Totally Awesome: Your Guide to the Great American Solar Eclipse of 2024 - David Gerdes

(upbeat music) (vocalist whistling and humming) - Welcome to the first "Saturday Morning Physics" of 2024. (attendees cheering and applauding) Fans. I love it. I'm Physics professor Tim Chupp. I'm one of the organizers of "Saturday Morning Physics," along with my colleague, Professor Rachel Goldman, Carol Raybuck and Monika Wood, and the staff of the Warren M. Smith Demo Lab. You know, I often need to explain to the inquisitive people that Saturday Morning Physics is not just physics. It's Satur

day morning astronomy, Saturday morning biology, Saturday morning chemistry, those are the ABCs, technology, Saturday morning art inspired by science and vice versa. Saturday morning science policy, family-friendly science extravaganzas and more. It's Saturday morning, not just physics, but I think we should keep the name. So we've got a great lineup for you over the next few months with presentations on robotics, materials engineering, the Van Loo graduate student lectures. And I would particul

arly wanna highlight the visit in the March 23rd "Saturday Morning Physics" by Dr. Laura Grego. She was a UM undergraduate and now a policy expert at the Union of Concerned Scientists. And she will discuss what we can do to reduce the nuclear threat in her presentation which is entitled, "The Future That Oppenheimer Feared." "Saturday Morning Physics" is made possible by you, your many generous contributions to "Saturday Morning Physics" and thank you so much for that. And for more about support

ing "Saturday Morning Physics," please go to saturdaymorningphysics.org. We also are very grateful to have the support of the Dr. Mary Lois Tiffany Endowment, the Hideko Tomozawa Family Endowment, the Van Loo Endowment, and the Pulikeshi Dayalu Astrophysics Fund. Today's speaker is professor of physics and astronomy and the chair of the physics department, David Gerdes. David has been a professor here at U of M since 1998 and we managed to steal him away from Johns Hopkins where he was an assist

ant professor on the faculty. And before that he was a postdoc here at Michigan. So we knew him well. He grew up in Ohio and he attended Carleton College as an undergrad and he received his PhD in 1992 from the University of Chicago. David started his research in high energy physics working at Fermilab, in particular on the discovery of the top quark, but now he's an astrophysicist. David and his students have been searching for and discovering numerous exoplanets among other research topics. He

has many honors actually, but one that I wanna highlight is that his exceptional teaching has been recognized as a University of Michigan Arthur Thurnau Professor. So I wanna note just after this talk that we will have the question and answer session as usual, those online or even those here, can email us questions or you'll also be able, those in the audience here will be able to ask them in person. Yeah, the email address is here, physics@umich.edu. So we're so fortunate to have you here to t

ell us about the upcoming solar eclipse and what an appropriate title for Saturday morning Physics, "Totally awesome." Thanks, David. - All right, thank you, Tim. (attendees applauding) Thank you and good morning, everybody, and good morning also to our online audience. The path of this eclipse passes through my hometown of Hudson, Ohio, and I'm sending out my greetings to my parents there and my old high school friends who I know are watching this online. A total solar eclipse is one of the mos

t awe-inspiring, awesome events that you can experience. It starts innocently enough, inconspicuously enough with just a tiny little nibble out of one limb of the Sun, barely noticeable. Certainly there's no visible effect on the surroundings at this point. Just like a mouse took a little bite out of the cookie. Time goes by, about 45 minutes later, things might look like this. And at about this point, you'll start to notice some changes in the environment. The sky will be getting a somewhat dee

per shade of blue colors might start to look a little more flat. Shadows start to look more sharp. Another little while later, and it's getting quite a bit dimmer. Shadows are getting really crisp. You might see crescent-shaped projections of the Sun onto the ground through leaves or gaps in the trees. And you'll start to notice changes in the natural surroundings too, birds and animals and insects might be starting to change their behavior and you will feel a sense of anticipation among the peo

ple that you're with. Then finally, the last ray of sunlight disappears, darkness will fall and you'll see the approach of the Moon's shadow at about 1,000 miles an hour and darkness will fall. (crowd cheering) Eclipse glasses coming off. (crowd cheering) (air horn honking) (Tim laughing) 360 degrees of twilight all around you. And that black hole in the sky where the Sun is supposed to be, just a stunning sight. The wind might be picking up, the temperature will drop, and then the diamond-ring

effect breaks through on the opposite limb of the Sun. The Sun bursts through some low valley on the edge of the Moon and the eclipses over very quickly; daylight returns. The partial phases rewind themselves in reverse order over the next hour and a half or so, and then the world returns to normal. But you will have experienced something amazing. You'll be changed. With that magnificent memory of the fine details in the corona and those prominences around the limb and all the natural world kind

of pausing and taking note of what's happening in the sky. It's a rare and amazing experience, but you will remember it for your whole life along with other really special milestone events in your life like the birth of a child or a wedding or a graduation or where you were when Michigan won the National Championship in football. (attendees laughing) And 51 days from now, we'll get the chance to have that experience again or perhaps for the first time. Can we see a show of hands in our audience

here, how many of you have never seen a total solar eclipse before? Lots of hands. How many have seen one solar eclipse before? And for how many of you was that the 2017 one a few years back? So how many of you have seen three or more total eclipses? How many of you have seen eight or more? (everyone laughing) About five? Anyone seen five? So at this point, I'm gonna sit down and let this guy give the talk. (everyone laughing) Because you probably know more about eclipses than I do. But here's

the path that the Moon's shadow will take across North America on April 8th, that will actually start out over the Pacific Ocean and then make landfall in Mexico and then move up, and that black dot there, let's see, that black dot there represents the point where totality will last the longest, about four and a half minutes, which is about two full minutes longer than the 2017 eclipse. Then it will pass up from the Southwest across the Midwest through Northeast Ohio, and then on into New York a

nd Canada and Maine. I'll be showing several maps like this and you can find them by just Googling Espenak and Meeus or Five Millennium Canon or something like that. And there's lots of tools there to build your own maps and references. An eclipse is possible because of an incredible cosmic coincidence. The Moon is about 400 times smaller in diameter than the Sun. The Moon is also 400 times closer than the Sun, or the Sun is 400 times further away, which means that they have the same apparent si

ze. That allows the Moon to just barely and exactly cover the disc of the Sun completely, but not too completely. If things were different, if the Moon were smaller, then we'd never have total eclipses, we'd have fancy partial eclipses. If the Moon were much bigger, we could have longer lasting total eclipses, but we'd never get that full view of the inner corona around the ring of the Sun. It would be more or less obscured in different places. This is really unique in our solar system. There's

no other combination of moon and planet that produces perfect eclipses like this. For all we know, it's fairly unique in the galaxy. And if a intergalactic or interstellar tourism industry ever springs up, it wouldn't surprise me if Earth is on some kind of a bucket list for people to come and see this experience for themselves in this magnificent solar system. We sometimes picture eclipses with diagrams that show like the Sun here and, you know, the Moon here and a shadow going onto the Earth.

