Astronomers, taking advantage of an opportunity that occurs only for a short period every 124 years, have witnessed for the first time an eclipse of the planet Pluto by its moon.

The observation, made early Sunday morning, confirms that Pluto, the smallest planet in the solar system, has a moon. Preliminary evidence of the moon emerged in 1978 and astronomers named the object Charon. But they needed confirmation to win official recognition of the object by the International Astronomical Union.

Observation of the eclipses, which are believed to occur every 3.2 days and which should remain visible for about five years, may improve estimates of Pluto's size, density and composition. The findings could shed light on one of the great mysteries surrounding the solar system's oddest planet -- whether it formed as a planet or is an escaped moon of Neptune. They may also confirm that Charon is so large -- perhaps half the size of Pluto -- that the pair ought to be considered a double-planet.

The eclipse was first seen by Richard Binzel, a graduate student at the University of Texas who has been looking for it from the McDonald Observatory in West Texas since 1982. "It's been a long, arduous search," Binzel said. "We thought we might be able to see it as early as '82, and we've been watching since then. I'm just glad we were looking now. If we hadn't started until a few years later, we would have had to wait 124 years."

The reason the eclipses are visible only every 124 years is that Pluto, Charon and the Earth line up in the same plane only that frequently. Unless one body passes in front of or behind the other -- as seen from Earth -- no eclipse is visible. While the orbit planes of all the other planets are relatively close together, Pluto's orbit swings above and below, and the viewing angle changes constantly. Only when Pluto crosses the plane of the Earth's orbit is the geometry right to see the eclipse. Because Pluto takes 248 years to circle the sun, it crosses the Earth's plane every 124 years.

This odd inclination of orbit is one of the factors that have led astronomers to wonder whether Pluto began as a planet. Another is Pluto's highly elliptical orbit, unlike the more nearly circular paths of the other planets. As a result, Pluto moved inside the orbit of Neptune in 1979 and will not swing outside of it again until 1999.

Pluto and Charon are so far away -- 2.8 billion miles from Earth -- and so close together -- only about 12,000 miles apart -- that they show up on telescopes as a single dot combining the amounts of sunlight reflected by both bodies. Any drop in the light level, therefore, indicates that one body is passing in front of the other.

Binzel's 36-inch telescope was attached to a device that measures the light coming from Pluto-Charon. Around 2 a.m. Sunday, the light began to dim steadily. It dropped by 4 percent, stayed low for about 2 1/2 hours, and then rose again. Charon, apparently, was just grazing Pluto.