In Our Time - Eclipses

Episode Date: December 31, 2020

To celebrate Melvyn Bragg’s 27 years presenting In Our Time, five well-known fans of the programme have chosen their favourite episodes. Guy Garvey, lyricist and lead singer of the band Elbow, has selected the episode on eclipses, first broadcast in December 2020. Solar eclipses are some of life’s most extraordinary moments, when day becomes night and the stars come out before day returns either all too soon or not soon enough, depending on what you understand to be happening. In ancient China, for example, there was a story that a dragon was eating the sun and it had to be scared away by banging pots and pans if the sun were to return. Total lunar eclipses are more frequent and last longer, with a blood moon coloured red like a sunrise or sunset. Both events have created the chance for scientists to learn something remarkable, from the speed of light, to the width of the Atlantic, to the roundness of the Earth, to discovering helium and proving Einstein’s Theory of General Relativity.With Carolin Crawford Public Astronomer based at the Institute of Astronomy, University of Cambridge and a fellow of Emmanuel CollegeFrank Close Emeritus Professor of Physics at the University of OxfordAndLucie Green Professor of Physics and a Royal Society University Research Fellow at Mullard Space Science Laboratory at University College LondonProducers: Simon Tillotson and Julia JohnsonSpanning history, religion, culture, science and philosophy, In Our Time from BBC Radio 4 is essential listening for the intellectually curious. In each episode, host Melvyn Bragg and expert guests explore the people, ideas, events and discoveries that have shaped our world. In Our Time is a BBC Studios production

BBC Sounds, music, radio, podcasts. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programs if you follow us on Twitter at BBC In Our Time. I hope you enjoy the programs. Hello, the experience of a total solar eclipse is one of life's most extraordinary, fleeting and intense moments, when day becomes night. The stars come out before day returns.

A total lunar eclipse is a more frequent. pleasure and lasts longer with a blood moon the colour of a shepherd's delight. And both events have created the chance for scientists to learn something remarkable, from the speed of light to the roundness of the earth, to proving Einstein's theory of general relativity, and much more besides. With me to discuss eclipses are Lucy Green, Professor of Physics and a Royal Society University Research Fellow at Mullard's Space Research Laboratory at University College London, Frank Close, Emeritus Professor of Physics at the University of Oxford,

and Carolyn Crawford, public astronomer based at the Institute of Astronomy, University of Cambridge and a fellow of Emmanuel College. Carolyn, can we define what we're talking about? What's a solar eclipse? A solar eclipse happens when the moon's orbit takes it directly on a path between the sun and the earth. So you need the sun, earth, moon completely lined up, and its shadow falls on the surface of an earth. So if you're a lucky observer standing within that shadow, the moon appears to move directly in front of the sun.

And if there's an exact alignment of the sun, earth and moon, the whole of the sun's disk becomes blocked out. And that's when you get the total solar eclipse. Otherwise, you might just see part of the solar disk blocked out. And that's known as the partial eclipse. What is it taking of these three objects to get in line? I mean, how do they get there? It happens rarely, I understand, but how rarely? And what's it got to do with speed and space?

Well, you would think that it should happen every month because a total serial eclipse can only happen in a new moon and we have a new moon once a month. But the inclination of the orbit of the moon around the earth and the earth around the sun are slightly tilted with respect to each other, only about five degrees. But that means that for most times there's a new moon, it passes either above or below the position of the sun and the sky.

There are only two times a year when you get this exact required alignment to reduce the total eclipse. So you need that alignment to happen in one of those two eclipse seasons at the same time that you get a new moon. So you've got two cycles that need to line up to get the total solar eclipse. In practice, you know, it's not that rare. You get a total solar eclipse about once every one and a half. years happening somewhere on Earth. The thing is that not many people get to see it each time for a couple of reasons. One is that the moon's shadows comparatively narrow. It covers less than 1% of the Earth's surface. Now, if you're standing in the path of this shadow as it moves across the earth, we call that

the path of totality, because only within that shadow do you get to see the total solar eclipse. people in a broad swaying the side of this path of totality only get to see a partial eclipse but most of the observers everywhere on earth miss the spectacle completely so not only do you get that very narrow path but the shadow of the main travels incredibly fast you know over a thousand miles an hour eastward over the surface of the earth so it's a very fleeting experience and that combines to make it such a special spectacle what about the lunar eclipse carlin well lunar eclipse happen at full moon because the positions of the Earth and the Moon have to be reversed

because you've got the Moon moving into the Earth's shadow. And they're more common. They happen about once or twice a year. And that's mainly because the Earth's shadow is so much larger than the size of the Moon. So you can have a less precise alignment of the Earth, Moon and Sun to still get an eclipse. So the totality in a lunar eclipse can last for up to an hour of 40 minutes because the Moon is relatively slowly through the earth's shadow. And the other thing is that everybody on the nighttime side of the earth

gets to see the eclipse in progress. So, well, everybody knows what they are now. So we can proceed, Caroline. Frank, Frank, close. Earth hasn't always experienced eclipses, I'm told, as I've read, as it does now, and won't always in the future. Can you give us a perspective on that, the past, the present and the future? Well, I think this is the total solar eclipse,

