 Well, good evening everyone. My name is joy white cough and I'm a student here at Berkeley. I'm studying economics political science computer science and legal studies. You know, something I love about Cal is our unwavering community. I think your presence here today says it all. And I also wanted to extend my deepest gratitude to all of the donors who make events like this possible. I'm being a region scholar myself. I know I wouldn't be here without you. So thank you. And now I'd like to introduce Alex for the Panko a member of both the National Academy of Sciences, and the American Academy of Arts and Sciences, and what is one of the world's most highly cited astrophysicists. For the Panko was voted best professor you see Berkeley a record of nine times. I know. And in 2006 he was named the case Carnegie National Professor of the Year among doctoral institutions. For the Panko has reduced by astronomy video courses with the great courses co offered an award winning astronomy textbook and appeared in more than 120 television documentaries 120 more than me. So, with a big round of applause, please give a warm welcome to Professor for the Panko. Thank you. Thank you very much for that kind introduction and what a treat to have oski with us Wow. Now you know, no one. Hardly anyone knows who he is I mean he or she he must or she or they must know who they are, and your employer must know who you are right. And how many other people know who you are. I don't know right. If you told me you'd have to shoot me right or something like that right a well guarded secret. In my nearly four decades here. I have still not figured out how to find out who oski really is. That is harder than any problem in astrophysics. Right. Dark matter dark energy. They don't hold a candle to the ultimate question of oski, who is oski. The question no the answer to the question of life, the universe and everything is not 42. It's oski. Right. Thank you very much, thank you too much credit oski. All right, very good. Thank you oski for coming. Oh, look, look, oski, wait a minute. Look what I have here. Bears, but this is from long ago. one of your ancestors maybe, right? So no, it's not you because this is from more than three or four years ago. And my guess is that you're an undergraduate. But I don't know that for sure. And if you are an undergraduate for more than about six or seven years, you've been here way too long. Okay. So all right, thank you for gracing us with your presence. All right, let's give a hand to Oscar. Very good. Thank you, Oscar. All right. So I'm glad everyone is here. Welcome to homecoming weekend. Should be great. The weather is almost too good, right? Boy, it is warm out there. My students who just took midterms and very stuffy classrooms. How was that? Probably not so great. But let me just minimize one thing right there. Okay, very good. All right. Well, that's funny. Why is suddenly the clicker not working? Now it is. There we go. Good. Okay. So there's the sun pretty familiar to all of you, I think. And generally something you should not stare at. All right. Maybe if it's highly attenuated, when it's just about to set, you can briefly glance at it looking for the green flash or whatever. But occasionally, it gets covered by the moon and you get either a partial or better yet, a total solar eclipse, more about the moon in a minute. But historically, solar eclipses were events to be feared, right? Before people understood what's going on, they might think that the gods are angry at them or some monster is eating the sun. Yeah, here's some of the general explanations. A monster, dog or dragon attempts to destroy the sun or maybe if it's even Oscar, maybe it's a bear trying to destroy the sun. Look, he's, he's yellow or gold, I should say not yellow gold, right? So you think he wants some golden sunlight there, although the sun is a white star by the way, but let's not get into that. Anyway, the sun fights with its lover, the moon. That's a possible explanation. The sun and moon make love and hide discreetly in the darkness. Oh, hiding. Or the sun grows angry, sad, sick, and neglectful. So what did these ancient cultures do some still do do this? They would sacrifice various animals. They would sacrifice virgins. They would beat pots and pans and scream loudly. And you know what? It worked every time. The sun would become uncovered the gods would subside in their anger. The dragons would no longer think that this is adequate punishment for whatever it is that the humans did wrong. Well, the true physical explanation removes some of this fanciful stuff. But it is just as grand in many ways. The moon gets in the way at its new phase every once in a while, not every new moon. And it's pretty much exactly the right size to get in the way. The sun is about 400 times the diameter of the moon. But it's also about 400 times farther away. So they look the same size in the sky. This is true of any two objects. The ratio of whose diameters is equal to the ratio of their distances. So here, a tennis ball is almost exactly twice the diameter of a ping pong ball. Looks more than that. But your eyes, your brain is judging the volume. It's twice the diameter. So if I adjust the distance here and the ping pong ball is too close, it appears too big. It more than covers the tennis ball. And if I make it too far away, it looks too small. It doesn't quite cover the tennis ball. But if I arrange it so that it's two to one, I think you can see that from different parts of the room here. I'll try to show that two to one ratio distances, they appear the same size to my eye. So anytime you satisfy this relationship, you'll get the same angle in the sky for the two objects. And there's no reason there's no physical reason that the moon and the sun had to be arranged in this way. There is a physical reason, for example, that we see the same side of the moon all the time. I won't get into that. Now it's not so relevant for eclipses, but you can ask it in the Q&A session if you want. But this is just a coincidence. And in fact, the moon is slowly going away from earth because of the effect of tides. So you'd better see a total solar eclipse soon in the next half a billion years or so. Because beyond that time, the moon will be always too far away to fully cover the sun. So another way to think about this is that the moon is casting a shadow on earth. And at this location right there, you have co linearity, and you have a potential total solar eclipse then. But if you're away from this region right here, the moon won't be exactly aligned, and you'll get a partial eclipse. Or if you're too far off, you won't get any eclipse at all. So this is the geometry of a total solar eclipse. A total lunar eclipse is sort of the other