The Essential Galileo

by Galilei, Galileo, Finocchiaro, Maurice A.

  The events of 1616 marked a turning point in Galileo’s career. He was no longer free to research the earth’s motion, since he was not supposed to hold or defend it but was to limit himself to discussing it hypothetically or instrumentalistically. He might have taken this opportunity to abandon the Copernican research program and go back to the earlier studies of falling bodies, which he had set aside in 1609. But apparently he felt the Copernican possibility was too important to be set aside completely. At any rate, the import of the new restrictions was not clear; perhaps he could do some useful work while operating under them. This turned out to be the case on the occasion of the controversy over comets that led to The Assayer.7

  [§0.8] In 1618 three comets appeared in succession, the third being especially bright and long-lasting. As usual, the appearance of such phenomena produced considerable discussion. Sometime between mid-December 1618 and mid-January 1619, they were the subject of a lecture at the Jesuit Roman College. The lecture was published the following month with the title Astronomical Disputation on the Three Comets of the Year 1618. The author was anonymous, but the pamphlet indicated that he was a professor at the Roman College. He was soon identified as Orazio Grassi (1590–1654).

  Due to ill health, Galileo was unable to make any observations of those comets. However, many people were soliciting his opinion. Thus, he decided to collaborate with a friend and disciple named Mario Guiducci (1585–1646) in writing a short work on the subject entitled Discourse on the Comets. This book was published in Florence in June 1619, but only under Guiducci’s name. Since the Discourse was critical of the views advanced in the Astronomical Disputation, Grassi immediately wrote a lengthy reply, publishing it under the pseudonym of Lotario Sarsi, who was allegedly one of his students. Grassi’s reply appeared in Perugia in October of the same year with the title Astronomical and Philosophical Balance, to convey the idea that Galileo’s and Guiducci’s ideas were being carefully weighed.

  By then the discussion had become so heated that Galileo was being urged to publish under his own name a reply to Grassi’s Balance. This pressure was coming especially from the other members of the Lincean Academy, who had come to dislike not only Grassi’s view of comets but also the Jesuits’ claims of leadership in mathematics, astronomy, and natural philosophy. Galileo worked on his reply for about two years, and then it took another two years for the manuscript to be revised, to be issued the imprimatur, and to be printed. Entitled The Assayer, the book was published in Rome in October 1623, under the sponsorship of the Lincean Academy. Moreover, it so happened that Cardinal Maffeo Barberini, who was an admirer of Galileo and a friend of the Lincean Academy, was elected Pope Urban VIII in the summer of 1623; so the new book was dedicated to the new pope, who appreciated the gesture very much.

  The Assayer is written in the form of a letter to Virginio Cesarini (1595–1624), chamberlain to Pope Urban VIII, member of the Lincean Academy, poet, and friend of Galileo’s. Additionally, it is structured as a series of long quotations from the Balance, each followed by a lengthy critical analysis. The controversy with Grassi did not end then, for in 1626 the latter published in Paris an even more voluminous work entitled Comparison of the Weights of the Assayer and the Balance. However, Galileo felt no need for a further reply.

  There were several astronomical and philosophical issues underlying this controversy. One was whether comets were located in the earth’s atmosphere (as Aristotle had claimed) or in the heavens (as Tycho Brahe had argued based on his study of the comet of 1577). A related question was whether or not the heavenly location of comets implied the existence of heavenly changes and so further undermined the earth-heaven dichotomy. Another issue was whether the precise trajectory followed by the third comet of 1618 could be explained in a geostatic geocentric theory, or whether the explanation had to include the Copernican hypothesis that the earth possesses an annual heliocentric motion. There was also the question whether the Tychonic system of the world was correct; this was the arrangement according to which the planets do revolve around the sun, but the sun (together with the planetary system) revolves both diurnally and annually around the motionless central earth. And there were more general issues: whether natural science should try to reduce secondary qualities, e.g., colors and sounds, to primary qualities, e.g., position and motion (as the corpuscular or atomistic worldview claimed); and what is the relative role of authority and independentmindedness in scientific inquiry. Galileo and Grassi disagreed on almost all these questions. Even when they happened to agree (e.g., on the heavenly location of comets), they disagreed about the manner of arriving at the conclusion.

