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.
The causes of celestial motion As early as Plato, philosophers considered the heavens to be moved by immaterial agents. Plato believed the cause to be a world-soul, created according to mathematical principles, which governed the daily motion of the heavens (the motion of the Same) and the opposed motions of the planets along the zodiac (the motion of the Different).Cornford 1957, pp. 57–93. Aristotle proposed the existence of divine unmoved movers which act as final causes; the celestial spheres mimic the movers, as best they could, by moving with uniform circular motion.Lloyd 1968, pp. 139–147.Grant 1994, p. 515. In his Metaphysics, Aristotle maintained that an individual unmoved mover would be required to insure each individual motion in the heavens. While stipulating that the number of spheres, and thus gods, is subject to revision by astronomers, he estimated the total as 47 or 55, depending on whether one followed the model of Eudoxus or Callippus.Lloyd 1968, pp. 147–53. In On the Heavens, Aristotle presented an alternate view of eternal circular motion as moving itself, in the manner of Plato's world-soul, which lent support to three principles of celestial motion: an internal soul, an external unmoved mover, and the celestial material (aether).Sorabji 1988, pp. 224–6.
Later Greek interpreters In his Planetary Hypotheses, Ptolemy () rejected the Aristotelian concept of an external prime mover, maintaining instead that the planets have souls and move themselves with a voluntary motion. Each planet sends out motive emissions that direct its own motion and the motions of the epicycle and deferent that make up its system, just as a bird sends out emissions to its nerves that direct the motions of its feet and wings.
John Philoponus (490–570) considered that the heavens were made of fire, not of aether, yet maintained that circular motion is one of the two natural motions of fire.Wildberg 1988, p. 239. In a theological work, On the Creation of the World (De opificio mundi), he denied that the heavens are moved by either a soul or by angels, proposing that "it is not impossible that God, who created all these things, imparted a motive force to the Moon, the Sun, and other stars – just as the inclination to heavy and light bodies, and the movements due to the internal soul to all living beings – in order that the angels do not move them by force."Wildberg 1988, p. 243.Wolfson 1962, p. 70.Sorabji 1988, p. 232. This is interpreted as an application of the concept of impetus to the motion of the celestial spheres.Sorabji 1988, pp. 231–7.Wildberg 1988, pp. 243–6.Sorabji 1988, pp. 232. In an earlier commentary on Aristotle's Physics, Philoponus compared the innate power or nature that accounts for the rotation of the heavens to the innate power or nature that accounts for the fall of rocks.Sorabji 1988, p. 234.
Islamic interpreters The Islamic philosophers al-Farabi () and Avicenna (–1037), following Plotinus, maintained that Aristotle's movers, called intelligences, came into being through a series of emanations beginning with God. A first intelligence emanated from God, and from the first intelligence emanated a sphere, its soul, and a second intelligence. The process continued down through the celestial spheres until the sphere of the Moon, its soul, and a final intelligence. They considered that each sphere was moved continually by its soul, seeking to emulate the perfection of its intelligence.Wolfson 1958, pp. 243–4. Avicenna maintained that besides an intelligence and its soul, each sphere was also moved by a natural inclination (mayl).Wolfson 1962, pp. 82–3.
An interpreter of Aristotle from Muslim Spain, al-Bitruji (d. ), proposed a radical transformation of astronomy that did away with epicycles and eccentrics, in which the celestial spheres were driven by a single unmoved mover at the periphery of the universe. The spheres thus moved with a "natural nonviolent motion".al-Biţrūjī. (1971) On the Principles of Astronomy, 1.60-62, trans. Bernard R. Goldstein, vol. 1, pp. 78–9. New Haven: Yale University Press. . The mover's power diminished with increasing distance from the periphery so that the lower spheres lagged behind in their daily motion around the Earth; this power reached even as far as the sphere of water, producing the tides.Dales 1980, pp. 538–9.Weisheipl 1961, p. 297.