way around. Instead of being at new moon, it can occur at full moon. It's a true eclipse where one object goes into the shadow of another. The solar eclipse isn't quite that but we call it an eclipse anyway. But here the moon goes into the shadow cast into space by earth. And it gets darkened and you can see earth shadow is about three times as big as the moon at the moon's distance. Now, the moon isn't completely dark there because some sunlight can filter through the atmosphere bent be bent by the atmosphere and land on the moon and reflect off of it so the moon doesn't look completely dark. But that's the geometry of a lunar eclipse. And as you're exiting the room, we won't let you out. Okay, it's like Hotel California until you take a quiz. So I'm gonna prepare you for the quiz here. A solar eclipse. Okay. A lunar eclipse. Everyone got that so far so good. And a paka lips. All right. So now you know the three possible geometries. All right. So let's discuss solar eclipses and go through these partial stages first. It takes about an hour and a half for the moon to gradually move into co linearity with the sun. Great anticipation during this time. And then the last little bit of the sun is peeking through not quite covered by the moon. And you get what's called the diamond ring effect where either to your eyes or to a photograph. The last remaining part of the sun is overexposed. So it looks sort of like a jewel a diamond. But the inner parts of the corona hot, tenuous gas begin to appear and a thin layer called the chromosphere begins to appear. And this part as I'll discuss is safe to look at with the unaided eye more on that later. So you can look at this and it looks like a ring in the sky. It's just gorgeous and it changes very rapidly. This is the very exciting part, the beginning of totality. There is a less overexposed photograph showing that little bit of the sun still shining. Okay. And then you have the total eclipse. And this photograph shows this thin layer called the chromosphere chromo for color. It's got this pinkish glow because electrons and hydrogen atoms are jumping from the third energy level to the second energy level and helium atoms are doing things with their electrons as well, etc, etc. But the color is this pinkish color. Okay. And you also then start seeing the very tenuous faint corona, the very hot outer atmosphere of the sun, which you can't normally see by just sticking a ball up and blocking the sun, because the sunlight has already scattered off of our atmosphere, the molecules in our atmosphere. And that makes the sky brighter than the corona. That's why you can't do this with an object inside our atmosphere, you have to block the sunlight with an object outside our atmosphere. So as our atmosphere doesn't become too bright. But the corona is this beautiful thing. It changes shape from one eclipse to another. Because these are hot ionized gases, they have lost one or more electron, and they tend to follow along the magnetic field lines of the sun. The sun is like a giant magnet. But in detail, it's magnetic field structure changes with time. And so that's one reason there are many. But that's one reason to see more than one total solar eclipse because the corona changes and it's just so magical. There's another beautiful corona. And your eye has what's called a large dynamic range. It sees bright things and faint things pretty well, simultaneously. So you see both the outer corona and details in the inner corona at the same time. A typical photograph, especially with cameras say more than five or 10 years ago that couldn't adjust very well to differences in brightness. You would tend to expose well for one part of the corona, in this case the outer corona, but you'd overexpose the inner corona. And to get that, you know, you'd need a different photograph. Now cameras are pretty good at adjusting for different intensity levels. It's still good not to have a bright sunny sky behind someone whose photograph you're taking up close, right? Because your camera will still get fooled. But these things are pretty good. But many cameras still, especially ones that don't have these adjustment techniques, they don't do a good job in terms of their dynamic range. Here's a photograph I took of Mount Lassen in Northern California, exposing well for the forest and the meadow, but not so well for the sky, it's overexposed, and even for the the snow and the glaciers. And now here's a photo exposed well for the clouds in the sky, and the mountain and the snow. But now you don't see many details at all in the meadow and in the forest. So your eye has a wonderful dynamic range, especially compared with older cameras. You get 360 degree twilight sky colors all around you, because the shadow falls on only a tiny region of the world. I'll show you how small in a few minutes. Just a few tens of miles away will be regions where only 99 or 98 percent of the sun is covered. So some light sunlight is coming in scattering off of molecules and dust particles in the atmosphere and filtering its way toward you. Well, that's sort of what happens near sunset. There's not that much sunlight, and it's kind of filtering its way toward you. So you get these yellows, oranges and reds, but you get those colors all around during totality. And the sky gets somewhat dark, not super dark, kind of like twilight, it varies from one eclipse to another, but some stars and bright planets do come out. I don't spend a lot of my time looking which stars and planets happen to be out, because you know, you can see those kinds of things usually at night. I have a friend who tries to figure out what is the faintest star he can see during totality? I tell him, that's not a good use of your time during totality. Okay, but he has seen many total solar eclipses. So now he chooses to study one phenomenon or another during each eclipse. But I don't spend a lot of time looking at the stars. All too quickly, totality is over. By quickly, I mean just a few minutes. The theoretical maximum is seven and one third minutes. Those are extraordinarily rare. My lifetime average in 19 eclipses is somewhere around two minutes per eclipse. Very short, go to the bathroom before totality. But there's the chromosphere reappearing and the diamond ring. You get to see two of them. If you're lucky, I often miss the first one for reasons I'll tell you in a minute. And then the partial phase is unfold for nearly an hour and a half. And most people don't care. They're busy partying. But for those who do care, don't bother them. Let them have the full experience. Okay, just as you would not want to be bothered if you wanted to immerse yourself in the full three hours. But most people don't care after totality is over. And they