Why is it so hard to predict solar eclipses? How did Newton and Halley team up to solve it? What have we learned from solar eclipses since then? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
It's summer. Hot. It's been a dry week. You've been wondering when the rains will come again. Still, you attend to the fields.
Working through the day, you're looking forward to dinner at home this evening. The sun is high in the sky, beating down on you. Sweat stings your eyes. What you would give for a moment of respite. As you work, you notice a shift.
You realize it's been happening for a while, but you only just noticed it. Your shadow looks strange, a little unfamiliar. Like it's more in focus, more real. Like your shadow is taking on a reality of its own. Still you shake it off.
It must be your imagination. You continue to work. A while later you trek over to the nearest tree taking a moment in the shade. As you look at your feet though it seems as if your eyes are playing tricks on you. In the dappled sunlight falling between the leaves, you notice on the ground countless crescent shapes repeated over and over and over and over.
As you watch, the crescents slowly become thinner and thinner. When they are razor thin, that's when you begin to notice. It's not hot anymore. The sunlight is getting dimmer. A cool evening breeze picks up.
You feel the familiar nighttime chill in the air. You're you're confused. How long were you under the street? Did you fall asleep? You look to the distant horizon.
It's not nightfall. The sun is still high overhead, but as you scan the horizon you see what appears to be a shadow approaching you from every direction. Something is tugging at you. You can't resist. You look up at the sun.
Through pained squinted eyes you see something you can't even comprehend. The sun itself is disappearing. A single thin line of blazing brightness surrounded by blackness. The stars come out. Nighttime insects begin their calls.
Birds flock to their nests. The shadows surrounding you on the horizon creep closer surrounding you, engulfing you. In a flash, you are plunged into darkness. Trembling, you make yourself look up and you fall to your knees in horror and awe. The sun is gone.
In its place, a black void surrounded by a ring of fire. You'd fall to the ground. This is what the stories meant. This is the end of our age. The end of our time.
Surely, surely, this is the end of the world. Perhaps the most frightening thing about solar eclipses is their unpredictability. For millennia, court astronomers have grappled with trying to predict eclipses. And I use the word astronomer. Really, they were astrologers.
They were concerned about the movement of objects in the heavens, not for the sake of understanding the movement of objects in the heavens, but because they believed it predicted and influenced and informed our daily lives. It wasn't really until the eighteen hundreds that astrologer and astronomer became different things. So all the astronomers that we associate with the birth of science, like Kepler and Copernicus, they they were more of an astrological meant and certainly the ones before them, but they they got really good at predicting motions of the stars and the planets and the moon and lunar eclipses, but solar eclipses are so elusive. One of the challenges is that when the solar eclipse occurs and the shadow of the moon crosses over the Earth, that's a pretty small area. So any one area part of the Earth, like the ground you're standing on right now, even though a solar eclipse will happen once every two years ish, the spot you're standing on will only have an eclipse every few hundred years at best.
So if you're in charge of an area the size of a country, you might experience total solar eclipses every few decades, but that's not a lot. That's not a lot to get a a handle on, get statistics on. The ancients attempted to predict solar eclipses, and they developed what we call the Saros Cycles, which are just just cycles upon cycles. Like like, you try to read about this stuff and your head just spins because it's like, oh, yeah. Yeah.
Yeah. An eclipse will occur in our country, every twenty years unless it's an odd numbered year, and then it's gonna be every twenty three years. And then we're gonna add six months to it, but then we're gonna subtract fourteen days for every lunar cycle since the last Saros. Like, it just goes on and on and on and on, and your heart goes out to them. Like, it's just so hard to predict a solar eclipse.
To the ancient astronomers, solar eclipses were essentially unpredictable. They could kinda sorta guess, like, yeah, there might be a solar eclipse coming, but that's the best they could do. So it's no wonder in twenty one fifty nine BC, there was a total solar eclipse, and the court astrologers of the emperor Kang or Chang of ancient imperial China missed it. They missed it. And this is one of our oldest pieces of writings we have from China, and it's the story of the two astrologers slash astronomers, he and Ho, who missed it.
Now according to the story, they were out drinking the night before. Like, maybe that was their excuse. They thought get away with it. Like, really, they just, period, couldn't predict the eclipse. And then, you know, the emperor dragged him in and said, why?
This eclipse is a pretty big deal. I've got some rituals to do. There's some purifications, etcetera, etcetera. Why do you guys drop the ball? And they couldn't just say, we can't predict solar eclipses because their whole job is to predict stuff that happens in the sky, so they went with option number two, which is we were out drinking all night.
