Dialogue Concerning the Two Chief World Systems

Lore & Background

While writing the book, Galileo referred to it as his *Dialogue on the Tides*, and when the manuscript went to the Inquisition for approval, the title was *Dialogue on the Ebb and Flow of the Sea*. He was ordered to remove all mention of tides from the title and to change the preface. The formal title on the title page became *Dialogue*, followed by Galileo's name and a long subtitle.

Reader's Guide

The *Dialogue* is significant as a central text in the conflict between science and religious authority. Although presented formally as a consideration of both systems, the Copernican side clearly gets the better of the argument. The work contains a classic exposition of the inertial frame of reference through a thought experiment involving a man below decks on a ship. Galileo's arguments include rebuttals to traditional objections, observations incompatible with the Ptolemaic model (such as the phases of Venus and sunspots), and a failed direct physical argument for Earth's motion based on tides. The book omits discussion of the Tychonic system and Kepler's elliptical orbits, which were known at the time.

Did You Know?

• The book was originally titled *Dialogue on the Tides* before the Inquisition ordered the removal of tides from the title.
• The character Simplicio is named after Simplicius of Cilicia, but the name was suspected to be a double entendre meaning 'simple-minded'.
• Galileo's tidal theory, presented on the 'Fourth Day,' is considered a failure and was described by Einstein as resulting from Galileo's temperament getting the better of him.