A ‘Literally Awesome’ Phenomenon
With us today is Daniel Kennefick, professor of physics and a researcher at the Arkansas Center for Space and Planetary Sciences. Kennefick is the author of a book on a British expedition to observe a 1919 eclipse, to test Albert Einstein’s general relativity theory. He is co-author of An Einstein Encyclopedia, published by Princeton University Press, and he is a scientific editor for the Collected Papers of Albert Einstein.
Welcome, Dan, and thanks for being here.
Daniel Kennefick: Thank you for having me.
MM: Every day I see in the news there’s another article about the eclipse that’s coming up on April eight. So the excitement is building for this. I think most people are familiar with what an eclipse is, but I’d like for you to kind of, more from a scientific perspective, tell us what is happening during a total solar eclipse. It seems simple, but it’s not. You’ve got a couple of orbits. You have an orbit within an orbit. The moon is orbiting the Earth. The earth is orbiting the sun, and these things have to line up. So, the question then is what you know, what conditions must occur for there to be a total solar eclipse?
DK: Well, the most important, as you know, is that the moon must get in between us and the sun. What happens during a total solar eclipse is that the shadow of the moon falls on us. And of course, that’s something that wasn’t understood in very early times. It already took some scientists or perhaps natural philosophers, as they might have thought of themselves back then, at least in ancient Greek times, at least that far back, to realize that that’s what was going on. And if we think about it from the point of view of our position here on the earth, trying to simplify things by ignoring our motion around the sun and just thinking of it as being the moon’s motion that’s taking place, there are really three things that have to happen for a total solar eclipse to occur and two of those are more important. The first, obviously, is pretty straightforward. The moon has to be in the direction that the sun is lying. And so that’s to do with the angular of motion of the moon. And that, of course, in a general way, occurs once a month at the new moon. A total solar eclipse can only take place at the new moon. And that, of course, is when the moon is in the same direction as the sun from an angular point of view. And in those situations, of course, we can only see the side of the moon that’s not lit up by the sun. So we’re not usually aware of the new moon. It’s rather impossible to see. But a second thing that’s very important and critical for the whole process is that the orbit of the moon is actually tilted with respect to the orbit of the earth around the sun. It does not lie in what’s called the plane of the ecliptic, which is the plane of the Earth’s orbit around the sun. And it gets its name because the ecliptic is where eclipses take place. The moon in its orbit on its plane, intersects with the orbit of the ecliptic twice a month at places called nodes. And the eclipse will take place if you have a new moon, which occurs precisely at one of these nodes in the lunar orbit. So these are the two main requirements for a total solar eclipse. Now, you may know that months can be defined in different ways. A month is somehow to do with one orbit of the moon. But because the moon’s orbit is relatively complex, there are several definitions of the month and there are none of them exactly the same length. The most important month that we’re familiar with is the Synodic month, which is the time between new moons. But in addition, there has to, in this case, also obviously be the time that it takes to go from a node in the lunar orbit back around to the next node, and that’s called the Draconic month. And it almost certainly gets its name from the ancient Greeks because of the ancient belief that a monster like a dragon was responsible for eating the sun during an eclipse. And therefore the dragon must have lived not only on the ecliptic but at the node. And so the time between that is still named after that ancient belief about a dragon that the Draconic month. And so those two months are not the same, but every 223 synoptic months that is to say, a little more than eight years, there will be a… they agree again. And so every eclipse is followed by another eclipse in the same cycle known as the Saros cycle from the ancient Babylonians, who were the first people to figure this out. And every eight years in a little bit, there’ll be another eclipse in the same cycle. Although I should mention at any given moment, there are several eclipse cycles going on. So we do see eclipses more often than that.
MM: So there are many cycles and you mentioned that the moon has a different kind of orbit of the earth, so that kind of leads me to the next question. And we’ve had one of these recently. Can you can you explain what an annular eclipse is? How is it different from a total solar eclipse?
DK: Absolutely. So we’ve already discussed two aspects of the equation, and that is what to do with two of the dimensions of motion that the moon has in this universe that we live in. There are three dimensions we’ve spoken about two going around in the circle, being either above or below the sun. But of course, the other is how far away from the earth is the moon. The moon’s orbit is actually reasonably close to a circle, but it’s not a circle. And that means that sometimes it’s a little further away. Sometimes it’s a little closer. And of course, when it’s closer, it looks bigger in our sky. Now, it’s already a remarkable coincidence when you consider how small the moon is compared to the sun, that the moon happens to be at just the right distance, to appear to be the same size in our sky as the sun is. It’s a pretty unique situation in the solar system. We’re privileged to have total solar eclipses because other planets typically don’t. Of course, sometimes the moon is a little further away and looks a little smaller on those occasions when the right situation lines up and we have a solar eclipse, we will, instead of having a total eclipse, we’ll have the annular eclipse because the moon won’t quite block out the whole of the sun. So it is in front of the sun, as it would be with a total. But it’s not blocking out.
