The best books on Galileo Galilei

Before we get to the books, a general question: in his lifetime, Galileo won celebrity status because of his telescope. What would you say was his biggest contribution as a scientist?

I think Galileo’s scientific achievement can be divided into a few different categories:

One is transforming the Dutch spyglass into a scientific instrument. He did not invent the telescope as an object, but he transformed a report of this object into an observational instrument. Astronomy changes, because of Galileo. It becomes observational in a different way. We extend sight with the help of instruments and use the telescope to see what we can see. That’s a major innovation.

Another is that Galileo’s mechanics is the foundation of a lot of basic high school physics. When we do inclined plane experiments, we’ve replicated some of his basic insights about how weight, mass, and motion behave—what became Galileo’s other lifetime work. People who study physics don’t forget this about Galileo, but others often do because of the infamy of his trial. They treat Galileo as if he died in 1633. But he lives another nine years and the thing that he is most concerned about during this period is the completion of his Two New Sciences, which is his mechanics. Galileo begins the process of really rethinking motion in ways that are very important for subsequent work, for instance, that of Isaac Newton.

Galileo also has very important comments on scientific methodology. The more his ideas are challenged, the more he thinks about scientific methodology, and ultimately writes this very interesting treatise called The Assayer that questions a lot of the presuppositions about how we know and what we know. Here Galileo is thinking about epistemology. In many ways it’s both a philosophical and a demonstrative piece of work.

Last, but hardly least, is his Letter to the Grand Duchess Christina. Galileo is probably the most important person trying to define the relationship between science and religion in writing, again, because he is put to the test about these things. His unpublished Letter to the Grand Duchess Christina, which he writes and really perfects in 1615 and begins to circulate is, today, a foundational document for people who are trying to understand the relationship between these two important ways of knowing and believing.

So, in spite of the trial, he was able to continue doing important science after 1633?

Yes, under great difficulty. He is under house arrest. He’s old and has a lot of health problems. He goes blind. But he also has loyal, younger colleagues who assist him. We sometimes forget his grit and determination to finish longstanding projects in this period. Also, from house imprisonment, he is able to protect and secure his reputation by getting books, like his prohibited Dialogue, translated into Latin and published in Protestant Europe. He has his Letter to the Grand Duchess Christina translated and published for the first time—not in Italy, obviously. It won’t be published in Italy until the 18th century, and even then it will be published illegally.

What do you think motivated him?

There are many different Galileos in the story of Galileo and that’s one of the reasons he’s such a fascinating figure. He’s human. He’s complex and he’s passionate and he’s arrogant and he’s egotistical and he’s smart and he’s insightful and he’s eloquent. But I think, fundamentally, what motivates Galileo and is probably the reason of his downfall—despite the fact that he’s savvy and a player, when the world starts to get to know him—is that he cares about the truth.

“Galileo’s mechanics is the foundation of a lot of basic high school physics”

He may at times seem to be prevaricating, but I think of him as somebody who, as a scientist, was fundamentally concerned with the science of motion. Astronomy is a long, productive and controversial diversion that interests him initially because it’s a place to explore problems of motion and a discipline full of unanswered questions. He is also somebody who likes to tinker, who likes machines, to know how things work. That combination is a natural for somebody perfecting an instrument that can assist in studying problems of motion by studying astronomy, so he becomes an astronomer.

But what does he do at the end of his life? He finishes his work on mechanics, which includes many of the building blocks on the way to a science of dynamics. We’re not there yet. It’s about statics, largely, but he raises questions that will get us there. He also raises questions about the mathematics for this new physics that will also eventually, over time, get us to places he couldn’t imagine like Newton’s and Leibniz’s calculus.

Let’s look at the books you’re recommending for anyone wanting to learn more about Galileo, starting with the Bertolt Brecht play, Life of Galileo. Why do you like this and is it historically accurate?

First of all, there are three different versions of the Brecht play. That’s what is so fascinating. Brecht creates this play in three different contexts in the 30s, 40s and 50s. As a piece of interpretation, as a demonstration that there are many different Galileos that can be different kinds of object lessons, it’s amazing.

There are many inaccuracies. He takes license and liberty. Galileo has two daughters who are sent to a convent way too early for the older one to be thwarted in her marriage aspirations by Galileo’s increasingly controversial reputation, as Brecht would have us believe. But that’s a perfect dramatic move, so one can forgive Brecht for deciding that he has to do that. And who knows exactly how much he knew about the life of the daughters? After all, it’s only very recently, thanks to Dava Sobel’s work, that we know a lot about Galileo’s eldest daughter Virginia (Suor Maria Celeste).

Nonetheless, I like Brecht. When you put aside both the deliberate decisions that he makes, the omissions and alterations and the things he just didn’t know in the way that a historian would, he does capture some things about Galileo as a person. He loved life. When the actor Topol played him, he really embodied this vision Brecht has of Galileo as a very human figure. That’s what theatre can do, it can bring that humanity to life in the same way that good fiction does. It’s also a performance and I think that Galileo and his world were naturally quite theatrical. Galileo was a contemporary of Shakespeare, after all.

