Kepler's laws of planetary motion

Johannes Kepler did not discover his laws through genius or insight; he discovered them through years of failed attempts to make the data fit preconceived models. His willingness to abandon beautiful theories when they contradicted observations—particularly the ancient ideal of circular orbits—produced the first accurate description of planetary motion.

Kepler began with Tycho Brahe's observations, the most precise astronomical data ever collected. When Tycho died in 1601, Kepler gained access to decades of positional measurements of Mars. He spent years trying to fit Mars's orbit to circles, combinations of circles, and egg-shaped curves. The discrepancy between his models and Tycho's data was eight arc-minutes—about a quarter the width of the full moon.

Most astronomers would have blamed observational error. Kepler trusted Tycho's data more than two millennia of circular-orbit tradition. In 1605, he finally tried an ellipse and found it fit perfectly. The first law: planetary orbits are ellipses with the Sun at one focus.

The second law followed from the same data: planets sweep equal areas in equal times. A planet moves faster when closer to the Sun, slower when farther. This was bizarre—why would an inanimate body know to speed up and slow down? Newton would explain it sixty years later, but Kepler simply reported what the numbers demanded.

The third law came in 1619: the square of a planet's orbital period is proportional to the cube of its average distance from the Sun. This mathematical regularity suggested deep structure in the solar system, structure Newton's gravity would illuminate.

The adjacent possible required Copernican heliocentrism to make planetary calculation tractable, Tycho's data to provide sufficient precision, and Kepler's mathematical skill combined with obsessive persistence. Earlier astronomers had lacked one or more of these elements. Later astronomers would have Kepler's work to build upon.

Kepler's laws enabled Newton's gravitational mechanics. They remain accurate enough for orbital calculations today. An astronomer who refused to privilege theory over data transformed humanity's understanding of cosmic motion.