If the Sun Is 93 Million Miles Away, Why Can't We Look Directly at It?

During next month's Great American Total Solar Eclipse, you may be tempted to take in the historic event by gazing directly at the sun, but you absolutely should not do this without the proper eye protection, experts say.

That's because, even though the sun is some 93 million miles (150 million kilometers) away, it can still cause serious, and sometimes irreversible, eye damage.

"Even very short direct observation of the sun has the potential to cause damage," said Dr. Russell Van Gelder, a clinical spokesman for the American Academy of Ophthalmology (AAO) and director of the University of Washington Medicine Eye Institute in Seattle. [The Best ISO-Certified Gear to See the 2017 Solar Eclipse]

On Aug. 21, 2017, the moon will pass between the Earth and the sun, causing a total solar eclipse that will be visible from parts of the United States, along a narrow path from Oregon to South Carolina. It will be the first time since 1918 that a total solar eclipse will be visible across the continental United States (from the West Coast to the East Coast), according to the American Astronomical Society (AAS). People outside the path of the total solar eclipse will see a partial solar eclipse.

Regardless of where you observe the eclipse, it's important not to look directly at the sun with the naked eye. To understand why, think of a child using a magnifying glass outside to burn holes in paper. "Focusing the sun's rays on a single point creates a lot of energy," Van Gelder said. And the lens in your eye is about four times as powerful as the type of magnifying glass a child might play with, Van Gelder said.

"If you take a lens that has that much power and point it directly at the sun, the energy becomes very high," and is enough to literally burn holes in the retina, or the light-sensitive cells at the back of the eye, Van Gelder said. [The 8 Most Famous Solar Eclipses in History]

Patients with this condition, known as solar retinopathy, show a very characteristic pattern of eye damage during an exam. "It looks like someone took a hole punch and just punched out the photoreceptive cells in the retina," Van Gelder told Live Science.

It's thought that this damage happens when photons (light particles) create free radicals, which are highly reactive molecules that can "poison" cells and kill them, Van Gelder said. The damage occurs in the fovea, a spot in the retina that is responsible for sharp, central vision. As a result, patients with solar retinopathy may have blurry vision or a central blind point in their eyes, according to the AAO.

Many patients with solar retinopathy recover from their symptoms, but some have lasting vision problems. For example, in a 2002 study of 15 patients in England with solar retinopathy from viewing an eclipse in 1999, all but two had normal vision on an eye exam 8 to 12 months later. Still, even some patients with normal vision on an eye test had subtle eye symptoms, such as a small blind spot in their vision.

In theory, a person could become legally blind — vision of 20/200 or worse — from staring at the sun. But staring at the sun is unlikely to result in total blindness, or loss of both central and peripheral vision, because solar retinopathy typically doesn't damage peripheral vision, Van Gelder said.

Because of the dangers, the AAO recommends that people not spend any time looking directly at the sun with their naked eyes. There is one exception to this rule — if you're in the path of a total solar eclipse, you may look at the sun with your naked eyes during the brief time when the sun is in "totality," meaning the sun's bright face is completely blocked by the moon. (The length of totality will vary depending on where you view the eclipse, but at most, this event will last 2 minutes and 40 seconds, according to the AAS.)

But there is a way to view the entire solar eclipse event safety, using special "eclipse glasses" or handheld solar viewers that contain solar filters, according to the AAS. You'll need to use these glasses if you want to look at the uneclipsed or partially eclipsed sun. The four manufacturers with certified eclipse glasses and handheld solar viewers are:  Rainbow Symphony, American Paper Optics, Thousand Oaks Optical and TSE 17, according to the AAS.

It's important to note that you should never look at the sun through an unfiltered camera, telescope or binoculars, regardless of whether you're wearing eclipse glasses.  That's because these devices will focus the sun's rays even more than your eyes do, Van Gelder said, and this can cause serious eye injury.

REMEMBER: Looking directly at the sun, even when it is partially covered by the moon, can cause serious eye damage or blindness. NEVER look at a partial solar eclipse without proper eye protection. Our sister site Space.com has a complete guide for how to view an eclipse safely.

Original article on Live Science.