IRA FLATOW: This is Science Friday. I’m Ira Flatow. Now, after years of anticipation, the 2024 total solar eclipse is almost here. This Monday, April 8, the moon will line up perfectly between the sun and the Earth, and for a few short minutes, turn daylight into twilight in parts of North America creating a spectacular light show for those of us fortunate enough to be in the path of totality.

And that means more than 30 million people live in that path where the moon completely blocks out the disk of the sun. It’s going to stretch from Northwest Mexico across the US and to Newfoundland, the Northeastern part of Canada. And depending how far you are from the path, you might experience a partial eclipse, still exciting.

Here to preview this celestial sensation and answer some of your questions are my guests, Mark Breen, meteorologist and planetarium director at the Fairbanks Museum and Planetarium– that’s in St. Johnsbury, Vermont. He’s also host of Eye on the Sky for Vermont Public. And Dr. Padi Boyd, astrophysicist at NASA and host of the agency’s podcast, Curious Universe. Dr. Boyd is based in Greenbelt, Maryland. Welcome, both of you, to Science Friday.

PADI BOYD: Thank you, Ira. Great to be here.

MARK BREEN: Thank you, Ira.

IRA FLATOW: Nice to have you both. Mark, I recall seeing my first total solar eclipse during the last one in the US that I could see in 2017. And even though I had always heard about what to expect, this event entirely blew me away. I imagine the same thing is going to happen to folks this time. Very exciting, right?

MARK BREEN: Yes, and actually, this will be my first total solar eclipse. I’ve never experienced one before, and so my anticipation is as high as it can get.

IRA FLATOW: It’s true. And Padi, what do you say to folks who are like, nah, another eclipse? No big deal.

PADI BOYD: I honestly don’t know anyone who ever says anything like that. There’s such rare and beautiful phenomena. If you’re in the path of totality, it’s a few minutes of an otherworldly scene that you rarely get to be a part of in real time. So if you can, look up.

IRA FLATOW: Mark, we have a note from Maria in Valley Stream, New York, who writes and wants to know, what’s the difference between a solar and a lunar eclipse, and what determines how long they last, Mark?

MARK BREEN: Sure. So a solar eclipse is eclipsing the sun, solar, and so a lunar eclipse, the moon becomes eclipsed by the Earth. So they’re separate situations. The moon is in between the sun and the Earth during a solar eclipse.

It’s also a relatively brief period of time and seen over a relatively select area whereas a lunar eclipse covers basically anybody that’s on the night side of the Earth that has a view of the moon. It also lasts quite a bit longer. The totality during a lunar eclipse can be well over one hour.

IRA FLATOW: And Padi, what are we actually going to witness if we’re in the path of totality? Will we be able to see part of the sun?

PADI BOYD: So if you’re in the path of totality and you have clear skies, you’re going to see some amazing things. Make sure you’ve got eclipse glasses. They will protect your eyes. But you’ll be able to basically watch the moon pass right over the face of the sun. So the sun is this big bright ball in our sky.

The moon is going to be at new moon phase, so it’s going to be a dark ball in the sky. And you’re going to watch the moon continuously cover the face of the sun until, when you get to totality, the phase of the moon is completely covering what we usually see as the phase of the sun.

That’s called totality. Leading up to it, you’ll see some beautiful effects on the surface of the sun that are due to the moon’s surface. But then when you’re in mid totality, you’re going to see the corona, which is this very faint outer atmosphere of the sun not possible to see at any other time than during a total solar eclipse.

And we’re so excited here because not only will we be looking at the corona during this eclipse, but this is during a period of the maximum solar activity called solar max, so we’re expecting the corona to look quite active, especially compared to what it looked like in 2017.

IRA FLATOW: OK. So you say you want to go out. You want to see the eclipse. You’re getting all your equipment together. One thing you don’t need, Mark, is a pair of binoculars. And you don’t want that, right?

MARK BREEN: No, no, no, no. In fact, it’s one of those tempting things to really want to enhance your view that way. And yet, enhancing light from the sun just isn’t a good idea. And there are, of course, some special filters, but that’s generally recommended for somebody that really has some experience in terms of what they’re doing. For the casual observer, there are a lot of other ways that you can safely view the eclipse.

IRA FLATOW: Give me a few.

MARK BREEN: Sure. Actually, one of the fun things that people have done in the past, actually with the partial phase of this, is you take your fingers, and you basically make a little crosshatch, like a tic-tac-toe pattern.

And in between little spots between your fingers, a little bit of sunlight goes down through, so the sun is behind your shoulder. And as it shines on the ground, you’ll get these little tiny crescent suns. And you can do the same thing with a spaghetti strainer. All the little holes, you get these little tiny crescent suns that appear on the ground.

