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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.
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" on account of fire's mobility. 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, Aether (Αἰθήρ), the bright, untainted upper atmosphere in which the gods dwell, as distinct from the dense lower atmosphere, Aer (Ἀήρ). 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. 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.
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). 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. 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. 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).
In his Planetary Hypotheses, Ptolemy (c. 90 – c. 168) 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. 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." This is interpreted as an application of the concept of impetus to the motion of the celestial spheres. 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.
The Islamic philosophers al-Farabi (c. 872 – c. 950) and Avicenna (c. 980–1037), 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. Avicenna maintained that besides an intelligence and its soul, each sphere was also moved by a natural inclination (mayl).
An interpreter of Aristotle from Muslim Spain, al-Bitruji (d. c. 1024), 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". 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.
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. 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.
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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.
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" on account of fire's mobility. 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, Aether (Αἰθήρ), the bright, untainted upper atmosphere in which the gods dwell, as distinct from the dense lower atmosphere, Aer (Ἀήρ). 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. 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.
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). 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. 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. 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).
In his Planetary Hypotheses, Ptolemy (c. 90 – c. 168) 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. 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." This is interpreted as an application of the concept of impetus to the motion of the celestial spheres. 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.
The Islamic philosophers al-Farabi (c. 872 – c. 950) and Avicenna (c. 980–1037), 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. Avicenna maintained that besides an intelligence and its soul, each sphere was also moved by a natural inclination (mayl).
An interpreter of Aristotle from Muslim Spain, al-Bitruji (d. c. 1024), 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". 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.
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. 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.