On Christmas Day, 2000, step outside and get a rare Christmas present-a partial solar eclipse!

Sky watchers living in the continental United States, Canada, Mexico, and the Caribbean will have a perfect view of the partially eclipsed Sun. Across North America, this partial eclipse will reach its maximum phase at 1735 Universal Time (12:35 p.m. EST) on Dec. 25th when 72 percent of the Sun's diameter will be covered by the Moon as seen from northern Greenland.

A map of the partial solar eclipse. SEE FULL PAGE VERSION Photo: NASA-GSFC

In other places, the eclipse magnitude will vary from over 60 percent in the northeastern United States to less than 20 percent in the far southwest. (Eclipse magnitude is the percent of the Sun's diameter covered by the Moon.) The exact time of maximum eclipse depends on your geographic position and time zone. Eclipse times for several hundred cities are listed on a NASA web site.

Solar eclipses occur during the New Moon, and under the condition that the Moon passes between the Sun and Earth. In the case of a partial solar eclipse, timing is everything. If the Moon's shadow happens to fall upon the Earth's surface then viewers can observe a partial covering of the Sun, says astrophysicist Fred Espenak at NASA's Goddard Space Flight Center in Greenbelt, Md.

Scientists strongly urge observers to take precautions when viewing the eclipse.

"Since solar eclipses are a rare occurrence, human curiosity impels some people to stare directly at the Sun during an eclipse and this can cause permanent damage to your eyesight," states Espenak. "Disregard the temptation, and never look at the Sun with the naked eye or through any optical device such as [unfiltered] telescopes or binoculars."

Ground observers can safely watch the eclipse as long as they project an image of the Sun onto a screen through a properly shielded telescope or a shielded pair of binoculars.

The Moon's shadow has two parts. The dark inner shadow is the umbra. When the umbra strikes the Earth, a total eclipse is seen there. The second component is the penumbra, the faint outer shadow where only part of the Sun's light is blocked. "The Moon's penumbral shadow will sweep across North America producing the partial eclipse on December 25," says Espenak.

And, if by chance, you oversleep Christmas morning, you'll have another chance next year on December 14, 2001. That eclipse will be visible from most of the U.S. except the northeast and will also be a partial eclipse.