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The Dialogue Concerning the Two Chief World Systems (Italian: Dialogo sopra i due massimi sistemi del mondo) is a work of natural philosophy by the Italian astronomer and mathematician Galileo Galilei, published in Florence on 22 February 1632. Written in Italian rather than Latin, the book is structured as a conversation over four days among three fictional characters: Salviati, who argues for the Copernican heliocentric model; Simplicio, who defends the Ptolemaic geocentric model; and Sagredo, a neutral interlocutor. The work was intended to present both systems impartially, as required by the imprimatur Galileo obtained from Roman Catholic Church censors, but its handling of the two positions led directly to his trial before the Inquisition.

The Dialogue occupies a significant place in the history of science and of the conflict between empirical inquiry and institutional authority. The book marshals telescopic observations, mechanical arguments, and tidal theory in support of the Copernican system, while the defense of Ptolemy assigned to Simplicio has been characterized by many historians as weak, and the assignment of Urban VIII's favored arguments to that character became central to the Church's case against Galileo. Pope Urban VIII, formerly a supporter of Galileo, reportedly identified his own arguments in Simplicio's mouth, a matter of disputed intent. Following its publication, the Dialogue was condemned, Galileo was tried, and he was compelled to abjure the heliocentric view; the book was placed on the Index Librorum Prohibitorum, where it remained until 1835. For a full account of these events, see Dialogue - History.

The scientific arguments in the Dialogue are of uneven quality by modern assessment. Galileo's tidal argument - which he regarded as decisive proof of Earth's motion - is now considered incorrect. His treatment of stellar parallax and his dismissal of Tycho Brahe's geo-heliocentric model have also drawn scholarly criticism. At the same time, the Dialogue advanced the principle of mechanical relativity (the ship's cabin thought experiment) and contributed substantially to the displacement of Aristotelian physics. The precise relationship between the book's rhetorical strategy and Galileo's personal convictions remains a subject of scholarly discussion.

There is broad consensus in the history of science that the Dialogue was a landmark in the Copernican Revolution and in the development of the scientific method. See History of Science Consensus. The character of Galileo's conflict with the Church - whether primarily theological, political, or personal - is less settled; see Historiography Consensus.

• The Dialogue as scientific turning point - The work is read as a decisive step in the displacement of Aristotelian cosmology and a model of empirical reasoning presented for a broad audience. See Scientific Significance Viewpoint.
• The Dialogue as rhetorical performance - Some scholars argue the book is as much a work of persuasion and literary craft as of natural philosophy, and that its scientific arguments should be read in that context. See Rhetoric Viewpoint.
• The Galileo Affair as science-religion conflict - A common reading holds that the condemnation of the Dialogue exemplifies a structural antagonism between scientific inquiry and religious authority. See Science-Religion Conflict Viewpoint.
• The Galileo Affair as political and personal dispute - An alternative reading holds that the condemnation was driven primarily by Galileo's alienation of Urban VIII and by curial politics, not by theological principle. See Political Dispute Viewpoint.
• The Church's position reappraised - Some scholars and Catholic commentators argue that the Church's concern about the premature assertion of heliocentrism as physical fact - rather than hypothesis - was epistemically defensible given the state of evidence in 1632. See Church Position Viewpoint.

• Dialogue - History
• Dialogue - Debate
• History of Science Consensus
• Historiography Consensus
• Galileo Galilei
• Copernican Revolution
• The Galileo Affair
• Index Librorum Prohibitorum
• Ptolemaic Model
• Tycho Brahe

1. Galileo Galilei, Dialogo sopra i due massimi sistemi del mondo (Florence: Landini, 1632). Stillman Drake, Dialogue Concerning the Two Chief World Systems (Berkeley: University of California Press, 1953).
2. Maurice Finocchiaro, The Galileo Affair: A Documentary History (Berkeley: University of California Press, 1989), pp. 1-38, on the imprimatur and the trial proceedings.
3. Pietro Redondi, Galileo Heretic (Princeton: Princeton University Press, 1987), for the political and personal dimensions of the Affair.
4. Stillman Drake, Galileo at Work (Chicago: University of Chicago Press, 1978), pp. 292-299, on the tidal argument and its reception.
5. William Shea and Mariano Artigas, Galileo in Rome (Oxford: Oxford University Press, 2003), on Urban VIII and the question of Simplicio.
6. The Dialogue was removed from the Index in 1835; see J.L. Heilbron, Galileo (Oxford: Oxford University Press, 2010), p. 361.