  Galileo’s The Assayer is thus not only an explicit discussion of comets and scientific method and an explicit critique of the Tychonic system, but also an implicit defense of the banned Copernican system.

  [§0.9] The election of Barberini as Pope Urban VIII, and his enthusiasm for The Assayer, encouraged Galileo to pursue the Copernican research program. So Galileo went to work to write the book on the system of the world which he had conceived earlier and to adapt its form to the new restrictions.

  Thus, he wrote the book in the form of a dialogue among three characters who discuss all the cosmological, astronomical, physical, and epistemological arguments on both sides of the questions; but no biblical or theological arguments are critically examined. This Dialogue was published in 1632 in Florence, and its key thesis is best stated as follows: the arguments and evidence in favor of the geokinetic theory are much stronger that those in favor of the geostatic view, and in that sense the earth’s motion is much more probable than geostaticism. When so stated, the thesis is successfully established. In the process, Galileo managed to incorporate into the discussion the new telescopic discoveries, his conclusions about the physics of moving bodies, a geokinetic explanation of the tides, and various methodological reflections. From the viewpoint of the ecclesiastic restrictions, Galileo must have felt that the book did not “hold” the theory of the earth’s motion because it was not claiming that the geokinetic arguments were conclusive; that it was not “defending” the geokinetic theory because it was merely a critical examination of the arguments on both sides; and that it was a hypothetical discussion because the earth’s motion was being presented as a hypothesis that happened to be better than the alternative.

  However, Galileo’s enemies raised all kinds of charges against the book. One was that the book did not treat the earth’s motion as a hypothesis, because it did not regard it merely as a convenient instrument of calculation and prediction, but also as a real possibility; that is, the proposition that the earth moves was regarded as a description of physical reality that could be true or false, even if one could not yet be sure as to which was the case. Another charge was that the book defended the earth’s motion because the arguments against it were criticized but the arguments for it were favorably presented. Both of these points involved an alleged violation of the decree of the Index and of Bellarmine’s warning. But there was a third charge: that the book violated a special injunction which Galileo had been given in 1616 and which prohibited him from discussing the earth’s motion in any way whatsoever; a document reporting on this special injunction had been found in the file of the earlier Inquisition proceedings of 1615–16. Thus Galileo was summoned to Rome to stand trial.

  After various delays, Galileo finally arrived in Rome in February 1633, although the proceedings did not begin until April. At the first hearing, Galileo was asked about the Dialogue and the events of 1616. He admitted receiving from Bellarmine the warning that the earth’s motion could not be held or defended, but only discussed hypothetically. He denied receiving a special injunction not to discuss the topic “in any way whatever,” and in his defense he introduced a certificate he had obtained from Bellarmine in 1616, which only mentioned the prohibition to hold or defend. Galileo also claimed that the book did not defend the earth’s motion, but rather suggested that the favorable arguments were inconclusive, and so did
not violate Bellarmine’s warning.

  The special injunction must have surprised Galileo as much as Bel-larmine’s certificate did the inquisitors. In fact, it took three weeks before they decided on the next step. The inquisitors opted for some out-of-court plea bargaining: they would not press the most serious but most questionable charge (violation of the special injunction), but Galileo would have to plead guilty to a lesser but more provable charge (transgression of the warning not to defend Copernicanism). He requested a few days to devise a dignified way of pleading guilty to the lesser charge. Thus, at later hearings, he stated that the first deposition had prompted him to reread his book; he was surprised to find that it gave readers the impression that the author was defending the earth’s motion, even though this had not been his intention. He attributed his error to wanting to appear clever by making the weaker side look stronger. He was sorry and ready to make amends.

