Dialogue Concerning the Two Chief World Systems

A dialogue in Italian, set over four days in Venice, comparing the Ptolemaic system, with the earth at rest at the centre, and the Copernican system, with the earth moving round the sun. Salviati argues the Copernican case, Simplicio defends Aristotle, and Sagredo, an intelligent layman, judges between them. Day One attacks the Aristotelian division between a perfect, unchanging heaven and a corruptible earth, using telescopic evidence such as mountains on the moon. Day Two answers the objections to the earth's daily rotation, including the stone dropped from a tower, with the image of a ship's cabin in which motion cannot be detected from inside. Day Three treats the annual motion, drawing on Mars, Venus and sunspots. Day Four offers the tides as proof of the earth's motion, an argument now known to be wrong. Because the book seemed to defend Copernicanism outright, Galileo was tried by the Inquisition in 1633 and spent his last years under house arrest.

It is a classic of scientific argument written for ordinary readers, and its trial remains the standard case in debates about science, authority and religion.