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Galileo’s Discoveries After 400 Years

Noel M. Swerdlow

GALILEO. J. L. Heilbron. xiv + 508 pp. Oxford University Press, 2010. $34.95.

GALILEO: Watcher of the Skies. David Wootton. Yale University Press, 2010. xii + 328 pp. $35.

The year 2010 marked the 400th anniversary of Galileo’s discoveries with the telescope, and the event and its author were commemorated with conferences, lectures, exhibits and publications all over the world. The literature on Galileo already exceeds in quantity the literature on every other scientist. (His closest competitors are Einstein and Darwin; even Newton falls far behind.) Why should this be? There must be a dozen scientists whose contribution is of greater importance. Also, much of Galileo’s own work is defective, some of it in ways he was himself aware of. And yet this attention to Galileo is not misplaced and has every promise of continuing for as long as the history of science and science itself are pursued. Consider Galileo’s life and work.

Galileo Galilei was born in 1564, the son of Vincenzio Galilei, a Florentine cloth merchant and musician best known for his theoretical writings on ancient and modern music. From Vincenzio, Galileo learned to play the lute and sing, to discover by observation and experience, and to be skeptical of authority and the supernatural. His father wished him to study medicine, to which end he attended the University of Pisa, but he greatly preferred mathematics, which he studied outside the university. He left in 1585 without a degree, for four years taught mathematics privately, and in 1589 received an appointment teaching mathematics at Pisa even though he was still without a degree. Three years later he received a call to lecture on mathematics at the Studio di Padova, then the most distinguished university in Europe. That Galileo, who had as yet published nothing, was appointed to such a post can only show recognition of his greatest quality, his genius.

2011-05BREVSwerdlowFA.jpgClick to Enlarge ImageHis principal interest was mechanics—the application of mathematics to statics, kinematics and dynamics—and he understood that the object of mechanics is to reduce these subjects to geometry, without the need for hidden forces. During the 18 years he spent in Padua, he made his most important discoveries in mechanics, both experimental and theoretical: the acceleration of a falling body, the parabolic trajectory of a projectile and the resistance of solids to fracture. Up to this point in his life, Galileo’s concern with astronomy was slight, although he did compute any number of horoscopes. But when Johannes Kepler sent him a copy of his Mysterium cosmographicum in 1597, the first important successor to Nicolaus Copernicus’s De revolutionibus of 1543, Galileo wrote back that he had himself arrived at the Copernican opinion many years ago and from that assumption had discovered the cause of many natural effects. Kepler guessed, correctly, that Galileo was referring to the tides.

Through all his years in Padua Galileo published little and was barely known outside his circle of friends and students in Padua, Venice and Florence. Then everything changed. In the early summer of 1609, he learned of an optical device recently invented in the Netherlands, which made distant objects appear close. He immediately figured out how to make such a thing from spectacle lenses—a “spyglass” giving an upright image of 2 or 3 times magnification, which he soon improved to 8 or 9 times. In September he offered it to the Republic of Venice as a military secret for spotting distant ships at sea, and he was rewarded with a lifetime appointment at Padua and a doubling of his salary.

But he had bigger things in mind. By December he had made an instrument of 20 times magnification and soon after that 30, and he began looking to the heavens. In two months he made more discoveries that changed the world than anyone before or since. He discovered, and confirmed, that the surface of the moon, of which he made strikingly realistic drawings, was rough and mountainous. He found that there are a vast number of stars that are smaller—we would say fainter—than those visible to the unaided eye, and saw that the Milky Way is made up of stars beyond counting. Most surprising of all, he saw that Jupiter is accompanied by four small stars that move around it. (Kepler soon introduced the term satellites—from satelles, an attendant upon an important person—to refer to them.) The discovery of Jupiter’s little stars made Galileo resolve to publish quickly, before someone else had the bright idea of turning the new optical device on Jupiter. He recorded nightly observations of the configurations of Jupiter and its companions from January 7 to March 2, and by March 13, 1610, published his new discoveries in the Sidereal Messenger. The little book was soon known throughout Europe, and Galileo became the most famous natural philosopher in the world.

