Galileo's Dialogue Concerning the Two Chief World Systems was published in 1632.
Written in Italian, the book presents four fictionalized days of discussion between Salviati, who argues for Copernican heliocentrism; Sagredo, an intelligent neutral observer; and Simplicio, who defends Aristotelian and Ptolemaic cosmology. Galileo used this conversational format to compare a Sun-centered universe with an Earth-centered one while making the work accessible beyond Latin-reading scholars.
The book drew on Galileo's telescopic discoveries, including the moons of Jupiter, the phases of Venus, lunar mountains, and sunspots. These observations challenged the idea that the heavens were perfect, unchanging spheres. Although the work was formally presented as a balanced examination, its arguments strongly favored Copernicanism.
A common mix-up is 1635: that was the year a Latin translation, Systema cosmicum, appeared. The original Italian edition was published in Florence in 1632, and its controversy helped lead to Galileo's trial by the Roman Inquisition in 1633.