But that's actually not really the scale of the situation. So if this basketball is the Earth, this tennis ball is roughly the same scale as the Moon and the distance between the Earth and the Moon on this scale is like this. And on this same scale, the Sun would be about 86 feet in diameter and it would be a mile and a half over that way. We couldn't do that demo today. But you can see that in that scale we're kind of a small target. You know, imagine this long conical shadow extending this way

from the Moon. You know, the Earth is easy to miss, which is why eclipses don't happen all the time. And we'll talk more about why they don't happen all the time a few slides from now. So that picture looks like this and you see what I mean about the scale and diagrams not necessarily being physically representative. So that conical, dark shadow in which the Sun is completely covered is called the umbra. And the path that that umbra takes across the Earth is what defines the path of totality. O

utside of the umbra there is the penumbra and where the Sun is partially eclipsed, that's much larger than the umbral path, which is about 80 or 100 miles wide. The penumbra can be many hundreds or thousands of miles wide. Even those observers see a partial eclipse. And what it actually looks like from space is something like this. That dark inner core is where the umbra is. The people right there are experiencing a total eclipse. Outside of that, you see a partial eclipse. To see all the effect

s that I have described, the corona, the darkness, darkness roughly equivalent to what it's like about 45 minutes after sunset, dark enough to see stars and planets in the sky. You must be in the umbral path, inside that narrow path of totality. Here's what that path will look like. Here's an animation that I found on a website, it's gonna look like this starting out over the Pacific. The path of totality is that inner orange dot, goes like that. So you see these weird projection effects of proj

ecting a a three-dimensional conical shadow onto a sphere. That's why the umbral path is not always a round circle. So let's look a little closer at where the path will come near us. Ann Arbor will experience a 98.5% partial eclipse. That's going to be extremely dark, but not night. One and a half percent of the Sun's light is actually a lot of sunlight. You cannot look at that safely with your eyes. You will not be able to see the corona, you will not be able to see the profound changes in natu

re that occur during totality. And so I'd like to say that 98.5% partial is 0% total. (attendees speaking faintly) This is kind of like, you know, being in Ann Arbor for this is kind of like being outside Michigan Stadium when you hear the crowds cheer, you know, you know something big just happened. You were close, but you weren't there and it was totally different for the people inside who saw it. So if you are able to do so, I strongly encourage you to travel somewhere within that path of tot

ality. And as I show you later, this is by far the best chance people in our part of the country will have for many decades to see an eclipse this close. Now there's a great deal of excitement about this eclipse in part because we have experienced another total eclipse in this country fairly recently. And as you showed me at the beginning, many of you experienced this 2017 eclipse firsthand. It was in fact the most watched eclipse in history. About 154 million American adults saw the eclipse, ei

ther the total or the partial phase. That's 88% of the whole country. 61 million more people viewed it online. About 20 million people traveled to see it. It was the first coast-to-coast eclipse in a century since I think the 1930s. And it was the first total eclipse visible in the U.S. in 38 years. So a lot of people were really new to this thing. It also happened to be a beautiful day and most people along the path actually got to see it. It was a big deal in terms of interest. And this is som

e search terms from Google. Google Trends is fascinating by the way. You can play around with it a lot. So it had much greater interest than the Super Bowl. Now this was before the Super Bowl involved Taylor Swift, but, you know... (attendees laughing) So it was just a kind of normal Super Bowl. That happened to be a big hurricane season. That was Hurricane Irma and Hurricane Harvey and I think there was one other one that fall so that more or less matched it and maybe the area under the Christm

as curve is a little larger, but the peak was higher for the eclipse. So my point is that this, that the kind of online imprint of that total eclipse was really comparable to major, major national and world events. It was not uniform. (David and attendees laughing) You might recognize a little trace of where people were searching the most for information about eclipses. That's because a lot of people were traveling and... ♪ Once upon a time ♪ ♪ There was light in my life ♪ (attendees laughing) -

People were also streaming (laughs) this song in a sort of correlated way with where totality was happening. (attendees laughing) You're welcome for getting that song stuck in your head. (attendees laughing) And afterwards there was quite a bit of traffic. So one of my recommendations if you choose to travel for the eclipse is, you know, take your time afterwards, have a picnic, chat with your friends. No need to rush home. It's gonna be pretty crowded, I think. Now eclipses are special in part

because they seem very rare. I mentioned 38 years between eclipses in the continental U.S. But how rare are they really? Here's a set of statistics for eclipses over a 5,000-year period. They can be predicted quite accurately. And you see that over a 5,000-year period there's nearly 12,000 eclipses of some sort or another that's, you know, two or three per year. They roughly break down, you know, more or less equally between partial eclipses; eclipses that are total nowhere, annular eclipses; w

here the Moon does not completely cover the Sun and so you see this, so-called ring of fire and total eclipses. I'll talk about hybrid eclipses in a second. And here is the paths that some of those eclipses will take over the next 20 or so years. So total eclipses themselves are not rare. You see that in 5,000 years, there's 3,000 of them. So there's a total eclipse somewhere roughly every year or two. But because that umbral path, that path of totality is only 80 miles wide, you generally have

to travel to go see them. If you wait in one place for a total eclipse to occur, you'll wait on average about 400 years between total solar eclipses. So you have to travel. But that's the other thing that makes eclipses special, is that in contrast with many other marvelous natural events, their time and location can be predicted with great precision, years, decades in advance. So you have plenty of time to plan. In fact, I was looking to see when the earliest prediction was when someone said, "

You know, it would be cool to be "in the Midwestern United States on April 8th, 2024." And the earliest prediction I could find was from a catalog of eclipses that was published in 1887. There's the map, you can zoom in and see, you know, the writing is upside down, but if you flip it that's April 8th, 2024. The 2017 one is above it, right here. So, you know, I'm not sure that the details were as exact as we we know them now, but that's absolutely the right idea. And in fact in literature of tha

t day, they talk about how you do it. We're making a lot of detailed calculations and many abridgments may be made, which will readily occur to the skillful computer, in the 1890s a computer was a person who did these calculations on paper. And here's how you did it on paper. You use this certain kind of paper, it should be ruled so you don't, you know, lose track of your figures. 85 lines of page, a total eclipse calculation takes about three pages. The lunar ones are easier. Okay? So I snuck i

n a fourth kind of eclipse on that earlier chart; a hybrid eclipse. Those are the most rare kind of eclipse, but it happens when the distance between the Earth and the Moon is such that that umbral path makes landfall, so to speak, over only a portion of the path. The Earth's surface is curved and so different parts are at different distances from the Moon and it is possible under the exact right circumstances to get a total eclipse, say here and then a little further away you see an annular ecl