The fact that the moon can perfectly blot out the sun, no more, no less, is a wonderful coincidence. The fact that the diameter of the moon is 400 times smaller than the diameter of the sun, and also by chance it's roughly 400 times nearer to us than the sun is. So by that coincidence of perspective, it is possible for the moon to line up and precisely block out the sun. Can I interrupt for a second, Frank? Can you just develop the word coincidence? I mean, it sounds a wonderful coincidence. I mean that we're very lucky because it happens that the moon is in just the right distance from us now,

but we know that the moon is gradually moving away from us at about the same rate that fingernails grow, which means that in the distant future it will be further away than it is now, and so will appear smaller in the sky than it does now, so it won't totally blot out the sun. And in the past, it was nearer to the earth and would have been much larger. So we happen to live at a very fortunate time in history that this coincidence happens and we can experience total eclipses once in a while. But that's going to change as it shrinks. It will, in the future, it will get further away.

There will come a time somewhere in the future that there will be no more total solar eclipses. That's probably a million years or more away. Whether the national debt is sold before that, I don't know. Frank, can you tell us what is like to experience a totally eclipse and then perhaps Carolyn and Lucy can give their experience too? Well, I suppose most people will have experienced a partial eclipse and the question that immediately comes is, well, what's special? I mean, what's the big deal difference between a 99.9% partial eclipse

and a total eclipse? And one analogy that somebody gave was that it's just like the difference. between being outside Covent Garden when the opera is taking place and being in a box seat at the front. They're totally different experiences. Psychologically, I think, we see partial eclipses of the sun.

What we perceive is not the moon passing in front of the sun. We know that's what's happening because we've been told it. What you actually see is the sun getting thinner and thinner, forming smaller and smaller crescents. You get the impression that the sun is literally disappearing. Eventually, there's just a thinnest sliver of the sun left,

99.9% partial, and you still can't see the moon. And then you can suddenly see the stars and the night sky, and in the blinker eye, something happens. There's a flash of light, which is the last rays of the sun passing through the valleys on the moon's surface, and the shimmering lights of the sun's career. The corona begins to appear around the outside of the moon, giving what's called a diamond ring effect.

But this comes out from nowhere, and then you suddenly realise that there's this black silhouette of the moon hovering there in the middle of this apparition. There's this eerie, shimmering light around the outside. And this halo, the corona, the stars, maybe you can see Venus in the night sky. You look down towards the horizon and the dark sky, the shadow of the moon turns a deep maroon and then around the horizon you're

able to look beyond the moon shadow and see what looks like a 360 degree sunset and you're left with this feeling that where the sun was there's been a black hole has been born out of nowhere somebody said oh it looks like the eye of God or it's like a black sunflower

with gossamer leaves all around the outside it's utterly weird I'll just conclude to say that this perception that the sun has been disappearing for over an hour and then suddenly this black hole has been born out of nowhere. You understand how it was the ancient Chinese, were convinced that a dragon was eating the sun, and you today even have this feeling that everything that we take for granted that gives us light and heat and warmth and the things of life,

the sun has disappeared and energy has been sucked off, deep into space and it is totally awe-inspiring. I'm lucky enough to have seen three total solar eclipses so far because I term also a self-confessed eclipse junkie if it all possible. But just to stress what Frank is saying that it's more than just the spectacle of the light disappearing, the sun disappearing, even though that is fantastic. And you can see that in the photographs, but it really is this full body experience. if you're looking out over the sea

What do you mean by that? Well, I'm going to tell you. Good. If you know, if you're hypermountain or you're looking over the sea, you have the sense of the moon shadow rushing towards you at phenomenal speed. The temperature drops.

A winds will pick up. You get the eerie sounds. It goes quiet and the birds and the animals are confused. They'll make sort of roosting noises. And altogether, it is the most eerie feeling imaginable. The hairs on the back of your nose. net go up. Your whole body knows that something very, very strange is happening. And it's a

completely awe-inspiring experience. Somebody once said it makes poets of scientists and scientists of poets. Lucy, what was your experience? Oh, I'm lucky enough to have seen two as well. And it's been great listening to Carolyn and Frank talking because it kind of, it just brings that experience back. And so I was lucky enough to see my first eclipse in 2009. And this was the longest eclipse of the century. But keep in mind, I'd been studying the sun's corona for a decade by that point in time, yet never having had a photon from the sun's, or at least never having had a clear view of the corona directly by my own eyes. So the ability to look at the corona that I've been studying for a decade was extremely moving for me. And I remember the purple hue of the

sky that was taken on during the total eclipse. But perhaps sort of another thing to add in is I saw one last year, 2019 from Argentina, and that eclipse was very close to the horizon. So my previous one was high up in the sky and it looked to me like a gun had shot a bullet through the sky and it was this small black dot where the moon was now and then the corona around it. But near the horizon, the sort of what's called the moon illusion took in. So when you see the moon near the horizon, your brain thinks it's closer to you and you sort of perceive it to be bigger. and the whole eclipse experience, the eclipsed sun to me looked bigger.

