The Dialogue Concerning the Two Chief World Systems was Galileo's comparison of the Copernican system, in which the Earth and other planets orbit the Sun, with the traditional Ptolemaic system, in which everything in the Universe circles around the Earth. The book was published in Florence in 1632 under a formal license from the 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 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.

While writing the book, Galileo referred to it as his Dialogue on the Tides; and this was its title when the manuscript went to the Inquisition for approval: 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 granting approval to such a title would look like approval of his theory of the tides, which attempted to prove the motion of the Earth physically. As a result, the formal title on the title page is Dialogue, which is followed by Galileo's name and academic posts, followed by a long subtitle. The name by which the work is now known is extracted from deep within the subtitle. This must be kept in mind when discussing Galileo's motives for writing the book.

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

  • Salviati argues for the Copernican position and presents some of Galileo's views directly, calling him the "Academician" in honor of Galileo's membership in the Academia dei Lincei. He is named after Galileo's friend Filipo Salviati (1582 - 1614).

  • Sagredo is an intelligent layman who is initially neutral. He is named after Galileo's friend Giovanfrancesco Sagredo (1571 - 1620).

  • Simplicio is a dedicated follower of Ptolemy and Aristotle, who presents the traditional views and the arguments against the Copernican position. He is modeled after Ludovico delle Colombe (1565 - 1616?) and Cesare Cremonini (1550 - 1631), both of whom were conservative philosophers. The character's name is not "Simpleton", but is taken from the sixth-century philosopher Simplicius, who wrote notable commentaries on Aristotle.

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. Significantly, Galileo fails to mention the Tychonian system which was the preferred system of the Catholic church at the time of publication. The main problem with the Tychonian system is that it is mathematically equivalent to the Copernican system and therefore there was at the time no valid disproof of it on empirical grounds.

The reason for the absence of Tycho's system (in spite of many references to Tycho and his work in the book) may be sought in Galileo's theory of the tides, which provided the original title and organizing principle of the Dialogue. For, while the Copernican and Tychonic systems are equivalent geometrically, they are quite different dynamically. Galileo's tidal theory entailed the actual, physical movement of the Earth; that is, if true, it would have provided the kind of proof that Foucault's pendulum actually provided two centuries later. With reference to Galileo's tidal theory, there would be no difference bewteen the Ptolemaic and Tychonic systems.

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 the William Gilbert's work on magnetism. Other parts are important to the debate, answering erroneous arguments against the Earth's motion. In this category 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 just 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 miles 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:

  • Rebuttals to the objections raised by traditional philosophers; for example, the thought experiment on the ship.

  • Observations that are incompatible with the Ptolemaic model; for instance, the phases of Venus, which simply couldn't happen, or the observed motions of sunspots, for which a Ptolemaic account would be extremely complicated and physically outlandish.

  • Arguments showing that the elegant unified theory of the Heavens that the philosophers held, which was believed to prove that the Earth was stationary, was incorrect; for instance, the mountains of the Moon, the moons of Jupiter, and the very existence of sunspots, none of which could be accommodated by the old astronomy.

By and large, these arguments have held up well in terms of the knowledge of the next 350 years. 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:

  • Direct physical argument for the Earth's motion, by means of an explanation of tides.

As an account of the causation of tides or a proof of the Earth's motion, it is a failure. But Galileo was fond of the argument and devoted the "Fourth Day" of the discussion to it. The degree of its failure is, like nearly anything having to do with Galileo, a matter of controversy. On the one hand, the whole thing has recently been described in print as "cockamamie." On the other hand, Einstein used a rather different description:
It was Galileo's longing for a mechanical proof of the motion of the earth which misled him into formulating a wrong theory of the tides. The fascinating arguments in the last conversation would hardly have been accepted as proof by Galileo, had his temperament not got the better of him. [Emphasis added]

Editions in print