Sphere (geocentric)
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In the geocentric model of Classical Greek astronomy according to Plato, Aristotle and others, the stars and planets were carried around the Earth on spheres or circles, arranged in the order (outwards from the center):
- Moon (lunar sphere)
- Sun,
- Venus,
- Mercury,
- Mars,
- Jupiter,
- Saturn,
- fixed stars (heavenly sphere), the firmament, including the zodiac, corresponding to the celestial sphere
In the fully developed Aristotelian system, the spherical Earth is at the center of the universe. All heavenly bodies are attached to 56 concentric spheres which rotate around the Earth. (The number is so high because several transparent spheres are needed for each planet.) The Moon is on the innermost sphere. Thus it touches the realm of Earth, which contaminates it, causing the dark spots (macula) and the ability to go through lunar phases. It is not perfect like the other heavenly bodies, which shine by their own light.
In philosophy, the pristine perfection of the mechanics of the spheres was associated with the perfect harmony of the Musica universalis (attributed to Pythagoras, whose system was not geocentric but had the Earth together with the other planets circle around a mystical central fire). In contrast, the mortal world below the lunar sphere (the sublunary sphere) was affected by corruption and decay.
Although the basic tenets of Greek geocentrism were established by the time of Aristotle, the details of his system did not become standard. This honor was reserved for the Ptolemaic system, espoused by the Greco-Roman astronomer Claudius Ptolemaeus in the 2nd century. His main astronomical book, the Almagest, was the culmination of centuries of work by Greek astronomers; it was accepted for over a millennium as the correct cosmological model by European and Islamic astronomers. Because of Ptolemy's influence, geocentric model is sometimes used synonymously with Ptolemaic system
In the Ptolemaic system, each planet is moved by two or more spheres: one sphere is its deferent which is centered on the Earth, and the other sphere is the epicycle which is embedded in the deferent. The planet is embedded in the epicycle sphere, allowing the model to account for retrograde motion in principle, although the equant needed to be introduced to account for the extent actually observed.
In the Paradiso of Dante's Divine Comedy, a ninth sphere of the Primum Mobile ("Prime Mover") is added, inhabited by angels, from where Dante ascends to a substance beyond physical existence, called the Empyrean Heaven. Here he comes face-to-face with God Himself, and is granted understanding of the Divine and of human nature.
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