ipse. So let's talk about how to make a total eclipse. What does it take and why don't we have one every month? So the first thing of course you need is a new moon. I hope that's fairly obvious and we do get those every month. So clearly you need something more than that. It also must occur when the Moon crosses, what we call, the ecliptic plane. The ecliptic plane is the plane of the Earth's orbit. So if you like to think of it from an Earth-centric point of view, the ecliptic is the path that

the Sun takes across the sky over the course of a year. It's the path through the zodiac that the Sun will trace over the course of a year. The orbits of the other planets lie more or less in the ecliptic plane, typically within a degree or two of it. But the Moon's orbit is tilted a little bit more. It's tilted about five degrees with respect to the ecliptic plane. So if the Sun is this 86-foot sphere a mile and a half away, the Moon is not orbiting in this same plane. It's orbiting in this til

ted plane that's five degrees above. And so in a typical month, the new moon might occur up here and that long conical shadow hits the wall instead of the Earth or down here and it, you know, hits the podium. So it has to be just in this plane as it's making its orbit and crossing the ecliptic plane, right when the Moon is new, those points where the Moon's orbit crosses the ecliptic are called nodes, the ascending node or the descending node. And the new moon needs to occur at a node so that it

has a chance of lining up with the Sun. So there's an illustration of that. And if the Moon's new phase is happening, you know, up here, it just misses the Earth. Okay. And then finally there's one more thing, as pictures of annular eclipses show you the Moon's size is not always perfectly big enough to cover the Sun. And that's because the Moon's orbit, besides being tilted with respect to the ecliptic, is also not circular, it's elliptical. And that means it is closer and further away from us

at different points in its orbit. And its apparent size is bigger or smaller at different points in its orbit. And our cosmic coincidence is just close enough that that difference matters. That's the apparent size of the Moon or a relative comparison when it is at its furthest point called apogee and when it's at its closest point called perigee. This will be the most technical slide in my talk. And so if you're baffled by what I'm about to show, relax, it'll get easier again. So what I've show

n here is a graph of the phases of the Moon for the first couple of months of this year. So the full moon is the open circles and the new moons are the dark circles and the in-between phases are the other phases. And then what I've plotted here is the elevation of the Moon with respect to the ecliptic plane. So this ecliptic plane here at zero is where the Sun's path goes. And here's this plus and minus five-degree tilt of the Moon's orbit. So we have conditions that are ripe for an eclipse if t

hose dark circles; the new moons, happen inside that yellow band where the Moon is at a node. So what's the period of the Moon's orbit and how can we use that to help us understand eclipses? It turns out that's not as simple of a question as you might think. There's different ways of talking about, of defining the period of the Moon's orbit. I think most of us when asked what's the period of the Moon's orbit would think in terms of the time between new moons or the time between full moons. And t

hat's what I've shown here in the blue band. That's the time between new moons and that is something called a synodic month and it's about 29 1/2 days. But there's other periods in this graph too, and they are all important for eclipses. There's also the time between crossings of the node, between when the Moon is in the ecliptic plane and the next time it's in the ecliptic plane and/or the next time it's passing through the ecliptic plane in the same direction. And that is called a draconic mon

th. I love these antiquated, astronomical terms and that's quite a bit different actually. It's 27.2 days. The difference is roughly because, you know, in the month between new moons, the Earth has moved 1/12th of the way around the Sun. And so the angle of illumination from the sunlight is different, but the Moon's orbit remains kind of fixed in orientation and space. So you can see that when we get a new moon, at a node like here, that's when we can get an eclipse. But is it a total eclipse? T

hat's the third period that is important. So this is the same time graph, but now on the vertical axis, I have plotted the apparent size in a funny unit called arc seconds. 1,800 arc seconds is about half a degree. To picture half a degree hold your pinky fingernail out at arm's length, and that's about one degree. So, you know, the apparent size of the Moon or the Sun is about, you know, half the size of your pinky nail. So here is the apparent size of the Moon fluctuating from month to month.

That's the blue curve. The yellow curve is the apparent size of the Sun and it's actually not constant. And that's because the Earth's orbit isn't circular either. Our orbit is slightly eccentric as we go around the Sun and you see that the Sun is actually at its largest apparent size here in January. We're closest to the Sun in January. We don't have winter because we're far from the Sun. We have winter because our northern hemisphere is tipped away from the Sun in January. But we're actually c

losest to the Sun than we are in July. So we have an opportunity to have a total eclipse when the Moon's apparent size is above the yellow curve, when the Moon is bigger than the Sun. And lo and behold, that is also what's happening here in April. In fact, the Moon is about as big as it ever gets. When the full moon occurs at the peak, we sometimes call it a supermoon. When the Moon is that extra large size, this is kind of a super new moon in April. And that period from perigee to perigee, from

peak to peak of the apparent size is yet another way of thinking about the period of the Moon. And that's something called an anomalistic month. And that's 27.5 days, a couple of hours different from the draconic month. So when we have all three of these things, a new moon at a node, when the Moon's apparent size is bigger than the Sun, we can get a total eclipse. And that's what's happening here in April. There's... - Why is the maximum size of the Moon changing in that? - Oh, that's a really

good question. The question is, "Why is the maximum size of the Moon changing?" And I thought someone might ask me that. And what I think the answer to that is, is that I have done this computation from Ann Arbor. And so Ann Arbor is at different distances from the Moon, you know, depending on the Earth rotation and and so on at the moment of... So it's not always equally big every month. I think if I had made this plot from the center of mass of the Earth-Moon system, it would be the same. Ther

e was actually a warmup act for this eclipse that some of you may have seen. There was an annular eclipse that was also visible from the continental U.S. this past October. It wasn't... It was a fairly low-grade partial eclipse from here. And I think the weather was bad that day. And so you probably wouldn't have noticed it unless, you know, you knew, it was cloudy anyway, but it was visible. So here we have a new moon, I think my green line's a little off, but we have a new moon about here cros

sing the plane. That's all good. But the Moon was further away and wasn't able to cover the solar disk that eclipse passed through the Southwestern U.S. I went with a group of students to Albuquerque. And this is a composite image of what we saw there from Albuquerque. And by the way, I took these photos with this rig over here, just a small telephoto lens with a little camera on it and the solar filter. You must put a solar filter over your optics on the big end, not the little end. (everyone l

aughing) And also remember to take the lens cap off. (attendees laughing) This picture was taken during the ring of fire phase of the annular eclipse. And so you see everybody wearing their glasses and you see like rays from the Sun, the Sun, in this case, was about 93% obscured. So a little more sunlight than we will have in Ann Arbor on April 8th. But you see there's still a lot of daylight and you cannot observe this eclipse without eye protection. One of the cool things you will see during a

