The harvest moon gets eclipsed by the Earth tonight, and Washington area residents can watch as a terrestrial shade covers the southern side of the lunar landscape at midnight.

Officially, astronomers call this type of celestial event a penumbral eclipse because a lighter portion of the Earth's shadow partly obscures the moon. There are two gradations of darkness behind the Earth. The outer fringe of the darkness is penumbral, while the center is umbral.

"It's a weak excuse for a lunar eclipse," said Geoff Chester of the Einstein Planetarium at the Smithsonian's Air and Space Museum.

The moon will be full and bright early in the evening. As the lunar disk enters the eclipse phase, the moon will gradually turn gray. At the height of the event, just after midnight, astronomers predict that the southern edge of moon will slightly darken. Then, the moon will return to its normal brightness.

Beginning at 9:53 p.m., the moon enters the eclipse. Sky watchers won't notice any difference in the moon's light until the middle of the event. At two minutes past midnight, the eclipse reaches its peak stage when the southern side of the moon nicks the Earth's upper umbral shadow.

"The darkening will be on the southern side, which is generally brighter, down where the {man in the moon's} chin is," Chester said. He also explained that if someone were standing on the south side of the moon, the Earth would appear to cover the sun.

Easing out of its shadowy phase, this penumbral eclipse ends at 2:10 a.m.

"To the experienced observer it will be interesting to see how dark the moon gets and to see whether there is a recognizable demarcation line," according to LeRoy Doggett, an astronomer at the Naval Observatory. "It's certainly less spectacular than a total solar eclipse or supernova."

Spectacular or not, this kind of eclipse is safe to watch. Area residents need not construct special viewing devices to catch the event, simply find the moon and look.

The full harvest moon hangs high in the southern sky. Close to the moon, but to the left, the brilliant Jupiter shines. Aside from the sun and the moon, Jupiter is third brightest object in the sky, at least for the next two weeks. Venus, brighter than Jupiter, begins shining in the early evening later this month.

Most of the world gets a shot at viewing the eclipse. North America (except Alaska), Central and South America, Africa, eastern Asia, the eastern Pacific Ocean and Europe can catch tonight's penumbral eclipse, according to the Naval Observatory's Astronomical Almanac for 1987.

The next total lunar eclipse visible throughout North America will be Aug. 17, 1989.