Coming of Age in the Milky Way - Timothy Ferris (2003)



Pure logical thinking cannot yield us any knowledge of the empirical world; all knowledge of reality starts from experience and ends in it…. Because Galileo saw this, and particularly because he drummed it into the scientific world, he is the father of modern physics—indeed, of modern science altogether.


         What if the Sun
Be Center to the World, and …
The Planet Earth, so stedfast though she seem,
Insensibly three different Motions move?

—Milton, Paradise Lost

           History plays on the great the trick of calcifying them into symbols; their legend becomes like the big house on the hill, whose owner is much talked about but seldom seen. For no scientist has this been more true than for Galileo Galilei. Galileo dropping a cannonball and a musket ball from atop the Leaning Tower of Pisa, thus demonstrating that objects of unequal weight fall at the same rate of acceleration, has come to symbolize the growing importance of observation and experiment in the Renaissance. Galileo fashioning the first telescope symbolizes the importance of technology in opening human eyes to nature on the large scale. Galileo on his knees before the Inquisition symbolizes the conflict between science and religion.

Such mental snapshots, though useful as cultural mnemonic devices, extract their price in accuracy. The story of Galileo at the Leaning Tower is almost certainly apocryphal. It appears in a romantic biography written by his student Vincenzio Viciani, but Galileo himself makes no mention of it, and in any event the experiment would not have worked: Owing to air resistance, the heavier object would have hit the ground first. Nor did Galileo invent the telescope, though he improved it, and applied it to astronomy. And, while Galileo was indeed persecuted by the Roman Catholic Church, and on trumped-up charges at that, he did as much as anyone outside of a few hard-core Vatican extremists to lay his body across the tracks of martyrdom.

Still, these distortions in the popular conception of Galileo work to his favor, and that would have pleased him. A devoted careerist with a genius for public relations, he was ahead of his time in more ways than one. His mission, as he put it, was “to win some fame.”1

Galileo was born in Pisa, on February 15, 1564, twenty years after the publication of Copernicus’s On the Revolutions. From his father, Vincenzo Galilei, a professional lute player and amateur mathematician, Galileo inherited a biting wit, a penchant for the dialogue form of argument, and a vehement distrust of authority. Vincenzo had written a book, Dialogue of Ancient and Modern Music, that encouraged Kepler in his search for Pythagorean harmonies. One of the characters in it utters a declaration that could have been the motto of the younger Galileo:

It appears to me that they who in proof of any assertion rely simply on the weight of authority, without adducing any argument in support of it, act very absurdly. I, on the contrary, wish to be allowed freely to question and freely to answer you without any sort of adulation, as well becomes those who are in search of truth.2

Galileo prospered so long as he remained true to that independent creed. Disaster beset him once he neglected it and began demanding that questions be decided on the pronouncements of his own authority.

As a young man, however, Galileo waged glorious campaigns against those who, as he was to write, “think that our intellect should be enslaved to that of some other man.”3 An incandescent speaker and pamphleteer, he was known during his student days at the University of Pisa as “the wrangler” for the sarcastic aplomb with which he skewered the Scholastic professors.

At his parents’ behest Galileo studied medicine, but he found little there to gratify his appetite for empirical knowledge. Medical lecturers typically taught from a volume of Galen, who had been dead for fifteen hundred years, and their laboratory sessions were hindered by a Church prohibition against dissection of human bodies. Galileo soon dropped out. He then spent four irresponsible, productive years lazing about at home, reading Virgil and Ovid, building little machines, and studying mathematics with a tutor, Ostilio Ricci, with whom he shared a devotion to the works of Archimedes.

Galileo was twenty-five years old when a scientifically inclined nobleman, Francesco Cardinal del Monte, took an interest in his abilities and got him appointed professor of mathematics at Pisa. There he lectured on astronomy, poetry, and mathematics and resumed his hectoring of the Aristotelians, at one point circulating a satirical poem poking fun at the Scholastics’ habit of coming to school in togas, like little wax Aristotles. The students were delighted but the Scholastics were in the majority on the faculty, and when Galileo’s contract expired he was let go.

