The nation is preparing for the Aug. 21 “Great American” total solar eclipse, which is the first in 99 years to cross coast-to-coast.
That means buying special eclipse glasses because normal sun glasses – even those with the darkest lenses – aren’t enough to protect eyes from damaging rays.
It’s not that the sun is any stronger during an eclipse, but where you would squint, blink and turn away from the full sun, it can be more comfortable to look at the sun as the moon moves over the bright disk.
That doesn’t mean it’s safe. You can damage your eyes without immediately realizing it if you don’t wear eclipse glasses or look through a special eclipse viewer.
Related: Best places to see the 2017 solar eclipse.
Rick Fienberg, the press officer for the American Astronomical Society, said ordinary sun glasses transmit 10 to 20 percent of the light that falls on them.
This makes the landscape on a bright sunny day easier to look at without squinting, and cuts down on glare.
Eclipse glasses allow just 0.0001 percent of the light that falls on them through.
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“That’s at least 100,000 times darker than ordinary sunglasses,” Fienberg said. “Nothing can get through such glasses except the sun itself – just enough to be comfortable for viewing.”
The only time it’s safe to look at the eclipse is if you are in the path of totality and the fleeting moments when the sun is completely covered by the moon.
Related: Check your eclipse forecast.
About 12 million people live in the path of totality for the Aug. 21 eclipse. Millions more will travel to get into the path.
“The sun can be viewed safely with the naked eye only during the few brief seconds or minutes of a total solar eclipse,” NASA says on its eclipse website. “Do not attempt to observe the partial or annular phases of any eclipse with the naked eye.”
Proper eclipse glasses are marked with ISO (International Organization for Standardization) and 12312-2.
Some older solar-viewing glasses may meet previous standards for eye protection, but not the new international standard, Fienberg said.
NASA recommends glasses from Rainbow Symphony, American Paper Optics, Thousand Oaks Optical and TSE 17.
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Ralph Chou, professor emeritus at the School of Optometry & Vision Science at the University of Waterloo in Canada, told Space.com that he has seen patients with crescents burned into the back of their eyes after watching an eclipse without protection.
“Lifetime exposure to solar ultraviolet radiation is an established contributor to accelerated aging of the outer layers of the eye and skin and the development of cataracts,” Chou wrote in a Sky and Telescope article. “But more immediate damage takes place from directly observing the Sun with inadequate eye protection.”
A solar eclipse seen from space.