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A Moment under the Sun
| Article
# : |
14440 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
6 / 1988 |
924 Words |
| Author
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Barbara Tufty
Barbara Tufty is a free-lance natural science writer
who lives in Washington, D.C. |
At precisely high noon on June 21 this year, the sun will pass directly over 13,035-foot Mt. Niltake on the island of Taiwan. Throughout the modern world, the moment will probably pass without fanfare. No bells will ring, no trumpets blare. No television coverage, perhaps not even a line in a newspaper. In fact, many people will not even notice. Yet at that particular instant, on that particular spot of the Tropic of Cancer, the sun stands still in its northward journey. It stands perpendicularly overhead, beating straight down upon the mountain slopes.
This is the moment of solstice: summer solstice in the Northern Hemisphere, winter in the Southern Hemisphere. The word solstice comes from the Latin sol. Meaning sun, and sistere, meaning to stand.
At that precise second, the sun has reached its northernmost point in the Northern Hemisphere. Now all the land and seas of the Arctic are bathed in 24 hours of sunlight, from the North Pole to the edge of the Arctic Circle, as the sun brushes the horizon without setting, even at midnight. On the opposite side of the world, all Antarctica is dark in the wintry shadow of the earth, as the rays of the sun skim the Antarctic Circle at a tangent, shooting out into space.
Perhaps the reason few people notice this moment of solstice is that the sun has been excruciatingly prompt at keeping this appointment for billions of years, flooding the Northern Hemisphere with summer light and drenching the Southern Hemisphere with shadow. It's one of the great on-time performances of the universe, but so constant that, instead of appearing a miracle, it's only ho-hum.
It only seems simple, however. The mechanisms that bring this about are exquisitely complex.
As the earth makes its annual circuit around the sun, its axis remains constantly tilted--at an angle 23.5 degrees perpendicular to the plane of its orbit. The tilt causes a changing slant of the sun' rays falling on earth and the resulting change of seasons. In spring, the sun appears to move northward until it is directly above the invisible circle called the Tropic of Cancer. Now summer begins across North America, Europe, and Asia. The sun immediately begins its southward progression, ending six months later when the sun's rays favor the Southern Hemisphere. The overhead rays then fall perpendicularly on the Tropic of Capricorn, and summer arrives in the southern parts of South America and Africa and in Australia. Everything below the
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