Don’t Look! (A Field Guide to the Solar Eclipse)

Astronomer Shane Larson offers insights and tips for experiencing the August 21 stellar event

August 10, 2017

What is a Total Solar Eclipse?

Everything the light of the sun touches casts a shadow. You cast a shadow, tress cast a shadow, and the Earth and the moon cast shadows. When planetary sized objects cast shadows into the solar system, other objects sometimes pass through those shadows, and when they do astronomers call it an eclipse. The next total solar eclipse is set to cross the continental United States from west to east on August 21. During a total solar eclipse, the moon passes directly between the sun and the Earth, casting its shadow on the Earth. As the moon moves along its orbit, the shadow moves across the planet. If you are along the shadow’s path, it will get dark for a few minutes as the moon passes directly between you and the sun.

“I always tell everyone that whether you are a scientist or not, a total solar eclipse is one of the most profound experiences you can have as part of an astronomical event,” says Larson, physics and astronomy. “In a total solar eclipse, if you are standing in totality, it is going to get completely dark at a time it should not.”

Larson, a member of Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), adds that even if the eclipse occurs during a completely cloudy day, every point along the eclipse centerline will experience a total blackout.

Eclipse Centerline

The moon’s shadow will come ashore in Oregon, and move southeast across the United States, heading into the Atlantic Ocean off the shore of South Carolina. The centerline is the path that is in the direct center of the moon’s shadow as it moves across the country. Those individuals lucky enough to be in an area along the centerline (Carbondale, Illinois, for instance) will get the maximum eclipse experience.

Anyone in the area roughly 35 miles north and south of the centerline is in the path of totality — the full shadow of the moon passes over you, but the eclipse is shorter the farther from centerline you are.

“In Evanston we'll see essentially what people will see in Chicago — the partially eclipsed sun rather than darkness,” says Larson.  “Keep in mind that during the entire eclipse, the sun will be too bright to look out without specialized glasses.”

Please note: Northwestern will not be hosting a public viewing event, due to CIERA astronomers traveling elsewhere to view the eclipse in its totality.

How will the eclipse look?

Only in totality will the sun be completely covered by the moon. Everywhere else in the continental United States, people will see a partial eclipse. Nowhere in the US will see less than 60 percent coverage. Most of Canada and Mexico will see more than 40 percent, with only the extreme north and south seeing 20 percent coverage.

“I often like to think about a time before we had astronomy and before we had an understanding of the sky and what the solar system was doing,” says Larson. “Imagine what it would be like if you were hanging out, just doing your early human civilization thing, and the sun disappeared. You might be a little worried, and uncertain if it was going to come back at all. These are profound events, so if you have a chance to travel to see the total solar eclipse, you should definitely do it.”

How to Watch the Eclipse 1: Projection

One of the easiest ways to see the eclipse is with projection. If light from the eclipse passes through a tiny hole poked in a thin sheet of paper or foil, it will make an image of the eclipse. You can make a projection viewer with a box or tube.

Cut a hole in one end, tape aluminum foil over the hole, and poke a hole in the foil with a sharp nail or pushpin. Tape paper on the inside of the box, opposite the hole. On the side in between, cut a hole big enough for you to look in and see the paper. Point the foil at the sun, and light will pass through the hole, making an image on the paper. The longer the box, the bigger the image and the easier it will be to see — try using a mailing tube.

How to Watch the Eclipse 2: Eclipse Glasses

You can purchase “solar eclipse viewing glasses” before the eclipse. These are paper glasses that have special solar filters over the eyeholes that stop harmful amounts of light from reaching your eye. The result is a dark view of the sky, but a clear view of the sun’s disk as it is being hidden behind the moon. If you purchase some, make sure to buy them from a reputable dealer, like your local planetarium or science museum, or an online telescope dealer. Make sure you get your solar eclipse glasses early — supplies may run short as we get closer to the eclipse!

How to Watch the Eclipse 3: Happenstance

Anything with holes in it will project images of the eclipse when it is happening. Look at the dappled light streaming through trees, make cross-hatches with your fingers, or hold up a spaghetti colander or slotted spoon! Be creative, and make sure to take a picture.

It is not safe to view a solar eclipse without proper eye protection. For more information and tips on viewing the eclipse, see CIERA’s online total solar eclipse guide.

Shane Larson