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.

NASA

"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!"

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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.

The moon's shadow over Madras, OR, as it heads inland as charted with Google Maps by eclipse tracker Xavier M. Jubier. Xavier M. Jubier

The temperature will drop, bright stars like Regulus will appear and the sun's super-heated corona will shimmer into view like a halo around the unseen moon. The light coming in from around the horizon, from beyond the edge of the moon's shadow, will appear in pinkish-orange hues like a 360-degree sunset.

"It's not as dark as night, for the most part," said Rick Fienberg a spokesman for the American Astronomical Society who holds a doctorate in astronomy. "This one, the shadow's only about 70 miles wide. So 35 miles away from you, in every direction, it's light. The sky during a total solar eclipse is not black, it's blue."

But the star of the show, so to speak, is the sun's corona, a magnetically energized region above the sun's visible surface -- the photosphere -- where the temperature suddenly -- and inexplicably -- climbs from about 10,000 degrees Fahrenheit to nearly 4 million degrees.

A sequence shot from the deck of a cruise ship off the coast of Africa in November 2013 showing the various stages of a solar eclipse, starting with a partial eclipse at upper right, through totality and then back to partial stages as the moon moved along in its orbit. Rick Fienberg / TravelQuest International / Wilderness Travel

The solar wind, a torrent of electrically charged particles, is accelerated to enormous velocities in the corona, spewing into space in all directions and reaching the farthest corners of the solar system. What drives the corona's extreme temperatures and accelerates the solar wind is a mystery, in part because the region is so difficult to study.

The inner regions of the corona can only be studied during a total solar eclipse when the moon completely blocks the sun's glare. Satellite instruments and ground-based telescopes create their own artificial eclipses, inserting a circular sun shade into a camera's light path that acts like a miniature moon.

But to keep from blinding sensitive detectors, such coronagraphs must block the sun's light out to more than a solar diameter. During a total solar eclipse, the actual moon does a much better job, revealing the corona's mysterious innermost regions.

"What's particularly special about a natural eclipse is that the moon is a perfect occulter, it blocks the surface of the sun just perfectly so you can see very low into the solar atmosphere," Black said. "Scientists are particularly interested in the low corona, because that's where lots of activity is, that's where the origins of space weather are."

With the Aug. 21 eclipse's coast-to-coast track across America, astronomers and citizen scientists will have a golden opportunity to learn more about the processes at work in the corona as millions along the path of totality get a chance to witness the sun's fiery halo.

A total solar eclipse occurs when the moon's orbit carries it directly between the Earth and sun, casting a shadow along the path of totality. NASA

"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.

"And the light, the quality of the light is very unusual, too," he added. "You're looking at glowing ionized gas, which is like what's in a fluorescent bulb or something, it's a mix of that and a mix of sunlight reflected off electrons and sunlight reflected off dust in the solar system, so it's a very different kind of light than ordinary daylight."

Unlike the partial phases of a solar eclipse, when even a sliver of sunlight can cause eye damage if viewed for extended periods or any sort of optical aid, the fully eclipsed sun is safe to look at during the few minutes the moon completely covers the sun's disk.

Never look directly at the sun during an eclipse, experts warn, except during the few minutes the sun is completely blocked by the sun along the "path of totality." At any other time, viewers should use inexpensive "eclipse glasses" or other filters that meet the ISO 12312-2 standard. Jay M. Pasachoff

"The total amount of light coming from the eclipsed sun, that's about as much light as comes from a full moon," Fienberg said. "That's why we keep telling people, it's perfectly safe to look at the totally eclipsed sun. Once every last part of the sun's bright everyday face is covered, what's left, the corona, is about as bright as the full moon. So it's no more dangerous to look at."

Fienberg will be in Madras with a tour group, his 13th trip to witness a total solar eclipse. His advice to first-time eclipse watchers: don't worry about telescopes or sophisticated optics. Just take in one of nature's grandest shows with the unaided eye.

"If you just look at it with a telescope, you're going to see the solar corona and you're going to see all kinds of interesting loops and streamers and things, but you're going to miss the broader effect. You might not see the sunset colors around the horizon, you might not realize how blue the sky is because in the telescope it'll just look black," he said.

"You won't see bright stars and planets that come out around the sun because they're going to be outside the field of view of the telescope. I'll tell you what I'm going to do. I'm going to alternate between using low-power binoculars and my naked eye."

The view in binoculars is "really breathtaking," he said, "because you see a lot more detail in the corona, but you also see it in context, you see the whole sun and the whole corona and you get the bluish sky in the background."