He then managed to gain an appointment to the chair of mathematics at the University of Padua, in the free Republic of Venice.* (Another applicant for the post was Giordano Bruno, but he was in chains by the time Galileo arrived at the university in September 1592 and was burned alive eight years later for refusing to abjure many heresies, among them his insistence that the stars are suns.) Galileo remained at Padua for eighteen years, writing, lecturing, conducting experiments, and inventing scientific instruments, among them the thermometer.

During this time his financial troubles, always onerous, became insupportable. His father had died in 1591, leaving Galileo to pay his two sisters’ dowries, each of which equaled several years’ worth of his university salary. In addition he was obliged to send money to his brother Michelangelo, a wandering musician who demonstrated his contempt for cash by squandering it as rapidly as he could get his hands on it. By the age of forty-five, Galileo was a respected scientist and teacher with a couple of books to his credit, but his contract was coming up for renewal, his debts were mounting, and he needed something to elevate his career from the creditable to the extraordinary. It came to him in 1609. It was the telescope.

During one of his frequent visits to nearby Venice, Galileo learned that telescopes were being constructed in Holland. Quick to grasp the principles involved, he returned home to Padua and built a telescope for himself. “Placing my eye near the concave lens,” he recalled, “I perceived objects satisfactorily large and near, for they appeared three times closer and nine times larger than when seen with the naked eye alone. Next I constructed another one, more accurate, which represented objects as enlarged more than sixty times.”4

Galileo did not need to be told that the telescope would have great practical value. Venice was an unwalled city, and its citizens depended for their defense upon their ability to spot approaching enemy ships in time to dispatch a fleet to engage them while they were still at sea; the telescope would greatly improve this early-warning system. The Venetians, furthermore, made their living from sea trade, and frequently kept an anxious watch, from the lookout towers (campanili) that dotted the city, for galleys returning with their holds full of cornmeal from the Levant, spices from Constantinople, and silver from Spain; an investor might be ruined if his ship were lost, or double his money once “his ship came in.” A lookout using a telescope could spot the flag flying from an incoming trading ship much sooner than with the unaided eye.

Galileo accordingly arranged a demonstration for the authorities. On August 25, 1609, he led a procession of Venetian senators across the Piazza San Marco and up the Campanile for their first look through his first telescope. As he recalled the scene:

Very many were the patricians and senators who, although aged, have more than once climbed the stairs of the highest campanili of Venice, to detect sails and vessels on the sea, so far away that coming under full sail toward the harbor, two hours or more passed before they could be seen without my eyeglass; because in fact the effect of this instrument is to represent an object that is, for example, fifty miles off, as large and near as if it were only five miles away.5

The senators, suitably impressed, doubled Galileo’s salary and granted him a lifelong appointment at Padua; as we would say today, Galileo got tenure. But his triumph was darkened by a cloud of deception. He permitted the senators to assume that he had invented the telescope. This was not strictly true, and his silence as to the stimulus of his greatest invention became embarrassing once telescopes produced by Dutch and Italian spectacle-makers began turning up in the marketplaces of Venice. In Bertolt Brecht’s play Galileo, Priuli the Venetian curator upbraids Galileo for his guile:


There it is—your “miraculous optical tube.” Do you know that this invention he so picturesquely termed “the fruit of seventeen years’ research” will be on sale tomorrow for two scudi apiece at every street corner in Venice? A shipload of them has just arrived from Holland.


Oh, dear! Galileo turns his back and adjusts the telescope.


When I think of the poor gentlemen of the Senate who believed they were getting an invention they could monopolize for their own profit…. Why, when they took their first look through the glass, it was only by the merest chance that they didn’t see a peddler, seven times enlarged, selling tubes exactly like it at the corner of the street.6

But while the senators trained their telescopes on the horizon, Galileo trained his on the night skies. He was the first scientist to do so (or one of the first; Thomas Harriot in England observed the moon through a telescope that same summer) and what he saw spelled the beginning of the end of the closed, geocentric cosmos, and the opening up of the depths of space.

