The Essential Galileo

by Galilei, Galileo, Finocchiaro, Maurice A.

  [§0.3] A main reason for this delay was that beginning in 1609 Galileo became actively involved in astronomy. To be sure, he had been previously acquainted with the new theory of a moving earth published by Nicolaus Copernicus in 1543. He had been appreciative of the fact that Copernicus had advanced a novel argument supporting that ancient idea, namely, a detailed mathematical demonstration that the known facts about the motion of the heavenly bodies could be explained more systematically and coherently (not just more simply) if we attribute to the earth a daily axial rotation and an annual heliocentric revolution. Galileo had also acquired the general impression that this geokinetic theory was more consistent with the new physics he was researching than was the geostatic theory. In particular, he had also been attracted to Copernicanism because he thought that the earth’s motion could best explain why the tides occur. But he had not articulated, let alone published, this general impression and this particular feeling.

  On the other hand, Galileo had been acutely aware of the considerable evidence against Copernicanism. The earth’s motion seemed epistemologically absurd because it contradicted direct sense experience. It seemed astronomically false because it had consequences that could not be observed, such as the similarity between terrestrial and heavenly bodies, Venus’ phases, and annual stellar parallax. It seemed mechanically impossible because the available laws of motion implied that bodies on a rotating earth would, for example, follow a slanted rather than vertical path in free fall, and would be thrown off by centrifugal force. And it seemed theologically heretical because it contradicted the literal meaning and the traditional interpretation of some passages in the Bible. Until 1609 Galileo apparently judged that the anti-Copernican arguments far outweighed the pro-Copernican ones. Thus we find him teaching geostatic astronomy in his courses and reacting in a lukewarm and evasive manner when an enthusiastic Copernican like Johannes Kepler tried to engage him.

  [§0.4] However, the telescopic discoveries that began in 1609 led Galileo to a major reassessment of Copernicanism, and so for the next seven years he was seriously and explicitly involved in astronomical research and discussions. In 1609 he perfected the telescope to such an extent as to make it an astronomically useful instrument that could not be duplicated by others for some time. By its means he made several startling discoveries, which he immediately published in The Sidereal Messenger (Venice, 1610): that the moon’s surface is full of mountains and valleys; that innumerable other stars exist besides those visible with the naked eye; that the Milky Way and the nebulas are dense collections of large numbers of individual stars; and that the planet Jupiter has four moons revolving around it at different distances and with different periods. As a result, Galileo became a celebrity, resigned his professorship at Padua, was appointed Philosopher and Chief Mathematician to the grand duke of Tuscany, and moved to Florence the same year. Soon thereafter, he also discovered sunspots and the phases of Venus.

  Although most of these discoveries were also made independently by other observers, no one understood their significance as well as Galileo. Their importance was threefold. Methodologically, the telescope implied a revolution in astronomy insofar at it was a new instrument that enabled the gathering of a new kind of data transcending the previous reliance on naked-eye observation. Substantively, those particular discoveries significantly strengthened the case in favor of the physical truth of Copernicanism by refuting almost all empirical astronomical objections and providing some new supporting observational evidence. Finally, this enhancement of the evidentiary solidity of Copernicanism was not equivalent to a settling of the issue or a conclusive establishment of its truth, for several reasons: there was still some astronomical counterevidence (e.g., the lack of annual stellar parallax); the criticism of the mechanical objections and the physics of a moving earth had not yet been articulated (although, as stated above, Galileo had been working on both projects); and the theological objections had not yet been dealt with. Thus, Galileo began to conceive of a work on the system of the world in which all these aspects of the question would be discussed. This synthesis of Galileo’s astronomy, physics, and methodology was not to be published for another twenty years, until his Dialogue on the Two Chief World Systems, Ptolemaic and Copernican (Florence, 1632).

  This particular delay happened because Galileo got involved in several controversies over floating bodies, sunspots, the astronomical authority of Scripture, and comets. These discussions turned out to be fateful developments that had a drastic and permanent effect on the evolution of his life and career.

  [§0.5] In July 1611, Galileo became involved in a controversy with some Tuscan Aristotelian philosophers over the behavior of solid bodies in water.4 The occasion was provided by a casual remark Galileo made to the effect that ice is rarified water, since it floats in water, and hence it is lighter (in specific weight) than water. This was in accordance with the hydrostatic principles of Archimedes. But it contradicted the Aristotelian claim that ice is condensed water, and that it floats because its shape prevents it from overcoming the resistance of the water. The discussion soon turned to the cause of floating, sinking, and motion in water, and the relative role of shape and density. At one point, the Aristotelians introduced the allegedly crucial experiment that ebony wood sinks when shaped into a ball, but floats when shaped into a large thin flat plate; they regarded this experiment as a conclusive demonstration that shape, not specific weight, determines whether a body floats or sinks.

