The moon will throw its spell over the sun Thursday, and for a few brief moments the light that grants life will grow dark.

High atop a volcano on the Big Island of Hawaii, scores of astronomers will turn their telescopes toward the sun as the shadow of the moon, most fortuitously, passes directly over the world's leading observatory.

On the beaches below, many who have traveled from distant lands will feel the air grow cooler and the sky grow dim.

Far away, on the tip of Baja California, thousands will wait for their moment without a sun.

To the south, millions more from Mexico to Brazil will hope for a parting of seasonal clouds so that they might share in the moment.

It will be like that, come Thursday, as more people watch a solar eclipse than at any time in history.

For astronomers, it will be a special moment because a solar eclipse gives them their best opportunity to study the atmosphere of the sun without being blinded by it. Only when the sun's luminosity is blacked out can they turn some of their most sensitive instruments on the regions around the star. Why, they wonder, should the sun's upper atmosphere, called its corona, be 300 times hotter than the surface of the sun?

Only an eclipse offers scientists such a rich opportunity to answer such questions, and this eclipse will be something special. Not only does it pass directly over two observatories on the island of Hawaii, its passage over the equatorial region will make it linger far longer than most eclipses. Many who would do almost anything for just a few seconds of "totality" will have from four to 6 1/2 minutes to enjoy this one.

Instruments around the world, not just those in the path of the eclipse, will study the sun simultaneously to give scientists an unprecedented treasure chest when it is all over.

At the exact moment that the total eclipse reaches Hawaii, Leon Golub and several colleagues from the Harvard-Smithsonian Center for Astrophysics will launch a rocket from White Sands, N.M. The rocket will carry an X-ray telescope on a five-minute flight--almost the exact length of the total eclipse--to make X-ray images of turbulent areas in the sun's corona.

Meanwhile, half a world away, scientists in the Canary Islands will study the sun with different instruments.

While both experiments are under way, scientists from the University of Hawaii expect to capture spectacular images of the corona as the moon blocks out the sun.