A right thumb, a finger, a tooth. These were the contents of a reliquary acquired several years ago by a collector at an auction in Florence. Little did he know that for centuries the remains had been objects of profane devotion. Last seen in 1905, they had been sliced from the corpse of Galileo, along with another finger and a vertebra, during his highly publicized reburial in the Basilica of Santa Croce in 1737 almost 100 years after his death, and preserved in a slender case fashioned of glass and wood and crowned with a carved bust of the scientist. The reliquary’s new owner consulted Galileo experts about his find, and after the authenticity of its contents had been verified he donated it to the Museo Galileo, which is tucked behind the Uffizi in a quiet piazza overlooking the River Arno. (A dentist asked by the museum to examine the tooth concluded that Galileo suffered from gastric acid reflux and ground his teeth in his sleep.) The rediscovered reliquary is displayed adjacent to a smaller one containing Galileo’s other finger, a prized museum possession since 1927. Nearby are several artifacts of Galileo’s scientific genius: a telescope presented to the Medici and the broken objective lens of the original device with which Galileo sighted Jupiter’s four satellites in 1610.

Galileo was not the first scientist whose corpse was as revered as his corpus. That honor belongs to René Descartes, who was reburied numerous times after his death in 1650, initially to secure the return of his body to French soil and subsequently to install him in the pantheon of French genius. Yet Galileo’s remains in Florence have an added meaning. In 1633 the scientist was tried for heresy, having been accused of violating a 1616 papal decree condemning as contrary to Scripture the idea of a heliocentric universe, first described by Copernicus in On the Revolutions of the Heavenly Spheres (1543). The Florentines who snatched a few of Galileo’s bones in 1737 sought to canonize the scientist as a counter-saint, even as the Roman Catholic Church, with a century of hindsight, relented on its decision to deny Galileo a public burial and monument worthy of his fame when he died. Times were changing, but not rapidly enough for Galileo’s most ardent disciples. Their veneration of a few body parts privately commemorated his martyrdom for the cause of science. The church’s interment of his other remains in a sepulcher adjacent to Michelangelo’s in Santa Croce designated him a heroic embodiment of Tuscan genius and creativity.

Understanding Galileo has been the task of historians ever since he became a mythical figure. His youngest disciple, Vincenzo Viviani, spent more than half a century trying to get his biography right, never quite managing to meet his own impossibly high expectations of how to write about a great scientist. Bertolt Brecht was so mesmerized by the particulars of Galileo’s life that he wrote three versions of it for the stage, the first while living in exile in Denmark, the second while in postwar America after Hiroshima and Nagasaki, and the third during his voluntary exile from McCarthy’s America in communist East Berlin. Brecht’s Galileo was simultaneously the victim of a tragedy perpetuated by his society, and the tragedy himself. As Brecht witnessed the evolving role of the scientist in the mid-twentieth century, he began to see similarities between Galileo and J. Robert Oppenheimer, who paid a high price for attempting to work on the Manhattan Project while resisting its core values. In this respect, it may be fairly said that Brecht got Galileo right more than any modern historian in recognizing that he belonged to the ages, and that our perspective on him would be ever changing.

The publication of two recent biographies of Galileo, by John Heilbron and David Wootton, coincided with the 400th anniversary of the publication of Starry Messenger (1610), the treatise in which Galileo reported the astronomical observations he had made with the instrument not yet called the telescope. Heilbron, a distinguished historian of physics and mathematics, has spent many years studying the relations between science and religion, including how the Roman Catholic Church stimulated and materially supported a research program of Catholic astronomy. Wootton has previously written on the history of atheism and unbelief, and about Galileo’s controversial Venetian friend Paolo Sarpi—a theologian and tireless critic of the papacy. In Venice there is a statue of Fra Paolo in Campo Santa Fosca commemorating his survival of a botched assassination attempt in October 1607. The cutthroats were sheltered and paid by Rome, yet Sarpi continued to defend freedom of thought and belief, both in conversation and in print, and to discuss science with Galileo. In Heilbron’s account, Galileo is a versatile connoisseur and critic; in Wootton’s, he is all but a modern scientist without faith.

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Before the appearance of Starry Messenger, Galileo was known as a poorly dressed, occasionally sarcastic and mechanically adroit college dropout who kept a mistress and had sired three illegitimate children. He admired and imitated the prose of Dante, Machiavelli and Ariosto; he enjoyed reading poetry and liked to draw and tell a good joke. He learned a fair bit of music from his father, Vincenzo Galilei, a Medici court musician, yet rebelled against his father’s desire that he become a physician. He frequently quarreled with his mother, Giulia Ammannati, who seems to have thought that a session with the Florentine Inquisitors might curb her son’s insolence. Wootton makes these fraught familial relations the basis of his depiction of Galileo as a proud, stubborn and sensitive man, a portrait reminiscent of Arthur Koestler’s 1959 account of Galileo as an anti-hero. But Wootton overreaches when he makes some imaginative and not particularly well-substantiated hypotheses about a third (illegitimate) daughter and a late-blooming love affair.

