Eclipse week 5 – 1919

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It was nearly 10 years ago that I had the chance to witness my first – and only – total solar eclipse. It was every bit what is cracked up to be, and the feeling that you’re witnessing something special comes effortlessly. You don’t have to be an Astronomer to enjoy the uniqueness and transiency of such a brief moment, and millions of people in Asia had the same pleasure this week that I most certainly hope to have again in the near future.

But you probably do have to be an Astronomer to turn an eclipse into an event which will change the face of Physics. So today we’re not here to talk about the 1999 or the 2009 eclipses, but rather the total eclipse of the 29th of May, 1919. This puts us four years after Albert Einstein published his theory of General Relativity, in 1915.

General Relativity, at its most basic, is a theory of gravitation. It provides a theoretical framework which we can use to explain the behaviour of gravitationally interacting systems that we observe – like the solar system, or the Earth-Moon system, the Milky Way, etc. Most importantly, like any other scientific theory, it also allows us to make predictions about how gravity should affect these, and other, systems.

Prior to Einstein and General Relativity, our understanding of gravity was motivated by Newton’s theory of gravitation, often also referred to as the classical theory. In what we may call “everyday situations”, both theories make the same predictions, but General Relativity makes significantly different predictions, or presents very different explanations, for things like the geometry of space, the passage of time and how light propagates. Crucially, General Relativity predicts that the mass of an object affects space(time – the merging of space and time into a single mathematical object is also a consequence of General Relativity), and that the shape of spacetime affects the way light travels. These are stunningly anti-intuitive ideas, because they are only noticeable in regimes far detached from our everyday experiences. For example, we need a very large amount of mass to notice a very small deflection in light’s path.

But Einstein was not the first to suggest that mass affects the path of light. In 1801, Johann Georg von Soldner used Newton’s corpuscular theory of light – which states that beams of light are streams of particles of tiny mass – to calculate how mass would affect the path of a beam of light. He arrived to a result which is known as the Newtonian result for the bending of light. However, because we know now that light is in fact made of massless photons, there is no formal way to correctly treat the bending of light in Newtonian gravity. This was not known at the time, so the result stuck. Unaware of Soldner’s calculations, Einstein in 1911 calculated what the bending of light should be in his new theory of General Relativity, which at the time was work in progress. He reached the same value that Soldner had done, over 100 years previously. Crucially, once his theory was finalised in 1919, Einstein revisited this problem and realised he had made an error – the bending of light, once one takes into account the curving of space, should be twice that of the Newtonian result. Such a clear distinction between the predictions of two competing theories is a blessing – it gives scientists the chance to design an experiment which can show which one, if either, is correct.

We couldn’t look in the Earth for a suitable system to measure this effect in, but in 1919 Astronomer Royal Sir Frank Dyson and Plumian Professor of Astronomy in Cambridge Arthur Eddington decided to look elsewhere – they turned the Sun, the Moon, and a distant cluster of stars called the Hyades into their own laboratory. The idea is that the mass of the Sun is large enough to bend the light which passes nearby, as that of distant stars which are sitting behind or very near the Sun from the Earth’s perspective. This is of course happening all the time, but we simply can’t see the light of distance stars near the Sun because the Sun is so much brighter than the stars we are trying to observe. Unless something really big gets in the way and blocks the light from the Sun – say the Moon (which is actually much smaller than the Sun, but sits just at the right distance – see Emma’s post). A total eclipse makes therefore the perfect opportunity to see how the position of stars in the sky changes when their light has to travel close to the Sun on their way to us.

1919_eclipse_negative The eclipse of 1919 was a good and timely opportunity to measure this effect, and expeditions in the island of Principe (off the west coast of Africa) and Sobral in Brazil were planned to do precisely so. The Moon would block the light from the Sun for almost 7 minutes – an exceptionally long period of totality! What no Astronomer can ever do, however, is plan to the weather. And so, for over 6 minutes, Eddington waited and stared at a cloud.. and prayed. The cloud did disappear, giving Eddington and his team 10 seconds – 10 precious seconds! – to take the needed photo. You can see the negative of this photo on the left, and if you look carefully you can see the horizontal lines which mark the positions of the stars. The expedition in Brazil got perfect weather, but later a flaw in the telescope setup meant that the results could not be used. So Eddington and Dyson went home to analyse the data, while the community – and Einstein – waited.