celebrate, perhaps with a corona extra. I own no stock in any beer company. Okay, but it's not totality without a corona. All right. Yeah, I know. My first total solar eclipse. I was a senior in college at UC Santa Barbara. My advisor said, Hey, let's go up to Oregon to see a total solar eclipse. I thought, Well, what's the big deal? The moon gets in front of the sun. So what? There weren't very good photographs back then there weren't good videos. And even the photographs and videos now don't really tell you what it's all about. You've got to be there. All right. But I said, Yeah, yeah, let's go. It was raining and cloudy up until the partial phases began. And then the clouds started to part almost magically. We were on the cascade range in Oregon. And you can see we're well into the partial phases here. There's still some clouds. You can see the scattering off of them. It looks a bit fuzzy. And then as the first diamond ring occurred, this last cloud moved away. And it was almost like we were just in the right spot at the right time. And these are photos I actually took. It was it was amazing. And I came and visited my parents and I was crying with emotion because it's a very emotional experience. I know that I sound a bit like a lunatic, right? Pardon the pun. But once you experience one, not just photographs or videos, you too will become a lunatic, if you're like most people, some don't get affected this way. They are emotionally dead, or compromised, I should say. If you've been to one and you didn't care for it, that's okay. Each person is different. Just don't tell me. Okay. Well, so here's a video. And I'm not paying anyone to have these reactions. Okay, a few minutes, shadow. Wow. See how downers in the shadow. Okay, then you're calm down a little bit. It's more than flavor. I'm a sphere in the bottom. That gives you some idea of the reaction people have. So why isn't there a total solar eclipse or even a partial one every new moon? Well, that's because there's a five degree angle between the moon's orbit around Earth and Earth's orbit around the sun. So usually the moon is a little bit below or above the sun. But every once in a while, typically every 16 months or so, although it's not perfectly periodic, they will line up. All right. There's usually a few lunar and solar eclipses per year, mostly partial. The most you can possibly get in combination is seven. Usually there's just four or five, but anyway. And then this location of the shadow traces out a path on Earth, because of both the rotation of Earth, and more importantly, the rotation or the revolution of the moon. So where this little spot lands, moves across Earth. And it moves across Earth, one or 2000 miles per hour, it depends on where on Earth it's landing and things like that. So here you can see the July 11, 1991 eclipse, a super long one, nearly seven minutes. This is the place of totality. This is where you don't have coloniarity. So that's that big gray area is the partial eclipse. No big deal. You want to be there. It went right over Hawaii, actually, but most of Hawaii was clouded out, went over the tip of Baja. That's where I took my parents to that one, actually. I vowed to take them and I did eventually. I don't know if I said that. But so you can see that over most of the US, there was only a partial eclipse. But it did go over Mexico City. Unfortunately, most of the people there were told the wrong thing in terms of viewing it. So most of them only watched it on TV at best. It is perfectly safe to view if you know how to safely view it. Here in the 20 year interval, you can see the paths of eclipses. A typical path covers only half a percent to one percent of Earth's surface. So total solar eclipses are rare. Typically, you have to wait 380 years for one to visit you. Okay. Generally, it won't come to you. You have to go see it. So I've seen 19. I'm not 19 times 380 years old. But it's a great excuse to vacation in exotic lands. If you have the time and the means and the desire to see the world, but no particular order in which you need to see it, you might as well go where there's going to be a total solar eclipse because that'll be the cherry on top, right? Yeah. Indonesia, right? Yes. April 20th, April 20th, 2023 and before that in November of 2012 and other years as well. My former students. Good to see you, Fira. All right. You graduated last year. Congratulations. Okay. So, so here's this one. Fira was one of my star students from Indonesia. Yay, where I was on April 20th, 2023. That little tiny dot in that particular case. This one only lasted one minute. My shortest ever was 30 seconds. Can you see it? It's that little dot. Yeah, I actually was at totality. I was right here. Sorry about Fira. But then we went and visited these places here. Okay. All right. So that was a beautiful one. Absolutely gorgeous total solar eclipse over the Indian Ocean and Indonesia. Beautiful corona. August 21st, 2017, folks. Perhaps you saw it. Raise your hands if you're in the path of totality. Oh, many more, many fewer hands. Okay. Well, those of you who saw partial eclipse, if you think that that's all there is to it, maybe you were in a place of 95% totality. No, you were not 95% of the experience. It's sort of like being pregnant versus not pregnant. There's a very large difference. But it was awesome. And you have another chance coming up more on that in a minute. But I have shirts from nearly every eclipse. And one of my hallmarks here at Cal is that for every lecture in my introductory astronomy class, I wear a T shirt that is in some way relevant to that day's lecture. So there you go. Okay. The chance you've got coming up is April 8, 2024. A long one, folks. Twice more than twice as long as my lifetime average. Coming in from Mexico through the US. Okay, more on that in a minute. But first, let me tell you about an annular eclipse. Not annual, but annual are an annulus, a ring of fire remains. It's a special case of a partial eclipse. What's going on? Didn't I say that the moon perfectly covers the sun? Well, the moon has an elliptical orbit around Earth. This is now Earth Earth like that. And we'll all be Earth. Sometimes it's far away. Sometimes it's close. And so sometimes it looks too small. When it's new to fully cover the sun because of this elliptical orbit. And sometimes it's near us when it covers the sun. That's what leads to a long a seven minute long eclipse because it takes a while for the moon to go across the sun because the moon looks big, a super moon, so to speak, but they're not so super. Imagine ordering a 16 inch pizza and getting a 17 inch pizza. It's not a big deal. There are three or four super moons per year. That's not a big deal. But when a super new moon occurs, and you have co linearity, that's