Solar eclipse: When do you need to put on your eclipse glasses?

We'll soon be able to see the solar eclipse - but how soon do you need to put your eclipse glasses on?

(NEXSTAR) — The total solar eclipse is finally here, meaning millions across the U.S. will be turning their eyes skyward to catch a celestial event we won't experience (at least here) for another roughly two decades.

Before you look up to the sky, though, you should don a pair of eclipse glasses, or employ some other method to safely watch the moon slide in front of the sun. If you're within the path of totality, you'll have the opportunity to take your glasses off for a short time to experience "sudden nightfall," but outside of that, you'll need to protect your eyes for much longer.

While everyone is looking forward to the peak of the eclipse, the whole event is relatively long.

In Cleveland, Ohio, for example, the partial eclipse is set to begin around 2 p.m. ET, but totality won't happen until around 3:13 p.m. ET. Then, after totality ends around 3:17 p.m., the moon will still partially cover the sun until 4:29 p.m. ET.

Over the roughly two and a half hours that you'll have a chance to see the eclipse, when should you put on your eclipse glasses (or welding mask)?

The short answer is that as soon as the moon starts moving in front of the sun, you'll want to put your eclipse glasses on before looking up.

If you're in the path of totality, you'll need to keep those on until the moon totally blocks out the sun. At this point, you can take off the glasses and experience the stars and planets, as well as the sun's corona. As soon as the sun begins to appear again (it'll start peaking out on the right side), you'll need to put the glasses back on.

Except for the brief moments when the moon is entirely covering the sun, those glasses will need to remain on during all phases of the eclipse.

If you're outside and not looking at the sun, you don't need to put on your eclipse glasses. The same is true if you're driving — experts say you should not put the special glasses on while behind the wheel, as they are extremely dark and will prevent you from seeing anything.

Why do I need to wear eclipse glasses?

NASA says that viewing the sun during the eclipse without the proper protection “will instantly cause severe eye injury.” The damage caused by looking at an eclipse without eye protection has earned its own name: eclipse blindness, or solar retinopathy. 

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.

Dr. Andrew Neighbors, a Seattle-based optometrist, told Nexstar’s KXAN that this damage can heal over time, but it’s also possible it becomes permanent, as there is little that can be done to repair it.

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

“Anywhere that the retina was exposed to that level of energy, it can burn it permanently and hurt your rods and cones, which can give you bad central vision, distorted vision, and it can affect your color vision as well,” optometrist Dr. James Forde told Nexstar’s WSYR.

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.

Can I wear sunglasses or a welding mask to watch the eclipse?

Let's start with the first one: No, sunglasses won't provide proper eye protection during the eclipse, no matter how dark they are. Eclipse glasses are very, very dark — you won't be able to see anything except the sun while wearing them. Your sunglasses can't do that.

Welding helmets, however, are able to get much darker than sunglasses. A welding helmet with glass that is Shade 12 or higher is safe for direct viewing of the sun during an eclipse, NASA previously explained.

However, ISO-approved eclipse glasses are still the preferred route to go when observing the eclipse.

Are your eclipse glasses eclipse-ready?

Solar eclipse glasses, according to the American Optometric Association, are usually made of paper with a dark film that covers your eyes. That film should meet the international standard of ISO 12312-2. Experts say that code, as well as the authentic ISO certification label, need to be on any eclipse glasses you pick up to ensure your eyes are protected. 

Your glasses should also fit properly, come from a reputable manufacturer and vendor, and not have any defects to the film intended to protect your eyes.

You can also create your own eclipse viewing device. Speaking with Nexstar’s KRQE, Dr. Robert Avery, professor of ophthalmology at the Department of Ophthalmology and Visual Sciences at the UNM School of Medicine, suggested people make a pinhole projector that allows “the sun’s image to be indirectly projected onto a piece of cardboard or, or paper so that you can view the eclipse indirectly without staring directly at the sun,” he said. NASA’s video tutorial on how to make a pinhole projector can be found by clicking here.

The total solar eclipse will take place on April 8. You can view this map to determine what you’ll be able to see and when you’ll get to see it.

The Associated Press contributed to this report.