Keplerâ€™s Laws of Planetary Motion
While Copernicus rightly observed that the planets revolve around
the Sun, it was Kepler who correctly defined their orbits. At the age of
27, Kepler became the assistant of a wealthy astronomer, Tycho Brahe,
who asked him to define the orbit of Mars. Brahe had collected a
lifetime of astronomical observations, which, on his death, passed into
Keplerâ€™s hands. (Brahe, who had his own Earth-centered model of the
Universe, withheld the bulk of his observations from Kepler at least in
part because he did not want Kepler to use them to prove Copernican
theory correct.) Using these observations, Kepler found that the orbits
of the planets followed three laws.