IRA FLATOW: You know, I once saw that many years ago made by tree leaves.

MARK BREEN: Yes.

IRA FLATOW: Isn’t that amazing? The trees on the ground, you see little crescents on the ground under a tree.

MARK BREEN: The only reason that I’m not thinking about it is because where I am in Northern Vermont, we don’t have any leaves just yet. In fact, it’s snowing outside right now.

[LAUGHTER]

IRA FLATOW: I’m sorry to hear that. And what about those special glasses, the foil on them?

PADI BOYD: You’re going to want to have your eclipse glasses during the partial phases of the eclipse, which everybody will see. Even if you’re in the path of totality, those partial phases happen before and after totality. It’s very important to protect your eyes. You can find the glasses in many, many places. Many public libraries have them, schools, museums.

Some people still have their eclipse glasses lying around from 2017. But if you’re in the path of totality, for those brief moments when the moon totally covers the bright surface of the sun, then you must take your eclipse glasses away from your eyes and view that totality moment with your full being.

IRA FLATOW: Let’s go to another listener. Jim from Libertyville, Illinois, asks, is it just a coincidence that we see the sun’s corona during a solar eclipse given their relative sizes and distance from Earth? I mean, Padi, it is amazing how the moon is just the perfect size, isn’t it?

PADI BOYD: Yes, it is. So the sun is quite a bit bigger than the moon, 400 times bigger. But it’s also 400 times further away from the Earth than the moon is. The relationship of their distances and their sizes is a perfect cosmic coincidence that they have the same apparent diameter in our sky and that when the orbits are all aligning, that moon diameter passes right in front of the sun’s apparent diameter to show us the corona. It is a coincidence, and it’s such a wonderful lucky cosmic coincidence for us here on Earth.

IRA FLATOW: Because the moon is still moving away from Earth. And sometime in the future, this is not going to happen. I mean, many years in the future.

PADI BOYD: Like millions of years. We lose about– it gets about 3.68 centimeters further from the Earth per year. It’s slowly receding away. So yes, in the far distant future, we will still have eclipses, but they will all be a type of partial eclipse, which is called an annular eclipse. So when everything lines up perfectly, you’ll still have this bright ring of the sun’s surface around the moon’s disk. We got a while.

IRA FLATOW: Yeah. Not worried about that one. Mark, instead of the future, let’s talk about history, the history of eclipses. How did ancient civilizations keep track of them if they happen so rarely?

MARK BREEN: Well, they were certainly rare. And yet at the same time, you’re basically taking out the life giving force that everybody knew about. I mean, the sun was the source of light, the source of warmth, and you take that away. And so that became really important. And various civilizations, especially those that kept relatively long records– I’m thinking of, for example, the Chinese records, which were fairly detailed and extensive.

And so as you go even from dynasty to dynasty, they were keeping track of such things. Same thing could be true of the Aztecs and the Mayans, keeping very careful track. And so they had various systems in place, so to say.

If suddenly the sun was disappearing, they had to, well, in many cases, make noise. Something was eating the sun. It could have been a dragon. I just heard from a gentleman in Vietnam– he was talking about a toad that would eat the sun. And they made noise so that they would basically regurgitate the sun.

IRA FLATOW: The Babylonians were pretty clever, right? Didn’t they figure out that there was a cycle going on there?

MARK BREEN: Yes. That actually still amazes me that over 3,000 years ago, they did such careful observations that they knew there were those periods of time when the sun and the moon and the Earth were all lined up. Now, they didn’t have the same concept that we do in terms of them being physical bodies out in space, but they certainly, in terms of their observations from where they were, detailed records that they kept. They came up with, eventually, a cycle of 223 months in which the moon repeated its motions.

IRA FLATOW: Speaking of history, I’m such a science history nut that I remember studying about the eclipse of 1919 where Einstein’s theory of relativity was tested and supported during an eclipse then, right, Padi?

PADI BOYD: Right. Einstein’s general theory of relativity is expansive and beautiful, but not quite as easy to test as many other scientific theories. But one of the really astounding predictions and the model of space time that he presented in general relativity was that mass can bend the fabric of space time.

And so that something is very massive but is in front of a light source, that instead of those light rays coming straight to you, the mass itself should bend that light around, and that would mean that the light would appear to be in a different place than it would be otherwise if the mass wasn’t there. So the sun is massive, and when the moon blocks out the bright sun, we can see stars.

And so in 1919 in that total solar eclipse, astronomers were able to test the theory. Was the sun actually bending light rays as they passed around its mass to get to our eyeballs and our telescopes here on Earth? And yes, indeed. They found it to be the case, and it was just a beautiful observational proof of Einstein’s theory of general relativity, which stands to this day.

IRA FLATOW: And Mark, we have learned lots of other things scientifically from past eclipses, haven’t we?

