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GEO-HELIOCENTRIC PLANETARY MODELS
Dr Robert A. Hatch - University of Florida
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| There are a number of little-known planetary models that foreshadowed the Tychonic Model. In addition to historical illustrations, I have provided several early modern illustrations as well as several modern diagrams (which I have drawn) to help illustrate how Tycho's model worked. One of the principal advantages, not celebrated in practice until after Tycho's death, is that the model accounts for the change of phases (the apparent illumination, similar to earth's moon) undergone by Mercury and Venus. The fact is, these two inferior planets go through phases (most notably Venus) ranging from a nearly perfect full disk to a thin large crescent shape. That the Tychonic Model could account (indeed would predict) such changes, notably in the wake of Galileo's telescopic discoveries, proved a very strong attraction. These changes of phase could not be accounted for in the Ptolemaic Model, which assumed that Mercury and Venus moved on circles on a line between the earth and Sun, hence, they could not be imagined to go through phase. In Tycho's Model, as in the Copernican, Mercury and Venus were understood to revolve around the Sun and, hence, they would go through phases like the earth's moon. A final note must be underscored. The Tychonic Model was not a cosmological 'half-way house', a 'red herring', or a 'conceptual bridge.' It was a legitimate, powerful alternative. Only in retrospect does it appear to fill the uneasy gap between geocentric and heliocentric models. Finally, as a corollary, the Tychonic Model was not a conservative, spine-less, past-embracing compromise, quite the contrary. A moments thought suggests that it was entirely contrary to Aristotle, his theory of elements, his theory of place, his theory of motion. Elsewhere, with the Star of 1572, Tycho called into question the doctrine of the immutability of the heavens. Elsewhere, with the Comet of 1577, Tycho called into question the doctrine of the crystalline spheres. What remained was the problem of the planets. What makes the planets move? What remained was the problem of the cosmos. What kind of material accounts for the Sun, the Moon, the Stars, and the Planets and hence, what accounts for their motion and the change that now seems so apparent? It is important to understand the descriptive power and cosmological subtlety of the Tychonic Model in order to appreciate its historical influence. To that end, I supply the following Clicks. |
rah.jan.99
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Copyright 1998 - Dr Robert A. Hatch - All Rights Reserved