The biggest and brightest of the 900 known comets, bearing the name of 17th-century astronomer Sir Edmond Halley, has swung from behind the sun heading toward its closest pass to Earth since 1910, and three astronomers have confirmed the first sightings of the rare visitor as it starts on the last leg of its voyage.

Halley's comet will not disappear behind the sun until early February, moving away from Earth toward the deep freeze of space, not to appear again until 2061.

The comet is beginning the stretch run of its 30th recorded visit to Earth. Though nameless on all but the last two of its recorded visits, Halley's comet has been seen by more people and more civilizations on Earth than any other celestial phenomenon.

Its long journey around the sun late this year and in early 1986 will be history's most observed, examined, studied, photographed and picked-over astronomical event. By the time the comet begins to darken and fade from sight as it speeds away late next spring, it will have been measured, pictured and analyzed hundreds of millions of times by astronomers using telescopes on the ground, in the air and in space.

"There will be times when Halley is so close to us that 200 of the world's best telescopes will be observing the comet at the same time," said Raymond Newburn of the Jet Propulsion Laboratory in Pasadena, Calif. "Nothing like it has ever happened before."

Astronomers consider Halley's their favorite comet, in part because it is so big and so bright, very far from being burned out and the only active comet to demonstrate a well-determined orbit and a reliable behavior.

The Soviet Union plans to have at least 10 major observatories watch Halley's and has even moved two smaller telescopes to South America to get southern exposures of the comet. Great Britain has constructed a telescope in the Canary Islands, and a European consortium has built a new telescope in southern Spain, all to observe the comet.

All major U.S. observatories will be looking at Halley's, including the four largest telescopes in Hawaii, which will provide this nation's best viewing post because of the islands' favorable position -- 19 degrees north latitude -- in relation to the comet's path from the sun.

The comet's visit has focused the consciousness of the world's astronomers as they seek to unravel its secrets and those of the first moments of the formation of the universe, when Halley's probably first congealed.

Comets are among the most unusual objects in the solar system and are at least as old as the system.

Despite their appearance as burning stars with fiery tails, comets are made of primordial ice imbedded with dust-sized bits of rock, formed into a something like a dirty snowball about the time the solar system was created, according to current theory.

Though comets seem to be coming from deepest space, all inhabit the neighborhood of the solar system, swinging around the sun or clinging in a great cloud of trillions of iceballs, called the Oort Cloud after a Dutch astronomer.

Away from the sun, Halley's is cold, lifeless ice and rock dust; when it nears the sun, "solar wind" radiation pouring from the sun vaporizes some of the comet's surface, creating a great "coma" or head around the mile-wide comet and blowing dust and gas in the direction away from the sun. This, the comet's tail, is illuminated by sunlight reflected from the shower and fluorescence created as the gas is heated.

Thus, on each trip past the sun, Halley's loses part of its substance as it creates a show for Earth.

Comets' origins have fascinated people for centuries. One medieval idea was that all comets were one object moving back and forth across the sky. Early astronomers thought comets exploded out of the giant planets Jupiter and Saturn. Some 18th- and 19th-century astronomers, deciding that no explosive volcanos existed there, concluded that comets exploded out of Jupiter's moons.

Jupiter has influenced all comets seen from Earth by pulling them from more distant orbits or nudging them off courses they routinely take toward the sun. Halley made one of his major contributions to cometary astronomy when he took into account Jupiter's perturbative effects in predicting the return of "his" comet in 1875.

Donald K. Yeomans of the Jet Propulsion Laboratory has said Halley's passage closest to the sun will occur 8.6 hours earlier this time than predicted because of a Jovian disturbance.

The only thing of which scientists are certain about comets is that all look as if they came from the same place beyond the edge of the solar system. "If you look at the orbits of all the observable comets, most of them appear to have come from a single distance of space about a thousand times as far from the sun as Pluto," Yeomans said.

No comet has appeared to come from as far as even a neighboring star. "We've never seen a comet coming in with the kind of hyperbolic velocity you'd need to escape another star," Newburn said. "This is what you'd see if you captured an interstellar comet, and we've never seen it."

That means comets are attached to the solar system, even if they originate just outside it. How did they get there? "It could be the same way they get in. They could have been kicked out there one by one by the major planets as the planets were being formed," Yeomans said.

If this happened, comets formed at the same time and place as the big outer planets, which seem made of exotic gases and ices akin to those in comets. One theory is that Uranus and Neptune were formed from millions of comets. Harvard University's Fred Whipple believes that all planets were formed from comets and that the leftover debris in space is also comets.

