ABOARD THE CANBERRA, in the Atlantic off Mauritania, June 30 — Today in one of the longest eclipses of modern times the moon's shadow swept across the entire width of Africa. Scientists on this ship off the African coast and elsewhere along its path worked frantically to record the event, and on the mainland native populations gazed aloft in wonder and, perhaps in some cases, trepidation.

While those on the ground, including thousands of encamped tourists, watched the dazzling corona spring into view as the last vestige of brilliant sunlight was snuffed out, a French supersonic Concorde jet airliner raced to keep up with the speeding shadow of the moon, which at times was moving at almost 1,400 miles per hour.

The event, which will not be matched for duration until the year 2150, began as the sun rose over the easternmost part of South America. It was then that the moon began nibbling at the solar face, and by the time sunrise had reached the Brazil-Guyana border the sun was fully hidden.

The lunar shadow sped across the Atlantic becoming larger and the eclipse more prolonged. By the time it reached this ship and others gathered off the coast of Mauritania the eclipse's duration on the centerline was about six minutes.

A great shout went up from the 2,600 people on the upper decks of this ship as the final crescent of sunlight shrank into a brilliant diamond on the edge of the black lunar disk, then vanished.

Thousands of instruments from giant one-ton telescopes to small hand-held cameras were aimed at the spectacle. Although the ship has six acres of upper decks — more, it is said, than any passenger ship except the Queen Elizabeth II — there was hardly a square foot not occupied by a telescope mount or a camera mount.

Some passengers referred to the decks as the "tripod national forest." It was estimated that at least a million dollars worth of equipment was deployed there. Included were sophisticated devices such as a massive scope from the Los Alamos Scientific Laboratory in New Mexico that automatically locked its aim on the sun and held there despite the ship's rolls.

There were also such homemade telescopes as one whose optics were held in place by plywood and lead pipe.

As the shadow sped on westward it crossed the Sahara, whose torrid sands suddenly became cool. There, in Central Africa, the eclipse lasted more than seven minutes.

At oases famous in colonial history, camels stared uneasily at the sudden onset of night. Birds in the path of the shadow sought their nighttime roosts.

While batteries of astronomical cameras clicked, magnetic tape flowed through electronic recording devices, and scientists sought in a variety of ways to deepen their understanding of the sun as well as the envelope of gas that surrounds it, extending beyond the earth itself. That envelope is visible nearest the sun as the glowing corona that can be seen only when the light of the sun itself is obscured as in an eclipse. The corona is of major interest because its thin outer region extends beyond the earth's orbit contributing to the environment within which the earth makes its annual journey around the sun.

A number of observations made from the ground along the eclipse path from the air and with rockets were designed to learn what happened to the daytime upper atmosphere and statosphere of the earth when sunlight is suddenly cut off.

This is a region where a complex series of reactions between atmospheric gases occurs under the stimulus of sunlight — particularly ultraviolet light. The reactions then, reverse themselves at night, emitting a glow at various characteristic wave lengths.

The rates and interdependence of these reactions have recently become of major concern in view of fears that exhausts from heavy supersonic transport traffic would upset their equilibrium and enable harmful ultra-violet rays through.

The sudden shutting off and reappearance of the sun made possible a far more precise measurement of these processes than the gradual changes that occur at sunset and sunrise. Similar observations remain as the effects on higher ionized (or electrified) layers of the upper air suspend radio waves and are used for long-range communications.

A solar observation focused on a variety of phenomena that relate to the functioning of our parent star that cannot be observed when the blinding light of the sun is visible. They included brief glimpses of the chromosphere — the lowest level of the solar atmosphere that hug the sun and is not bright enough to be seen when the sun itself is visible.

More extensive were observations of the overlying corona. The latter was visible throughout the eclipse whereas the chromosphere was hidden by the moon except at the very start and ending of the moon's passage across the face of the sun.

Within the corona, gases are exposed to extremely high temperatures, strong and complex magnetic fields, violent movements and intense bursts of radiation.

The behavior of an ionized gas, or plasma, under these conditions is of special interest to those on earth seeking to compress and heat the plasma enough to bring about the fusion of atoms. Such fusion provides the energy sources of the hydrogen bomb, but no one so far has tamed it in workable fashion.

This ship was one of two sent into the shadow zone by Eclipse Cruises, Inc. The latter is an enterprise based in New York City headed by Dr. Phil S. Sigler, a social scientist who teaches at Staten Island Community College.

The other vessel of this project is the Cunard Adventurer, which lay in the eclipse path further east. She reported completely clear skies, whereas here 150 miles northeast of the Cape Verde Islands a barely perceptible haze had been blown from a dust storm in the Sahara 300 miles to the west.

Placing this ship under clear skies was a dodging game between dust clouds nearer the coast and water clouds further to sea. Final observing conditions were almost ideal with the anchors lowered a short distance to provide drag and hold the ship into the wind. There were almost no waves.

To minimize vibration, all engines were shut down except one generator. Most interior lights were out and elevators halted, which presented no problem since almost the entire crew and all 1,868 passengers (except one who allegedly slept through the eclipse) were on deck.

Almost all the professional experiments seem to have gone well. The one from Los Alamos using a telescope system that obscured the inner corona recorded on infra-red film the extent to which wave motion in fainter light from the middle and outer corona was polarized.

A group from Carleton College in Minnesota recorded from the corona the wave lengths of light emitted by a highly ionized form of iron (one in which 14 electrons have been blown off by high temperature). The results should reveal motions within the corona.

A group from Dowling College on Long Island led by Dr. Henry C. Courten continued his search for a hypothetical object orbiting the sun within the orbit of Mercury. He has recorded evidence for such an object or objects in photographs taken when something that near the sun might become visible.

In 1845 Leverrier, A French astronomer, reported a planet inside the orbit of Mercury, which was named Vulcan, and others claimed to have seen it. However, any substantial object in that region would by its gravity affect the orbit of Mercury in ways that have not been observed.

Dr. Courten's controversial claim is that one of more smaller objects such as cometary fragments may be there. Haze, he said, may have prevented his astronomical camera from recording, but he hopes others along the eclipse path have done so.

The decks bustled with instruments brought from as far away as Australia, Mexico, France and Germany. Signs proclaimed many astronomical societies and clubs such as the "Eclipse maniacs" from the University of Bridgeport, the "young scientists" from the Natural Sciences Institute of the State University of New York at Albany and the "old scientists" from Van Nuys, Calif.

The eclipse over, one child, accustomed to the shows produced by her father, a planetarium director turned to him in delight and said, "Do it again, Daddy!"