National Geographic : 1992 May
THE EMERGING SUN AT THE END OF TOTALITY ON JULY 11 DAZZLES WITH ITS "DIAMOND RING." THE DARKNESS THAT ENLIGHTENS By JAY M. PASACHOFF IT ISACURIOUS FACT of astronomy that one of the best ways to study the sun is to have its glowing disk hidden from view. With its great light seemingly extinguished, the usual glare of earth's atmosphere drops away. Then, in a sky as dark as night, the sun's faint outer atmosphere-its veil-like corona-comes boldly into view. The total solar eclipse that swept a cone of darkness across the earth last July 11 dramatically banished the daylight. For unforgettable minutes hun dreds of scientists and millions of other watchers saw fantastic prominences masses of gas-looping from the sun's surface out into the corona. It was a special eclipse. Usually astronomers and their equipment go to the eclipse. This time the eclipse came to the astronomers, passing directly over the world's largest array of giant telescopes, on Mauna Kea volcano in Hawaii. Mauna Kea's altitude- 13,796 feet-and its clear, dry air gave astronomers and their machines an unmatched earthly view of the phenomenon. The eclipse darkened the great urban mass of Mexico City. The moon's central shadow drew a swath of darkness 9,300 miles long and as much as 160 miles wide, bringing nearly seven minutes of totality in some areas-a duration that occurs only every 18 years, 11 days. Why do astronomers find eclipses so fascinating? While telescopes can be adapted to partly simulate an eclipse, a real eclipse offers the best visibility for observing the corona. At eclipse we see the corona as a crown of light around the sun; its shape is sometimes more round, sometimes more elliptical. We see its glow extending a million miles from the edge of the sun. The corona is composed of the same gases as the rest of the sun: 90 percent hydrogen, almost 10 percent helium, and a tiny quantity of the other elements. The corona flows outward into the solar system as the solar wind-streams of charged particles. These travel 93 million miles to earth and even pass beyond the outer planets. Eclipses are a time-tested tool of solar astronomy. Observers pursuing the 1868 eclipse determined through spectroscopy the existence of the gas JAY M. PASACHOFF is professor of astronomy and director of the Hopkins Observatory at Williams College in Massachusetts. He is author of Astronomy: From the Earth to the Universe and A Field Guide to the Stars and Planets.