Total solar eclipse mania is settling in around the world.
It’s the first total solar eclipse in almost 100 years, making it a once-in-a-lifetime event for most of us.
Millions of people will be in at least partial view of the August 21 solar eclipse, with millions more traveling to somewhere with the "path of totality," or the 100-mile wide strip from Oregon to South Carolina where the moon will completely engulf the sun.
It’s sure to be an awe-inspiring sight, but looking directly at it could actually damage your eyesight, according to NASA.
Why looking at the eclipse can damage your vision
"Danger to the eye is from infrared radiation (heat), ultraviolet radiation (IV) and from excessive blue light," Dick Land of The Schepens Eye Research Institute at Harvard University wrote in an article. "The heat risk is perhaps the best understood, since we are familiar with using a lens to focus the sun to burn paper."
A quick glance of the sun on the normal day does put heat on the retina, but our natural reflexes make us turn around before any damage occurs, other than a brief image of the bright light when we divert our eyes.
However UV rays can affect the eyes much like a sunburn, but instead of peeling skin they cause pain and vision loss.
"The retina is at risk from a very small part of the UV that is transmitted through the ocular media and lens," Land wrote. "This risk is greatest for young eyes, and in general adults beyond 30 years of age have enough yellow in the lens and absorption in the media that UV after atmospheric absorption is less of a problem than the heat.
Blue light is the least understood of the three types of light, but more evidence shows that "excessive exposure to blue light may, in most individuals, result in Macular Degeneration and blindness in people when they become older."
Damage might not be apparent right away.
"I made the mistake of looking at it with my naked eye," Bill Hanlon told The Express of the 1952 eclipse that seriously damaged his eyesight. "People warn you about the dangers, but I didn't think anything would happen to me." He was only 13 at the time. "As a result I have only got about 10 per cent of my vision left because the light burnt through the centers of both my eyes," he told the newspaper.
"I can make out shapes and colors but everything else is gone, I can't see people's faces."
Can you look directly at the total solar eclipse?
It’s safe to look at the total solar eclipse — or the moment when the moon completely passes in front of the sun — without solar eclipse glasses.
"As the moon moves in front of the sun, there comes a time when several bright points of light shine around the moon's edges," NASA advises on its website. "Known as Baily's Beads, these are light rays from the sun streaming through the valleys along the moon's horizon."
The beads disappear as the moon continues to move until there’s only a bright spot that looks "like a giant diamond ring," according to NASA. "It is still not safe to look at the sun at this point! Only when that bright spot completely disappears can you safely look at the sun."
This period is short, so be sure to keep your eye on the moon as it moves from its place in front of the sun. Put your glasses back on when that happens, or use an indirect method to watch, like by looking at nearby trees.
Where to buy solar eclipse glasses
Retailers looking to cash in on eclipse mania are now selling the special glasses you need to view safely, but NASA says that not all glasses are safe.
It has come to our attention that some vendors (including apparently Amazon), are selling solar viewing glasses that are unsafe," the organization wrote in a Facebook post. "Some are specifically being marketed to children. We at NASA are concerned that people may have purchased some of these for use at event sites."
They’re legit if they’re made in the United States by Rainbow Symphony, Thousand Oaks, TSE or American Paper Optics, according to NASA. Look for "ISO 12312-2" or "ISO 12312-2:2015" printed on the filters, and don’t use if your filters are torn, scratched or damaged in any way. "No glasses made in China are safe."
Space.com columnist Joe Rao says that you can also head to the hardware store and pick up the same glasses welders use — welder’s glass no. 13 or no. 14 — to safely view the once-in-a-lifetime event.