What group of 800 American intellectuals would miss both the Democratic National Convention and the opening of the World Chess Championship to make a round trip to a nondescript point 900 miles out in the cold North Atlantic?

Eclipse freaks, of course.

Amateur and professional scientists took over the Greek Lines' Olympia earlier this month for a week's cruise, not to the sun, but to the moon's shadow, to observe the most spectacular of celestial events: a total solar eclipse. The eclipse was only partial in the United States, and partial eclipses display none of the impressive phenomena that can be observed by those for whom the sun's disc is completely covered by the moon, those in the narrow path which the lunar shadow tracks across the earth. American eclipse buffs who hoped to witness totality had to choose, therefore, among an arduous journey to Alaska's North Slope, a high risk of cloudiness in Nova Scotia and Prince Edward Island and a voyage on the Olympia to the mid-Atlantic, where ocean waves would make photography more difficult but the probability of avoiding cloudy weather would be enhanced by the vessel's maneuverability.

To increase the attractiveness of the cruise, its sponsors hired a staff of scientists, including Scott Carpenter, the former astronaut, to give passengers week-long mini-courses in astronomy, navigation, oceanography and astrophotography and to lead star-identification sessions on deck.

Predictably, the group aboard ship were not exactly typical cruise passengers. One third were high school teachers or had university affiliations. Nearly all were sophisticated photographers and most had brought aboard telescopes, long telescopic camera lenses or both. About half had traveled to earlier eclipses. (The observer who stays in one place must wait 360 years, on average, before totality comes to him.)

Dorothy May, an amateur observer from Nashville, had seen two previous eclipses, one in Argentina, the other in the Canary Islands, in her 69 years. She had been rained out of three others, including one for which she had participated in a two-day eclipse train excursion from Winnipeg, Manitoba, to Hudson Bay in 1963. Like many eclipse veterans, Mrs. May chose the cruise because community observation and discussion promised to heighten the experience.

A team from the Scripps Institute of Oceanography in La Jolla, Calif., would conduct an eclipse experiment possible only at sea; the would lower a 300-pound echo chamber over the Olympia's bow to determine whether the layer of microscopic ocean animals called plankton rises at night merely because it gets dark or because of an internal biological clock that would not be triggered by the extraordinary, sudden darkness of the eclipse.

The trip had its share of children, as well, but they were, of course, of the post-astronaut generation. Lectures on " Astronomy for Young People", particularly the under-10's, were far more advanced than those for the passengers generally and explored ionization in the solar atmosphere, element formation by nuclear fusion in stars and the correlation between the temperature of stars and their color.

The ship's common rooms converted well enough to floating classrooms, and the Greek Lines' regular cruise staff finished a poor second to the scientists in competing for passengers' attention. From Dawn (100 early risers made it to the 6 A.M. bird-watching sessions) to late at night, passengers crowded into the Olympia's cabarets and bars and sat in velvet armchairs or on low cocktail tables to watch, under romantically dim lighting, as Dr. Joseph Chamberlain, director of the Adler Planetarium in Chicago, explained how to use a sextant.

The ship's regular staff knew that nearly every passenger had signed up for courses but gaped nonetheless when, on the half hour, the ship's corridors were filled, like the halls of a college, with changing classes. "This afternoon at 2:30 I have meteorology and at 3:30 I have oceanography," said a man rushing to a class. "Then I think I'll have a drink."

The ship's calisthenics instructor was surprised to have only a single pupil, while oceanography attracted 580 of the 800 passengers. The dance teacher was utterly unemployed. "The ship's social staff had developed a complex," said Ami Vassilliadis, the Greek Lines' executive vice present, who took the cruise to witness his first total eclipse.

The bartenders, too, had little to do, a source of real surprise and greater distress to the liner's management. Unable to attract a quorum, the ship's horse races had to be canceled. Each day the Olympia Times announced, "Tonight's Dress: FORMAL", each evening the passengers collectively mutinied against the stricture.

The cruise organizers attempted to integrate astronomical activities with the customary social events of an ocean liner, which may have been more a gesture to placate the Olympia's temporarily, superfluous cruise director than a fulfillment of passengers' requests. One night clouds obscured the sky and the stargazing sessions was replaced by dancing. But several determined sky enthusiasts remained on deck until midnight, when the clouds broke. Then they demanded instruction. "It's criminal the way they are carrying on down there," said a middle-aged woman, indicating the throbbing music coming from the ballroom. "Why can't they at least get a telescope on the deck?" Some of the staff's efforts to weave an astronomical theme into normal cruise routine quickly wore thin, however, like a joke that goes on too long. Most passengers probably appreciated the band's humor when, on the first evening, it accorded each scientist as he was introduced on the cabaret stage a musical fanfare normally reserved for singers or comedians. This was followed, however, not only by a special three-person dance (for sun, moon, and earth) called the Eclypso, but also by a continuing succession of sun and moon songs, such as " Canadian Sunset" (" the big song for this cruise," said the cruise director), "Moon River" and "Shine On Harvest Moon"

Consider that, on top of this, first-run movies were replaced by old science-fiction films such as "2001" and "20,000 Leagues Under the Sea" and even older flicks featuring eclipses, such as "A Connecticut Yankee in King Arthur's Court," and you begin to get the picture.