In the course of that year Galileo went on to discover the peculiar shape of Saturn, which was correctly explained as a ring nearly 50 years later by Christiaan Huygens; the phases of Venus, showing that it must move about the Sun and shine by reflected sunlight; and spots on the Sun that move and appear and disappear, showing that the Sun rotates and the heavens are subject to change. And that is why after 400 years, we are having conferences, lectures, exhibits and publications, and it is one reason that Galileo is still the most famous natural philosopher in the world.

But there are other reasons. The Sidereal Messenger is Copernican—Galileo states that he will prove that the Earth is a planet in an intended work to be titled System of the World—and so too are the Letters on Sunspots of 1613, in which he reports his later discoveries. By the summer of 1610 he had left Padua to take the positions of Mathematician and Philosopher to the Grand Duke of Tuscany in Florence and Chief Mathematician of the University of Pisa, with no teaching responsibilities. He had all he wanted, but he had made what turned out to be a serious mistake, for when trouble came, the Grand Duke could not provide the protection he would have received in Padua from the Republic of Venice.

2011-05BREVSwerdlowFB.jpgClick to Enlarge ImageIn December 1613, in answering a letter from his former student Benedetto Castelli concerning scriptural objections to Copernican theory, Galileo set out his own views of Scripture and science, offering an ingenious interpretation of Joshua’s making the Sun stand still to show that not only does Holy Scripture not oppose Copernican theory, it actually supports it. Castelli allowed copies of the letter to pass into circulation, and it fell into the wrong hands. In December 1614 a sermon was preached in Florence criticizing the opinion of Galileo for being contrary to Scripture and inimical to the Catholic faith, and soon a complaint was sent to the Holy Office in Rome. Early in 1615, Paolo Antonio Foscarini published a book, with admiring references to Galileo, showing how completely Scripture supports Copernican theory. He sent the book to Cardinal Robert Bellarmine, defender of the Church against Protestants and heretics. Bellarmine wrote a letter to Foscarini, also intended for Galileo, making these points: Although the Sun at the center and the Earth in motion may be taken as a supposition, to maintain it as true endangers faith by making Holy Scripture false, for Holy Scripture must be interpreted in accordance with the Fathers of the Church, all of whom agree that the Earth is immobile at the center of the world. Galileo drafted a reply to Bellarmine, arguing that the motions of the Earth are not just a supposition but can be proved, but he never completed or sent it.

By late 1615 there had been more complaints to the Holy Office. Galileo, as he told the Grand Duke, was stung by the vain slandering of his adversaries that he held erroneous opinions in his works, and resolved to come to Rome for the purpose of vindicating himself from such accusations and making clear the truth and his righteous and pious intention. He never should have gone; had he not done so, history would have been different. In Rome he did what he did best: He talked, for no one could talk more brilliantly, more convincingly. And what he talked of was Copernican theory, arguing, demonstrating, refuting, devising far more damaging objections to it than could anyone else, and refuting them too. But it did no good and probably did much harm.

Then Pope Paul V turned the matter over to Bellarmine, who asked eleven Father Theologians to the Holy Office to assess two propositions, the immobility of the Sun and the mobility of the Earth. On February 24, 1616, they ruled that the first was “foolish and absurd in philosophy and formally heretical as it expressly contradicts the teachings of Holy Scripture,” and that the second “receives the same censure in philosophy, and with regard to theological truth is at least erroneous in faith.” What began as a difference over interpretation of Scripture had become a condemnation of Copernican theory. On February 26, Galileo was warned to abandon entirely the opinion that the Sun is the center of the world and immobile and the Earth moves, and he was admonished to “in the future not hold, teach or defend it in any way either by speech or writing”; otherwise, the Holy Office would proceed against him. He acquiesced to this order and promised to obey.

