Dialogue Concerning the Two Chief World Systems

''Dialogue Concerning the Two Chief World Systems'' (''Dialogo sopra i due massimi sistemi del mondo'') is a 1632 book by Galileo Galilei comparing Nicolaus Copernicus's heliocentric system model with Ptolemy's geocentric model. Written in Italian, it was translated into Latin as ''Systema cosmicum'' (''Cosmic System'') in 1635 by Matthias Bernegger. The book was dedicated to Galileo's patron, Ferdinando II de' Medici, Grand Duke of Tuscany, who received the first printed copy on February 22, 1632. It consists of four Socratic dialogues between the Copernican Salviati, the educated layman Sagredo and the geocentrist Simplicio. They discuss the findings of their "mutual friend the Academician" (Galileo).

In the heliocentric 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 found to be "vehemently suspect 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 in Catholic countries. Provided by Wikipedia