Fundamentally, Brecht recognized that Galileo is not just of his times, but for the ages. As times change, we learn different things from him—and Brecht did. That’s why he kept rewriting the play. One version is for the Weimar Republic, on the eve of Nazi Germany, and as Brecht leaves that world. Another version is about the social responsibility of the scientist after the dropping of the atomic bomb. Another version is in the midst of the Cold War. By then Brecht is thinking about the American reaction to his own communism in the McCarthy era and becoming increasingly interested in migrating to Russia to explore these issues. In all of them, he can bring Galileo with him.

And it’s fun. I started reading it this afternoon and I was enjoying it.

In many ways, it’s a great starting point for understanding Galileo. You can watch the version with Topol online. It used to be one of these bootleg things that was hard to find, but now nothing is hard to find anymore. It gets you into a lot of the issues, even if it flattens the complexity, because it’s trying to make these simple dichotomies between good and bad. But even in the simplicity, the fact he changes the punch line—which version of Brecht’s play are we watching?—means you have to ask yourself what meaning we should extract from Galileo’s trial and his response to it. There is a great moment in Brecht’s play when Galileo returns to Rome in the midst of Carnival. His papal adversaries observe that he’s not wearing a mask. In making this observation, Brecht reveals something fundamental about Galileo.

Let’s go on to the second of the books you’ve chosen, which is Galileo’s Telescope: A European Story, written by three Italian historians of science, Massimo Bucciantini, Michele Camerota and Franco Giudice. Tell me why you chose it and what you like about it.

I’ve chosen this book because, first of all, it’s the combinatorial power of three really good Galileo scholars, who’ve thought long and hard about different aspects of Galileo’s work and his world. It’s three experts getting together and writing a book that is seamless, it doesn’t look like three different people wrote it, it’s very well integrated, it’s very accessible.

What I especially like is that it’s a real time story of the history of the telescope, which we don’t often get. What they decided to do was watch it unfolding like a news episode. How is this happening? Who is generating the news? How is the story of the telescope being transformed in each and every context? It’s not a story that starts with Galileo and it’s certainly not a story that ends with Galileo. Indeed, they leave us with a coda in which you see that the telescope has now travelled, via the commercial and the Jesuit missionary networks, all the way to India and to China. The telescope travels very far, in the early 17th century. In fact, when I was reviewing the book, I commented that by the late 17th century, people were using telescopes in the exploration of Baja California. They had been shipped to New Spain.

What the book gets you to do is to think about this as an object that has to travel through very concrete networks. At each stage, people respond to the fact of the telescope and to Galileo. It takes us back to a world of diplomacy and commerce. How does the telescope—through word of mouth, letters, relationships—travel through the networks that link different parts of the world?

I know they go way beyond this, but just focusing on the basics, I like the way at the beginning they have pictures of the telescope and explain the physical object he used. I think they say it’s 14x strength—so people really could see a lot more with this telescope than with the naked eye?

I think the original version was maybe eight to 10 times magnification. This is where Franco Giudice, who does a lot of work on the history of the instrument, comes in. They’re really good at getting you thinking about the problems of this telescope. Most of us have never used a replica of a Galilean telescope—we’ve used the kind of telescope that Kepler modifies, that changes how you view the image through a different combination of lenses. But at roughly the same magnification, cardboard replicas are kits for elementary school students. The first thing you learn is that you can’t even see the moon in its entirety through it, even with this magnification. You learn how narrow the focal point of the lens is and then you start realizing how many observations you would have to make before you could put together the description of the new things you see in the heavens that we see in Galileo’s Sidereal Messenger in 1610. You realize that he makes it look so easy when he presents his initial observations to the public, but it’s actually really hard work. To see Jupiter’s satellites—what Galileo calls the four moons of Jupiter—or the phases of Venus, is extremely difficult. That material insight into the limits of this new scientific instrument helps you understand that we have a lot of steps to go before we get to the Hubble telescope. Galileo has taken this very important first step, but he has to do a lot of hard work.

“Galileo was a great communicator of science to the public. He wanted to involve society in the project of science”

What they also help us understand is that critiquing Galileo is not just about resisting new ideas; this instrument doesn’t always work. The glass is made artisanally. Sometimes you get better lenses, sometimes you get worse, even when they’re crafted by Galileo. A seventeenth-century lense is artisan – it doesn’t come out the same each time. Even when he has made one, tested it, and sent it to someone else, they don’t have the benefit of all the hard work he’s done to generate observations. Everyone has to learn how to use this instrument, and they have to get a good version of it. Even then, there are still going to be questions about the implications of the ideas.

Bucciantini, Camerota and Giudice do an excellent job of showing that the questions are there from the start. It’s not simply a reactive thing that happens because Galileo pushes too hard, which is the simple version of the story. There really are a lot of questions and a lot of interest, even on the part of critics. What are people in places like Milan thinking about the telescope? There we see that the archbishop of the city, Federico Borromeo, is super interested in science. He is getting everything that’s new, but he’s also an archbishop. He’s a good example of how somebody outside of Rome is thinking along similar lines to what we see within Rome, that of course shaped the story of science and religion.

