When the moon begins its three-hour traverse across the sun on Aug. 21, millions of people throughout the U.S. will put life on hold to gaze up and bear witness to the first coast-to-coast total solar eclipse in nearly a century. For many, the experience will be life-changing; for still others, it will result in serious eye damage or blindness.

"The thing is, this eclipse will be so fascinating, I’m telling you, you’ll be amazed," Bill Nye the Science Guy recently told Newsweek. "So the danger is, you just stare at it, you get transfixed. It’s something that’s easy to avoid. 'It hurts when I do that,' so don’t do that. But it’s so striking, it’s so out of your everyday experience, that the danger is people just don’t look away."

That danger can lead to something called "solar retinopathy," when bright light from the sun overwhelms the retina on the back of the eyeball. This in turn burns the retina and damages the light-sensitive tissue that helps you see. Since none of us actively go looking for an excuse to stare at the sun during our normal day-to-day, solar retinopathy is generally associated with people incorrectly looking at the sun during a total solar eclipse.

The risk is significant enough that many schools are closing for the day or keeping students inside for recess. In many parts of the U.S., the eclipse is expected to peak around the time of dismissal, and school officials don't want children watching it without proper eye protection.

How to avoid burning your retinas

A map showing how the Great American Eclipse will appear from different points in the United States. A map showing how the Great American Eclipse will appear from different points in the United States. (Photo: NASA)

For those in the path of totality, a narrow band some 70 miles wide where the moon will completely cover the sun, a few minutes will occur where viewing the solar eclipse without safety glasses will be possible. Unfortunately, this brief safety window will not exist for a large majority of the rest of the viewing public. If you're not in the path of totality, you're going to want to use special safety glasses to protect your retinas.

“Even when 99 percent of the sun is blocked out by the moon, the amount of light is still 10,000 times stronger than a full moon," Alex Young, associate director for science in the heliophysics division of NASA’s Goddard Space Flight Center," told The Atlantic. "So even when there’s 1 percent of the sun still visible, it’s still too bright."

As for those lucky enough to find themselves in the path of totality, do not let the moments before and after totality beguile you into believing you're 100 percent safe. As RG Coleman of the site Chasing Totality warns, removing your glasses too soon can ruin the rest of the experience.

"Solar retinopathy can occur at any point during the partial phase — including the final seconds before totality — and the most important reason to shield your eyes occurs in the very last two or three seconds before blackout: looking directly at the sun during this time can cause temporary vision distortion that can bork your enjoyment of the scant, precious minutes of totality," he writes.

Where can I get eye protection?

Special 'eclipse glasses,' which filter out harmful infrared, ultra-violet, and intense visible light, are required accessories for proper viewing of solar eclipses. Special eclipse glasses, which filter out harmful infrared, ultraviolet, and intense visible light, are required accessories for proper viewing of solar eclipses. (Photo: Ken Lund/flickr)

If you have solar eclipse glasses, then good job planning ahead! If you don't, try these DIY options instead. It's important to note that regular sunglasses will not work one bit as they lack the protective solar filters for blocking out the sun's rays. It's also not a good idea to hack together glasses using homemade materials or using alternatives like standard welder's glasses.

NASA recommends looking for language that specifies that the solar eclipse glasses meet the ISO 12312-2 international standard, as well as a clearly marked manufacturer’s name and address printed somewhere on the product. If you have an old pair of eclipse glasses gathering dust from some earlier solar event, toss them. NASA recommends anything over three years old, with wrinkled or scratched lenses, be replaced.

Solar eclipse glasses range from your basic (top, $0.56) to your more stylish (bottom, $22).

Solar eclipse glasses range from your basic (top, $0.56) to your more stylish (bottom, $22). (Photo: Amazon)

“These things block out such a huge amount of light they’re hundreds of thousands of times stronger than regular sunglasses,” Young added. “When you look outside you only see the sun.”

Be on the lookout for knockoffs, warns the American Astronomical Society (AAS). Reuters reports that bogus glasses are on the market:

The lenses of some obvious fakes allow the penetration of light from such relatively faint sources as fluorescent lamps, while the only thing one should see through authentic solar-safe filters when looking at objects fainter than the sun is pitch blackness. Other bogus glasses have come stamped with forged logos of reputable manufacturers or with phony safety labels.

How do I know I've hurt my eyes?

As the video above explains, solar retinopathy can cause blurred vision, a central black spot, sensitivity to light, headaches, or complete blindness in one or both eyes. Recovery from the damage can take anywhere from a few hours and days to several months. A study on solar retinopathy after the 1999 solar eclipse in East Sussex, U.K. found that six months appeared to be the average, though the researchers did note that some patients suffered long-term damage.

"Visual acuity can improve considerably in the majority of eclipse-related solar burns," they wrote. "However, the persistence of visual symptoms in those with mild burns would suggest that even brief glimpses of a solar eclipse should be avoided."

Don't risk it –– wear your glasses.

Editor's note: This story has been updated since it was originally published in July 2017.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.