Johannes Kepler

A life in turbulent times

Johannes Kepler (1571–1630) lived through the upheaval of the Thirty Years’ War.^[1] He became assistant to Tycho Brahe, the great astronomer who had kept a continuous record of the Sun, Moon and planets for almost 20 years.^[1]

Twenty years with Tycho’s data

After Tycho died in 1601, Kepler obtained his records and spent more than 20 years analysing them. He concluded that the orbits of the planets are ellipses.^[1]

The laws of planetary motion

Kepler published his first two laws in 1609 in The New Astronomy.^[1] Together with his third law, they describe how every planet moves:^[1]

1. Each planet moves in an ellipse, with the Sun at one focus.
2. The line joining a planet and the Sun sweeps out equal areas in equal times.
3. The square of a planet’s period equals the cube of its average distance from the Sun (P² = a³).

See Kepler’s Laws for what they mean and how to use them.

Kepler’s legacy

Kepler described how the planets move. It took Isaac Newton’s law of gravity to explain why: an inverse-square force of gravity produces exactly the orbits Kepler found.^[2]