Épicycle
The épicycle is an essential component of the astronomical system of Ptolémée (IIe century) (geocentric system which will be used until Copernic (XVIe century) at least).
To explain the movements of planets (or wandering stars ), which seems sometimes to retrogress, the Greek astronomers introduce during the III E the épicycle : it is about a circle whose center describes a circle called Déférent, which (in the first outline of the system in any case) is centered on the Earth. This system replaces that of the " gradually; spheres homocentriques" of Eudoxe de Cnide, which had the favor of Aristote (IV E) but which supposed wrongly that the planets were at a constant distance from the Earth.
The introduction of the équant by Ptolémée (2nd century) makes it possible to improve the model, in particular while giving better an account of apparent speeds of planets.
Extension
Indicate a theory preferred with the contempt of the Principe of economy or Rasoir of Ockham, often by dogma. It is a reference to Galileo which fought against this theory sophisticated by explaining the movement of planets in a very simple way: it is the Ground which turned around the Sun.
External bonds
- an Applet cabriJava which illustrated the principle of the épicycle
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