n eclipse, at least if you're someplace where there are leaves on April 8th, which is probably not here, is these... The gaps in the leaves act like little pinhole cameras and they project that image of the Sun onto the ground. And that's what we saw. (attendees speaking faintly) I'll mention lunar eclipses for completeness. They're often paired with a solar eclipse roughly two weeks apart because if the Moon is near a node, you know, for a total solar eclipse, probably half an orbit away, it wa

s also near, you know, a node for the full moon. So if the full moon is crossing that node, you can get a lunar eclipse. They're often paired with solar eclipses in proximity. And I believe there's a partial lunar eclipse in March of this year. Yeah, there it is, on March 25th. So we have those three periods: The synodic, draconian, and anomalistic months. And there are all these kind of random numbers that aren't quite the same. But if you have a condition where you get a total eclipse, when mi

ght those same conditions repeat? So if you go 223 synodic months, that's 6,585.32 days, 242 draconic months are nearly the same. And 239 anomalistic months are also just about the same amount of time. That is 18 years, 11 days and 8 hours. And the eight hours is important. That's about a third of a day. But what this is saying is that if you have an a condition that produces a solar eclipse, then those same conditions or very nearly the same conditions will recur 18 years, 11 days, and 8 hours

later, you'll get sort of a copy of that eclipse on intervals of that time. This is called a saros cycle. And this eclipse in April belongs to something called saros 139. So here is our April 8th eclipse, but if you rewind 18 years and whatever I said, 11 days and 8 hours, you'll get these earlier eclipses, which look really similar, but that eight-hour difference is 1/3 of a day. And so you see that the eclipses happen, you know, eight hours or before, you know, shifted eight time zones before

or after the one in question. And in the future there will be saros twins of this eclipse in 2042 over the South Pacific and 2060 over Africa and Asia. And then on the third round, we'll kind of be back to where we started and we'll get another copy of this eclipse visible from the Southeastern U.S. in 2078. So make your plans. (attendees laughing) Airbnb prices are gonna be exorbitant by 2077, so make your reservations early. So this particular cycle started in 1501 with a partial eclipse visib

le near the North Pole and will end in 2763 in Antarctica. The cycle consists of 16 partial; 43 total. And then for some reason, and don't ask me why, I don't know, zero annular and this eclipse cycle seems to be abundantly overproducing hybrid eclipses. I don't know why, (laughs) but that's the lineage of our April eclipse. So they begin near the poles, they move north or south depending, typically produce several dozens of eclipses, and the whole thing lasts 12 or 1,500 years. So let's go back

now to April of 2024 and talk about how that will go. This eclipse actually passes through even more population centers than the 2017 one did. It passes through some good-sized cities in Mexico. It will also pass in or near San Antonio, Austin, Dallas, Indianapolis, Dayton, Ohio, Rochester, New York, Buffalo, New York. Something like 32 million people live in the path of totality. Now the question is, where should you go? If you have a choice, where should you go? We all know what the weather c

an be like here in early April. So what does the science say? What does the weather say? This is an analysis of historical cloud cover data for April 8th done by a meteorologist named Jay Anderson. So what you can see here is that, so blue means clearer and red means higher fraction of cloudy days. So, you know, the smart money is in Mexico. (attendees speaking faintly) Go to Durango, go to Mazatlan. That's really where it's gold. Generally speaking, the fraction of cloud cover increases as you

move to the northwest. So for example, here in like south of Toledo, which is the closest point to us, or in the Cleveland area, it's about 60, 70% chance of clouds. There's actually, and this is more clear if I show you the next plot. So this is a plot of the cloudiness fraction along the center line. And certain cities have been labeled here. So, you know, Mexico, only a 20 or 30% chance of clouds. When you cross into Texas, you're getting to be more like about a 50-50 proposition. Indiana doe

sn't look so great. But then something interesting happens as the path approaches, comes up through Western Ohio and then sort of approaches Lake Erie around Cleveland and Berea, there's this dip. And people who know more about the weather than I do say that that's actually, kind of, sort of a reverse lake effect. At this time of year, the lake is very cold, it's colder than the air, and that somehow acts to suppress cloud cover. So if you're looking to make a shorter trip within a few hours dri

ve of here, but don't wait till the morning of the eclipse to do it because of that traffic map, then I would recommend going to Cleveland. There's actually this little dip here that makes it about as good as Texas. Don't go to Quebec, you know, there's another dip here. And then this is like Nova Scotia and stuff, okay? So, you know, it's dicey it's gonna be different than that August, 2017 one. And if you have the flexibility, I would be looking at cloud cover predictions that start to get ver

y good within a few days of the event. And if you have the freedom to control your whereabouts and make a strategic decision about where to drive, have that flexibility if you're able to do it. I'm gonna be in Texas. That's where I'm placing my bet. I talked a little bit at the beginning about the experience of the eclipse on eclipse day. Now if you're here in Ann Arbor, if you're unable to travel out of this general area, as I said, you will see a 0% total eclipse. But if you travel a little wa

ys from here, these times are roughly similar within, you know, 5 or 10 minutes. So the partial phase will start a little before 2:00 in the afternoon. It will be this maximum eclipse of about 98.5%, a little after 3:00, and by 4:30 it's done. These times came from clicking on an interactive map. The link is here and I've also put a QR code, if you don't wanna write down the link, you can just scan that QR code, bring up an interactive Google map of the whole eclipse path, and then play around a

nd click in different places to see what and where the eclipse will be like. Safety reminders. You must wear a pair of eclipse glasses when looking at the Sun at any time except during totality. These are widely available online. They look something like this. They're just a piece of really dark film. And you can hardly see anything through eclipse glasses except the Sun. I can very dimly see the overhead lights. The floodlights in the back are tiny, little glowing things, but you really can't s

ee anything except the Sun with these glasses. The Sun is really bright. You must wear those to be safe. You also, as I have done here, must cover the objective end of your optics. Don't put the eclipse glasses like behind the eyepiece. You might burn up your optics and you're concentrating maybe more light than the glasses are built to take. So block the light at the front. So you might see this diamond-ring effect. This is really one of the most unforgettable moments of an eclipse. It lasts on

ly a brief second or so before you have to, you know, look away at the end or, you know, when you can start to put your eyes up there at the beginning of the eclipse. The lunar limb is uneven in that lowest point, where the Sun can first break through is where you see the diamond ring. As the diamond ring disappears you might see Baily's beads, these last little bits of light. You might be able to see prominences, solar flares along the solar limb. It's perfectly safe to look at all this through

