THE  TYCHONIC  PLANETARY  MODEL
Dr Robert A. Hatch - University of Florida


The most serious cosmological challenge to the Copernican hypothesis was undoubtedly the model presented by the 'Prince of Astronomers', the great Danish naked-eye observer, Tycho Brahe (1546-1601).  In the Tychonic Model, illustrated above, Tycho assumed, as had Aristotle, Ptolemy, and virtually all the ancients, that the great sphere of  Fixed Stars revolved on its axis each day East to West, and with that motion (the First Motion) all the heavenly bodies circled the fixed, central earth (which was geocentric and geostatic).  Although similar models can be traced to antiquity, Tycho introduced a critical new variation.  In addressing the problem of the annual motions of the planets (the Second Motion) Tycho assumed, as had the ancients, that the Sun revolved around the earth once each year (we would say earth year).  His innovation was to argue that, in turn, all of the other planets revolved around the Sun (with their appropriate annual period).  This arrangement is known as a Geo-Heliocentric Model.  Here the earth (Geo-) is assumed to be at the center of the cosmos, while the Sun (Helio-) is assumed to be the center of the planetary motions (excluding the Moon).  The above illustration first appeared in Boulliau's Philolaus (Amsterdam 1639).  For other illustrations of the Tychonic Model (old & new) Click Here.

   
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