Total Solar Eclipse 2017: How to Watch Safely and When

7-24-17 Total Solar Eclipse Safety
A girl looks up to try to view a partial solar eclipse outside the Planetarium of the Royal Observatory of Belgium, in Brussels on March 20, 2015. Yves Herman/Reuters

The total solar eclipse being called the "Great American Eclipse" will pass through 14 states from Oregon to South Carolina on August 21. Though eclipses are not rare per se, it's uncommon for a total solar eclipse to sweep across the third most populous country in the world.

The mainland United States has not experienced such a celestial event since 1979. The rarity of these events means many of us may not be aware of the potential dangers. Fortunately, NASA and other experts are here to help.

Related: Total solar eclipse 2017: Bill Nye on how, when, where and why to watch

Watching an eclipse can be a mesmerizing, unforgettable event, but it can also cause permanent eye damage without the proper safety precautions. With the countdown just past the one-month point, NASA has published a set of safety tips for those who are planning to watch, so that viewers have the right safeguards in place before they become transfixed by the incredible sight.

"NASA isn't trying to be the eclipse safety glasses 'police,'" Alex Young, associate director for science in the Heliophysics Science Division at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. But "it's important that individuals take the responsibility to check they have the proper solar eclipse viewing glasses."

The only safe way to look directly at the sun is with special solar filters. Those can come in the form of glasses or handheld solar viewers. NASA's guidelines advise viewers to use only glasses or viewers with certification information with a designated ISO 12312-2 international standard. They should also have a manufacturer's name and address printed on them.

People also should ensure that their glasses or viewers are not older than three years and don't have scratched or wrinkled lenses. NASA warns against using any homemade filters and ordinary sunglasses, even if the lenses seem very dark.

Related: Total solar eclipse viewing 2017: Where to buy glasses and how to make a homemade viewer

Alan MacRobert, senior editor at Sky & Telescope magazine, says that staring at the sun is akin to using a magnifying glass to set paper on fire. "Your eye lens is magnifying glass, and your retina is the piece of paper."

Watching an eclipse without the appropriate protection can cause solar retinopathy, which the American Academy of Ophthalmologists describes as an injury to retinal tissues commonly associated with sun gazing or eclipse viewing that can result in impaired vision. Recovery is unpredictable and uncertain. Sometimes, the damage can be permanent.

But there is one safe time during the eclipse: the main event. When the moon is blocking the sun completely, the danger of the retina burning is absent. "The main thing is if you can see any of the surface of the sun directly, don't look," says William Keel, a University of Alabama astronomer.

"During totality, look. That's what you're there for," he says, explaining that those who are in the eclipse's path of totality can remove their glasses once the moon is completely blocking, or eclipsing, the sun. "Even out of the corner of your eye, you will see that last brilliant spark of the sun dying away," he says. The diamond ring effect, which occurs just before totality, is characterized by an "intensely brilliant glare on one side."

Even when a sliver of the sun is still visible, unprotecting gazing is unsafe. But during the roughly two and a half minutes of totality, when daylight briefly turns to twilight and the temperature drops, eclipse watchers can set their glasses aside until the diamond ring effect returns on the opposite side.

The total solar eclipse will take place at different times that Monday, depending on the location. In Madras, Oregon, for example, the eclipse will begin at 9:06 a.m. PDT and end at 11:41 a.m., with totality occurring between 10:19 and 10:21 a.m. On the other side of the country in Columbia, South Carolina, the eclipse will begin at 1:03 p.m. EDT and end at 4:06 p.m., with totality occurring between 2:41 and 2:44 p.m.

Alternatives to Glasses

There are alternatives to special solar glasses. Amateur and professional astronomers who want to use a telescope, binoculars or a camera should make sure to use a special filter. If you're not sure whether it's properly filtered for sun gazing, it's probably not, Keel says. Such filters should be installed on the front of the instrument, not where people peer through, he adds. That's the wrong side to try to filter the sunlight, and the little glass filters that screw into the eyepieces can break without warning.

Fred Espenak, a retired NASA astrophysicist at the Goddard Space Flight Center and an eclipse expert, warns in his safety guide that solar filters for telescopes and cameras, which offer adequate protection, should not be confused with eyepieces. The latter have a tendency to absorb heat and crack, which would allow concentrated sunlight coming through the telescope to enter the eye.

NASA cautions against any homemade filters. But die-hard DIY enthusiasts can still make their own pinhole projector. The technique is simple: The sun streams through a tiny hole in a piece of paper or cardboard and projects an image onto a flat surface, enabling viewers to see the eclipse indirectly. Keel points out that in areas with many trees, the leaves can create natural, accidental pinholes, projecting images of the sun onto the ground. Eclipse watchers in such environments might notice the ground covered with a pattern of crescents as the moon begins to obscure the sun.

Eclipse watchers can also use a telescope or a pair of binoculars to project an image of the sun onto a makeshift screen behind it. MacRobert reminds watchers not to look through such instruments but to hold them out and aim them toward the sun to let the light flood through.

Finally, MacRobert urges eclipse watchers not to forget common sense. If you're en route while the eclipse is taking place, he says, "don't watch sky while driving a car. Watch the road ahead."