In his formative years Galileo cultivated a highly fertile geometric imagination that would nourish his study of mathematics and physics, and especially mechanics. His invention, when he was in his 20s, of a lightweight hydrostatic balance earned him the admiration of senior mathematicians in Italy. Heilbron lovingly explores Galileo’s resourcefulness by explaining, recalculating and diagramming all his most important insights into the nature of things. This work has been done piecemeal by other historians of science, but it is Heilbron’s accomplishment to have created a complete, accessible yet technical synthesis of Galileo’s findings. Such reading is not for the mathematically faint of heart, but it is essential for understanding Galileo’s science. By integrating this material into a sharp-witted and ironic narrative of Galileo as a man of culture and learning, Heilbron portrays Galileo as a child of the Renaissance, a man who saw the lunar mountains not only through the lens of his telescope and by the point of his compass but also in the context of Ariosto’s fantastic descriptions of them in Orlando Furioso.

In 1589 Galileo became a mathematics professor—no degree was required for the sixteenth-century version of this job—first in Pisa (1589–92) and then in Padua (1592–1610), though he was never fully comfortable with university life. Wootton presents Galileo’s years in the Venetian Republic as the period when his best work ripened, even if he had yet to enjoy the international celebrity and recognition that would come with his triumphant return to Florence in 1610. Galileo had a lot of fascinating ideas but virtually no publications or discoveries of any particular significance to his name; his meager salary was devoured by bills. But he did have ambitions. In Venice he received a patent for a horse-driven pump, but nothing came of it. Like all good seventeenth-century astronomers, he cast horoscopes (the Paduan Inquisitors who investigated new charges against him in 1604 found this activity benign because he did not prognosticate the future). He taught, and talked with his Venetian friends.

Eventually Galileo’s efforts began to pay off. Between 1597 and 1604 he perfected a geometric and military compass—a versatile sector with which to calculate interest and exchange rates, extract square roots, measure volumes and distances, and design to scale. He ran out of town an ex-student who had tried to plagiarize a pamphlet he sold to students to supplement his income. The next year he won a plum assignment teaching mathematics to the Medici prince who, as Grand Duke Cosimo II, would become an important patron.

In 1609–10 the world changed for Galileo. Drawing on reports of a Dutch spyglass that magnified things at a distance, he made a powerful observational instrument through which new data poured from the heavens. He rushed to publish The Starry Messenger, which earned him a substantial raise and a new position with no teaching responsibilities at the Medici court in Florence. He revealed to people the craters and valleys of the moon, the many stars of Orion’s belt and the Milky Way; he named the four satellites of Jupiter the Medicean stars in honor of his most important patron. He won the admiration of the German mathematician and astrologer Johannes Kepler and eventually convinced skeptical astronomers such as the renowned Jesuit mathematician Christopher Clavius that he had found novelties in the heavens.

Galileo’s writings about his spectacular celestial discoveries made him justly famous as a pioneer of modern science. They also delayed by more than a quarter-century the publication of Two New Sciences (1638), an account of his fundamental insights into the physical world. Its ideas are commemorated every time a high school physics class does experiments with inclined planes and pendulums, or charts the parabolic path of a projectile, or discusses the problem of inertia and the acceleration of falling bodies.

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A crucial question for Galileo’s biographers has been how a mathematical physicist became the brilliant observational astronomer and advocate of a new and increasingly controversial cosmology. In Wootton’s reconstruction Galileo turned Copernican in Padua between 1592 and 1597, when his reading of the Polish polymath’s On the Revolutions of the Heavenly Spheres, among other works, made him realize that the physics of motion and the question of a moving earth were interrelated problems. Drawing on Sarpi’s unpublished notebooks to eavesdrop on the theologian’s conversations with Galileo, Wootton makes the case that Galileo began aspects of his controversial Dialogue Concerning the Two Chief World Systems (1632), especially his theory that tides were a product of the earth’s movements, as early as 1592. The nature of the freewheeling and philosophically probing conversations that occurred in the Venetian Republic is essential to Wootton’s argument, because it was in this environment that Galileo first opened his mind to new and daring ideas.

