History Weblecture for Unit 26
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Lecture outline:
This week we start on the Scientific Revolution in physical science. The first major challenge to Aristotle (as opposed to Galen) was in planetary theory. The Ptolemaic system worked, sort of, but it wasn't always accurate enough and it was very clumsy.
We discussed at some length already the physical models of Aristotle and the model Ptolemy proposed for calculating the positions of the planets. Review these concepts if you need to before looking at the new models of the Scientific Revolution.
Three people challenged the Ptolemaic-Aristotelian view of the solar system: Copernicus, Kepler, and Galileo; we'll cover Kepler and Galileo next time.
In a way, Copernicus was the most conservative of the three. He didn't like some aspects of the Ptolemaic theory, thought it could be made much simpler if it was reorganized a little. He realized that placing the Sun at the center of the planetary system and setting the earth in motion would have certain ramifications in circle of ecclesiastical power, circles which were already being challenged by the rise of Protestant groups in northern Europe and England, so he wrote the main text of his work very carefully. Historians of science still have debates over whether Copernicus believed that the physical system he proposed was real, or just a better model for figuring out the planetary positions than Ptolemy's.
Read a bit about Copernicus' background in the St. Andrew's biography of him.
Study the diagrams below for the Ptolemaic system, which places the Earth at the center of the Solar System.
Planet at opposition.
Planet past opposition.
Study the diagrams below from the Copernican planetary system.
Planet at superior conjunction.
Planet at opposition.
These are based on the simulators available from the astronomy department at the University of Nebraska, Lincoln
Despite the radical reposition of the Sun, however, Copernicus' system closely resembles Ptolemy's in many aspects. Copernicus still had to use epicycles and eccentrics to explain changing rates of speed in the planetary motions (although he didn't need them to explain retrograde motion any more). His system was no more simple to use for calculations than Ptolemy's.
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