Absolutely fantastic. And also we saw shadow bands, sort of shimmering effects on the ground because the winds that Carolyn talked about, they cause turbulence in the atmosphere of the earth and you get a fraction of light. Everything is new. Everything is different and it's very sort of discombobulating as well as being extremely moving. We're requested to tell the listeners that, and we call it, can't emphasize enough that it's not safe to look directly at the sun. And so what do you do

when you're studying a solar eclipse, Lucy? Absolutely right. Never look at the sun outside of the total phase of the solar eclipse. So you will need eclipse glasses or projection effect, a projection device. And my favorite one is to use a colander that you're just using your kitchen. And then you can make a pinhole camera many times over and you can see the eclipse sun on the ground. when we're looking at the different eclipse phases so we have different phases that get described so first contact is when partial the partial phase begins and the moon just starts to become visible

on the sun's disc, that first bite taken out of the sun. Then we have second contact where the entire disk of the sun is covered and you might see Bayle's beads which are the rays of sunlight coming through the mountains on the moon and then you have the diamond ring effect that Frank talked about just before totality where you see literally it looks like a diamond ring in the sky. So a thin circle around the eclipsed, around the moon,

and then one bright spot, the last rays of sunlight before the sun is completely gone. And then you're in totality. And then the moon starts moving away all too soon. So just as you're stood there, you've taken off your eclipse glasses, you look at the corona, look at those photons coming straight to your eyes. Then it's over. The moon starts moving away again. You see the diamond ring again, the bailies beads again.

You also, during the total phase, I should say, might see some prominences and some structures in the lower atmosphere of the sun as well as the corona itself. But third contact is when the moon starts moving away. Sunlight comes back again. Everybody, you know, tries to gather their emotions again. And fourth contact is then when the moon has completely slid away from the sun's disc and everything is back to normal daytime again.

During the total phase, that's when we will get out. our scientific kit. So there'll be a lot of people during solar eclipses on the science research side who are trying to sort of get that sweet spot of getting the data that they want during a total eclipse, but also experiencing it for themselves as well. And perhaps I should say that we create artificial solar eclipses all the time with instruments called coronagraphs that we have on the ground or we fly in space because the scientific value of studying the sun during a total solar eclipse is so great. Thank you very much. Thank you. Caroline, what was the early understanding of eclipses in the ancient world?

We start with China, do we? Is that a good place to start? That's certainly where the earliest records we have of total solar eclipses. That's where they come from from the Chinese astrologers. I mean, it's really not surprising. You've already got a sense of how dramatic inexperience a total solar eclipse is. So it's not surprising they've been recorded for millennia. And the earliest record state from about 2000 BC, where you have the Chinese astronomers,

well, in fact, astrologers, as they were at the time, recording these events because this was such a dramatic event. It was seen as perhaps a message from the gods or a bad omen where the sun, or if you like, the son's representative here on earth, the emperor, was under attack. You know, eat, there are tales, from different cultures of it being eaten by a dragon, as Frank suggested, or a wolf or a bear, or indeed being stolen away. And there are tales of how it was the people's duty to come out

and bang on their pots and their pangs and make such a loud noise that they would scare the beast away and leave the sun alone. And it always worked. Yeah, it's amazing. It always worked. So you have early records from the Chinese and also from ancient Babylonians recording on their clay tablets, about 1,300 BCE, the solar eclipses, the lunar eclipses. And then we go to Greece. And then we go to Greece. And what's interesting with the Babylonians, they by then were beginning to pick up that there was a certain repeatability, irregularity in the pattern of eclipses.

By the time you get to ancient Greece, so say about 500 BCE, the interpretation has switched from being sort of purely supernatural, but trying to use the eclipse to have an understanding about the geometry of the solar system. So you have Aristotle trying, you know, making the point that the earth must be spherical because it always casts a circular shadow on the moon during a lunar eclipse. And you have Aristarchus trying to estimate the size of the moon by timing how long it takes to pass through the earth's shadow. And from that and the fact that it's got the same size in the sky as the sun, estimating the distance of the moon relative to the distance to the sun.

So you start to have interpretation in terms of science and how the solar system works in action. Frank, what does our knowledge of eclipses, how does it throw light on events in history? People in the past were so awestruck by eclipses that they recorded them. And of course, we now understand what's going on and we can back calculate and work out precisely when and where eclipses, in particular solar total eclipses, took place,

and compare the dates that come out with the historical record. For example, in Genesis, there's a reference to Abraham being in Canaan and a great darkness descending, and it's possible to calculate that as having taken place in 1533 BC. I suppose that the most famous biblical one, at least, is the whole question of the crucifixion and the darkness at noon. and many artists have painted the crucifixion showing an eclipsed sun. But as Carolyn said at the very start, that cannot have happened.

