May 20, 2012 at 4:44 AM ET
It's time to put on your eclipse glasses, prepare your pinhole projectors or scout out a sun-watching website: The first annular solar eclipse to pass through the United States in 18 years is on its way.
The moon will start eating away at the sun's disk around 5 p.m. ET today — although that's early Monday morning in Asia, where the eclipse begins. A wide swath of the world between south China and the American Midwest will see a partial solar eclipse, due to the moon's position between Earth and sun. And along a roughly 200-mile-wide track, skywatchers can witness a "Ring of Fire," in which just a thin ring of the sun's disk remains uncovered. There'll be no total eclipse this time around, because the moon is too far away in orbit to match the sun's apparent size. Nevertheless, it's a sight not to be missed.
Here are seven things you need to know about witnessing the eclipse:
Seeing the big picture: Solar eclipses occur when the moon, sun and Earth line up closely enough for the moon to throw its shadow on earthly locations. Annular eclipses, which create that fiery ring around the moon, are actually rarer than total eclipses because the moon has to be relatively far away in its orbit. Check out this interactive for the graphic details.
Seeing it in Asia: The moon's shadow races eastward across Earth's surface at more than 2,000 mph, starting in China's southern Guangxi Province. Theoretically, the "Ring of Fire" could be visible after 6 p.m. ET over Asian urban centers such as Guangzhou, Hong Kong, Macau, Taipei, Osaka and Tokyo. But for many of those cities, the weather outlook isn't that great: Cloudy skies or even thundershowers are in the forecast.
Seeing it in America: The partial eclipse begins over the U.S. West Coast and Canada around 8 p.m. ET, and even earlier in Alaska. Skywatchers in portions of Oregon, California, Nevada, Utah, Colorado, Arizona, New Mexico and Texas can witness the "Ring of Fire" effect at its peak after 9 p.m. ET. As you go farther east, sunset becomes the limiting factor. The U.S. East Coast, for example, will miss out on all phases of the eclipse. Consult NASA's clickable map to find out what will be visible from your locale. The times are listed as UTC, so subtract five hours for Central Daylight Time, six hours for Mountain Time, and seven hours for Pacific Time. Don't forget to check the weather, too.
Seeing it safely: Experts emphasize that you should never gaze at the sun without appropriate eye protection, even when the solar disk is almost completely blocked by the moon during an annular eclipse. You can look at the sun through specially designed eclipse-viewing glasses, or through welder's glass. Those are in short supply now, but you might still be able to find the right equipment at national parks, science centers and other eclipse hot spots. You can also create a pinhole projector with supplies as simple as a sheet of paper, aluminum foil and a box. Or you can just look at the weird crescent-shaped and ring-shaped patterns created when sunlight streams through the trees. Check out this safety guide for more tips.
Seeing it online: If you're outside the eclipse zone, or if cloudy skies spoil your view, you can still choose from more than a dozen webcasts that are promising to follow the phenomenon. If one webcast isn't working, try another. Here's a list of the webcasts we've come across.
Sharing what you see: If you're a Twitter user, you'll want to use a hashtag like #eclipse, #eclipse2012 or #annulareclipse to let the world know about your sky sighting. And if you snap a great picture of the eclipse, won't you please share it with us, via Twitter or Instagram or Facebook? Flag your submission with the #EclipseMSNBC hashtag. We have an extra special option for DSLR users: Just upload your images using the drag-and-drop feature on this PhotoBlog page.
Seeing the next sky spectacular: The annular eclipse is a treat, but it's not the end of this year's big sky shows. If you have the right equipment — for example, a telescope or a pair of binoculars equipped with solar filters — you can watch the June 5 transit of Venus. (This will be the last such transit until 2117.) There's a total solar eclipse coming up on Nov. 13 that can be seen from Australia and the Pacific, as well as via the Internet. And in five years, totality will make a huge splash across the United States, for the first time since 1979. Sunday's event will provide good practice for all these coming attractions.
More about the annular solar eclipse:
Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.