On April 8, as millions of people try to watch a solar eclipse sweep over North America, eye doctors across the United States will be on high alert.
That's because, while a solar eclipse is a stunning celestial event, it can also be dangerous. Looking at any part of the exposed sun without the right kind of protection can permanently injure the eye's light-sensitive retina.
And if past eclipses are prologue, it's likely that some eclipse-gazers will show up at doctors' offices with significant eye damage.
In 2017, during the solar eclipse seen across the United States, that happened to multiple people despite abundant media coverage about the danger of looking at the sun when it is anything less than fully and completely covered by the moon.
In New York City, for example, one young woman came to the New York Eye and Ear Infirmary of Mount Sinai, complaining of blurred and distorted vision.
She had peeked up at the crescent sun without eyewear at first, then looked at it longer while wearing what she thought were appropriate eclipse glasses.
"But the problem was she was handed glasses from someone else," says ophthalmologist Avnish Deobhakta, so she didn't know if the eyewear really met safety standards.
Doctors found a permanent, crescent-shaped wound on her retina; there's no treatment for that kind of injury, which is similar to the kind of light-induced damage caused by pointing a laser into the eye.
Other eclipse-related eye injuries were reported in California and Utah.
Given that more than 150 million people directly viewed either a partial eclipse or a total solar eclipse, however, the number who suffered eye problems may seem relatively small.
"We've got less than 100 cases across Canada and the U.S.," says Ralph Chou, an eclipse eye safety expert with the University of Waterloo in Canada.
But no one knows for sure how many people damaged their eyes in 2017, he says, because not every case gets written up for a medical journal, and people may not seek help for less severe vision troubles.
"A lot of them, if they actually happened, were probably relatively minor and, you know, they resolved on their own within weeks or months," says Chou, who says that about half of those who experience significant blurring on the day after an eclipse will recover almost completely.
Some of that recovery may just be the brain learning to compensate and "fill in" the blanks, says Deobhakta, who notes that "there's two eyes, and often there's asymmetric injury. Your brain kind of gets used to it."
He notes that there are ways to enjoy the eclipse without looking up at all; everyday household objects like colanders allow you to create pinhole projectors that let you watch an image of the sun becoming more and more crescent-shaped.
"My advice is to not look at the sun, because you may not realize that it is affecting your retina. It does not hurt. It doesn't burn at the time. It's not as if you feel it," says Deobhakta.
If you do choose to look up at the sun when it is partially eclipsed, says Deobhakta, "make sure you really are sure that you have the standard glasses that have the right filters."
The American Astronomical Society has a list of vetted suppliers.
If you still have reliable eclipse viewers from 2017 that are in good condition, those should still work fine, says Chou.
He notes that eclipse viewers usually have a "best by" date on them, but that is to satisfy European regulations related to personal protective equipment.
"It's essentially meaningless because the filters do not age," says Chou. "If you've taken good care of the viewers from 2017, they haven't been crushed or folded or whatever to damage the mountings, then they're perfectly safe to use for this eclipse."
Despite the warnings, some people try to glimpse the partially-eclipsed sun without eye protection, thinking that a quick look won't cause any harm. While an initial glance at the sun may not cause lasting damage, says Chou, repeated peeks do add up.
"At some point, you may tip yourself over the critical threshold," says Chou. "Unfortunately, you don't realize that until far too late."
The eye damage only becomes apparent hours after it occurs. Typically, people wake up the morning after observing an eclipse and see a spot of extreme fuzziness in the center of their field of vision.
There is one time when it's safe to look up at the sun with the naked eye, experts say, and that's when the sun is totally covered by the moon.
This eclipse phase is only visible from the so-called "path of totality," a stretch of land from Texas to Maine. And the experience of totality doesn't last long — up to four and a half minutes or so, depending on your location.
When the sun is 100% obscured, the sky abruptly darkens and the once-bright sun becomes a dark circle surrounded by a ghostly white ring called the corona.
If people wear super-dark eclipse eyewear during these dramatic moments, they'll miss the whole show.
"People get so concerned to not hurt their eyes, which of course is super important, that they don't take their glasses off when the moon completely covers the sun," says Laura Peticolas, a space physicist at Sonoma State University. "And then they're like, 'I never saw the corona.'"
So knowing when to take the glasses off, and when to put them on, is key.
Chou says that in the last moments before the sun gets totally covered, the thin crescent of the bright sun breaks into discrete points of bright light. These are called "Baily's beads," and they are the last bits of light from the disk of the sun shining through the valleys on the edge of the moon.
"And as they go out, their disappearance is a signal that it is now safe to remove the filters and look at the sun without a protective filter," he explains.
As soon as the sun starts to re-emerge, the glasses need to immediately go back on.
"It is possible to observe the eclipse in perfect safety," says Chou, who has seen 19 total solar eclipses.
He encourages people to go out and enjoy an event that won't happen again in the United States until 2044, even as he realizes that some people will be too fearful of eye damage.
"I recognize that there are going to be people who just don't trust the science and just don't trust the public service announcements and are just going to ignore the eclipse as much as they can," says Chou. "It's an unfortunate thing."
