphericity of the heavens and identified the standard classical order of the planets as the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. His book On the Reckoning of Time, became an important manual for chronology and the construction of calendars. The practice of dating events by the number of years that have elapsed since the birth of Christ was begun by Bede.

During the twelfth century a few industrious scholars, notably Gerard of Cremona (1114-1187) in Toledo and Adelard of Bath (1076-1160), produced Latin translations of Arabic scientific works as well as Latin translations of Arabic editions of Greek works. Euclid's Elements, Ptolemy's Almagest, and Aristotle's major philosophical works circulated in Latin. A translation of al-Farghani's Elements by John of Spain (1110-1180) helped to make Ptolemaic astronomy more widely known, and the more advanced work of al-Battani also circulated in the Latin West.

The classical geocentric cosmology of Aristotle and Ptolemy was described to European readers by Johannes de Sacrobosco (1195-1256), also known as John of Hollywood, who lived in Paris in the first half of the thirteenth century and wrote On the Sphere in 1220. This book was supplemented later in the century by the anonymous Theory of the Planets, which gave a more detailed account of the planetary theory sketched in the last part of On the Sphere. The latter work superseded al-Farghani's Elements and became the most widely read treatise on astronomy and the principal popular source for European views about cosmology for the next three centuries. The first part of the work contains a description of the Aristotelian physical universe:

The elementary region, existing subject to continual alternation is divided into four. For there is Earth, placed as it were, as the center in the middle of all, about which is water, about water air, about air fire, which is pure and not turbid there and reaches to the sphere of the moon ... and these are called the "four elements"____

Around the elementary region revolves with continuous circular motion the ethereal ... of which there are nine spheres, as we have just said: namely, of the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the fixed stars, and the last heaven. Each of these spheres encloses its inferior spherically. (Thorndike 1949, 119)

Sacrobosco's Sphere articulated the physical views of Aristotle, while the Theory of the Planets provided an elementary account of Ptolemaic astronomy. During the thirteenth century Aristotle's writings on theology and philosophy became established as the main source of theoretical reflection about the world; with the Bible they formed the basis of the medieval scholastic canon. The two most influential interpreters of Aristotle were Albertus Magnus (1200—1280) and his student Thomas Aquinas, both members of the Dominican order and prolific authors. Aquinas formulated the principle concerning the relationship between religion and science that has guided Christianity ever since (with the exception a few fundamentalist sects), namely, that truths of faith and truths of reason will never come into conflict. In the history of astronomy, Aquinas is primarily remembered for his commentary on Aristotle's On Heavens, a work in which he brought Peripatetic astronomical doctrines into the compass of Christian medieval thought.

The translation into Latin of the astronomical works of al-Rushd (Averroes) and al-Bitraji (Alpetragius) gave rise in the thirteenth century to the adoption by some thinkers of Aristotle's cosmological scheme involving concentric spheres, in opposition to Ptolemy's system of deferents and epicycles. Albertus Magnus was one such figure, and the English scholar Robert Grosseteste (1168-1253) also rejected Ptolemy on various Aristotelian grounds. Although Aristotle's physics continued to provide the basis for all speculation about the physical world, his system of homocentric spheres was unsatisfactory: it was qualitative and unable to account for planetary motions. The schools of Paris and Oxford rejected Aristotle's cosmology, and during the later Middle Ages the Ptolemaic system was widely adopted, although not universally accepted, by commentators on astronomy. The rivalry (such as it was) between the two systems did become mixed up with a question of genuine interest, namely, the extent to which Ptolemy's models corresponded to physically real mechanisms in the heavens. Aquinas is noteworthy for having explicitly raised this question. According to Aquinas, the fact that Ptolemy's epicycles and deferents save the phenomena, that is, account for the observations, does not imply that they are real because it may turn out that there are other theories that account equally well for the phenomena. By contrast, Aquinas believed that in physics one arrives at physically true principles, for example, the principle that the natural motion of celestial bodies is uniform and circular.

