On Aug. 21, the moon will slip between Earth and sun, casting a roughly 70-mile-wide shadow that will race across the United States from Oregon to South Carolina, giving tens of millions of Americans a chance to enjoy -- and study -- a fleeting but sublime spectacle, the first coast-to-coast total solar eclipse in 99 years.
It has been dubbed, appropriately enough, the "Great American Eclipse."
"It really is fortunate," said Matthew Penn, an astronomer with the National Optical Astronomy Observatory who is leading a nationwide effort to photograph the eclipse from 68 sites along the path of totality.
"The U.S. only covers 2 percent of the globe, so we get very few eclipses," he said. "And to have one travel across the entire country is an unprecedented sort of opportunity. It'll be a heck of day. The best thing is, it can't be cloudy everywhere!"
The first inklings of what's to come will be visible from the Oregon coast, weather permitting, around 9:05 a.m. local time (12:05 p.m. EDT) when viewers with safety filters, from inexpensive cardboard "solar glasses" to more sophisticated aids, will see the moon begin to take a bite out of the sun, the start of a partial solar eclipse.
A partial eclipse will be visible across the entire United States, including Alaska and Hawaii, Canada, Central America and northern South America, with more than half of the sun obscured for residents across the lower 48 states.
But for millions of viewers who make their way into the narrow path of totality, a partial eclipse, a thrilling sight in its own right, will serve as an appetizer for the main course, the all-too-fleeting moments when the moon completely blocks out the sun.
All of North America will see at least a partial solar eclipse on Aug. 21, 2017, and a 70-mile-wide swath of the country from coast to coast will get a total eclipse.
Michael Zeiler, www.GreatAmericanEclipse.com
Moving across the Pacific Ocean at more than 2,400 mph, the dark inner heart of the moon's shadow -- the umbra -- will move ashore near Lincoln Beach, Ore., at 10:16 a.m. local time (1:16 p.m. EDT), and then sweep across 14 states and 20 national parks over the next hour and a half.
More than 200 million people live within a day's drive of the path of totality, potentially turning the Great American Eclipse into one of the most heavily viewed and shared events in recent memory.
"The Great American Eclipse on Aug. 21 is such an exciting event, it is a really singular and unique event in human history, really, where in the 21st century we have this amazing technology with social media," said Carrie Black, associate program director of the National Science Foundation's Division of Atmospheric and Geospace Sciences.
"And it's happening across the United States, so the entire country can participate," she said. "There's never been an event like this in human history where so many people could participate and with such unique technology."
A total solar eclipse as seen from Easter Island on July 11, 2010. The super-heated corona, normally invisible in the glare of the unobstructed sun, extends into space in a shimmering halo that will be visible to millions during a coast-to-coast total eclipse Aug. 21.
American Astronomical Society/Dennis di Cicco/Sky & Telescope
When the eclipse arrives
Two minutes and 145 miles after the leading edge of the moon's shadow crosses the coast of Oregon, a partial eclipse will begin in the small town of Madras at 9:07 a.m. local time (12:07 p.m. EDT). With clear skies the rule this time of year, thousands of tourists and astronomers, both amateur and professional, are expected to join 6,400 area residents to take in the show.
One hour and 12 minutes after the start of the partial eclipse, at 10:19 a.m., the moon will completely block out the sun above Madras, turning a bright morning into deep twilight for the next two minutes and two seconds.
What you see is the jet black silhouette of the moon framed by the beautiful white of the corona, streaming out in all directions, usually several diameters of the sun away," Fienberg said. "What makes it especially wonderful is the quality of the light, and the fact that the corona behaves like iron filings around a bar magnet, it traces the sun's magnetic field. So you see loops and streamers ... and it's just spectacular.