Astronomy

The Great American Solar Eclipse: Here’s Everything You Need to Know

A total solar eclipse will sweep across the continental United States on Aug. 21, providing a rare opportunity for much of the country to experience the celestial wonder.

The roughly 12.2 million people who live directly along the eclipse’s so-called path of totality — a 70-mile wide swath of darkness caused by the moon passing directly in front of the sun relative to Earth’s line of sight — will need only step outside to witness the lunar shadow falling on the planet and turning daylight into twilight.

Sky-watchers should wear special protective glasses when the sun is partly blocked, but during totality, which occurs when the moon completely obscures the face of the sun, it is perfectly safe — and highly advisable — to gaze at the total eclipse as the sun’s sparkling outer atmosphere, or corona, becomes visible. The shimmering light will appear to undulate due to solar magnetic fields, as jets of light stream out in plumes from around the bright orb.

Another 50 million Americans reside within 100 miles of the path of totality, which will travel from Oregon to South Carolina in little more than 90 minutes.

Except for the northern tip of Maine, the entire population of the continental United States lives within 900 miles — a two-day drive — of the eclipse’s path, which will be the first to stretch from coast to coast since 1918. Anyone outside of the path of totality will only be able to see a partial eclipse, with the moon obscuring a portion of the sun.

The last time a total solar eclipse touched any part of the continental United States was in 1979.

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“To go through life without seeing a total eclipse of the sun would be kind of like going through life without ever falling in love,” said Rick Fienberg, an astronomer with the American Astronomical Society.

But advance planning is key.

“There’s a lot of people who are going to be trying to get to the path of totality,” added Alex Young, a solar scientist with NASA’s Goddard Space Flight Center who is spearheading the agency’s eclipse outreach efforts. “There just aren’t enough porta-potties in the world to handle this many people.”

Traffic heading to parks and other sites along the path of totality is expected to spike as the eclipse approaches, with people from surrounding areas streaming toward prime viewing areas. When a total solar eclipse tracked through Mexico in 1991, border patrols would only allow visitors with confirmed hotel reservations to cross into country, recalled University of Missouri astronomer Angela Speck, who heads the American Astronomical Society’s eclipse outreach efforts.

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Fienberg, like many enthusiastic eclipse watchers, intends to witness the event with a group in Madras, Oregon, one of dozens of cities that will experience totality and currently the odds-on favorite for clear viewing conditions.

Other notable cities within the path of totality include Salem, Oregon; Idaho Falls, Idaho; Casper, Wyoming; Lincoln, Nebraska; Jefferson City, Kansas City, and St. Louis, Missouri; Carbondale, Illinois; Paducah, Kentucky; Nashville, Tennessee; Clayton, Georgia; and Columbia, Greenville, and Charleston, South Carolina.

Astronauts aboard the International Space Station will have one of the most unusual views. The station, which orbits about 250 miles above Earth, is  expected to be over Canada at the time of the eclipse, giving the crew an oblique perspective of the moon’s dark shadow crossing the North American continent.

NASA is taking advantage of the eclipse to fund 11 science investigations to study the sun and its effects on Earth, and to test instruments that could be installed on future satellites. The experiments will take place on the ground, aboard high-altitude balloons and on aircraft that will fly inside the path of totality. The moon’s shadow will hustle across the continent at about 2,000 mph, so the aircraft won’t be able to fly fast enough to follow the eclipse for long.

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Due to Earth’s geometric shape, the moon’s shadow, or umbra, will travel fastest at the beginning of its cross-country trek and slower in the middle. An interactive Google map created by the French eclipse-watcher Xavier Jubier, a member of the International Astronomical Union’s eclipse outreach group, calculates that the moon’s shadow will zip along at 2,955 mph in Western Oregon, 1,747 mph in central Nebraska and 1,502 mph near Charleston, South Carolina.

The slower the umbra moves, the longer the period of totality lasts, though the biggest impact on the duration of totality is how close an observer is to the center of the umbra’s path. The upcoming eclipse will last longest — two minutes and 42 seconds — at Giant City State Park, just south of Carbondale, Illinois, according to Jubier’s map.

All of North America, the northern part of South America, western Europe, and Africa will experience a partial eclipse, though the difference between that and the path of totality is literally the difference between day and night, Feinberg noted.

“During a total eclipse, the moon blocks the bright sun and reveals the sun’s outer atmosphere, the corona, which you don’t see even under a 99 percent eclipse,” he said. “The temperature goes down quiet noticeably, the horizon is ringed by pretty sunset colors, the sky gets deep twilight blue, and bright stars and planets come out. Animals and birds behave strangely, like it’s the end of the day. It’s a whole constellation of things happening over the course of a few minutes.”

“During a partial eclipse, even a 75 percent partial, unless someone told you it was happening you might not even know because the landscape doesn’t change,” he added.

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Before and after totality, and during any partial eclipse, special eclipse glasses must be worn to protect the eyes, scientists warn. Solar radiation can be very damaging to eyes, and retinal burns can occur if someone doesn’t take the proper precautions during an eclipse, resulting in a dimming of vision popularly known as “eclipse blindness.”

When the face of the sun is fully blocked by the moon, however, it is perfectly safe to look at the eclipse, which if skies are clear will appear about as bright as a full moon.

“You’d be an idiot not to look at it. It’s one of the most spectacular sights in all of nature,” Fienberg said. “You may think this is an astronomical event, but really it’s just a life event. It’s just a beautiful thing.”