Used with
permission of Maiken Naylor, University at Buffalo, Buffalo, NY, USA,
http://ublib.buffalo.edu/libraries/units/sel/exhibits/stamps
Johannes Kepler (1571-1630) was a German mathematician who is remembered for his three laws of planetary motion, derived empirically from Tycho Brahe's data and observations. The laws describe the solar system as having the sun at the focus of elliptic planetary orbits. In his writings on conic sections he introduced the word focus into mathematical language.