A collection of writings defending the heliocentric system against theological and scientific objections
What the Work Is
The Letters on Copernicanism refers to a series of writings composed by Galileo Galilei between 1613 and 1615, during the period when his telescopic discoveries had made him the most prominent advocate of the Copernican system. The two principal texts are an eight-page letter to Benedetto Castelli, written in December 1613, and a substantially expanded forty-page letter to Grand Duchess Christina of Florence, composed two years later. Both were written in Italian and circulated widely in manuscript form, though neither was formally published at the time. The letters represent Galileo’s first sustained attempt to address the religious objections to heliocentrism—the model, revived by Nicolaus Copernicus, in which the Earth and other planets orbit the Sun rather than the reverse.
What the Book Is About
The letters address a controversy that erupted in December 1613, when Grand Duchess Christina confronted Benedetto Castelli, a friend and follower of Galileo, with biblical objections to the motion of the Earth. The incident prompted Galileo to write first to Castelli and later to Christina herself, arguing that heliocentrism was not contrary to biblical texts. His central claim was that the Bible is an authority on faith and morals, not on science—a distinction that would become foundational to the relationship between religion and natural philosophy. The letters defend the Copernican system against both religious and scientific objections, arguing that scriptural passages implying the Earth’s fixity should be interpreted figuratively when they conflict with demonstrated physical truths.
Key Ideas and Themes
The letters develop a sophisticated hermeneutical argument about how to read Scripture in light of scientific knowledge. Galileo contends that the Bible was written for the common understanding of its original audience and speaks in ordinary language about natural phenomena, not as a scientific textbook. When a literal reading of Scripture appears to contradict what can be demonstrated about the natural world, he argues, the scriptural passage should be reinterpreted rather than the scientific finding rejected. This principle—that God is revealed both in Scripture and in nature, and that these two revelations cannot ultimately conflict—forms the theological core of the letters.
Galileo also addresses the scientific objections to heliocentrism that were current in his day. The most formidable of these came from Tycho Brahe, who argued that if the Earth orbited the Sun, the apparent positions of the stars should shift over the course of the year—a phenomenon called annual stellar parallax. Since no such shift had been observed, Brahe concluded that heliocentrism must be false. Copernicus and the ancient Greek astronomer Aristarchus had correctly responded that parallax would be negligible because the stars are so distant, but Brahe countered that stars appear to have measurable angular size. If they were truly as distant as heliocentrism required, he argued, they would have to be far larger than the Sun or even the Earth’s orbit—an absurdity. Galileo’s letters engage with this line of reasoning, though the full development of his response would come later in his astronomical writings.
The letters also reflect Galileo’s conviction that heliocentrism was supported by his telescopic observations of 1609 and the years following. These observations—including the phases of Venus, the four largest moons of Jupiter, and the rugged surface of the Moon—had undermined the Aristotelian cosmology that dominated European universities and had provided powerful evidence for the Copernican model. The letters to Castelli and Christina represent Galileo’s attempt to clear the intellectual ground for these discoveries by removing the theological obstacles that stood in their way.
Structure and Approach
The two letters differ significantly in length and scope. The letter to Castelli, written in the immediate aftermath of the Grand Duchess’s challenge, is a concise eight-page argument that sets out the basic principles of Galileo’s position. The letter to Christina, composed two years later, expands these arguments to forty pages, developing them more fully and addressing a wider range of objections. Both letters circulated in manuscript among Galileo’s correspondents and supporters, spreading his ideas through the informal networks of learned correspondence that characterized intellectual life in early seventeenth-century Europe.
The letters were not published in Galileo’s lifetime, but they circulated widely enough to attract the attention of church authorities. In 1615, Father Niccolò Lorini submitted Galileo’s writings on heliocentrism to the Roman Inquisition, claiming that Galileo and his followers were attempting to reinterpret the Bible—a violation of the Council of Trent that looked dangerously like Protestantism. Lorini specifically cited the letter to Castelli. Galileo traveled to Rome to defend himself and his ideas, but the controversy would culminate in the Inquisition’s 1616 decree declaring heliocentrism “foolish and absurd in philosophy, and formally heretical since it explicitly contradicts in many places the sense of Holy Scripture.”
Significance and Influence
The Letters on Copernicanism occupy a crucial place in the history of the relationship between science and religion. They articulate a principle that would become central to modern scientific thought: that empirical investigation of the natural world and theological interpretation of Scripture operate in distinct spheres and need not conflict. Galileo’s argument that the Bible speaks about faith and morals rather than natural philosophy anticipated later developments in biblical hermeneutics and helped establish the intellectual framework within which scientific inquiry could proceed independently of religious authority.
