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The Dialogue Concerning the Two Chief World Systems (Italian: Dialogo sopra i due massimi sistemi del mondo) was a 1632 book by Galileo, comparing the Copernican system with the traditional Ptolemaic system.

In the Copernican 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 Roman Inquisition. In 1633, Galileo was convicted of "grave suspicion of heresy" based on the book, which was then placed on the Index of Forbidden Books, from which it was not removed until 1835.

The dialogue depicts three fictional characters: Saviati, the Copernican (and Galileo's stand-in); Simplicio, the Ptolemaic, and; Sagredo, an initially neutral layman.

Dialogue Concerning the Two Chief World Systems in "But It Does Move"[]

As part of his analysis of Galileo in 1633, Cardinal Sigismondo Gioioso suggested that the Dialogue Concerning the Two Chief World Systems was a subconscious act of rebellion against both the Catholic Church and Galileo's own father.

After contemplating this argument, Galileo saw it had some merit.