binoculars once you're in the total phase. It is also helpful to have some kind of a timer to keep track of when totality will start and end. I'm not a commercial recommendation service, but I like an app called "Solar Eclipse Timer" that is a... You know, it's an app for your phone. It is aware of your position and if you're in the path of totality, you can set it up to give audio cues of when totality will start and give you a countdown and things like that. "Solar Eclipse Timer." The view in

the sky, the wide view is really spectacular. Like I said, it's about as dark as 45 minutes or so after sunset. And that is plenty dark enough to see brighter stars and planets. On April 8th, there will be a really nice conjunction of Saturn and Mars. They'll be very close together over here. Saturn is a little closer and these numbers next to them are their visual magnitude. So Saturn and Mars are of equal brightness. Mars is the red one. Venus will be extremely prominent. In fact, you should

be able to see, if you know where to look, you should be able to see Venus maybe as early as a half an hour before totality if you have good eyes. And then Jupiter will be over here. Mercury is somewhere over here, but it's gonna be like fifth magnitude. I don't think you'll be able to see it with your eyes, but you'll see it with binoculars if you know where to look. (attendees laughing) What you may experience; all kinds of things, including some, you know, not positive words. And we kind of c

huckle about it. And I think the people who are attending an eclipse talk are maybe more likely to experience these positive motions. But, you know, historically disorientation and distress have been more associated with eclipses than feelings of joy and awe. I don't know of any primitive or ancient culture that has taken eclipses as omens of years of peace and prosperity and good fortune. (attendees laughing) Usually when an eclipse happens, it means the crops are gonna die and the king is gonn

a... You know, the kingdom will fall. And, you know, people... Since bad things are always happening, the correlation between bad things and eclipses is really what sticks in people's minds. But even, you know, in contemporary times, even if you know what's happening, and a common remark you hear from people who have experienced a total eclipse is, "Can you imagine what it would be like "to not know why this is happening, "to not know what this is about?" The sight of a black hole in the sky whe

re the Sun is supposed to be, is so profoundly disconnected from our everyday experience that it can be, you know, upsetting at a kind of a primitive level. And here's a quote from Annie Dillard writing about the 1979 eclipse. "More moving photos than those of the Sun's corona "can appear in magazines. "But I pray you'll never see anything "more awful in the sky." Eclipses have been depicted historically in artwork. Here is St. Benedict having a vision during the diamond-ring effect. Here is som

e sort of a drawing of Chinese astronomers, I hope he's got his solar filter on, (attendees laughing) observing an eclipse while others are sort of prostrate and in prayer. Eclipses, for the most part, nowadays are just kind of a treat for the eyes. But they have historically also been associated with some extremely important scientific measurements. And the most important of those took place in 1919. Einstein had a new theory of gravity, general relativity, that was really not clear if it appli

ed to reality. It did correctly predict the anomalous behavior of Mercury's orbit. But no one had yet tested one of its other critical predictions, which is that light is bent by gravitational fields. And that meant that if you could observe starlight passing near the Sun, and notice that the stars were shifted because the light had been bent, what we call gravitational lensing, then that would be an important confirmation of this critical prediction of Einstein's theory. The only opportunity yo

u have to do that is during a solar eclipse. And so in 1919 when England and Germany were at war, an English expedition led by Sir Arthur Eddington traveled to off the coast of Africa and to Brazil to observe the solar eclipse and test the predictions of this German scientist. There's one of their setups. Here's a closeup of one of their photographic plates. The star was supposed to be here, it was over there. And the shift was exactly the amount predicted by Einstein. There is a way you can pre

dict this in Newtonian gravity and it predicts the shift half as big. So this led to one of my favorite headlines in the New York Times, "Men of Science More or Less Agog." (attendees laughing) Some of them were more agog, others less agog. (attendees laughing) There's also a story... This was actually the event that turned Einstein from a scientist who was well known in the physics community to a global celebrity. And what caused the celebrity was not only this the strange prediction of "Lights

All Askew in the Heavens," but the fact that his theory had a reputation of being so very difficult to understand. In fact, it's still an advanced undergraduate topic in our physics courses or a graduate course. But in 1919, this was said to be something that only a very few people in the world could understand. There's actually a story that Arthur Eddington, the British astronomer who led this expedition to observe the total eclipse was asked if it was true that general relativity was a theory

that only three people understood. And Eddington paused and said, "Well, I'm trying to think who the third one would be." (everyone laughing) So back to the eclipse day experience. So this is... And if you saw my talk a few years ago, you've seen this, this is a video shot by a drone at our site in Oregon in 2017. So you've seen that, I showed you that animation of the path of the Moon's shadow across the Earth. It's traveling at something like 1,000 miles an hour. It's gonna be actually a litt

le faster than that by the time it gets to Michigan and April 8th. So here's the drone looking to the west in the moments approaching totality. I've sped this up by a factor of a few. So you see that, there it gets dark, twilight all around, and then here comes the... Oh, now we're looking east, back west, and here comes that shadow across the land, you can almost see it moving. It gets really dark and now it's over. I'll say a few words about photographing totality, if that's something you choo

se to do. This is a photograph I took with a setup very similar to this in 2017. Actually it's an HDR stack of a group of photographs. My first piece of advice actually is that if this is your first eclipse, don't try very hard to photograph it. I somewhat regret at past eclipses spending a little too much time messing around with equipment and not paying attention to what was going on around me. It's only 4 minutes and 20 seconds or so outta your whole life. So make the most of it. There will b

e many, many excellent photos of this eclipse. But your memories, your experiences are not replaceable. They belong to you. And I suggest, you know, focusing on those. Another reason is that actually photographing eclipses in really good detail is quite difficult. That's because the corona is extremely high contrast. It's very bright near the rim of the Sun and it kind of trails off into these delicate tendrils that blend into the sky. And there's a real gradient from in to out that typically ca

n't be captured in one exposure. I'll show you an example of that in a second. Your eye, however, is actually much better than any camera at detecting gradations of contrast. And finally, the sights and sounds all around you are really an important part of what the experience feels like. You're not gonna listen to me, you're gonna take pictures anyway. (attendees laughing) Fine. So having said all that, almost anything you do will result in something, an iPhone photo will show you, you know, som

ething, that picture I showed earlier of a bunch of people standing in darkness with the eclipse in the sky, that was an iPhone picture. The best part of an eclipse video is actually often the sound, people react in really great spontaneous ways to that moment when the Sun disappears. Again, once the total phase is underway, you can take off your glasses. A good way to know when to take off your eclipse glasses is when you look up at the Sun and you can't see anything, then you know it's safe. Y

ou can observe and photograph totality without special filters. I do recommend that you again use some kind of a timer so that you are prepared for when totality ends and you can protect your eyes and your optics as soon as that diamond ring brightens to the point where you can't look at the Sun anymore. So here's what I mean about that high contrast in the corona. This is a set of exposures ranging from very short to over a second. And you see that at each exposure setting, there's a sort of di