As beginning observers have done ever since, Galileo looked first at the moon, and the sight of its mountains and craters immediately impressed him with the fact that it was not a wafer composed of heavenly aether, but a rocky, dusty, sovereign world. Aristotle to the contrary, the moon is “not robed in a smooth and polished surface,” wrote Galileo, but is “… rough and uneven, covered everywhere, just like the earth’s surface, with huge prominences, deep valleys, and chasms.”7

Turning his telescope to Jupiter, Galileo discovered four moons orbiting that giant planet, their positions changing perceptibly in the course of just a few hours’ observation. Jupiter, he was to conclude, constituted a Copernican solar system in miniature, and proof as well that the earth is not unique in having a moon. Galileo called it

a fine and elegant argument for quieting the doubts of those who, while accepting with tranquil mind the revolutions of the planets about the sun in the Copernican system, are mightily disturbed to have the moon alone revolve about the earth and accompany it in an annual rotation about the sun. Some have believed that this structure of the universe should be rejected as impossible. But now we have not just one planet rotating about another while both run through a great orbit around the sun; our own eyes show us the four stars [i.e., satellites, a term coined by Kepler] which wander around Jupiter as does the moon around the earth, while all together trace out a grand revolution about the sun in the space of twelve years.8

When Galileo observed the bright white planet Venus, he found that it exhibits phases like those of the moon, and that it appears much larger when in the crescent phase than when almost full. The obvious explanation was that Venus orbits the sun and not the earth, exhibiting a crescent face when it stands nearer to the earth than does the sun and a gibbous face when it is on the far side of the sun. “These things leave no room for doubt about the orbit of Venus,” Galileo wrote. “With absolute necessity we shall conclude, in agreement with the theories of the Pythagoreans and of Copernicus, that Venus revolves about the sun just as do all the other planets.”9

The greatest surprise was the stars. The telescope suggested, as the unaided eye could not, that the sky has depth, that the stars are not studded along the inner surface of an Aristotelian sphere, but range out deep into space. “You will behold through the telescope a host of other stars, which escape the unassisted sight, so numerous as to be almost beyond belief,” Galileo reported. Moreover, the stars were organized into definite structures, of which the most imposing was the Milky Way:

I have observed the nature and the material of the Milky Way…. The galaxy is, in fact, nothing but a congeries of innumerable stars grouped together in clusters. Upon whatever part of it the telescope is directed, a vast crowd of stars is immediately presented to view. Many of them are rather large and quite bright, while the number of smaller ones is quite beyond calculation.10

The phases of Venus, observed by Galileo through his telescope, proved that Venus lies closer to the sun than does the earth.

Galileo’s account of his visions through the telescope were first published in March 1610, in his Sidereus Nuncius, or Starry Messenger. The book was an instant success, and soon readers as far away as China were reading its reports of the rocky reality of the moon, the satellites of Jupiter, and the multitude of previously unseen stars in the sky. Here was observational evidence that we live in a Copernican solar system in a gigantic universe.

Galileo, who was principally a physicist and had been a Copernican before he ever looked through a telescope, understood that the task now facing science was to bring physics into accord with the reality of a moving Earth. The old anti-Copernican arguments had been turned inside out: Given that the earth really does rotate on its axis, why don’t arrows shot into the air fly off to the west, or east winds constantly blow across the land? Why, in short, does a moving Earth act as if it were at rest? Finding the answers to these questions would require a greatly improved understanding of the concepts of gravitation and inertia. Galileo struggled with both.

In Aristotelian physics, heavy objects were said to fall faster than light ones. Early on, probably while still at Pisa, Galileo had realized that this commonsensical view was wrong—that in a vacuum, where air resistance would have no effect, a feather would fall as fast as a cannonball.* Having no means of creating a vacuum, Galileo tested his hypothesis by rolling spheres of various weights down inclined planes. This slowed their rate of descent as compared to free fall, making it easier to observe that all were accelerating at approximately the same rate.

But these experiments, which form the basis for the myth of the Leaning Tower, served to verify rather than to instigate Galileo’s thesis. More important were his “thought experiments,” the careful thinking through of procedures that he could not actually carry out. To be sure, Galileo recognized, as he put it, that “reason must step in” only “where the senses fail us.” But since he lived in a time when the senses were aided by none but the most rudimentary experimental apparatus—he had, for instance, no timepiece more accurate than his pulse—Galileo found that reason had to step in rather often. In the words of Albert Einstein, the greatest all-time master of the thought experiment, “The experimental methods of Galileo’s disposal were so imperfect that only the boldest speculation could possibly bridge the gaps between empirical data.”12 Consequently it was more by thinking than by experimentation that Galileo arrived at new insights into the law of falling bodies.

