Equant

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Related to equants: Epicycles

Equant

 

an auxiliary point in the Ptolemaic system (the point E in the figure), introduced by Ptolemy to match observed planetary motions with motions previously calculated with deferents and epicycles.

Figure 1

Describing the motion of the planets with circles and constant angular velocities, Ptolemy placed the earth (T) outside the center of the deferent (C). He assumed that the center of the epicycle (O) of a planet moved nonuniformly along the deferent, but in such a manner that the motion seemed uniform to an observer located at the equant, which was symmetric to the position of the earth relative to the center of the deferent.