What Siouxland Families should know about the 8.21.17 Solar Eclipse

A solar eclipse is coming! On Monday, August 21st a solar eclipse will be crossing the US. In a strip crossing sort of diagonally from Oregon to South Carolina the eclipse will be a total solar eclipse, and in most of the rest of the country a partial eclipse will be visible. This is the first solar eclipse visible from the entire contiguous US since 1918! I talked to Morningside Astronomy and Physics Professor Laura Kinnaman to find out more about what a solar eclipse is and what Sioux City kids, teens, and parents should know about watching the 2017 eclipse. Here's what I found out :

What is an eclipse?

A solar eclipse happens when the moon gets directly between the earth and the sun. (A lunar eclipse, which is much more common, happens when the earth gets directly between the moon and the sun. The sun never gets directly between the earth and the moon, which is good because we’d all die!) A solar eclipse can only happen when the moon is new—a new moon is when the side of the moon that’s lit up is away from us, and new moons rise at 6 am and set at 6 pm (roughly). It’s highest in the sky at noon. The question is, if there’s a new moon every month, why isn’t there a solar eclipse every month? To answer that, we have to look at orbits.

The earth orbits the sun, in the plane of the solar system. If you drew a picture of the sun in the middle and a circle around it, that’s actually a pretty decent representation. The moon also orbits the earth, and it *almost* does so in the same plane—you’d have to draw the moon’s orbit on a different piece of paper, and it’d be poking out of the sun/earth plane a bit. I’m attaching a picture of what I mean here. The exact place where the moon’s orbit intersects the plane of the sun/earth orbit changes every month. Some months during a new moon, the moon is above that plane (called the “ecliptic” because it’s related to eclipses); some months, the new moon is below that plane. But every once in a while, the new moon intersects the ecliptic plane, which means that the sun, moon, and earth all line up directly. 

When all three line up, then the moon is close enough to the earth that its shadow lands on the earth’s surface. If you stand in that shadow, you see the eclipse—just like standing in the shade of an umbrella means that the umbrella is blocking the sun from your view. There’s actually a fantastic cosmic coincidence here: the moon is 400 times smaller than the sun, but it’s also 400 times closer to us, so the moon appears to be the same size in the sky, and it perfectly covers the sun’s face!

In Sioux City

In Sioux City, a partial eclipse will be visible from approximately 11:37am to 2:37pm, with maximum obscuration--the sun will be about 94% blocked--at around 1:02pm. These are the times for downtown Sioux City, and the farther you live away from Sioux City, the less accurate those times will be. You can check NASA's interactive map here to see the exact times for where you will be at the time of the eclipse (but they are not in central time so you will have to convert them). Professor Kinnaman explained what we should expect to see during a partial eclipse, and how to keep your eyes safe: 

The difference between a partial eclipse and a total eclipse is pretty stark. While most of the sun will be covered by the moon during the eclipse, as seen from Sioux City, most people won’t even notice. You have to be looking at the sun with eclipse glasses in order to see that the moon is covering the sun at all. 
You can also use a pinhole camera—that is, a stiff piece of paper with a small hole in it, where the light goes through the hole and projects an image onto another piece of paper. It’ll show the circle of the sun with what looks like a bite taken out of it by the moon. Otherwise, it might just seem slightly dimmer—like the sun went behind a bit of cloud. Partial eclipses are really cool, don’t get me wrong! But they’re nothing compared to a total eclipse (or so I’ve been told; I’ve seen the former but never yet the latter).

**I do not recommend buying eclipse glasses on Amazon. Even if you purchase from one of the NASA approved companies, Amazon does not screen for fakes and does not stand by their products. For more info on Amazon's shady practices please see this article, originally written about cloth diapers but applicable to any type of product sold by Amazon.

Be sure to look for glasses with ISO certification, as looking at the sun with inadequate glasses can result in harm to your eyes. Regular sunglasses are not enough!

Visiting the Umbra

Why can some cities see a total eclipse and others only a partial one? Professor Kinnaman explains:

The shadow that the moon casts on the earth’s surface isn’t just a single dark shape. It’s actually got a darkest part (the “umbra”), a small circle where none of the sun’s light hits, and the less dark part (the “penumbra”) where some light, but not all is blocked. I’ve included a picture of this, too. Sioux City will be in the penumbra, while Beatrice, NE, will be in the umbra.

If you would like to travel to the umbra, which I have also seen referred to as the "totality zone"--as it is the area where the eclipse will be a total, rather than partial, solar eclipse--you should make your hotel reservations quickly or plan to stay with friends or relatives, as many hotels in the zone are totally booked! Some of the cities nearest to Siouxland where the eclipse will be a total one include Grand Island and Lincoln in Nebraska, and St Joseph and Kansas City (north only) in Missouri. You can find a full list of cities in the umbra here. You can also find more information about the eclipse in general and the expected time and length of total eclipse at specific places by checking out NASA's 2017 eclipse website.

Whether you stay home in the penumbra and watch the partial eclipse, or travel to the umbra to watch the total eclipse, be sure to (safely) notice the sky on August 21st. You won't regret it!

Besides being awesome to look at, eclipses are also useful in studying physics and in particular wave theory! If you're interested in learning more, check out Professor Kinnaman's book, The Student's Guide to Wave Theory. Thank you to Professor Kinnaman for all her help in writing this post and for helping Siouxland families understand the physics behind the eclipse!

For the latest news on where to watch the eclipse in Sioux City, check out the official eclipse thread in the Siouxland Family Friendly Events group on Facebook!