Kepler Unveils Laws: Planets Move by Math

Mars would not cooperate. Johannes Kepler spent five years wrestling with its orbit, filling 900 pages with calculations that kept failing because he assumed circles. Perfect circles, that was what the heavens demanded, what 2,000 years of astronomy insisted upon from Aristotle through Copernicus. But Mars deviated by just eight arc minutes from the circular predictions, a difference so tiny that most astronomers would have blamed their instruments. Kepler did not. He torched his circular models and tried ellipses instead. It worked. He published his first two laws of planetary motion in 1609 in Astronomia Nova, and on May 15, 1618, he discovered his third law: the square of a planet's orbital period is proportional to the cube of its average distance from the Sun. The third law linked the geometry of orbits to the timing of planetary motion, revealing a mathematical harmony in the solar system that Kepler, a deeply mystical thinker, interpreted as evidence of divine design. The three laws together replaced two millennia of epicycles, deferents, and equants with three clean mathematical relationships. Newton would not explain why planets moved in ellipses for another sixty-nine years, but Kepler had already proven that they did. Born in Weil der Stadt in 1571, he studied theology at Tubingen before his mathematics professor Michael Maestlin introduced him to Copernican astronomy. He worked as imperial mathematician to Rudolf II in Prague, inheriting the position and the observational data from Tycho Brahe, whose meticulous measurements of Mars's position provided the eight-arc-minute discrepancy that shattered the ancient assumption. He died in Regensburg in 1630, impoverished and in search of back pay the emperor owed him.