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Astronomy with Dante

March 29th, 2023 | 24 min read

By Susannah Black Roberts

Sperello di Serego Alighieri is an Italian astronomer. Beginning in 1990, he was at the Arcetri Astrophysical Observatory in Florence, but he has retired from that. More recently, his interest has turned to his ancestor, the poet Dante Alighieri. This past year was the 700th year since Dante’s death, and to mark the occasion, he wrote a book, The Sun and the Other Stars of Dante Alighieri, which has just been translated into English.

Susannah: Sperello, this is actually the third interview that I’ve done with you; you’ve been very kind to speak with me about a variety of topics. In this interview, we’re going to talk about one particular canto in the Paradiso. When I asked you which section of the Commedia was most important to you in your work, or that spoke to you most as an astronomer, as a scientist, you mentioned Canto II of the Paradiso. Can you say more?

Sperello: Yes. Well, there are things to say first. One important thing is the fact that I’m going to talk only about some aspects, actually one aspect of Dante, which is just one of the many aspects of Dante. And this is very important to understand because Dante, being a medieval man and a genius, he was able to have in his mind all the knowledge available at this time in all fields, and he could have all this knowledge, as well, in depth. This is something that we cannot even imagine because we are not able to do this.

Of course, the obvious reason is that every field of knowledge has developed since Dante. And so it’s not possible for a single mind, probably not even to Dante, if he was living today, to have command of all the knowledge fields in detail. We people who are busy with knowledge in various fields, if we want to contribute to knowledge, which means to make something new, we have to specialize in a specific field. And for the other fields, we cannot have them in depth. We can only have some of them, very little of them. So we are not used to having all the knowledge in depth.

And this makes for us a big difference compared with Dante. We have to understand this if we want to understand Dante. If you speak to one person, that person will only give you one specific field, only one aspect. In order to get all the aspects, you have to listen to different persons.

So don’t trust somebody who says, I know everything about Dante. It’s not possible. He’s lying to you. Just go ahead and listen to different people. So I’m only going to talk to you about one aspect of Dante, which is astronomy, cosmography, in Dante, because this is the only area in which I could be an expert.

Of course, I must say that when it comes to Dante, it’s very difficult, almost impossible to say something new because everybody has been working on him for centuries. So everything has been said already. But what one can do is try to collect knowledge and understand it, elaborate it and diffuse it.

Okay. So we are going to talk about science and astronomy specifically in Dante. And for this, indeed, it is very important, the second canto of the Paradiso, because it deals with a mistake, an error.

For us scientists, error is not something to be afraid of. We are used to errors. We know that we are making errors in science. Actually, many times science progresses by understanding an error. These are many examples of this. So in this Canto, Dante talks about error. And he talks about it in a scientific way.

The canto deals with the moon spots. This is the beginning of the Paradiso. So Dante has just left Virgilio, who led him through Hell and Purgatory to the Earthly Paradise, which is on the top of the Mountain of Purgatory. But then Virgilio had to abandon Dante because he could not progress further since he was not baptized. So Dante had to choose another leader.

And not by chance, it was Beatrice. Of course, Beatrice! Everybody knows about Beatrice. And we are going to learn more about Beatrice today.

So they go up to the first sky, the first heaven, the sphere around the Earth, which is the sphere of the moon, which is the closest to the Earth. They get to the Moon and Dante asks a question to Beatrice.

I think I’m going to read some verses first in Italian, and then I will give you the translation in English.

So Dante says,

“Ma ditemi: che son li segni bui
di questo corpo, che là giuso in terra
fan di Cain favoleggiare altrui?”

Dante says, “But tell me, what are the dark marks in this body, that make people down there on the Earth tell fables about Cain?”[1]

He asks Beatrice, What are the moon spots? The moon spots — at the time, people thought that the spots on the moon resembled the face of Cain. Of course you can think that! Surely it doesn’t matter.

But it’s actually important. The point is that the only heavenly body which you can distinguish at night by naked eye is the moon. All other heavenly bodies by naked eye — and they didn’t have instruments at the time of Dante, of course, they didn’t have telescopes or binoculars; nothing but the naked eye. All heavenly bodies at night are a point. Jupiter, Mars, Venus, all points. The only one which is not a point, where you can distinguish features, is the moon.