More influential for later Christian thinkers were the teachings of Averroes (1126–1198), who agreed with Avicenna that the intelligences and souls combine to move the spheres but rejected his concept of emanation.Wolfson 1962, p. 85. Considering how the soul acts, he maintained that the soul moves its sphere without effort, for the celestial material has no tendency to a contrary motion.Grant 1994, pp. 539–40.
Later in the century, the mutakallim Adud al-Din al-Iji (1281–1355) rejected the principle of uniform and circular motion, following the Ash'ari doctrine of atomism, which maintained that all physical effects were caused directly by God's will rather than by natural causes. He maintained that the celestial spheres were "imaginary things" and "more tenuous than a spider's web".pp. 55–57 of His views were challenged by al-Jurjani (1339–1413), who argued that even if the celestial spheres "do not have an external reality, yet they are things that are correctly imagined and correspond to what [exists] in actuality."
Medieval Western Europe In the Early Middle Ages, Plato's picture of the heavens was dominant among European philosophers, which led Christian thinkers to question the role and nature of the world-soul. With the recovery of Aristotle's works in the twelfth and thirteenth centuries, Aristotle's views supplanted the earlier Platonism, and a new set of questions regarding the relationships of the unmoved movers to the spheres and to God emerged.Grant 1994, pp. 707–717.
In the early phases of the Western recovery of Aristotle, Robert Grosseteste (–1253), influenced by medieval Platonism and by the astronomy of al-Bitruji, rejected the idea that the heavens are moved by either souls or intelligences.Dales 1980, pp. 540–3. Adam Marsh's (–1259) treatise On the Ebb and Flow of the Sea, which was formerly attributed to Grosseteste, maintained al-Bitruji's opinion that the celestial spheres and the seas are moved by a peripheral mover whose motion weakens with distance.Dales 1980, p. 540.
Thomas Aquinas (–1274), following Avicenna, interpreted Aristotle to mean that there were two immaterial substances responsible for the motion of each celestial sphere, a soul that was an integral part of its sphere, and an intelligence that was separate from its sphere. The soul shares the motion of its sphere and causes the sphere to move through its love and desire for the unmoved separate intelligence.Wolfson 1962, pp. 87-90.Grant 1994, pp. 516–17. Avicenna, al-Ghazali, Moses Maimonides, and most Christian scholastic philosophers identified Aristotle's intelligences with the angels of revelation, thereby associating an angel with each of the spheres.Grant 1994, p. 525. Moreover, Aquinas rejected the idea that celestial bodies are moved by an internal nature, similar to the heaviness and lightness that moves terrestrial bodies.Weisheipl 1961, p. 322. Attributing souls to the spheres was theologically controversial, as that could make them animals. After the Condemnations of 1277, most philosophers came to reject the idea that the celestial spheres had souls.Grant 1994, pp. 546–7.
Robert Kilwardby (–1279) discussed three alternative explanations of the motions of the celestial spheres, rejecting the views that celestial bodies are animated and are moved by their own spirits or souls, or that the celestial bodies are moved by angelic spirits, which govern and move them. He maintained, instead, that "celestial bodies are moved by their own natural inclinations similar to weight."Weisheipl 1961, p. 315. Just as heavy bodies are naturally moved by their own weight, which is an intrinsic active principle, so the celestial bodies are naturally moved by a similar intrinsic principle. Since the heavens are spherical, the only motion that could be natural to them is rotation. Kilwardby's idea had been earlier held by another Oxford scholar, John Blund (–1248).Weisheipl 1961, pp. 316-317.Grant 1994, pp. 547–548.
In two slightly different discussions, John Buridan () suggested that when God created the celestial spheres, he began to move them, impressing in them a circular impetus that would be neither corrupted nor diminished, since there was neither an inclination to other movements nor any resistance in the celestial region. He noted that this would allow God to rest on the seventh day, but he left the matter to be resolved by the theologians.Dales 1980, pp. 547–8.