The orbit of Mars, 1609.
© Science Museum / Science & Society Picture Library
Engraving from 'Astronomia nova' (New astronomy) by German astronomer Johannes Kepler (1571-1630) published in Heidelberg in 1609. Modern astronomy started with the publication here of Kepler's first two laws of planetary motion: 1. The orbit of a planet is an ellipse of which the sun is in one focus; 2. In describing its orbit, the radius vector of a planet traverses equal areas in equal time. Using Tycho Brahe's planetary tables, Kepler realised that the orbit of Mars was elliptical; (in this diagram earlier theories of planetary motion by Copernicus, Brahe and Ptolemy are compared).