  Although the authorities accepted this confession of guilt, they were unsure about Galileo’s denial of a malicious intention. Thus, in accordance with standard practice, they decided to subject him to an interrogation under the verbal threat of torture. This occurred on June 21, and the transcript indicates that Galileo was threatened with torture but was not actually tortured, and that he was willing to be tortured rather than admit his transgression to have been intentional (thus vindicating the purity of his intention).

  The trial ended on 22 June 1633 with a harsher sentence than Galileo had been led to believe he would receive. The verdict found him guilty of a category of heresy intermediate between the most and the least serious, called “vehement suspicion of heresy”; the objectionable beliefs were the cosmological thesis that the earth moves and the methodological principle that the Bible is not a scientific authority. Thus he was forced to recite a humiliating “abjuration.” And the Dialogue was banned.

  The sentence also states that he was to be held in prison indefinitely. However, this particular penalty was immediately commuted to house arrest. Accordingly, for about one week he was confined to the Villa Medici, a sumptuous palace in Rome belonging to the Tuscan grand duke. Then for about five months he was sent to the residence of Siena’s archbishop, who was a good friend of Galileo’s. Finally, in December 1633 he was allowed to live in seclusion at his own villa in Arcetri, near Florence.

  One of the ironic results of this condemnation was that, to keep his sanity, Galileo went back to his earlier research on motion, organized his notes, and five years later published his most important contribution to physics, the Two New Sciences (1638). Without the tragedy of the trial, he might have never done it. This book was written in the form of a dialogue among the same characters (Salviati, Sagredo, and Simplicio) who appeared in the earlier Dialogue, with the addition that at various points in the discussion Salviati reads from the manuscript of a treatise on motion written by a so-called Academician; this is a reference to Galileo himself, who was proud of being a member of the Lincean Academy.

  Galileo died in Arcetri in 1642, assisted and surrounded by his son Vincenzio and his disciples Vincenzio Vivani and Evangelista Torricelli.

  1. Einstein 1954, 271; Hawking 1988, 179; 1992, xvii.

  2. Here I am adapting Gingerich’s (1982) eloquent formulation.

  3. Here, I overlook a fourth aspect, which cannot be appreciated in translation. That is, Galileo also happened to be one of the greatest writers in the (800-year) history of the Italian language, and his writings can be appreciated from the literary and aesthetic point of view.

  4. For more details on this controversy, see Drake 1978, 169–79, and 1981; Biagioli 1993, 159–209; Camerota 2004, 227–38.

  5. Galilei 1890–1909, 4: 35; cf. Camerota 2004, 230.

  6. For more details on the sunspots controversy, see Drake 1978, 179–213; Camerota 2004, 238–59; Biagioli 2006, 135–217; Reeves and van Helden forthcoming.

  7. For more details on the comets controversy, see Drake and O’Malley 1960; Drake 1978, 263–88; Biagioli 1993, 267–312; Camerota 2004, 363–98.

  Chronology of Galileo’s Career and Aftermath

  1543 Polish astronomer Nicolaus Copernicus publishes his book On the Revolutions of the Heavenly Spheres; he dies the same year.

  1545 The Catholic Church convenes the Council of Trent to deal with the Protestant Reformation, and the Catholic Counter- Reformation begins; the Council will not conclude its work until 1563.

  1564 15 February: Galileo is born in Pisa, which is part of the Grand Duchy of Tuscany ruled by the House of Medici.

  1574 Galileo’s family moves to Florence, the capital of the Grand Duchy.

  1581 Galileo enrolls at the University of Pisa in medicine and studies mathematics privately. In 1585, he leaves without a degree.

  1589 Galileo becomes professor of mathematics at the University of Pisa, where he teaches for the next three years. While there, he writes a work On Motion but does not publish it.

  1592 Galileo leaves Pisa and becomes professor of mathematics at the University of Padua, which is part of the Republic of Venice.

  1593 Galileo writes a Treatise on Fortifications for the use of his students, but does not publish it.

  1594 Galileo finishes writing a treatise on practical Mechanics begun the previous year; again, it is for student use and is not published.

  1597 Galileo writes, again for student use and without publishing it, a traditionally oriented Treatise on the Sphere, or Cosmography.