Comets could have been formed in areas of the solar nebula before the planets were formed, at the same time they were formed or even a little later. They could have congealed out of gas and dust from a nearby star at its birth. Either way, comets appear to originate in a place that puts them outside the solar system 40 million times the Earth's distance from the sun.

Historians say the first astronomer to recognize the existence of this comet limbo was Ernst Opik, an Estonian who calculated as long ago as 1932 that a cloud of comets, invisible from Earth, could have existed an enormous distance from the sun and survived that deep freeze since the birth of the solar system.

In 1950, Dutch astronomer Jan Oort calculated the paths of several comets believed to have rounded the sun for the first time and determined that all came from beyond Pluto, the most distant planet from the sun. Oort figured that as many as 2 trillion comets may exist in a spherical shell surrounding the sun as far away as 4 trillion miles. That would confirm the guess of 17th-century astronomer Johannes Kepler that there are "more comets in the sky than there are fishes in the sea."

Today's astronomers believe that the Oort Cloud exists, even though they cannot see it. Comets wait in the cosmic deep freeze until a star passes perhaps one to three light years away and knocks them from the cloud, starting their 2-million-year journey toward the sun. Many comets in the Oort Cloud may be at distances halfway to the nearest stars, far enough so their orbital motion could be changed by a passing star.

Other comets in the cloud could be close enough to the edge of Earth's solar system to be pulled toward Earth by the largest of the outer planets. Yeomans believes that "something" must be resupplying the comet stock to keep their numbers at almost 1,000, and Newburn believes that the inner solar system must pull in at least one new comet every 100 years to keep intact the inventory of short-lived comets.

Very bright comets that appear unannounced and seemingly out of nowhere are probably cleared out of the Oort Cloud by a passing star. These fresh comets plummet so fast into the solar system that they undergo dramatic changes, making them spectacular sights from Earth. This would explain why a comet known as West was so bright and broke into four separate comets when discovered in 1976.

Ed Danielson, an astronomer at the California Institute of Technology, which operates Palomar Observatory on a mountaintop east of San Diego, is one attracted by Halley's imminent passage. He works on the 200-inch telescope at Palomar.

On the night of Oct. 16, 1982, Danielson and eight other astronomers reserved time on the telescope as they raced astronomers throughout the world to make the first sighting of Halley's comet on its way back to another once-in-a-lifetime visit to Earth.

The comet had last been seen at the end of May in 1911, heading away from the sun, then 40 years ago slowed until the sun began pulling it back toward Earth. It has been racing back ever since.

The night was perfect for viewing at Palomar. Danielson and his weary crew had been looking in vain for Halley's for two years. There was no moon as the big telescope was slued in the direction of the constellation Canis Minor.

An object was found near the expected position of the comet, moving in the expected direction at the expected speed. Determining whether it was Halley's depended on a subsequent sighting.

Three nights later, Danielson was ready to look again, but sky coordinates he was given for the comet were awash in the light of a bright star. Danielson swung the telescope to where he thought he had seen the comet three nights earlier. It was not there. "That's one way to verify comet recovery. Comets move," he said.

At the time they determined that they had seen Halley's, it was 2,127,000,000 miles away, the most distant comet ever seen and the faintest ever recorded.

Danielson won the race to see Halley's first and subsequently saw it in January and February 1983, January and December 1984 and January and March this year. He has watched it move across the skies from constellation to constellation, at the edge of Orion in towards Taurus and out into Gemini.

Then this summer, as it passed out of Earth's view behind the sun on the last swing that would bring it back toward Earth, astronomers throughout the world watched for it daily.

The first tentative sighting came July 19 at the European Southern Observatory in Chile, but it was unconfirmed. Another sighting came from Japan. Then, finally, at Palomar again, in the last days of July, James Gibson locked onto the comet for three successive nights. He measured and image-enhanced it by computer and, within the last week, confirmed that it was Halley's.

In the next few months, the comet will become increasingly visible to non-astronomers.

Danielson said he plans to watch it at every opportunity in the coming months until it disappears again into the void past Neptune. "After all, we're talking once in a lifetime," he said.

The best way to observe the comet's passage will be with instruments mounted on spacecraft flying nearest it. The European Space Agency's Giotto craft, two Soviet craft and one from Japan named Planet A are expected to be in the comet's vicinity next March.

Two planned U.S. space shuttle flights next January and March are to be dedicated to observations of the comet from low Earth orbit.

Some American astronomers have expressed surprise and disappointment that no other U.S. spacecraft will be in position to watch Halley's.

"Your ancestors and mine sat in a cave and worried about this thing," Joseph Veverka of Cornell University said about Halley's comet. "Our children may not forgive us."