While it might seem contrary to countless warnings, using binoculars during totality, when the sun's light is completely blocked, is not dangerous in and of itself. And the observer will see clear signs totality is ending in plenty of time to look away and switch back to a filtered view.

But aside from the few minutes of totality, one should never look at the sun directly, experts warn, with or without optical aid, even during a nearly full partial eclipse, without using an internationally certified filter designed to reduce the sun's glare to a safe level.

"There's probably going to be an uptick in eye injuries no matter how much you tell people and how easy it is to get your hands on solar filters," Fienberg said. "There are always people who will either ignore the messaging or just take unnecessary risks. But ... it's really hard to hurt yourself by looking at the sun too long. It's very uncomfortable."

Cross-country path of eclipse

From Madras, the moon's shadow will race across Oregon and into Idaho, passing just north of Boise before moving on across Idaho Falls at 11:33 a.m. local time (1:33 p.m. EDT), Casper, Wyoming, at 11:42 a.m. (1:42 p.m. EDT), Grand Island, Nebraska, at 12:58 p.m. (1:58 p.m. EDT), St. Joseph, Missouri, at 1:06 p.m. (2:06 p.m. EDT) and nearby Columbia six minutes later. By this point, the shadow will have slowed to about 1,500 mph.

Residents of the northeast corner of Kansas City, just inside the path of totality, will enjoy about a minute of darkness around 1:08 p.m. (2:08 p.m. EDT) as will residents in southwestern St. Louis a few minutes after that.

Crossing the Mississippi River, the center of the moon's shadow will pass just south of Carbondale, Illinois, at 1:20 p.m. (2:20 p.m. EDT) before moving over Paducah and Hopkinsville, Kentucky, at 1:24 p.m. (2:24 p.m. EDT).

Along this stretch, eclipse watchers will enjoy the maximum duration of the eclipse, about two minutes and 40 seconds of totality.

From Kentucky, the shadow will move across Clarksville and then Nashville, Tennessee, the largest city in the path of totality, at 1:27 p.m. (2:27 p.m. EDT) crossing over the Smokey Mountains, the southwest corner of North Carolina and then racing over the heart of South Carolina where viewers in Greenville, Columbia, Charleston will enjoy totality between 2:38 p.m. and 2:47 p.m. EDT

The moon's shadow then will move off shore and out over the Atlantic Ocean, one hour and 33 minutes after the umbra crossed the coast of Oregon some 2,500 miles away.

Given its coast-to-coast track through the heartland of the United States, many observers believe the 2017 eclipse will be one of the most viewed in history and certainly the most scientifically studied.

"The other reason this is going to be so well observed is because we now live in a social media era," Fienberg said. "Before, there were no smartphones, there was no Facebook, there was no Twitter. You have to literally be locked in an underground bunker to not be able to find out what's going on outside.

"So, everybody's going to hear about this, one way or another, and with any luck, they're going to get excited. ... Even if they don't make it into the path of totality, a deep partial eclipse, observed safely, can be fun."

Williams College astronomer Jay Pasachoff, author of an article outlining planned eclipse research in the August issue of Scientific American magazine, plans to observe his 66th solar eclipse from the campus of Willamette University in Salem, Ore., along with an international team of researchers and a battery of telescopes, cameras and spectrographs.

"We've done preliminary versions of this before, but things get refined, the theoretical models get refined and the instruments get better," he said in an interview. "We only have about two minutes, so you do the measurements you can. Each time we have an eclipse, the loops of gas, the shape of the corona, the magnetic field is different. So you want to take those couple of minutes whenever you can and find out what's going on there."

Pasachoff team is one of scores that will be stationed along -- and above -- the path of totality. At least 11 NASA spacecraft will train their instruments on the sun, more than 50 high-altitude balloons will take photographs and study the effects of the eclipse on Earth's atmosphere, along with aircraft carrying a variety of instruments.

Hundreds of ground-based observatories will swing into action with numerous focused teams of researchers spread out along the path of totality. Veteran eclipse astronomer Shadia Habbal of the University of Hawaii will lead five such teams that, like Pasachoff's, will be studying the inner corona.

Penn is leading a team made up of more than 200 volunteers using identical telescopes, cameras and computers at 68 locations along the path of totality.