The reason is that the crucifixion took place at Passover, and Passover is the time of the full moon. And at full moon, you can have a lunar eclipse, but not a solar eclipse. However, there are some records that claim that the night of the crucifixion, the moon rose blood red. And as we already heard, that is one of the signals of a total lunar eclipse. And you can have a lunar eclipse at Passover. And indeed, you do the calculations back.

And there was a total lunar eclipse on Friday the 3rd of April 33 AD. And it was just after sunset in what is now modern Israel. That sends shivers down your spine, doesn't it? It does. I've heard that before, yes. Amazing. Lucy, Lucy Green, let's move on now, the Renaissance and beyond into the Enlightenment.

How did Kepler and then Halley put the eclipse at the forefront of science? Yeah, so once we're coming into the 1600s, we have the development of science, of knowledge, but also of technology as well. So we have the development of the telescope for astronomical studies. And as you say, Johann Kepler, who was born in what's now southwest Germany, he was very much inspired by the Copernican view of the universe. And that placed the sun at the centre and it has the Earth orbiting around it. And in the early 1600s, Kepler came up with three laws of planetary motion then that described what he had found.

One was that planets move in ellipses around the sun. The second is that the planets move more quickly in their orbits when they're at the closest point to the, the Sun. And third, is that how long a planet takes to orbit depends on its distance. So the further away the planet is, the longer it takes. Then we also need to bring in Isaac Newton. And gravity explained the force responsible for the planetary motions that had been described by Kepler. Now you've got a physics theory that you can combine with your observations to make accurate predictions of total solar eclipses. And Edmund Halley was one of the first. We also I have John Flamsted, who was Astronomer Royal, the latter half of the 1600s making accurate

total solar eclipse predictions. But Edmund Halley made an accurate prediction of a total solar eclipse in 1715. It was one of the early examples of a citizen science project. So Edmund Halley knew that the total solar eclipse of 1715 would pass over England and Wales. And in fact, it was the first total solar eclipse seen in England for 500 years and a great opportunity to gather data. So Halley requested that the curious of the country who were along the path of totality observe what they could. And in order to facilitate this, he published an eclipse map and allowed people to go out, make a note of what they saw, make a note of the timing and the duration. And from that he was able to work out the speed at which the eclipse shadow raced across the earth. Amazing. Carolyn,

Given that he was trying to encourage not only the public but fellow scientists to make observations of the eclipse, there's this, he's entertainingly disparaging in his record about the efforts by the Professor of Astronomy at Cambridge and the quotas, you know, where he had the misfortune to be oppressed by too much company so that the heavens were very favourable, yet he missed both the time of the beginning of the eclipse and that of total darkness. I love that. It's a wonderful record that he made of the account. I mean, it's there for people to read on the internet to get the paper from the Royal Society. And then maybe sort of just to come back to your point, Melvin,

that Hallie really wanted the public to be informed about the eclipse as a scientific and scientifically understood a natural event rather than an event to be taken by surprise and to be superstitious about it's not a bad omen. This is just a natural consequence of the ordering and the clockwork motion in the solar system. And Colin, how did solar eclipse lead to the, discover of helium? Well, the first evidence for the existence of helium was discovered during a total solar eclipse. And we're talking now about the mid-19th century where astronomers are in using the new technique of spectroscopy, which is where you gather the light through the telescope.

He also dispersed the light into its constituent colours. And at the time, astronomers were away, you could determine the chemical composition of distant stars by matching features in this dispersed light, which we call a spectrum, to features that are given off within a lab experience. And astronomers were also interested, of course, in what gases made up the sun. And Lucy earlier talked about the pink streamers, the sort of effects that you see during the eclipse, this pink ring round the sun, which is the lower part of its outer atmosphere. And the idea was it was possible within a total solar eclipse to isolate the light from that cremosphere and then pass it through a spectroscope. And this was first done successfully by

the French astronomer Jules Johnson in August 1868 and he went to a total solar eclipse in Madras in India and collected the light during the eclipse. And most of it you could see was due to extremely hot hydrogen. But there was also one feature, what we call an emission line, a bright patch of emission, which did not match the emission seen by any known element in the lab. And he made the announcement on the same time, same day, that Norman Lockyer also had independently done the same observation of the light from the sun. And they shared the discovery, which was very appropriately named after the sun god Helius and became helium. And it turns out that helium is the second most abundant element in the universe, and yet it is amazing that we only see it through a total