I don’t know whether it was in this book or another one, but the author(s) makes the point that everybody called it ‘Galileo’s telescope’ even though he hadn’t invented it. It was the telescope that make him an internationally renowned scientist, is that right?

Absolutely. This is why the news of the telescope is so important, because it’s also news of Galileo. Who was Galileo before he invented the telescope? He was a mathematics professor in his 40s who had published virtually nothing. He’d been teaching, people knew he was smart and interesting and witty and sarcastic. They knew that he had an agile mind. But he hadn’t done anything to gain the world’s attention. And then he did. I don’t think even he expected it to be as wildly popular, successful and controversial as it turned out to be.

Galileo’s Telescope also pays close attention to what other people do with news of the Dutch spyglass. It’s the story of the simultaneous discovery of new ways of thinking about how we see the heavens. The Englishman Thomas Harriot is a fascinating figure. He writes this famous account A Briefe and True Report of the New Found Land of Virginia. He’s a mathematician and many other things. Harriot is simultaneously designing his own telescope, doing observations, and then he hears about Galileo’s work. It’s not clear that Galileo ever hears about Harriot’s work, which is interesting, that the network does not go in two directions. Then Harriot makes observations that suggest he’s now looking at Galileo’s Sidereal Messenger, looking at how Galileo presents his own illustrations of what he’s seen, as well as describing them. And yet, we don’t remember Harriot unless we resuscitate him from the historical record. He can stand in for many of these figures who appear in the book, who are also simultaneously exploring the possibilities of telescopes, whether they’ve read Galileo or they’ve gotten interested on their own. Most of them have read Galileo and heard about what he’s doing.

Let’s move on to the third book you’ve recommended to learn more about Galileo, which is the letters that his oldest daughter sent to him, Letters to Father: Suor Maria Celeste to Galileo. She was sent to a convent age 13, which seems really sad. Can you explain the context of this and why you think these letters are important to read?

Galileo has three children with Marina Gamba, a woman he never marries. We don’t know a lot about this woman, so she’s been an object of some fascination. He has a son and he has two daughters. Virginia Galilei—with whom he has this correspondence—is the oldest daughter and perhaps the child who seems to be closest to his heart and most interested in what he does. They have a very loving relationship and we’re very fortunate that this half of the correspondence, her letters to him, have survived. And we’re all very sad that the other half of the correspondence has not survived, because then we would have the whole story of what really is a fascinating family dynamic.

First of all, we owe an enormous debt to Dava Sobel who not only wrote a book, Galileo’s Daughter, which if I were allowed to recommend six books I would recommend, but also had the insight to recognize that she should make her translation of the sources available. That’s not something a writer for a general audience often thinks of doing, and I want to fully credit her for having recognized not only that it would be great to write the story of Galileo from the perspective of his relationship with his oldest daughter, but that we should hear her in her own words.

Here again, we see a very human Galileo. We see a Galileo whose daughter loves him and worries about him, and who increasingly becomes aware of the problems that he’s facing, probably in ways that she hasn’t had to be until the trial occurs. We really get to know Galileo as a father, but also as somebody vehemently suspected of heresy while his own daughters, who are nuns, are praying for him. After he is condemned, Suor Maria Celeste demands access to read the terms of the condemnation and his abjuration, and then she volunteers to take on his penance of prayer. Shortly after they’re reunited, when he finally is allowed to return from Rome and comes to Arcetri—where he’s under house imprisonment and she’s in the convent nearby—she gets sick and dies. So the ending is tragic in ways that Brecht should have incorporated into his play. It was one of Galileo’s many tragedies.

The story of Galileo’s daughter provides a lot of insight into this network of family and close friends who are very protective of him. Another moment I especially like in Galileo’s correspondence with his daughter is when, through this network of family and friends, he arranges for her to hand over the keys to his villa at Arcetri so that one of the trusted members of this larger family can go in and make some documents at least temporarily disappear. She may be a nun, but she does not hesitate to protect her father from further recriminations, should there be anything in the house that might get him into further trouble—or simply get lost if the Inquisition decides to come over and rifle through it. Some people have argued—and many of us now think—that it’s the manuscript of what becomes his Two New Sciences that is the thing they’re most concerned about. It goes ‘missing’, but it’s surely not the only part of his archive that disappears.

Another great moment in the letters is when she thinks about how she can use her female network to help her father. She’s outside of Florence, in this rather impoverished convent. They’re basically living a subsistence life: this is not one of these aristocratic convents in Florence, where people are eating jellied candies all day. But even from there she mobilizes all the women she knows. She imagines that maybe through talking to people, who can then intervene with the wife of the Florentine ambassador in Rome, she might assist her father in the problems that he’s having during the trial. That gives us some insight into the kind of person that she was, that she would reach out in these ways, as well as hand over the keys to her father’s house. She is actively trying to figure out what’s going on and what role she can play. I love this. We see a very different Galileo than if we just read his publications.