His reasoning went something like this: Suppose that a cannonball takes a given time—say, two pulse beats—to fall from the top of a tower to the ground. Now saw the cannonball in half, and let the two resulting demiballs fall. If Aristotle is right, each demiball, since it weighs only half as much as the full cannonball, should fall more slowly than did the original, full-size cannonball. If, therefore, we drop the two demiballs side by side, they should descend at an identical, relatively slow velocity. Now tie the demiballs together, with a bit of string or a strand of hair. Will this object, or “system,” in Galileo’s words, fall fast, as if it knew it were a reconstituted cannonball, or slowly, as if it still thought of itself as consisting of two half cannonballs?

Galileo phrased his reductio ad absurdum this way, in his Dialogues Concerning Two New Sciences:

[Were Aristotle right that] a large stone moves with a speed of, say, eight while a smaller moves with a speed of four, then when they are united, the system will move with a speed of less than eight; but the two stones when tied together make a stone larger than that which before moved with a speed of eight. Hence the heavier body moves with less speed than the lighter; an effect which is contrary to [Aristotle’s] supposition. Thus you see how, from your assumption that the heavier body moves more rapidly than the lighter one, I infer that the heavier body moves more slowly.13

This line of reasoning pointed directly to the second major question facing post-Copernican physics, that of inertia. If a cannonball and a feather fall at the same rate in a vacuum, then what is the difference between them? There must be some difference: The cannonball, after all, weighs more than the feather, will make more of an impression if dropped on one’s head from atop the Leaning Tower, and is harder to kick along the ground. We would say today that the feather and the cannonball have differing mass, and that the amount of their mass determines their inertia—their tendency to resist changes in their state of motion. It is precisely because the heavier object possesses greater inertia that it takes longer for gravity to get it going, which is why it falls no faster than the lighter object. But these are Newtonian conceptions, unknown to Galileo, who had to make his way on his own.

Galileo’s thought experiment: According to Aristotle, if a one-pound cannonball falls a given distance in a given time (1), then if the ball is cut in half, each half-pound ball should fall less far in the same interval (2). But, reasoned Galileo, what happens if the two half-balls are attached, by a thread or a stick (3)? Thus was Aristotle’s physics of falling bodies reduced to absurdity.

Aristotle had defined half of the concept of inertia, that bodies at rest tend to remain at rest. This was sufficient for dealing with an immobile Earth, but was of no use in explicating the physics of an earth in motion in a Copernican universe. Galileo groped his way toward the other half of the concept—that bodies in motion tend to remain in motion, i.e., that the cannonball’s inertial mass makes it just as difficult to stop as to start. Sometimes he came close, as in his charming comparison of the residents of planet Earth with voyagers aboard a ship:

Shut yourself up with some friend in the main cabin below decks on some large ship, and have with you there some flies, butterflies, and other small flying animals. Have a large bowl of water with some fish in it; hang up a bottle that empties drop by drop into a wide vessel beneath it. With the ship standing still, observe carefully how the little animals fly with equal speed to all sides of the cabin. The fish swim indifferently in all directions; the drops fall into the vessel beneath; and, in throwing something to your friend, you need throw it no more strongly in one direction than another, the distances being equal; jumping with your feet together, you pass equal spaces in every direction. When you have observed all these things carefully … have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still.14

But here Galileo bogged down. He was still a captive of Aristotle’s erroneous supposition that the behavior of objects results from an internal tendency, or “desire,” rather than simply from their inertial mass and the application of force:

I seem to have observed that physical bodies have physical inclination to some motion (as heavy bodies downward), which motion is exercised by them through an intrinsic property and without need of a particular external mover, whenever they are not impeded by some obstacle. And to some other motion they have a repugnance (as the same heavy bodies to motion upward), and therefore they never move in that manner unless thrown violently by an external mover. Finally, to some movements they are indifferent, as are these same heavy bodies to horizontal motion, to which they have neither inclination … or repugnance…. And therefore, all external impediments removed, a heavy body on a spherical surface concentric with the earth will be indifferent to rest and to movements toward any part of the horizon. And it will maintain itself in that state in which it has once been placed; that is, if placed in a state of rest, it will conserve that; and if placed in movement toward the west (for example) it will maintain itself in that movement.15

Some of these words anticipate Newton’s explanation of inertia; bodies “placed in movement” tend to remain in motion, those “at rest” to remain at rest. Others remain ensnared in Aristotle’s dusty web, as when Galileo asserts that objects have an inherent “inclination” or “repugnance” for certain sorts of motion. Galileo never really freed himself of confusion on this point, and his “law” of falling bodies, stated in 1604 and often called the first law of classical physics, was fraught with error.