MM: So we know these events are rare. A total solar eclipse. And I read that the next total solar eclipse to hit part of North America won’t come until 2023, for example. So that one and that one really… I think that said that it would only affect part of the Arctic in Alaska. So it’s technically the territory of the U.S., but in the way not many people are going to see it. Yeah, so but what’s interesting about this is we’ve had two to sorry, two total solar eclipses in the in the continental U.S. even to come to the Midwest even in recent years. We’ll have after this one on April 8. So why is this special, and maybe a different way of asking this question, for geography nuts like me… we talked about this recently… A different way of asking this question is why is Carbondale, Illinois, special?
DK: Well, Carbondale and the region around it in southern Illinois, especially because it lies at the intersection of the tracks of totality of these two eclipses that we’ve been very privileged, as you say, to experience in recent years. Back in 2017, the track went from west to east, across the continental United States. This one will go more southwest to northeast. Eclipse tracks always generally go run from west to east, but they can go south or north or other ways. In addition to that. And where the two tracks cross is, of course, the place where you’ll get to see both eclipses without having to leave your home, which is a very unusual, just as you said. Now, I should mention that solar eclipses do take place on average every year and a half or so. So most years will have a solar eclipse that’s visible somewhere in the world. But the track of totality, the shadow of the moon, is actually pretty small. For instance, not sufficient to cover the whole of the state of Arkansas when it passes over us on April the eighth. So that means that your chances of actually being in line to see totality are pretty slim. And you have to remember that the sun is so bright that unless all of it is obscured, you may not even notice it. Here in Fayetteville, we’ll probably be close enough totality so that it will be pretty obvious, but much further away than that and you may actually miss it. So that’s means that the total solar eclipse really is striking. And because of the narrowness of the track and the size of the earth to get one to I’m sorry, every seven years, as is happening for Carbondale, is extremely unusual. To give you a point of comparison, my own home country of Ireland hasn’t experienced a total solar eclipse since 1724 and won’t again until 2090. So most Irish people will go their whole lives without ever seeing a total solar eclipse. Unless, of course, they travel. And that actually is probably more typical. It’s probably more on the order of a century or two between total solar eclipses for most regions of the world. So Carbondale is pretty lucky.
MM: So this one of those this is one of those interesting situations where science and popular culture meet and yes, about an eclipse like this and so people get excited to observe it. And so there’s a lot of popular cultural importance. But historically, what I think is also interesting is that eclipses have also been used as sort of science, including your field, physics. So can you talk a little bit about your book, specifically the 1919 British expedition to observe a total eclipse solar eclipse? And how are those? You’re a Einstein historian. So how are those related to some of Einstein’s theories?
DK: Yes. Yes. Well, they were central not only to Einstein science and to his theories, but actually to his world fame. Of course, we’re very used to the idea that Einstein is more famous probably than any other scientist in history. And the reason for that actually is these eclipse expeditions that we’re speaking of. He became famous almost literally overnight as a result of these two English expeditions that were to different English observatories involved, though they collaborated very closely. And it’s remarkable to think, because this was, of course, before the Internet news had to travel in the old slow way. And yet within days or weeks of the announcement of the results, he actually shot to world fame. And the reason is that he was overthrowing the work of the then most well-known scientists in history, Isaac Newton, and replacing Newton’s theory of gravity with his own theory of general relativity. And what had taken place was that Einstein had realized in the course of his work on relativity theory that it was required to be able to address gravitation and that he probably would need to replace Newton’s theory. And one of the key insights that he had was that he realized, given his famous equation from the very earliest days of his work on relativity theory, E Equals M.S. Squared, that was specifically came into his mind because he realized that if light has energy, it’s probably necessary to conclude that it also has mass. And then he realized, if it has mass, why wouldn’t it be affected by gravity? Now, of course, light travels so quickly that we’re normally not conscious of the idea that light falls. It will have shot past us before we have a chance to notice anything falling about it. But in our solar system, there is one very massive object the sun. And if light passes close by the sun, it may be just possible to measure it’s falling. And this would be noticeable if a star close to the sun was shifted in its position a little bit by the near presence of the sun. That would be evidence that the light from the star, the star light had weight and fell marginally as it passed the sun, and this would deflect it from its path. And you can think of it like the sun acting like a gravitational lens, which is the technical term for this kind of thing. So Einstein, in about 1912, in 1913, when he was still working on his theory, hadn’t finished it, but he had seen enough to know that this was a very interesting question. He began writing and talking to astronomers and asking them, Could you test this? And of course, they pointed out that you can’t see stars close to the sun, and there’s only one time when you can, and that is during a total solar eclipse. And once he’d finished the theory, a number of astronomers and physicists realized, wow, this is actually pretty important. We really should make a particular effort to test it. And so the English, in spite of the First World War, which was raging at the time that Einstein finished his theory, in spite of difficulties, weather and equipment made enormous pains to travel because, again, of course, you typically have to travel if you want to see an eclipse to South America, Brazil specifically, and to the coast of Africa. Two different observatories, the Cambridge Observatory, the Greenwich Observatory and they came back study the results. It was a very difficult measurement and agreed that Einstein was right. And subsequent tests have amply confirmed his theory. And this, of course, is why he became the famous icon of science that he is today.
