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Dynamics of the celestial spheres
Ancient, medieval and Renaissance astronomers and philosophers developed many different theories about the dynamics of the celestial spheres. They explained the motions of the various nested spheres in terms of the materials of which they were made, external movers such as celestial intelligences, and internal movers such as motive souls or impressed forces. Most of these models were qualitative, although a few of them incorporated quantitative analyses that related speed, motive force and resistance.
==The celestial material and its natural motions==
In considering the physics of the celestial spheres, scholars followed two different views about the material composition of the celestial spheres. For Plato, the celestial regions were made "mostly out of fire"〔Plato, ''Timaeus'' 40a2–4〕〔Cornford 1957, p. 118.〕 on account of fire's mobility.〔Plato, ''Timaeus'', 55d-56a.〕 Later Platonists, such as Plotinus, maintained that although fire moves naturally upward in a straight line toward its natural place at the periphery of the universe, when it arrived there, it would either rest or move naturally in a circle. This account was compatible with Aristotle's meteorology of a fiery region in the upper air, dragged along underneath the circular motion of the lunar sphere. For Aristotle, however, the spheres themselves were made entirely of a special fifth element,〔Grant 1994, p. 189.〕 ''Aether'' (Αἰθήρ), the bright, untainted upper atmosphere in which the gods dwell, as distinct from the dense lower atmosphere, ''Aer'' (Ἀήρ).〔''Aether'' was personified in mythology as the offspring of night and the dwelling place of Zeus.〕 While the four terrestrial elements (earth, water, air and fire) gave rise to the generation and corruption of natural substances by their mutual transformations, aether was unchanging, moving always with a uniform circular motion that was uniquely suited to the celestial spheres, which were eternal.〔Lloyd 1968, pp. 134–9.〕〔"by simple bodies I mean those which possess a principle of movement in their own nature, such as fire and earth and their kinds ... there must necessarily be some simple body which revolves naturally and by virtue of its own nature with a circular movement." Aristotle, ''On the heavens'', 268b28-269a8.〕 Earth and water had a natural heaviness (''gravitas''), which they expressed by moving downward toward the center of the universe. Fire and air had a natural lightness (''levitas''), such that they moved upward, away from the center. Aether, being neither heavy nor light, moved naturally around the center.〔Grant 1994, pp. 196–7.〕
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