Staring directly at the sun during a solar eclipse or at any other time can lead to permanent eye damage. The eclipse is only safe to witness with the naked eye during totality, or the period of total darkness when the moon completely covers the sun.

Those eager to experience the eclipse should acquire eclipse glasses from a reputable vendor. Sunglasses are not protective enough, and binoculars and telescopes without a proper solar filter can magnify light from the sun, making them unsafe.

“Please, please put those glasses on,” NASA Administrator Bill Nelson said.

Since counterfeit glasses abound, consider purchasing glasses from a local science museum or order online from a seller cleared on the American Astronomical Society’s website.

Eclipse safety experts say legitimate eclipse glasses should block out ultraviolet light from the sun and nearly all visible light. When worn indoors, only very bright lights should be faintly visible — not household furniture or wallpaper.

Old eclipse glasses from the 2017 total solar eclipse or October’s “ring of fire” annular eclipse are safe to reuse, as long as they aren’t warped and don’t have scratches or holes.

Glasses should say they comply with ISO 12312-2 standards, though fake suppliers can also print this language on their products. NASA does not approve or certify eclipse glasses.

If you don’t have eclipse glasses, you can still enjoy the spectacle through indirect ways such as making a pinhole projector using household materials.

Poke a hole through a piece of cardstock or cardboard, hold it up during the eclipse and look down to see a partial crescent projected below. Holding up a colander or a cracker will produce a similar effect.

Another trick: Peering at the ground under a shady tree can yield crescent shadows as the sunlight filters through branches and leaves.

Eye experts warn against viewing the eclipse through a phone camera. The sun’s bright rays can also damage a phone’s digital components.

Eye damage can occur without proper protection. The sun’s bright rays can burn cells in the retina at the back of the eye. The retina doesn’t have pain receptors, so there’s no way to feel the damage as it happens. Once the cells die, they don’t come back.

Symptoms of solar eye damage, called solar retinopathy, include blurred vision and color distortion.

In a rare case of eclipse eye damage, a woman who viewed the 2017 eclipse without adequate protection came to Mount Sinai’s New York Eye and Ear Infirmary, complaining of a black spot in her vision. Doctors discovered retinal damage that corresponded to the eclipse’s shape.

“The dark spot she was describing was in the shape of a crescent,” said Dr. Avnish Deobhakta, a Mount Sinai ophthalmologist.

There’s no set rule for how long of a glance can lead to permanent damage. Severity varies based on cloudiness, air pollution and a person’s vantage point.

But doctors say looking at a solar eclipse for even a few seconds unprotected isn’t worth the risk. There are reports of solar retinopathy after every solar eclipse, and U.S. eye doctors saw dozens of extra visits after the one in 2017.

Spectators who plan ahead can secure a stress-free eclipse viewing experience.

“It can be dangerous if we aren’t careful, but it’s also very safe if we take the basic precautions,” said Dr. Geoffrey Emerson, a board member of the American Society of Retina Specialists.

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Eclipse-watching tips

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NASA offers the following eclipse tips:

– When watching the partial phases of the solar eclipse directly with your eyes, which happens before and after totality, you must look through safe solar viewing glasses (“eclipse glasses”) or a safe handheld solar viewer at all times. Eclipse glasses are NOT regular sunglasses; regular sunglasses, no matter how dark, are not safe for viewing the sun. Safe solar viewers are thousands of times darker and ought to comply with the ISO 12312-2 international standard. NASA does not approve any particular brand of solar viewers.

– You can view the eclipse directly without proper eye protection only when the moon completely obscures the sun’s bright face — during the brief and spectacular period known as totality. (You’ll know it’s safe when you can no longer see any part of the sun through eclipse glasses or a solar viewer.)

– As soon as you see even a little bit of the bright sun reappear after totality, immediately put your eclipse glasses back on or use a handheld solar viewer to look at the sun.

– Always inspect your eclipse glasses or handheld viewer before use; if torn, scratched, or otherwise damaged, discard the device. Always supervise children using solar viewers.

– Do NOT use eclipse glasses or handheld viewers with cameras, binoculars or telescopes; the concentrated solar rays will burn through the filter and cause serious eye injury. Those require different types of solar filters. When viewing the partial phases of the eclipse through cameras, binoculars or telescopes equipped with proper solar filters, you do not need to wear eclipse glasses. (The solar filters do the same job as the eclipse glasses to protect your eyes.)

– If you don’t have eclipse glasses or a handheld solar viewer, you can use an indirect viewing method, which does not involve looking directly at the sun. One way is to use a pinhole projector, which has a small opening (for example, a hole punched in an index card) and projects an image of the sun onto a nearby surface. With the sun at your back, you can then safely view the projected image. Do NOT look at the sun through the pinhole.

– Seek expert advice from an astronomer before using a solar filter with a camera, telescope, binoculars, or any other optical device. Note that solar filters must be attached to the front of any telescope, binoculars, camera lens, or other optics.

Visit science.nasa.gov/eclipses/future-eclipses/eclipse-2024 for more information.