That was the end of Galileo’s intention to prove Copernican theory in his System of the World. He occupied himself with finding accurate periods of Jupiter’s satellites to compute tables for determination of longitude, and, following the appearance of the bright comet of 1618, engaged in a controversy on comets and telescopes, and empiricism and authority in science, in The Assayer.

Then in 1623 Galileo’s old friend and supporter from Florence, Cardinal Maffeo Barberini, was elected Pope Urban VIII. In audiences with the Pope the following year Galileo was given to understand that it was now permissible to write on the system of the world, provided that he do so hypothetically and stay away from Scripture. The Pope also repeated something he had said to him years earlier: Granting everything devised concerning the heavens and the motion of the Earth, does not God have the power or knowledge to arrange the spheres or stars in another way such that everything can be saved? And if God has the power and knowledge to arrange these in another way, we must not restrict the divine power and knowledge to this one way. Having heard this, Galileo was silent. This was more than a question about Galileo’s science; it was a stern warning. And it makes all certainty in science impossible.

Galileo returned to Florence and began, or resumed, work on what became the Dialogue on the Two Great Systems of the World, Ptolemaic and Copernican, completed in 1630 and published in 1632. A more accurate title would be Dialogue on the Two Great Systems of the World, Aristotelian and Galilean, although Galileo, for all his nerve, knew enough not to say that. He did stay away from Scripture, but he did not treat the subject hypothetically, even though he made occasional remarks to that effect. What he wrote was a sustained argument of nearly 500 pages to prove Aristotle wrong and Copernicus—or, better, Galileo—right. He takes up, in order, these topics: circular motion and the nature of the heavens, which are subject to change; the diurnal rotation of the Earth, which it must have, and why it is not detected; the annual heliocentric motion of the Earth, which it must have and which can in fact be proved; and the cause of the tides from the diurnal and annual motions of the Earth, which proves that the Earth must have both motions. The Dialogue is brilliant and compelling: The arguments against Aristotle are devastating—any remaining Aristotelians must be fools—and the arguments for the motions of the Earth are effective, even though Galileo himself knew that some were flawed and Newtonian mechanics has shown others to be defective. He had accomplished just what he set out to do, prove Aristotle wrong and Copernicus right, and for this he expected celebrity and gratitude.

Galileo received both from his friends and former students and many throughout Europe, but he did not receive either from the person he was most interested in winning over. He believed that when his old friend Maffeo Barberini saw the strength of his arguments, saw the truth, he would rescind the prohibition against Copernicus and congratulate Galileo for proving the system of the world once and for all and for rescuing the Church from an unfortunate error. For if Galileo stood for anything, it was that in science, which is concerned only with truth, truth will prevail. But the Pope believed his own truth, that God has the power and wisdom to do things in ways no one can understand, and that to deny this challenges the omnipotence of God, which is dangerous to religion and faith. And Galileo had not only violated this truth but had treated it ironically and dismissively in the Dialogue. The Pope believed himself betrayed and insulted, and he appointed a commission to examine the Dialogue.

In September 1632 the commission presented a report reviewing the Dialogue and raising various objections. But it also found the record of the Holy Office from 1616 stating that Galileo had been warned to abandon entirely his opinion that the Sun is the center of the world and the Earth moves and “in the future not hold, teach or defend it in any way either by speech or writing.” Here was all that was needed to remove the matter to the Holy Office, over which the Pope himself presided.

Galileo had four appearances before the Holy Office in Rome in April through June of 1633. In the first he insisted, against the evidence of the record, that he had received or could remember no such warning, and against the evidence of his book, that he neither held nor defended the mobility of the Earth and the stability of the Sun, but showed “the contrary of Copernicus’s opinion and that Copernicus’s reasons are invalid and inconclusive.” This was, to say the least, hard to believe, and three subsequent reports concluded that Galileo held, taught and defended the Copernican opinion. After three further appearances, in which the most he would admit to was making the arguments for Copernicus—arguments he said he intended to refute—convincing rather than easy to answer, he repeatedly denied the charges against him. On June 22, in accordance with the order of the Pope, he was found “vehemently suspected of heresy,” a formal term meaning in effect “seriously guilty of heresy.” He was given the opportunity, really the necessity, to abjure his heresies, which he did, reading a humiliating statement written for him, as the alternative was imprisonment or worse.

