Life of Galileo opens on Galileo Galilei, a professor of mathematics at Padua University. He’s talking to Andrea (his housekeeper’s young son), who has just brought him breakfast. They’re discussing the solar system and how it works. Galileo shows Andrea a wooden model that illustrates the current, generally accepted understanding of the planets. In it, the Earth is in the middle of the universe and is surrounded by eight crystal spheres. These spheres represent the moon, the sun, and all the planets. People have believed this model for two-thousand years, Galileo says, but as mankind progresses in technology and knowledge, he suspects they won’t believe it for much longer. He teaches the ideas of Nicolaus Copernicus to Andrea (who calls Copernicus “Copper Knickers”). The new ideas place the Sun at the center of the solar system, with the Earth and planets revolving around it. All the other stars in the night sky are at the center of their own systems. Galileo uses the wooden model as well as a series of common-sense demonstrations with an apple to show Andrea how Copernicus’ theory could be true. Andrea believes him somewhat, but also questions Galileo whenever an argument seems weak. When Andrea’s mother, Mrs. Sarti, arrives, she expresses serious concerns about what Galileo is teaching Andrea, since it goes against the Church’s approved model and could therefore get Andrea into trouble at school.
Throughout all of this, another concern repeatedly appears: money. Galileo doesn’t have any, but he needs it—not just to continue his research and buy books—but also to do simple tasks like pay the milkman. So when Ludovico arrives, hoping to hire Galileo on as a tutor, Mrs. Sarti insists that Galileo accept the offer. He does, though not happily. Shortly afterwards, Galileo’s supervisor at Padua University (the Procurator) arrives to tell Galileo that his recent request for a raise has been denied. The Procurator suggests that, if the mathematician needs more money than his teaching job provides, he should invent something useful. He reminds Galileo that, while Padua (and more broadly, Venice) might not pay much, it at least offers freedom from persecution by the Church, which he might experience in other, better funded places (like Florence). Galileo responds that such freedom of thought may be nice, but it is meaningless if he spends all of his free time working to make ends meet instead of thinking.
Ludovico, however, provides a possible solution to Galileo’s problem: a new invention by the Dutch called the telescope. It’s still unheard of in Italy, but Ludovico has seen it put to wondrous uses abroad. Galileo instantly understands the mechanics behind the device and quickly replicates one, pawning it off as his own original invention. The Procurator, seeing the great many uses that the telescope could be put to, guarantees Galileo his raise. Shortly thereafter, however, a Dutch merchant arrives in Venice with a boatload of telescopes and Galileo’s deception is revealed. It doesn’t matter, though. He’s already used the telescope to empirically prove Copernicus’ theory (which he’d previously only been able to prove theoretically using mathematics). He excitedly tries to show this proof to his friend Sagredo, but Sagredo only reminds him that a man was burned at the stake for quoting Copernicus only a few months before. Undeterred, Galileo remains confident that the Church will be unable to avoid the truth when it’s right before their eyes. This confidence causes him to move to Florence where, despite being under strict religious censure, he believes he will have the time and money to explore his new findings.
With Galileo newly settled in, Cosimo Medici, the Grand Duke of Florence (who is still just a child), is brought by his counsellors to see the telescope at work. Among Cosimo’s party are a theologian, a mathematician, and a philosopher. All of them are wholly skeptical of Galileo’s latest findings and, after some heated debate with him, they decide that he’s a waste of time at best if not an outright lunatic. In the end, they won’t even look through the telescope to see the simple, observable evidence that Galileo presents as proof, though they do agree (in a way that seems less than sincere) to present Galileo’s information to the Church’s chief scientist, Clavius. Shortly thereafter, a deadly plague rips through Florence. Galileo, his daughter Virginia, Mrs. Sarti, and Andrea are given the chance to flee, but Galileo declines it, citing his need to work. Mrs. Sarti decides to stay behind with him, but they send Virginia and Andrea away. Andrea, however, opts to return despite the danger so that he can continue assisting Galileo.
All manage to avoid the plague and Galileo soon finds himself at the Vatican awaiting Clavius’ review of his work. The scene plays out in much the same way that the confrontation in Florence did: the Church’s scholars are simply too dedicated to the Church’s existing understanding of the universe to entertain alternatives. They all feel that Galileo’s telescope is a dangerous object and that his questioning of age-old wisdom is even more dangerous. A kind of fever overtakes the discussion and at one point an older cardinal faints while berating Galileo. Nevertheless, the scene ends with Clavius confirming that Galileo is correct. His words are followed up by “deadly silence.”
Though Galileo understandably feels that his work has been vindicated by Clavius, he soon discovers that the Inquisition has other ideas. They’ve decided that Copernicus remains heretical and cannot be taught. Paradoxically, though, they’ve accepted Galileo’s findings. What this means is that the Church has decided to allow Galileo to continue his research but not to publish it to the outside world. Galileo is upset by this, but also slightly overwhelmed—he is, after all, a devout Catholic who doesn’t wish to go against his Church, and these orders come from the highest levels of authority.
In the following scene, the Little Monk visits Galileo. He has looked through a telescope and observed the same things Galileo has. The discovery has shaken his faith, and in order to recover that faith, he has decided to abandon astronomy. He visits Galileo to explain why—perhaps in an effort to convince Galileo to do the same. Their long conversation doesn’t go quite as planned, however, and Galileo ends up converting the Little Monk into one of his students by offering him his manuscripts. Galileo compares these to “an apple from the tree of knowledge,” something he knows the Little Monk won’t be able to resist. Kept from publishing, Galileo has instead spread his knowledge to his students, who now include the Little Monk, Andrea, and Galileo’s telescope lens manufacturer, Federzoni.
Meanwhile, the Pope is dying and it seems likely that his successor will be Cardinal Barberini, a mathematician with whom Galileo has had favorable interactions in the past. Assuming that Barberini will be far more receptive to his work than the previous Pope, Galileo resumes publication. His ideas spread far and wide, seemingly overnight: he even becomes the subject of ballads sung at public fairs and carnivals. Naturally, this catches the eye of the Inquisition, who summon Galileo to the Vatican. While Barberini does indeed agree with Galileo, the politics behind supporting him are just too risky and complicated. Therefore, the new Pope has given the Inquisition the right to imprison Galileo, and even to threaten him with torture, in order to force him to renounce his work. Their plan succeeds, and Galileo recants his doctrine. His students can hardly believe it, and they turn their backs on him. They feel that Galileo has abandoned their hard and important work to save his own skin.
Nearly a decade passes. Galileo has been imprisoned in his home by the Inquisition and will remain so for the rest of his life. He’s forced to write dissertations approving the Church’s opinion on a number of banal matters, all of them below his abilities. These texts are carefully checked by a monk for any heresies they might contain, and any other writing is forbidden. Nevertheless, Galileo has, in secret, finished his magnum opus, The Discourses and Mathematical Demonstrations Relating to Two New Sciences. One day, Andrea comes to visit (the first of his old pupils to do so). At first, Andrea is cold towards his old mentor. Galileo reveals, however, that he did not recant his work in order to save his life. Rather, he recanted it so that he could continue it in secret. With Andrea’s help, Galileo manages to sneak The Discourse out of the country and into Holland, where it is published without censure.