Reusing eclipse glasses? Here’s how to make sure they’re still safe to use

The total solar eclipse on April 8 is not to be missed. The last such event that will be visible from North America for 20 years, it’s a chance for millions of people to catch an awe-inspiring sight, and many won’t even have to leave their backyards.

But while a total solar eclipse may be a rare occurrence, partial and annular eclipses are more common. So many people may have a pair of eclipse glasses (or a handheld eclipse viewer) tucked away in a drawer somewhere.

But are those glasses still safe to use? Here’s how to check. 

How to tell if your old eclipse glasses are still safe

There are two factors to consider when assessing a pair of previously-used eclipse glasses. The first is whether they were safe when you purchased them. The second is their current state.

The latter is easier to explain, so let’s start there. 

Hold the lenses up to a light. (Not the sun!) Inspect them for scratches, punctures, tears, or any other damage. Then check to make sure that the lenses aren’t coming loose from the frames. If there’s any issue, discard the glasses; they are not safe to use.

If they appear undamaged but are smudged or dusty, you can clean them gently with a microfiber cloth. Then check them again. If they still appear undamaged, then you’re in the clear – that is if they were safe when you acquired them.

How can you tell which eclipse glasses are safe?

Eclipse glasses need to meet the ISO 12312-2 international standard for direct sun viewing, according to the American Astronomical Society. If they meet that standard, they may be marked accordingly.

That international safety standard (sometimes written as ISO 12312-2:2015) means that the glasses will, per the AAS, "not only reduce visible sunlight to safe and comfortable levels but also block all but a tiny fraction of solar UV and IR radiation."

However, according to the AAS, counterfeit glasses have become a problem. So how can you tell if yours are the real deal?

Counterfeit eclipse glasses

Unfortunately, some vendors have taken to indicating that their product is ISO 12312-2 compliant when it hasn’t been properly tested. 

There’s no way to test whether or not your glasses are compliant without access to complex lab equipment. But the AAS has a few more tips to offer.

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First, it’s possible to check whether the glasses are unsafe. Per the AAS:

"If you can see shaded lamps or other common household light fixtures (not bare bulbs) of more ordinary brightness through your eclipse glasses or handheld viewer, and you're not sure the product came from a reputable vendor, don't use it. Safe solar filters produce a view of the Sun that is comfortably bright (like the full Moon) and in focus. If you glance at the Sun through your solar filter and find it uncomfortably bright, don't use it."

Second, consider the source. If you can remember where you got them and the source is reputable – a museum or planetarium, an astronomy organization, a college or university, an optometrist, or one of the vendors on the AAS safe suppliers list – you’re almost certainly good to go.

But if you can’t remember the source or you have doubts about whether or not the purchaser would know about the safety standard, it’s probably best to seek out another viewing solution. 

How long can you safely look at an eclipse?

If you’re wearing eclipse glasses, you’ll still want to look away every few minutes. NASA warns that "staring at the Sun for minutes at a time even with proper filters can still overheat the tissues and fluids in the eye," which can be dangerous. So even though it’s riveting to watch, you’ll want to glance away often to keep all those important eye fluids at a reasonable temperature. 

Can you use sunglasses as eclipse glasses? 

No! Firm no! While sunglasses are useful for protecting the eyes from everyday exposure to the sun, they can’t stand up to intense direct sunlight. Standard sunglasses – even those with UV protection – transmit thousands of times too much sunlight. 

Eclipse glasses, on the other hand, are specifically designed for direct solar viewing. They are equipped with special-purpose solar filters that safely block out the dangerous infrared and ultraviolet light, as well as reduce the sun’s brightness to a safe and comfortable level, allowing you to observe an eclipse without risking your eyesight.

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Can you watch the eclipse through your phone?

It’s not a great idea to watch the eclipse through your phone. 

The American Astronomical Society urges that a special-purpose solar filter should remain on the lenses of all cameras (including smartphones) and telescopes during the eclipse. Just like your eyes, photo lenses can be damaged if pointed directly at the sun. 

Why is looking at an eclipse unsafe?