The emperor beheaded them anyway because that's not a good enough reason. Most of us would probably get fired from our jobs. That was, like, the twenty one fifty nine BC version of getting fired from your job as court astrologer. So the relationship between astronomers and solar eclipses, or I should say the bad relationship between astronomers and solar eclipses, started thousands of years ago. They just couldn't do it, but it changed.
It changed on a very important date. It changed on 05/03/1715. '17 '15. We had spent, by this time, at least four thousand years unable to predict one of the most spectacular events in nature. And solar eclipses aren't a one time thing.
You get forgiveness if it was a one time thing or like, oh, wow. We didn't see that coming. That's cool. But solar eclipses happen often. If you're in charge of a big country, they happen every, say, decade or two decades, and you can't predict them.
You can't nail them down. But a solution came in 1750, and the solution is owed to the one of the greatest assists in scientific history. You see, Newton, Isaac Newton, in the late sixteen hundreds had figured out this thing called the universal gravity, which is tremendously huge. I would love to do an episode just on Newton's gravity. I did Einstein's gravity, but Newton's gravity, one might argue, and I might if I did the episode, that Newton's understanding of gravity was a bigger intellectual leap than Einstein's gravity.
Because what Newton did was completely open up our vision of what gravity is and means. Of course, people knew about gravity. It's the thing that makes things fall. Whoop de doo. So it's not like Newton invented gravity.
People had studied gravity. People had understood gravity. People had thought about gravity going back since forever, but Newton flipped the idea of gravity on its head. And he got the idea according to himself according to himself. We have this story of Newton seeing the apple fall from the tree, and he has the big realization.
And it sounds like one of these, like, legends, but according to himself, he was chilling at his mom's house. There was a plague happening, so he's like, no interest in being plagued. So I'm gonna go to my mom's farm, and I'm just gonna do my Newton stuff there. He was sitting under an apple tree. He saw the apple fall from a tree, and it clicked.
It snapped. It popped. Many, many, many puzzle pieces went right into place in that moment. You see, by this time, he had figured out what we call his laws of motion, that force is equal to times mass times an acceleration, that every action has an equal and opposite reaction, that things move in straight lines unless there's a force that deflects them, etcetera, etcetera. You know these.
We saw the apple fall from a tree, and he saw it fall in a straight line. Like, it fell straight down. He's like, oh, okay. It's falling in a straight line. So whatever is pulling on the apple is pulling straight down, not off to the side.
It's going straight down. And he noticed that in order to fall, the apple had to accelerate because at first, it was not moving and then it was moving. That is kind of one of the definitions of acceleration. So if it's accelerating, then there must be a force being applied to the apple because force equals mass times acceleration. If you've got mass and you're accelerating, there's a force being applied to you.
And this force is making the apple come straight down. So the force points in the direction of the center of the Earth. It doesn't go off to the side, not left hand. It goes around. It goes straight down.
And since every action has an equal and opposite reaction, if the Earth is pulling on the apple by applying a force to the apple, then the apple must be applying a force to the Earth, equal and opposite. But in our experience, the apple is doing all the movement while the Earth is doing all the work. Really, the apple and the Earth are pulling on each other. The forces are equal and opposite. It's just the earth has a little bit more mass than an apple, so it has a very, very tiny acceleration.
The apple is pretty tiny compared to the earth, so it has a big acceleration. So it looks like the apple is falling, but really the apple and the Earth are attracted to each other. So the gravity of the Earth being applied to the apple must be matched by gravity, the apple being attracted to the Earth. That means all objects all objects gravitationally attract all other objects, that this gravity that we assign to the Earth is universal. If you have mass, you have gravity.
Period. End of story. That is a huge leap in understanding. That's giant to make that click. And sometimes I think, you know, Einstein, absolute genius.
Man, Newton was pretty smart, so I've heard. He used this newfound universal theory of gravity to solve all sorts of like, he just he figured out the speed of the moon's orbit. He was able to figure out Kepler's laws. He was able to explain the the orbits of moons around their planets. Like like, just all sorts of systems all you know, just if there's a gravitational problem, Newton solved it using his new found theory.
It was pretty awesome. By the way, he kinda sorta had to invent calculus to do this, but that was like you know, that was that took him a couple hours, I guess. And he wrote it all up, this newfound theory of gravity, all the work that went into it, and he is like, I don't think anyone would be interested in this. I'll put it on a shelf. I'll put it in the drawer, The drawer of, you know, other ideas that weren't very interesting to him and he didn't think would be interesting to anyone else.
And it sat there for years. Years. He didn't like, he figured it out. He was like, whatever. I'll go do something else.
