Dialogue Concerning the Two Chief World Systems

"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. 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 . 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. Get the weekly Five Books newsletter 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. 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. 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."