what gives you a long total solar eclipse. And so that's what's going to be going on in 2024. But this annular eclipse, fun to see, but it's definitely not a total eclipse. Still, no reason to be too grumpy. You can enjoy the Ring of Fire and be creative here in San Diego. I think 1991 or 92. Dennis Mamana took this picture. And then more recently, there's a bunch of phases of of an annular eclipse. The reason I mentioned this is that a week from tomorrow, folks, there's an annular eclipse in our same neck of the woods. Indeed, Texas gets it twice, right? Because they get the total one going through like this. So a week from Saturday. Yeah, you're like Linda Mucca. Is that what you said? Someone said, Well, anyway, it goes through parts of Oregon, Nevada, Utah, little bits of Arizona, little bits of Colorado, a lot of New Mexico, Texas. It's not that long a drive either to Nevada or Oregon from here. So I don't usually travel to the far side of the world to see an annular eclipse because you don't get to see the chromosphere and the corona and all that great stuff. But but I'll go to somewhere in Nevada for this one might as well. Family trip next Saturday. Okay. So during the partial phases, which is any phase during an annular eclipse, eye safety is of paramount importance to directly view a partial eclipse, you must use a special filter that blocks all but one part in 100,000 of the visible light and preferably 100% point 00 of the ultraviolet and infrared regular or polarized sunglasses. No way you can harm your eyes. Shade 14 welders glass is good stuff to have. Okay, here it is. It gives the sun and clouds a green tinge. Here it looks white because it's overexposed, but it's green. And you can mount it on cardboard to make it easier to hold. And it blocks then the cardboard blocks your face from the sunlight but put on sunscreen anyway. Here my brother and I are in Mongolia in 2008, viewing the partial eclipse, and then totality without the filter more on that in a minute. If you want to shield the sides of your head, you know, your cheeks, you can bend the cardboard. And if you're totally anti social, you can put a box over your head. And you can be creative. And, you know, have various designs on it and stuff. So this is very durable. I keep some in my pocket. Sometimes if I'm standing in some boring line, I'll take it out and I'll see if there are sunspots, little dark spots in the sun and stuff. So if you wish, I have a bunch of these. After the lecture, you can get one. I sell them at cost. I save you the trouble of going to a welding supply store. But these will suffice as well. They're just not as sturdy. You can't ram them in your pocket and stuff. They'll they'll bend. So take care of these. But any of you who do not yet have one of these, I think I have enough to have a free one per person. So raise your hands and if you don't yet have one of these, where is my box of these? But maybe Susan can give you one. If you still have some, Susan, I don't know where the box is. If there are any spares left over, Susan, then give them to me at the end. But anyway, you get a free one. You know, but if you want one of these more sturdy things come to me after class. So here's someone in Peru enjoying the eclipse with one of these cheap cardboard things. If you're using binoculars or a telescope, be sure to use this glass or mylar or other CE certified. That's the that's the key phrase CE certified. I'm not sure what stands for but but it means that it's safe and put it at the top end of the binocular or the telescope, not at the eyepiece end because otherwise a big huge mirror or lens is gathering sunlight, focusing it into the eyepiece and the filter could heat up and crack and then expose your eyes to huge amount of sunlight. You know, another way to view the eclipse and this is totally safe is to just punch a pencil hole through a piece of cardboard and then allow the cardboard to cast a shadow on the ground on a viewing screen and the sun, a luminous object will emit light rays that will go through the hole and produce an inverted version of the sun but who cares what's up and down with the sun. So you can do this you can put more than one hole in I put here the name of my oldest son Simon and of course you might as well observe the celestial phenomenon with celestially themed candy eclipse gum or orbit gum I own no stock in any candy supply store or brand but it is kind of fun to have eclipse gum or orbit gum. So here you see a whole bunch of little crescent simons a calendar works real well because it's very sharp holes right if you punch a hole through a piece of cardboard with a pencil it'll look a little bit ratty and that'll make it a little bit fuzzy but a calendar has nice sharp holes. It's still the case that the images are not perfectly sharp and that's because the hole has some non zero diameter. You'll get sharper but fainter images with a smaller hole, brighter but larger sorry brighter but less in focus images with a larger hole. And then what's this? I'm viewing the ground on my way to the bathroom well before totality in 1991. What am I seeing here? What was this caused by a tree that's right holes between the trees and you might say well the holes, why would they be crescent shape? They're not. You're getting an image of the sun, not an image of the holes. The holes are acting like pinhole cameras. Okay. Telescope binoculars no filter. I got to accelerate here a little bit. I got a bit of a later start when I thought because I want to leave time for questions. You can project I forgot to bring them but you can if you have a telescope you can project the sun's image onto a sheet of paper by holding the telescope aligned with the sun. In that case don't have a filter because you want to have a bright image of the sun, a larger, brighter image of the sun. So here I've got I forgot to bring it. Maybe I didn't forget to bring it. I have a bag of tricks here. Nope. I forgot to bring it. Oh well, it's just a little telescope aligned with the sun. So there it is casting a shadow. If you have two telescopes that would be a binocular and you would get two images on the shadow. I call this the C cups. Okay. So there you go. You have plenty of time during the incoming partial phases to amuse yourself an hour and a half or so. So you can play various games like this. Backman. If skies are clear, you will be happy. And you're viewing now through pinhole camera, or with the shade 14, or with these glasses. Okay, now totality is coming up. A few seconds before totality, you can take the filter off. More about that in a minute. Totality can and should be viewed with no filter whatsoever. If everyone around you is yelling and screaming, and you're saying what's the big