A spectacular total lunar eclipse graces the evening sky on January 20, 2000.

Next Thursday evening, January 20, the full moon launches itself over the eastern horizon near the time of sunset, 5 p.m. By 6:30, however, a ghostly pallor starts spreading across the ascending moon. A denser shadow follows, making landfall on the moon at 7:01 p.m. By 8:05 the formerly sunny face of the moon completely succumbs to Earth's shadow.

Lunar eclipses, often overlooked in favor of the more spectacular solar eclipses, deserve more respect than they get. Unlike a total solar eclipse, which is visible only along a narrow "path of totality" sweeping across Earth's surface, lunar eclipses can be observed from anywhere on Earth's nighttime hemisphere, weather permitting. Our last opportunity (here in San Diego) to see a totally eclipsed moon was in 1996; the next, after this January 20, will occur more than three years hence.

Although the sky over urban San Diego may be crisply clear next Thursday, the darker skies of far East County will allow you to appreciate the finer points of the upcoming eclipse. The eclipsed moon will lie below and to the left of the glittery winter constellations, which together contain nearly half of all the bright stars in the sky. These stars will shine opulently when seen from a location well away from the city lights.

Rather than being black or very dark, the totally eclipsed moon is expected to appear as a faintly glowing, red- or orange- colored disk. The glow exists because a small fraction of the sun's light is refracted (bent) into Earth's shadow when passing through the atmospheric layers encircling Earth. The long passage through the atmosphere reddens the light in the same way that sunlight is reddened near the time of sunset and sunrise. To understand this, imagine standing on the moon when it's being eclipsed: you could look upward at the black disk of Earth hiding the sun and see only a thin, reddish ring of filtered sunlight streaming around this disk.

Since the moon will plunge noncentrally across Earth's shadow during next Thursday's eclipse, the reddish glow on the moon will be irregularly bright at all times during totality. The part of the moon appearing brightest within the shadow is the part closest to the edge of the shadow. To see the subtle and colorful gradations of light promised by this eclipse, it's best to use binoculars or a small telescope.

Totality lasts between 8:05 p.m. and 9:22 p.m. Following this are the anticlimactic, tedious closing partial phases. These end at 10:25 p.m.

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