And, well, it’s a problem — because the moon has spots. Well, they told us that everything in the sky is perfect. Circles, spheres, everything perfect. But the only body that we can see has spots! So it’s not perfect.[2] Come on, how’s that? So this is a very important question to ask, and Dante takes the opportunity to ask it of Beatrice.

The answer is quite long, takes up almost all this canto, and that’s what we are going to talk about. So first, “Ella sorrise alquanto.” This means she smiled a little. And when Beatrice smiles, something important is going to happen. Then, she says

“S’elli erra
l’oppinïon”, mi disse, “d’i mortali
dove chiave di senso non diserra,
certo non ti dovrien punger li strali
d’ammirazione omai, poi dietro ai sensi
vedi che la ragione ha corte l’ali.”

She says, “If the opinion of mortals errs where no key of sense unlocks the truth, surely the arrow of wonder ought not to pierce you now, since you see that reason has short wings even when following the senses.”

So essentially, Beatrice says, “well, be careful, because the first thing that comes to your mind may not be the right one.” But then comes something that we are very used to, because every one of us has been a pupil at school. Some of us have been masters or professors.

She says, “Ma dimmi quel che tu da te ne pensi” – “But tell me what you think of this yourself.”

When a pupil asks his professor a question, often the professor turns the question back to the pupil. This is very common. And so this is what Beatrice does. Well, Dante has to say something! So he says,

“Ciò che n’appar qua sù diverso
credo che fanno i corpi rari e densi.”

“What looks different to us up here is caused, I think, by bodies that are rare [that is, rarified] and dense.” So essentially, Dante says that maybe the moon spots are due to different densities in the moon.

This is an idea that was not new. It’s an idea of Averroes, and also Albert the Great, and it’s also recalling the Convivio.

Susannah: So the idea is that if there were denser spots, it would be darker or something like that?

Sperello: Well, yes, but we’re going to go into details about this. So, yes, the idea was denser parts would be probably darker. But Beatrice says,

“Certo assai vedrai sommerso
nel falso il creder tuo –”

so this means “You certainly will find that your belief is much submerged in error” So, you see, this is about error. Then,

“se bene ascolti
l’argomentar ch’io li farò avverso.”

“If you listen carefully to the argument I shall make against it.” So, “Listen to me: You’re wrong. And I’m going to show you how.”

So she goes on,

“The eighth sphere displays to you many lights, which you can see, both in quality and size, are stars with different faces.[3] If rarity and density alone caused those differences, then one sole power would be in all of them, distributed into greater or lesser, or sometimes equal force. But different powers must necessarily be the fruit of different formal principles.[4] Then these, except for one, would, according to your way of thinking, be destroyed.”

So Beatrice essentially tells Dante, “You’re wrong. Listen to me.” The first thing she does to show that what Dante said is wrong is actually some kind of similitudine, I don’t know the English word.

Susannah: A similitude — she’s almost doing a reductio ad absurdum: She’s saying, if your theory is right, then these other things would be the case, would follow, and those conclusions are obviously wrong.

Sperello: Yeah. Okay, so she talks about the eighth sphere. The eighth sphere is the sphere of the fixed stars. It’s actually the only sphere which does not have just one heavenly body, it has many bodies, all the fixed stars. And Beatrice essentially says, You look at the stars, and there are some that are brighter, some dimmer. So if this difference were due to density, as you posit, for the moon spots, then the stars would only have one virtue, which is density.

And this is not possible because every star has its own virtue and it transmits it to the Earth, to humanity. This is a sort of philosophical and also, in a sense, theological argument.

Susannah: Virtue, in this sense, would mean something like power.

Sperello: Yes, virtù, we say in Italian. So this first confutation, if you like, is not very scientific, but then it goes on in a more scientific way. Beatrice says,

Ancor, se raro fosse di quel bruno
cagion che tu dimandi, o d’oltre in parte
fora di sua materia sì digiuno
esto pianeto, o, sì come comparte
lo grasso e ’l magro un corpo, così questo
nel suo volume cangerebbe carte.