But why was he so poor that he had to put his daughter in a convent, if he was such a successful scientist? Do we know why he didn’t marry her mother?

We have no idea why they didn’t marry, the assumption is that she’s not of the appropriate social class, since he facilitates her marrying someone else in Padua after he leaves for Florence. Clearly the children matter a lot to him because he brings them all with him to Florence—and not her. Whatever the relationship was, marriage was never in the offing. But the children matter a great deal. He frets a lot over his son, who like so many sons of famous figures is not all his father would want him to be. With the girls, I think there is a problem about the cost of a dowry. It costs a lot less to give somebody a convent dowry than a dowry for marriage. I think there’s a decision made that it’s too costly to marry these daughters, that’s part of it. They are on the younger side when they take their vows. There’s been a bit of a rush. There’s also a clear sense that the youngest daughter, Livia, who’s known as Suor Arcangela, has some kind of psychic distress, she has some problems and that the older sister is looking after her. Maybe that was part of the bargain, too, that they couldn’t be separated. But, again, we have to read between the lines about why this all is. Did the oldest one ultimately have a spiritual vocation? Maybe, but no more or less than many women who enter convents under similar circumstances in this period.

Brecht presents him as desperate for money at the beginning of the play. Is that accurate then, that Galileo was always short of money?

One of the reasons he accepts this new position in Florence in 1610 is not only to go back to his native Tuscany, but because he hopes that it will be better financially and so better for his work, because he won’t have to be teaching and housing all these students. He’s very quick to cash in on the benefits of inventing the telescope. One senses he has a messy, needy family in which he is the best source of income for many people—like his brother, the musician, who then dies. He’s constantly having to bail people out.

Is he always needy? That’s unclear. One presumes the move to Florence did assist a number of these things. But, of course, with fame come even more expectations by those who want things from you. It was probably a never-ending cycle. I wouldn’t say that he ultimately was poor, but it never seems to have been quite enough for all the people who needed things from him, or perhaps even for his own expectations of who he was.

Okay, we’re now at the fourth of the books you’re recommending on Galileo, a book which deals with the trial and his 1633 condemnation as a suspected heretic by the Roman Inquisition. It’s called On Trial for Reason and it’s by Maurice Finocchiaro, an emeritus professor of philosophy at the University of Nevada Las Vegas. The book is actually also quite a good introduction to Galileo in general, quite an opinionated one. Tell me why you chose it.

There are so many good books on Galileo and I hope these five will encourage people to read and dig in further. I’ve chosen this one because it was published recently, it came out in 2019. It’s specifically written for a general audience, but is by somebody, Maurice Finocchiaro, who has spent his life thinking about Galileo. Finocchiaro is a philosopher who is also deeply interested in the history of science, but has done more translating than almost anyone—save possibly for Stillman Drake (a great Galileo scholar of an earlier generation) and my colleagues Al van Helden and Eileen Reeves—and therefore has a very close and careful reading of Galileo’s words. When we want to read the documents of Galileo’s trial in English, and many allied documents, we turn to the works of  Maurice Finocchiaro. He’s spent his entire career thinking about Galileo. I want people to benefit from that long, hard thinking, because this is the kind of subject that deserves that.

What I like about On Trial for Reason is that it very economically gives you a lot of basic things that you want to know about Galileo. What exactly is the nature of Galileo’s scientific innovation? What has he done? What are the controversies? What are the problems of it from a scientific perspective, from a philosophical perspective, and then, of course, ultimately, from a religious perspective? He also reads the trial like a forensic analyst. He’s not a lawyer, he’s a philosopher, and he’s very interested in questions of logic. He does this logical analysis with the full benefit of a lot of the interesting new documents that have come to light over the last few decades, as people have done more and more archival research. Believe it or not, it’s still possible to find new documents related to the trial of Galileo. In fact, I just got an email this week from a young Italian researcher, Leonardo Anatrini, telling me about his forthcoming publication of one of the new documents that he found.

Wow.

Finocchiaro is up to date on all the things that have happened up to the point the book is published, and he’s going to give you the full benefit of his reading these different, tricky, documents. For example, the famous injunction that Cardinal Robert Bellarmine personally gives to Galileo, in Rome, in 1616, after the Congregation of the Index has placed Copernicus’s On the Revolution of the Heavenly Spheres on the Index of Prohibited Books, pending its revision to conform with Catholic doctrine. Finocchiaro does a careful reading of this document that resurfaces during the trial that people have long forgotten about, and Galileo now has to reckon with. There have been many different interpretations of the slight differences in wording in the sparse documentation of this injunction by Robert Bellarmine. What did it really mean? What did he really say to Galileo? Why does it seem to be somewhat different in the inquisition’s archive versus Galileo’s own version? Finocchiaro does a very good job of sorting through these kinds of subtleties and discrepancies. That’s why I think this is a terrific starting point – then you can go from his reading of these things into the documents themselves.