His sentence included prohibition of the Dialogue, swearing that he would never again say or assert, in speech or in writing, things through which one could have similar suspicion of him (meaning the immobility of the Sun and the mobility of the Earth), and imprisonment at the pleasure of the Holy Office. In the following months, the sentence and abjuration were read to professors of philosophy and mathematics at universities throughout Catholic Europe, “so that they will understand the gravity of the error committed by Galileo in order to avoid it along with the punishment they would receive were they to fall into it.” Galileo’s imprisonment was later commuted to house arrest in his villa in Arcetri outside Florence, where he remained under close supervision that lasted the rest of his life, with repeated warnings that he speak with no one about the motions of the Earth.

But Galileo was not defeated, for under the severe conditions of confinement, he wrote the Discourses and Mathematical Demonstrations Concerning Two New Sciences, which was sent by way of Venice to Leiden and appeared in 1638. Here at last he published his discoveries in mechanics—some made many years earlier and now revised into their final form, others made very recently and worked out along with the revision. The mathematics, which is cumbersome and often obscure, was soon superseded in brevity and clarity by that of his successors—in truth Galileo was not a good mathematician. But the discussion in dialogue form is brilliant and filled with ingenious illustrations of the mechanics, paradoxes that continue to astound, and as devastating an attack on Aristotelian physics as anything in the Dialogue. By the time the book appeared, Galileo was blind, but that did not keep him from working. He kept up his correspondence through dictation, and in late 1641 began another dialogue eventually published as an addition to the Two New Sciences. He died on January 8, 1642.

Galileo’s lasting fame rests on his discoveries with the telescope, the reflections on science in The Assayer, the arguments for Copernicus and against Aristotle in the Dialogue, and the mechanics of the Two New Sciences, but most of all on his condemnation by the Church—the most famous event in the history of science, whose pertinence to contemporary issues never seems to fade. As long as there is a conflict, a contradiction, between science and religion, between reason and faith, between the natural and the supernatural—which appears inevitable from the essential character of each—Galileo will remain the most famous of all scientists.

The enormous literature on Galileo continues to increase at a rate faster than anyone can keep up with, certainly faster than I can. The foundation of modern studies is Antonio Favaro’s monumental edition Le Opere di Galileo Galilei, published between 1890 and 1909; it is the source of all serious scholarship on Galileo, along with Favaro’s many other publications. But these works are for scholars of Galileo. For everyone else, the two books I consider the most interesting and essential are Leonardo Olschki’s Galilei und seine Zeit (1927) and Stillman Drake’s Galileo at Work (1978). Olschki’s is the third volume of a longer work on the practical mathematical and technical literature of the Renaissance, from which Galileo’s own interests in mechanics arose and of which his own writing in the vernacular is a continuation. Drake is the foremost scholar of Galileo after Favaro, known for his translations of most of Galileo’s important works and a lifetime of publication of books and articles on him. Galileo at Work is a life and work with the emphasis on the work as it took place, shown in letters and manuscripts.

The two most recent biographies, J. L. Heilbron’s Galileo and David Wootton’s Galileo: Watcher of the Skies, are both the product of immersion in Favaro’s edition, from which, even after a century, new discoveries can be made. Both are also based on extensive research in primary and secondary literature. And both tell the story of Galileo’s life in detail, with any number of incidents that will be new to nearly every reader. Both books, in my opinion, are fascinating, even difficult to put down. They are, however, quite different, and the principal difference concerns science.