solar eclipse initially. It wasn't discovered on Earth, I mean it's comparatively rare enough, till another 14 years after this event, and that's through collecting the spectrum, the light of glowing lava and Mount Vesuvius, and wasn't observed in the lab until 1895. Frank, how did an eclipse support Einstein's theory of general relativity? Well, Einstein's theory of general relativity had a prediction that gravity would bend the rays of light and the biggest sort of gravity in the solar system is the sun. And the idea being that if there were some stars that were nearly in line of sights with the sun, the sun's gravity would bend the rays from them and their positions would be

slightly shifted from where they would normally be seen. The problem of course is you can't go and do that because daylight blots all the stars out. So the idea was to wait for a total solar eclipse, which blots out the sun's light, but of course the sun is still there with its gravity still acting. And that was the idea was to go and measure the positions of some stars that were nearly, I think it was the Hydees cluster that was nearly in the line of sight of the sun during a total eclipse in 1919 and compare the images with where they were relatively six months beforehand, and when you would see them in the normal night sky,

with the sun all out of the way. And the story of this is well known, that there was a shift scene, and everybody said this is remarkable. New York Times had this amazing headline, lights all askew in the heavens, and Einstein was overnight a celebrity. What people at large tend not to know

is that Newton's theory of gravity also predicts that gravity will bend light rays, and it's all a factor of two. Einstein's theory predicts that they would be twice as big. And in fact, big here means about one, two thousandth of a degree. So it's a very sensitive measurement that was being taken in 1919. But the results were sufficient to confirm his theory. Lucy, can we talk briefly about eclipses away from the earth around Jupiter, for example, or Mars?

Can I just throw those into the pot? Yeah, it's a lovely thing to think about what you would see from other points in the solar system. And, you know, we live on a planet that has an unusually large moon for its size. So we have these beautiful and perfect, coincidentally, or alignment of the moon and the sun in terms of its size. Mercury and Venus have no moons, so there'll be no eclipses to be seen there. But Mars has two small moons. And in fact, rovers on the surface of Mars, NASA rovers, have observed these two small moons that only have a diameter of about, I think it's 14 miles and 8 miles across. Very small moons.

but the cameras on the rovers, the Mars rovers, have seen these moons move in front of the sun. So not a total solar eclipse, but still an eclipse. And fantastic to see these rocky sort of uneven moons moving in front of the sun from that perspective. But if you want to see perhaps a better eclipse, your best bet might be to go to Jupiter. So it's about five times further out from the sun than the Earth is. And stand on one of the moons of Jupiter, Ganymede, or Europa, for example, and then watch one of the other moons of Jupiter eclipse the sun. The sun is only small in the sky as seen from Jupiter, so just a fifth of the size that we see it,

but you would still get an almost perfect total solar eclipse seen from the moons of Jupiter. So maybe one day with some telescope on the spacecraft sometime. Caroline, still trawling away from the Earth and the Sun and the Moon at the moment. Can you tell us about eclipse and binary stars? Well, yes, because the sun is fairly unusual in being on its own. There are many stars across our galaxy that exist in pairs that are bound together by their gravity, and they orbit to a sort of common centre of mass. And the things, they're far too close together, and they're too far away from us,

to sort of separate out the light of the stars. It just appears as a single point of light. But if you watch that and you map how the brightness varies with time, you find that you get periodic dips of brightness and this is when one star blocks the light of the other. And just from mapping out these variations in light, you can start to tell fiscal parameters, so things like the size and the orbital size

and the surface temperature of the star. So, you know, duration and periodicity of the dimming of the eclipses that you observe tells you the relative sizes of the stars and the size of their orbits. How much light is diminished? tells you something about their surface temperatures. But the neatest thing is, it gives you a way to find the mass of stars.

Because if you see an eclipsing binary, you know that you were viewing it edge on. And therefore, the velocity of the stars, one around each other that you measure, is the true velocity. And you measure that through the Doppler shift in the light from the stars. And the great thing is, is the velocity of the stars around one another. It's completely dependent on mass because the stars interact through each other. through gravity. And so it's a wonderful way, in a very accurate way, to estimate the masses of very distant stars. Frank, Frank Close, before eclipses were as well understood as they are now, how did people use their knowledge of them? I mean, how reliable were they for measuring distances

in the Atlantic and the Mediterranean, for instance? When there is a lunar eclipse, it's visible everywhere on the nighttime side of the earth, and it lasts for over an hour. And it starts at the same instant for everybody on the night side of the earth. Back in the time when the colonists were in Virginia, they knew where they were in latitude. That's relatively easy to find out how far north of the equator you are, but how far west of here they were was a more tricky thing. And a lunar eclipse was due to take place. The almanacs predicted that. And the idea was to measure the time that this started in Greenwich. I think it was starting here at one time in our night. I have to get the east of the west flow of time right here.

