Dialogue Concerning the Two Chief World Systems

Galileo Galilei’s *Dialogue Concerning the Two Chief World Systems* (original Italian title: *Dialogo sopra i due massimi sistemi del mondo*) was published in 1632. It compares the heliocentric model of Nicolaus Copernicus, in which Earth and the other planets orbit the Sun, with the geocentric model of Ptolemy, where everything in the universe revolves around Earth. Written in Italian, the book was later translated into Latin as *Systema cosmicum* in 1635 by Matthias Bernegger. Galileo dedicated it to his patron, Ferdinando II de’ Medici, Grand Duke of Tuscany, who received the first printed copy on February 22, 1632.

The text is structured as four Socratic dialogues among three characters. Salviati, named after Galileo’s friend Filippo Salviati, argues for the Copernican position and often presents Galileo’s own views, referring to him as “the Academician” because of Galileo’s membership in the Accademia dei Lincei. Sagredo, named after Giovanni Francesco Sagredo, is an intelligent layman who starts out neutral. Simplicio, a devoted follower of Ptolemy and Aristotle, defends the traditional geocentric view. He is supposedly named after the sixth-century commentator Simplicius of Cilicia, but the name was suspected to be a double entendre—the Italian word “semplice” means “simple-minded.” Simplicio was modeled on two contemporary conservative philosophers: Lodovico delle Colombe, a Florentine opponent of Galileo whose followers were sometimes called “the pigeon league,” and Cesare Cremonini, a Paduan colleague who refused to look through a telescope.

While writing the book, Galileo called it his *Dialogue on the Tides*. When the manuscript went to the Inquisition for approval, the title became *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 approving such a title would imply endorsement of his theory that tides were caused by Earth’s motion. Consequently, the formal title page reads simply *Dialogue*, followed by Galileo’s name, academic posts, and a long subtitle. The name by which the work is now known was extracted by the printer from the title page description when permission was granted for a reprint in 1744, which included an approved preface by a Catholic theologian. This history is important for understanding Galileo’s motives. Although the book is formally presented as a balanced consideration of both systems—necessary to get it published—the Copernican side clearly wins the argument.

The discussion ranges widely over contemporary science, not just astronomy. For example, it includes a favorable discussion of William Gilbert’s work on magnetism. It also addresses erroneous arguments against Earth’s motion. A classic thought experiment involves a man below decks on a ship who cannot tell whether the ship is docked or moving smoothly: he observes water dripping from a bottle, fish swimming in a tank, and butterflies flying, and their behavior is the same in both cases. This illustrates the concept of an inertial frame of reference and refutes the objection that if Earth rotated at about 1700 km/h at the equator, a dropped object would fall behind and drift westward.

Galileo’s arguments fall into three main classes. First, rebuttals to objections from traditional philosophers, such as the ship thought experiment. Second, observations incompatible with the Ptolemaic model—for instance, the phases of Venus, which could not occur in that system, and the apparent motions of sunspots, which in the Ptolemaic or Tychonic systems would require an implausibly complicated precession of the Sun’s axis. Third, arguments showing that the old unified theory of the heavens, believed to prove Earth was stationary, was incorrect: examples include the mountains on the Moon, the moons of Jupiter, and the existence of sunspots, none of which were part of ancient astronomy. These arguments have generally held up well over the following four centuries, though how convincing they should have been to an impartial reader in 1632 remains debated. Galileo also attempted a fourth class of argument: a direct physical proof of Earth’s motion through an explanation of tides. As a theory of tides or a proof of Earth’s motion, this part fails.

The *Dialogue* was published in Florence under a formal license from the Inquisition. In 1633, Galileo was found “vehemently suspect of heresy” based on the book, which was then placed on the Index of Forbidden Books. It remained there until 1835, after the theories it discussed had been permitted in print in 1822. In an action not announced at the time, the publication of anything else Galileo had written or might ever write was also banned in Catholic countries.