deal? You've still got your filter on during totality. The corona is very faint. Very low surface brightness, brightness per unit area. You won't see anything. Okay. So look at the total solar eclipse with the unaided eye binoculars, telescope, no filter, no $1 glasses. Okay. The diamond ring is the controversial part. The official recommendation of the American Astronomical Society is that the diamond ring should not be looked at without a filter. To put that another way, there are a lot of negatives there. Use a filter to look at the diamond ring. I don't agree. You'll see the diamond, but you won't see the ring. The American Astronomical Society gives this recommendation because most people in the US don't know how to listen and act accordingly. But you, Cal graduates, parents of current or former students, or in some other way affiliated with a university, you are attentive, well educated, intelligent, and you can listen. So the diamond ring with the unaided eye is safe to view for one or two seconds. Don't be looking through a telescope, because although only a little bit of the sun will not be covered, that part will be focused on one part of your retina. So the energy density on that one part of your retina will still be very high. No optical aid, but also no filter. I miss the first diamond ring half the time, because I'm so uncertain about when to take the filter off. It's hard to judge when you're only two seconds away from totality. I'm not looking at a clock half the time I'm on a boat somewhere. I don't even know exactly where I am. And the timing depends on exactly where you are. You want to be looking up, not at some clock. And so half the time I miss it. That's okay. Then you watch totality for two or three or four minutes. And then you can watch the second diamond ring emerge. And what you need to do is have your wits about you and designate people who have had not too much alcohol and stuff and have their wits about them, maybe have seen one or two eclipses so they know what to expect. They will yell filters on. But if no one yells it, it's up to you after a couple of seconds to put the filter back on. The view is mesmerizing for the second diamond ring. And so it's very easy to just be saying that are my God, what am I looking at, you know, and to continue staring. That's the danger. Put the filter back on. Okay, filters or pinhole camera definitely required during all other partial phases. So there's the diamond ring. Let's call it the second diamond ring. Great view. And you are watching it. And things are changing so quickly. To me, the two diamond rings are at least the second one, which is the one I see most often. That's in a sense the most special moment of the whole total eclipse, even better really in my opinion than the corona, but to each their own. The corona is beautiful too. So the eclipse coming up. Let's focus on that and then we'll do a Q&A. There is this map which showing a partial eclipse overall of North America, basically. The big deal. You want to go to where it's going to be total, right? I tell my students, if I find that you did not go to the path of totality, but didn't have a good reason not to go, I will retroactively fail you. Fira, didn't I tell you that? I did not retroactively fail, Fira. Okay. Students from long before 2017 have come back to me and said, first of all, can't fail me. Second of all, I undersold it. I try to undersell it because I don't want to ruin the experience for you. I sometimes worry that by giving these lectures, I will play it up too much and you will end up being disappointed because I had no warning, essentially, before I saw my first in 1979. But people have told me, no, it was good that I gave the lecture and I didn't spoil it for them, but I worry about that sometimes. And so students tell me that I undersold it. Where should you go? Well, first of all, it takes about an hour to cross the USA, starts in Texas around 1.30 p.m., central time, Vermont 3.30. Well, it took an hour, but it moved into another time zone. So that's why there's a two hour difference there, but it zips across the U.S. from west to east, starting in Mexico. The single most important variable for you is to be within the path of totality. If you live in Houston and you think you're close enough, you're not close enough. Okay? If you live in Chicago and you think you're close enough, you're not close enough. You've got to be within that path. The second variable, second most important, is to be in a place with good weather. So here's something based on climate models and records of 40 or 50 years of satellite photos done by the world's premier eclipse climatologist, in a sense, Jay Anderson. This is cloud cover, mean median cloud cover fraction on April 8th, going back decades. Blue is good, not many clouds. Red is very bad. You can see that Mexico has the best chance. Durango, Mazatlan, great chance in terms of cloud cover. Real nice. But if you want to stay in the U.S., southern, south, well, south central Texas is sort of bluish green, and then there are variations, but monotonically things tend to get cloudier as you move off to the northeast, okay, with some variations. So I would suggest you go to wherever you want to go, where there are friends or relatives or whatever, and then a few days in advance, start monitoring the weather carefully. And the night before, look at the forecast for the next day very carefully. And if it looks like it's going to be cloudy where you are, but a much greater chance of clear skies a couple hundred miles away, have some mobility, have a car available. And at one or two in the morning, pack your sleeping bag, start heading to where it's going to be clear. Because once sunrise comes, everyone and their grandmother is going to be trying to head to the clearer places. You want to already be there. Get there in the middle of the night, lay out your sleeping bag, or you'll be so excited that you'll just pull on all night or whatever. It gets a couple of hours of sleep at 10 in the evening, whatever. Just get there. Stay within the path of totality. If it looks like it's going to be clear in Albuquerque, that's not a good place to go for this total eclipse. Third variable, eclipse duration. You don't have to have the full four and a half minutes. But it's good to have at least 30 seconds. And the longer than the better, all other things being equal. Near the edges of the path, you get a very short eclipse. San Antonio. South San Antonio, they won't even get totality. North San Antonio, it's not so bad. But if I were living in South San Antonio, I would head northeast, northwest, toward this green center line. Near the center line, you get the full duration. I'll show you in a second that you can get pretty far off