by John Heilbron
Oxford, 528 pp., $34.95

Watcher of the Skies
by David Wootton
Yale, 354 pp., $35


In 1610, Galileo published The Starry Messenger, a brief report of wonders seen through his state‑of‑the‑art telescope: mountains on the moon, vast multitudes of stars invisible to the naked eye, moons circling Jupiter, earthshine (the illumination of our moon by sunlight reflected from Earth). It promised a future work to refute “those who argue that the earth must be excluded from the dancing whirl of stars” and show that it is “a wandering body surpassing the moon in splendor, and not the sink of all dull refuse of the universe.”

Earth was a “sink of all dull refuse” according to then-dominant Aristotelian cosmology and physics, whereby each of the four elements (earth, water, air, fire) composing terrestrial objects has a proper place and, unless obstructed, proceeds by “natural motion” toward it. The proper place for the element earth is the center of the universe. The coarse and gross objects in which it predominates are heavy because they naturally move toward that center and thereby accumulate on Earth. Bodies composed of these four elements are confined to the region between Earth and the moon​—​the “sublunary sphere”​—​which, since the four can metamorphose into one another, is the domain of change and decay. The sun, moon, planets, and stars​—​which circle Earth in the superlunary heavens​—​are composed of the fifth element, unchanging ether.

The Aristotelian account is consistent with Ptolemy’s geocentric model of the universe, but cannot be reconciled with a scarred and pitted moon, an Earth that shines like a star, or a planet having moons of its own. The Starry Messenger tells us, says David Wootton, that “the earth is a heavenly body and the heavens contain earthly bodies.”  The 400th anniversary of its publication has been the occasion for two substantial biographies of Galileo, both scholarly but accessible to the general reader.

John Heilbron depicts a Renaissance humanist: not only a great scientist but a skilled musician and artist; a polemicist, literary stylist, poet, and playwright; a brilliant debater; a lover of wine. He shows us Galileo’s intellectual and social circles, complicated politico‑theological intrigues, court spectacles intended “to stupefy every viewer with their grandeur,” and the chaos within Galileo’s (at best) semifunctional family. David Wootton provides an intellectual biography concerned primarily with the development of Galileo’s scientific ideas. He gives lucid accounts of the intellectual stakes in Galileo’s scientific controversies and striking interpretations (and reinterpretations) of important historical questions. Scholars will evaluate the book’s novelties; to this nonscholar, most of its pages ranged from absorbing to dazzling.

 A quick example: Galileo’s contemporaries had good reasons to be cautious about astronomical evidence from telescopes. Its reliability could not be directly verified by comparing telescopic images of the heavens with close-up observation by the naked eye; nor could it be inferred from experiments with terrestrial objects​—​since similarity of the celestial and terrestrial realms is precisely the matter in dispute. But, says Wootton, Galileo faced a more radical difficulty because he lived in a culture unlike our own, one in which seeing was not believing. Sight was the most easily deceived of the senses​—​what was perspective drawing but systematic trickery?​—​so why trust it “to provide information on a world [we] would never touch, hear, or smell”?

When The Starry Messenger appeared, Galileo was a 46-year-old mathematics professor at the University of Padua who had published nothing significant. Yet he had by then made all his major discoveries and founded the very notion of an experimental science. How had he thought his way out of the Aristotelian physics and Ptolemaic astronomy of his teachers and mentors? Answers must be conjectural: We lack not only published work but, for some important periods, even notes and papers, and those that survive can be difficult to date.

Galileo dedicated his booklet to the grand duke of Tuscany and became the duke’s official mathematician and philosopher​—​giving up a lifetime position at Padua and the protection that the powerful Republic of Venice would have offered against the Roman Inquisition. Principally, Heilbron and Wootton agree, he wanted access to the Jesuits so that he could convert the intellectual world of Rome to Copernican astronomy.

Heilbron’s Galileo is an artist. His genius as an observer lay in the ability to interpret what he saw, and it was skill in drawing and perspective and chiaroscuro that helped him recognize changing contrasts visible on the surface of the moon as plays of light and shadow on a mountainous landscape. His first known public lecture discussed the geometry of Dante’s hell. He loved Ariosto’s Orlando Furioso and his critical essay on Ariosto and Tasso is still read. Heilbron finds significance in what Galileo criticizes about Tasso (an alleged poverty of invention) and admires in Ariosto (“the realistic treatment of the marvelous”). He suggests that Galileo could not respond to Tasso’s psychological depth, preferring to judge fictional characters (as he judged real-life friends and opponents) in black and white terms​—​and that this preference for surfaces is one psychological root of his physics of “accidents” rather than “essences,” of laws rather than causes.

