German astronomer Johannes Kepler was born on 27 December 1571 into a colourful family: his father, Heinrich, was a mercenary who disappeared when Johannes was five years old, and his mother, Katharina, was a herbalist and healer who was later tried for witchcraft. (Kepler himself defended her – and won.)
As a young man Kepler’s work as a mathematician led him to travel widely in Europe – including to Graz, Austria, and Prague, Czech Republic – and he worked with some of the leading astronomers of his day, including Tycho Brahe, as well as serving as ‘court mathematician’ to the Holy Roman Emperor Rudolf II. His most influential work was Astronomia Nova, published in 1609, which is considered the founding work of modern astronomy in its approach to verifiable data and in which he explained his theory of planetary motion. His first two ‘laws’ proved that planets move in elliptical orbits with the sun as their focus and that planets are closer to the sun during different parts of their orbit. (His third law, that planets closer to the sun have a faster orbit than those further away, was published in 1619.) Kepler did not call these breakthroughs ‘laws’, but rather ‘celestial harmonies’ as his Lutheran beliefs led him to see them as part of an intelligible God-created universe. Kepler died in 1630, and while his grave was destroyed during the Thirty Years War the epitaph he wrote for himself survived:
I used to measure the heavens,
Now I shall measure the shadows of the Earth.
Although my soul was from heaven,
The shadow of my body lies here.
Kepler’s work was not universally accepted in his lifetime, but his influence grew over time, particularly after Sir Isaac Newton arrived at similar conclusions in his theory of universal gravitation, and his discoveries are today acknowledged as constituting the founding tenets of modern astronomy. Since his death, Johannes Kepler has had a lunar crater named after him, as well as the second European Automated Transfer Vehicle and a NASA mission, which was launched in March 2009, to discover Earth-like planets orbiting other stars.