Kepler Publishes Harmonices Mundi

In summer 1619, Johannes Kepler published 'Harmonices Mundi' (The Harmony of the World) in Linz, Austria. The work contained his third law of planetary motion, demonstrating that the squares of planets' orbital periods are proportional to the cubes of their semi-major axes. Kepler had discovered this mathematical relationship on May 15, 1618. The publication integrated astronomy, geometry, music theory, and numerology to explain astronomical and planetary relationships. The work supported the heliocentric model of the solar system.