The Facts: The Great American Eclipse of 2017
Professor of Physics Maggie Sherer answers questions about the Aug. 21 total solar eclipse visible from Jewell.

What is a total solar eclipse?
A solar eclipse occurs when the moon passes directly between the Earth and sun. You can observe a total eclipse if you’re in the moon’s umbra, which is the darkest part of its shadow. Here, the moon completely covers the sun’s visible surface (the photosphere). If you’re outside the umbra, but still in the penumbra, you observe a partial solar eclipse. The moon moves across the sun, but never fully covers it.

Why will Jewell have a long period of darkness?
The path of totality is about a 70-mile wide circular shadow (the umbra). Those closer to the center of the shadow stay under it for a longer amount of time, and therefore observe a longer period of totality. At Jewell, we will experience darkness for more than two minutes.

What is the corona of the sun I’ll see during the eclipse?
The corona—Latin for “crown”— is the super-heated upper atmosphere of the sun, primarily consisting of hydrogen and helium nuclei that have lost all their electrons. It appears white and wispy, and the sun’s constantly changing magnetic field determines its shape. The sun’s luminosity blocks the corona from view, so we can only see it during total solar eclipses and with special instruments like coronagraphs.

Do I need eye protection?
Seeing any amount of photosphere is damaging to our eyes because the light rays carry a very high energy. Therefore, you should wear eye protection at all times except during absolute totality, which lasts about two minutes. It’s especially important to be cautious at the end of totality because the darkness of the sky causes your eyes to dilate, so the burst of intense light can be damaging.

How frequently does Earth have total solar eclipses?
In any one place on Earth, total solar eclipses occur every 100 years or so. However, by chance, there are some places in the Midwest that will be able to see totality both on August 21, 2017, and on April 8,  2024.

Total solar eclipses happen once or twice a year, but you have to be in the path of the moon’s shadow to see them. If the moon orbited in the same plane as the Earth and sun, somewhere on Earth would have a total solar eclipse every new moon. However, since the plane of the moon’s orbit is tilted about 5º to the Earth’s orbit, the moon’s shadow often falls above or below the Earth into space. Additionally, the moon’s elliptical orbit causes its distance from the Earth to change, so sometimes the moon is too far away to fully cover the sun. This event is called an annular eclipse.

When is the next total eclipse?
The next total solar eclipse to cross the U.S. will be April 8, 2024, passing from Atlantic Canada through Mexico. This may be the most viewed eclipse in history because of the extensive road system in the United States, which allows more people to travel to the path of totality that crosses from coast to coast. Additionally, it has been 99 years since the last total eclipse in the United States that was coast to coast, so it will be the first time for many people to see one.