On August 21, Americans in the right place at the right time will have a chance (weather permitting) to witness a total solar eclipse. This means that, for up to 2 minutes and 43 seconds (depending on one's location), the moon will completely block out the sun, and the wispy, ephemeral glow of the sun’s corona, its outer atmosphere, will be visible.

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A total solar eclipse happens about once every 18 months somewhere on Earth, says Nicholeen Viall, Ph.D., a NASA research astrophysicist who studies the sun. “But most of the world is ocean,” she notes. The last total eclipse over the continental U.S. was in 1979, and the next one won’t be until 2024, so you may want to try to catch a glimpse of this one.

This summer's total solar eclipse will be visible along a narrow band from Oregon to South Carolina, but everyone in North America will have a chance to catch at least a partial eclipse. The closer you are to the “path of totality,” the more of the sun the moon will cover up.

“An eclipse is one of nature’s greatest shows, and people want to see it,” says Joel Schuman, M.D., chair of the department of ophthalmology at NYU Langone Health. But looking directly at the sun can cause irreparable damage to your eyesight.

Fortunately, there are simple ways to protect yourself. Here’s what you need to know about the dangers of solar-eclipse-gazing, and what to do to be safe.

What Happens When You Stare at the Sun

Your pupils expand and contract depending on the amount of light that comes in. But the intensity of the light when you look directly at the sun—even when it’s partially covered by the moon—isn’t something your eye is equipped to handle.

When you look at the sun, your eye's lens concentrates the light energy on your retina, the nerve tissue at the back of your eye that allows you to see, Schuman says. Look for just a second or two, and you’ll see temporary spots in your visual field that fade. This happens when you look at any very bright light. But look for a few seconds longer, and the sheer amount of light energy focused on one spot can burn your retina, permanently damaging it.

“If the retina is damaged, you won’t be able to see in the area of that damage,” Schuman says. You wouldn’t lose your peripheral vision, but you can permanently lose or impair the sharpest part of your field of vision, the part you use for reading, driving, and many other important tasks.

You don’t normally have a reason to stare at the sun. During an eclipse, though, you might think that looking at the partially blocked sun is not so risky. But staring directly at even a partly obscured sun has been known to damage people’s eyes, says Schuman.

And a study in the Journal of Optometry detailed four cases of people who looked too long at the sun while trying to see a solar eclipse in 2011 that was visible in Europe, North Africa, and Central Asia. Three of those people took weeks to regain their normal eyesight; the fourth one's vision was permanently impaired.

Even if you'll be in the path the total solar eclipse will take, you need to take these precautions to protect your eyes during every moment other than when the moon completely blocks the sun.

How to Safely View the 2017 Solar Eclipse

The most important thing to remember is that you shouldn't view the eclipse without carefully protecting your eyes.

There are two main ways to do this. First you can view the eclipse directly if you're wearing special protective eyewear, or you can view the eclipse indirectly, using a projection device like a pinhole camera.

Direct viewing. One simple way to look at the eclipse is to use “eclipse glasses,” which have a special solar filter that makes it safe to look directly at the sun. According to Viall, museums and other community centers may be handing out free glasses in advance of the event. You can also purchase them online, but you need to order them from a trusted manufacturer.

Some sites are taking advantage of the demand for eclipse glasses by selling counterfeit ones that may not protect your eyes. The American Astronomical Society (AAS) has published a guide to finding approved glasses, and you can visit the the AAS’s site here for a list of approved makers and vendors.

Keep in mind that even though a seller may claim to offer glasses that meet the international safety standard for solar filters—ISO-12312-2—anyone can print that code on their product without actually having tested the safety of their glasses.

You should also check your eclipse glasses to make sure they’re free of scratches or any damage. And don’t look through a camera, binoculars, or a telescope while wearing the glasses, since the lenses in these devices will concentrate the light enough to damage the glasses’ filters and your eyes. (If you have a telescope at home, you may be able to purchase a sun-viewing filter attachment specifically for looking at an eclipse.)

Some welding glasses are also safe to use, but only if they’re made with #14 welder’s glass. A lighter shade of glass isn’t safe.

Most important, don't rely on your own sunglasses. No matter how dark they are, they’re not going to be dark enough to protect your eyes.

In fact, all these viewing devices are so dark you won’t be able to see anything through them—except for the sun.

Indirect viewing. You can easily create a simple device to project an image of the sun onto a surface. The simplest kind is a pinhole camera, which passes light through a small hole and projects an image of the sun onto a surface. Here are directions from NASA Jet Propulsion Laboratory on how to do it:

  • Take two sheets of stiff white card stock and cut a square hole in the middle of one of them.
  • Tape aluminum foil over the hole.
  • Use a pin or paper clip to poke a hole in the foil.
  • Place your second piece of card stock on the ground and hold the piece with the foil above it with the foil facing up.
  • Stand with the sun behind you and view the projected image on the card stock below.
  • Adjust until you can see a small circle of light, projected from the pinhole, on the other sheet of paper. That’s a tiny image of the sun, which will become crescent-shaped as the moon moves across it.

You can make more sophisticated versions of these types of projectors. For example, this cereal box pinhole camera (PDF) projects the sun’s image on the inside of a cereal box, blocking out other light and allowing you to see more clearly. Here are a couple more guides to making a projector you can try:

What If You’re in the Solar Eclipse Sweet Spot?

If you live inside or are planning to travel to the path in which it’s possible to see the total solar eclipse, there is a small window of time in which you can take off your protective eyewear and look directly at the eclipse. You can only do this safely when the moon entirely covers the sun.

To find out precisely when that will be, NASA has provided this interactive map. Find where you’ll be located on the map and click to see the start and end times of the total solar eclipse. Give yourself several seconds of cushion.

Want to watch the total solar eclipse? This NASA map shows the path of the event.
Illustration: NASA's Scientific Visualization Studio