The goal of Penn's Citizen CATE (Continental-America Telescopic Eclipse) project is to assemble a carefully calibrated movie of the eclipse to learn more about how the sun's magnetic field accelerates particles in the corona. Data will be collected using identical telescopes, cameras and computers operated by volunteers, including students and astronomy club members.

"The thing that excites me the most about this eclipse is that millions of people across the country, in the path of totality, can walk out on their porch in their slippers and collect world-class data," Penn said. "They won't have to travel to a large telescope on top of a mountain, you can go into your backyard using fairly modest-size instruments and collect really important research data."

A more public project, the Eclipse Megamovie, is led by Hugh Hudson, a solar physicist at the University of California at Berkeley. His team is soliciting imagery from volunteers "as an outreach thing," he said, "and make a big movie that incorporates all the images."

"It will be complicated, but in principle you can ... create an actual movie," he said. "That's never been done before as far as I can tell, and this is a wonderful opportunity for us."

In an interview, Pasachoff advised the public to take in the partial phases of the eclipse with low-cost solar filters. For viewers in the path of totality, "just keep looking through them and when that presence diminishes and becomes invisible through the glasses you put the glasses down and watch for a couple of minutes."

"If you want to look up in those two minutes, which is between two minutes on the Oregon coast on the central line and two minutes 40 seconds in Illinois, then you can glance with binoculars for a few seconds to see some of the detail in the corona. But it's also nice to look around the sky and see what stars and planets you can see and look at that sunset effect on the horizon all around you."

The shape of the corona changes during the sun's 11-year sunspot cycle as the number of spots wax and wane. The sun currently is moving into the minimum phase of the cycle "and that means that the corona will be very extended at the sun's equator and it won't hide the plumes of hot gas, the million-degree-gas, that's coming out of the poles," Pasachoff said.

"So we'll have an opportunity to study this gas at the poles for the first time in about eight or 10 years."

For eclipse novices taking in their first total solar eclipse, Pasachoff said astronomers will "be glad to give you our pictures."

"Don't cut into your enjoyment by being too busy with your camera, just watch," he said, echoing Fienberg's advice.

Does the view ever get old?

"No, it gets better and better," he said.

Will weather conditions cooperate?

But all that assumes the weather cooperates. Canadian meteorologist and eclipse enthusiast Jay Anderson has produced a chart based on more than 15 years of data to give a sense of how much cloud cover might be expected in late August along the path of totality.

In Madras, for example, eclipse chasers can expect average cloud cover between around 15 and 20 percent. Moving east, cloud cover generally increases depending on topography and other factors. In Nashville, up to 70 percent of the afternoon sky can be cloud covered.

But as Fienberg noted, climate is what one expects, weather is what one actually gets, "the idea being that you can predict the statistical likelihood of clear skies based on past history, but weather is a very local phenomenon."

That said, "the northwest is going to have a much better chance of clear skies than the southeast."

"And so, many of the people who are going to be making a sincere effort to see totality are going to be heading to the northwest," he said. "Oregon is very popular, Wyoming, Idaho and Nebraska, all through those states the path of totality is predicted to be 80 percent or 70 percent chance of clear skies. The only exception being the Oregon coast itself."

The weather might be better in the northwest, but because of the moon's orbital track and changing distance from the Earth, totality will last longer in the southeast.

"Two minutes and 40 seconds in southern Illinois and western Kentucky, whereas it's only going to be a couple of seconds over two minutes in Oregon," Fienberg said. "You want to go where the eclipse will be long, so you tend to go to the point where it's maximum and as close to the center of the path as you can. But you also want to go where the weather prospects are good.

"In this case, a lot of people are going to the northwest," he said. "Even though the eclipse won't be as long, the chance of clear skies is better."

And if the weather doesn't cooperate, your car breaks down or your flight is canceled before you can reach the path of totality, you'll get a second chance in 2024 due to the nature of the moon's orbit and Earth's path around the sun.

During the 2024 eclipse, the moon's shadow will cross the U.S. border in southern Texas and move up into the eastern half of the United States, passing over Dallas-Fort Worth, Cleveland, Ohio, Buffalo, New York, and Montreal.

The moon will be closer during the 2024 eclipse, its shadow will be wider and totality will last more than four minutes along most of its track across the United States. The paths of the 2017 and 2024 eclipses will cross near Carbondale, Illinois, where residents will witness their second bout of totality in just six and a half years.

"So if you can't get to totality this time, you're going to see all the pictures and movies on the internet, you're going to hear from people who went, you're going to be really motivated for the next one," Fienberg said. "And you've got six and a half years to plan for it."