"It's maybe the greatest natural occurrence one can witness," said Larry Toy, a Chabot College astronomy professor. "The phenomenon is just awe-inspiring."; Solar eclipses occur twice a year, when the sun, moon and earth line up. (This line-up would occur at every new moon, except that the moon's orbit is tilted so it only crosses the sun's path every six months.); Because of variations in the moon's orbit, fewer than a third of the solar eclipses are total eclipses. In these cases the moon's shadow, up to 200 miles wide on the earth's surface, races west to east along the ground at 1,000 to 5,000 miles per hour for a few thousand miles.; Most total eclipses get little public attention. They occur over water, over the poles or in difficult-to-reach terrain. On average, any point on Earth is eclipsed just once every 360 years.; But when the shadow's narrow path does cross inhabited areas, people flock there to experience "totality" -- to watch the light snap off and the stars snap on, and to gaze at the sun's shaggy corona hanging in the dark sky for a few minutes before daylight abruptly returns.; In the United States, the opportunity has come twice in the past couple of decades -- along the East Coast in 1970 and across the Northwest and Northern Plains in 1979.; This Thursday the moon's shadow will be back for another performance, one of the century's best.; It's unusual for several reasons. At its peak, observers will see one of the longest-lasting eclipses this century -- six minutes, 53 seconds. Also, its 150-mile-wide path across Hawaii, Baja California, Mexico City and four Central American capital cities "puts more people in the moon's shadow than any other eclipse in history," says Alan Dyer, an editor of Astronomy magazine.; Finally, since the center of the shadow will pass right across the world's largest cluster of astronomical observatories, the audience will include several dozen solar scientists intent on getting the best view they've ever had.; "It's going to be great from up there on the mountain," said Harold Zirin, a California Institute of Technology astronomer who will be at one of the observatories atop Mauna Kea, a dormant volcano that makes up a large piece of the island of Hawaii.; Historic events; For science, two eclipses have proved especially historic: In 1868, spectroscopic study of the sun's halo turned up evidence of a new element, which was named helium, from Helios, the Greek sun god. And in 1919, astronomers spotted a star, which should have been behind the sun, instead peeking around the edge of the sun. That was the first confirmation of Albert Einstein's prediction that a star's gravity will bend the path of light.; But in recent years solar eclipses have declined in significance for researchers.; For one thing, chasing eclipses by trekking to remote regions with delicate astronomical instruments to capture a few minutes of data has lost much of its glamour, especially in times of tight budgets.; For another, some of the information now can be gathered any day of the year by using instruments that simulate a solar eclipse.; But this time, the eclipse is coming to them, and they can't resist a look. At 13,800 feet, Mauna Kea is the world's tallest island mountain. The air above it is still, dry and clean, making it one of the world's choicest spots for viewing the cosmos.; It's so high that oxygen deprivation is a serious concern. And, unlike most people headed for Hawaii, astronomers will be packing long underwear and down jackets in case, as sometimes happens even in July, the temperature at the summit drops below freezing.; 7 observatories in use; Seven of the nine observatories atop Mauna Kea will be used Thursday morning. (The Keck Observatory, being built by the University of California and the California Institute of Technology, is behind its original construction schedule and will not be completed in time for the eclipse.); What they want to know is more about the temperature and composition of the sun's upper atmosphere -- layers called the chromosphere and the corona -- which normally is difficult to examine against the glare of the sun.; They hope the information will provide clues to what causes the sun's 11-year cycle of sunspots and flares (now near its peak), what propels the solar wind and why the sun's outer atmosphere is, paradoxically, so much hotter than its lower atmosphere.; Solar wind studies; At some observatories, astronomers will take repeated photographs of the corona during the eclipse to study it close up and to watch for material accelerating outward to become solar wind.; Others will use telescopes to study the sun's emissions at microwave frequencies to learn more details about how its temperature varies with altitude.; But, frankly, the astronomers also want to watch the lights go out.; They'll be fighting the temptation to abandon their instruments and go outside to watch.; "The biggest problem," Zirin said, "is whether we're going to see the eclipse."
Solar eclipses occur somewhere on earth every six months but, on average, at any given point only once every 360 years. Fewer than a third are total. Because the moon's orbit is tilted, it crosses the sun's path twice a year, otherwise an eclipse would occur with each new moon. The eclipse to occur on June 11, is of great interest because of its long duration and wide path encompassing heavily populated areas. Eclipse study in 1868 provided evidence of helium and in 1919, astronomers saw a star at the edge of the sun that should have been behind the sun, thus confirming an Einstein prediction that star's gravity can bend light.