Galileo might have made more progress in understanding inertia and gravitation had he collaborated with Kepler. Kepler, too, had only part of the answer; he, like Galileo, thought of inertia chiefly as a tendency of objects to remain at rest, and, consequently, he conceived of gravity as having not only to hold planets in thrall to the sun but also to tug them along in their orbits. But he was ahead of Galileo in some ways, as when he proposed that the gravitational attraction of the moon is responsible for the tides. Galileo dismissed Kepler’s theories of gravity as mere mysticism. “I am … astonished at Kepler,” he wrote. “… Despite his open and acute mind, and though he has at his fingertips the motions attributed to the earth, he has nevertheless lent his ear and his assent to the moon’s dominion over the waters, to occult properties, and to such puerilities.”16

The differences between the two men were pronounced. Galileo was an urbane gentleman who loved wine (which he described as “light held together by moisture”), women (he had three children by his mistress, Marina Gamba), and song (he was an accomplished musician). Kepler sneezed when he drank wine, had little luck with women, and heard his music in the stars. The deep organ-tones of religiosity and mysticism that resounded through Kepler’s works struck Galileo as anachronistic and more than a bit embarrassing. Kepler suspected as much, and pled with Galileo to please “not hold against me my rambling and my free way of speaking about nature.” Galileo never answered his letter. Einstein remarked near the end of his life that “it has always hurt me to think that Galilei did not acknowledge the work of Kepler…. That, alas, is vanity,” Einstein added. “You find it in so many scientists.”17

Nowhere is Galileo’s disdain for Kepler more painful to recount than in the matter of the telescope. Kepler was by this time recognized as the most accomplished astronomer in the world, and his enthusiastic endorsement of Galileo’s Starry Messenger had helped stave off criticism by those who dismissed the telescope as a kalei-doscopelike toy that produced not magnification but illusion. (This was not an entirely unreasonable suspicion; Galileo’s early telescopes produced spurious colors, and they presented such a dim image, in so narrow a field of view, that it was not immediately obvious that they magnified at all.) But astronomy hereafter would require telescopes, and Kepler, though he understood the optical principles involved much better than Galileo did, could not obtain lenses of quality in Prague. With his customary earnestness and lack of restraint, Kepler wrote to Galileo in 1610, asking him for a telescope or at least a decent lens, “so that at last I too can enjoy, like yourself, the spectacle of the skies.”

O telescope, instrument of much knowledge, more precious than any scepter! … How the subtle mind of Galileo, in my opinion the first philosopher of the day, uses this telescope of ours like a sort of ladder, scales the furthest and loftiest walls of the visible world, surveys all things with his own eyes, and, from the position he has gained, darts the glances of his most acute intellect upon these petty abodes of ours—the planetary spheres I mean—and compares with keenest reasoning the distant with the near, the lofty with the deep.18

Galileo ignored Kepler’s entreaties. Possibly he feared that his observations might be eclipsed by what an astronomer of Kepler’s abilities could accomplish if he, too, had a telescope at hand. In any event, he had other fish to fry. He was busy parlaying his rapidly growing celebrity into a position at Cosimo de’ Medici’s court in Tuscany. He passed the request along to Cosimo’s ambassador, who advised him to, by all means, send the estimable Kepler a spyglass. Galileo instead told Kepler that he had no telescopes to spare, and that to make a new one would require too much time. Meanwhile, he was making presents of telescopes to royal patrons whose favor might advance his career. One of the beneficiaries of Galileo’s gifts, the elector of Cologne, summered in Prague that year and loaned Kepler his telescope. For one month, Kepler could gaze with delight at the craters of the moon and the stars of the Milky Way. Then the elector left town, taking the telescope with him.

