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Eclipsing the Sun
| Article
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19271 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
7 / 1991 |
1,578 Words |
| Author
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Naomi Pasachoff and Jay M. Pasachoff
Naomi Pasachoff, research associate in astronomy at Williams
College in Williamstown, Massachusetts, has viewed six total
solar eclipses. Jay M. Pasachoff, director of the Hopkins
Observatory at Williams College, has made 15 eclipse
expeditions. They are coauthors of science textbooks on the
elementary, junior high, and college levels. |
"The rim of black spread slowly into the sun's disk," said the Connecticut Yankee in Mark Twain's story. "The multitude groaned with horror to feel the cold uncanny night breezes and see the stars come out." It is no surprise that King Arthur was willing to exchange the Yankee's freedom for the sun's return.
Twain's Connecticut Yankee was making use of a remarkable celestial coincidence: The moon is 400 times closer to the earth than the sun, and it is also 400 times smaller; thus, they appear to be the same size. This remarkable coincidence is not shared by any other planet and moon in our Solar System.
When the earth, moon, and sun are in a straight line we usually experience a new moon, but at irregular intervals, when the alignment is exact, we are fortunate enough to see the beautiful result of the apparent equal size of the sun and moon: a total solar eclipse.
From somewhere on earth about every 18 months, we can see the moon gradually covering the sun. Little by little, for an hour or so, the bright solar surface is hidden, though usually we wouldn't notice it happening unless we knew to look. It is only the height of the eclipse that brings dramatic change. The sky becomes eerily dark, and shadows sharpen strangely.
As night comes on, even at noontime, ripples of light and dark known as shadow bands flit over the landscape. The last bright bead of sunlight, visible at the edge of the moon, glows so brightly, compared with the surrounding darkness, that it seems like the diamond on a ring; thus, the phenomenon is known as the diamond-ring effect.
Then, the total part of the eclipse begins. The diamond ring is obliterated, and a white halo appears around the moon, as our eyes adjust to the darkness. We see this solar corona peering around the dramatic black silhouette of the moon as it hides the sun. Overlying the innermost corona, bright red loops and curves come out from a reddish border that is gradually covered over. This chromosphere, from the Greek words for "color sphere," is, like the corona, gas that surrounds the sun.
Totality, or total eclipse, may last only a fraction of a second, or it may last as long as 7 minutes. The seconds or minutes are an emotional time for those fortunate enough to view the spectacle. All too soon, the darkness is broken by the appearance of the other side of the chromosphere and then a
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