The encyclopedic writers of the early Middle Ages communicated a modest assortment of basic cosmological information, drawn from a variety of ancient sources, especially Platonic and Stoic. These writers proclaimed the sphericity of the earth, discussed its circumference, and defined its climatic zones and division into continents. They described the celestial sphere and the circles used to map it; many revealed at least an elementary understanding of the solar, lunar and other planetary motions. They discussed the nature and size of the sun and moon, the cause of eclipses, and a variety of metrological phenomena.
This picture was enriched in the twelfth century by renewed attention to the content of Plato's Timaeus (and Calcidius's commentary on it) and by contact with Greek and Arabic books in translation. One of the resulting changes was increased emphasis (surpassing that of the early church fathers) on reconciling Platonic cosmology and the biblical account of creation. Another novelty was the frequent argument of the twelfth-century authors that God limited His creative activity to the moment of creation; thereafter, they held, the natural causes that He had created directed the course of things. Twelfth-century cosmologists stressed the unified, organic character of the cosmos, ruled by a world soul and bound together by astrological forces and the macrocosm-microcosm relationship. In an important continuation of early medieval thought, twelfth-century scholars described a cosmos that was fundamentally homogeneous, composed of the same elements from top to bottom: Aristotle's quintessence or aether and his radical dichotomy between the celestial and terrestrial regions had not yet made their presence felt. Perhaps, one of the best example of twelfth-century cosmology would be The Cosmographia of Bernardus Silvestris. I would go so far to say that it can be considered the standard text for the subject.
Robert Grosseteste (ca. 1168-1253) is of great use to an attempt to build a solid view of medieval cosmology, especially since he wrote voluminously. Educated in the twelfth century, the majority of his writings, including all the important ones, date from the first half of the thirteenth century.
Central to Grosseteste's cosmology was light: the cosmos came into existence when God created a dimensionless point of matter and its form, a dimensionless point of light, which Grosseteste refers to as "first form" or "corporeal form." The point of light instantaneously diffused itself into a great sphere, drawing matter with it and giving rise to the corporeal cosmos. Subsequent radiation (from the outermost limit of the cosmos back toward the center) and differentiation gave rise to celestial spheres and the characteristic features of the sublunar region. In his early writings Grosseteste seems to have accepted the idea of a world soul - an idea from which he later retreated. The theme of microcosm and macrocosm is fundamental to Grosseteste's works: humanity represents the pinnacle of God's creative activity, simultaneously mirroring the divine nature and the structural principles of the created cosmos. Finally, Grosseteste shared the early medieval and twelfth-century belief in a homogeneous cosmos: the heavens in his cosmology are made of finer (specifically more rarified) stuff than are terrestrial substances, but the difference is quantitative rather than qualitative.
Cosmology, like so many other subjects, was transformed by the wholesale translation of Greek and Arabic sources in the twelfth and thirteenth centuries. Specifically, the Aristotelian tradition gained center stage in the thirteenth century and gradually substituted its conception of the cosmos for that of Plato and the early Middle Ages. This is not to suggest that Aristotle and Plato disagreed on all the important issues; on many of the basics they were in full accord. Aristotelians, like Platonists, conceived the cosmos to be a great (but unquestionably finite) sphere, with the havens above and the earth at the center. All agreed that it had a beginning in time - although some Aristotelians of the thirteenth century were prepared to argue that this could not be established by philosophical arguments. Nobody representing either school of thought doubted that the cosmos was unique: although nearly everybody acknowledged that God could have created multiple worlds, it is difficult to assume that anybody seriously believed He had done so.
However, where Aristotle and Plato disagreed, the Aristotelian world picture gradually displaced the Platonic. One of the major differences concerned the issue of homogeneity. Aristotle divided the cosmic sphere into two distinct regions, made of different stuff and operating according to different principles. Below the moon is the terrestrial region, formed out of the four elements. This region is the scene of generation and corruption, of birth and death, and of transient (typically rectilinear) motions. Above the moon are the celestial spheres, to which the fixed stars, the sun and the remaining planets are attached. This celestial region, composed of aether or the quintessence (the fifth element), is characterized by unchanging perfection and uniform circular motion. Other Aristotelian contributions to the cosmological picture were his elaborate system of planetary spheres and the principles of causation by which the celestial motions produced generation and corruption in the terrestrial realm.
A variety of Aristotelian features, then, merged with traditional cosmological beliefs to define the essentials of late medieval cosmology - a cosmology that became the shared intellectual property of educated Europeans in the course of the thirteenth century. Universal agreement of such magnitude emerged not because the educated felt compelled to yield to the authority of Aristotle, but because his cosmological picture offered a persuasive and satisfying account of the world as they perceived it. Nonetheless, certain elements of Aristotelian cosmology quickly became the objects of criticism and debate, and it is here, in the attempt to flesh out and fine-tine Aristotelian cosmology and bring it into harmony with the opinions of other authorities and with biblical teaching, that medieval scholars made their cosmological contribution. It is impossibly to cover the full body of this subject (Pierre Duhem devoted ten volumes to the subject!) and to tell the truth, it is not necessary for Ars Magica. Instead, it will be fruitful to limit ourselves to the most important and hotly debated questions of cosmology.
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Last modified: Sun Dec 27, 1998 / Jeremiah Genest