Dialogue Concerning the Two Chief World Systems

Dialogue Concerning the Two Chief World Systems 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 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".
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. 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.

Overview

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, there is no question that the Copernican side gets the better of the argument.

Structure

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.
Sagredo is an intelligent layman who is initially neutral. He is named after Galileo's friend Giovanni Francesco Sagredo.
Simplicio, a dedicated follower of Ptolemy and Aristotle, presents the traditional views and the arguments against the Copernican position. He is supposedly named after Simplicius of Cilicia, a sixth-century commentator on Aristotle, but it was suspected the name was a double entendre, as the Italian for "simple" is "semplice". Simplicio is modeled on two contemporary conservative philosophers, Lodovico delle Colombe, Galileo's opponent, and Cesare Cremonini, a Paduan colleague who had refused to look through the telescope. Colombe was the leader of a group of Florentine opponents of Galileo's, which some of the latter's friends referred to as "the pigeon league".
Content

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:

Rebuttals to the objections raised by traditional philosophers; for example, the thought experiment on the ship.
Observations that are incompatible with the Ptolemaic model: the phases of Venus, for instance, which simply could not happen, or the apparent motions of sunspots, which could only be explained in the Ptolemaic or Tychonic systems as resulting from an implausibly complicated precession of the Sun's axis of rotation.
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 was part of the old astronomy.

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:

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. The fundamental argument is internally inconsistent and actually leads to the conclusion that tides do not exist. 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.

Omissions

The Dialogue does not treat the Tychonic system, which was becoming the preferred system of many astronomers at the time of publication and which was ultimately proven incorrect. 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 in small circles, while the Sun in turn orbits a stationary Earth; Mars, Jupiter, and Saturn orbit the Sun in much larger circles, which means they also orbit the Earth. The Tychonian system is mathematically equivalent to the Copernican system, except that the Copernican system predicts a stellar parallax, while the Tychonian system predicts none. Stellar parallax was not measurable until the 19th century, and therefore there was at the time no valid disproof of the Tychonic system on empirical grounds, nor any decisive observational evidence for the Copernican system.
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 may be sought in Galileo's theory of the tides, which provided the original title and organizing principle of the Dialogue. 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 apparently provided two centuries later. Without reference to Galileo's tidal theory, there would be no difference between the Copernican and Tychonic systems.
Galileo fails to discuss the possibility of non-circular orbits, although Johannes Kepler had sent him a copy of his 1609 book, Astronomia nova, in which he proposes elliptical orbits—correctly calculating that of Mars. Prince Federico Cesi's letter to Galileo of 1612 treated the two laws of planetary motion presented in the book as common knowledge; Kepler's third law was published in 1619. Four and a half decades after Galileo's death, Isaac Newton published his laws of motion and gravity, from which a heliocentric system with planets in approximately elliptical orbits is deducible.

Summary

"Preface: To the Discerning Reader" refers to the ban on the "Pythagorean opinion that the earth moves" and says that the author "takes the Copernican side with a pure mathematical hypothesis". He introduces the friends Sagredo and Salviati with whom he had had discussions as well as the peripatetic philosopher Simplicio.

Day one

Salviati starts with Aristotle's proof of the completeness and perfection of the world because of its three dimensions. Simplicio points out that three was favoured by the Pythagoreans whereas Salviati cannot understand why three legs are better than two or four. He suggests that the numbers were "trifles which later spread among the vulgar" and that their definitions, such as those of straight lines and right angles, were more useful in establishing the dimensions. Simplicio's response was that Aristotle thought that in physical matters mathematical demonstration was not always needed.
Salviati attacks Aristotle's definition of the heavens as incorruptible and unchanging whilst only the lunar-bound zone shows change. He points to the changes seen in the skies: the new stars of 1572 and 1604 and sunspots, seen through the new telescope. There is a discussion about Aristotle's use of a priori arguments. Salviati suggests that Aristotle uses Aristotle’s personal experience to choose an appropriate argument to prove just as others do and that Aristotle would change his mind in the present circumstances.
Simplicio argues that sunspots could simply be small opaque objects passing in front of the Sun, but Salviati points out that some appear or disappear randomly and those at the edge are flattened, unlike separate bodies. Therefore, "it is better Aristotelian philosophy to say 'Heaven is alterable because my senses tell me' than 'Heaven is unalterable because Aristotle was so persuaded by reasoning.'" He adds "we possess a much better basis for reasoning about celestial things than Aristotle did...Now we, thanks to the telescope, have brought the heavens thirty or forty times closer to us than they were to Aristotle, so that we can discern many things in them that he could not see; among other things these sunspots, which were absolutely invisible to him." Experiments with a mirror are used to show that the Moon's surface must be opaque and not a perfect crystal sphere as Simplicio believes. He refuses to accept that mountains on the Moon cause shadows, or that reflected light from the Earth is responsible for the faint outline in a crescent moon.
Sagredo holds that he considers the Earth noble because of the changes in it whereas Simplicio says that change in the Moon or stars would be useless because they do not benefit man. Salviati points out that days on the Moon are a month long and despite the varied terrain that the telescope has disclosed, it would not sustain life. Humans acquire mathematical truths slowly and hesitantly, whereas God knows the full infinity of them intuitively. And when one looks into the marvelous things men have understood and contrived, then clearly the human mind is one of the most excellent of God's works.