This warning isn’t an old wives tale – sitting too close to your TV won’t make you go blind, but staring directly at an eclipse can do serious damage. Even though the sun becomes obscured partially or entirely during an eclipse, what remains visible can cause significant harm to the eyes, potentially leading to permanent vision trouble (blurred vision, dark or yellow spots, pain in bright light, or loss of vision in the center of the eye).

Retinal burn (also called solar retinopathy) happens when sunlight floods the retina. This can overstimulate the cells and cause destruction. What’s more, there’s no warning of the damage, because retinas don’t have pain receptors – so while our fingers warn us when they touch a hot stove, our retinas have no such alarm system. 

This isn’t a danger exclusive to eclipses and other astronomical events (you should never look directly at the sun). But because an eclipse is such a rare and often awe-inspiring occurrence, people are far more likely to risk it, thus exposing themselves to harmful solar radiation. Use eclipse glasses or another protective device, whether store-bought or homemade. 

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What does totality mean?

Totality in an eclipse is when the moon fully obscures the sun, allowing observers within the path of the moon's shadow to witness it. When totality occurs, the corona (the sun's outer atmosphere) becomes visible as a halo surrounding the moon. The day is plunged into twilight, stars and planets may become visible in the sky, and the temperature drops as much as 10 degrees. This period of totality is typically quite short – a few minutes, tops – but it can have a profound impact on those who experience it.

MORE: What happens if it's cloudy during the eclipse? The answer depends, researchers say

What does path of totality mean?

The path of totality is a specific pathway across the Earth's surface in which observers can witness a total solar eclipse. During a total solar eclipse, the moon completely covers the sun's disk, turning daylight into twilight – that’s totality. The duration of totality at any given point along this path is rarely more than approximately seven minutes. Outside the path of totality, observers may see a partial solar eclipse.

Total solar eclipse path

The April 2024 solar eclipse will be visible, at least in part, to nearly everyone in the U.S. But the path of totality – where the moon will completely block the sun – is a 115-mile-wide region that stretches from southern Texas up through Ohio, then over to northern Maine.

Large cities in the path of totality include:

• Austin, Texas
• Dallas, Texas
• Little Rock, Arkansas
• Carbondale, Illinois
• Indianapolis, Indiana
• Cleveland, Ohio
• Buffalo, New York
• Plattsburgh, New York
• Presque Isle, Maine

The farther you are from that path, less and less of the sun will appear to be blocked.

What time is the solar eclipse?

Southern Texas will see the peak of totality first, around 1:30 p.m. Central Daylight Time. Then Dallas at 1:42 p.m., with the time getting later and later as the moon’s shadow moves north. Indianapolis will see the peak around 3:05 p.m. Eastern Daylight Time; Cleveland at 3:15 p.m., and northern Maine around 3:30 p.m.

However, it will take several hours for the moon to move across the sun, so the actual eclipse event will start just over an hour before the peak of totality, with more and more of the sun slowly being blocked.

How long is the solar eclipse?

Again, that depends on where you are. Those closest to the center of the path will see total darkness for about four minutes at the peak of totality.

But because the moon moves slowly across the sun’s path, the entire eclipse event – from when the moon first clips the sun until the time it clears – will last from 90 minutes to over two hours for those in the path of totality.

Where do I look for the solar eclipse?

The easiest way to know may be to step outside in the days leading up to the eclipse and see where the sun is during the afternoon.

MORE: How to get the best view of the solar eclipse

Early afternoon on April 8, the sun will be pretty high in the sky. As always, though, the further north you are, the lower in the sky the sun will appear.

For example, in Austin, the sun will be at 67 degrees up from the horizon at the peak of totality. Remember, 90 degrees is straight up, so 67 degrees is just over two-thirds up into the sky from the horizon.

In Cleveland, meanwhile, the sun will be slightly lower, at only 49 degrees – just over halfway up in the sky.

When is the next total solar eclipse?

After 2024, NASA says, the next total solar eclipse visible from any point in the contiguous United States will occur in 2044. Totality will only be visible from North Dakota and Montana.

The next total solar eclipse that will travel across the lower 48 states from coast to coast is in 2045. ​

This story was reported from Chicago.