And it would be visible instantly in Virginia. And the idea was in Greenwich we would record the time of the start according to our clocks. And in Virginia they would do the same according to their clocks. And then eventually by comparing the two times, we could work out how far away we were. If you like in modern language, we discovered that they were five time zones away from us. so it was done much more accurately than that. And so from that, it was possible to measure the width of the Atlantic. The one that perhaps that has greater fate for history was back in 150 AD when Ptolemy used the data on a lunar eclipse

that had previously taken place to work out the length of the Mediterranean, using the same sort of idea as was later used to measure the Atlantic. But the data he had wasn't that great. and the result was that he thought the Mediterranean was about a thousand kilometres longer than it is in reality. But the effect of that was that Columbus thought that he knew he could sail out

through the Straits of Gibraltar and from the measurements he thought that meant that it wasn't that far to get to China heading westwards. And he managed to convince people to fund him and he set off to find China and when he landed in Hispaniola, he thought he'd arrive.

But in fact, the discovery of North America by Columbus was all because Ptolemy had got the wrong length of the Mediterranean from using the lunar eclipse. So that is one bit of serendipity that came out of all of this. Lucy, what is there still to be observed from real solar equips viewed from the Earth? Because as I understand it now, a lot of the viewing is now done from satellites. And so if you're standing on Earth, you feel that you're a bit behind a curve, as it were. But what can still be observed from real several clips, viewed from the earth? Yeah, and it is an important thing to still think about. I mean, you're right, we have permanent views of the eclipsed sun now using spacecraft.

So we put a disc in front of the sun and we see the corona. But the problem with that technique is that we are not able to see all of the corona. So when the moon moves in front of the sun, we can see everything from the photosphere, which is the surface of the sun pretty much up. When we have artificial solar eclipses, our disks have to be bigger than the size of the sun in the sky, and so we lose precious information about that lower corona. And there are so many important physical processes

that we want to study in that part. It's where the solar wind forms, which is a stream out into the solar system. It's where explosions and eruptions occur. It's where energy and mass propagate up through the atmosphere of the sun. So we have a limited view using our spacecraft. That's not to say that that won't change in the future. We're working on interesting new techniques like formation flying spacecraft,

so two spacecraft, 150 metres apart, creating an eclipse of the sun, which is allowing us to see a little, will allow us to see a little bit further down, but we don't see everything. And then there's the other aspect that spacecraft have limited data storage. So we've only got computers of a certain size on board, whereas if we observe eclipses from the ground, we can have massive hard drives and we can collect loads of data and look for activity in the atmosphere of the sun that takes place very quickly.

For example, waves and oscillations in the atmosphere of the sun. And I think there's also an aspect about flexibility. So by continuing to do scientific studies of total solar eclipses from the ground, we can build new instruments in short amounts of time and respond quickly to science questions. Conversely for spacecraft, it takes us maybe 20 or 30 years to get the spacecraft up. So, Caroline, would you concur that observations from satellites haven't made observations from Earth redundant? Yeah, I mean, they're complementary.

I mean, the great thing about the spacecraft, of course, is that they can monitor the emissions from the sun, the solar wind, and all these eruptions that Lucy mentions. You can monitor it continually and also not just not. just from Earth, from further around in Earth's orbit. So you get a side-on view for the sun and how these winds and the corona might be effect the Earth in the future if you're seeing sideways motion towards the Earth. So there's still a lot to be done with satellites.

But as Lucy says, you've got that limitation. If you're interested in weather, real power sources in the lower part of the atmosphere of the sun, you really do need to get to see it within the eclipse. Frank, what are eclipses to you now? Are they something to, are they still as exciting? Are they still as difficult? Are they still as defining? What do they mean to you now?

Well, everyone is unique. And although it's not my day job, I think chasing eclipses has now become an expensive hobby. Every time I see an eclipse, yes, there's always that sense of wonder. It shows the immense beauty. that there is in nature. And also it shows that it's all amenable to science. I think although we've talked about the total eclipses

and the wonderful spectacles, there is a moment when it begins, the first contact, the moment that the moon just begins to cross the sun's face, which always makes me sort of feel funny in the pit of my stomach. I'm looking through the dark spectacles, and I know that I'm told the eclipse is supposed to start at this particular moment and the little Nick appears in the corner of the sun,

is it it? And a few seconds later you can see, yes, the eclipse had started. And science works. It predicted that if you are at this spot on the earth's surface, at this moment in time, the moon will begin to cross the face of the sun. You can predict it with absolute certainty. What you can't predict is whether the weather will allow you to see it.

