FIFTH TRIP • 3 MAY–26 JUNE 1630
Galileo had returned to Florence in the summer of 1624 with the conviction that he was now free to write on the motion of the Earth as long as he avoided stating that it was physically true. Within a couple of months he had finished his Reply to Ingoli and could now return to his Dialogue. To emphasize what he considered his decisive argument for the motion of the Earth, Galileo planned to call the work The Discourse on the Tides. It is only when the pope objected that he changed it to The Dialogue on the Two Chief World Systems. The work is the record of a discussion spread over four days, like a play in four acts, among three friends who meet in a palace in Venice.
Galileo, who was now over 60, welcomed the opportunity to bring back to life two of his best friends. The first is Filippo Salviati, his host at the Villa delle Selve near Florence, where he had been frequently a visitor. He is Galileo's spokesman and he makes a brilliant case for Copernicanism. The second is the Venetian patrician Giovanfrancesco Sagredo, in whose palace the meeting is held. He is presented as open-minded and