Kepler's illustration to explain his discovery of the elliptical orbit of Mars, 1609. Working with data collected by the Danish astronomer Tycho Brahe, Johannes Kepler (1571-1630) determined that planetary orbits were elliptical rather than spherical. He formulated three laws of planetary motion, known as Kepler's laws, using them to accurately predict the Transit of Venus which occurred in 1631. From Astronomia Nova.de Motibus Stellae Martis by Johannes Kepler, 1609.