But he had a friend. As much as anyone could be a friend of Newton, I'd love to do an episode on him. You just need to ask. He had a friend named Edmond Halley. Edmond Halley, great astronomer, super smart guy, friend of Newton.
Of course, they got to talking someday. You know, I don't know how this played out, but I'm sure Newton, like, mentioned, like, yeah. I figured out all of gravity. And Halley's like, what? And Newton's like, yeah.
I figured out all of gravity, but, you know, whatever. It's boring. And Newton and Halley's like, dude. Dude. Dude.
Say that again. And, you know, Newton's probably getting annoyed by now. He's like, I I figured out all gravity. I wrote it all up, but I don't think it'd be interesting. Haley's like, I'm interested.
Tell me more, please. And over it took a while, like years. Edmond Halley got Newton to finally publish it, and then everyone realized that gravity is universal. And Halley was one of the biggest proponents of this new theory of universal gravity, and he applied it to all sorts of problems. So so Newton applied it to a a set of problems like, hey.
Look at all this stuff the universal gravity can explain. Halley went nuts with it. Like, if there's a problem, Halley's like, let's try universal gravity. And one of the tricks that Halley would pull often is he would dig back into historical records of astronomical events and use that to figure out and predict future events because you have universal gravity, but you need to know what came before, so you need the current status. So we'd look at, like, say, comet records.
And it's like, oh, wait. There's, like, comets that kind of appear regularly? Now that I have the inputs, I can input that into Newton's universal gravity, and I can predict when these comets will come back, Halley's Comet. Halley knew that coming up, there was going to be an eclipse, a solar eclipse over London. We we were kinda, sorta, you know, had a rough idea of when eclipses would come, but we weren't very good at it, and it required all sorts of cycles.
But we knew there was an eclipse coming. Halley looked at historical records of eclipses. He was able to use that and Newton's universal gravity to predict when the eclipse would occur. Not only did he predict that it would occur on 05/03/1715, he predicted the event of the eclipse to within four minutes. Not four days, not four hours.
He accurately predicted the eclipse to within four minutes. No computers, no calculators, just historical records in Newton's universal gravity. Four minute accuracy. Boom. For the first time ever, we had accurately predicted a solar eclipse.
It only took us thousands of years, and it took this major mental leap of Newtons to do it. Halley made a map showing Londoners when the eclipse would occur, how it would look like, oh, if you're standing here, these are the times of when you'll be in the shadow. Or if you're standing over here, here's the time for where you only get 80%. Like, if you've seen an eclipse map of modern day map of an eclipse and you go back and look at Halley's seventeen fifteen eclipse map, they look mighty similar, and there's a reason for that because Halley nailed it. Yeah.
He gets all the comet fame. I think the solar eclipse is like a bigger deal. The first person in human history to accurately predict a solar eclipse. Haley did something else. He noticed that when he went further back in time to look at eclipse records, that they didn't quite line up with the theory with universal gravity.
And you might be tempted to think, oh, well, then universal gravity is wrong or it gets a little sketchy at, you know, century or millennia timescales. Halley went in a different direction when he noticed that the, quote, unquote, back predictions, backwards predictions of eclipses using universal gravity didn't quite apply, didn't quite match up with actual eclipse records. He thought something might funny might be going on with the moon. One way to explain it is that the moon is getting further away, and so altering, lengthening slowly lengthening the time between eclipses. He just kind of wrote a footnote, and he's like, hey.
This is a little weird. Maybe the moon's going further away, but I got other stuff to do, and he left it alone. About a hundred, hundred fifty years later, someone actually figured out that, yes, the moon is getting further away. But Halley was the first one to call it. In the eighteen hundreds, skipping ahead a little bit, the flavor of what we learn from eclipses begins to change.
Like, 1715 was a big watershed moment where we used a solar eclipse to figure out gravity, really, to solidify our understanding of gravity. When the eighteen hundreds, after that was all settled and everyone understood it, in the eighteen hundreds, astronomers were obsessed with this newfound tool called spectroscopy, which is that when you heat up elements, they glow, and they glow with a very specific fingerprint of light. And so you can look for that fingerprint of light elsewhere and figure out what stuff it's made of. Chemists and astronomers and physicists had figured out spectroscopy before we really knew what atoms were or before we had pinned down the periodic table, before we knew about any of the physics of spectroscopy. We we didn't know what was going on, but we knew it was something cool.
