Galileo Galilei was born in Pisa, Italy in 1564, a place and time in which the understanding of the universe was dictated, for the most part, by the teachings of the Catholic Church. Galileo considered dedicating his life to the Church and joining the priesthood as a young man. However, he instead decided to attend the University of Pisa in order to pursue mathematics. He studied under several mentors in several different Italian universities before finally settling in Florence as the Grand Duke of Tuscany’s chief mathematician and philosopher. Thus began the career of one of the most accomplished scientists in human history.

Galileo published many scientific essays in his time, but arguably his most influential, and certainly his most controversial, was the Dialogue Concerning the Two Chief World Systems published in 1632. The primary focus of this particular essay was a discussion of the merits between the two existing theories of the organization of the universe at the time: the geocentric theory and the heliocentric theory. The former had been the accepted theory for thousands of years, beginning with its explanation and formalization by the Egyptian astronomer, Ptolemy, all the way until the time of Galileo. This theory places the Earth at the center of the universe with the Sun and planets in orbit around it, all surrounded by the stars which lie in the celestial sphere. The theory was endorsed by the Catholic Church due to its alignment with the Bible. However, in the mid-15th century, Dutch astronomer Nicolaus Copernicus published a conflicting theory in his scientific work On the Revolutions of the Celestial Spheres in 1543. The Copernican model hypothesized, and correctly so, that the Sun is actually positioned in the center of the Solar System, while the Earth and other planets lie in orbit around it.

Dialogue Concerning the Two Chief World Systems is organized as a discussion between two astronomers, Salviati who advocates for the Copernican model and Simplicio who argues for the Ptolemaic model, each competing to persuade the neutral Sagredo. The debate takes place over four days, ultimately culminating in Sagredo being convinced by Salviati’s more contemporary theories. The first day consists mostly of Salviati explaining basic principles of motion and various discoveries that Galileo had made with his signature new technology, the telescope (Galilei 67). They also discuss at length the relationship between the light of the Sun, Moon, and Earth and how each reflects onto the other (Galilei 86). On the second day, Salviati refutes many of the popular arguments that were used to reject the heliocentric theory, including the motion of the Earth. Copernicus’ conclusion that the Earth has some sort of tangential motion led to some erroneous assumptions by other astronomers. Why is it that when we throw something straight up in the air, it does not fall behind the spot from which it was thrown? Why can’t we feel the speed of the Earth moving at such a high velocity? The answer to both lies in the difference between acceleration and velocity. It is the same reason one only feels a resistive force in a car while it is speeding up, and not when it is at a constant speed (the example that Sagredo uses involves a ship at rest versus in motion [Galilei 116]). However, the mathematical principles behind velocity and acceleration would not be put forth until 100 years later by Isaac Newton. Sagredo’s (or rather, Galileo’s) conclusions are based purely on observation.

The third day is an argument about the most disputed principle for and against the Copernican system: retrograde motion. Because Mars’ year is longer than the Earth’s, after several months when the Earth passes Mars in its orbit, Mars appears to move backwards in its position in the sky. This is called retrograde motion, and it was explained by Ptolemy with the use of epicycles, which are small orbits around a fixed point on the planet’s larger orbit around the Earth. The involvement of epicycles made the Ptolemaic system very convoluted compared to the much more elegant Copernican one, and this ultimately sways Sagredo on the fourth day (Galilei 623).

The Church reacted very aggressively toward such an affront to the teachings of Catholicism. After the first publication of Galileo’s work in Florence in 1632, all other future publications were banned by Pope Urban VIII, and Galileo was accused of heresy. In 1633, the Dialogue Concerning the Two Chief World Systems was placed in the Vatican’s library of banned books and was not removed until 1835. In fact, the Catholic Church did not publicly accept Galileo’s conclusions until 1992 (Linder 1). Because the Dialogue was so popular in its initial release, it spread across Europe quickly into predominantly Protestant countries, where it was not hindered by the Catholic ban.

Galileo’s work with the telescope, and in particular his observations presented in the Dialogue, were extremely important contributions to the Scientific Revolution in Europe of the 17th and 18th centuries. His conflict with the Church, ultimately ending in his trial and sentencing brought on by the Dialogue Concerning the Two Chief World Systems, became an epitomizing moment in the ever-growing conflict between science and religion since the Renaissance era.

The transcript of the trial of Galileo in 1633 was recorded and maintained in the Vatican archives that remains to this day, and has even more recently been organized into an online database. After being previously admonished for his theories, Galileo Galilei received a summons by the Roman Inquisition in 1632. Despite Galileo’s being ill and in a significantly weakened state, he was forced to travel to Rome from Florence and appear in person before the cardinals that would be presiding over his trial. In his defense, Galileo claimed that his apparent support of the Copernican model was just what his observations and calculations showed to be more likely. He claimed that even with the evidence, both models were still just theories and he offered to write another book that would act as a Ptolemaic response to the Dialogue and as a complement to his first book (Linder 1). The Inquisition was not satisfied, and Galileo was sentenced to serve the remainder of his life in prison. However, Galileo signed a formal recantation in which he stated “…[he] must altogether abandon the false opinion that the Sun is the center of the world and immovable, and that the Earth is not the center of the world, and moves, and that [he] must not hold, defend, or teach in any way whatsoever, verbally or in writing, the said false doctrine…” (De Santillana 1). After this, he was finally allowed to return to his home in Tuscany and serve his sentence in his home without the opportunity to publish any more scientific theories.

These primary sources, the Dialogue Concerning the Two Chief World Systems found in the Simpson Library of the University of Mary Washington and the transcript of the trial of Galileo found online, each tell an important tale in the surge of scientific innovation of the 17th and 18th centuries. European scientists and inventors built upon each other’s work to generate a chain of discoveries, and the Dialogue is an excellent example of this. Galileo, an Italian astronomer, perfected a device created by a Dutch inventor in order to expand on and reinforce the conclusions of a different Dutch astronomer, culminating in a set of physical laws and mathematical principles by a British scientist and mathematician that still govern our universe to this day. The period of improvement in our understanding gave the Western Civilization an advantage over the rest, and the West became undisputedly the most powerful. However, the term “Scientific Revolution” implies resistance to these new ideas, particularly from religious organizations. The transcript from Galileo’s trial and his recantation are evidence of the conflict between scientific progress and religion that, in many ways, still exists to this day.








Copernicus, Nicolaus, and Charles Glenn Wallis. On the Revolutions of the Celestial Spheres; the First Translation into This Language of De Revolutionibus Orbium Caelestium; (from the Text of the Edition Published by the Societas Copernicana at Thorn, 1873). Annapolis: St. John’s tore, 1939. Print.

De Santillana, Giorgio. “Recantation of Galileo Galilei in 1633.” Recantation of Galileo Galilei in 1633. UMKC, n.d. Web. 22 Nov. 2015.

Galilei, Galileo, Stillman Drake, and Albert Einstein. Dialogue concerning the Two Chief World Systems: Ptolemaic and Copernican. Berkeley: U of California, 1967. Print.

Linder, Douglas. “The Trial of Galileo: A Chronology.” The Trial of Galileo: A Chronology. University of Missouri Kansas City, 2002. Web, accessed 15 Nov. 2015. <>.

Machamer, Peter. “Galileo Galilei.” Stanford University. Stanford University, 04 Mar. 2005. Web, accessed 17 Nov. 2015. <>.

Van Helden, Albert. “The Galileo Project.” The Galileo Project. Rice University, 1995. Web, accessed 17 Nov. 2015. <>.


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