MARK BREEN: One of the things that developed especially in the 1860s and ’70s was photography, and in particular, astrophotography. And by taking images of the sun, one, instead of that very momentary opportunity to look at the sun’s corona that Padi was talking about, we now have images of it that they can study in more detail.

But the other thing that was going on with telescopes is they were able to use the specific spectra that was going on. So in other words, you allow that light to come through the telescope through a special instrument, and so you can actually detect what the sun was made out of, hydrogen and helium. It was a huge breakthrough in terms of understanding what the sun was made out of and eventually what the stars were made out of.

IRA FLATOW: That is cool. I never knew that. I never knew that. Let’s go to another Twitter question. YoteMonster is asking, are there specific examinations of the sun that can only be accomplished when the moon blocks out most of its light, such as during an eclipse? Padi?

PADI BOYD: Absolutely. Not only are we able to see the corona at this time, and we can’t at other times, every time there’s an eclipse, there’s a little bit more technology development. So we can take our instrumentation on observatories or put instruments on things like airplanes or sounding rockets to really collect sophisticated data and images of the corona at various wavelengths, like Mark was just talking about, and see what is going on with the activity inside the corona.

The corona has clues to how the sun works overall and how the solar wind comes out, so we’re hoping to learn a lot about that during this particular eclipse. But it’s not just what we can learn about the sun, the sun influences the Earth and the Earth’s atmosphere, in particular, the upper layers of the Earth’s atmosphere, the ionosphere, charged particles.

They’re constantly in interplay with the sun. So we’re going to also be taking some measurements during this total solar eclipse when the ionosphere or parts of it are under the shadow cast by the moon to see how that changes and to get a better understanding of that sun-Earth connection, which is so important for many, many things on Earth, including like satellites, astronauts, communication.

IRA FLATOW: And does NASA have plans to do any of these studies during the eclipse?

PADI BOYD: Yes, indeed we do. Yes. There is an airplane that will be carrying about five different experiments. It’s called the WB-57. It’s a research jet, and those are meant to do basically exactly what I was talking about, really interrogate the corona during this short period of totality. But the really exciting thing about being on a jet is that you’re not fixed in your geographic location.

You can actually move the jet in such a way that you extend that period of totality. So it’s about 20% longer from the jet’s vantage point, which means we’re going to get much more data than you can get from the ground.

We’ll be above all the noise from the Earth’s atmosphere at that point in time. There’s also sounding rockets that will be going up from Wallops Flight Facility that will be looking at how the atmosphere changes before, during, and after totality.

IRA FLATOW: And I know, Padi, you host NASA’s podcast, Curious Universe, and you’ve been working on a series about the sun. So tell me, what are some of your favorite fun facts about the sun?

PADI BOYD: Oh, gosh. Thank you for asking. We’ve had so much fun with this podcast focused on the sun this year in particular because it gives us a great opportunity to talk about all of the NASA science that’s being done on the sun today but also the science writ large. Not just NASA is interested in the sun.

But I also love the fact that we’re bringing in the story of the history. I’m so happy we’re talking about it here today, because the history of eclipses and humans watching and trying to understand the patterns in the sky and what they might mean is so rich and so inspiring. When we’re talking about an eclipse we’re going to see in just a few days, we’re connected to those people who were looking at it thousands of years ago.

So there’s also an active investigation that will be going on during this particular eclipse that attempts to answer the question, were the Chacoans, the Indigenous people in Chaco Canyon– that’s out in the Southwest in the US– did they actually witness a solar eclipse, and did they record it in their cliff art? There’s some tantalizing clues that, in fact, they did.

And the cool thing about that is that particular eclipse that did pass over Chaco– we can time eclipse this, so we know that it happened, and we know when– it was also during solar maximum. And so there are some indications that they may be looking at a very active sun and seeing features on that edges of the sun when the totality was happening that they captured in their art.

And we’ll be able to compare that to some of the images that we’re seeing in this eclipse with our most sophisticated technology and hopefully put some meat on the bones of that question. Is this art in Chaco Canyon actually connected to humans viewing an eclipse and recording it in cliff art?

IRA FLATOW: That really is exciting. I want to thank both of you for joining us and wishing you all a happy eclipse and especially you, Mark, who’s going to see it for the first time.

MARK BREEN: Thank you.

PADI BOYD: Thank you, Ira. Happy eclipse to you.

IRA FLATOW: Mark Breen, meteorologist and planetarium director at the Fairbanks Museum and Planetarium in St. Johnsbury, Vermont, host of Eye on the Sky for Vermont Public. Dr. Padi Boyd, astrophysicist at NASA and host of the agency’s podcast, Curious Universe. Dr. Boyd is based in Greenbelt, Maryland.

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