At 1:33 a.m. yesterday, 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.

The moon began moving into the outer edges of the shadow about an hour earlier, but gave little visible sign at that time that a celebrated astronomical event was taking place.

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.

Washington residents who deferred their bedtimes by a few hours and those who returned late from July 4 weekend travels were rewarded by splendid conditions for eclipse watching.

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

For an hour and five minutes after 1:33 a.m., as some earthbound observers went to sleep and others continued staring with awe 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 was directly between the sun and the moon, preventing the direct rays of the sun from reaching and illuminating the lunar surface.

The period of totality lasted long enough to provide a possible glimpse of the eclipse to early risers. The moon remained fully immersed in the earth's inner shadow, or umbra, until 4:24 a.m. At that time the shadow began slowly to recede 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 once a year. Two circumstances, however, made yesterday 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 yesterday morning's eclipse the moon moved directly through the center of the shadow.

In addition, at the time of the eclipse the moon, which follows an elliptical orbit, was at its greatest distance from the earth. At this point, its speed is its slowest. Thus it required a maximum amount of time to travel across the earth's shadow.