the center line and have a slightly shorter duration. But the function is very interesting. Near the edges, it drops like a rock. Okay. So you can see a few durations here. But let me give you a specific example from Xavier Jubie's wonderful website. It's an interactive Google map. You can click on various places. It's not the only such map in existence, but it's a particularly good one. Centerline, east of Dallas, Texas. Four minutes, 22 seconds, and a bunch of other information like when. You don't want to be sleeping or something. Okay. Here's Dallas itself. Pretty far off the center line. Three minutes and 55 seconds. Pretty darn good. If I were living Dallas, I would just stay there. Why move? Austin is a good, another good place. Fort Worth, two minutes, 37 seconds. Not bad. Exceeds my lifetime average. But now, just a few miles west of Fort Worth, you're down to one minute and 11 seconds. That's what I had in Indonesia and northwest Australia on April 20th. That's the best I could do. But for this one, you can probably do better. And let's say you, your GPS messed up where you are and you're actually a couple of miles west of that, then it's tough luck for you. You get 99.5 percent, but I will retroactively fail you. Why do I show a picture of a random motel six? Because pretty much all the hotels on the path of totality are already booked and have been for a year. But you might find one. In 2017, motel six went for $800 a night, three night minimum in Oregon. Yeah. And people who had reservations years earlier at the usual low price before the proprietors knew what was going on. Why are people suddenly, reserving those days? They actually canceled the reservations and then people were left without a room and stuff because it got resold and there were lawsuits and stuff. And maybe the people even won the lawsuit, but if you missed the eclipse, that's not a, you got your money back, so what? You're 50 bucks. But you missed the eclipse. So if they're charging 50 bucks a night or something, you might want to tell them, you know, there's an eclipse coming up, microeconomics supply and demand, you could be charging a lot more. You might want to even tell them that so that they don't go off and cancel later on your $50 reservation when they find out that people are willing to pay 800,000 bucks a night. Okay. If you're even able to find one right now, but start looking as you go off the path, though, there are more and more hotels and just drive in that morning at two or three in the morning. Okay. So it's okay. The hotels on the path are booked up. Just get a hotel off the path 100 miles away. I bet you there's lots and lots of hotels available 100 miles away. Don't miss it because the next total story of eclipses in the USA will not be till 2044 and that one only covers North Dakota and what Montana here, it looks like, right? The next one that goes over mostly you are, well, sort of, you know, coast to coast is not until August 12th, 2045. So you've got a long dry spell coming up, folks. We had a long dry spell between 1979 and 2017. Now only seven years later, another total eclipse. Great. But we've got another dry spell coming up. There's a bunch of good websites. You can take a picture of this if you want, but it's use your favorite search engine. Total solar eclipse April 2024. There will be 10 blue websites, you know, 10 links that you can then go to. But here are some of my favorites. I want to thank you for interest and support. Homecoming weekend is a chance to come back and revel in your glory days when you were a student here and all that, but we can also appreciate that this is a nice time to give as well. We can't run this university without your support. So thank you very much. Here are some of my favorite things. You go to the Give to Cal website in the search box, type in astronomy or lick observatory. These are all good things to support because our students use these facilities and so you'll be supporting all our students. So thank you so much. I hope there's, I mean, it's six o'clock, but we didn't start until with Oskie and stuff. It was great having him here for so long, but hopefully I have a few minutes to answer questions. Thank you. And I'll repeat the questions for the benefit of those on Zoom. And this is also being taped and so you can come back and watch the recording later on. Yes. The question is, when did people first predict, begin to predict eclipses with different levels of accuracy thousands of years ago? Babylonians and others noticed patterns and from the patterns they could then predict them without a physical understanding of what's going on. But some, in some cases, not enough data had been gathered and not enough patterns had been seen. So roughly 2000 BCE, I might have the date not quite right and I'm only giving it to one significant digit, two Chinese astrologers, not astronomers, astrology is how it started out. She and Xu, I think were their names, failed to predict a total solar eclipse and the emperor, whatever you call the person who led the empire back then, beheaded them because a son or a daughter was born on or near that day. And that was a real bad omen and these dudes hadn't predicted it. And by that time, the nine month pattern for conception and birth had been noticed. And so these dudes should have really predicted this and told the emperor or whatever of the dynasty, whichever one it was, not to do this. But probably by around 2000 years ago, it had matured and there is a story of a region of what is now Turkey, I've forgotten exactly what it's called but it was the, I think it was the battle between the Linians and the Medes in 585, either AD or BC, shows how well I know my history. I'm off by 1100 years there, right? But one person supposedly predicted it. But then, I don't know whether the warring factions didn't pay attention or whatever, but when the eclipse then started, they were all, oh my, the gods are mad at us, let's settle our differences and make up and they did. But I believe Herod or someone like that predicted that one. But anyway, I don't know the history all that well, but it has been known for probably a couple of thousand years or certainly a thousand years. Okay, yeah, other questions, there must be other questions. There's always questions, yes, right there. And then over there, yes, go ahead. Ah, why is the corona hotter than the chromosphere and hotter than the so-called surface in quotes because it's a gaseous surface, but it's just the place from which the light comes. That's a very interesting question. But corona has a very high temperature, but not a very large heat content, more on that in a minute. But the high temperature, millions of degrees, is thought to be caused by energy injection from tangled