Early Astronomy in the University of Michigan Collections

The Theory of Mercury

The topic of Chapter V of the Theorica planetarum is the theory of the motions and circles of Venus and Mercury. The page shown here includes a two-color woodcut representing the motions of Jupiter. In this particular section, the text explains that the deferent center of Mercury moves upon the circumference of this small circle from east to west with a daily motion equal to the one with which the Sun moves contrary to the sphere of the firmament, carrying the aux with it. Therefore, in the same span of time in which the Sun goes around the firmament, the aux of the eccentric deferent and also any other point of it will go around the equant, and the center of the eccentric deferent will go round its small circle.

The Theory of Planets

Pre-Copernican Astronomy

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