And what’s his general conclusion? What was Galileo on trial for? It was a clash between science and religion—not in a simplistic way, obviously—but ultimately that’s what it was about.

That’s right. It goes back to the very thing that Galileo was trying to anticipate and provide his own answer for. I always tell my students that the trial of Galileo is like a murder mystery—except we already know who did it, we just don’t know what the crime was. What did he do? People have been asking this question ever since 1633.

Finocchiaro reminds us that the trial is a never-ending story. We’re still finding new ways to talk about it and rethink it. This is not a closed history, that happens and ends in 1633. It’s going to keep going, and we’re going to keep returning to it—and we need to look at the evolution of the ways people have responded to the trial ever since.

The title of the book reveals Finocchiaro’s understanding of the trial. Galileo’s use of reason over revelation, his foregrounding of the role of observation, of instruments, his definition of a hypothesis, his emphasis that when he is speaking hypothetically about something he is on a path to what he believes to be a better scientific truth: all of these things raise questions about the relationship between science and religion.

“I don’t think that Galileo, on his own, would ever have talked about religion and its relationship to science”

In 1543, when Copernicus published his book as a deathbed contribution, very few people—other than Copernicus himself—recognized that there might be a potential conflict. But that potential was written into the introduction of Copernicus’s book. It took almost a century to fully realize the level of the conflict that it engendered, that science is developing its own epistemology and its own path to truth, and that this may not fit comfortably with the way people understand nature from a religious perspective. In other words, if your point of reference is always how nature has been described in the Bible, and you allow no possibility for the distinction Galileo wishes to make—between why we write about nature in the Bible, for reasons of belief, versus how we write about nature, when understanding nature scientifically is our primary focus—what we see is a world of clashing expertise and, therefore, completely incompatible views about which truth to foreground.

Galileo’s way out of this is, I think, brilliant. He says, in the Letter to the Grand Duchess Christina, that two truths cannot contradict each other. Ultimately, he thinks that one has to trust that there will be a harmonious reconciliation, which is what the church believes nowadays. This is the great irony that Finocchiaro is really good at pointing out. What people refused to accept in Galileo’s own time we have accepted as a modern proposition. Not everyone, to be sure, but the Roman Catholic Church institutionally has accepted this since the end of the 19th century—and, in fact, moved towards it in stages ever since the trial of Galileo.

It ended badly for Galileo in the sense of he was under house arrest, but Giordano Bruno, who was also a Copernican, was burned at the stake. Why did Giordano Bruno end up dead and Galileo get off more lightly?

People have long puzzled over that. As a specialist, I will say that Bruno is an out-and-out heretic. First of all, he is an ex-Dominican who has not only left behind his religious order, but tried out every version of religion in Europe and been rejected or rejected himself from all of them. Then he returns to Italy, after having flirted with Protestantism in many different places. He’s a radical philosopher, a radical theologian. He believes in the plurality of worlds. I am quite confident that Galileo read Bruno and made sure to never once cite him explicitly because he understood how controversial that would be.

People still debate whether Galileo was actually a radical thinker from a religious perspective. Is he rejecting the Catholic Church? I have to say no, because there are many people in his world who do precisely that.  They tend to exit, if they can. Sometimes they come back, like Bruno, and challenge the church’s authority to define orthodoxy, and it ends badly. Galileo certainly could have fled. He could have stayed in Venice, where this might not have happened because while Catholic, Venice was very antagonistic to Rome. We’ll never know, of course. But Bruno sees Copernicanism as a kind of sign, he reads it allegorically. For him, it’s like an avatar. His The Ash Wednesday Supper, which is his most Copernican publication, is like a Renaissance Tao of Physics, if I can put it in those terms. He has a whole different style. He’s not interested in empiricism. He really is trying to create a new philosophy that also is tied to a new theology. He’s a radical thinker and utterly unrepentant about it.

“The more his ideas are challenged, the more he thinks about scientific methodology”

I don’t think that Galileo, on his own, would ever have talked about religion and its relationship to science. He was pressed to do so because, increasingly, the church was taking an interest in his science in a way that he didn’t like. He was trying to find a way out of this mounting difficulty.  Think of him as someone who finds himself at the center of a labyrinth and is trying to exit.

Returning to Brecht’s use of his condemnation at the trial and Galileo’s famous abjuration of everything he believed in—what kind of moment is this? Maybe Brecht’s vacillatio is a recognition that it’s hard to be an absolutely pure hero. And sometimes it’s hard—under these circumstances anyway—to be entirely a victim, let alone a collaborator. You’re all of the above.

