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An “American” Eclipse


Last fall, I got a pair of paper glasses in the mail. The flimsy plastic lenses were pitch black and the sides had eagles printed over an American flag. “What kind of propaganda is this?” I thought. Then I read the insert in the package. These were actually solar viewing glasses, to be used during the solar eclipse happening this year on August 21. The company that sent them may have jumped the gun by sending them a year early, but at least I am prepared.

My handy dandy solar viewing glasses

This is the first total solar eclipse that will be visible from the continental US since 1979, hence the American symbolism all over those glasses I received. Even more impressively, this is the first eclipse to cross both coasts since 1918, and the next eclipse like it won’t be gracing our presence until 2045. It can be seen as a partial eclipse from South America and Northwestern Europe, but the total eclipse will be visible only from a 70 km (about 45 miles) wide strip that extends from Oregon all the way across the US to South Carolina. The moon’s shadow will traverse the entire country in only 90 minutes, and since this shadow moves west to east, the eclipse will occur later in the day the farther east you are.  

The Path of Totality.

What is An Eclipse?

You may be wondering what makes this eclipse a “total” eclipse. Well, a solar eclipse occurs when a new moon passes between Earth and the Sun, blocking light from the Sun as viewed from Earth. The shadow of the moon causes darkness at some locations on Earth where there would otherwise be bright daylight.

That may seem simple enough, but there are different types of eclipses. The August eclipse will be a total eclipse, meaning the disk of the moon will entirely cover the disk of the sun and Earth will fall within the moon’s umbral shadow (the inner, dark part of the shadow). This leaves only the solar corona (not the brand of beer, but actually the active outer atmosphere of the sun) visible beyond the moon’s shadow. A total eclipse is the only time that the sun’s corona is clearly viewable to the naked eye, producing a spectacular display that you don’t want to miss.

Left: Total solar eclipse, complete with visible corona. Courtesy of Flickr. Right: Sun, Moon, and Earth positions. Wikimedia.

Total solar eclipses occur about once every year and a half, but they are only visible from certain portions of the globe; hence why I have never seen one. The other types of solar eclipses – annular and partial eclipses – occur more often though, about 2 to 5 times per year.

An annular eclipse and a total eclipse are actually very similar. Despite the fact that the sun is about 400 times larger, the sun and the full moon are approximately the angular size in the sky since the sun is also 400 times farther away from Earth. However, the moon can seem slightly larger or smaller depending on where it is at in its orbit around us – in fact, the moon varies in its distance from Earth by about 50,000 km (30,000 miles). Therefore, when the moon is farther away it appears smaller than the sun in the sky, especially if we Earthlings are closer to the sun in our own orbit. If this is the case during an eclipse, when the moon passes directly in front of the sun (as it does in a total eclipse), the moon’s disk does not entirely cover the sun’s disk. Instead of seeing the sun completely blocked out, we see a small “ring of fire” around the dark moon. Thus the term “annular,” meaning ring-shaped, is an appropriate descriptor for this type of eclipse. During annular eclipses, you cannot see the corona, but you can see a small sliver of the sun’s surface around the moon that is just as breathtaking.

Annular (left) and partial solar eclipse (right)

A partial eclipse occurs when only part of the sun is blocked by the moon. This is still pretty cool, mainly because it looks like the moon took a bite out of the sun. Such an eclipse happens when the outer, penumbral shadow of the moon passes over Earth. If you are not in the “path of totality” during a total or annular eclipse, you may only see a partial eclipse. The phrase “path of totality” may sound slightly terrifying, but it merely refers to the area where the umbral shadow falls and a total eclipse can be seen. The moon’s umbral shadow is not large enough to cover the whole earth, so this path is typically pretty slim – less than 50 miles in this case. The closer you are to the center of the path, the longer you will see a total eclipse.

For Your Viewing Pleasure

For those in the path of totality on August 21, 2017, a total eclipse will be visible for about 2 minutes and 40 seconds at a maximum. During this time, you actually won’t need those solar viewing glasses I was talking about, as these are used for viewing the sun as it comes out of (or goes into) eclipse. The sun, even during a partial or annular eclipse, is too bright to look at with the naked eye and can literally sunburn your corneas or permanently damage your central vision. The only safe time to look at the sun directly is during a total eclipse – otherwise, you should have some protective gear, which you can find here.

So if you’re on the right path, come August you can stare straight into that burning ball of gas above us (albeit for less than 3 minutes) and think of everyone who told you not to as a child. Or you can think about something more majestic, up to you. As for me, I will be traveling to Tennessee to witness this amazing phenomenon, since my hometown of Athens, GA is just outside the path of totality. Check out NASA’s interactive map to see if and when the eclipse will be visible to you!

About the author

image04 Lauren Sgro is a PhD student in the Physics department at the University of Georgia. Her research focus is in astronomy, specifically debris disks around young stars that may tell us more about planetary formation. Despite the all-consuming nature of graduate school, she enjoys doing yoga and occasionally hiking up a mountain. You can’t reach her on Twitter, but you can email her at lauren.sgro25@uga.edu. More from Lauren Sgro.

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