45Chapter 6Mechanics, Planetary Motion and
the Modern Science Revolution
The Copernican Revolution
The Copernican heliocentric theory of the universe posed an even
greater challenge to Aristotelian physics than the ideas of Jean Buridan
Albert of Saxony and Nicholas of Oresme. His theory also grew out of a
reinterpretation of the data rather than new observations. Copernicus
(1473–1543) was not the best of observers, accepting good and
bad observations indiscriminately. He was disturbed, however, by
certain discrepancies between the Ptolemaic system and the existing
experimental data. Although his system removed many of these
discrepancies others remained. There were other motivations, however.
Copernicus was disturbed by the contradiction between Ptolemy’s
claim that the Earth was at the center of the universe and the actual
details of the Ptolemaic system. In adhering to Aristotle’s principle
that the heavenly bodies move in circles, Ptolemy reduced the motion
of the planets to combinations of uniform circular orbits called epicycles.
An epicycle is generated by a circular orbit whose center moves about a
second circle and that circle could be moving about a third circle as
illustrated in Fig. 6.1 on the next page. Copernicus could accept the
idea of epicycles and indeed incorporated them into his own scheme.
What disturbed him about the Ptolemaic system, however, was the fact
that the Earth was not actually at the center of the epicyclical orbit of the
Sun and planets but slightly displaced from the center and hence
Ptolemy’s claim that the Earth was exactly at the center of the universe
was a deception.