Galileo certainly decided that he did not want to suffer the ultimate punishment for the conflict he has found himself in with the church. He wants détente, he wants to survive this moment. To do so, he has to demonstrate very publicly, in a way that is quite contrary to his intellectual beliefs and to his entire raison d’être, that he is an obedient Catholic more than anything else. So, he demonstrates that obedience in this official and public way and then he goes home and does a lot of things under house arrest that actually demonstrate quite the opposite. He gets into no trouble for it, even though he’s being closely monitored. That’s a really fascinating aspect of the story. This is something that Maurice doesn’t fully get into, but I’m especially fascinated with – what does it mean to live with the consequences of the trial for Galileo? For his society? For the church? How do they handle those initial years? What are people doing? No one ever calls him on his use of a network that allows his publications to appear in Protestant Europe, even those that are prohibited. Nobody ever says anything, even though it would have been fairly easy to trace some of this back directly to him if they wanted to.

And is that because the church recognizes that he’s probably right and so they’ll have to come around at some point?

Finocchiaro raises these issues in his book, as others have also done. Is Catholic regret there right from the start? We love to think of the Roman Inquisition as this bloodthirsty thing: they can’t wait to kill people. As I point out to my students, it’s bad PR for your religion to kill people. The best thing is to redeem and rehabilitate. Virtually no one really wants to do these things, least of all to Italy’s most famous scientist. He has already been given a lot of latitude—in 1616, he is warned but not condemned and his books are not put on the Index of Prohibited Books then, unlike 1633 when only one of his books is condemned this way. That’s important. There was an effort made to forestall what some of his most virulent critics already would have liked to see happen by 1616. Instead, it happens in 1633.

There are many different kinds of Catholics, even within the church, and that’s one of the things that you see in the story. Many people have accepted Galileo’s observations and the utility of the telescope; they recognize that these new observations might indeed open up new questions about how we understand the physical nature of the heavens, and that, over time, those questions might indeed lead us to want to discard old theories of the cosmos. They’re wondering what the implications will ultimately be for the relationship between cosmology as a scientific endeavour and faith. He’s hardly the only person thinking through these issues.

There are many Catholic astronomers who feel it was a mistake to condemn Galileo and who would have liked to see it otherwise. After the trial, there’s a reason the Vatican has an observatory, even today. Part of the legacy of the trial is that they become some of the great observational astronomers of the next couple of centuries. Many of them are actively trying to see if they can resolve some of the technical issues of observation that could be decisive in proving or disproving heliocentrism. At many moments, they think they’re almost on the verge of resolving these issues. Ultimately, though, you really do need better telescopes to be able to see and calculate things like stellar parallax. It is a matter of the time needed to develop the technology of a science and of course, the skills then to observe and calculate what you see. But there’s an enormous amount of optimism that if they just keep observing, eventually, they’ll be able to prove that Galileo is right.

“Critiquing Galileo is not just about resisting new ideas”

Catholic scholars lobby for a rethinking of Galileo’s condemnation, almost as soon as the trial ends. Maybe if I just talked to my friend in Rome, who really trusts me, he’ll see not only that this was wrong—maybe he already knows that this is wrong—and he will work with me to help figure out a way to resolve this. There were no easy answers for that in the short term. But, in the long term, by 1744, there are some concessions, as Finocchiaro discusses. The decision not to put any new books that advocate heliocentrism on the Index of Prohibited Books is important. It’s a sign of begrudging acknowledgment. Institutions are complex things. They move slowly, and they rarely acknowledge failure.

Some years ago, I wrote an essay on a Jesuit life of Galileo written in the late 17th century that was never published and all the correspondence around it. It is a good example of the negotiations underway, within as well as outside of the church, to rewrite this episode. It led me to write about how people talked about the trial in biographies of Galileo, both in his own lifetime and afterwards, and to examine how this curiosity led to a desire to reconstruct the Galileo archive in full that still animates the research we see today.

Let’s go on to the last book, which is actually by Galileo. The book of his you’ve chosen is The Dialogue Concerning the Two Chief World Systems. How many books did he actually publish?

Galileo  first published a pamphlet about his compass, Operation of the Geometrical and Military Compass. Many people forget that that was published before his Sidereal Messenger in 1610. There’s also a witty dialogue about seeing new stars (nova) that people have debated whether we should attribute to Galileo. That doesn’t appear under his own name, so we’ll put that aside though he was surely involved in it. Then he publishes his Letters on Sunspots in 1613, The Assayer in 1624, The Dialogue in 1632 and The Two New Sciences in 1638.

We should also count things published after the trial in Protestant Europe, like his Letter to the Grand Duchess, in 1636. It’s published after the trial, but not directly under his name, initially, so that’s also a little fuzzy.

Why did I pick The Dialogue? I like all of Galileo’s books, they’re all fascinating. Fundamentally, Galileo was a great communicator of science to the public. He wanted to involve society in the project of science, to appreciate it, to support it, to understand it. And he had the literary skills to do so. He was, after all, a member of the Florentine literary academy as well as the scientific one. He loves poetry, music, and art. He’s a very cultured, talented and versatile figure. He brings all of this to his writing in each of his publications but, for me, The Dialogue is like reading Shakespeare. An entire world is in it. It’s not a scientific work in any normal sense, by our standard, or even by the standards of his own time. It’s like reading Plato and Shakespeare together, learning a lot about astronomy, and being very entertained while it’s going on and while it’s long, you read the whole thing. Then you understand what The Dialogue is. Galileo has mustered all of his knowledge and understanding of the resources of his society, of every moment in his life about science, but also every cultural moment that’s been meaningful, and he’s brought them all to bear in this very theatrical style of writing. What is The Dialogue, but a play by other means?

