Watch This Weekend's Eclipse... Without Burning Your Eyes Out

Watch This Weekend's Eclipse... Without Burning Your Eyes Out

This Sunday, May 20, the moon will pass directly between the sun and the Earth, casting a shadow across the western United States, Canada and eastern Asia. Most people in western states will see a partial eclipse, like a moon-shaped bite has been taken out of the Sun. But those lucky enough to be in a 185-mile-wide swath from northern California through western Texas will witness the sun transformed into a spectacular ring of fire in a rare event known as an annular eclipse.

The moon will be at nearly the farthest it travels from Earth during its 29.5-day elliptical orbit, so it will not block out the Sun fully, as during a total solar eclipse. Instead, the smaller-seeming moon will superimpose over the sun, eclipsing about 94 percent of its disc. An annular eclipse has not tracked over the U.S. in almost 18 years. "It's pretty cool to be in a place where this symmetric circle of ordinary sunlight shines around the edge of the moon," Williams College astronomer Jay Pasachoff says.

Safely viewing such an astronomical arrangement, however, requires caution and preparation. We often glance up at the full-bore sun, but for a very short time; our eyes instinctually look away from its big, bright disc. Yet when much of the sun is eclipsed, we can focus on the scene for hazardously lengthy periods. "Part of the everyday sun is still visible, so you shouldn't look at it with your eyes unless you have protection," Pasachoff says. "Just looking through sunglasses is not at all safe." Staring at the sun unless it is completely eclipsed can cause permanent vision damage or even blindness.

To catch glimpses of this weekend's solar eclipse, go buy a proper solar filter. These filters come in a variety of formats, from a ttachable pieces for telescopes and binoculars to wearable "solar shades" that look like 3D glasses. Solar filters dramatically dim the appearance of the sun to allow for intermittent safe viewing. "Sunglasses knock down the brightness of the sun by perhaps 20 percent," Pasachoff says. "A solar filter knocks down the brightness by a factor of 100,000, so it's like more than a dozen sunglasses, one after the other." Crucially, a solar filter must reduce the transmission not only of visible light from the sun, but of invisible infrared light as well. "The infrared light that you can't see can cook the inside of your eyeball," Pasachoff says.

Solar filters often include particles of gold or silver, as these metals work well in scattering and absorbing visible and infrared light. Typically, solar filters are made of treated glass, though special polymers similar to developed film can also do the trick. Pasachoff suggests two companies in the U.S. for solar filters, Thousand Oaks Optical and Rainbow Symphony. Another product that doubles nicely as a solar filter is No. 14 welder's glass (the darkest kind), available through various online and welding supply outlets.

Regardless of the type of solar filter used, Pasachoff says, it is still good practice not to stare at an eclipse for very long. "Even when you do have a filter and know it's safe, only look for a second or two every few minutes," Pasachoff says. Partial eclipses can last more than a couple of hours, and the "ring of fire" annularity phase on Sunday will last about 4-1/2 minutes in Redding, Calif., and Albuquerque, N.M., for instance. "The eclipse doesn't change very rapidly," Pasachoff says. "It's fun to watch as it goes along."

If you don't have enough time to score a solar filter before the event, there is still a way to safely catch eclipses using solar projection techniques. Take a piece of paper and poke or cut out a hole 1/8 to 1/4 inch in diameter. When this pinhole is held up to the sun, a small bright dot will project onto a wall, the floor, or the ground. This miniature disc of the sun will pass through eclipse phases, letting you watch the show indirectly. "If you're standing under a tree, you might be able to look on the ground or on yourself and see an image of the eclipse falling from a natural pinhole of leaves," Pasachoff notes.

Binoculars or telescopes can also be utilized to throw a magnified, intensified image of the eclipsing sun onto a wall, he says. Just be extra careful not to look at the eclipse through the equipment, however, if a solar filter is not in place. "Looking through a scope is even more dangerous than to look at the sun with your eyes directly," Pasachoff says. The same advice goes for any photography of the eclipseâ€"make sure solar filters are in place before attempting to directly line up any shots.

Practice makes perfect. As celestial happenings go, solar eclipses (usually partial) transpire with some frequency, Pasachoff saysâ€"a few times per year. And this year there's a bonus of getting the hang of proper eclipse-viewing methods: You will be prepared to behold a once-in-a-lifetime astronomical event on June 6â€"the transit of Venus. At sunset in the U.S., the second rock from the sun will cross our star's face as a little black dot, a sight that earthbound humans will not see again until 2117.

"It's exciting to know we can predict these eclipses and planetary transits as well as we can," Pasachoff says, "but you do have to be careful."

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