Sun shade.

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On May 20, people in the southwestern United States will witness an amazing astronomical event. They'll see the first annular solar eclipse over the U.S. in 18 years.

An annular eclipse occurs when the moon passes between Earth and the sun (see diagram, below). The moon moves around Earth in an oval-shaped elliptical orbit, so its distance from Earth changes. During an annular eclipse, the moon is too far away to appear large enough in the sky to block the entire surface of the sun. This leaves an annulus, or halo of light, around the moon's silhouette.

"Everyone who can should get out and take a look," says Jay Pasachoff, an astronomer at Williams College in Massachusetts. But "even partly covered, you should never look directly at the sun without special equipment," he warns. To view the eclipse safely, you'll need glasses with solar filters, which you can buy online for less than $1, to block harmful sun rays.

How an annular eclipse happens

During an annular eclipse, the moon blocks most of the sun. Those not in the direct path of the eclipse see only a partial eclipse, where the moon appears to take a crescent-shape bite out of the sun.

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