She basically says, if the moon has different densities and these are causing the moon spots, then there can be only two cases. Either the dense parts do not cover the whole face of the moon or they cover it. Okay. Then she says, “Se ’l primo fosse,” “To validate the first case, that those areas of density don’t cover the whole face, then during a solar eclipse, you would see the sun’s light through the moon, through the more rarified parts of the moon.” But we all know that during a solar eclipse, when the moon is between us and the sun, the moon covers the sun completely, no sunlight can go through.

So the first case is not possible. Beatrice says

“Questo non è: però è da vedere
de l’altro; e s’elli avvien ch’io l’altro cassi,
falsificato fia lo tuo parere.”

“This is not so; so now we have to consider the other case — if I show that one wrong as well, then your hypothesis is completely falsified.”

So. Then Beatrice takes up the second hypothesis that she’s drawn out of Dante’s idea. She says,

S’elli è che questo raro non trapassi,
esser conviene un termine da onde
lo suo contrario più passar non lassi;
e indi l’altrui raggio si rifonde
così come color torna per vetro
lo qual di retro a sé piombo nasconde.
Or dirai tu ch’el si dimostra tetro
ivi lo raggio più che in altre parti,
per esser lì refratto più a retro.

It could be, she says, that dense areas do cover all of the moon, but they are at different depths inside the moon. And these different depths makes the spots.[5]

But, she says, this too is not possible. And in order to prove that this is not possible, Beatrice does a thought experiment, a Gedankenexperiment, like Einstein did many years later, to prove his theories. She says, OK, take three mirrors and place two of them at the same distance from you, but a bit separated. Then take the third, and put it further back than them, and between them. Okay. Then, take a light — of course, this would be a fire of some kind, a torch or something — and put it behind you so that it will be reflected in all three mirrors. Then turn to face the mirrors.

Now what you will see is that although the image of the light in the farthest mirror is smaller, all three have the same brightness. She says, Clearly,

Ben che nel quanto tanto non si stenda
la vista più lontana, lì vedrai
come convien ch’igualmente risplenda.

This means that even though the more distant image is not as extended in size — it is smaller — you will see that it is equally bright.

Now, this thing which Dante explains here, to us astronomers is a very well known fact: the fact that surface brightness does not depend on distance. This is the principle of the independence of brightness on distance. Now, astronomers, we know this very well. It’s very clear to us. It’s one of the fundamental rules for observing objects in the sky. It is a very simple thing, actually, because the surface brightness is just a ratio. If you have an extended source in the sky, you take the ratio between the total brightness of the source and the solid angle that the source subtends in the sky. It’s the ratio between these two. Since both these things, the total brightness and the solid angle, vary inversely as the square root of the distance, their ratio is independent of distance. This is very clear. It’s a very simple thing.

So there’s something really wonderful happening here — really wonderful. Well, I want to point out three things. First, Dante chooses a woman as a scientific master. This is something that we have to wait centuries for, before women would be even heard of in science. I can think of Marie Curie, or somebody like this, but this is only much later. Dante did it already seven centuries ago. I think this is wonderful.

The second thing, which is also amazing, is the fact that Dante uses a Gedankenexperiment, a thought experiment, in order to disprove an argument.

The third thing, which is also wonderful, is the fact that Dante anticipates the law about the independence of surface brightness on distance, which, of course, was not known at the time. And so I think this is really wonderful! I think we can give Dante a Ph.D. in physics.

Susannah: The other cool thing — there are two interesting things here. One is that Beatrice says at the beginning of that section, “You see that your reason, even when supported by the senses, has short wings.” Basically, your reason, even propped up by or checked by your senses, is not going to get you the answer here. Which is, you know, appropriate because he’s just left Virgil, and now he is in the realm of revelation rather than reason.

But then — Beatrice uses reason! She doesn’t dismiss reason, even though she is a representation of knowledge that comes through revelation rather than through reason. She still uses reason. And she’s got this amazing line right before the section that you just read, “Experience, if you let it be your guide, the fount for every stream of human art, can set you free from this objection, too.” That “Experience” is the Italian esperïenza. But Mandelbaum translates this as “Yet an experiment, were you to try it, could free you from your cavil — and the source of your arts’ course springs from experiment.”

She’s putting forward as a principle that experiment or experience, with reason, is what will lead you to the truth even after you’re in the realm of the Paradiso. It’s wonderful.

