By M.S.S. Gill

The Great American Eclipse of 2017

The Great American Eclipse of 2017 occurred on August 21 in a slightly less than 100-mile-wide strip. It entered the US off the northern Oregon coast and exited off the coast of South Carolina about two hours later (see, for example, here for a clickable path map)—and several KIPAC members made sure to station themselves within this strip, also known as the "Zone of Totality," hoping to experience firsthand this very rare life event (on any given place on the Earth, total eclipses occur only about every 100 years, though they occur somewhere on the Earth about every year and a half).

We'll get to the experiences of KIPAC'ers and their friends, but first let's discuss some generalities about eclipses.

The specifics of the Zone's properties, such as its width and the exact timing of totality at each location within it, are determined by detailed celestial mechanics calculations, known many years in advance. The next in the continental US will occur in April 2024, and the one after that, in 2044 (for example, see this Wiki page for the upcoming ones).

Total eclipses have a long and storied history in human affairs, starting from worries about the Sun being eaten in ancient cultures to it being an omen for many different occurrences in various societies over time. One very new social/economic phenomenon related to eclipses is a concern for how much electrical capacity would be temporarily lost from the grid in affected states (!) given the massive amount of solar power that now supplies significant a fraction of the grid in states from California all the way to the Eastern Coast.

Historical eclipses—Vindicating Einstein 

Turning to eclipses from the past, certainly one of the most legendary of them for physicists and astronomers, in particular, was that of 1919, when Sir Arthur Eddington took it upon himself to attempt to verify one of the most dramatic predictions of Albert Einstein's theory of gravity (commonly referred to by physicists as "General Relativity" or GR). This prediction forecast that the light from distant stars would be bent by gravity as it skimmed close by the surface of the Sun, altering where they looked like they were when their original images got near the edge of the Sun (see the figure below).

(Credit: NASA, Joseph Wudka.)

Granted, a significant "postdiction" (i.e. when scientists realize a theory automatically explains something they hadn't realized it would explain, and which had been confusing before) of GR, something called the "precession of the perihelion" of the orbit of Mercury had already been confirmed by that point (which is said to have given Einstein heart palpitations when he realized GR hit the number right on for this long-standing and vexing issue for astronomers, see e.g. Pais’ recounting of Einstein's famous quote at the beginning of this document). However, the dramatic prediction of stars looking like their location had deviated from their 'normal' positions in the sky had never yet been seen. So, good ole Art E. went out, off the coast of Africa (along with others simultaneously who traveled to a location off the coast of Brazil), looked up—and found that indeed, the star image locations had shifted by the exact amount that Doktor Einstein had predicted. Einstein had been completely right and his theory was totally vindicated, and that is when his visage was splashed across all the world's newspapers. (And though this has been confirmed multiple times later, it is incumbent on us to say that that initial discovery has certainly always been controversial and has an intriguing history all its own, see e.g. this historical paper for more details.)

What we don't yet understand about our closest star

Another thing we learn much about only during total eclipses, is the corona of the Sun, which in visible light is about a million times dimmer than the main face of the Sun, and thus impossible to see normally without specialized instruments that block out the main light of the Sun and let the surrounding area be seen. But during an eclipse, the face is blocked automatically, and the corona with its bright streamers going way out from the Sun show ups in all its opalescent glory, and we can start to look at some of the most important questions in solar physics in new detail. Some of the largest questions we don't understand are: how does the corona get to 1M degrees K, when the surface is only at 5,800K ? How does the Sun generate its large-scale magnetic field which reverses itself on a regular basis? What’s the exact origin of the 11-year sunspot cycle? And many others.

And—to be very clear—unlike many other astrophysical questions which may seem of extremely remote academic interest to our daily lives, the day-to-day properties and vicissitudes of the Sun matter a whole lot to us denizens of the Earth. To give one graphic example: if a coronal mass ejection (CME) of the scale of the Carrington Event of 1859 were launched towards the Earth today, the estimated damage to infrastructure in North America alone has been calculated to be up to $2.6 trillion—that’s TRILLION, with a T. And we know one of these will for certain happen again, at some point. It would very much behoove us to be able to predict better when these might be coming, with as much advance notice as possible, so we can prepare our electrical and electronic infrastructure on our vulnerable little third rock from the Sun.

Next up: Personal reactions of KIPAC members to the Sun’s disappearance

Read more 

Einstein, Eddington and the 1919 Eclipse by Peter Coles (2001)

Sometimes, What We "Know" Isn't Actually True: A solar eclipse, dark matter and the cautionary tale of the planet that wasn’t there (SciAm Blog, 2017)