The eclipse would occur on the third day of the cruise and anxiety over the weather mounted with every hour. Ominously, rain began falling minutes after the ship left the pier, causing considerable gloom. After all, a family of four had to spend more than $1,500 to witness an event that would last only 115 seconds, and a single cloud, not to mention fog or overcast, could spoil everything.

Prof. Edward Brooks, a Boston College meteorologist, was aboard to guide the ship to the clearest location, but he looked worried as he delivered his weather briefing to the passengers on the first evening.

"We can go to 41°N 55°W in the Gulf Stream, or in the other direction to the Gulf of St. Lawrence," he said. "But once we choose, we won't have time to change destinations. The trouble is that there is a major storm between these two positions, and we don't know which way it will move. We have to choose between the warm, partly cloudy, showery side and the cold, partly cloudy, showery side. Applying eight mathematical models and our best scientific skill, we can say that if the storm is going south we should go north, and if the storm is going north, we should go south. Unfortunately, I'd say that our chance of seeing the eclipse is now only 50-50."

Just before midnight, Professor Brooks collided with the ship's bureaucracy, and it began to look as though cruise ships could not really adjust to specialized charters. A passenger who lingered near the radio room heard the scientist arguing with a Greek mate. "I have to see the captain," he said. "I've been trying to find him for an hour."

The ship's officer looked bemused. "You must mean the staff captain, or the steward," he said. "No one sees the captain."

"I'm Dr. Brooks," protested the professor. "I'm sure he wants to see me. I have to tell him to turn the ship around. We have to set a new course."

Now the uniformed officer knew that he had a looney on his hands, "The captain has gone to bed," he announced. "He cannot be disturbed. You see him in the morning."

Just then another ship's officer ran up. "Dr. Brooks, Dr. Brooks," he said, a little breathless. "Where have you been? The captain has been looking for you all over the ship." And he hustled him away, to the astonished stare of the mate.

The course adjustment didn't seem to do much good, however; all the next day the sky ahead of the ship was filled with dark clouds. At 11 o'clock that night, with the eclipse less than 16 hours away, Brooks was on the bow, describing to a jittery audience the causes of the impressive lightning storm they were watching ahead of the ship. "Notice how that lightning is horizontal, from cloud to cloud, rather than vertical," he said. "We are about 200 miles from the eclipse zone now, and I'd say that storm is at least 100 miles deep. I don't think it's 200 miles, but I will still put our odds at 50-50."

By morning, however, the ship had passed through the storm, and only scattered clouds could be seen. Sunlight poured on the excited passengers. Brooks was on the bridge, aiming the ship precisely for a hole between groups of clouds on all sides. Passengers began setting up their equipment, until the starboard side on each of the decks was lined with telescopes and camera mounts.

The display of optical equipment included a surprising assortment of homebuilt assemblies, such as the telescopic camera that Stephen Greenberg of San Francisco had built from an Army-surplus aerial lens and a Nikon bellows camera. (Greenberg and his wife, Sandy Jo, actually had two cameras, for when they asked the Nikon company some technical questions and described the cruise, the firm lent them a 1000 mm. Telescopic lens.)

Most rigs were set up to photograph or measure a particular phenomenon. Long exposures would be most useful for filming the sun's corona, the brilliantly glowing streaked halo visible only during a total eclipse, that radiates from behind the black circle of darkened moon which covers the sun's face. Shorter exposures would be used to capture the solar prominences, poetically described by Dr. Hess as "leaping red tongues of incandescent gas" that explode from the sun's surface. A group from Brooklyn prepared, with high-speed film, to attempt a rare photograph of the "shadow bands," mysterious fast-moving parallel lines of darkness that sweep faintly across the landscape a few moments before totality in some, but not all, eclipses.

As the partial phases of the eclipse began, the ship's engines were turned off, to reduce vibration, as was all generator power, even for the air-conditioning. The bingo game scheduled by the cruise director for eclipse time was not officially canceled, but it went unattended. For an hour the passengers observed partiality with pinhole cameras and specially filtered glasses. Dr. Brooks had done well; the sky was perfectly clear.

Then, over the ship's speaker system, the countdown began. Five minutes. The sky darkened. A black shape, somewhat like a tornado (it was the approaching shadow of the moon) appeared in the northeast. The air became chilly — it would drop 25 degrees in 15 minutes — and children asked for sweaters. Three minutes. The horizon glowed eerily, with an electric color. The sky became increasingly dark, as dark as late evening, but deeply blue rather than gray. The pigeons of the ship's magician tucked tucked heads under their wings. One minute. Eight hundred passengers shivered — both from the sudden cold and the tension.