Sperello: Yes. I think the fact that, as you say, although we are in Paradise, in the cantos of faith, of the glory of God, Dante does not abandon reason. I think this is very much connected to the fact that in Dante’s mind, everything was uniformly together. There is no separation between things.

Susannah: The realm of reason against the realm of revelation.

Sperello: Right. Instead, he is able to join everything in the way that for him seems to be best. This is I think amazing and that makes this canto actually very beautiful.

I want to say one more thing. During the last year, I did many conferences about cosmography in Dante because it was Dante’s Centennial. And at one of these conferences, a professor came to see me after the conference and he said, “well, I’m a professor. I’m teaching Dante in high school. I like it very much. To my students, I read every single canto of the Divino Commedia, but I always skip the second canto of the Paradiso because I do not know how to explain it.” And then he said, “well, now I have understood. I had not understood before.”

Also, if you look into most Divina Commedia commentaries, they don’t tell you about the independence of surface brightness on distance. But this is essential to understand it. If you don’t know that, there’s no way you can understand it.

What I think is that in many cases a genius is able to work across time. A genius is not linked to his own time and level of knowledge. A genius is such because he can fly above and do things that others would do only centuries later. And, in fact, this is one aspect that had not been understood probably for some time after that.[6]

Susannah: I wonder if we could talk now about the problem of light. This is something that we actually talked about a little bit, and I hadn’t really thought about until you said something in the first interview about Dante’s vision of understanding of the rainbow.

Sperello: Let me tell you one story. I think it was the year 2015. UNESCO declared it to be the International Year of Light. In that year, I went to a conference in Rome, at the Pontifical University in Rome. And the title of the conference was “Fiat Lux.” You know what it means?

Susannah: “Let there be light.”

Sperello: Yes. And of course, at that conference, it was very interesting because there were many different people dealing with light. There were some priests, there were engineers, biologists — because there are animals that produce light — and so on and so forth. And there was also me, as an astronomer. I think there were a couple of other astronomers, too.

During my talk, I pointed out one very important thing: the fact that light is made of photons, and light is essentially our only information source about the universe. All we know about the universe is given to us by light, by photons. Well, there are some exceptions, like in the solar system. We can go to the moon, we send probes to Jupiter, Mars, et cetera.

But this is only a very limited part of the universe, so it doesn’t count very much. And there are also particles getting to us, like high speed electrons or protons. They give us some information, but it’s really very limited. All we generally know about the universe comes from light. You couldn’t tell anything, any information about the universe that comes from particles.

Well actually, there is one more important exception, which is gravitational waves. Gravitational waves have been discovered, and they’ve already given us important information about black holes and other things in the universe. But that’s only starting.

So more or less everything we know about the universe is due to light. For us this is obviously very important.

Susannah: Just to clarify, this means for example, when we’re looking out with a telescope, what we’re perceiving and what we’re understanding based on what we perceive, this is all based on what we see — obviously not necessarily with our naked eyes; there’s other forms of perception, detectors and so on. But they all have to do with light.

Sperello: Yes, because when I say light, I mean also X-rays, radio waves, UV light, infrared, microwave, these are all photons. It’s the same thing.

So what sort of information does a photon bring to us? Essentially three kinds of information. One is the direction the photon comes from, which is, if you like, described by two angles in the sky. Of course, when you have several photons, then you can make an image with all the directions. Okay, so direction is one kind of information.

The second kind of information is the frequency, or the energy, of the photon. Photons can have different energies and different frequencies which is equivalent to the wavelength of the light that we see. For example, X-rays have very short wavelengths; radio waves have very long wavelengths. So this is the other kind of information — energy.

And there is a third one which is not very well known, which is polarization. Polarization is not well known because our eyes cannot see it. Our eyes can see direction. Of course! And they can see the energy of the photons, because they can see color. But they cannot see polarization. So this is more mysterious for us, but it’s very simple.

Photons can be used as electromagnetic wave. An electromagnetic wave is made by an electric field which varies, and perpendicular to it is its magnetic field, which travels. Okay, now when the photon comes to you, the electric field oscillates in a plane which has a direction in the sky, you can see that.