MM: And that was only possible because of the situation created by the total solar eclipse?
DK: Exactly. But for the fact that during the eclipse, you can see a star that’s close to the sun, you wouldn’t be able to make this measurement, not with the science they had at that time.
MM: And they were able to do this by looking at the Corona?
DK: So the corona actually get really gets in the way of this. The corona plays a very important part in the story because, of course, scientists, astronomers specifically, I should say, were very used to attending eclipses before Einstein ever made his request. And the reason is that they wanted to study the corona, the corona, which is really the atmosphere of the sun, is normally totally obscured by the brilliance of the star itself. It’s quite bright, similar in brightness to the full moon. But of course, the star, the sun is so bright that you just can’t see the corona. The only time you can see it is when the moon neatly obscures the body of the sun, but leaves the corona unobscured. And so it’s a remarkable thing to watch when you see the eclipse. One of the most striking things about the eclipse and the main object of scientists interest in going to observe eclipses because the corona can tell us a great deal about the nature of the sun. Unfortunately for Einstein’s experiment, it actually gets in the way because it’s kind of tending to obscure the starlight. But they were able to do it. Nowadays it’s actually with the kind of technologies we have today. It’s certainly more possible to work around the corona, but in some sense the corona was a little bit of an obstacle that they had to contend with.
MM: Interesting. Okay. I could talk to you about this all day. It’s fascinating. But unfortunately, we have a limited amount of time. But I do want to get your reaction to something I read. I was just kind of preparing for this interview and I read looking at NASA’s website and there’s a lot of great information on there about the upcoming eclipse, especially the information about safety and wearing the right viewing. Yes. They make sure to mention that about 10,000 times. So when they’re not talking about that, this was something else I read. So this comes right off of NASA’s website. “During totality, take a few seconds to observe the world around you. You may be able to see a 360-degree sunset. You may also be able to see some particularly bright stars or planets in the darkened sky.” You referenced that just a second ago. “The air temperature will drop and often an eerie silence will settle around you. It is also worth stealing a peek at the people around you. Many people have a deep emotional response when the sun goes into totality.” I thought this was fascinating and I’d like to get your reaction to it. Some of it makes sense, you know, the moon is blocking the sun, so we know that light is heat, so therefore, it would make sense that it would be a little cooler. But it’s fascinating to me that we would actually sense that. But also this this idea of a 360 degree-sunset and especially this eerie silence among people observing it, which I kind of remember from 2017. So tell me your thoughts on this. What’s going on here? On this last point, it strikes me is that maybe is it just something that humans… Is it just humans reacting to kind of a rare celestial phenomenon? What are your thoughts on this?
DK: That’s certainly part of it. And it’s true to say that we humans find something about eclipses a striking and resonant. One of the things that is most interesting about it is that historically people found them a little bit frightening. They came upon them all of a sudden they weren’t expecting it. And a clear difference between the modern world and the medieval or ancient world is our ability to predict the eclipses. And that’s something that was developed over a couple of thousand years by scientists. And it is something I think that means a lot to humans. It’s comforting to think that our dream of being able to use science to predict the future certainly came true, at least in this one respect. And we can very accurately note when this will occur and where you will be able to see it from. And that was something that in the way that we do it today, wasn’t even worked out until the early 19th century. So it’s quite recent for the majority of people to really be able to plan and think, I’m going to watch this, I’m going to think about it a little bit instead of it happening and going, my God, That’s right. What is going on here? That’s right. I do recall for the eclipse of 2017, we went to see it in Missouri that one of my children was sort of saying, well, why are we going to the trouble to see it? Won’t we be able to watch it on YouTube? And even she was said afterwards, you know what, that actually was worth it to be there. And so I do agree. Certainly the temperature I’ve been to two total solar eclipses and one was pretty much clouded out. But on the other hand, the darkness, the drop in temperature, these things are pretty dramatic even if you really can’t see the sun. And of course, the silence is not only extending to humans, but birds who are used to falling quiet at night do so for an eclipse because they think night has fallen. So there is something pretty impressive and I suppose literally awesome about it. We are very fortunate that even if there are were people on the other planets of our solar system, this is something that they would not be able to experience. And furthermore, it’s not something that will last forever. The moon, as we now know from the Apollo astronauts work, is slowly drawing away from us. So eventually, a long time from now, it will actually get so far away that it will no longer be large enough in our sky to actually make the total solar or total solar eclipse happen. So we’re pretty lucky from a situation from our location and from our place in time to be able to observe this and makes sense to take advantage of the opportunity.
MM: Daniel Kennefick, thank you so much for being with us here on Short Talks from the Hill. I really appreciate it.
DK: Thank you for having me. It’s a pleasure.
MM: Short Talks from the Hill is now available wherever you get your podcasts. For more information and additional podcasts, visit arkansasresearch.uark.edu, the home of research and economic development news at the University of Arkansas. Music for Short Talks from the Hill was written and performed by local musician Ben Harris.