There are three characters and we get to know them, and also to love them, each in their own way. Two characters are named after two close friends from two different cities, who are both dead by 1633. He’s memorializing real friendships and even real conversations that he surely had over the years, not just with them, but with many, many, friends and colleagues. They each get to play a different role. The Florentine Salviati is a very committed Copernican and believes in heliocentrism. He’s all in and represents that view. Sagredo, named after a Venetian friend, is mostly persuaded but is constantly asking clarifying questions. He stands in for those of us who need to really understand and figure things out and aren’t just going to believe the first thing we hear because it’s a new and shiny prize. He’s willing to take the time to talk to people who are stubborn, and stick to their guns about old, outdated ideas, because Sagredo is the one who’s going to moderate their discussions with this third character called Simplicio. Yes, he could be named after an ancient philosopher, but one suspects it’s a joke and that this is the simpleton. This is part of what gets Galileo in trouble, being a little bit too funny with everything. Simplicio is a committed Aristotelian and not a sophisticated one. He’s the butt of the joke, he often doesn’t get the joke. He asks naive questions and doesn’t have profound insights. And yet, he allows us to see a position. Then, in essence, the fourth character in the story is Galileo himself, as the unnamed mathematician, who doesn’t get to speak but is discussed indirectly by the others as this kind of absent authority we might refer to. This book is the culmination of all of Galileo’s literary, philosophical and scientific aspirations for his astronomy.

And is it convincing in terms of the science? If we were back in the 17th century, and trying to figure out whether Galileo was right or not, should we be convinced by this book that the Earth does go round the sun and not vice versa?

We can see how friendly readers respond in surviving correspondence. And we can also witness an antagonistic reading by certain inquisitors in his trial documents.

Would we be convinced? Well, Galileo in this book builds a world over the days of his Dialogue, and many parts of it are quite persuasive, because they’re based on all his hard work with his instrument to observe the heavens. We should be convinced by what he says about the stars, the phases of Venus and the sunspots. Then we get to day four, and Galileo talking about his theories of the tides. And not only do we not find those convincing, from a scientific perspective, but many of his contemporaries did not think his theory of the tides was persuasive. As a culminating proof, it’s problematic.

There were many other problems about how people responded to The Dialogue, to be sure. But here Galileo’s desire to claim victory on everything—even on things that deserved a more careful reading—becomes his Achilles’ heel, not that the trial revolves around this, to be sure. He wanted the tides to prove the motion of the Earth in relationship to the sun; he basically excludes the Moon entirely. There is no lunar pull. He does this even though a careful reading of Kepler should have given him pause. Galileo does so because he thinks it’s too occult, it’s old, bad science to allow invisible forces to operate this way.  He misses the opportunity to rethink what the pull of the Moon is, what the physical relation is between the Moon, sun, and the Earth together.  And he also misses the opportunity to be much more empirical about the data he has on tides. People point out that the data he’s using on tides is not only incomplete, but even the data that he has, from places like Venice, if he’d read it more carefully, wasn’t adding up to what he argues.

“He’s complex and he’s passionate and he’s arrogant and he’s egotistical and he’s smart and he’s insightful and he’s eloquent”

So, here, we could say that he’s getting into an area of science, he hasn’t studied and thought through as carefully as he has observational astronomy. He leads with his desire to bring all these things together in one great culminating work. His instinct is right, but the science goes off in the wrong direction, and is incomplete.

From a literary perspective, it’s a perfect book. From a scientific perspective, it is a very interesting book that has a lot of sound insights, one big problem and a number of other unanswered questions. It is plausible, for many highly probable, but is it certain? This raises a philosophical question—and this is another way some inquisitors read the book and it is something that also interests Finocchiaro, by the way—of whether Galileo was a good philosopher. They say no, because he’s not following Aristotelian logic.  Of course he’s not, that’s not what Galileo wants to do! But they use that to critique the book, that this is not a work of good philosophy and that he’s failed the test of logic in a very formal sense. So that’s fascinating.

So he’s written a completely out of the box book, just like The Assayer is an unorthodox manifesto of his method and epistemology. There is no book like the Dialogue, even though there are dialogues that inspired it. Most of the others, we don’t think of as great literature—unless it’s Plato or Castiglione’s Book of the Courtier.

Did he have enough data to prove the Copernican system or not?

He had great preliminary data. The phases of Venus and his observations on sunspots are more decisive than the moons of Jupiter, spectacular as they are. They’re still trying to figure out what Saturn is. Galileo thinks it might have some moons as well. Later on, after his own lifetime, through some very interesting observational and deductive thinking, people figure out that, no, it’s a ring. That’s in the middle of the 17th century.

