Dialogue Concerning The Two Chief World Systems

The Dialogue Concerning the Two Chief World Systems (Dialogo sopra i due massimi sistemi del mondo) was a 1632 Italian language book by Galileo Galilei comparing the Copernican system with the traditional Ptolemaic system. It was translated to Latin as Systema cosmicum (English: cosmic system) in 1635 by Matthias Bernegger. The book, which was dedicated to Galileo's patron, Ferdinando II de' Medici, Grand Duke of Tuscany and delivered to him on February 22, 1632, was a bestseller.

In the Copernican system the Earth and other planets orbit the Sun, while in the Ptolemaic system everything in the Universe circles around the Earth. The Dialogue was published in Florence under a formal license from the Inquisition. In 1633, Galileo was convicted of "grave suspicion of heresy" based on the book, which was then placed on the Index of Forbidden Books, from which it was not removed until 1835 (after the theories it discussed had been permitted in print in 1822.) In an action that was not announced at the time, the publication of anything else he had written or ever might write was also banned.