So that direction, which can be expressed as an angle in the sky, if you like — a position angle, we call it — is the polarization. That is the additional information that the photon brings to us. And I’ve devoted much of my astronomical career to the study of polarization of astrophysical sources. It’s very important. It’s difficult to do because the instruments are difficult. But anyway, okay, it’s important to look at polarization if you want to understand the universe.

Because polarization can bring you geometrical information about a very distant source. Take a very distant source that you cannot distinguish because it’s too far. But still, if the light is polarized, the polarization brings you geometrical information about the source: that position angle.

Okay. Anyway, there is one more thing I want to say about light, which was actually very amusing. I gave a talk at the conference: the topic was light and darkness. Of course, for us scientists, light exists because it’s photons, electromagnetic waves. Darkness does not exist, it’s just a lack of light.

But since at the conference, there were many people, with different backgrounds, I thought, well, yeah, but the common people or the poets, for them, darkness exists just as well as light, as something real, something that they can talk about, that they can feel. You can feel the darkness in a way when you are scared. Or a poet, of course, can write a poem about darkness. Why not?

So for scientists, light exists. Darkness does not exist because it’s just a lack of light. But for the men in the street, both exist.

After I gave my talk, there was a coffee break and then a guy came to me and he said, well, what you said is very interesting. And he said, I’m a philosopher. And I work on — what do you call the kind of philosophy that deals with things that exist?

Susannah: Ontology.

Sperello: No.

Susannah: It could be metaphysics?

Sperello: Metaphysics! Yes! He said, I’m a metaphysician, so I deal with things that exist. Well, then I asked him, then you can tell me — you should be able to tell me whether light and darkness both exist.

Well, he said, well, this is a difficult question. Let me think about it. He went away, thinking about it. The following day, he came to me and said, well, for us metaphysicians, neither exists because only things exist. Light and darkness are just attributes of things — not real.

Well, now the panorama is complete! For the man in the street, both light and darkness exist. For the scientists, only light exists. For the metaphysicians, neither exists!

Well, my conclusion was that God thinks that we are right, we scientists are right, because he created the light. He did not create darkness. He created the light and separated it from darkness, which is exactly what we think. So we are correct.

Susannah: I agree with you! I don’t know what species of metaphysician he was, but I will tell you that if he were a Thomist, I think he would be closer to agreeing with you. Because there’s this idea that there’s a conversibility of goodness and being in Thomistic metaphysics. Everything that exists is, to a certain degree, good because everything that God creates shares in his attribute of being, and since he is simple, that’s not a separate thing from his attribute of goodness. And so when we talk about sin, or evil Again, the man in the street thinks that sin and evil are things. And they kind of are — like we can perceive when someone is being evil. Putin invades Ukraine. This is sinful. This is evil.

But in a certain way it’s a lack of virtue, there’s an absence of goodness. It’s not a thing in itself. And I think that often God makes physical realities that are in some way metaphors of spiritual realities. And I think that the physical reality of light existing and darkness not existing is a very good analogy for the way that goodness exists and evil or sin are not as real.

Sperello: Yeah, maybe… Yes. Anyway, I thought that was quite funny. And light is very important!

Susannah: Can you talk about the section on the rainbow?

Sperello: Yes. In the 29th canto of the Purgatorio, Dante describes the bright colors of the luminous train left by the seven candelabra carried in procession; shortly before the meeting with Beatrice, he referred to the rainbow and to the moon halo. The moon is mythologically assimilated to Diana, the Greek Artemis, born in Delos and called Delia.

In fact, this is quite interesting because both the rainbow and the moon halo are due to the same phenomenon, which is a refraction of light in drops of water. So it is quite interesting that Dante associates them here.