Heilbron agrees that this period of Galileo’s life was crucial to his refutation of geocentrism but is not convinced that it was more important than his experiences before 1592 or during the fertile period of 1610–15, when he returned to Florence and found allies in Rome, including the young aristocrat Federico Cesi, who enthusiastically admitted Galileo to his Academy of the Lynxes and began to underwrite the cost of Galileo’s publications. Heilbron provisionally dates the origins of Galileo’s Copernican sympathies to his time in Pisa, and points to his later reading of Kepler’s brilliant and imaginative synthesis of astronomy, physics and mathematics, The Cosmic Mystery (1596), as an experience that led him to consider the full complexities of celestial motions.

Wootton’s Galileo has a churlish theoretical mind focused on understanding first principles. Heilbron’s Galileo is restless and pragmatic—always on the move, in conversation with many people, hedging his bets. Both portraits have their merits. Wootton is right to present Sarpi as the ghost in the machine, bringing Galileo news of the Dutch spyglass from an associate in Paris, witnessing Galileo’s observations of Jupiter and worrying that Galileo lacked Kepler’s refined understanding of optical theory. Galileo’s Venetian friends understandably felt betrayed upon hearing of his decision to defect to Florence after having pocketed plenty of Venetian ducats to build a new and improved telescope. They warned him that Florence was too near Rome—and the Vatican—and that Rome could never be as tolerant of his scientific investigations as Venice. With hindsight, they understood Galileo perhaps better than he understood himself.

Heilbron demonstrates how Galileo adroitly developed a new way of writing about science in a prose style of such suppleness that it is still taught in Italian schools. In the university towns of Italy, Galileo and his friends had laughed at those philosophers who still wore togas, declaimed venerable old truths in hoary Latin and seemed disconnected from the new realities of the physical world. The Medici Grand Duke supported Galileo, and his Roman patron Cesi encouraged him to debate his opponents. A youthful and admiring cardinal of Florentine origin named Maffeo Barberini wrote poetry about Galileo’s discoveries. When he became Pope Urban VIII in 1623, everyone agreed that his poetry was sublime. When he had Galileo’s Dialogue Concerning the Two Chief World Systems confiscated and prohibited, and its author tried for heresy, condemned and placed under permanent house arrest from 1633 until his death in 1642, people ceased to talk about his paean to science.

Heilbron and Wootton carefully reconstruct the events leading up to Galileo’s trial, beginning with the uneasy climate of 1615, when Florentine Dominicans accused Galileo and his followers of unorthodox beliefs and made known their discontent about Galileo’s friendship with Fra Paolo. Wootton argues that the accusations were probably true, a claim that adds ballast to his case for presenting Galileo as an unbeliever rather than a good, if imprudent, Catholic. Heilbron reminds us that the Medici Grand Duke personally certified Galileo’s status as an observant Catholic, quoting Galileo’s statements about the purity of his intent and the Venetian ambassador’s description of Galileo in Rome as a changed man who frequently took Communion. He also notes that several years after the Vatican’s 1616 condemnation of Copernicus, Galileo took a pilgrimage to Loreto to make a votive offering at the Virgin’s shrine in the hope that his health would improve. Heilbron’s Galileo becomes ever more Catholic in light of mounting questions about his piety and orthodoxy. By contrast, Wootton’s Galileo is another Sarpi (or possibly a precursor of David Hume), going through the motions of faith to save his skin while waiting for the right moment to unleash a radically naturalistic account of miracles that would further rationalize, if not topple, Christian faith.

How to explain this divergence? In his desire to make a strong argument about Galileo’s relationship to faith, Wootton transforms a few opaque phrases in Galileo’s correspondence into a lost treatise on miracles that makes his Letter to the Grand Duchess Christina—Galileo’s famous 1615 manifesto declaring science and religion to be separate but complementary truths—sound like the product of a pious but misspent youth. According to Wootton, Galileo’s experiences of 1615–16 radicalized his soul. Put a different way, he rediscovered his inner Venetian while conforming outwardly to Roman appearance.

Galileo would have been in good company wearing such masks. Heilbron reminds us that Sarpi described himself in just these dualities. But in Heilbron’s portrait Galileo is not so subtle or cunning in his actions. He is instead a scientific Don Quixote, charging forward where no one ought to tread. Galileo did not shy away from seeking material comfort from the church. In 1630, as he put the finishing touches on his long-awaited Dialogue, he began to wear a clerical tonsure and to recite the Divine Office in order to reap the benefits of a church pension his son had neglected to take up. Its monetary value would be reduced in 1633 by the irascible Barberini pope. None of these outwardly pious acts are proof that Galileo was a spiritually devout man. Yet throughout his life he was surrounded by clerical friends and disciples; not every Catholic scientist in religious orders was Galileo’s enemy, and there were many different ways to be a Catholic in good standing.

