The fact that the planets move around the Sun, not in circles but in ellipses, was first demonstrated by Johannes Kepler. Galileo is credited with several important inventions, including the geometric compass, thermometer and pendulum clock. Martin Luther, O.S.A. was a German professor of theology, composer, priest, monk, and a seminal figure in the Protestant Reformation. Nicolaus Copernicus was instrumental in establishing the concept of a heliocentric system, in which planets revolve around the sun.