Giordano Bruno motion of the stars to be an illusion caused by the rotation of the Earth on its axis; he also preserved the notion of an immobile center, but it was the Sun rather than the Earth. Copernicus also argued the Earth was a planet orbiting the Sun once every year. However he maintained the Ptolemaic hypothesis that the orbits of the planets were composed of perfect circles—deferents and epicycles—and that the stars were fixed on a stationary outer sphere. Despite the widespread publication of Copernicus' work "De revolutionibus orbium coelestium", during Bruno's time most educated Catholics subscribed to the Aristotelian
Age of the Earth it would take for the near-surface to cool to its present temperature. His calculations did not account for heat produced via radioactive decay (a process then unknown to science) or, more significantly, convection inside the Earth, which allows more heat to escape from the interior to warm rocks near the surface. Even more constraining were Kelvin's estimates of the age of the Sun, which were based on estimates of its thermal output and a theory that the Sun obtains its energy from gravitational collapse; Kelvin estimated that the Sun is about 20 million years old. Geologists such as Charles Lyell