J. L. Heilbron is a noted historian of science, primarily of physics, who has written on a broad range of subjects from the early modern period to the 20th century. In 1999 he published The Sun in the Church: Cathedrals as Solar Observatories, a history of the research in astronomy encouraged and sponsored by the Church in the 17th and early 18th centuries. His new biography considers all the varied aspects of Galileo’s life, including his interests in art, literature and poetry, beginning with his lectures in 1588 on the plan of Dante’s Inferno. Heilbron also provides an account of Galileo’s scientific work that is based on detailed knowledge and understanding of the science, which he explains very well. No one since Drake has treated as much of Galileo’s scientific work. Because Drake’s accounts are spread among numerous papers, collections and books, Heilbron’s book, to the best of my knowledge, explains more of Galileo’s science than any other single book. It is all covered here, from the early Archimedean theorems on the center of gravity of solids and writings on motion and machines, through the experimental and theoretical researches on accelerated motion and the parabolic trajectory of projectiles recorded in Galileo’s manuscript notes, to the discoveries with the telescope, the hydrostatics of floating bodies, the controversies on sunspots and, later, comets and telescopes, to the arguments and demonstrations concerning the motions of the Earth in the Dialogue and the final explication of mechanics in the Two New Sciences. In places the exposition, with the mathematics, is compressed and will take some concentration to follow, but the many diagrams clarify the mathematics, and the full range of Galileo’s science is explained so that anyone who studies it carefully can understand it. The conflicts with the Church in 1616 and 1633 are set out in appropriate detail and without the strange conspiracy theories that have grown up around this subject. Above all for its exposition of Galileo’s science, Heilbron’s book is worthy to take its place beside those of Olschki and Drake.

David Wootton is a historian, principally of political philosophy and of religious skepticism in the early modern period; most pertinent to the present volume is his 1983 book, Paolo Sarpi: Between Renaissance and Enlightenment. Sarpi, a Servite monk who became theologian to the Republic of Venice during the controversy over the Interdict of 1606, was a friend and correspondent of Galileo’s during his years in Padua and later. If any contemporary was Galileo’s equal, it was Sarpi, who was as radical as Galileo and as brilliant a writer. In his History of the Council of Trent (1619), Sarpi did to the Counter-Reformation Church what Galileo did to Aristotelian natural philosophy.

Galileo: Watcher of the Skies is more a biography than an account of Galileo’s science. The book is divided into 39 sections, most fairly brief, which read like a series of essays on aspects of Galileo’s life. There is nothing wrong with this, and many of the essays are lively and on subjects not often treated, such as Galileo’s family and his relations with them—which consisted mostly, it appears, in giving them money. I wish, however, that Wootton knew more of the science and could explain it better. There is no real mathematics, and there are no diagrams, which precludes a proper exposition of Galileo’s mechanics. And the treatment of astronomy is not better, with curious reflections on Galileo’s “premature Copernicanism,” which might not seem premature if the evidence that convinced Copernicus, Kepler and probably also Galileo were understood.

Wootton also falls for conspiracy theories of Galileo’s troubles with the Church. In particular, he invents a scene in which he imagines Galileo’s Inquisitor telling him that if he fails to cooperate in the proceedings against him, he will face an additional very serious charge, that of having denied transubstantiation—a charge based on the distinction he made between primary and secondary qualities in The Assayer. Wootton gives rather a lot of attention to arguing that Galileo was an unbeliever, as if this were something new. From all I know of Galileo and from his use of irony in his writing, particularly on anything to do with piety, it would surprise me if he were anything else. It would also surprise me if this were not obvious to the Holy Office when it compelled him to abjure with words that it was evident he could not possibly believe. And I wish that Wootton did not rail so much against writers with whom he disagrees, because no one who knows anything takes them seriously, and many of them are dead.

Still, Wootton has written a lively book that is interesting to read, and one can pass over the superfluous interpretations and concentrate on the fascinating details from the extensive research. And after all, it is about Galileo.

Noel M. Swerdlow, an emeritus professor in the department of astronomy and astrophysics and the department of history at the University of Chicago, is currently a visiting associate in history at the California Institute of Technology. He is the author of The Babylonian Theory of the Planets (Princeton University Press, 1998) and is coeditor with Trevor H. Levere of a collection of 80 papers by Stillman Drake, titled Essays on Galileo and the History and Philosophy of Science (University of Toronto Press, 1999).

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