At 1:33 a.m. today, the full moon that had brightened the southern sky for hours moved into the darkness of the earth's shadow, beginning the most vivid period of the longest lunar eclipse in more than a century.

Washington residents who chose to defer their bedtimes by a few hours and those who made late returns from July 4 weekend travels were rewarded with splendid conditions for watching the historic heavenly spectacle.

Temperatures were in the pleasant 60s and the bright face of the moon, haloed by a slight hint of haze, hung like a lantern moving on a long slow arc toward the western horizon.

For an hour and five minutes, as some earthbound observers went to sleep and others continued staring with awed intentness at the skies, the shadow of the earth spread across the face of the moon in implacable adherence to the laws of celestial mechanics.

At 2:38 a.m. the moon was enveloped inside the long conical shadow cast across the heavens by the earth, and the eclipse had achieved totality.

The moon, which produces no light of its own but shines by the reflected light of the sun, was now on the far side of the earth from the sun. The earth -- directly between the sun and the moon -- was preventing the direct rays of the sum from reaching and illuminating the lunar surface.

The relative positions of the sun, moon and earth made it possible to observe the eclipse throughout the Western Hemisphere, but what could be seen by viewers on the ground depended on local cloud and weather conditions.

The eclipse officially began at 12:22 a.m. [EDT] when the moon entered the penumbra -- the outer region of the earth's shadow -- in which the sun's rays were not yet fully prevented from reaching the moon's face. Little or no noticeable effect could be seen by the unaided eye at that time.

The period of totality, during which the moon was enveloped in the umbra -- the inner and darkest region of shadow -- was expected to last long enough to provide a possible glimpse of the eclipse to early risers.

It was calculated that the moon would remain in the region of totality until 4:24 a.m. At that time the shadow of the earth was to begin receding slowly from the lunar disc, vanishing by 5:29 a.m., just 20 minutes before sunrise.

Total eclipses of the moon occur relatively frequently, roughly at a rate of one a year. Two circumstances, however, made this morning's a once in a lifetime event. In many other eclipses, the moon cuts across the outer regions of the earth's shadow, and is immersed only briefly in it. In this morning's eclipse, the moon moved directly through the center of the shadow.

In addition, at time of the eclipse, the moon -- which follows an elliptical and not a circular orbit -- was at its greatest distance from the earth. At this point, its speed is slowest. Thus, it required a maximum amount of time to travel across the earth's shadow.