Heilbron traces the evolution of Galileo’s early scientific views from notes and drafts for an abandoned work on motion. These ridicule many Aristotelian propositions but argue primarily within a framework of Aristotelian (or Aristotle-friendly) categories such as natural motion. Heilbron imagines a dialogue between Galileo and an alter ego to illustrate how Galileo might have arrived at his radically different physics, rejecting not only Aristotle’s answers but also his questions. Galileo developed not a dynamics that explains what causes motion but a quantitative kinematics that relates different measurable properties of motion: time, distance, speed.

 A central concern is what we would now call free fall. He produces different deductions that begin from different (and incompatible) hypotheses, but conclude with the same law relating time and distance of fall. From this, according to Heilbron’s dialogue, he draws a moral: Mathematical exploration shows its power by leading to a precisely formulated kinematic law; that law is verified by experiment; and the fact that it could be deduced from different hypotheses, corresponding to different underlying causal mechanisms, shows that consideration of causes can be deferred. The role of experiment is merely illustrative, “conclusive when it confirms a quantitative rule, but only an inconvenience when it does not.”  Laws describe ideal, artificial, situations. Disagreement with experiment may simply show that ideal conditions are too difficult to reproduce.

I am grateful that Heilbron lays out these arguments in detail but found some of them difficult to follow. Pulling rank as a professional mathematician, I’d say that he sometimes gives us the words without the music, allowing those details to obscure essentials. (An unsound argument of Galileo’s is sometimes identified as such only in an endnote.) Wootton says his account is novel in emphasizing Galileo’s “reluctant empiricism,” “early Copernicanism,” and “private irreligion.” The reluctance, also illustrated in Heilbron’s dialogue, comes from a deep-seated preference for deductive reasoning over empirical tests. Wootton argues that the never‑resolved tension between that gut feeling and Galileo’s wish to ground knowledge in sensory experience was a fruitful one.

Galileo’s Copernican views were formed not merely early in his life but well before the empirical evidence was compelling, when all available astronomical data were consistent with Tycho Brahe’s geo‑heliocentric model: The moon and sun circle Earth, and the planets circle the sun. Tycho’s system had the scientific advantage that a stationary Earth did not require a radically new physics to explain everyday motion on Earth and the political advantage of being theologically orthodox. Yet something other than science must account for Galileo’s adamant insistence that Copernicus provided not merely the best astronomical model but the truth. Here, says Wootton, his science and irreligion were mutually supportive, each a rejection of the view “that the world was made for man, and that man was made in the image of God.” (He finds the psychological roots of that rejection in Galileo’s relations with his Gorgon-like mother.)

Against the commonly accepted view that Galileo was a pious Catholic, Wootton claims to establish a “very strong presumption” that he was not even Christian. The question is important not only to Galileo’s story but also to the significant body of scholarship on the Christian roots of the scientific revolution. Wootton’s arguments include the fact there is little record of Galileo expressing piety except when his piety was questioned and the claim that Galileo’s old and close friend Benedetto Castelli did not believe him to be Christian. The evidence for that claim is a letter Castelli sent to the aged Galileo expressing joy on hearing that Galileo had accepted Christ. The letter mentions the parable of the laborers in the vineyard and Luke’s account of the thieves crucified beside Jesus, both of which are traditionally understood as stories about last-minute conversions.

Wootton calls this letter his strongest argument, and it seems strong to me. But contrary evidence exists; for example, Galileo’s pilgrimage to the House of the Virgin Mary in Loreto in thanks for recovery from an illness, his request (accepted) that the Holy Office of the Inquisition not require him to confess to having lapsed from the behavior of a good Catholic. Let the scholars weigh in.

Wootton’s close readings are typically illuminating, but can seem a stretch, as when he interprets Galileo’s youthful satire Against the Wearing of the Toga in light of his conclusions about Galileo’s irreligion. This poem, mocking the requirement to wear academic robes, declares that in the golden age we were all naked and clothing was invented by the devil for concealment and deceit. Wootton argues that, since Genesis presents clothing not as diabolical but as “the first expression of shame,” it associates nakedness with the state of Eden. And therefore, by praising nudity, Galileo says there is no such thing as sin (or salvation): “His aversion to the wearing of the toga is only a pretext for a poem which attacks Christianity itself.” To Heilbron it is, more plausibly, an exercise in a bawdy, anticlerical genre.

