LECTURE 5: PTOLEMY TO COPERNICUS

Key Questions:






THE PTOLEMAIC MODEL

Geocentric theory of planetary motion perfected by Hipparchus (2nd century BC) and Ptolemy (2nd century AD).

Key innovation: the epicycle

To achieve high accuracy, allow deferent center to be off Earth ("eccentric").
Also fiddle with uniform circular motion.
If necessary, can add epicycles onto epicycles.







EVALUATION OF THE PTOLEMAIC MODEL

Strengths:

Weaknesses:






"SAVING THE APPEARANCES"

A strength or a weakness?

A key philosophical question: Should a theory of the universe seek to explain the motions of the stars, Sun, Moon, and planets in true physical terms, or should it merely try to predict them accurately ("save the appearances")?


LOSS AND REDISCOVERY








PAVING THE WAY FOR COPERNICUS

Nicolaus Copernicus (1473-1543), Polish lawyer, physician, and church canon, first developed his heliocentric model around 1510.
After 1500 years of Ptolemy, what paved the way for a change of view?
Numerous changes in the 14th and 15th centuries.

``Scientific'' developments:

Broader cultural developments:








THE COPERNICAN MODEL

Motivations:

Basics: Epicycles:






SUCCESSES OF THE COPERNICAN MODEL

Copernicus takes heliocentric model much further than Aristarchus, finding many powerful applications.








EVALUATION OF THE COPERNICAN MODEL

Strengths:

Weaknesses: From a modern perspective, Copernicus gets to largely the right answer for partly the wrong reasons (insistence on circular motions).
Not just luck: required guts, creativity, calculational skill, years of hard work.






RECEPTION OF COPERNICAN THEORY

Again the key question: Should an astronomical theory seek objective truth, or merely an adequate description of what is observed?
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Updated: 2005 April 3[dhw]