People with clear skies across most of the U.S., Canada, and Mexico will experience a partial eclipse of the Sun late this Sunday afternoon (May 20, 2012). Only those near the Eastern Seaboard will miss out on this awesome cosmic event.

And, if you happen to be in a swath of land running from Northern California to Texas, you'll also get a very special *kind* of partial eclipse: an annular eclipse, in which the rim of the Sun becomes a brilliant ring completely encircling the black silhouette of the Moon.

The Sun will be moving down the afternoon sky when a dark dent begins to intrude into one edge. The dent will deepen, eventually turning the Sun into a fat crescent - or, for the western half of the continent, a thin crescent. The dent is the silhouette of the new Moon traveling along its monthly orbit around the Earth.

Most Westerners can see the entire eclipse from beginning to end before sunset. Farther east, sunset puts an end to the show while the eclipse is still in progress - affording weird and spectacular sunset scenes just above the west-northwest horizon. "This is going to be a great photo opportunity," suggests Robert Naeye, editor in chief of Sky and Telescope magazine. For a how-to, see Sky and Telescope's article on photographing the eclipse:

The farther east you are, the earlier in the eclipse the Sun sets for your location. The Eastern Seaboard misses out completely; here the Sun sets before the eclipse even begins. The map on the web version of this release tells the story:

The eclipse will become annular (a "ring eclipse") for parts of southern Oregon, northern California, Nevada, Utah, Arizona, New Mexico, and a bit of Texas, as also shown on the maps. The annular aspect makes this the first "central" solar eclipse (meaning total or annular) to cross the United States since 1994. Untold numbers of people are planning to travel to the path of annularity for the grand event.

Americans are seeing the tail end of this eclipse. Its annular part begins at sunrise along the southern coast of China (where the local date is May 21st), then crosses parts of Japan, including Tokyo on the centerline. It then speeds across the North Pacific through much of the day before making landfall on the California-Oregon coast in the late afternoon of the 20th (local date).

What to Watch For
"Annular" means "ringlike." This is the kind of eclipse we see when the Moon is at its farthest from Earth and appears slightly smaller in the sky than the Sun does. The Moon will be practically at the apogee of its orbit - two weeks, or half an orbit, after the Moon was a "supermoon" at perigee when full on May 5th.

Only 88% of the Sun's surface area will be blocked during annularity. "This will cause less change in the daylight than you might think," says Alan MacRobert, a Sky and Telescope senior editor. "Moderately thin clouds would dim the sunlight more. And if you're where the eclipse is only partial, the dimming will be less."

The exposed part of the Sun will remain blindingly bright - literally so - so anyone viewing any part of this eclipse, partial or annular, must use a safe solar filter, such as a #13 or #14 rectangular arc-welder's glass or an astronomer's filter made specifically for Sun viewing. Ordinary dark glasses won't do. Watching the Sun through an inadequate filter (or none) can permanently damage your eyesight.

Sky and Telescope's website describes several recommended Sun-viewing methods:

"When the eclipse is deep or annular, the clear blue sky will become a darker, deeper blue than normal," says MacRobert. "Look for Venus - it's shining east of the Sun by about two fist-widths at arm's length. Jupiter and Mercury will be tougher. They're on the other side of the Sun by about a quarter and a third as far, respectively, and they're not as bright."

Other things to look for during this eclipse include a silvery or metallic quality to the light around the time of annularity or when the Sun is a thin crescent. Look for images of the crescent or ring Sun being cast under leafy trees; small openings between leaves often make "pinhole cameras" projecting images of the Sun on the ground.

A partial or annular eclipse is a rewarding experience in itself, but it's no match for a *total* eclipse of the Sun. "The next total eclipse of the Sun to cross the United States will be on August 21, 2017," says MacRobert. "So consider Sunday's event a warm-up."

But another solar spectacle is coming up much sooner. Just 16 days later, on the afternoon of June 5th, it's the planet Venus's turn to cross the face of the Sun. The silhouette of Venus will be a small black dot with just 3% the diameter of the Sun, compared to the Moon's 94% on May 20th. Read all about the transit of Venus on Sky and Telescope's website: