Most people who have heard of Johannes Kepler, pictured here at age 39, remember him primarily as an astronomer who changed our understanding of planetary motion. He is most famous for his discovery that planets move in elliptical orbits rather than in the pure circles theorized by those who came before him. He deemed Earth a planet like any other, one that revolved annually around the Sun. That belief made him one of the first to accept Copernicus’s heliocentric cosmos.
But Kepler did not view astronomy as his highest calling. In a letter from 1605, written only a few weeks after he formulated his theory of elliptical orbits—following a careful study of the orbit of Mars—he wrote the following to a friend in London: “If only God would set me free from astronomy so that I might turn to the care of my work on the harmony of the world.”...