And in August eighteenth of eighteen sixty eight, there was a solar eclipse in India, and there was a French astronomer, Jules Janssen, that observed it. And you he used a fancy new device called a spectrometer, which is a thing that, well, measures the spectrum as the name might suggest. It splits the light like a prism does except way better, makes a big, very detailed rainbow, and then you blast that rainbow onto, like, a grid or a ruler, and you say, well, that's how much that wavelength of light there is. That's how much that wavelength of light there is, etcetera, etcetera, etcetera. And was viewing the corona, which is that ring of fire when the moon covers up the sun, it blocks the surface of the sun itself, reveals the atmosphere of the sun, which we call the corona, and he saw some very, very bright emission lines.
That means the elements, whatever the elements were inside the corona, they were emitting light. And the only way to emit light is if you're hot. We didn't know at the time really the temperature of the sun and especially the temperature of the corona, but just the fact that these lines, these spectral lines, these like bright features in the spectrum were appearing, that was the suggestion that the corona was very, very hot. They were so bright, he figured out a way that they could be seen without any clips. Because they're so bright and so narrow, you can build a special device that that splits the light into its various colors and then just block out all the other colors just so you could observe those very specific wavelengths, those very specific lines, and you don't need they're so bright, they're so intense, and so different that you don't need a solar eclipse to see it.
And so months later, other astronomers like Lockyer were able to look at this without the eclipse because now scientists are looking at it all the time instead of just waiting for the next eclipse. He found one line was something new that had never been seen before. No elements on Earth made this kind of fingerprint of light, and so he proposed a new element. He called it helium, which is the Greek word for, you know, the sun. Helium was the first element discovered off the planet Earth, and it was thanks to observations of an eclipse.
This theme continued into the next year with August seventh of eighteen sixty nine. There was a German astronomer named, Walter Grotean. I apologize for my German accent. It's not nearly as fun for me to do as a French accent. But he was observing a total solar eclipse.
He was observing a corona, and he noticed some iron lines, some some emission from iron. Like, that's interesting. The corona has iron in it. That's cool, I guess. But the only way for iron to make this particular set of lines is if a bunch of electrons around the iron are ripped off and that changes the fingerprint of iron.
In fact, like 13 of the electrons had to be ripped off of the iron nucleus out of the iron atom in order to make this particular fingerprint, which means the corona had to be stupid hot. Like had figured out that the corona is hot or at least warm. Now, Valtor is figuring out that the corona is ridiculously hot, which is weird. Why is the atmosphere of the sun so much hotter than the surface of the sun? Shouldn't it be as you get further away from the surface that things cool down?
Apparently not. Why is the corona so hot? We don't know. I mean, we have some ideas here and there probably involving magnetic fields, probably involving contributions to Patreon. Go to patreon.com/pmstutter, and you can keep the corona of the sun hot.
That's right. The more you contribute, the hotter the corona gets. And, man, if you don't contribute, then the sun will shut down and we'll all die. So no pressure at all. It's purely voluntary, but I'm just, you know, outlining the consequences.
That's patreon.com/pmsutter, so you can keep this show going, and you can literally keep the lights on. Alright. The next date is 1919, and our odyssey began in 1715, '2 hundred years before this, with a revelation of gravity. Then we went through the eighteen hundreds with a couple events, with a revelation of what the universe is made of. And now, 1919, May '20 ninth '19 '19, we have another revelation of gravity.
And this is the Eddington expedition. This is a wonderful experiment performed by sir Arthur Eddington. That's right. It's sir to you and it's sir to me and it's sir to everybody else. Sir Arthur Eddington, absolutely brilliant astronomer, astrophysicist, all around smart dude, had heard about this newfangled thing called general relativity by this weirdo guy named Albert, but he took a shine to it.
He was like, oh, that's pretty interesting. And Albert himself had predicted back in 1911 that massive objects will curve space around them and will deflect the path of light. Massive objects do this because light always goes in straight directions. It just, poof, just go. Like, you shoot a laser.
It's not curving left or right, it's just going straight, and it's always gonna go straight. Like, that's all light knows how to do is to go straight. But if the space time underneath, quote unquote, because this is a four dimensional thing and it's kind of hard to quantify underneath, but it's the best word we got for the situation, If space time underneath that beam of light is bent, then the beam of light will continue following a straight path, but it has no choice but to curve because straight lines mean different things in curved space. Newton's universal gravity had already predicted that massive objects would bend the path of light, but this was an old, old theory, an old work. This was way before we even conceived of light as a wave where light was just a bunch of particles, a bunch of bullets zipping around, and you can work out the gravitational influence on those bullets and you'll get a curvature.