While Galileo cannot be transformed into a saint—though Heilbron cannot resist comparing Galileo’s modern rehabilitation by the Roman Catholic Church to the process of canonization—he should not be seen as a mirror image of Sarpi, let alone the second coming of the radical cosmologist, theosophist and Catholic martyr Giordano Bruno, whose death in 1600 atop a funeral pyre lit by the Roman Inquisition was singed into popular memory. Galileo certainly read Bruno’s dialogues, borrowed aspects of his witty theatrical style of writing and carefully avoided any discussion of the plurality of worlds, gods and souls, subjects that got Bruno burned. But he was not trying to rewrite faith, though in retrospect his Letter to the Grand Duchess became the basis for a new approach to the relationship between knowledge and faith. He was an accidental theologian trying to advance science.

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Galileo’s day of ignominy came on June 22, 1633. Standing before the inquisitors in Santa Maria sopra Minerva in Rome, dressed in white penitential robes, he was forced to deny the validity of belief in the earth’s motion, an idea he had advocated for more than two decades. The Special Commission appointed by Urban VIII to review accusations against Galileo after the publication of his Dialogue spent less than two months investigating his crimes before reaching a decision. For the next 359 years the Roman Catholic Church would keep revisiting and revising the meaning of Galileo’s trial.

Galileo’s friend the pope had very clear ideas about the circumstances under which he would permit Galileo to write favorably about heliocentrism. They discussed the issue at length in 1630 during Galileo’s trip to Rome. Galileo seems to have followed the literal spirit of the pope’s message without getting the point. He could never forget that Urban VIII, as a cardinal involved in the 1616 Inquisition proceedings, had thought that the condemnation of Copernicus’s book and the effect on the opinions of his followers had been a mistake.

Like the demented, wandering knight of Ariosto’s epic poem and the bumbling Don Quixote, Galileo did not stop to reflect on how the former cardinal, who had written a poem in praise of him after the 1616 condemnation and called him brother, would now view his situation as the pope. As it turned out, Urban VIII felt betrayed when presented with the official inquisitorial record of Galileo’s conversation with the learned and pious Cardinal Robert Bellarmine about the limitations the church planned to impose upon Galileo’s ability to teach, hold or defend in any way Copernican astronomy, an exchange that had occurred just before Rome’s decree against heliocentrism was made public. Galileo had not only misinterpreted the pope’s personal wishes but disobeyed the Inquisition by not disclosing the nature of this conversation during the long negotiations preceding the publication of his Dialogue. Galileo’s enemies in Florence and Rome finally had a legitimate platform for their pointed criticisms, which they repeated mercilessly. Galileo never published a book in Italy again. Chastened and publicly humiliated but still insisting that he was a good Catholic, he spent the final nine years of his life under house arrest at his country villa in Arcetri, where he was periodically reminded by visiting inquisitors not to talk with anyone about Copernican astronomy. He completed Two New Sciences, a work of mechanics that secured his reputation as the founder of modern experimental physics in 1638; that same year he went blind after decades of telescope gazing. Galileo published Two New Sciences in the Netherlands. His Dialogue remained on the Vatican’s Index of Prohibited Books until 1835.

Was belief in Copernicanism actually a heresy? Wootton thinks it was, because he sees it as an idea strongly associated with other known heresies and largely advocated by people whose religious views were otherwise suspect. Heilbron characteristically teases out the complexity of the issue, siding with recent research that characterizes the Vatican’s sentencing of Galileo as the result of a compromise between two different positions within the Special Commission that decided his fate. They made Galileo a vehemently suspected heretic of an unspecified heresy. He had been punished for playing with fire, yet it was unclear whether anyone else harboring these ideas would suffer the same consequences. In fact, no one did.

The specter of Galileo haunts conversations about science and religion to the present day. In a Republican primary debate in September 2011, Texas Governor Rick Perry compared his skepticism about global warming and climate change theory to the unpopular stance taken by Galileo in the face of church criticism. But Perry is no scientist, nor does his position carry the risks that Galileo took when he failed to restrain his opinions. Heilbron invites us to marvel at how someone with such sublime understanding of the natural world could know so little of human nature and the consequences of disobeying his society’s rules. Wootton instead sees Galileo’s actions as a deliberate provocation stemming from his commitment to a worldview incompatible with Christianity. It would be easy to dismiss aspects of Wootton’s thesis, where he pushes the argument too far on too little, yet there is a grain of truth in his vision of Galileo as a complex man who enjoyed the company of controversial and devoted friends. I am inclined to believe that people in search of the truth, as Galileo surely was, are not especially good at playing by the rules. Whether this makes one an unbeliever, however, is a different matter.