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Natural Science - Year I

Unit 26: The Copernican Revolution

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History Weblecture for Unit 26

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History Lecture for Unit 26: The Revolution in the Heavens

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Lecture outline:

The Copernican Revolution

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.

Remembering Ptolemy

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.
  • How was Copernicus educated and employed?
  • What was Copernicus' primary goal in reforming the Ptolemaic system?
  • What two works did he write about his heliocentric system?
  • What objections did Copernicus have to Ptolemy's system?
  • What changes to this system did he make? What theoretical improvements resulted?
  • What major observational evidence was required (but at the time, impossible to obtain) to support Copernicus' assertion that the earth revolved around the Sun?

    Study the Ptolemaic Planetary System model, which places the Earth at the center of the Solar System.
    • The simulation starts with Mars as the target planet.
    • Run the simulation (use "start animation") until Mars is on a line between the Earth and the Sun (superior conjunction.). The Planet P and Sun S should be superimposed on the Zodiac Strip.
    • Now run the animation until the planet and Sun are directly opposite one another with the Earth at the center. What happens to the path of the planet as approaches this opposition position?
    • Run the simulation again and watch the planet and Sun positions change on the Zodiac Strip.

    Planet at opposition.


    Planet past opposition.

    Study the Planetary Configurations (Copernican) Heliocentric system.
    • The simulation starts with the observer's planet as an inferior planet, closer to the Sun than the target planet.
    • Run the simulation until the planet reaches quadrature (use the timeline at the bottom to identify when you reach each stage). What are the planets' positions relative to the Sun and each other?
    • Run the animation through opposition to the next quadrature. Watch the planet and Sun positions on the Zodiac Strip. What happens as the planet approaches opposition? What does it do after opposition?
    • Do both systems predict the same motions of the planet relative to the Sun along the Zodiac Strip? Which system seems simpler?

    Planet at superior conjunction.


    Planet at opposition.

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.

Study/Discussion Questions:

Further Study/On Your Own