So that wasn't a big deal. Like, massive objects bending the path of light isn't a big deal. Einstein's prediction, however, through general relativity predicted a deflection twice as big as you might expect with Newtonian gravity, so this is a very clear cut test of general relativity. If you can observe the bending of the path of light, then you can measure how much it gets bent. If it's bent a little bit, then Newton's right and Einstein's wrong, and if it bends a lot, then Newton's wrong and Einstein's right.
Sir Arthur Eddington wanted to test this because it's a clear test like this is a big idea of Einstein, he had proposed a way to study it, a way to test it like any good scientist should. He didn't just sit there cooking up ideas, he'll say, here's Sarah, you can prove me right. Sir Arthur Eddington was gonna do it, but the problem is in order for the light to get bent around, say, something like the sun, the light has to come really, really close to the surface. Once you get light far away from the sun, then this effect goes away. The gravity from the sun is just too weak to really bend light at great distance.
So you have to look at light that just grazes, just skims the edge of the sun. Problem, sun is kind of bright. Hard to observe stars right on the edge of the sun. Solution, total solar eclipse. And what does a total solar eclipse do?
It blocks the face of the sun. The moon covers up the sun. Boom. Now, you can see stars right at the edge. Their their light is just grazing the edge of the Sun without any of that, you know, pesky Sun business getting in your way.
So, what I think to Den, this was an eclipse in South America, in some islands off the coast of South America. So he had a setup, he had some buddies with a setup, prepped up, giant telescopes, whole deal, real classic, like, early twentieth century expedition, and they did it. What they did is during the eclipse, they very, very carefully measured the positions of the stars near the sun, and then, like, that night after the eclipse was all done and everything was back to normal and that Earth wasn't gonna get destroyed, they looked at the exact same stars and measured their positions. And so during the eclipse with the light from that star passing very very close to the edge of the sun, the path will get deflected and so the star will appear to look like it's coming from a different direction. It'll appear to be in a different part of the sky, almost like a mirage effect.
You can compare and contrast, lo and behold, Einstein was right. So it's funny, almost two hundred years after Halley demonstrated the correctness of Newtonian gravity, Sir Arthur Eddington demonstrated the correctness of its successor, and they were both using eclipses. Fast forward a hundred years, we come to the great American eclipse of 08/17/2017, almost a hundred years after the the Sir Arthur Eddington expedition, we didn't do a lot of science. Nowadays, twenty first century, we don't do a lot of science. With solar eclipses.
Yes. There were science experiments. There was some legit research done studying the corona. One of the biggest experiments was actually more about how our Earth's atmosphere changes in response to different solar heating. So over the course of a couple hours, you get a lot of normal sunlight and then it dims very quickly and then it brightens very quickly.
You can see how the atmosphere responds to that and so that helps inform our understanding of the relation between our atmosphere and the sun, which is very, very cool, but also not very much astrophysics. And yet we're incredibly interested in the sun and especially the corona. The reason we don't need eclipses so much anymore is that we have technology. We made corona graphs, which are graphs of the corona. It's a little disc.
Okay? A corona graph is a little disc. It's just a fancy name for a little disc that you put in front of your telescope to block the light of the sun to make a pretend eclipse so you can see the corona. So if you're interested in the corona, you won't have to wait for an eclipse now because you can put a little just a little disc in front of your telescope. But you can call it a coronagraph so it sounds way more sciency and authentic, but it really is just a little disc.
And you can use that to block the light of the sun, pick out the corona, look at the corona, do all sorts of cool science. So nowadays, solar eclipses aren't used for that much science because we can make a solar eclipse in our telescope any time we feel like it. Too bad, he and ho, that you didn't have a way to just make an eclipse happen. Maybe you'd still have your heads. Thanks to Michael m on Facebook, Craig w on Facebook, Robert m on email for asking the questions that led to today's episodes, and thanks again to my top Patreon contributors.
We've got are you ready for this? Matthew k, Helgeb, Justin z, Matt w, Justin g, Kevin o, Duncan m, Corey d, Kirk b, Barbara k, Neuter to Chris c, Robert m, Nate h, Interaff, Chris l, John, Elizabeth w, Georgia, and Cameron l, and of course all the other supporters on patreon.com/pmsutter where you can join their lofty, lofty ranks. There will be no more total solar eclipses until my next goal in Patreon is achieved. And, yes, I am in control of that, just in case you were wondering. And if you can't do Patreon, that's cool.
Please go to iTunes, leave a review. You can also go to the website askaspaceman.com. You can shoot me some questions at askaspaceman@gmail.com, or you can just hit me up on social media. I'm at paul mattzutter on all channels. Thanks again for listening, everyone.
Have such a fun time sharing all this cool science with you, and see you next time for more complete knowledge of time and space.