The 2017 eclipse is predicted to be the largest mass migration to see a natural event in history. The Federal Highway Administration estimates that between two and seven million people will be traveling to be able to witness totality (a 70 mile swath reaching from Oregon to South Carolina).1 That's in addition to another 12 million that already live in that path. Alaska Airlines is offering a special flight to view from the air,2 United Airlines has worked with NASA on its flights, and Southwest has five flights that cross the path, for which they're offering special themed cocktails and viewing glasses.3 Business in the path of totality are shutting down for an hour. Thousands of eclipse viewing parties have been organized, at locations ranging from universities to bars. You can't function as a news outlet right now and NOT have an article about the eclipse. Most outlets have literally sold out of eclipse glasses. The excitement and commitment that people have for this event is incredible to me- and it's not due to a sporting event, or a once in lifetime concert, or a political rally; it's all about a natural, scientific phenomenon. That's amazing to me. The United States is currently divided about everything, and we're experiencing what many have called an era of renewed science denialism.4,5,6 And yet, somehow, a scientific event is bringing millions of people together, making us travel, interact, learn together, and experience something wondrous together. For someone who cares passionately about science literacy and engagement, I wish we, as a society, had this excitement all the time, but I'll take this as a start.
I'm pretty sure that if you're a science journalist and you don't write something about the eclipse, they take away your Twitter account and force you to go back to the bench. But as I mentioned, every news outlet has written something about the eclipse. So I feel a burning need to say something about the eclipse, but there aren't many stories that haven't been totally played out. Today, I'm taking a historical approach, focusing on the the things we've learned about the earth and our universe from similar eclipses in the past. Let's also take a moment to appreciate that I came up with a musical sun themed title that wasn't "Total Eclipse of the Heart".
|Babylonian eclipse tablet recording eclipses between 518 and 465 BCE|
Historically, it has been incredibly difficult to study the sun. The sun is bright. Really, really bright. Bright enough to do severe damage if you're looking directly at it, and bright enough to basically drown out anything around it. Now, astronomers use telescopes that can filter out specific spectrums of light and allow them to see the normally suppressed features, but for most of history, the only way to observe these features was while the bright part of the sun, the photosphere, was covered up. These features, specifically the chromosphere and the corona, are actually part of the sun's atmosphere. Some believe that the earliest recorded reference to the corona was carved into oracle bones in China during the Shang Dynasty (1766 to 1123 BCE), but it's a little ambiguous. Plutarch described what sounds like a corona during the eclipse of March 20, 71 CE in the book De Facie in Orbe Lunae: "Even if the moon, however, does sometimes cover the sun entirely, the eclipse does not have duration or extension; but a kind of light is visible about the rim which keeps the shadow from being profound and absolute." Byzantine historian Leo Diaconus reported on the eclipse of December 20, 968 CE in Constatinople: "...at the fourth hour of the day ... darkness covered the earth and all the brightest stars shone forth. And is was possible to see the disk of the sun, dull and unlit, and a dim and feeble glow like a narrow band shining in a circle around the edge of the disk".7 Pre-filtered telescope astronomers used the differences in the corona between eclipses in subsequent years to study changes over the course of the sun's eleven year cycle from solar maximum to solar minimum, as measured by the number of sunspots.8
|The corona photographed in 1871|
Eclipses are also responsible for the discovery of Coronal Mass Ejections (CMEs). CMEs are projections of plasma that snap off from the surface of the sun and go hurtling through space at thousands of miles an hour, carrying with them billions of kilograms of ions. This happens at least once a day, but most of them don't come anywhere near the Earth.9 When they do, however, they can cause power surges that can result in blackouts and damage electronic equipment. On the aesthetic side, they can also cause auroras, more commonly knows as the Northern or Southern Lights.10 The first CME was observed during the eclipse of July 18, 1860. Solar prominences, filaments of plasma that extend from the surface of the sun into the corona, were first described during the Russian eclipse of May 1, 1185. The observance of the corona and subsequent discoveries of CMEs and solar prominences tells us a lot about the sun's magnetic field. This magnetic field is a major driver of solar weather patterns, which can have huge effects on the Earth and the astronauts and equipment we have floating around it. However, just the Earth's magnetic field, the lines of the field aren't visible. The pattern of a corona is a reflection of the magnetic field, prominences are held in place by the field, and CMEs are a direct result of breaking the magnetic field lines. Their discovery allows us to better understand the seemingly distant phenomena that affect our world.11
During the solar eclipse of August 7, 1869, two astronomers independently observed an emission line in the corona in the green part of the spectrum. This line didn't correspond to anything known, so it was believed to be a new element dubbed "coronium". In the 1930s, it was discovered that this wasn't actually a new element, but rather was just iron.12 I say "just", but in reality, the lines weren't caused by your everyday iron, but incredibly hot iron. So hot, in fact, that it was much, much hotter than the sun. These lines indicated that the corona reached temperatures of 3.6 million degrees F, when the actual surface of the sun only reached about 10,000 degrees F.13 We don't know exactly how the atmosphere of the sun is hotter than the sun itself (which is presumably giving off the heat), but it's suspected to be due to "heat bombs", the result of the magnetic field crossing into the corona and realigning.14
