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Non Ptolemaic Geocentric Structures

Planetary models of the Almagest did not match the physical vision of the Cosmos preferred by philosophers. Ptolemy's eccentric deferents appeared to disagree with the Earth-centred spherical structure of the heavens. Further, the equants opposed the uniformity of celestial motion. This contradiction with the basic cosmological ideas of both Plato (4th century BC) and Aristotle produced a relevant conceptual reaction.

In the 13th and 14th centuries, several Islamic astronomers conceived systems of spheres brilliantly substituting eccentric deferents and equants. Successively, spreading knowledge of the Almagest produced reactions in Europe. Hans Muller of Konigsberg (1436-1476), also known as Regiomontanus, built simple concentric models for the Sun and the Moon. In Italy, Giovan Battista Amico (1511-1538), Alessandro Achillini (1463-1512), and Gerolamo Fracastoro (1483-1553) attempted to restore perfectly spherical heavens by recalling the planetary models of Eudoxus of Cnydus (4th century BC), based on sets of interconnected concentric spheres. However, the resulting models were extremely complex; for example, Fracastoro's ones required the considerable number of 11 revolving spheres to explain the motion of a single planet.

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