Galileo gives us many of the foundational building blocks and raises so many questions about traditional cosmology that there’s no going back. People at the time who criticize him about other things know that and there are very few pure Ptolemaic or Aristotelian philosophers wandering around Italy by 1633. Just to give you an example, by 1620 the Jesuits have decided that their official astronomy is that of Tycho Brahe, who is a Danish Lutheran. That’s perverse and yet it’s an important intermediate step. Tycho Brahe advocated for what’s called ‘geo-heliocentrism’ in which the Earth is still the stationary point of the cosmos, and the sun and the moon are going around the Earth, but all other planets revolve around the sun. Then you have to do these fancy calculations to make that work so they’re not colliding with each other.  It’s an awkward synthesis of the old and the new.

After all, if we think about this in the large scale of the cosmos—this was Kepler’s great insight—it’s all near the center. This is why Kepler writes science fiction, when he writes his Somnium and he imagines what the universe would look like from the Moon. He even does it with Mars, at one point. It’s all near the centre, mathematically, you can make it work, you just have to keep playing around. It’s harder from Saturn or anything further out, but anything that’s close to the centre can kind of be the centre. But that kind of playing around did not appeal to Galileo. He was a much more straightforward kind of thinker. I think that’s why he sometimes misses things that Kepler had much better insight into, like the elliptical path of planetary orbits. Galileo tells you in The Dialogue—that’s another thing that’s wrong­­—that everything moves in perfect circles. He’s still being more traditional than Kepler, who says, ‘No, it’s the circle and the line combined. It’s elliptical.’  He, not Galileo, begins to envision some invisible forces that produce this result, what Newton will call gravity.

So are you from a science background or a history background? Because to really understand Galileo, you need to be really on top of the science as well as the history.

Many of us in history of science come in with some combination of a background in science and history. I didn’t get my first degree in science. I’m one of those people who loved science in high school. Even when I entered college, I thought, ‘I’ll major in science, but also do something more humanistic.’ The problem was, I could never figure out which science I liked the most. What I found was that the history of science, especially the early history of science, was a wonderful place to ask these very foundational questions that we now take for granted and where there are not clear distinctions between disciplines. Galileo is such a great example. He is a Renaissance man, to use that classic phrase. He’s the son of a musician and his Dialogue is inspired partly by a dialogue his father had written between ancient and modern music that is part of the story of the birth of opera.

Galileo has interesting relationships with artists, he has strong opinions about what’s good literature. And he’s the great observational astronomer of his day who transforms this instrument into something productive, useful, and generative of an entire new way of thinking about astronomy. So he embodies that fluidity of disciplines. That’s what gives his Dialogue an especially distinctive flavour, because there’s not even a hint that it’s supposed to be a scientific textbook. That would be boring. It would be written in Latin and only for specialists (as Copernicus did) and be read by their students in the mathematics and astronomy classrooms of Renaissance Italy. Galileo wants to write for the world. He’s a very ambitious guy.

So are all his books written in Italian?

All of the works that he publishes directly are in Italian except for The Sidereal Messenger, which is in Latin. The reason is that he published his pamphlet on how to use his military and geometric compass in Italian in 1606 and one of the students, Baldassarre Capra, who took his tutorial, plagiarized it and published a Latin translation under his own name. Galileo then had an early intellectual property debate, where he asked the University of Padua to adjudicate in his favour, which they did. So, he’s already learned an important lesson—when you have a really valuable discovery and you publish it in your local language, somebody else can try to claim it. That was bruising, and it was recent and very fresh in his memory.

The telescope is a much bigger innovation than his compass. When he publishes the Sidereal Messenger, he deliberately chose to publish it initially in Latin because he wants it to be internationally received with no question that he is the author of this book, the person who has done these observations. In fact, he does this to such a degree that he actually erases the collaborations with his friends in Venice and Padua from the text, to their dismay, because he then takes a job elsewhere. So, he disses them on multiple levels. That’s another great thing about Galileo’s Telescope, the authors get you to see the way in which Galileo erases all of them, because he’s using this book to move on. It’s such a VC kind of story. It’s the IPO of 1610 and to do it he has to publish in Latin.

Then, the question is why he resumes publishing in Italian from 1613 till the end of his life. Doesn’t he want that international audience? Well, yes, but not in the same way. He sees these other publications as engaging the Italian-speaking public he is now cultivating after his celebrity in 1610. Galileo is a scientist who thinks a lot about the interface between science and society. When he writes The Sidereal Messenger, it has to be in Latin to be international, but when he wants people to appreciate the quality, style and flavour of his argument, he wants it to be in his own vernacular.  Ultimately, he is somebody who has not only read Dante and many other great Italian literary authors but he has also participated in the time-honoured Florentine project of calculating the shape and size of the Inferno (a lecture that he gives to the Florentine Academy). He’s very proud of his literary style and the way it communicates his ideas, not only to those who have some version of this as their written and spoken language, but also to foreigners who read and appreciate it. This is an era in which Italian is an international language of diplomacy and travel. He knows it’s not going to completely restrict his audience in the way that, say, Shakespeare’s English did at this time.