The “Galileo affair” continues to fascinate and provoke after 400 years. It was, in a way, both simple and very complicated. What was simple was its upshot: The great founder of modern science was tried, convicted and sentenced in 1633 to perpetual house arrest by the Catholic Church for defending the idea that the Earth goes around the sun, and was forced to recant under oath. This offense against freedom of thought, research and conscience can never cease to shock.

The complicated question is how and why it happened. It was not inevitable. Saint Augustine had warned eloquently in the 4th century against interpreting scripture contrary to what is known with certainty by reason and experience. This was a well-established principle, accepted by Cardinal Bellarmine, the church’s top theologian, who admitted in a famous letter to one of Galileo’s friends that “if there were a true demonstration that . . . the earth circles the sun, then one would have to proceed with great care in explaining the Scriptures that appear contrary, and say rather that we do not understand them, than that what is demonstrated is false.”

The Galileo affair involved a complex interplay of ancient and novel scientific ideas, scriptural exegesis, entrenched theological and philosophical positions, intellectual turf battles, academic rivalries, flawed characters, personality clashes, and ecclesiastical and secular politics, all unfolding over two decades in the fevered atmosphere of the struggle between Reformation and Counter-Reformation and the Thirty Years War. Few historical episodes are more fraught with subtleties, ironies and ambiguities. To tell it properly requires an unusual breadth of knowledge and the gifts of a great storyteller. Fortunately, Mario Livio is fully equipped for the task. In “Galileo: And the Science Deniers,” his mastery not only of the science (which is to be expected of a highly accomplished astrophysicist), but of the cultural and historical context, is impressive. Even more impressive perhaps, given that he is not Catholic, is his relatively sophisticated grasp of some of the theological arguments and issues.

To take just one example, he accurately identifies as a crucial misstep Cardinal Bellarmine’s sweeping interpretation of a rule laid down by the Council of Trent to deal with the challenge of Protestantism. The rule forbade interpreting scriptural passages in a way contrary to the consensus of the early church fathers on “matters of faith and morals.” Galileo quite reasonably argued that because astronomy did not pertain to faith or morals, the literal readings by the early fathers of certain passages (understandable in their day) need not be followed. Bellarmine rejected this, however, using a circular argument. He simply assumed the most literal interpretation of those passages, which logically entailed that the astronomical notions implicit in them were authoritatively taught by scripture and hence “matters of faith,” which in turn required (by the Trent decree) that one follow the fathers in interpreting them literally. Livio rightly calls this blunder of Bellarmine’s a theological “bombshell.” The kind of extreme literalism that it forced had not been the church’s rule previously, nor would it be later.

Even so, a proof of the Earth’s motion would have compelled a less literal reading, as Bellarmine conceded. Unfortunately, such proofs were unavailable at that time. Galileo did have proof, from his telescopic discoveries, that the 15-century-old geocentric theory of Ptolemy was wrong. Those discoveries were quickly confirmed by the Jesuit astronomers of the Collegio Romano and others, and the Ptolemaic theory was just as quickly abandoned. But Galileo’s findings were not enough to prove the Copernican theory correct or that the Earth moved, for there was a perfectly respectable third theory on the market, proposed years earlier by the great Danish astronomer Tycho Brahe.

In Tycho’s model, the planets orbited the sun, but the sun orbited a stationary Earth. This model was completely consistent with all the observations of that time. It avoided not only any scriptural issues but also a significant scientific objection to the Copernican theory, namely the failure to see “stellar parallax” (the slight apparent motions of distant stars that would be caused by the Earth’s shifting vantage point as it orbits the sun). This effect was not observed until 1838.

The scientific debate between the advocates of the Copernican and Tychonic models continued long after Galileo’s death and involved sophisticated scientific arguments on both sides. A definite scientific consensus that the Earth moves came only when Isaac Newton discovered the laws of gravity and motion in the late 17th century.

Near the end of his book and in many interpolated paragraphs throughout it, Livio draws parallels between Galileo’s opponents and the contemporary “science deniers” of the book’s subtitle, by which he means fundamentalist anti-evolutionists and global warming skeptics. But those parallels are rather strained. Nobody today has been tried and punished for defending the ideas of evolution or global warming, or coerced into recanting them. And, in contrast to the anti-evolutionists of today, who are tightly closing their eyes to overwhelming scientific evidence, the advocates of the Ptolemaic theory abandoned it immediately when faced with Galileo’s discoveries, adopting instead Tycho’s theory, which was quite viable and scientifically respectable until Newton came along, at which point it, too, was abandoned.

Livio’s occasional straying into the didactic, not to say homiletic, will be distracting or irritating for some readers. But they do not diminish the value of the rest of his book, which tells the story of Galileo in a perceptive, illuminating and balanced way.

Galileo

And the Science Deniers

Simon & Schuster. 286 pp. $28