Just when Galileo might have done the most to help bring physics to a Copernican maturity, he instead diverted his efforts to a quixotic campaign aimed at converting the Roman Catholic Church to the Copernican cosmology. Politics did not suit him, and soon he was demanding, like any blustering campaigner, that Copernicanism be accepted for little better reason than that he said it was correct. The old anti-Aristotelian was asking to be regarded as the new Aristotle, urging that it was now acceptable to ignore the planets in favor of the decree of a book, so long as the book was his own.

His situation grew more precarious when he abandoned the Venetian Republic for the glittering court at Tuscany, where he was named chief mathematician and philosopher to the grand duke. His friend Giovanni Sagredo warned him that he was making a mistake. “Who knows what the infinite and incomprehensible events of the world may cause if aided by the impostures of evil and envious men,” he wrote Galileo in a letter from the Levant, where he was serving as the Venetian consul. “… I am very much worried by your being in a place where the authority of the friends of the Jesuits counts heavily.”19 But Galileo could resist neither the glory nor the wealth of the Medician court, nor the prospect of being relieved of his teaching duties at Padua: “I deem it my greatest glory to be able to teach princes,” he wrote. “I prefer not to teach others.”20

The initial reaction against Galileo’s campaign came less from priests than from pedants. The reactionaries whom the world remembers for their obstinate refusal to look through his telescope —“pigeons” and “blockheads” as Galileo called them—were not clerics but professors, and they were worried less about impiety than about threats to their academic authority. The Church, initially, was more tolerant. The Vatican praised Galileo’s research with the telescope and honored him with a day of ceremonies at the Jesuit Roman College, and when a Dominican monk named Thommaso Caccini preached a sermon against Galileo in Florence, he was promptly rebuked by the preacher general of the Dominican Order, Father Luigi Maraffi, who apologized to Galileo for the fact that he was sometimes obliged “to answer for all the idiocies that thirty or forty thousand brothers may or do actually commit.”21

But Galileo’s appetites had evolved from praise to power. Carried away by zeal for his cause, he began insisting that the Copernican cosmology was sufficiently well established scientifically that Scriptures must be conformed to it. Cardinal Robert Bellarmine, Master of Controversial Questions at the Roman College and the greatest theologian of the day, had reservations on this score. He agreed, he wrote in a letter dated April 4, 1615, “that, if there were a real proof that the sun is the center of the universe, that the earth is in the third sphere, and that the sun does not go round the earth but the earth round the sun, then we should have to proceed with great circumspection in explaining passages of Scripture which appear to teach the contrary…. But,” he added, “I do not think there is any such proof since none has been shown to me.”22 In the absence of such a demonstration, Bellarmine cautioned Galileo, to teach Copernicanism as bald fact would be “a very dangerous attitude and one calculated not only to arouse all scholastic philosophers and theologians but also to injure our holy faith by contradicting the Scriptures.”23

Galileo replied that he could prove that Copernicus was right, “but how can I do this, and not be merely wasting my time, when those Peripatetics who must be convinced show themselves incapable of following even the simplest and easiest of arguments?”24

This was pure sophistry. Galileo did not, in fact, have definitive proof of the Copernican theory. What he proffered instead were a series of analogies (the planets go around the sun as Jupiter’s moons go around Jupiter, each is a world just as the moon evidently is a world, etc.) and the phases of Venus, which could be explained as readily by the geocentric model of Tycho as by the heliocentric model of Copernicus.

When in Rome, Galileo ridiculed the anti-Copernicans at every opportunity, and promised that he would finally reveal his irrefutable proof of the Copernican theory. This turned out to be his erroneous account of the tides—Kepler’s more nearly correct theory having, as usual, been ignored by Galileo. His friends, ecclesiastical and secular alike, warned him not to press the point too far. “This is no place to come to argue about the moon,” the Florentine ambassador cautioned him. Galileo persisted, regardless. “I cannot and must not neglect that assistance which is afforded to me by my conscience as a zealous Christian and Catholic,” he wrote.25

The result of his efforts was that the pope referred the matter to the Holy Office, which declared Copernicanism contrary to Scriptures and put Copernicus’s De Revolutionibus on the Index of forbidden books. Kepler, for once, lost patience. “Some,” he fumed, “through their imprudent behavior, have brought things to such a point that the reading of the work of Copernicus, which remained absolutely free for eighty years, is now prohibited.”26

Enjoined by the Church against espousing Copernicanism, Galileo returned to Florence and there wrote Il Saggiatore (The Assayer) a sarcastic attack on the Jesuit thinker Horatio Grassi. In doing so he added to his growing list of enemies many Jesuits who had been among his allies. (Cardinal Bellarmine, the most powerful of the Jesuits sympathetic to Galileo, had by this time died.)