fferent regime of the corona that is shown in good detail, the rest is too dark or it's too bright. So that image I showed you earlier is a HDR stack of a group of these images. Okay, what if you miss the 2024 eclipse? What if it's cloudy? What if you just hate to travel and you wanna stay put? I'm gonna wait in Ann Arbor for the next total eclipse. I have good news. There's gonna be another one that's gonna be visible from here. I also have bad news; it's in 2099. (everyone laughing) (attendees

speaking faintly) That's 75 years from now. But I think some of our younger members of the audience might be around for that. So remember when you're an old man, you know, that this professor told you once that this thing was gonna happen. So this is September 14th, 2099. And look where it goes. (attendees speaking faintly) It's actually cutting it really close. (everyone laughing) Don't go north of the Huron River, go south of the Huron River. In fact, probably, you know, go a little further s

outh altogether because when you're at the very edge of the band of totality, the totality doesn't last very long. You want to be as close to that center line as you can. The details of this path, the details of these boundaries actually depend quite sensitively on things we don't know yet, like how the length of the day will vary slightly between now and 2099, you know, leap seconds and and so on. So I advise you to check back for updated predictions around 2096 or so and we'll have a better id

ea of which side of the Huron River you should be on. September 14th, 2099 is a Monday. (attendees laughing) I have a great idea for a "Saturday Morning Physics" talk on September 12th, 2099. And I hope Tim will, you know, be on that. Yeah. Once you're on this committee, Tim, you can't get off. It's just like, you're it. Okay, so as many of you know, once you've seen one eclipse, you want more. So if you love the one in 2017 and you're excited about this one, if you see this one and you're reall

y excited to go see a next one, when are the next opportunities? Well, like I said, there's a total eclipse somewhere in the world once every year or two. After April's eclipse, the next available opportunity is in August, 2026. You can either go off the coast of Iceland or go kind of, you know, to Spain or off the coast of the UK. These lines here indicate twilight. There'll be an evening eclipse here, it'll be a a more of a... The Sun will be higher up here. In 2027, this will be cool, in 2027

, there's this eclipse going across Northern Africa and through Egypt, good opportunity to include some sightseeing in your eclipse travels. And then in 2028 there will be an eclipse that crosses Australia and also hits Christmas Island. (attendees speaking faintly) None of these will be as easy as driving to Toledo. (everyone laughing) So try it. And the next eclipse that will be like the 2017 one and like this one, which is visible more or less coast to coast across the continental U.S., won't

happen for another 21 years in August of 2045. And this will be a really special one because totality will last almost six minutes for that one. The longest a total eclipse can possibly last I think is something like seven minutes. So this is really quite unique. That's a long way away, but it's not that far away and I think that's something that many of us can hope to look forward to. Okay, so that's all, I'll leave some time for questions. Thank you very much. I'm wishing you clear skies and

a wonderful experience on April 8th. (attendees applauding) And did you want to pause for a minute, Tim, or just go right to questions? - Gotta get set up, but it's like, we'll start the question and answer in just a minute. There will be microphones in both aisles. So we request that if you have a question that you make your way to the aisle. We also have online questions and we expect more. (attendees speaking faintly) David, I'm gonna ask a question that came online. "Where did the Moon come

from "and is it, in some sense, "responsible for this coincidence and tourist destination "that the Earth-Moon system should be?" - So the question is where did the Moon come from? So we think that the Moon probably formed in the solar system's very early days when there was still a lot of debris floating around from the planetesimal disk. And this is the same planetesimal disk that was causing larger objects to, you know, accrete and form into planets. Once you have made a big massive blob of s

tuff, it becomes sort of a gravitational magnet for more stuff. But a planet like the Earth was also quite hot being in close to the Sun, still much more of it was molten than is molten today. And it is believed that the Moon arose from some kind of a giant impact that caused the proto-Earth to split into a larger and a smaller blob. But it wasn't such a violent impact that that remnant was thrown off entirely, it was captured into orbit. The fact that the size is what it is, I think is pure coi

ncidence. Lots of other planets have moons formed through similar mechanisms and we just got lucky. - So. - Question here. - Oh, is it on? So ideally I'd like to be able to view the solar eclipse this year while flying an airplane. And so I was wondering if that would be possible to see the eclipse from 3,000 feet in the air? - That's a great question. I'll give you two answers to that. The first is that yes, it's possible, let me go back a few slides to this. (attendees laughing) Oh yeah, you'l

l like this. That's one of the Donovan Edwards touchdowns. - (faintly speaking) or something. - So the Sun will be about 50 degrees above the horizon. - Yeah, so just one. - So you want to make sure that you have a good view, you know, like not just sideways, but up. The Moon's shadow is traveling at about 1,000 miles per hour or maybe more than that. So unless you're in a really special plane, you probably won't be able to prolong the duration of totality very much. But you will succeed in doin

g that a little. I wanna say we have an expert on flying planes along the path of solar eclipses in the room here. Josh Cassada has joined us and he got to experience the 2017 eclipse by flying, what was it, an F-14 along the path? - T-38. - A T-38. How long did that eclipse last for you? - Probably an extra 15 seconds. (attendees laughing) - Well, I think that's worth it at that point, so. - Yeah. (laughs) So yeah, if that extra 15 seconds means a lot to you, try to hitch a ride with Josh. - So

we were doing about 600 knots over the ground and we were in a two ship, we had four people total in the formation and - of the three, - Mic. - I was in the lead so the other three - Mic - had their glasses on. And we found out when we got back that those glasses that you'll be wearing in April are really rated for sea level and not 30,000 feet. - Oh. (laughs) Okay. - Thank you. - Even through the glass? Yeah. - Yeah, I don't think there's a filter there. - Yeah. (Josh speaking faintly) Okay. Q

uestion over here. Go ahead. - Good morning, doctor. We are expecting an increase- - Use the mic. - It's- - Turn it on. - You gotta turn it on. - Case, please. (attendee speaking faintly) - Okay, can you hear me now? - Yep. - Okay. So we are expecting more solar activity this year. What is going to show up in the eclipse? - So the question is about solar activity and as you pointed out, we're actually near a peak of the solar cycle now. So if you are able to look at... This is why there's been a

lot of activity with northern lights lately. In fact, even earlier this week I think there was a chance of seeing northern lights from Michigan. So periods of peak solar activity are associated with things like northern lights, solar flares, more sunspots, and from the Eclipsophile's point of view, more activity in the corona, bigger streamers, maybe more, you know, features in the corona than a sort of uniform thing and more of those prominences and flares around the rim. You can also look in

the days immediately preceding the eclipse, there are sometimes forecasts that you can find of what the corona may look like and what the solar activity profile could be. So this is gonna be a good eclipse for that too. Thanks for pointing that out. - Not to miss it. - Right. - How about a question from online? - Okay. - And I don't completely understand this question, you probably do, which is how did you choose the factors in the saros cycle calculations? - Oh, so I showed a slide about the sa

