Kepler, born in 1571, proved planets orbit in ellipses

Johannes Kepler was born on December 27, 1571, in Weil der Stadt, in what is now south-west Germany, and grew up to discover the three laws of planetary motion, which showed that planets travel in ellipses, not perfect circles.

His raw material came from Tycho Brahe, the Danish astronomer who was one of the foremost observers of the age. Kepler settled in Prague to work with Brahe after being banished from Graz on 2 August 1600 amid the religious upheavals of the time. From Brahe’s records of Mars, Kepler concluded that its orbit is an ellipse with the Sun at one focus (his First Law), and that a line from planet to Sun sweeps out equal areas in equal times, so planets speed up near the Sun (his Second Law). He published both in Astronomia nova in 1609, then showed they held for the other planets too.

None of it was a flash of insight. Kepler called the Mars problem his “war with Mars”, and nearly a thousand folio sheets of his hand calculations survive. The orbit he extracted matches modern results so closely that comparisons have to allow for slow changes in Mars’s orbit over the centuries since.

His Third Law, linking the square of a planet’s orbital period to the cube of its average distance from the Sun, appeared in Harmonice mundi in 1619. Kepler recorded that the idea first came to him on 8 March 1618, failed when he tested it, and finally held up on 15 May:

“So strong was the support from the combination of my labour of seventeen years on the observations of Brahe and the present study, which conspired together, that at first I believed I was dreaming.” — Johannes Kepler, Harmonice mundi

He finished that book under strain. His mother, Katharina Kepler, had been charged with witchcraft, and he enlisted Tübingen’s law faculty and travelled to Württemberg to defend her. She was eventually released, partly because the authorities had not followed correct legal procedure in the use of torture.