  1600 Apostate Dominican friar Giordano Bruno is convicted of heresy and burned at the stake by the Inquisition in Rome. Galileo and his common-law wife, Marina Gamba, have a daughter named Virginia.

  1601 Danish astronomer Tycho Brahe dies. A second daughter, Livia, is born to Galileo and Marina.

  1604 Galileo is convinced of the truth of two laws of falling bodies and is attempting to derive them from some more fundamental principle: the law of squares, according to which the distance traversed by a freely falling body is proportional to the square of the time elapsed; and the law of odd numbers, according to which in free fall, the distances covered in successive equal times increase as the odd numbers from unity.

  1605 During the summer vacation, Galileo tutors the fifteen-year-old prince Cosimo II de’ Medici.

  1606 Galileo publishes in Padua a booklet entitled Operations of the Geometric and Military Compass, containing instructions on using an instrument of his own invention that makes rapid calculations to solve engineering and military problems. A son named Vincenzio is born to Galileo and Marina.

  1609 German astronomer Johannes Kepler publishes his New Astronomy, containing the first two of his famous three laws of elliptical planetary motion.

  February: Cosimo II becomes grand duke of Tuscany, after the death of his father Ferdinando I.

  June: Galileo claims to have arrived at several correct theoretical principles underlying the laws of falling bodies. Summer: Galileo builds his first telescope.

  Fall: He begins to observe the heavens with the telescope and to make various discoveries.

  1610 13 March: Galileo’s Sidereal Messenger is published in Venice, describing his discovery of mountains on the moon, satellites of Jupiter, new fixed stars, and the stellar composition of the Milky Way and nebulas.

  19 April: Kepler sends his Conversation with the Sidereal Messenger to Galileo, supporting the new discoveries.

  10 July: Cosimo II de’ Medici appoints Galileo “Philosopher and Chief Mathematician” to the grand duke of Tuscany.

  Summer: Galileo observes Saturn as “three-bodied,” a puzzle that was only solved after his death when better observations showed Saturn to have rings.

  September: Galileo leaves Padua and moves permanently to Florence.

  Fall: Galileo observes the phases of Venus.

  1611 Kepler publishes in Frankfurt an account of his observations of Jupiter’s satellites, further supporting Galileo.

  25 April: Galileo is made a member of the Lincean Academy.

  13 May: The Jesui
t Roman College holds a special meeting at which, in the presence of Galileo, Father Odo van Maelcote delivers a lecture praising The Sidereal Messenger.

  1612 Galileo publishes in Florence his Discourse on Bodies in Water, attempting to resolve a controversy with Aristotelian philosophers.

  1613 22 March: Galileo’s History and Demonstrations Concerning Sunspots is published in Rome, sponsored by the Lincean Academy; it contains a collection of letters exchanged with German Jesuit astronomer Christoph Scheiner.

  21 December: After his former pupil Benedetto Castelli reports that Galileo’s views have been criticized at the ducal court on scriptural grounds, Galileo writes a refutation of the argument that Copernicanism is wrong because it contradicts Scripture; the refutation is in the form of a private letter to Castelli.

  1614 21 December: At the church of Santa Maria Novella in Florence, Dominican friar Tommaso Caccini preaches a sermon against mathematicians in general and Galileo in particular, on the grounds that they hold beliefs contrary to Scripture and so are heretics.

  1615 January: Carmelite friar Paolo Antonio Foscarini publishes in Naples a book entitled Letter on the Opinion, Held by Pythagoreans and by Copernicus, of the Earth’s Motion and Sun’s Stability and of the New Pythagorean World System; it argues that Copernicanism is compatible with Scripture and probably true.

  6 February: Christoph Scheiner sends to Galileo, together with a courteous letter, a copy of a book (Mathematical Investigations on Astronomical Novelties and Controversies) written by one of his disciples ( Johannes Locher); in it the proponents of the earth’s motion are violently attacked. Galileo will include some harsh criticism of this book in his Dialogue (1632).