Do you still have things to learn scientifically from the experience you've just described. Yeah, absolutely. So there's a lot of science that is still open about the sun. And the thing about studying the sun is that we are working towards getting a really detailed understanding of our local star. So it's kind of incremental in a way that we don't necessarily have in other areas of astrophysics, where there are still big discoveries being made because you might look at an object or a certain class of objects in detail for the first time. With the sun, we've been making detailed observations for a long time.

time now. But one of the areas, for example, is around why the corona is so hot. It has a temperature that's about a million Kelvin, million degrees, whereas the surface of the sun is about 6,000. That was that high temperature of the corona actually tracks back, our understanding tracks back, or the discovery tracks back to total solar eclipses, as we've been hearing all through the programme. So many scientific discoveries made during total solar eclipses. So we want to know why the atmosphere is so hot, and during total solar eclipses, we can look at precisely the right part of the atmosphere of the sun and start to or continue to understand why that is the case. And that's just one scientific question. We get to study the magnetic field of the sun, which is the energy reservoir for all the activity.

We get to study the plasma, the material, the density, the temperature, the composition. What's it made of? Why does it differ throughout the atmosphere of the sun? as compared to the surface, lots and lots of science that we still want to do. I'm not a solar physicist and I will confess when I go to a solar eclipse. I'm in awe of people like Lucy who can have enough sense about them to actually carry out scientific experiments. You know, when there's all this absolute joy and awe going on above your heads, you have the corona spread out across the whole sky.

So I'm afraid I shall stick to being a solar eclipse junkie and just actually enjoy the spectacle for it. its own value and let Lucy and others do the science. I was just going to add one thing that the first eclipse that I saw where it was visible was in Zambia, which of course is in the southern hemisphere. And the sun, of course, goes across the sky in the opposite direction. I mean, it goes from east to west, but it goes from right to left rather than left to right. And the moon crosses the sun's face in the opposite direction to what it does up here in the Northern Hemisphere. And I was on the flight going out talking to some solar physicists, and I

commented to them on the way back that that was the thing that I had been totally unprepared for. And they said, yes, we only realised that six months ago and had to redesign the experiment. Do you know, I think there were some colleagues of mine who went out. Somebody who joined me after their PhD said one of their earliest studies was that eclipse in Zambia. So can we come back to you to round this phase off, Frank? You are right that these eclipses do make poets out of scientists, we've just had that more or less demonstrated. Is there a book?

Eclipses we have known? I read a sort of semi-autobiographical, which told how I started as a scientist because of an eclipse a long time ago, but that's another story than today. Why should it be another story? How did that set you all as a scientist? In the 1950s, when I was at junior school, there was a total eclipse across the Shetland Islands, and down in Peterborough, where I lived, it was about 80%,

and we were taken 80% partial. And we were taken out into the school yard by our teacher and Mr. Laxton. And he had seen the last total eclipse in Britain in 1927 when he was a teenager. And he set up a telescope, projected the partial phases of the sun on the ground, and one group of students were tracing that. Another group were making notes of the time. And I was part of a group that measured the temperature. That was the very first science experiment in a way that I ever did.

The next day he had all the... of this data collected in the schoolroom. And being a sportsman, he told us what had happened. He used a football to represent the earth, a cricket ball for the moon, and a flashlight to cast the shadow of the moon onto the earth. And he explained everything that had happened. And then he answered the question,

why isn't an eclipse every month? And somebody asked, when would the next total eclipse be visible in England? And he said, 1999. Now, in the 1950s, when you're 70 years old, that's like infinity. But in 1999, I went to see my first total eclipse in Britain, and it was completely cloud-covered.

At that moment, I thought we've got to find where the next one is and go, and it was in Zambria, and I've been spending money ever since. I didn't realize that there were eclipse watchers gathered together all around the world in different sides at different times. It must be quite an exclusive club. Anyway, thank you all very much. I thought that was terrific. Thanks to Lucy Green, Frank Close, and Carolyn Crawford.

next we'll talk about the Great Gatsby by Scott Fitzgerald and that will be on January the 14th, 2021. Thanks for listening and happy new year. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. I was going to just add a postscript about my school teacher Mr. Laxton who got me started back in the 1950s that he had told us that the next.

The next total eclipse would be visible in England would be 1999. So when 1999 finally arrived, 40 years later, I and my wife and daughter, elder daughter, went down and camped the night before, woke up to it being totally cloudy. The thing I learnt later was that Mr. Laxton lived to be 100 years old and finished his life living in a care home in Torquay. And Torquay, the clouds were cleared. and I wonder to this day, did Mr. Laxton actually succeed in seeing that 1999 total eclipse? Wow. You can be sure he did. Yeah.

I just have one sort of thought that I wanted to leave with the listeners. And that's about the eclipse in 2021 that will be seen on the 4th of December and seen from Antarctica. So one thing we haven't mentioned, I'm not sure, maybe we did. is that the eclipse shadow moving across the earth normally goes from west to east. But for this eclipse from Argentina, which happens in the summer in the southern hemisphere, December, so the tilt of the earth means that the southern hemisphere is pointed towards the sun. And the result of this is that the eclipse shadow for that eclipse will go east to west. So the opposite direction.