ros cycle, which... Where is that? Here we are. So I said that... Sorry. Anyway. There. How did I choose those factors? So those factors were chosen to find the least common multiple of those three periods. So those are the various integers I need to multiply those periods by to get nearly the same number. Okay? And you see it, they're not exactly the same. And that's why those eclipse siblings I showed you aren't carbon copies of the original eclipse. They're not completely commensurate. There

are other ways of finding other multiples that also add up to, that combine to give you, you know, the same period. But the saros cycle is the most immediate one. And it's actually, you know, 18 years and 11 days is kind of a human timescale. And so this was worked out by ancient astronomers. People actually built mechanisms with gears, you know, to work out the appearance of the next eclipse in a saros cycle. If you saw the Indiana Jones movie this summer, the Antikythera mechanism was actually

an eclipse predictor based on the saros cycle. It's not a portal back in time, but it does predict eclipses. (attendees laughing) - Question over here, David, on the left. - Oh, I'm sorry. - On your left. Sorry. - Then you're next. - You showed those Chinese observers using a telescope and presumably not protecting their eyes. Can you tell us what part of the solar spectrum is damaging to eyes and what kinds of damage can occur? - The question is "What part of the solar spectrum is damaging to

the eyes?" Really the answer is all of it, because it's not so much, you know, it's not just the particular wavelength, it's the intensity. It's just how much light is getting through. And so we think of ultraviolet light as being the most damaging kind of light from the point of view of sunburn and so on. But at those intensities, it's all dangerous. So the kind of damage that it does, you know, your eye is a magnifying glass. And if you have ever started a fire by putting a magnifying glass wi

th sunlight, you know, onto a piece of paper or some leaves or something like that, that's what's happening to your retina when you focus unfiltered sunlight onto your eyes, it will do permanent damage. So don't do that. (chuckles) Question. - Okay. This is a question about photographing the eclipse. During the last partial eclipse, I got my spotting telescope, got a large whiteboard and projected the image on that large whiteboard. You know, it was a perfect image that I took photographs of tha

t image on the whiteboard. I got some fantastic pictures, the sunspots, everything there. Would that technique work for doing the eclipse this time? - So the question is about photographing a partial eclipse, if I understood you correctly, by using a small telescope and just projecting the image, you know, from the eyepiece straight onto a... Yes, that will work. I caution you that when you focus that intense sunlight through your optical path, which has multiple lenses and sometimes there's glu

e and adhesives and things in there holding it in place, or glass has a, you know, coefficient of thermal expansion. It can possibly break when a piece of it gets really hot. So you're placing your optics at some risk by doing that. But what I would suggest doing, if you want to try a technique like that, is actually put a little filter over your lens that's just got a tiny little hole in it. 'Cause that'll let plenty of light through, but it is less likely to damage your optics. Yeah, you can m

ake a really low rent, even if you don't have eclipse glasses, you can make a really low-rent eclipse viewer by just taking a, like a three-by-five index card and poking a hole in it with a pin and projecting it onto another piece of paper. And you'll see those nice, like, rings like I showed you in the annular eclipse. You'll see those nice pinhole images of the Sun's disk. - How about using that technique just for, during the total eclipse? - During the total eclipse, - Will I get all the imag

es- - I don't think that will work because there's not enough light. So during the total eclipse, during the total eclipse, there's no reason to avoid just looking at it and photographing it. Yeah. - There's actually about 200 people online as well, David, so it's - All right. Hello - very popular topic. And one question that we've gotten from more than one online viewer is to try and explain again why the differences in the length of totality from eclipse to eclipse? - So I think the easiest wa

y to account for differences in the length of totality has to do with this graph here. So roughly speaking, the larger the apparent size of the Moon relative to the apparent size of the Sun, the longer the eclipse will last. So a situation like this one right here, where the Moon's size is just barely the same as the Sun's. Of course in this particular case, there's not an eclipse because it's not at a node. But if conditions were otherwise ripe for an eclipse, something like this would correspo

nd to an extremely short-lived total eclipse or maybe a really big annular eclipse. Whereas in the eclipse that we will have in April, you can see that the Moon's size is actually pretty big, which is why this eclipse in April lasts four and a half minutes. It mostly has to do with the apparent size of the Moon. - On the left. - So this is actually a question that I'm asking on behalf of a far distant-future descendant of mine who, at a time when climate change was fixed, there's no war, all nuc

lear weapons have been eliminated and people are living forever. - I'm really glad to hear that's what it's like. - I was happy to hear that too. But her question was where on Earth could she live permanently to have the maximum percent of her time be spent in totality? And I assume this might depend on latitude (everyone laughing) and not longitude. - Yeah, what a great question. (attendees laughing) They can't just teleport wherever they want by then? So I mentioned that eclipses happen on ave

rage about every 400 years. I think it's actually 380 years or something for a given place on Earth. But that probability is actually not uniform. And as the questionnaire alluded, there's a latitude dependence to the frequency of eclipses and eclipses are somewhat more frequent near the equator. So I think anywhere on the equator, because I think there's not a longitude dependence. So anywhere on the equator will maximize your chance of experiencing the most totality. Yeah. We could probably an

swer that question explicitly with the help of that 5,000 year canon. There are probably particular places... There is some place on Earth that has more totality than any place else in the next 3,000 years. And I don't know where that is, but I that that's something one could look up. (attendees speaking faintly) Statistically, be on the equator - Question for the person who went on the flights for the eclipse, you said the glasses were not rated for 2,000 feet. I was wondering what effects- - 3

0,000 feet, - 30,000 feet. I was wondering what effects the people who are flying with you experienced. (Josh speaking indistinctly) - It's on. - It's on. - I'm pretty sure that they weren't looking prior to us being in total eclipse. So in that sense, I don't think any damage was actually done, but we thought we were being extra safe by having them with us. And then when we came back, we started having some conversations. I actually don't even know how much assumption is put into the atmospheri

c effects into those glasses because you can imagine, I think we used the same glasses in Colorado that we would, you know, here in Ann Arbor. So I'm imagining the effect was not significant in any way and we weren't dumb enough to be looking before... 'Cause we had a pretty good run and it was coming from behind us, so it would've been really hard to look before we were in totality anyway. So I think we're all good and nobody's actually said anything, right? - In general at 30,000 feet, there's

only about 1/3 as much atmosphere, which means there is three times more, there's three times less absorption of light. So all that UV light, you know, just like when you climb a mountain, you need to put on lots of sunscreen, so. - Just an online question and I really like, it's actually more than one, but I really, really like it. "So if you could give some examples of the kind of science "that will be done in April, et cetera, "astronomy, biology, atmospheric chemistry, et cetera." - Actuall