Within the margin of error, the shifts observed in the positions of the stars were more in agreement with Einstein’s predictions that Newton’s, and Einstein was thrown into stardom once the results were announced. It is worth mentioning that there was healthy controversy at the time, and the data analysis of Eddington was challenged – as it should have been – by the scientific community. Such a leap in scientific thinking never gets an easy ride! But results in following expeditions confirmed the 1919 results, as did other experiments which measured slight departures from Newtonian predictions in different systems (the orbit of Mercury being one example). General Relativity has been proven time and time again to be accurate within our measurement errors, and it’s a deep and beautiful theory. Given another chance I will tell you why and how some scientists feel the need to adapt General Relativity to explain some recent observations of the distant Universe, and how controversial and interesting a topic that is. But not now..

Personally, I find it rather interesting that this event really catapulted Einstein into the public eye. Einstein_theory_triumphsYou can see on the right one of the headlines at the time, and in fact the story was picked up by newspapers and magazines all over the world. It is particularly interesting given how complex General Relativity is, which doesn’t make it a readily accessible theory to the public. This didn’t stop the world from taking an interest, and hopefully encouraged many people to try and understand it, even if only a small part.

I also finding it amusing that two other expeditions, planned for 1912 and 1914, failed due to bad weather and the war. But had they happened before Einstein corrected his predictions (in 1915), his theory would have been proven wrong – before he had a chance to correct it!

I’ve stayed too long, but for those who want to know more let me recommend Peter Coles’ excellent exposition of this subject here.

I believe this concludes our first mini-series of posts! I hope you enjoyed it, and learnt a thing or two. If you have any ideas for more mini-series just leave us a comment – we’ll be happy to consider it.

Eclipse Week 4: Beware of Assyrians bearing crowns

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Solar eclipses were often thought of during early history as being portents of the supernatural. In ancient China for instance it was thought that the Sun was being eaten by a dragon. These events were almost always associated with bad luck.

Perhaps the most striking example of superstition in ancient societies comes from Assyrian Empire. There lunar or solar eclipses were regarded as portents for the death of the king. Hence for the period around a solar eclipse a temporary king or “sar puhi” was appointed. This was usually a man condemned to death who was given the title of king while the actual king was disguised as a peasant. After the period around the eclipse was over (usually 100 days) the true king was reinstated to the throne. Some sources suggest the temporary king was then bumped off.

However as science developed, superstitions became much less common. En route to a battle against the Spartans, the Athenian general Pericles noticed that his troops were terrified by a solar eclipse. Removing his cloak he demonstrated that an eclipse was just a shadow cast on the Earth remarking “What is the difference, then, between this and the eclipse, except that the eclipse has been caused by something bigger than my cloak?”

As Arab, Chinese and European astronomers became more and more advanced the idea that the eclipse was the moon casting a shadow on the Earth (probably first proposed by Chinese astronomers) became more accepted and refined and eclipses became easier to predict.

By the 19th century eclipses were being used to make new scientific discoveries. During a solar eclipse in India in 1868 Frenchman Pierre Janssen observed a previously unknown spectral line. It looked similar to lines from sodium so was put down to that element. however later that year the line was also observed by English astronomer Norman Lockyer. He correctly identified it as a new element which he named Helium after the Greek word for the Sun.

And then of course there was the eclipse of 1919……

Just to add: as has been pointed out to me, the Assyrian temporary king ritual could be performed for many omens, a solar eclipse being just one of them.

Eclipse week 3: A different perspective

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As stunning as an eclipse is to see from the Earth it’s not the only vantage point we have on the event. Having sent probes in to space now for a number of years we can look back on the Earth to witness this event from the outside in. During the last major eclipse astronauts on board the ISS turned their cameras back to the earth and recorded these frankly terrifying images:

You can clearly see the ominous shadow that the Moon is casting on the Earth. In the center of that dark spot many onlookers are experiencing a total eclipse while around the periphery where the shadow is not so deep will be a partial eclipse. This shadow slowly makes its way across the face of the Earth. It’s a stunning and largely foreign perspective on an already amazing event.