In 1623, in what seemed a stroke of good fortune, Galileo’s friend and admirer Maffeo Barberini was elected pope. Intelligent, vital, learned, and vain, Barberini had much in common with Galileo. As Galileo’s biographer Arthur Koestler writes, the pope’s “famous statement that he ‘knew better than all the Cardinals put together’ was only equalled by Galileo’s that he alone had discovered everything new in the sky. They both considered themselves supermen and started on a basis of mutual adulation—a type of relationship which, as a rule, comes to a bitter end.”27 Galileo enjoyed six audiences with the new pope, Urban VIII, and was rewarded with lavish gifts and a declaration of “fatherly love” for “this great man, whose fame shines in the heavens.”28 Warmed by the newly risen papal sun, Galileo spent the next four years writing an exposition of the Copernican manifesto, his Dialogue … Concerning the Two Chief World Systems, Ptolemaic and Copernican. Cleared by Church censors, chief among whom now was Galileo’s former pupil Father Niccoló Riccardi, it was published in 1632.

The dialogue form was a device, transparent as Aristotle’s crystalline spheres, through which Galileo could argue for Copernicanism without violating the letter of the papal edict. Two of the conversante, Salviati and Sagredo, are learned gentlemen who sympathize with the Copernican scheme; they serve to speed the argument along on wheels of mutual agreement. Simplicio, the third participant, represents the Scholastics, and is presented as little better than a fool. In a typical passage, Simplicio maintains that “if the terrestrial globe must move in a year around the circumference of a circle—that is, around the zodiac—it is impossible for it at the same time to be in the center of the zodiac. But the earth is at that center, as is proved in many ways by Aristotle, Ptolemy, and others.” To which Salviati, dripping sarcasm, replies: “Very well argued. There can be no doubt that anyone who wants to have the earth move along the circumference of a circle must first prove that it is not at the center of that circle.”29

Galileo’s enemies were quick to point out to the pope that the official cosmology of the Roman Catholic Church had been put into the mouth of the Simplicio the simpleton. It is Simplicio, for instance, who gives voice to a (scientifically accurate, by the way) statement that the pope had ordered inserted into the manuscript, to the effect that Galileo’s theory of the tides does not prove that the earth revolves on its axis. The pope, angered, ordered an investigation, and in August 1632, the Inquisition banned further sales of the Dialogue and ordered all extant copies confiscated.

Galileo responded with the political naivete that was fast becoming his hallmark. He prevailed upon his protector, the grand duke of Tuscany, to send the pope a strongly worded objection to the ban. The pope, who had been elected with the support of Francophile cardinals, was under attack from pro-Spanish factions in the Vatican—a controversy sufficiently heated that he feared assassination—and Galileo’s duke supported Spain. The letter presented the pope with an irresistible opportunity to demonstrate his resolve by quashing an ally of the Francs. The only cost would be his friendship with Galileo, a brilliant but increasingly troublesome old man.

Thus was the clutch released from the wheels of persecution.* Galileo was ordered to appear before the Inquisition in Rome, either voluntarily or to be brought “to the prisons of this supreme tribunal in chains.” He confidently awaited intervention by his friend the pope; it never came. He took refuge for a time in the thought that “everyone will understand that I have been moved to become involved in this task only by zeal for the Holy Church, and to give to its ministers that information which my long studies have brought to me.” The ambassador, whose predecessor had warned him that Rome was “no place to argue about the moon,” quietly acquainted Galileo with the facts of life. There would be no debate concerning the scientific merits of the Copernican system. The issue was obedience. Too late, Galileo realized his position. “He is much afflicted about it,” the ambassador reported back to Florence. “I myself have seen him from yesterday to the present time so dejected that I have feared for his very life.”30

