The moon glides directly in front of the sun this month. A total eclipse? Strangely, no. An odd annular eclipse will unfold, sweeping across the United States in the premier celestial event of the year.
That the moon and sun normally appear to be the same size is a happy accident. The sun is 400 times larger than the moon, but it happens to lie 400 times farther away. The result is a near perfect match, allowing the two to align occasionally for the stunning phenomenon of darkness at noon. But because the moon’s elliptical orbit carries it 28 to 32 Earth diameters from us, the moon now and then fails to block the sun totally even when the alignment is right. That’s what will happen on May 10. Because the moon will be near its far orbital point when it eclipses the sun, it won’t appear quite large enough to fully cover the solar disk. Instead, a thin ring--or annulus--of sunlight will surround it.
Does the sun transformed into a brilliant ring sound intriguing? You’re in luck if you live in a 130-mile-wide path that angles across the United States from El Paso to Detroit to Buffalo to the entire coastline of Maine. If you live anywhere else, the consolation prize is a fine partial eclipse; the sun will be reduced to an eerie crescent, the stuff of dreams.
You’ll need eye protection either way. True, eclipses are not as dangerous as commonly thought. The dire warnings of immediate eye damage from squinting at the sun, repeated before every eclipse, have given the public a good case of heliophobia. Perhaps this misinformation is good. Some people may go overboard if they’re told it’s okay to take a quick sunward glance. In any case, never look through a telescope at the sun; the focused light could blind you in a second.
To be really safe, you can create a pinhole camera by poking a hole in a piece of paper with a needle, then projecting the sun’s image through it and onto a second sheet held underneath. But it’s far more striking to observe the sun-moon minuet more directly, through a filter. Shade number 14 welders’ goggles, from any welding supply store, are safe and inexpensive. Just the rectangular glass replacement filter is enough; you don’t need the whole goggles unless you’ll be repairing your car at the same time.
Then just sit back--the sun will be high up, especially from the country’s eastern half. Look for sunspots. We’re now near the minimum point of the 11-year solar cycle, so the sun is nearly free of blemishes. But occasional storms materialize anyway, and if any black dots appear to your unaided (but filter-protected) eye, they’re larger than our planet!
By an interesting coincidence, the moon’s 2,200-mile-per-hour orbital speed propels it through a distance equal to its own diameter every hour. At night it creeps against the background stars at this speed, and on May 10 it covers the sun at about the same rate. It’s a two-hour event: the moon needs an hour to obscure the sun and another to get out of the way.
Look beneath trees around mid-eclipse, shortly after 1 P.M. eastern, 10 A.M. Pacific time. Each of the innumerable spaces between the leaves acts as its own pinhole camera, creating a bizarre profusion of bright crescents or rings. The ground will teem with Cheshire cat smiles, just one of the strange ways nature will celebrate that sun and moon and the United States have formed a perfectly straight line in space--for the last time until 2012.