"There it is," someone yelled, and for the shortest two minutes in the experience of anyone aboard, novice and veteran alike, passengers and crew gaped into the dark blue sky at the black and while eye that was the total eclipse. A few had the presence of mind to look around them rather than stare transfixed at the most impressive sight, the corona; these saw the colors of a late afternoon sunset in the east, the planet Mercury, the star Procyon (normally a winter star, for it is overhead during the day in summer) and hues so strange that hundreds of people travel thousands of miles to see them during total eclipses. As for sound, imagine a babble becoming a hush, followed by the clicking of closely spaced cameras snapping 8,000 to 10,000 pictures within two minutes, mixed with the kind of "oohs" and "ahs" that escape from crowds watching fireworks displays.

Then the inevitable words from the loudspeaker — "10 seconds left," and the appearance of the "diamond ring" as the corona swelled in one small brilliant spot — the first glimmer of real sunlight escaping between two mountains on the moon, now moving off center.

Eight hundred observers gasped collectively, then looked away to avoid eye damage; the crew hoisted an eclipse flag on the ship's mast and the band struck up "You are My Sunshine." The tension subsided; many passengers might then have danced for the first time, but the deck was too littered with tripods.

No one was disappointed. "I've been to six eclipses since I watched the 1925 eclipse at the age of 11," said Morton Schiff of Washington, a past secretary of the National Capital Astronomers, " and I was more nervous this time than ever before. But this was one of the most beautiful and pearly coronas I ever saw, comparable only to the one in Wells, Me., in 1932." The professionals agreed.

"That was the most magnificent corona I have ever seen," said Dr. Hess. The Olympia's captain, John Katsikis, who had once seen an eclipse in Greece, was somewhat more blasé. "It was very beautiful," he said. The Scripps scientists were also pleased: The plankton rose.

The cruise and its classes continued for four more days with stops at Gaspé,Quebec, and Sydney, Nova Scotia, but the climax had clearly passed. Most passengers felt simultaneously elated and sad, because the continental United States had now entered a long dry spell for total solar eclipses; the only one before 2017 that it will witness will occur on Feb. 26, 1979 in the State of Washington. However, almost all of the 21 total eclipses that will take place between now and the year 2000 will cross an ocean at some point, so eclipse cruises may multiply.

The Olympia cruise was the idea of Dr. Phil Sigler, a teacher of social science at Staten Island Community College, and his brother-in-law, Ted Pedas, who directs the planetarium at Youngstown (Ohio) State University. An earlier attempt on their part to combine business and astronomical pleasure had failed; the city fathers of Eclipse, Va., along the path of totality of the March 7, 1970 eclipse, had refused to let them conduct a rock festival in connection with the event, fearing an influx of hippies and drugs.

When Sigler and Pedas tried to find a steamship company that would be willing to send a liner to the path of this year's eclipse, most flatly refused. According to Pedas, the owners could see no point in diverting a ship from the Caribbean, trading an almost surely full complement of passengers for the risk of an unprofitable voyage in chilly waters, and so they demanded that the organizers guarantee a full house. Finally, though, the idea caught the imagination of Ami Vassilliadis and as the Greek Lines' executive vice president he was able to commit his company to the venture.

Travel agents, however, were skeptical that the cruise would sell; they did not push it and sold only 10 per cent of the bookings. But the gamble paid off through announcements at planetariums and advertisements in astronomical journals: Every stateroom was sold. Even a room made available by a last-minute cancellation was grabbed by a man who appeared at Pier 97, cash in hand, an hour before sailing.

The next total eclipse will be June 30, 1973, over the western Sahara and a part of the Atlantic. Sigler and Pedas are about to announce a 14-day cruise to that eclipse aboard the Cunard Line's Canberra, and the trip is likely to include a land tour of parts of west Africa. The eclipse will be the longest until 2150, lasting more than seven minutes.

Sigler and Pedas plan to introduce a new element: eclipse insurance, which will return passengers' fares if the eclipse is rained out or clouded over (a near impossibility near the Sahara). Pedas feels confident the new venture will succeed despite a higher price tag. "Leisure time is so plentiful now," he says. "especially for professionals. The pressure to have fun, fun, fun is passé —people want more than that."

...to see a total eclipse of the sun, you will have to station yourself on the right day along a path thousands of miles long but only 100 miles wide. Total eclipses occur once every year or two, but most of them are over water or unpopulated areas. New York, which last witnessed a total eclipse in 1925 (totality was visible only for those north of 80th Street) will have to wait until April 8, 2024.

Some good bets (and some exotic locations) for eclipse viewing in the near future, aside from the Sahara eclipse next year, are Australia on June 20, 1974, northern South America on Oct. 12, 1977, Java on June 11, 1983, Antarctica on Nov. 12, 1985, Finland on July 22, 1990 and Hawaii on July 11, 1991. Younger readers will undoubtedly be pleased to learn that the United States will enjoy many total eclipses during the 21st century.

Information about location, causes and effects of eclipses can be obtained from planetariums. Sky and Telescope Magazine, available in most public and university libraries, usually runs a detailed story about a year before a solar eclipse occurs.