magnetic fields. Magnetic fields have a large amount of energy content. And occasionally, if you have tangled ones, if a North-South pointing one that way meets up with a North-South pointing one this way, they annihilate each other, but the energy has to go somewhere. So it goes into heating the particles around it. And sunspots are regions with strong tangled magnetic fields. So those are regions where both the chromosphere and the corona tend to get heated. Now, macroscopic gas motions might contribute to some of the heating as well. We see the gases going up and down, cooling and heating, a process called convection when they're coming up and down. That might mechanically heat at least the chromosphere, but probably not so much the corona. Probably it's the magnetic heating. However, that goes up to a high temperature, but it is not a large heat content. And here's something that Billy and others remember from my class. I say, don't try this at home, but if you were to go near the sun, like the Parker Solar Probe, which is a satellite up there right now studying the corona, go over there into the corona, block the sun's surface, the photosphere. Well, you would freeze to death immersed in this corona because although the particles that hit you are moving very quickly, that's what we mean by a high temperature, millions of degrees, they're moving very quickly. They're jiggling around. There are so few particles hitting you and transferring their energy to you that you cool by radiating energy to the cold, dark world out there faster than the rate at which you are absorbing energy from the occasional rapidly moving particles that hit you. So you would freeze to death in a 2 million degree corona, but don't try this at home. Okay, so it doesn't have much heat content even though it has a very high temperature, which is just a measure of the random motions of the gaffes. Okay, there was a question up back there first and then you, yeah. Yeah, cases where both the moon and the sun have been above the horizon. So if the sun is above the horizon and it's a solar eclipse by geometrical construction, the moon is above the horizon as well. So I'm not exactly sure I'm getting the gist of your question. I don't know what Centihelion is because you can't have the moon over there and the sun over here and have a solar eclipse. Not possible. Well, if Ptolemy said they were both above Ptolemy may have been referring to the partially eclipse sun being eclipsed by the moon, but he couldn't have been referring to the moon over there and the sun over here because it's just a physical impossibility. And I don't care how smart Ptolemy was. It is a physical impossibility. And by the way, there are all kinds of crazy videos online now, right, the fake news and stuff, but there's this one that's gone viral. Maybe some of you have seen it. It's from an Arctic region. And they show this big, old huge moon and it comes up above the horizon and it's way bigger than the sun and it blots out the sun and then it comes back down below the horizon. Has anyone seen that video? It's sort of a viral video. Yeah, you've seen it. Complete nonsense. There are so many things wrong with that video. Okay, but people can now make very, you know, realistic looking things. So be careful what you absorb on the internet. I don't think I have to say that to Cal people, but you had a question here. Oh yeah, when I go, I just enjoy it. I do no scientific experiments whatsoever. Yeah, so what kinds of things can you do if you're scientifically interested in them? So that's very interesting, of course. You know, you can, one thing you can do is study the inner corona. Now, there's a satellite called SOHO, the Solar and Heliospheric Observatory, which is monitoring the sun 24-7 and they have what's called an occulting disc, like a quarter that's just blocking the sun all the time. And they study coronal mass ejections, which are big ejections of charged particles from the sun and so on and so forth. But they have to cover not only the sun's bright part, but the inner corona as well. Because otherwise, what happens is the inner corona is so bright, relatively speaking, compared to these faint outer parts that they want to study, that the light would bend around and mess up the data. So I don't have an image here, but the occulting disc of SOHO is about this big. So they can study the mid and outer corona, but they can't study the inner corona. The inner corona really can only be studied during the few brief minutes of a total solar eclipse. And my co-author on the textbook that was mentioned in the kind introduction, Jay Pasikoff, who just passed away last November, I'd worked with him for decades on five editions of this textbook and he was an eclipse addict even more than I am. I mean, he was older and had seen more, but he would do these observations. He would go and set up giant numbers of telescopes and cameras, which is why he could never do it on cruise ships because they're not stable enough. And one time in March of 2015, he went to Svalbard, which is this island north of Norway. I said, Jay, the odds are 5% that you're going to see it. He says, Alex, I got to go there because that's where my equipment will be stable. Guess what? He saw it. It was clear in Svalbard. I was on an airplane above the North Atlantic, which was pretty uniformly clouded over. That was the one I had seen from an airplane, but you miss out on the changing temperatures. You miss out on the changing light levels. I think it's good that I did it once from an airplane, but I think the experience overall is better from the ground or from a ship, although the duration from the airplane was longer because we could follow the shadow at 500 miles an hour and it's going one to 2,000. One to 2,000. So we couldn't make it arbitrarily long, but we increased the duration by about 50%. Anyway, back to your question. You can study the chromosphere and inner corona and changes that might occur over a few minute time interval. And in some cases, there are injections of energy over a sufficiently short time scale that you might reasonably see a change. So those are interesting. But the historically most important scientific use of a total solar eclipse was Arthur Eddington's expedition in May of 1919 to an island off the west coast of Africa and also to Brazil. And he had two teams take photographs of the sun at the time of an eclipse and they could see stars. You can't see any of them here because it's too bright in this room, but you can see some stars. And that was to verify Einstein's qualitative and quantitative prediction that the stars in the background of our sun will have their light bent by the gravitational field of the sun, the warping of space and time in the