The most observed, the most examined, the most studied and the most photographed celestial event in history will be when Halley's Comet starts its eight-month swing around the sun in September, 1985.

By the time the comet begins to darken and fade from sight as it speeds away from Earth in May, 1986, it will have been watched, measured, pictured and analyzed hundreds of millions of times by most of the world's astronomers using telescopes on the ground, in the air and even in space.

That does not count the serious amateur astronomers, who number in the thousands and who are already organizing hundreds of trips to the Southern Hemisphere early in 1986, when the comet will be at its brightest and where it will be at its highest elevation in the sky.

"I know of no other astronomical effort even comparable to the gathering interest in Halley," said the Jet Propulsion Laboratory's Ray Newburn Jr., who is coordinating the International Halley Watch for the world's astronomers.

"There will be times when Halley is so close to the Earth that 200 of the world's best telescopes will be observing Halley at the same time. Nothing like it has ever happened before."

Why study Halley's Comet so intently? Because Halley is the only comet with a large nucleus, coma (gas envelope around it) and tail that will be seen this century. Also because Halley swarms with exotic dust particles and complex molecules of sodium, carbon, oxygen, nitrogen and hydrogen that have been part of the comet since the dawn of time. Because comets like Halley are the source of the zodiacal light and the meteor showers that rain on the Earth throughout the year. And because there is nothing in the heavens like a comet.

Few events occur in the heavens like the apparition of Halley's Comet, which last appeared in the skies above the Earth in 1910 and which bears the name of 18th Century British astronomer Sir Edmund Halley. It was Halley who observed that the comets appearing in 1531, 1607 and 1682 behaved the same and therefore must have been the same comet returning every 76 years.

"Halley predicted it would return again in 1758 and it did," said the JPL's Dr. Donald Yeomans, a Halley historian and a leader of one of numerous scientific teams organized to observe the comet's return in 1985. "He was the first scientist to figure correctly that comets travel in predictable parabolic orbits around the sun."

For the people who witness it, the passage of Halley's Comet around the sun in 1985 and 1986 will be a spectacular sight. Circling the sun at speeds up to 140,000 mph, the comet's head will grow to more than 100,000 miles in diameter and its tail will spread to more than 50 million miles in length.

Lit by the sun for four months on the way in and four months on the way out, Halley's reappearance will be seen by more people than have seen any other celestial apparition in history.

Last seen by the naked eye as it headed away from Earth 71 years ago, Halley's Comet was "recovered" by the 200-inch telescope at California's Palomar Observatory Oct. 16 as it raced back in toward its 76-year reunion with Earth. Halley is now more than 1 billion miles from Earth, outside the orbit of Saturn, a path it will cross sometime in June.

The comet will light up from the heat of the sun in August, 1985, when it is about 270 million miles from Earth. Moving faster and faster as it is pulled in by the sun's enormous gravity, Halley will begin to form a tail in late November that will grow to more than 50 million miles long at the comet's closest approach to the sun on Feb. 9, 1986. On that date, Halley will be about 87 million miles from Earth.

By then, the world's major telescopes will have photographed Halley millions of times. The Soviet Union has dedicated the time of at least 10 major observatories to watching Halley, and is even moving two smaller telescopes to South America to get southern exposures of the comet.

Australia is building a new telescope at Siding Springs, Great Britain is constructing a new telescope in the Canary Islands and a European consortium is building a new telescope in southern Spain in time to observe Halley.

All of the major American telescopes will be Halley-watching, including the Hale Observatory on Mount Palomar, the Whipple Observatory on Mount Hopkins and the Kitt Peak Observatory in Arizona and the four largest telescopes in Hawaii, which will be the best viewing post (only 19 degrees north latitude) in the United States.

Because of its location, South America will be a beehive of Halley activity. The busiest will be the European Southern Observatory and the Mount Wilson and Kitt Peak Southern Stations, all in the foothills of the Andes Mountains, about 7,500 feet high.

Even the world's radio telescopes will be "watching" Halley, mostly to get the telltale signatures of the molecules spewing off the comet when it is close to the sun and washed out of sight of the optical telescopes. At last count, the JPL's Newburn said, at least 40 of the world's largest radio telescopes had assigned time to Halley.

When the comet swings behind the sun and heads away from Earth, it will be greeted by four spacecraft from the Soviet Union, the European Space Agency and Japan. The Soviet spacecraft will come as close as 6,200 miles, while the European spacecraft will fly less than 400 miles from the comet's nucleus.

The best space pictures of Halley could come from the $1 billion Space Telescope to be carried into orbit in 1985 by the space shuttle. Though it will come nowhere near as close as the other spacecraft, the Space Telescope has such elegant optics it is expected to furnish some of the best pictures.

"It will be one of the most difficult observations because the comet will be moving so fast," Newburn said. "At the same time, it will be the only spaceborne telescope that will be able to photograph the outer coma of the comet plus its tail. The others will be too close to get the whole thing."

Though the reappearance of Halley's Comet will disappoint some as it swings low in the northern skies in February, it will be a rewarding sight almost everywhere on Earth in March and April as it flies toward its next destination between the orbits of Neptune and Pluto.

Still lit by the sun and still bearing a tail, the comet will be best seen from Earth on April 11, 1986. Less than two weeks later, on April 24, the world will be treated to an even more wondrous sight. On that night, Halley's Comet will be visible in the same region of the sky as the full moon when it undergoes a total eclipse by the Earth, an event so rare that astronomers cannot even estimate how rare it is.