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Ptolemy
Kepler
Comparison In the 3 applets above (if you can't see them, make sure you have JAVA and JAVASCRIPT enabled in your browser), you have the two competing "world views" for explaining the motion of the planet Mars against the stars. Ptolemy used "epicycles" or circles inside circles, (Applet 1) Kepler used ellipses centered on the Sun (Applet 2). The data that Ptolemy had to work with couldn't tell the difference between the two "world views". Kepler had the data from Brahe's observatory, which was much better than the Greeks (or Chinese, or Mayans or Aryans...). Compare the two predictions against the different quality data sets using the hidden buttons at the bottom (Applet 3). Now answer the following questions. What explanation does Ptolemy have for "retrograde" or reverse motion of Mars? What exactly does his explanation "mean"? What explanation does Kepler(Copernicus) have for retrograde motion? Does it "mean" anything more than Ptolemy's explanation? What purpose (meaning) is there is making measurements of something that has already been measured? (E.g., what are the metaphysical reasons for repeated measurements?)
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