So it's a bit of homework for the listeners to think in their minds. get an orange or something, set it up, tilt it, and think about why the shadow moves east to west. But it's a rare event only possible in the polar regions, making the 2021 eclipse from Antarctica even more special. The only other thing I wanted to add about, Jals Janssen, who was the French astronomer who discovered helium during the total solar eclipse, I was reading up about him and it's fascinating because he continued to go and observe solar eclipses. there at least four that he went to in the late part of the 19th century. This is wonderful tale of him actually escaping Paris in a hot air balloon because it was under siege for the Franco-Prussian War

so that he could go and observe the eclipse from Algiers in 1870. I think that's a talk about a life of daring do for a scientist. Somebody mentioned shadow bands and I had not seen these at all. They're very sort of ephemeral. I was in the Sahara Desert and there, through the corner of my eye, I saw what looked like millions of ants sort of running across the white sand. But the person in my family who saw them first was in 2017, we all went out to North America. And I took my grandsons and family along as well. And my grandson who was then

seven years old, the same age as I had been back those years before when I got started on this, he was with us and he just about half a minute before totality he started jumping up and down saying the road was moving the road was moving and there was this amazing sight of these subtly dark and light bands moving down the road giving the impression

that it was rippling and it was absolutely astonishing so my grandson saw them before I did it is a wonderful thing to see I remember holding out my arms to create a shadow and sort of it looked like my fingers were getting really long and really wavy and we stood just mesmerized by that just totally different experience.

And I wasn't even looking out for them. I didn't think that we would see them. This was during the eclipse in Argentina last year, which we went to see with, I don't know how many of my colleagues were there, a bunch of solar physicists. So Frank, what you were saying about, you know, the noise of humans during a solar eclipse, we roared and cheered. Oh, my goodness, it was so exciting.

Yes. I mean, I discovered that you can actually hear the passage of an eclipse, because when we were in Wyoming in 2017, we were on a hill. We all cheered out, and then a second or two later, some people further down the valley cheered out as this arrived. And you made a wise decision going to Argentina last year. I decided to give it a miss because I wanted to go this year in the summertime. And of course, COVID happened.

Yeah, it's a real shame. Yeah, and I wonder for the eclipse next year to Antarctica, you'll need to be on a cruise ship at that point to see that eclipse. So how likely it will be that many people will see it, I don't know. And the really sad thing is that we're not actually going to have a total solar eclipse visible from the UK now to 2090. And so that's, so we're going to have, you know, people are going to have to travel to go and witness this experience for themselves. It sounds to me like a biologist like you are and see the world, isn't it? I don't know which content we haven't been on in this last half hour or so.

Two of you are lucky because you can at least charge it to your scientific research. Because my research is in particle physics, I have to do it out of my own personal budget. But at least you get about 18 months to save up between one and the next. But the decision that we took way back in Cornwall was, where's the next one? This must be a great way of visiting places on the earth that you would never otherwise go to with the eclipse as the icing on the cake. And that's indeed how it's turned out. And there is also that element of danger that actually you will travel through to Siberia or somewhere really remote or Antarctica that Lucy's talking about. And it'll be a thick cloud and you miss the eclipse completely.

So it's never guaranteed what your experience will be. Does that make you more exciting for you? Well, it makes it worthwhile ensuring that you are going to visit something even if you don't see the eclipse. So use the eclipse as a reason for going somewhere, which is interesting in its own right. And even if it's cloudy, I mean, from my first experience, if it's totally cloudy, you still get an awesome impression.

of the moon shadow. It's a different experience. It's not a loss totally. I don't think it's possible to have a complete loss of totality. Each one is unique and different. The angle in the sky, and I can't remember whether it was Lucille

Carlin mentioned the one that they saw low down in the sky and you got the effect of the enlarged moon. I saw one in the South Pacific that was low down in the sky. And the light scattering off in the distance from the beyond the moon shadow off the ocean gave a sort of greenish tinge to the outside of the moon.

And in the Sahara, the Sahara was so bright. I mean, there's million square miles of white sand reflecting, I mean, from beyond the moon shadow where I was underneath, there's another million square miles reflecting sunlight back up onto the rear side of the moon. And we saw the mares of the moon against the silhouette. It was totally unexpected. So has every eclipse experience been different for you, Frank? Absolutely, yes.

I mean, you have some qualitative expectation, as you know beforehand, but when it happens, there's one thing that is always true. I think one of you mentioned this in passing. Time seems to change that you know the eclipse is supposed to last for three minutes and you think it lasts about three seconds. Well, thank you all very much. Thank you. I'm sure listeners are going to really love it. In our time with Melvin Bragg is produced by Simon Tillotson.

Before you go, I'm Miles, the producer of a brand new podcast for Radio 4 called Tricky. This is how it works. Four people from across the UK meet up and without a presenter breathing down their necks talk about issues they really care about. Sex work is quite complicated for a lot of people and it's okay to be against it but not to shame someone because of their professional. Across the series we'll hear anger, shock and even the odd laugh. Another thing that really gets to me is when people say,

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