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Geocentric World Systems in 1450-1550

In On the sky Aristotle (384-322 BC) pictures the Earth, immovable at the centre of the Cosmos, surrounded by eight groups of heavenly spheres rotating uniformly carrying the Moon, the Sun, the planets, and the fixed stars. In the Middle Ages, such groups of spheres were incorporated into a ninth sphere, or "first movable", and thereafter into a tenth one, the immovable "empyrean sky'.

The planetary models that Claudius Ptolemy (2nd century A.D.) described in his Almagest solved some problems of the Aristotelian architecture of the Cosmos. However, the new models were founded on a series of circles which were difficult to arrange into a system of heavenly spheres. Moreover, the "eccentric" and "equant circles" and "epicycles" employed by Ptolemy often appeared to contradict the Platonic concept of the uniform circularity of celestial motion.

Between 1450 and 1550, many problems related to the Aristotelian or Ptolemaic concepts of the Cosmos remained unsolved, and the astronomical debate gave rise to at least three geocentric conceptions. Therefore, the idea that the so-called "astronomical revolution" determined the dominance of the heliocentric system against the Aristotle-Ptolemaic one, only approximates the historical facts.


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