Nicolaus Copernicus'epoch-making De Revolutionibus Orbium Caelestium was published in 1543, and when Galileo, some fifty years later, became interested in astronomical problems, the heliocentric theory was no longer a novel idea. It was common knowledge in educated circles that several Humanists had tried to establish the genuine world-picture by returning to the Ancients. They had sought help from Ptolemy, and when he had failed to give them the required assistance, they had taken the next step of examining those notions of Greek astronomy which the Ptolemaic system had replaced. Thus, while Copernicus had returned to Aristarchus, Fracastoro had reverted to Eudoxus and Aristotle, and Tycho Brahe had revised the views of Heraclides of Pontus. The great astronomical controversy of the sixteenth century was fought by scholars over ideas which they often believed daring and revolutionary but hardly ever original.

Things changed with Galileo's succession of telescopic discoveries: the rugged surface of the Moon, the existence of hitherto unknown stars, the nature of the Milky Way, the satellites of Jupiter, and the phases of Venus. But the battle was by no means won. A mere looking-glass could not dispel a theory about the structure of the world. The case had to be argued and we shall try to show in this paper how brilliantly this was done by Galileo in his Dialogue on the Two Chief World Systems of 1632.