Jay Pasachoff

Getting ready for the solar eclipse

On August 21, a solar eclipse will be viewable across the United States. Those who want to take a look will need eye protection, though. Looking directly at a solar eclipse without such protection can damage the eyes.

Eeriness creeps in. Colors change and shadows sharpen. The last minutes before a total eclipse of the sun triggers a primal reaction, says astronomer Jay Pasachoff.

“You don’t know what’s going on,” says Pasachoff. “But you know something is wrong.”

Millions of people will encounter this reaction on August 21, 2017. That’s when a total eclipse of the sun will sweep across the continental United States. This will be the first eclipse to grace the country since 1979. And it will be the first since 1918 to bring a total, if temporary, blackout coast-to-coast. Its path will be roughly 120 kilometers (75 miles) wide. Created by the moon’s shadow, this so-called totality will pass through 12 states, from Oregon to South Carolina.

Researchers and the public alike have been gearing up to make the most of this rare spectacle. After all, U.S. communities won’t get another chance until 2024.

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eclipse path
On August 21, a solar eclipse will race across the United States.

The mystery of the eclipse

A solar eclipse occurs when the moon passes between the sun and our planet, casting its shadow on Earth. For someone sitting on Earth viewing the eclipse (with some kind of eye protection), it appears that the moon nearly blots out the sun. A total solar eclipse occurs about once every 18 months.

Eclipse enthusiasts will travel from all over the world to experience up to nearly three minutes of midday twilight and glimpse the sun’s corona (Koh-ROH-nah). The outer layer of the sun, it is an intensely hot, ionized gas, or plasma. During a total eclipse, this seldom-seen halo of light will frame the blacked-out sun.

eclipse corona
During a solar eclipse, the moon blocks out the sun. That allows people to see the sun’s corona (as visible in this picture from a 1999 eclipse).

At its sight, “People cheer and people cry,” Pasachoff says of Williams College in Williamstown, Mass. And he should know. He’s already witnessed 33 total solar eclipses and 30 partial ones.

People like him will often have to travel to the far ends of the Earth to experience an eclipse. That’s because eclipses that pass over densely populated portions of the planet are fairly rare. This explains why the 2017 event is so special. Millions will be able to experience it firsthand, often without leaving home.

For scientists, an eclipse offers more than an interesting experience, though. It’s a time to study the corona. Though some of it is visible all of the time to a few telescopes in space, the region where the corona meets the surface is masked by the sun’s intensity. “Only on days of eclipses can we put together a complete view of the sun,” Pasachoff explains. For researchers, the 2017 eclipse is thus another chance to connect what they see on the surface of the sun to what’s happening in the outer reaches of its corona.

One enduring mystery relates to temperatures on the sun. The sun’s surface is a relatively balmy 5,500° Celsius (nearly 10,000° Fahrenheit). But the corona is millions of degrees hotter. Scientists still aren’t really sure why. “The consensus is that the sun’s magnetic field is responsible,” says Paul Bryans. “But it’s not clear how,” he adds. Bryans…