Galileo spent years on a book that set the Copernican model of a Sun-centred universe against the old Ptolemaic system, which put the Earth at the centre. He wrote it in Italian rather than Latin, as a conversation among three characters, so that readers outside the universities could follow it. It came out in Florence under a formal licence from the Inquisition, and the first printed copy went to his patron, Grand Duke Ferdinando II de' Medici. The licence did not protect him for long. In 1633 the book brought Galileo before the Inquisition, and it was put on the Index of Forbidden Books.
The Dialogue takes place over four days of talk among three men. Salviati argues for Copernicus. Sagredo is an educated layman with an open mind. Simplicio defends the Earth-centred view handed down from Aristotle and Ptolemy. They often discuss the findings of their "mutual friend the Academician", who is plainly Galileo himself. Galileo claimed the book was an even-handed comparison of the two systems. Most readers could see which side he was on.
That was his mistake. Church authorities had approved the book, but they now saw it as open support for an idea they had already condemned. In 1633 the Inquisition found Galileo "vehemently suspect of heresy". He was forced to recant and spent the rest of his life under house arrest.
The book went on the Index of Forbidden Books and stayed there for about two centuries, until it was left out of the 1835 edition. The name Galileo gave his geocentrist mattered too. Simplicio nominally honoured an ancient commentator on Aristotle, but in Italian it sounds uncomfortably close to "simpleton".
Key people: Galileo Galilei, Ferdinando II de' Medici