Although coronium wasn't really a unique element, the discovery of new elements during eclipses was not without precedent. A total eclipse in Guntur, India on August 18, 1868 revealed a bright yellow line in the spectrum of the chromosphere that didn't correspond to any known element. The element was named for the sun where it was first discovered- helium.15 Helium wouldn't be observed on Earth for almost thirty more years.16
Sometime around 150 BCE, Hipparchus of Nicaea realized that the solar eclipse could help the calculation of the distance between the Earth and the moon. For that particular eclipse, northwestern Turkey experienced totality. Alexandria, Egypt, about 1,000 km away, reported only 80% totality. Since a solar eclipse occurs when the moon drifts between the Earth and the sun, this gave him enough information to be able to use trigonometry to figure out the distance between the Earth and the moon. It should be noted that he was wrong (off by about 20%), but it was still a pretty good estimate for the tools of 150 BCE.17
This one is my personal favorite. For a long time, scientists predicted that there was a tiny planet in between Mercury and the sun, because Mercury's orbit was a little more unstable than it should be. They called this planet Vulcan, I assume because it would have to be so hot and Vulcan was the god of fire and volcanoes, and not because the crew of the Enterprise broke the Prime Directive. During the eclipse of July 29, 1878 two different astronomers reported observing a red planet object very near the sun, and believed this to be confirmation of Vulcan.18 The next year, one of those astronomers, James Craig Watson, along with Maria Mitchell and Thomas Edison (yes, that Thomas Edison), tried again to observe Vulcan during an eclipse and failed, probably because it's not there.
In 1915, a theory was postulated that could explain the wobbly orbit without needing another planet: Einstein's theory of relativity. The basic idea of general relativity is that spacetime curves around mass, which is an exactly useless explanation for anyone who isn't a physicist, and probably many who are. I'm going to try to break this idea down.
- Spacetime is a model that combines the three dimensions of space (up-down, left-right, front-back) with one dimension of time (which I'll call forward-backward). It's kind of hard to picture, because we're so used to visually conceptualizing 2D (think about graphing in grade school) or 3D (all objects around us). But all objects DO have a fourth dimension- time. The table my feet are propped up on right now has the dimensions that were listed on the website, (length, width, and height), but it also exists in different moments of time, essentially right now.....and right now.....and right now.....and right now. Graphing the time component of the table would look something like this one dimensional line:
The time dimension is tricky because for every day objects, it feels less finite than height, width, and length. If you think about the expanse of the universe, though, height, width, and l length are also decidedly less finite. If this is still confusing to you, I don't blame you, and there are a bunch of videos you can google for visual explanations.
- So we have a fabric that we'll call spacetime. It stretches out in all dimensions. Think of it like a stretchy trampoline material. If you place a baseball on a trampoline, it will sag a little under the baseball. If you put a bowling ball on the trampoline, it will sag more, because the bowling ball is heavier. If you put a really round hippo on the trampoline, it's going to sag a lot. That sagging is what we mean by spacetime being curved, and just like in our trampoline example, objects with different masses make it curve different amounts.
- Now imagine you have a marble on the trampoline. Flick it across the surface, and it rolls steadily in a specific path according to Newton's laws: it'll keep going in a straight line at the same speed until it gets pushed or friction slows it down. Now push the marble across the trampoline when the hippo is sitting on it; it doesn't move in a straight line. When it hits the sag, it starts to spiral inward, like those penny donation cones at the mall. This is the idea of relativity.
Relativity explains why orbits precess in a way that isn't consistent with classical physics and ideas on gravity, and also predicts that, much like the path of our marble, light itself bends in the gravitational field of a large object.
You might imagine this to be a difficult thing to prove, but fortunately, there was another solar eclipse coming up on May 29, 1919. This eclipse occurred as the sun was passing the Hyades star cluster. Light from these stars would have to pass through the sun's gravitational field to reach Earth, and thanks to the eclipse dimming the sun, it would be dark enough to see them. By comparing measurements of the locations of these stars on a normal day and measurements taken in Principe during the eclipse, Sir Arthur Eddington was able to confirm that the light was was being bent by the gravity of the sun by more than what would be predicted by classical physics and exactly as much as would be predicted by general relativity. The morning after Eddington reported his findings, Einstein became a scientific celebrity, a position he's never faltered in.19
Historically, eclipses have portended doom and been signs from God. They've caused fear and hope. They've inspired astronomers and brought people together. They've changed the way we view our universe. There a hundreds of experiments that will be taking place on August 21st, and who knows what they'll change.
Follow me on Facebook, follow me on Twitter, and for the love of all that is holy, don't look directly at the sun.