Galileo's Dialogue Concerning the Two Chief World Systems was written in Italian.
Published in Florence in 1632, the book's original title was Dialogo sopra i due massimi sistemi del mondo. Galileo used Italian rather than Latin to reach a broader educated readership beyond university scholars and specialists. The conversational format also made difficult astronomical arguments more accessible.
The book presents four days of discussion among Salviati, Sagredo, and Simplicio. Their conversation compares the Copernican heliocentric system with the Ptolemaic geocentric system, although Galileo's presentation strongly favors the Copernican side. The work also discusses motion, astronomy, observation, and Galileo's theory of the tides.
Italian did not mean the book escaped controversy. After its publication, Galileo was tried in 1633 and found vehemently suspect of heresy. The Dialogue was placed on the Index of Forbidden Books. A Latin translation, titled Systema cosmicum, appeared in 1635, which may cause confusion about the original language. Latin was the later translation, not the language in which Galileo first wrote the Dialogue.