The Tychonian planetary model, conceived by Tycho around 1583, was an unconvincing attempt to reintroduce geocentrism in the Copernican planetary system. From his observations of the 1572 nova and 1577 comet, Tycho was convinced of the falsity of the Ptolemaic system. In Tycho's system the Earth is absolutely fixed, so that the daily motion of the fixed stars is ascribed to a daily rotation of the outermost sphere, as in the Ptolemaic system. A similar planetary system was proposed in antiquity by Heraklides of Pontus (ca. 388--310 BC) who, however, ascribed to the Earth a daily axial rotation.
From the standpoint of apparent planetary motions as seen from Earth, this system is observationally indistinguishable from the Copernican model, yet maintains the fixity of the Earth. The latter belief was held by Tycho to the end of his life, largely because he had been unable to detect the annual parallax of the fixed stars predicted by the Copernican model, despite the unprecedented accuracy of the observations carried out with his giant instruments at Uraniborg. Tycho could measure parallax down to 2 minutes of arc (1/30 of a degree); his lack of parallax detection for fixed stars implied that the latter would have to be located 700 times [CHECK/CALCULATE] farther away than Saturn, the outermost planet known at the time. Image reproduced from Hevelius' Selenographia.
-Written and last revised 20 December 1997 by paulchar@ucar.edu.
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