by William Harwood: 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.
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 .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 inexpensiveto 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.
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.”
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, 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.
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.
“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 florescent 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, whenif 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.”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.”