Galileo, now seventy years old, was interrogated at length and threatened with torture. The case against him was sealed by forged “minutes” of his 1616 meeting with Cardinal Bellarmine, reporting that he had been enjoined from holding, teaching, or defending Copernicanism in any way, even as a hypothesis. This was stronger than the warning that had in truth been given him at the time. Left defenseless, Galileo took the only reasonable option available to him, and on June 22, 1633, he recited the prescribed abjuration, from his knees, in the great hall of the Dominican convent of Santa Maria Sopre Minera:

Wishing to remove from the minds of your Eminences and of every true Christian this vehement suspicion justly cast upon me, with sincere heart and unfeigned faith I do abjure, damn, and detest the said errors and heresies, and generally each and every other error, heresy, and sect contrary to the Holy Church; and I do swear for the future that I shall never again speak or assert, orally or in writing, such things as might bring me under similar suspicion….31*

Galileo spent the remaining eight years of his life under house arrest in his villa outside Florence. There he wrote his finest book, the Dialogues Concerning Two New Sciences, a study of motion and inertia. His daughter Sister Marie Celeste, whom he had sent to a convent against her wishes twenty-three years earlier, stayed with him and said the seven daily psalms of penitence ordered by the Holy Office as part of his sentence. He observed the moon and planets through his telescope up until only a few months before he lost his sight, in 1637. “This universe that I have extended a thousand times … has now shrunk to the narrow confines of my own body,” he wrote.33

Milton visited Galileo, and may have gained from him something of the sense of vast spaces that permeates Paradise Lost. Milton’s universe, however, remained earth-centered, and his poem contains a warning against cosmological presumption. In it, a Miltonic angel advises Adam:

Sollicit not thy thoughts with matters hid,
Leave them to God above, him serve and feare;
Of other Creatures, as him pleases best,
Wherever plac’t, let him dispose: joy thou
In what he gives to thee, this Paradise
And thy fair Eve: Heav’n is for thee too high
To know what passes there; be lowlie wise:
Think onely what concernes thee and thy being;
Dream not of other Worlds.34

But that paradise had indeed been lost. Humankind was awakening from a dream of immobility to find itself in a waking fall, its planet plummeting through boundless space. The weight of authority that brought Galileo to his knees succeeded only in halting the growth of science in the Mediterranean. Thereafter, the great advances came in the north countries. The physics of the Copernican universe was to be elucidated by Isaac Newton, born in Woolsthorpe, Lincolnshire, on Christmas Day, 1642, the year of Galileo’s death.

* Ruled not by a feudal aristocracy but by a thriving merchant class, Venice was relatively liberal, innovative, and inquisitive, an excellent place for a freethinker like Galileo. The difference was evident in the way the anatomy classes were conducted: The proscription against dissection, generally obeyed in Pisa, was circumvented at Padua by means of a laboratory table that could be lowered to an underground river, where corpses brought to the university by boat in the dark of night were raised into the hall for dissection in the advanced anatomy class. Proctors kept a lookout, and if the authorities approached the body was lowered away, its place was taken by the usual volume of Hippocrates or Galen, and the lecturer resumed teaching in the conventional fashion.

* He was not unprecedented in making this suggestion. Lucretius in the first century B.C. wrote that “through undisturbed vacuum all bodies must travel at equal speed though impelled by unequal weights,” and some of Galileo’s Renaissance colleagues had proposed the same hypothesis. But none argued for it as convincingly, or investigated the question with greater experimental care, than did Galileo. And, in any event, there is more to science than precedence. As Whitehead remarked, “Everything of importance has been said before by somebody who did not discover it.”11

* Pietro Redoni argues, in his book Galileo: Heretic, that Vatican objections to Galileo may have had less to do with Copernicanism than with his advocacy of atomism and a corpuscular theory of light. Certainly the motives behind Galileo’s persecution were complicated, and are likely to be debated among historians for some time yet to come.

* Three centuries later, in 1980, Pope John Paul II ordered a reexamination of the case of Galileo. Speaking at a ceremony honoring the centenary of Einstein’s birth, the Pope declared that Galileo had “suffered at the hands of men and institutions of the Church,” adding that “research performed in a truly scientific manner can never be in contrast with faith because both profane and religious realities have their origin in the same God.”32