Going on. Because this metaphor is scientifically very appropriate because, in both cases, colors are due to refraction within drops of water. Then Dante in canto 25 of the Purgatorio mentioned the fact that the rainbow is due, as we well know, to refraction of the sun’s rays in the raindrops. And he says

E come l’aere, quand’ è ben pïorno,
per l’altrui raggio che ’n sé si reflette,
di diversi color diventa addorno —

This means “and just as air when it is very moist, becomes adorned with various colors because it reflects another’s rays…”

So he has this idea about rays being reflected, which is in fact true. And then there is another citation. We can also read this again in Canto XII of the Paradiso. There, the poet describes the double crown of spirits that appeared during the speech of Thomas Aquinas by using the double arc of the rainbow — because you know the rainbow has a double arc. We’re going to talk about this. And then he says,

Come si volgon per tenera nube
due archi paralelli e concolori,
quando Iunone a sua ancella iube,
nascendo di quel d’entro quel di fori,
a guisa del parlar di quella vaga
ch’amor consunse come sol vapori,
e fanno qui la gente esser presaga,
per lo patto che Dio con Noè puose,
del mondo che già mai più non s’allaga:

So this means, “as through a tenuous cloud, two arcs curve parallel and colored alike, when Juno commands her handmaiden, the outer born from the inner one, like the speech that the desirous nymph whom love consumed as the sun does vapors, which cause people here to predict the weather, thanks to the pact God made with Noah that the world will never again be flooded.”

So this is very interesting because Dante talks about the two arcs of the rainbow and he says, well, first they are parallel, which is quite obvious.

But then he says “the outer born from the inner one,” which is exactly what happens because the outer is due to a double reflection inside the drops of water. So it is, in a sense, born from the inner one.

Susannah: This is one of these things where you’re like, how could he possibly have known? I mean, he could perceive the double arc because sometimes you can see it, even though one is fainter. But how could he know that that was the cause?

Sperello: Because probably he had noticed that the colors in the two arcs are inverted. And so this probably gave him the idea that the outer was born from the inner one because there is some kind of reflection, additional reflection, one reflected from the other. I think you could see that. But of course, in order to describe this in a few words, so precisely, it’s very difficult. And he was able to do it.

Susannah: Yeah. This is, again, one of these examples of his synoptic vision, the completeness of his vision, because he brings classical myth and Biblical reference and scientific observation together in these incredibly condensed ways.

I was disappointed that you didn’t have a whole chapter on light in your book. I think that it would be really interesting for you to write, at some point, a piece that was just looking at light in the Commedia, but also maybe looking at the way that optics and light have been a driver for our understanding of cosmology through the centuries.

Sperello: Yes. Well, you are right. I would have to study for this because I know that there are many references to light sometimes in a scientific way, though not only in a scientific way, but also in that which have nothing to do with cosmography. So not in this book. It could be in a book for the next Centenary.

Susannah: Don’t make me wait that long! If you ever want to write this, I will help edit it and I will publish it. I would love this.

And here’s another metaphor. Right after the section in Canto II in the Paradiso that you were reading before, Beatrice says — this is the Esolen translation —

I wish to fill your intellect with light,
Light so aflame with life that cannot cease
In your eyes it will tremble like a star.

And obviously, she’s speaking of an intellectual understanding of the world. But it’s another one of these things where it seems to me like the way that God has made the world means that our metaphors are very powerful reflections of reality. As I think you mentioned in one of our earlier interviews, the only knowledge that we can have of the world, including the physical extent of the universe, and also its extent into the past has to be within the light cone of the big bang.

Sperello: Yes, of course.

Susannah: The horizon of the possibility of human understanding is specifically within that perceptible created order. That’s very interesting to me. Obviously, I don’t know what I’m talking about, I’m an English major, but I do really enjoy reading around the edges of things like this.

But it is also one of the things that makes me irritated when people get very excited about trying to hypothesize about string theory and so on, about the “cause” of the Big Bang, or about the idea of oscillating universes. It seems to me that, first of all , they’re trying to avoid the idea of an origin like that because it seems too much like creation. And second of all, it seems to me that it’s literally speculating about things that, in principle, we can’t know because they’re outside that light cone of the Big Bang.

Sperello: OK, so, one thing: the Big Bang is the wrong name, it implies wrong things: “bang,” “explosion,” means that something explodes inside something else. But this is completely wrong. What people call the Big Bang is actually a singularity. It’s the start of space and time in one point. This is very important, actually. For scientists, this is quite obvious. In that point, the singularity, there was nothing existing before, or at least, we cannot know about whether there was something existing before. So it’s not a matter for science. Everything that we can talk about started then in that singularity.

