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A Chip Off the Sun

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A Chip Off the Sun

Sky watchers can enjoy a solar eclipse this Friday when the Moon's shadow sweeps across the Pacific Ocean and parts of the Americas.

NASA
Marshall Space Flight Center

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December 12, 2001:  On Friday, Dec. 14th, the Sun over Costa Rica and Nicaragua will dim -- but not because of cloudy weather. Those countries are in the direct path of an annular (ring-like) solar eclipse. Shortly before the Sun sets over that part of the world, the Moon will glide in front of the Sun covering about 96% of its disk, and a "ring of fire" will emerge around the lunar limb.

Elsewhere in the Americas people will experience a partial eclipse. Whether the Moon takes a little nibble or a big bite out of the Sun depends on where you live. The Sun over the Hawaiian islands, for example, will become a thin (79%) crescent early in the morning Hawaiian time. The event will be less spectacular but still pleasing across the continental United States, where 10% to 60% of the afternoon Sun will be hidden by the Moon.

Above: Olivier Staiger captured this image of the crescent Sun just before the peak of an annular eclipse in Ecuador in 1995. Copyright O. Staiger, all rights reserved.

A list of eclipse times for locations in Hawaii and North America is available from Sky & Telescope magazine. NASA's Goddard Space Flight Center has published a similar list covering Central America, the Caribbean, and South America .

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Above: A Dec. 14, 2001, eclipse map for the continental United States. See also maps for North America, the world, or a detailed map of Costa Rica and Nicaragua where the full annular eclipse will be visible. Image credit: NASA/GSFC. [more information]

Once you've discovered when the eclipse will happen in your part of the world, what should you do?

Actually, the most important thing is what you should not do: Never look directly at the Sun! Staring at our star without suitable eye protection -- even during an eclipse -- can cause permanent eye damage.

Fortunately, there are safe and easy ways to view the event. Chief among them is projecting an image of the Sun onto a screen through a properly shielded telescope or a properly shielded pair of binoculars. If you don't have a telescope or binoculars, even a simple pinhole poked through a piece of paper can serve as a makeshift projector. Such techniques are good because you don't need to face the Sun to see the show.

Above: Projected images of a partial eclipse, such as this one captured by Evan J. Gnam of Madison, Wisconsin, on Dec. 25, 2000, can be remarkably beautiful. [more]

Dark-lens filters also work, but using them means looking toward the Sun itself, so they aren't recommended for children. Welder's glass #14 is sufficiently dark, and can be bought inexpensively at many hardware stores. In years past, eclipse watchers who couldn't find welder's glass often made their own filter using exposed, developed black-and-white camera film. That's rarely a good idea nowadays because some modern black-and-white films don't contain the sun-blocking silver that's in the older-style film. Unless you're absolutely certain the film contains silver, it's best to steer clear of this old trick -- after all, your vision could be at stake!

Eclipse watchers are occasionally tempted to glance at the Sun during the moments of maximum coverage. Don't! The Sun's disk is never completely obscured during an annular eclipse.

Jay Pasachoff, Field Memorial Professor of astronomy at Williams College in Williamstown, Massachusetts, explains: "[During an annular eclipse], the Moon is slightly farther away from the Earth than average on its elliptical orbit, and so it appears a little smaller in the sky than usual. In fact, it's too small to cover the Sun entirely, so a bright ring of sunlight remains around the Moon."

Above:  Annular eclipses get their name from the ring of sunlight around the edge of the Moon at the peak of the eclipse. Another word for ring is "annulus." This picture of an annular solar eclipse on May 10, 1994, was taken from the Holloman US Air Force Base.

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Within the so-called "path of annularity" where the eclipse coverage is greatest, the Sun will dim for minutes as if a cloud had passed overhead. Otherwise it will seem much like ordinary daylight to observers.

Total eclipses are different. They cast a darker shadow than annular eclipses do -- about 10,000 times darker, in fact. However, notes NASA astronomer Mitzi Adams, the shadow of a total eclipse is not like, say, the dark of a moonless night. "It's rather surreal and unlike anything we normally experience. Straight overhead the sky is very dark. Near the horizon we can see the edge of the Moon's shadow glowing like a sunrise or sunset. But unlike a sunrise, which appears only in the east, this glow circles the entire horizon -- a full 360 degrees."

Annular eclipses are less spellbinding than total eclipses, but they are nevertheless beautiful. For example, during an annular eclipse observers can see "Baily's Beads," a string of bright spots along the Moon's edge caused by sunlight glimmering through canyons and craters. Near the center of the narrow track where the eclipse will be annular, Baily's Beads are short-lived. But at the edge of that track -- where many eclipse watchers will station themselves -- the lovely beads can persist for more than a minute.

Above: "Baily's Beads" are rays of sunlight lancing through lunar mountains and craters. Image copyright 1999 by Fred Espenak of MrEclipse.com.

Although Pasachoff has already seen 32 solar eclipses, he's by no means bored by them. Indeed, he's traveling a great distance to San Juanillo, Costa Rica, where he will view this week's eclipse.

"Oh yes, for me it's tremendously exciting," he says. "I just don't know of anything more exciting. We try to bring as many people to these eclipses as possible."

Some of the onlookers this week are undoubtedly first-time observers who will become -- like Pasachoff himself -- dedicated eclipse-chasers. It only takes one good eclipse, say veteran sky watchers, to hook you for life!

Web Links

How to Watch a Partial Solar Eclipse Safely -- from Sky & Telescope

Annular solar eclipse of Dec. 14, 2001 -- information about the eclipse from NASA's Goddard Space Flight Center.

Annular Eclipse of the Sun -- from Sky & Telescope

Solar Eclipse Gallery -- North Americans also experienced a partial solar eclipse last year. This photo gallery from SpaceWeather.com displays images of the event and well as photos of the equipment used to safely view the Sun.

Right: Photographer Dan Hawrylkiw captured this picture of the partially eclipsed Sun rising over Sedona, AZ, on Dec. 25, 2000. [more]

Eclipse Webcams -- a list maintained by Costa Rica's Fundación para el Centro Nacional de la Ciencia y la Tecnolgía.

Eclipse Anular en Costa Rica -- a Spanish-language site about the annular eclipse.

Oliver Steiger's Almost-Live Webcast -- direct from Costa Rica

Observing eclipses safely -- from MrEclipse.com

Solar eclipses for beginners -- a tutorial on the basics of solar eclipses from MrEclipse.com.


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