A lunar eclipse is the result of a complex celestial dance:

Earth races around the sun at 66,500 m.p.h. (about 107,000 kilometers per hour). As it circles, it casts behind it a long, conical shadow a million miles (1.6 million kilometers) long. The moon, in turn, orbits Earth at more than 2,000 m.p.h. (3,200 kilometers per hour) relative to the Earth, and it too carries a conical shadow, but the smaller moon casts a shadow "only" a quarter million miles (402,325 kilometers) long.

Approximately every six months Earth, sun and moon line up in such a way that Earth and moon can be touched by each other's shadows. We are now in that time, and two weeks ago the moon's shadow touched Earth, resulting in a solar eclipse seen along a narrow strip on the planet. (Because it was an imperfect alignment this was only a partial eclipse.)

It's now two weeks later and the moon has circled to the other side of Earth, where it can be touched by Earth's shadow. But whereas the moon's shadow is very small and covers only a tiny spot on Earth, our planet's shadow is larger than the whole moon. Because of this, the moon will be completely engulfed by it; the effect will be a lunar eclipse.

Unlike a solar eclipse, which lasts less than five minutes and is visible only from a few places, an eclipse of the moon occurs over several hours as the moon moves into, through and out of Earth's cone of shadow. During this time the eclipse can be observed from anywhere on Earth where the moon is visible.

Last September's total lunar eclipse disappointed observers in the eastern and midwestern United States because of cloudy skies. However, the Midcourse Space Experiment satellite in Earth orbit took this infrared image.

Unfortunately, viewers in the Americas will not see much of this eclipse. Along the eastern seaboard the moon will have set before the eclipse begins; in the far west it will set just as the eclipse is beginning.

The eclipse will be at its best along the western Pacific and far-eastern Asia, where it will occur with the moon high in the sky. Viewers in that part of the world will see the moon darken slightly as it enters the faint outer fringe of the Earth's shadow (the area of shadow known as the penumbra), then deeper darkness creeps over its face as it enters the central shadow region (the umbra).

During the peak of the eclipse the moon will appear a dusky blood-red or dark orange color -- the color coming from sunlight being diffused through Earth's atmosphere and onto the moon. About three hours after it first started to darken, the moon will begin to emerge from the shadow, regaining its silver shine as it races out of Earth's cone of darkness.

In the end, the moon will be back as it started, having shown us not only Earth's dark shadow, but the whirling dance that is taking place all around us.