It's the 400th anniversary this year of the publication of Kepler's

*Astronomia Nova*. While thinking about his contributions, I realized Kepler probably made a larger contribution to the scientific revolution in astronomy than Copernicus did. While Copernicus made use of the epicycle, Kepler made it obsolete. More important, though, before Kepler, the systems of Ptolemy, Copernicus, and Brahe had roughly the same power to predict celestial motion. It's easy to forget, but the mathematics of Copernicus' heliocentric system was still firmly rooted in Ptolemy and methods employed by Arab astronomers in the middle ages, especially al-Tusi. Kepler's decision to employ the geometry of conic sections is really what made the qualitative leap from a geocentric to a heliocentric system possible. Or I should say it was a necessary but not a sufficient condition. Galileo's observations of the phases of Venus and the moons of Jupiter provided the other pillar necessary to make the transition. But while Galileo's observations made a transition to a heliocentric model necessary, it was Kepler's mathematics of planetary motion that gave such a model the predictive edge over competing theories.

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