Copernicus had challenged the geocentric principle, a thing that had been done many times before him; indeed with increasing frequency during the fifteenth and sixteenth centuries. Never did Copernicus question the Circularity Principle. As we have seen, Tycho Brahe was also unflinchingly committed to that principle. Very likely, no one had ever seriously challenged the idea that all celestial objects move in perfect circles, despite a tremor to the contrary in the works of certain Oriental astronomers, in some of the writings of Nicholas of Cusa, and in the astonishing concession of Tycho Brahe—wherein the paths of comets are said to be ovular. But these departures are aberrations. The young Kepler was as deeply committed to the Circularity Principle as anyone had been during the two millennia of astronomy before him. The powerful grip this principle had on astronomical thought up until the seventeenth century is illustrated by an argument in Kepler’s Mysterium Cosmographicum: