Galileo's Dialogue Concerning the Two Chief World Systems

Locale Rome and Florence (now in Italy)

Date 1632

Galileo published his great work Dialogue Concerning the Two Chief World Systems, leading to widespread acceptance of the heliocentric system of Copernicus while precipitating condemnation of Copernican theory by the Roman Catholic Church.

Key Figures

• Tycho Brahe (1546-1601), Danish astronomer who modified the geocentric system of planets
• Nicolaus Copernicus (1473-1543), Polish astronomer who developed the heliocentric system
• Galileo (1564-1642), Italian mathematician who began the swing toward accepting Copernicus’ theories
• Johannes Kepler (1571-1630), German astronomer who formulated new laws of the planets
• Sir Isaac Newton (1642-1727), English physicist who established scientific basis for the heliocentric system
• Ptolemy (c. 100-c. 178 c.e.), ancient Greek astronomer who developed a geocentric system

Summary of Event

Although Galileo’s Dialogo sopra i due massimi sistemi del mondo, tolemaico e copernicano (1632; Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican, 1661) was condemned by the Roman Catholic Church and placed on the Index of Forbidden Books, it fueled the Scientific Revolution and led to increasing acceptance of the heliocentric (sun-centered) system of Nicolaus Copernicus, culminating in the Newtonian synthesis and the eighteenth century Enlightenment. The classic Greek system of the planets was completed by Ptolemy in about the year 150. This Ptolemaic system was geocentric (Earth-centered) and could accurately account for the positions of the planets by a complicated combination of circles known as epicycles. It was further developed by Arabic scientists and was incorporated into Catholic theology by Thomas Aquinas in the thirteenth century.

The heliocentric system was developed by Copernicus and published in 1543 as De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres, 1939; better known as De revolutionibus ), in an attempt to simplify astronomy; but it still required complicated combinations of circles to match the accuracy of Ptolemy, and it provided no explanation of how Earth could rotate on its axis and revolve around the sun. The annual revolution of Earth about the sun implied that the positions of the stars should shift as Earth moves, but no such shifting of the stars was observed. Since motion of Earth also seemed to contradict the philosophical and theological ideas of the time, only a few astronomers gave serious consideration to the Copernican system.

The last great astronomer before the introduction of the telescope was Tycho Brahe . By building very large instruments for measuring celestial positions, he increased the accuracy of astronomy about ten times that of the Greeks. However, he was still unable to measure the annual shifting of the stars required by the Copernican system. He recognized the mathematical advantages of heliocentric astronomy but could not accept the idea of a moving Earth. Thus, he proposed a compromise system in which the sun revolved around a stationary Earth, but all the other planets revolved in circular orbits around the sun. The Tychonic system gained a significant following among astronomers, so that by the end of the sixteenth century there were three competing systems of the world: the Ptolemaic (geocentric) system, the Copernican (heliocentric) system, and the Tychonic (combined geo-heliocentric) system.

The two great champions of the Copernican system at the beginning of the seventeenth century were Galileo and Johannes Kepler, even though neither was able to detect direct evidence of Earth’s motion. Kepler became Tycho’s assistant in 1600. A year later, Tycho died and Kepler began to develop the Copernican system using the accurate data Tycho had accumulated. In 1609, he published his analysis of the orbit of Mars in his Astronomia nova (New Astronomy , 1992), which established that the planets move in elliptical orbits. This simplified the Copernican system, since only a single ellipse was required to account for the motion of each planet rather than a complicated combination of circles.

Although Galileo corresponded with Kepler and shared many of his heliocentric views, he never endorsed elliptical orbits and retained a strong emphasis on the importance of circles in astronomy. Galileo was born at Pisa in northern Italy in the same year that Michelangelo died. His father, Vincenzo Galilei, was a musician whose book Dialogo della musica antica, et della moderna(1581; Dialogue of Ancient and Modern Music , 2003) was used by Kepler in his attempt to apply the principles of harmony to astronomy. In 1581, Galileo went to the University of Pisa in the Republic of Venice to study medicine, but after four years he had to drop out for lack of funds. After further private study of mathematics, he was appointed as a professor of mathematics at the University of Pisa in 1589. Conflict with Aristotelian colleagues led him to resign after three years and take an appointment at the University of Padua, where he concentrated on the study of motion.

Galileo interrupted his work on motion in July of 1609 when word reached Venice about a magnifying tube made with a combination of lenses by a Dutch lens grinder, Hans Lippershey . After hearing these reports, Galileo ground lenses and tried several arrangements before finding a combination that gave a magnifying power of about thirty. When he presented one of his telescopes to the Venetian senate, it renewed his professorship for life and doubled his salary. He loaned another telescope to Kepler, who worked out the geometry of image formation by two lenses. Galileo recognized that the primary value of the telescope was in astronomy, opening up new vistas of space. Few of his contemporaries realized how valuable this would be for astronomy, and some even opposed its use as deceitful. In 1610, Galileo published his initial discoveries, including the four largest moons of Jupiter, in a small booklet called Sidereus nuncius (The Sidereal Messenger , 1880; also known as The Starry Messenger). This success led to his appointment as chief mathematician of the University of Pisa and recognition in Rome by Pope Paul V.

