Shadow Over Sahara

Water wells are miles apart. Under the blistering sun, the temperature of the sand often reaches 180DEG F. Despite these forbidding conditions, foreigners have lately been scurrying in and out of the West African republic of Mauritania, at the western end of the Sahara --hiring the few available trucks, renting plots of land and even booking rooms in an old French Foreign Legion post. Told that the strangers are there to watch the moon black out the sun, some believers in the oasis town of Chinguetti--the seventh holiest city of Islam--are incredulous. "How can you tell the sun will darken?" a herdsman asks. "Only God can know that."

The herdsman's faith may soon be shaken. Chinguetti lies directly in the path of totality* of a major solar eclipse that will be visible across northern Africa on June 30. Although solar eclipses are not rare--at least two occur every year--this one will be unusual. Where the borders of Mali, Algeria and Nigeria meet, totality will last 7 min. 4 sec., making the eclipse the second longest in 1,433 years; the only longer one --7 min. 7 sec.--took place in 1955. A comparable eclipse will not occur again until the year 2150. Thus scientists from a dozen countries--as well as hundreds of amateur eclipse watchers--are gathering in Africa for a glimpse of the prolonged celestial spectacular.

The eclipse is more than a curiosity. Its unusual length will provide crucial extra moments for numerous wide-ranging experiments that can best be performed during such a solar blackout. Scientists, for instance, will search for comets and other bodies close to the sun--possibly even a small undiscovered planet--that would normally be hidden by solar glare. They will also test Einstein's general theory of relativity by measuring the degree to which light from distant stars is bent by solar gravity as the rays pass near the sun. It is during an eclipse that scientists can fully observe the sun's spectacular halo, or corona, believed to be caused by the outrushing of solar gases. Understanding the corona, in turn, may shed new light on the sun's thermonuclear reactions.

Not the least of the observations are those related to the terrestrial environment, involving the sun's influence on the earth's atmosphere, weather and magnetic field.

There will also be studies of phenomena on the ground itself. Some scientists will record the reaction of animals to the sudden darkening. Others will remain behind after the eclipse to assess its impact on Africa's tribesmen.

The Borana in Kenya and Ethiopia, for instance, regard eclipses as an evil omen. Even if nothing untoward happens after the event, they may use it as an excuse to kick out any unpopular ritual leaders, called Kallu.

The main U.S. scientific contingent, consisting of some 100 observers from two dozen universities and other institutions, will be divided into two camps, on opposite sides of Africa. The smaller group will set up its instruments in Mauritania, where the hot dry air should offer good viewing. But because Mauritania has experienced a severe drought for the past few years, sudden winds could blow up obscuring clouds of dust particles. Scientists are hedging their bets by establishing another camp on Kenya's Lake Rudolf, near Loiyengalani. Even more primitive than some of the sites in Mauritania, the village is accessible only by small planes or by Land Rovers on a two-day trip over rutted bush roads.

Not all eclipse watchers will endure harsh conditions. Three cruise ships, carrying hundreds of scientifically minded tourists, will station themselves off the coast of West Africa in the path of totality, while lecturers on board discuss the fine points of the event overhead. The best view should be obtained by seven French, British and American scientists. On June 30, they will board a supersonic Concorde jet in Las Palmas, in the Canary Islands, fly south and intersect the path of totality near Chinguetti. Then they will race the moon's shadow across some 1,700 miles of Africa at nearly Mach 2 speeds (1,300 m.p.h.). Traveling at times only slightly slower than the shadow itself, they will see the blackout for some 80 minutes, longer than anyone has watched a total eclipse before.

* The narrow strip of earth along which the shadow of the moon travels as the lunar disk completely obscures the sun.

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