  On more than one occasion, this discussion acquired the character of a formal debate. One of these debates took place at the house of Filippo Salviati, Galileo’s Florentine friend whom he later immortalized as one the speakers in the Dialogue and Two New Sciences. Another debate occurred at the court of Grand Duke Cosimo II, in the presence of cardinals Ferdinando Gonzaga and Maffeo Barberini (1568–1644); the latter, who would become Pope Urban VIII in 1623, sided with Galileo. This sort of debate soon convinced Galileo that the philosophical discussion was degenerating into a competitive sport,5 and so he decided to write down his thoughts. The result was the Discourse on Bodies in Water, which was published in the spring of 1612. However, rather than ending the controversy, this publication merely moved it into the print medium. In fact, within about a year four books against Galileo’s Bodies in Water were published by various Aristotelians, including one by Lodovico delle Colombe, who was also criticizing Galileo’s ideas on the motion of the earth. Finally, a lengthy reply to these critics was written jointly by Galileo and his disciple Benedetto Castelli, and published only under Castelli’s name in the spring of 1615.

  [§0.6] At about the same time that Galileo was studying and disputing about bodies in water, he was also studying and disputing about sunspots.6 It is uncertain when Galileo first observed sunspots with the telescope, but it is certain that while in Rome in the spring of 1611 he showed them to a number of people. It is also clear that he did not publish or write anything on the topic until stimulated by German Jesuit Christoph Scheiner (1573–1650).

  In November and December 1611, Scheiner wrote three letters about sunspots to Marc Welser, an official of the German city of Augsburg. These were published under the pseudonym of Apelles in January 1612 in a small book entitled Three Letters on Sunspots. Welser immediately sent Galileo a copy, requesting his opinion. In May, Galileo replied with a long letter to Welser, criticizing Scheiner’s views and observations and advancing his own. A second Galilean letter to Welser followed in August. In the meantime, after reading Galileo’s first letter, Scheiner wrote another essay, which he published in September under the same pseudonym and with the title A More Accurate Inquiry on Sunspots. Galileo again received a copy from Welser and then in December wrote him a third long letter. Finally, in March 1613 the Lincean Academy published in Rome a volume containing Galileo’s three letters and an appendix with Scheiner’s two booklets. Such was the origin of Galileo’s History and Demonstrations Concerning Sunspots.

  Part of the dispute between Sc
heiner and Galileo involved priority of discovery. In 1612–13, this aspect of the controversy was relatively subdued, and the most sensible thing to say is that the phenomenon was discovered independently by both. Later, the priority dispute became bitter and nasty, as it came to encompass other aspects of the phenomenon (such as the inclination of the solar axis of rotation) and other more general issues (such as the Copernican controversy). However, the most significant intellectual aspect of the controversy between Galileo and Scheiner concerned the interpretation of the sunspots and their implications for the Copernican theory. Echoes of, and new twists on, the sunspot controversy can be found in almost all of Scheiner’s and Galileo’s subsequent writings.

  In the 1612–13 discussions, Scheiner held that sunspots were swarms of small planets orbiting the sun at small distances. Individually they were invisible; but when several simultaneously reached the line of sight (of an observer from earth), then they appeared as dark spots projected onto the sun. Scheiner’s interpretation saved an essential part of the Aristotelian worldview, namely, the earth-heaven dichotomy; according to this doctrine heavenly bodies and terrestrial bodies were very different, insofar as only the latter were subject to physical changes, such as generation and destruction. For Scheiner the only novelty required by sunspots was the existence of some previously unknown planets.

  On the other hand, Galileo’s interpretation was that sunspots were phenomena occurring on the body of the sun, individually subject to sporadic production and dissolution, but collectively undergoing regular eastward motion. This implied that the sun rotates on its axis (with a period of about one month), and that it undergoes physical changes similar to those on earth (sunspots being analogous to terrestrial clouds). And this in turn undermined a key tenet of the Aristotelian worldview—the earth-heaven dichotomy.

  [§0.7] As it became known that Galileo was convinced that the new telescopic evidence rendered the Copernican theory of the earth’s motion a serious contender for real physical truth, he came increasingly under attack from conservative philosophers and clergymen in Florence. They started arguing that Galileo was a heretic because he believed in the earth’s motion and the earth’s motion contradicted the Bible. Underlying this personal attack was the biblical argument against Copernicanism: the earth cannot move, because many biblical passages state or imply that it stands still, and the Bible cannot err. The most frequently mentioned biblical passage was Joshua 10:12–13 (King James Version): “Then spake Joshua to the Lord in the day when the Lord delivered up the Amorites before the children of Israel, and he said in the sight of Israel, ‘Sun, stand thou still upon Gibeon; and thou, Moon, in the valley of Ajalon.’ And the sun stood still, and the moon staid, until the people had avenged themselves upon their enemies. Is not this written in the book of Jasher? So the sun stood still in the midst of heaven, and hasteth not to go down about a whole day.”

  Although Galileo was aware of the potentially explosive nature of this particular issue, he felt he could not remain silent, but decided to refute the argument. Because of the circumstances of the attacks and to avoid scandalous publicity, he wrote his criticism in the form of long private letters, in December 1613 to his former student Benedetto Castelli, a Benedictine monk and professor of mathematics at Pisa, and in spring 1615 to the grand duchess dowager Christina.

