The detailed map from NASA identifies the eclipse’s exact path of totality through U.S. and
Georgia on Aug. 21. NASA
On Monday, Aug. 21, a swath of the U.S. will experience a total solar eclipse, spurring gatherings around the country to watch the rare celestial event. Here are a few facts for skygazers:
What is a solar eclipse and why is this one special?
A solar eclipse happens when the moon passes between the sun and the earth, temporarily blocking the sun and plunging areas of the earth into the darkness of the moon’s shadow. They happen every year or two around the globe, but only very rarely in any one particular location. A total eclipse was visible in the Pacific Northwest in 1979, but this is the first time since 1918 that a total eclipse will occur coast to coast across the U.S. The next one in the U.S. will be in 2024, cutting a smaller path.
During the Aug. 21 eclipse, a narrow band of the country from Oregon to South Carolina will experience so-called “totality” in which the moon completely covers the disk of the sun. The rest of the contiguous 48 states will experience a partial eclipse covering at least 50% of the sun. The totality will pass through 14 states.
How long will it last?
It will take about an hour and a half for the moon’s shadow to traverse the country from west to east. As the shadow moves across the U.S., locations along the “path of totality” will experience complete darkness generally lasting over two minutes, with the longest period near Carbondale, Ill., at 2 minutes and 40 seconds.
The eclipse will happen in five stages as the moon begins to encroach on the sun, covers it and then moves off, starting a little after 9 a.m. Pacific time on the coast of Oregon. There, totality will begin a little after 10:15 a.m. Pacific time and last for about two minutes, and the eclipse will be over by about 11:40.
The 2017 Eclipse
If skies are clear during the Aug. 21 solar eclipse, the entire U.S. will be able to see at least partial obscuration. Percentage of the sun’s area that will be covered by the moon during the event:
The eclipse will finish its continental journey on the coast of South Carolina, where totality will start after 2:40 p.m. Eastern time. The tail end of the shadow will leave land a little after 4 p.m. Eastern time and will pass out to sea.
Why do I need special glasses to look at it?
It isn’t so much the eclipse itself that poses a risk; it is the act of staring at the sun. During actual totality, when the sun is completely covered, it is safe to look, but still risky. That’s because if you watch during the partial phases, even when the sun is 99.5% covered, solar radiation could harm the light-detecting cells in the back of the eye, known as photoreceptors.
“The amount of energy that is pouring [in]…is just too much for the photoreceptors to handle” and can lead to a so-called retinal burn, said Ralph Chou, a professor emeritus of optometry and vision science at the University of Waterloo in Canada.
Intense amounts of light set in motion chemical reactions that can kill or injure photoreceptors, leading to vision loss that can be permanent in the worst cases, he said. Because the back of the eye lacks pain sensors, you may not know they have suffered a retinal burn until hours later.
To be safe, viewers should wear special eclipse glasses, which filter out tens of thousands of times as much light and radiation from the sun as sunglasses. When wearing the proper type of eclipse glasses, you shouldn’t be able to see anything other than the sun and other sources of extremely bright light.
Counterfeits have been flooding the market, so be careful of fakes. The American Astronomical Society lists on its website trusted brands and vendors. Legitimate glasses should have the manufacturer’s contact information printed on them. Some knockoffs have disclaimers printed on them or let in too much light.
Skygazers should watch for the sun’s so-called Bailey’s Beads, formed by sunlight peaking through the moon’s rugged topography. The sun’s atmosphere, or corona, soon leaps into view after that. During the totality phase, only the corona is visible. After totality, Bailey’s Beads come into view again, and “that’s when you have to get the viewers back in place,” said Dr. Chou.
Experts warn that people shouldn’t look through “unfiltered” lenses, such as binoculars or camera viewfinders, even if they are wearing eclipse glasses.
“Binoculars and telescopes gather lots more light than our eyes and concentrate it into a very bright image,” which can be dangerous, said Rick Fienberg, an astronomer and a spokesman for the American Astronomical Society. “You need to block most of that light before it enters the optics.”
Eclipse glasses are sold out! How can I watch?
“If you don’t have the right glasses, don’t stare at the sun,” said Bill Nye, the CEO of the Planetary Society. Instead, try making a pinhole viewer, he said.
You can make one out of a piece of paper or cardboard. Punch a small hole and hold it in front of you with the sun behind you, allowing the light to shine through the hole. You can see an image of the sun on any surface the light shines on—another piece of paper or even the sidewalk.
Here are instructions for making a slightly more ambitious pinhole viewing box.
Can I take a picture of it?
Yes, but make sure your equipment has the proper filters. Concentrated, unfiltered sunlight can damage your single lens reflex camera’s sensor and cause eye damage when viewed through the optics of a range finder camera. Apple says photographing the eclipse won’t damage an iPhone’s camera. But you’ll get a better shot by holding a pair of eclipse glasses in front of the lens as you snap the shutter.
According to NASA, the filter will eliminate the “sun blooming” effect—which results in a disappointing photo of a bright blob.
If you want to get fancy, check here for more tips.