If you’re one of the lucky ones living across a broad swath of the United States from the Pacific Northwest to north Texas, you’re in for a rare treat this evening.
It’s called an “annular solar eclipse,” which happens when the moon slides over the sun, blocking out the center and leaving a glowing ring called an “annulus.”
We could get into “umbra,” “penumbra,” and “antumbra” but you’ll have to Google those yourselves. The main thing is, the difference between an annular solar eclipse and a total eclipse of the sun has to do with how far away the moon is from the earth.
The closer to the earth it is, the larger the moon appears – blocking out more of the sun. When the moon is farther away from the earth, it appears smaller, leaving that “ring of fire” when it appears to pass in front of the sun. Right now, the moon is at its greatest distance from the earth, hence an especially wide fiery ring.
Actually, it’s not only Americans along that swath who’ll be able to witness this very occasional solar-lunar event. (The last one was in 1994.)
It’ll start in southern China, then move across the southern coast of Japan, crossing the Pacific just south of the Aleutian Islands before hitting the US near the Oregon-California border. (Speaking of “ring of fire,” the Aleutians are part of another “ring of fire” – the volcanic rim around the Pacific from South America to New Zealand. It has nothing to do with Johnny Cash, however.)
These graphics by the United Kingdom Hydrographic Office are especially good.
The main advice in observing this evening’s annular eclipse and its “ring of fire” is be careful.
Ron Hipschman, who describes himself on Twitter as the “loose cannon at the Exploratorium” in San Francisco, offers this warning:
“As a kid, did you ever take a magnifying glass out into the sun and burn leaves? If so, you probably remember that when the focused sunlight coming through the lens was refracted and concentrated to a small spot, the energy available there was truly remarkable. Guess what? You have a lens just like that in your eye. If you look at the sun, your eye-lens will concentrate the sun's light and focus it to a very small spot on the back of your retina. This can cause permanent eye damage or blindness. Additionally, there are no pain sensors back there so you won't even know it's happening! Have I scared the willies out of you? Good!”
On this website, Mr. Hipschman gives directions for safely observing any solar eclipse.
As you might have done in your junior high school science class, you can build a pinhole projector using a long box, a piece of aluminum foil, a pin, and a sheet of white paper. What a great opportunity to do something with your kids!
This site shows how to build such a projector using a dark-colored plastic cup, wax paper, scissors, a rubber band, and a pushpin. The Clark Planetarium in Salt Lake City has good directions for making a simple pinhole projector here.
“Getting even more basic, you can use your own hands,” Hipschman explains. “Just hold up both hands with your fingers overlapping at right angles. The holes between your fingers make pinholes.”
Clark Planetarium director Seth Jarvis says forget about the old suggestions of putting soot from a candle flame on a piece of glass, using exposed black-and-white film, or doubling up on sunglasses. None offer adequate protection, and he recommends against trying to use welder’s goggles to peer at the sun unless they are rated for arc-welding (at least grade 14).
So enjoy this evening’s light show, but do it carefully.