Dialogue Concerning the Two Chief World Systems

Dialogue Concerning the Two Chief World Systems
Frontispiece and title page of the Dialogue

The Dialogue Concerning the Two Chief World Systems (Dialogo sopra i due massimi sistemi del mondo) was a 1632 Italian language book by Galileo Galilei comparing the Copernican system with the traditional Ptolemaic system. It was translated to Latin as Systema cosmicum[1] in 1635 by Matthias Bernegger.[2] The book, which was dedicated to Galileo's patron, Ferdinando II de' Medici, Grand Duke of Tuscany and delivered to him on February 22, 1632,[3] was a bestseller.[4]

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.)[5] 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.[6]



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 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 Giovanni Francesco Sagredo (1571–1620).
  • 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" (as in "simple minded") is "semplice".[7] Simplicio is modeled on two contemporary conservative philosophers, Ludovico delle Colombe (1565-1616?), Galileo's fiercest detractor, and Cesare Cremonini (1550–1631), a Paduan colleague who had refused to look through the telescope.[8] 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".[9]

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 one-sided treatment, many[who?] cite this as a classic example of a Straw man argument.

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 (as in the Copernican system) 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 no stellar parallax. 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. The Copernican system is deducible from Newton's laws of motion and gravity, but these were not published until 1687.

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.

A classic argument against earth motion is the lack of speed sensations of the earth surface, though it moves at 1600 km/h. 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 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: the phases of Venus, for instance, which simply couldn't 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.[10]
  • 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 (though these are of somewhat doubtful relevance, as none of these phenomena dealt directly with the question of the motion of the earth or sun).

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. (In fact, 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."[11] 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][12][13]

Editions in print


  1. ^ Maurice A. Finocchiaro: Retrying Galileo, 1633-1992, University of California Press, 2007 ISBN 0-520-25387-6, 9780520253872
  2. ^ Journal for the history of astronomy, 2005
  3. ^ Gindikin, Semen Grigorʹevich (1988). Tales of physicists and mathematicians. Birkhäuser. p. 62. ISBN 9780817633172. http://books.google.com/books?id=Vhx--jfFN8IC&pg=PA62. Retrieved 22 February 2011. 
  4. ^ http://v1.theglobeandmail.com/servlet/story/RTGAM.20080829.w50booksdialogue/BNStory/DAVA+SOBEL
  5. ^ The Trial of Galileo: A Chronology
  6. ^ See Galileo affair for more details, including sources.
  7. ^ Arthur Koestler, The Sleepwalkers: A History of Man's Changing Vision of the Universe (1959), Penguin Books, 1986 edition: ISBN 0-14-055212-X, 978014055212X 1990 reprint: ISBN 0-14-019246-8, 9780140192469 [1]
  8. ^ Stillman Drake: Galileo at Work: His Scientific Biography, Courier Dover Publications, 2003, ISBN 0-486-49542-6, page 355 : Cremonini and delle Colombe
  9. ^ "La legha del pippione". "Pippione" is a pun on Colombe's surname—which is the plural of the Italian word for dove. Galileo's friends, the painter, Lodovico Cardi da Cigoli (Italian), his former student, Benedetto Castelli, and a couple of his other correspondents often referred to Colombe as "il Colombo", which means "the Pigeon". Galileo himself used this term a couple of times in a letter to Cigoli of October, 1611 (Edizione Nazionale 11:214). The more derisive nickname, "il Pippione", sometimes used by Cigoli (Edizione Nazionale 11:176, 11:229, 11:476,11:502), is a now archaic Italian word with a triple entendre. Besides meaning "young pigeon", it is also a jocular term for a testicle, and a Tuscan dialect word for a fool.
  10. ^ Drake, (1970, pp.191–196), Linton (2004, pp.211–12), Sharratt (1994, p.166). This is not true, however, for geocentric systems—such as that proposed by Longomontanus—in which the Earth rotated. In such systems the apparent motion of sunspots could be accounted for just as easily as in Copernicus's.
  11. ^ Timothy Moy (September 2001). "Science, Religion, and the Galileo Affair". Skeptical Inquirer. 
  12. ^ "Foreword; By Albert Einstein; Authorized Translation by Sonja Bargmann". http://milestones.buffalolib.org/books/books/dialogo/impact.htm. (passages omitted)
  13. ^ Paul Mainwood (2003-08-09). "Thought Experiments in Galileo and Newton’s Mathematical Philosophy". 7th Annual Oxford Philosophy Graduate Conference. 7th Annual Oxford Philosophy Graduate Conference. http://www.philosophy.ox.ac.uk/gradconf/2003gradconf/Selected_papers(PROGRAM2)/thought_expirements.pdf. , quoting page xvii of Einstein's foreword in G. Galilei (1632/1953). Dialogue Concerning the Two Chief World Systems. Translated by Stillman Drake. Berkeley and Los Angeles, CA: The University of California Press. 


  • Drake, Stillman (1970). Galileo Studies. Ann Arbor: The University of Michigan Press. ISBN 0-472-08283-3. 
  • Linton, Christopher M. (2004). From Eudoxus to Einstein—A History of Mathematical Astronomy. Cambridge: Cambridge University Press. ISBN 978-0-521-82750-8. 
  • Sharratt, Michael (1994). Galileo: Decisive Innovator. Cambridge: Cambridge University Press. ISBN 0-521-56671-1. 

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