As Emma discussed last post we are pretty much just lucky that the Sun and Moon appear to be the same size in the sky but one place in the Solar system in which this is obviously not the case is our own Moon. On the Moon the Earth appears in the sky much as the Moon does in ours and can on occasion eclipse the sun as well. Solar eclipses on the Moon though are a little different form here on Earth, for a start the Earth moves across the sky much slower on the Moon: a lunar day is about as long as our month meaning the Earth moves across the sky roughly 29 times slower than the Moon moves in our skies. The Earth also appears much larger in the sky both of which mean that Eclipses on the Moon last much longer than ours.

We don’t just have to imagine what an eclipse on the moon would like: there have been two Solar eclipses observed from the Moon over the years, one by human eyes and one by robotic. The first was witnessed by the Apollo 12 crew on their way home from the moon and snapped this image:

Apollo 12 sees a lunar eclipse

Apollo 12 sees a lunar eclipse

The small slither of light still visible is filtering through the Earth’s atmosphere, where if you just squint you might be able to make out some details of clouds. This is something we are generally unaccustomed to seeing in an eclipse as the Moon has no atmosphere.

The second eclipse  in which the shadow is cast by us was recorded in wonderful HD by the Japanese Kaguya probe which is currently orbiting the Moon. To check out the full HD video of the eclipse you can here.

It’s hopeful to think that with our return to the Moon that such sights might become much more common.

Join us tomorrow when Niall will be talking about what eclipses have meant to people down through the years.

Eclipse live webcasts

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Quick post for everyone who’s not lucky enough to be in the path of tomorrow’s solar eclipse. I’ve come across two websites which will be doing live webcasts of the event: this one from China and this one from Japan. If anyone knows of any other good (or better) sites along the same lines leave a link to them in the comments box below.

Eclipse Week 2: The importance of luck

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Have you ever considered why the Moon and the Sun appear to be about the same size in the sky? I think it’s one of those things you just accept and never get round to examining how remarkably lucky that is, and what it would mean for eclipses if it wasn’t the case.

It turns out that the Sun is around 400 times bigger than the Moon but also just happens to be around 400 times further away from the Earth – it is this coincidence which causes the two objects to seem to have similar disk sizes. This means that during an eclipse the Moon is able to completely obscure the Sun, giving us a spectacular show in the process.

We do have some experience of what eclipses would be like if the Moon were further away. In an annular eclipse (see Stuart’s post yesterday for an explanation) the small ring of Sun still visible is enough to drown out the beautiful, faint, solar corona. A more extreme example comes from NASA’s STEREO-B solar satellite which is about 4 times further from the Moon than we are. It therefore sees eclipses like this:


As far as we know there’s no physical reason for the Sun/Moon size coincidence. In fact, since the Moon is actually moving away from us by several centimetres per year, in several million years our eclipses will be a lot more boring! Equally the dinosaurs (if any of them wanted to look) would have seen a Moon which appeared larger than the Sun. Basically we’re just lucky to be around at the optimum photograph time.

Interestingly, the reason we know the Moon is moving away is because of the Apollo missions which are being talked about so much at the moment because of the 40th anniversary of the lunar landings. One of the things the astronauts left behind them were reflector arrays (the one left by Apollo 11 is pictured to the right).

Optical telescopes have been sending pulses of laser light at these arrays ever since – timing how long it takes the light to return gives the distance to the Moon at that moment. The initially narrow laser beam is several kilometres wide by the time it reaches its destination but, given that the Moon is over 3000 kilometres in diameter, hitting the correct area is a challenging task!

Well I think that’s all I’ve got to say about eclipses, but don’t worry as there’s lots more to come from everyone else! Tune in tomorrow when Stuart’s back again to talk about what an eclipse would look like from the Moon’s perspective.