Dialogue Concerning the Two Chief World Systems 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 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).