  Galileo’s critique may be summarized as follows. The biblical argument attempts to prove a conclusion (the earth’s rest) on the basis of a premise (the Bible’s commitment to the geostatic system) that can only be ascertained with a knowledge of that conclusion in the first place. In fact, the interpretation of the Bible is a serious business, and normally the proper meaning of its statements about natural phenomena can be determined only after we know what is true in nature; thus, the business of biblical interpretation is dependent on physical investigation, and to base a controversial physical conclusion on the Bible is to put the cart before the horse. Second, the biblical objection is a non sequitur, since the Bible is an authority only in matters of faith and morals, not in scientific ones; thus, its saying something about a natural phenomenon does not make it so, and therefore its statements do not constitute valid reasons for drawing corresponding scientific conclusions. Finally, it is questionable whether the earth’s motion really contradicts the Bible, as one can show by an analysis of the passage (Joshua 10:12–13) where God stopped the sun to prolong daylight and give Joshua enough time to win a battle before nighttime; according to Galileo, a careful analysis shows that this passage cannot be easily interpreted in accordance with the geostatic theory, but that it accords better with the geokinetic view, especially as improved by Galileo’s own discovery of solar axial rotation. The biblical objection is therefore groundless, aside from its other flaws.

  Galileo’s letter to Castelli was widely circulated, and the conservatives got increasingly upset. The situation was exacerbated in January 1615 when Galileo received the unexpected but welcome support of a Carmelite friar named Paolo Antonio Foscarini, who published 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. Although this was written in the form of a letter to the head of the Carmelite order, the book was a public document. Moreover, although Foscarini’s arguments overlapped with Galileo’s, they had a distinct flavor and original emphasis: that the earth’s motion was probably true and compatible with Scripture.

  The publication of Foscarini’s Letter did not go unnoticed. The Inquisition ordered an evaluation of the book, and a consultant wrote a very critical opinion. But before any formal proceedings started, Foscarini learned about the censure and informally contacted Cardinal Robert Bellarmine, the most authoritative theologian of that time and a member of the Congregation of the Inquisition as well as of the Index. Foscarini sent Bellarmine a copy of his book together with a long letter defending it from the type of criticism contained in the Inquisition consultant’s report. Bellarmine replied courteously in a famous letter directed to Galileo as well as to Foscarini and discussing epistemological as well as scriptural issues. Galileo soon received a copy of Bellarmine’s letter to Foscarini and immediately started writing a reply now known as “Considerations on the Copernican Opinion,” in three parts. This reply was, however, never published, delivered, or even completely finished because other, more formal Inquisition proceedings soon became the center of attention.

  In February 1615, a Dominican friar named Niccolò Lorini, from Florence, filed a written complaint against Galileo with the Inquisition in Rome, enclosing his “Letter to Castelli” as incriminating evidence. Then in March, another Dominican, Tommaso Caccini, made a personal deposition against Galileo with the Roman Inquisition. An investigation was launched that lasted about a year. As part of this inquiry, a committee of Inquisition consultants reported that the earth’s motion is absurd and false as a matter of natural philosophy and heretical, or at least erroneous, as a matter of religion and theology. This judgment reflects the weight of the traditional objections to the earth’s motion; the failure to know or appreciate the new arguments in its favor; and the unwillingness to question the biblical fundamentalism according to which the Bible is an authority on physical questions, as well as on questions of faith and morals. The Inquisition also interrogated other witnesses. Galileo himself was not summoned or interrogated partly because the key witnesses exonerated him and partly because Galileo’s letters had not been published, whereas his published writings did not contain either a categorical assertion of Copernicanism or a denial of the scientific authority of the Bible.

  However, in December 1615 Galileo went to Rome of his own accord to defend his views. He was able to talk to many influential Church officials and was received in a friendly and courteous manner; and he may be given some credit for having prevented the worst, insofar as the Inquisition did not issue a formal condemnation of Copernicanism as a heresy, in accordance with the consultants’ report. Instead t
wo milder consequences followed. In February 1616, Galileo himself was given a private warning by Cardinal Bellarmine (in the name of the Inquisition) to the effect that he was forbidden to hold or defend the truth of the earth’s motion; Galileo agreed to comply. And in March, the Congregation of the Index published a decree which, without mentioning Galileo at all, declared that the earth’s motion was physically false and contradicted the Bible; that Foscarini’s Letter on the Earth’s Motion was to be condemned and permanently banned; and that Copernicus’ book On the Revolutions of the Heavenly Spheres (1543) was temporarily banned until appropriately corrected. These corrections were not specified until 1620 when the Congregation of the Index issued another decree explaining how a dozen passages in Copernicus’ book were to be deleted or reworded in order to eliminate from it any suggestions that the earth’s motion was or could be physically true and compatible with the Bible; the revisions were also meant to make it clear that the book was treating the earth’s motion merely as a hypothesis, which in that context meant a mere instrument of astronomical calculation and prediction.