Galileo’s successes with the telescope led him into a bolder polemic for the Copernican system, bordering on propaganda. Although none of his observations provided conclusive evidence for a moving Earth, taken together they began to turn the tide toward its wider acceptance. Resistance to Galileo’s ideas began to build. In 1616, he was warned by the Holy Office in Rome that the idea of the moving Earth was expressly condemned. After the election of Pope Urban VIII in 1623, Galileo went to Rome, where he had several audiences with the new pope and received permission to write about the motion of Earth as a scientific hypothesis. During the next six years, he worked on his masterpiece, the Dialogue Concerning the Two Chief World Systems. Supposedly an evenhanded comparison of the Copernican and Ptolemaic systems, it ended up as a highly persuasive book in favor of the heliocentric system while largely ignoring the Tychonic system. To make matters worse for him, Galileo wrote it in vernacular Italian, accessible to a wide audience, instead of the usual scholarly Latin.

Using the dialectical form of Plato, Galileo developed his arguments through the voices of three persons: the traditional Simplicio, the Copernican Salviati, and the open-minded Sagredo. On the first of four days (chapters), the dialogue compares Simplicio’s Aristotelian (geocentric) arguments on celestial perfection with telescopic evidence of such apparent imperfections as sunspots, mountains on the moon, and bulges (rings) on Saturn. The phases of Venus are given as evidence against the Ptolemaic system and for the Copernican system without mention of the rival Tychonic system, which could also account for them. The second and third days include arguments on the rotation and revolution of Earth, indicating his failure to detect the annual shift in the positions of the stars as evidence for large stellar distances. The moons of Jupiter are offered as an example of a nongeocentric system. On the fourth day, Salviati presents Galileo’s erroneous theory of the tides as conclusive evidence for the Copernican system.

Galileo submitted his manuscript to the chief censor at Rome in 1630. After several delays and minor revisions, permission was finally granted in both Rome and in Florence, where it was published in 1632. The closing paragraph of the dialogue included a statement suggested by Pope Urban that the Copernican theory was “neither true nor conclusive” and that no one should “limit the divine power and wisdom to one particular fancy of his own.” Unfortunately, Galileo put these words in the mouth of Simplicio, leading to the accusation that all of his views represented those of the pope. Sale of the book was stopped, and Galileo was summoned to Rome.

Significance

In the winter of 1633, the gravely ill Galileo was carried by litter to Rome. After trial by the Inquisition, in which he vigorously denied that he had intended to teach the truth of the heliocentric system, he was judged guilty and the Dialogue Concerning the Two Chief World Systems was totally forbidden. He was then sentenced to house arrest at his country estate near Florence, with no visitors allowed except by special permission. Although he lost his eyesight in the last decade of his life, he returned to the study of matter and motion, which was eventually developed by Isaac Newton to establish the physical basis for the Copernican system.

Bibliography

Alioto, Anthony. A History of Western Science. Englewood Cliffs, N.J.: Prentice Hall, 1993. Chapter 15 gives a succinct historical account of Galileo’s work.

Boorstin, Daniel J. The Discoverers. New York: Random House, 1983. In chapters 40 and 41, Boorstin provides a brief but authoritative historical account of Galileo’s scientific work.

Drake, Stillman. Galileo at Work: His Scientific Biography. Chicago: University of Chicago Press, 1978. An excellent biography of Galileo written by a noted scholar.

Galileo Galilei. Dialogue Concerning the Two Chief World Systems. Translated by Stillman Drake. Berkeley: University of California Press, 1967. The standard English translation of Galileo’s chief work.

Koyré, Alexandre. Galileo Studies. Translated by John Mepham. Atlantic Highlands, N.J.: Humanities Press, 1978. Ably translated into English, these scholarly studies of Galileo provide intriguing insights into Galileo’s work.

Machamer, Peter, ed. The Cambridge Companion to Galileo. New York: Cambridge University Press, 1998. A collection of essays exploring the many facets of Galileo’s work. Chapter 7 discusses “Galileo’s Discoveries with the Telescope and Their Evidence for the Copernican Theory.”

Rowland, Wade. Galileo’s Mistake: A New Look at the Epic Confrontation Between Galileo and the Church. New York: Arcade, 2003. Rowland argues that Galileo and Church officials disagreed about something more significant than whether Earth revolved around the sun: They were really in dispute about the nature of truth and how people acquire the truth.

Sharratt, Michael. Galileo: Decisive Innovator. Oxford, England: Blackwell, 1994. Concise biography that closely examines Dialogue Concerning the Two Chief World Systems.