Looming over any account of Galileo’s life is his trial, whose crucial events unfolded over many years. In 1616, seven decades after its publication, Copernicus’s On the Revolutions of the Celestial Spheres was placed on the Index of Forbidden Books pending “correction” of several sentences, so that it would present the heliocentric system as a “hypothesis” rather than a “truth.” Galileo, a conspicuous advocate for Copernicus, was warned that a heliocentric model must not be held or taught as true. The main theological sticking point was not a moving Earth, something merely anti-Aristotelian, but a stationary sun​—​which contradicted, for example, the biblical story where Joshua asks the Lord to make the sun stand still.

In 1623, Galileo’s longtime acquaintance and admirer Maffeo Barberini became Pope Urban VIII, received him royally, and encouraged him to complete the works eventually published as the Dialogue on Two World Systems and Two New Sciences. (Wootton characteristically asks why only two systems​—​Ptolemy and Copernicus, but not Tycho​—​and sees a rhetorical strategy: demolish Ptolemy in a way that presents Copernicus as the only alternative.) The pope’s approval came with a condition. The Dialogue must conclude by affirming his view that astronomy and theology could not be incompatible because astronomical models could be neither true nor false. The observed phenomena can be explained by many different models, and God could choose any of them, so to insist on one was not only to go beyond the evidence but to limit God’s omnipotence. Permission to publish the Dialogue was obtained without close scrutiny of all the final text, which put the pope’s views in the mouth of the dullard Simplicio.

The book was published in 1632. The Inquisition soon issued a summons. According to a document then retrieved from the Vatican archives, Galileo had (in 1616) received not just a friendly warning to observe the difference between a “hypothesis” and a “truth” but a formal injunction forbidding him even to discuss Copernicus hypothetically (though a document in Galileo’s possession seemed to contradict that). He was charged with disobeying that injunction.

Vincenzo Maculano, the Inquisition’s commissioner general, held an informal, unrecorded meeting with Galileo, intended to let Galileo off the hook without undermining the tribunal’s authority. Galileo agreed to plead guilty of appearing to defend Copernicus; he expected no punishment except the banning of his book. Wootton speculates on the details of that meeting: Galileo would surely have been threatened with torture, but that threat was present all along ​—​torture being a best practice in both civil and ecclesiastical inquiries. What new threat could have changed Galileo’s mind? Additional charges, Wootton suggests. He backs that up with detective work, finding in the Vatican archives a recommendation that Galileo be investigated for denying the fundamental dogma of transubstantiation. Wootton argues that the recommendation, although not part of the trial record, preceded the trial and was not pursued because Maculano used it as a bargaining chip.

The pope insisted that Galileo be found guilty of “vehement suspicion” of heresy​—​thereby making Copernicanism retrospectively heretical​—​and be required, humiliatingly, to abjure. The Dialogue was banned and Galileo placed under house arrest for the rest of his life, spent mostly at his villa in Arcetri. There he continued to work, smuggling out Two New Sciences for publication in 1638. Of course, Copernicus did not ultimately need Galileo’s help. And to be a Copernican, you don't have to live in the new mental world Galileo introduced in this volume, the truly revolutionary book that founds modern mathematical physics.

Heilbron and Wootton agree that the collision between Galileo and Urban, though not inevitable logically, may have been so psychologically. It was not a conflict, Wootton says, of impersonal forces and institutions but a falling-out between friends. Given the state of knowledge, the limitation to teach Copernicanism as a useful hypothesis, and not a demonstrated truth, was perfectly reasonable. Galileo overreached and brought his troubles on himself: “In the world of Counter Reformation Italy, heresy often went unpunished; disloyalty and ingratitude  .  .  . were never tolerated.” Heilbron puts it symbolically: The Latin title of Galileo’s booklet can be translated as Starry Messenger or Starry Message. Galileo originally thought of himself as reporting a message, but in his “megalomaniacal middle age” came to see himself as a prophetic messenger, or a knight errant in his favorite poem.

A lay reader can find much pleasure and profit in either of these volumes, which are mind‑expanding in a way that popular accounts of current physics and cosmology cannot be. They invite us to recapture, imaginatively, the position of someone who has not​—​as you almost surely have​—​adopted modern science as an unexamined prejudice.


David Guaspari is a writer in Ithaca.