Johannes Kepler had poor eyesight, so he needed Tycho Brahe's data
Johannes Kepler fell for the sky early: at six his mother took him up high to see the Great Comet of 1577, and at nine he watched a blood-red eclipsed Moon. Then smallpox left him with weak vision and damaged hands. The future founder of modern astronomy could never be much of an observer.
His family had come down in the world. His grandfather had been lord mayor of Weil der Stadt, but his father scraped by as a mercenary and walked out when Johannes was five. His mother, an innkeeper's daughter, worked as a healer and herbalist, and the boy amazed guests at his grandfather's inn with his arithmetic. At the University of Tübingen he trained for the Lutheran ministry, cast horoscopes for classmates and learned both the Earth-centred and Sun-centred systems from Michael Maestlin, siding with Copernicus. The church refused to ordain him because his beliefs clashed with the Formula of Concord, so in 1594, aged 22, he went to teach mathematics in Graz.
There he published Mysterium Cosmographicum in 1596 and sent copies to leading astronomers. Tycho Brahe replied with a blunt critique, pointing out that Kepler relied on inaccurate numbers inherited from Copernicus. Kepler knew the complaint was fair, and without Tycho's far better measurements he could go no further. In 1597 he married Barbara Müller, a twice-widowed heiress whose father had thought him too poor.
Religious politics pushed him out. Archduke Ferdinand, set on restoring Catholicism, ordered Protestant preachers and teachers to leave in September 1598. Kepler was exempted but felt unsafe. On 1 January 1600, before Tycho's invitation had even reached him, he set out for Prague. The two met on 4 February at Benátky, where Tycho was building a new observatory. Tycho guarded his records jealously, releasing observations of Mars bit by bit; talks over a formal job collapsed in an angry row before the pair made up and agreed on pay and lodging.
Kepler went on to become imperial mathematician to Rudolf II and two later emperors. He also transformed optics, designed an improved refracting telescope that bears his name, and wrote Somnium, which has earned him the label father of science fiction. His work helped lay foundations for Newton's theory of gravitation.
Source: Johannes Kepler
