Dialogue Concerning the Two Chief World Systems

Dialogue Concerning the Two Chief World Systems (Dialogo sopra i due massimi sistemi del mondo) 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 (Cosmic System) 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).

In the heliocentric 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 Inquisition. In 1633, Galileo was found to be "vehemently suspect of heresy" based on the book, which was then placed on the Index of Forbidden Books, from which it was not removed until 1835 (after the theories it discussed had been permitted in print in 1822). In an action that was not announced at the time, the publication of anything else he had written or ever might write was also banned in Catholic countries.

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 because not granting approval to such a title would look like approval of his theory of the tides using the motion of the Earth as proof. As a result, the formal title on the title page is Dialogue, which is followed by Galileo's name, academic posts, and followed by a long subtitle. The name by which the work is now known was extracted by the printer from the description on the title page when permission was given to reprint it with an approved preface by a Catholic theologian in 1744. This must be kept in mind when discussing Galileo's motives for writing the book. Although the book is presented formally as a consideration of both systems (as it needed to be in order to be published at all), there is no question that the Copernican side gets the better of the argument.

The book is presented as a series of discussions, over a span of four days, among two philosophers and a layman:

The discussion is not narrowly limited to astronomical topics, but ranges over much of contemporary science. Some of this is to show what Galileo considered good science, such as the discussion of William Gilbert's work on magnetism. Other parts are important to the debate, answering erroneous arguments against the Earth's motion.

A classic argument against Earth motion is the lack of speed sensations of the Earth surface, though it moves, by the Earth's rotation, at about 1700 km/h at the equator. In this category there is a thought experiment in which a man is below decks on a ship and cannot tell whether the ship is docked or is moving smoothly through the water: he observes water dripping from a bottle, fish swimming in a tank, butterflies flying, and so on; and their behavior is the same whether the ship is moving or not. This is a classic exposition of the inertial frame of reference and refutes the objection that if we were moving hundreds of kilometres an hour as the Earth rotated, anything that one dropped would rapidly fall behind and drift to the west.

The bulk of Galileo's arguments may be divided into three classes:

Generally, these arguments have held up well in terms of the knowledge of the next four centuries. Just how convincing they ought to have been to an impartial reader in 1632 remains a contentious issue. Galileo attempted a fourth class of argument: