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 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 (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 .
The book is presented as a series of discussions, over a span of four days, among two philosophers and a layman:
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. Because of this onesided treatment, many cite this is as a classic example of a Straw man argument. What the discussion would have been like if Simplicio had been as smart and well informed as Salviati is a matter of speculation, as no one has attempted to construct a version of the dialogue in which the traditionalists come out ahead.
The dialogue does not treat the Tychonic system which was becoming the preferred system of the Catholic church at the time of publication. The Tychonic system is a motionless Earth system but not a Ptolemaic system; it is a hybrid system of the Copernican and Ptolemaic models. Mercury and Venus orbit the Sun (like the Copernican system) which in turn orbits a stationary Earth; Mars, Jupiter, and Saturn orbit Earth in respectively larger circles. The Tychonian system is mathematically equivalent to the Copernican system, and therefore there was at the time no valid disproof of it on empirical grounds. Galileo never took Tycho's system seriously, as can be seen in his correspondence, regarding it as an inadequate and physically unsatisfactory compromise.
A 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 between 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 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:
By and large, 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:
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]