The writings of Aquinas and Sacrobosco provide the background for one of the most famous works of Western literature, Dante Alighieri's (1265-1321) Divine Comedy. This poem is often cited for its symbolic literary integration of classical geocentric cosmology and traditional Christian thought. In it Dante is conducted by the Roman poet Virgil on a trip to the center of the Earth. Traversing the successive concentric rings of hell, the travelers encounter the devil in hell at the center of the Earth. They continue their travels to the opposite side of the Earth, where they scale the heights of purgatory, a pyramidal-shaped mountain. At the peak of purgatory Virgil is replaced as Dante's escort by Beatrice, a woman from Dante's youth representing idealized love, who accompanies him as they ascend upward to the successive heavenly spheres. The order of the spheres is the characteristic Ptolemaic order of the planets: the Moon, Mercury, Venus, Earth, Mars, Jupiter, Saturn, and the fixed stars. Beyond the fixed stars, there is a ninth sphere, the primum mobile, or prime mover, of all of the spheres, and beyond it, finally, the empyrean and the throne of God.

In The Divine Comedy, physical and moral-religious visions of the universe are intertwined. Man lives on the Earth in the outer part of the sublunary world, near the boundary between the corruptible terrestrial and the sublime celestial realms. Similarly, man is balanced morally between a striving for the good and an inclination to submit to vice and temptation. There is a symmetry between the concentric rings of hell within the Earth and the celestial orbits surrounding the Earth. The universe is a concrete, finite, and morally ordered world, and the central place of man in both a literal and spiritual sense is guaranteed as part of the natural order.

Dante was a literary figure who contributed nothing to astronomy or cosmology itself. In the fourteenth century the French bishop Nicole Oresme (1323—1382) developed new mathematical methods that would prove useful in natural philosophy. Oresme was perhaps the leading representative of late medieval scientific thought and someone whose ideas were to be an important stimulus for the early modern growth of science. In mathematics he invented the graphical representation of qualities, his so-called latitude of forms, which consisted of a mathema-tization of Aristotelian qualities. In the work of Oresme and his Oxford contemporaries, there was an erosion of the peripatetic distinction between physics, which concerns that which is corruptible and subject to change, and mathematics, which concerns that which is unchanging and eternal.

Although astrology continued to gain in popularity during the Middle Ages, it faced determined opposition from Oresme and other advanced thinkers. In his book Commensurability or Incommensurability of Celestial Motions Oresme attempted to counter a notion popular in astrology known as the "great return" or the "great year." This was an event of major significance that would take place when the Sun, Moon, and planets had returned once again to the exact same positions in the sky that they had occupied at some earlier epoch. Oresme considered the periods of rotation of the celestial bodies and examined their ratios or proportions. (The argument that Oresme developed

Figure 4.3: Dante's universe.

applies to both the Aristotelian and Ptolemaic systems.) The theory of proportions was a central part of Greek mathematics and had, by the fourteenth century, become a subject of interest in theoretical mathematics in Latin Europe. Oresme reasoned that the ratio of the periods of rotation of any two celestial bodies was an incommensurable magnitude, in modern terminology, an irrational number. The ratio of the periods would never be equal to the ratio between two whole numbers. From this fact it followed that there would never be a time when the two bodies have exactly returned to any given earlier configuration, and the concept of the great return was an illusion.

However foreign Oresme's reasoning may seem from the perspective of today's science, it at least involved sophisticated mathematical considerations and showed a healthy aversion to astrological speculation. Oresme also took up subjects directly related to the subsequent history of cosmology. In a translation into French and commentary on Aristotle's On the Heavens he considered the question of the rotation of the Earth and the various arguments given by Aristotle for the immobility of the Earth. While finally accepting the general validity of Aristotle's views, his vigorous critique of peripatetic arguments was important in encouraging later thinkers to question the absolute correctness of Aristotelian physical doctrine.

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