The Dialogue Concerning the Two Chief World Systems (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 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[1]). 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 [2].

Overview

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 Accademia dei Lincei. He is named after Galileo's friend Filippo 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[3] on Cesare Cremonini (1550-1631, his Paduan colleague, who refused to look into the telescope) and Ludovico delle Colombe (1565-1616?, his fiercest detractor, whom some of his friends nicknamed "the Pigeon"[4]) , both of whom were conservative philosophers. The character's name is that of sixth-century philosopher Simplicius, but the Inquisition would object to its resemblance to "simpleton."

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 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:

* 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 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:

* 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."[5] 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][6][7]

Editions in print

* Dialogue Concerning the Two Chief World Systems", translated by Stillman Drake, University of California Press, 1953 (revised 1967). Also Modern Library paperback.

* Galileo on the World Systems, translated and abridged by Maurice A. Finocchiaro, University of California Press, 1997.

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