The letters also mark a turning point in Galileo’s own career. They brought him into direct conflict with the Roman Inquisition and set the stage for the events that would lead to his condemnation in 1633. The controversy they provoked—over the interpretation of Scripture, the authority of the Church in scientific matters, and the proper relationship between faith and reason—would resonate for centuries. The letters remain significant not only as historical documents but as an early and influential statement of the case for the independence of scientific inquiry from theological constraint.
The letters to Castelli and Christina were part of a wider controversy that had already begun to involve the Church hierarchy before the Grand Duchess’s challenge. In 1611, the same year that Jesuit members of the Collegio Romano acknowledged Galileo’s telescopic discoveries, a commission of cardinals began investigating him, inquiring whether he had been involved in the trial of Cesare Cremonini, a colleague from the University of Padua who had been charged with heresy. These inquiries marked the first time Galileo’s name was mentioned by the Roman Inquisition.
The scientific opposition to heliocentrism that the letters addressed was rooted in the work of Tycho Brahe, whose arguments were later formalized in an essay sent to Galileo by Francesco Ingoli at the start of 1616. Ingoli’s essay, which Galileo later stated he believed was instrumental in the subsequent action against Copernicanism, contained eighteen physical and mathematical arguments against the system, borrowing primarily from Brahe’s claims—notably that heliocentrism would require the stars to be much larger than the Sun. The essay also included four theological arguments, but Ingoli suggested Galileo focus on the physical and mathematical ones, and he did not mention Galileo’s biblical ideas. Ingoli may have been commissioned by the Inquisition to write an expert opinion on the controversy, with the essay providing the basis for the Inquisition’s actions.
The Immediate Aftermath
The controversy provoked by the letters culminated in February 1616, when an Inquisitorial commission declared heliocentrism to be “foolish and absurd in philosophy, and formally heretical since it explicitly contradicts in many places the sense of Holy Scripture.” The Inquisition further found that the idea of the Earth’s movement “receives the same judgement in philosophy and … in regard to theological truth, it is at least erroneous in faith.” Pope Paul V instructed Cardinal Bellarmine to deliver this finding to Galileo and to order him to abandon heliocentrism. On 26 February, Galileo was called to Bellarmine’s residence and ordered “to abandon completely … the opinion that the sun stands still at the centre of the world and the Earth moves, and henceforth not to hold, teach, or defend it in any way whatever, either orally or in writing.”
Galileo’s Broader Astronomical Context
The letters were written during a period of intense astronomical activity for Galileo, whose telescopic observations provided the empirical foundation for his defense of heliocentrism. In 1610, he observed Saturn, initially mistaking its rings for planets and thinking it was a three-bodied system; when the rings later oriented edge-on to Earth, he thought two of the bodies had disappeared, and their reappearance in 1616 further confused him. He also observed Neptune in 1612, noting it in his notebooks as one of many unremarkable dim stars without realizing it was a planet, though he did note its motion relative to the stars before losing track of it.
Galileo’s observations of sunspots, made with both naked eye and telescope, raised another difficulty with the unchanging perfection of the heavens posited by Aristotelian celestial physics. An apparent annual variation in their trajectories, observed by Francesco Sizzi and others in 1612–1613, provided a powerful argument against both the Ptolemaic system and the geoheliocentric system of Tycho Brahe. A dispute over priority in the discovery and interpretation of sunspots led to a long and bitter feud with the Jesuit Christoph Scheiner, with Mark Welser acting as intermediary; both were unaware of Johannes Fabricius’s earlier observation and publication on the subject.
The Question of Stellar Sizes
The letters’ engagement with Brahe’s argument about stellar size was part of a larger ongoing debate. In his Starry Messenger, Galileo had reported that stars appeared as mere blazes of light, essentially unaltered by the telescope, unlike planets which appeared as discs. Shortly thereafter, in his Letters on Sunspots, he reported that the telescope revealed the shapes of both stars and planets to be “quite round,” and he continued to report that telescopes showed stars measuring a few seconds of arc in diameter. He devised a method for measuring the apparent size of a star without a telescope, hanging a thin rope in his line of sight and measuring the maximum distance from which it would wholly obscure the star, then calculating the angle subtended from that distance and the rope’s width.
In his later Dialogue, Galileo reported finding the apparent diameter of a first-magnitude star to be no more than 5 arcseconds, and that of a sixth-magnitude star about 5/6 arcseconds. Like most astronomers of his day, he did not recognize that these apparent sizes were spurious, caused by diffraction and atmospheric distortion. However, his values were much smaller than previous estimates, such as those made by Brahe, and enabled him to counter anti-Copernican arguments that such stars would have to be absurdly large for their annual parallaxes to be undetectable. Other astronomers such as Simon Marius, Giovanni Battista Riccioli, and Martinus Hortensius made similar measurements, with Marius and Riccioli concluding the smaller sizes were not small enough to answer Tycho’s argument.