Now, whether that is similar to creation, it doesn’t matter for a scientist. Well, it matters. Actually, people, before there was evidence about the Big Bang thought that the universe was infinite and existed forever, which is very much against creation, but it made sense. Why? If you just look, well, time goes on. It went on in the past. Why should it have started at some time from a physical point of view?

So there were theories about the universe being infinite. And actually, even when it was obvious that the universe is expanding, I mean, galaxies are getting farther and farther away with time, there were people like, what’s his name? Gamow? Not Gamow. A British guy.

Susannah: Bethe? No, not Bethe.

Sperello: Anyway, this guy made a theory by which there was continuous creation of matter to compensate for the decrease in density due to the expansion of the universe.

Susannah: Right! What was his name?

Sperello: I met him… We talked a lot. He’s a nice person. He’s a very nice person. Very bright. What’s his name?

Susannah: It’s going to drive me crazy. I know exactly who you mean.

Sperello: He also wrote books for the public….

Susannah: Hoyle! Hoyle!

Sperello: Fred Hoyle! He was very much against creation, I think, for philosophical reasons. And he had his theory about the universe going on forever, and being essentially uniform and constant. And when it became obvious that the universe was expanding, he made a theory by which there was creation of matter to compensate for the decrease in density. So you see how crazy one can get — the spontaneous creation of matter! Well, come on!

Susannah: In a weird way, it’s the desire to save the appearances, like with the Ptolemaic system and its epicycles. We have this idea of uniformity of causes and general conditions, this idea that the universe has basically always been the same, and then we see that the galaxies are receding from each other, you need to admit that there’s got to be a cause that is not a cause that we see operating.

Sperello: Well, first of all, you have to admit that there is a change, right, in a global property of the universe which is density. Because if galaxies get far away, then the density must decrease again. This is exactly what Dante is running into. How can there be change in the superlunary world? In the spheres above the terrestrial sphere? How can there be change?

Susannah: I feel as though again, we could go on for a long time.

Sperello: Yeah, but you have to get married.

Susannah: I do have to get married! I have to get back to my wedding planning. This has, as usual, been completely fun. Happy Easter!

Sperello: Happy Easter!

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  1. It used to be a commonplace that the “man in the moon” was Cain, the brother of Abel
  2. The sublunar world, the world of the earth and of human life, was thought to be the domain of change and imperfection, wounded by sin but also naturally changeable, whereas the superlunary cosmos was perfect.
  3. i.e. it’s plain to see that these are different stars with different qualities, colors, sizes, luminosities, etc.
  4. I.e. stars have different “powers,” virtues, and these must be the fruit of different forms/formal principles; Beatrice is drawing a conclusion based on the premises of hylomorphism, i.e. Aristotelian/Thomistic physics, and on what is obvious to Dante, that each of the stars influences human affairs in different ways.
  5. In other words, perhaps the sun’s light is reflected more dimly to us by the farther-away dense parts of the interior of the moon; Anaxagoras (5th century BC) had already known that the moon shines only by reflected sunlight; Beatrice here takes that for granted.She says, essentially, “If some parts of the interior of the moon are denser, then those dense parts will act like the lead at the back of a mirror, throwing the light back out in reflection. Now, you’d probably argue that that’s the explanation, that’s what makes the spots: that when a ray of light is reflected from a dense part of the interior of the moon that’s further back, deeper in the moon rather than on its surface, that reflection would be dimmer, and that’s how you get your moon-spots. But you’d be wrong.” And then, as Dr. Alighieri describes, she shows how this would be confuted by an experiment.
  6. I noticed, as I was editing this interview, one more amazing thing: Dante uses a thought experiment having to do with earthly things, mirrors and a torch, to talk about superlunary things, the light of the sun reflecting off the moon. That idea of uniformity of cause or uniformity of law in the sublunary and superlunary realms is one that you only begin to get in the 16th century with Copernicus and the idea that the heavens might be a place of imperfection and change like earth, and with Brahe’s observation of a nova, a new star, and which only came to fruition with Newton’s recognition that the same law of gravity described the motion of things on earth and in the heavens.

Susannah Black Roberts

Susannah Black Roberts is senior editor at Plough. She is a native Manhattanite. She and her husband, the theologian Alastair Roberts, split their time between Manhattan and the West Midlands of the UK.