What was the main difference between Copernicus' model and Kepler's model?

Answer 1

Copernicus (1473-1543)Ptolemaic System

• Planets appear to reverse motions at times.
• Ptolemy explained motions in terms of orbits (epicycles) carried on a larger orbit (deferent).
• Epicycle deferent ratios were very close to modern values of planet/earth orbit ratios. System worked very well.
• Contrary to popular myths, Ptolemy's system was not overly cumbersome, and it accounted for subtleties like the uneven motion of the Sun
• It is not Ptolemy's fault he did such a good job that it took 1500 years to improve on him!

Emerging Problems

• System began to seem cumbersome and inelegant.
• System inaccurate. Alfonsine tables out of date by 1500.

Possible clues to Copernican idea

• Epicycle motions for Venus and Mercury opposite other planets.
• Epicycle for Sun's motion appeared in schemes for all other planets.
• References to now-lost ideas of Aristarchus of Samos

Heliocentric Idea

• Copernicus replaced epicycles with orbital motion of Earth.
• Less accurate than Ptolemaic system but conceptually simpler.
• Published as De Revolutionibus Orbium Coelestium (on the Revolutions of the Heavenly Spheres) 1542.
• Little immediate hostility. Earlier speculations on moving earth had been decreed heretical, hence moving earth idea could be lightly dismissed.
• One of most vigorous critics was Martin Luther.

Kepler (1572-1630), Brahe (1546-1601)Kepler was a medieval mystic.

• One of the last of the "scientific astrologers."
• Attempted to explain spacing of planet orbits by reference to Platonic solids.
• Kepler was reluctant to abandon perfectly circular motion (but despite his mystic tendencies, he did when the evidence required it).
• Kepler found many numerological relationships among the planets, of which "Kepler's Laws" are the three that have proven to have a physical basis.
• Kepler's concept of the Sun as center of solar system may have had a mystical basis.

Need for observational data.

• Copernican theory based on same observational data as Ptolemaic, hence no more accurate.
• Kepler turned to Brahe, who had the most advanced observational data.

Kepler's Laws

• Orbits are elliptical, Sun at one focus, nothing at other.
• Radius vector sweeps out equal areas in equal times.
• Square of period proportional to cube of distance.

Kepler and Galileo

• Galileo and Kepler corresponded.
• Galileo defended Copernican astronomy but never wrote about Kepler's model.
• Galileo may have been repelled by Kepler's mysticism.
• Moral: even the best and most innovative workers can sometimes fail to recognize a major advance.

Answer 2

The main difference between Copernicus' and Kepler's models of the universe was the shape of the orbits. Copernicus proposed a heliocentric model with circular orbits, while Kepler later proposed an elliptical orbit model. Kepler's model was able to better explain the observed motions of the planets.