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Johnnes Kepler 1571 - 1630

     Johannes Kepler firmly believed in the Copernican heliocentric picture.  Having been raised in the Greek geometric tradition, he believed God must have had some geometric reason for placing the six planets at the particular distances from the sun that they occupied.  He thought of their orbits as being on spheres, one inside the other.  One day, he suddenly remembered that there were just five perfect Platonic solids, and this gave a reason for there being six planets - the orbit spheres were maybe just such that between two successive ones a perfect solid would just fit.  He convinced himself that, given the uncertainties of observation at the time, this picture might be the right one.  However, that was before Tycho's results were used.  Kepler realized that Tycho's work could settle the question one way or the other, so he went to work with Tycho Brahe in 1600.  Tycho died the next year and Kepler stole the data, and worked with it for the next nine years.

     He reluctantly concluded that his geometric scheme was wrong.  In its place, he founded these three laws of planetary motion:

     These are the laws that Isaac Newton used as a basis to establish his theory of universal gravitation.  Kepler was the first to state clearly that the way to understand the motion of the planets was in terms of some kind of force from the sun.  However, in contrast to Galileo, Kepler thought that a continuous force was necessary to maintain motion, so he visualized the force from the sun like a rotating spoke pushing the planet around its orbit.

     On the other hand, Kepler correctly ascertained that the tides were caused by the moon's gravity.  Galileo Galilei mocked him for this suggestion.

     Awards, Honors, and Associations

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