context of his general theory of relativity. And that was at least quantitatively not the prediction that Newtonian gravity gave. And Arthur Eddington made the measurements. They were not completely definitive, but at the time they were considered good enough. They're certainly not definitive by today's standards, but much better experiments have been done. But back then, 104 years ago, it was done by Eddington and that turned Einstein into an overnight celebrity worldwide. He was already very well known among physicists by 1919, but he became a household name, most famous scientist or physicist, at least since Newton. You could argue whether Darwin was more, that's why I switched scientists to physicists. That was the single most important historical scientific use of the total solar eclipse. Yes, over there. Yeah. Okay. So why do we see the same side of the moon? So the moon started spinning when it was formed early on in the solar system's history. We think a Mars-sized object hit Earth, splattered a bunch of the crust and upper mantle away, which formed a disk, which then coalesced into the moon. That would have been spinning quickly. But just as the moon raises tides on Earth, Earth raises tides on the moon. The moon is sort of elongated toward Earth. And if it's spinning rapidly, normally I have a balloon. Let's see if I have a balloon here. Gosh, I forgot my bag of tricks. It's because I picked up the big box of filters and I don't have my balloon. Oh, well, I'll try to, I mean, you can imagine a balloon being squished out toward you, but you're Earth. But now the moon is rotating really quickly. So now the moon, you know, different rocks are now pointing toward you. And now the pen, the pen rocks are pointing toward you. And now it rotates over there. And now some other rocks are, I mean, in other words, the deformation of the moon is deforming the whole thing. It's not just one or two rocks. It's going squish, squish, squish, squish. And so like rubbing your hands together, that'll dissipate energy. Right? Frictional energy. That energy has to come from somewhere. It comes from the rotation rate of the moon. So the moon slowed down more and more and more until it reached what we call synchronous rotation. That is, it rotates on its axis with exactly the same rate or the same period as its revolution around Earth. In that case, the same side points toward it. Because suppose I'm the moon and that's Earth. Suppose I didn't rotate at all. Well, then, you know, different sides would point toward it and the moon would keep on getting all deformed in different directions. But if the same side is pointing toward it, then it's deformed. It's the same rocks facing the moon at all times. But notice that with a quarter of a revolution around Earth, I also rotated a quarter and now another quarter and now another quarter and now another quarter. So it's not dissipating energy anymore. It's locked into the synchronous rotation. Earth rotates more quickly than the lunar orbit. The lunar orbit is 30 days, roughly. Earth rotates in 24 hours. The moon exerts a tide on Earth, mostly the oceans. They're sloshing around. They're dissipating energy. Earth's day-night cycle is slowing down by two seconds every 100,000 years and not discontinuously, just continuously, OK? So we're slowing down as well, but it's going to be many, many, many billions of years before we're locked into synchronous rotation with the moon. So why is the green flash so rare? The green flash is this little tiny bit of sunlight that still remains sometimes when the sun is setting. And you can look at that little tiny bit of sunlight with no filter and it's safe because there's so little sunlight and it looks green. So that's because, first of all, the light goes through our atmosphere, which is not a vacuum, so it bends the light like a raindrop, but not as much, OK? Because our atmosphere isn't a vacuum, so it's bending the light as the function of wavelength into a little tiny rainbow, just a little tiny, tiny rainbow. And the violet, so it spreads the sun out into a violet sun, a blue sun, a green sun, a yellow sun, an orange sun, a red sun. The violet sun is the highest, just a tiny rainbow, but violet is easily scattered by our atmosphere and absorbed by dust and our eyes aren't very sensitive to violet and the sun doesn't emit much violet light. All four reasons, very little violet. For those same reasons, but not quite as much, not much blue. It gets scattered, that's why you have a blue sky, gets absorbed by dust, our eyes aren't very sensitive to blue. Compared to green and yellow, and the sun doesn't emit as much blue light compared to green and yellow. So the next one down is green. Lots of green sunlight, our eyes are sensitive to green, the molecules don't scatter green all that well, and the dust doesn't absorb green all that much. So if you're lucky and the sky is really clear and there isn't much dust and there's not ashes from forest fires or whatever, that green won't be filtered out, and you'll see that little tiny button of green. But more likely than not, there is some junk in the atmosphere, some smog, some wildfire stuff, some volcanic eruption, whatever. A tree in the way, that messes you up for sure, a building in a way, you need a very true horizon. So it's so rare because the conditions of a really clear sky are so rare. And slight thing, I know I'm about to be pulled off the stage here, by the hook, my peripheral vision noticed, Susan. The other thing that's a bit of a subtlety is that what helps the green flash is not just that geometrical thing that I just mentioned, but a density structure in the atmosphere that leads to a mirage. Lifting the sun up a little bit. And there are different kinds of mirages. You know, the mirages you see on hot asphalt, those are called inferior mirages. There are superior mirages. There are different kinds of mirages, but if you have the right density structure in the atmosphere, that little leftover button of green light will be lifted up a little bit, making the phenomenon last a second or two. Maybe even three or four if you're really lucky, instead of just a tiny fraction of a second, which is what just the so-called geometric optics without mirages would give you. So you need not only clear skies, but the right atmospheric conditions to give you the right kind of mirage. How many of you have ever seen a green flash, do you think? Yeah? Yeah, they're hard to see, but they do exist and they've been photographed, and those are not fake photographs if you're looking at the right kind of photographs. Thank you so much for your attention. Go Bears, okay? And if you want the shade 14 glass, it's just more durable, no pressure whatsoever, but you can get one here or more.