May 20, 2012— -- If you live in a band across the southwestern United States, twilight likely seemed to come early this 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 it is 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 was visible this afternoon in a strip that began on the California-Oregon coast and stretched southeastward across Reno, Nev., the Grand Canyon, and Albuquerque, N.M., and ended at sunset near Lubbock, Texas. In the map we've provided, the best viewing was in the yellow band; outside it, people saw a partial eclipse.
San Francisco, Sacramento, Yosemite, Las Vegas, Salt Lake City and Denver all saw a partial eclipse -- the sun dwindling to a crescent. Even some distant cities, including Chicago, Dallas and Buffalo, saw a fair portion of the sun blocked by the passing moon.
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 was today -- 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 laws of orbital mechanics make solar and lunar eclipses fairly common, actually -- just not necessarily visible from where you live. If you were underwhelmed by today's annular eclipse, there will be a total eclipse on Nov. 13 -- but it will only be visible from Australia and the South Pacific.