y I'm glad you mentioned that because I am part of a NASA-funded citizen-science project called the Citizen DEP Project, which stands for Dynamic Eclipse Broadcast Project, run by my friend Matt Penn in Tucson. And this is a DEB telescope. So we will be deploying a bunch of citizen teams, a lot of student-led teams and community organizations with telescopes like this along the path of totality, but also outside the path to monitor the Sun and white light and also assemble a big movie. You know,

the eclipse only lasts four minutes in any one place, but the whole thing lasts about an hour and a half. And if you can piece all those movies of totality together, you have a hour-and-a-half-long movie of what's going on in the inner corona. We did something very similar in 2017 and were able to actually observe some coronal mass ejections and how they evolved and, you know, evolved in this complicated magnetic environment of the Sun. You can only do that kind of science during a total eclips

e. A coronagraph isn't good enough to do it. So it's the one time we really have access to directly what's going on with the corona in invisible light. So that's an astronomy example. I'm sure there are many others, but that's the one I'm most familiar with. - Another online question, this I think is from one of our younger listeners. "Where can I find out more information "about the 2099 eclipse?" (everyone laughing) - Well, it won't be from my "Saturday Morning Physics" talk in 2099, I'll tell

you that much. So I will put back up the QR code and I will find this. Here. Here. So this QR code will take you actually to this website, which is a Google map of the upcoming eclipse. But if you look at the first part of this website address, which is this X-U-B-A.free.fr it has links to interactive data about all the upcoming eclipses in the next many hundred years. And you can find that 2099 one and start playing with maps, start plotting your observing site. And I think that's the main inf

ormation you want at this point about that eclipse. For information about... Because that will tell you about the track and the duration and the timing and so on. For information about eclipses in general, if you want to learn more about how to observe them, about what we have learned from them and so on, I would recommend two pages. NASA has some really nice eclipse pages and there's also a site called greatamericaneclipse.com that has a wealth of resources focused on this eclipse, but much of

it applies to eclipses in general, and you can use that to think forward to future eclipses too. - It's a amazing coincidence, this majestic, but as I understand it, the Moon's moving away from planet Earth. At what point will we no longer have this phenomena? - Great question. The Moon's orbit is, as the questioner mentioned, moving very slowly away from Earth. I wanna say off the top of my head, it is getting more distant by something like five or 10 centimeters per year. (attendees speaking i

ndistinctly) That means eventually the Moon will never be big enough to cover the Sun. And how eventually is eventually, I should have the answer to that. It's a really long time. It is of order 100 million years. It's less than a billion, it's more than 10 million. I wanna say it's a of order 100 million years. So plenty of time for us to capitalize on the interstellar tourism trade. - Another online question about eclipses that occur, I guess near sunset or sunrise, are they different in any w

ay or appear different? - That's a great question. I think, I've never seen one of those myself. They do occur near morning or evening with some regularity and including this one that's happening in 2026. It'll be an evening eclipse off the coast of the UK and Spain. The Sun is low on the horizon, which means that you have a really cool view of a landscape in conjunction with the eclipse. It also means there is a greater chance that you might encounter clouds. You're looking through more of the

Earth's atmosphere, depending on how high above the horizon the eclipse takes place, you might see more, you know, interesting colors and so on during the partial phase, the twilight might look a little different, but other than that you will observe the same other kinds of effects that you see in other total eclipses. It will get as dark, nature will respond to that, animals will go into their nighttime mode even though it's not quite night, et cetera. That would be a really cool thing. And tha

t's on my list to do at some point. - I observed the 2017 eclipse in Tennessee and coming up to the eclipse, about an hour before, it was cloudy and everyone was bummed out that, you know, we weren't gonna see the eclipse clearly, but as the eclipse proceeded, the clouds just kind of faded away. The grayness just disappeared and we had this beautiful blue sky and it was just a spectacular observation. How does the eclipse affect the metrology at the time? How does that affect the heat? - Yeah, t

hat's a great question. Actually I'd like you to come to my site because it sounds like you're good luck. (David and attendees laughing) So the question is about how the eclipse affects the weather. One of the effects I didn't mention, and I'm glad you brought it up, is with the temperature change that comes from the encroaching darkness and the the Moon's shadow, you also often see that the wind picks up because wind responds to these, you know, convective currents are induced by the temperatur

e changes in this local region that's traveling. And so that can produce weather patterns that can act on the clouds. I can't guarantee it will move clouds away, it might move clouds in, but there may be local effects like that because of winds and changes in the winds brought about by these temperature changes over a swath that's 100 miles wide. Yeah. - Thank you. - If I wanted to replicate Eddington's experiment from 1919, do you have any suggestions for any stellar candidates? - That's a grea

t question. There is a bright star that will be in the field of view of these cameras. We have been talking in our group about what's in the field of view besides the Sun and how can we use it to align our images. There is a fourth or fifth magnitude star, I can look up the name for you and let you know offline, Case, but it's... There's a star that will be less than half a degree away. Now how much will that star move from its apparent position and how do you know? That's a more complicated que

stion. I mean, this is not a big effect. This is a small fraction of an arc second. So I would suspect that you would need, well, you saw the kinds of telescopes they were using in 1919. It wasn't some little thing like this because I think what you need to do, you know, with just one star, you don't really know if it moved or not. So what you need to see is a few stars in the field and notice that the ones closer to the Sun have moved by more than the ones, you know, a little further out. At so

me point there's no measurable effect. And so you need a telescope big enough to capture several stars at several different distances. The best candidate is this fourth or fifth magnitude star in Pisces, I think. But it's not a simple experiment to replicate. Yeah. - All right. I have a quick one. Is it ever possible to see the northern lights or southern lights during an eclipse? - Oh wow. (attendee laughing) Wouldn't that be amazing- - Alaska in 2033 is what I'm thinking. - Yeah. I don't see w

hy not. I mean, northern lights are caused by solar activity, charged particles that have been ejected from the Sun, that arrive at the Earth, hit our magnetic field, start spiralling around and giving off light and exciting fluorescence. The northern lights happen during daytime too. We just can't see it. This is a, you know, continuous phenomenon when it's happening. So I guess I don't see why you couldn't have northern lights together with a solar eclipse at a sufficiently, you know, northern

or southern latitude. - All right. Thanks. New bucket list item. - Wow. Yeah, wouldn't that be incredible? (attendees speaking faintly) Okay. - All right. (attendees speaking faintly) - Any more questions from the audience? Okay, well, I think we should thank David again. (attendees applauding) "Totally Awesome," you all agree. And we look forward to seeing you at the next "Saturday Morning Physics," which is on March 9th. Have a great weekend, everyone. (attendees speaking faintly)