Kepler publishes Astronomia Nova

Johannes Kepler used the meticulous observations of Tycho Brahe, especially those of Mars, to show that planets do not move in circles at constant speed. His New Astronomy of 1609 states two laws: each planet moves in an ellipse with the Sun at one focus, and a line from the Sun to the planet sweeps equal areas in equal times. After years of failed calculations he gave up the ancient dogma of circular motion because the data would not fit. A third law, linking orbital period to distance, followed in 1619. Newton later derived all three from his law of gravity.

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