May 18, 2012 — -- If you live in a band across the southwestern United States, twilight will seem to come early on Sunday afternoon, well before the sun actually sets.
The cause: a rare annular solar eclipse -- a ring of sunlight as the new moon, passing between Earth and the sun, blocks most, but not all, of the sun's disc.
This is not the kind of total eclipse of which you usually see pictures -- the moon blocking the sun completely, creating a few moments of near-night in the middle of the day, with only the sun's ethereal corona visible around the moon's edges. The sky will darken a bit, but there will still be a blindingly bright ring (an "annulus" in Latin) of sun, and it's dangerous to look directly at it.
Still, there will be a striking sight to see, if you look at a heavily-filtered image projected onto a screen through binoculars or a small telescope, or protect your eyes with No. 14 arcwelders glass (not something found at most hardware stores).
The ring will be visible Sunday afternoon in a strip that begins on the California-Oregon coast and stretches southeastward across Reno, Nev., the Grand Canyon, and Albuquerque, N.M., and ends at sunset near Lubbock, Texas. In the map we've provided, the best viewing is in the yellow band; outside it, people will see a partial eclipse.
The moon's shadow moves quickly -- about 1,200 mph. Some times when the moon's disc will be most centered over the sun's are as follows:
Why an annular eclipse instead of a total one? Because the moon, constant in size as it may appear to us, does not move in a perfect circle around Earth. Its orbit is slightly elliptical. On average, it's about 239,000 miles away, but at its closest it comes within about 225,000 miles of us. At its farthest -- as it will be Sunday -- it's a little more than 250,000 miles away. It's just enough of a difference so that the moon will only cover 88 percent of the sun.
(You may recall the "super moon" of two weeks ago; that night the full moon coincided with the low point of the moon's orbit, making it look a little more vivid than usual.)
The Interior Department points out that a number of national parks -- Redwoods and Lassen in California, Zion in Utah, Glen Canyon National Recreation Area in Arizona, Petroglyph National Monument in New Mexico -- will all be in the zone from which the ring will be visible. More information from the National Park Service here.
San Francisco, Sacramento, Yosemite, Las Vegas, Salt Lake City and Denver will all see a partial eclipse -- the sun dwindling to a crescent. Even some distant cities, including Chicago, Dallas and Buffalo, will see a fair portion of the sun blocked by the passing moon.
But if you're in the eclipse path, you really just need a place with a good clear view westward. (Check our weather page for a local forecast, though most of the eclipse zone can expect clear skies, at the moment.) You may want to go to a local observatory or planetarium, where viewing parties are likely.
And if you don't feel like investing in welder's glasses, you may be happy -- seriously -- with a piece of paper, or leafy trees around you. Prick a small hole in the paper and it will act as a tiny lens, projecting a miniscule image of the sun onto the pavement. Likewise, take advantage of the natural pinholes in many leaves. As the eclipse approaches maximum, look down, not up. If you're lucky, you'll see hundreds of little eclipse images dancing on the ground beneath your feet.
The laws of orbital mechanics make solar and lunar eclipses fairly common, actually -- just not necessarily visible from where you live. If you're underwhelmed by Sunday's annular eclipse, there will be a total eclipse on Nov. 13 -- but it will only be visible from Australia and the South Pacific.