Were Galileos discoveries the most important astronomical events of 1609?
Maybe not! Galileo’s discoveries were world-shaking without a doubt. Less heralded at the time was the publication of Johann Kepler’s book New Astronomy, which appeared in 1609. In it, Kepler showed that the orbit of Mars is an ellipse (and probably the orbits of the other planets as well). It is hard today to understand how shocking that was to a world that took for granted that all celestial motions were perfect circles (including, by the way, Galileo, who never accepted Kepler’s work). Perfect circles had been the unquestioned foundation of astronomy since the ancient Greeks invented mathematical astronomy more than 2000 years before Galileo and Kepler were born. To throw away celestial circles was one of the most revolutionary ideas a scientist has ever presented. How did Kepler settle on ellipses? He used years of celestial observations compiled by Tycho Brahe, who had never seen a telescope (Tycho died in 1600), and was able to show that observational data demanded that the orbit
