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National Geographic : 1947 Nov
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Unlocking Secrets of the Northern Lights Wallace J. Trapilo This Aurora Is in the Form of a Drapery Viewed Edgewise It was photographed at Narsarssuak, Greenland, February 18, 1945. The white dot in the lower right corner is the planet Venus. The larger light at the extreme upper left is the moon, which, although actually only a crescent, appears full because of the length of exposure (pages 679, 701, 703). particles from the sun begin to excite the atoms of the air. The highest aurora so far known was measured by Professor Stormer in Norway and extended more than 600 miles (1,000 kms.) above the earth. This indicates, incidentally, that the earth's atmosphere must extend up to at least that height, for there can be no aurora where there is no air to be excited by incoming particles from the sun. Probably in no other way could we determine how far upward the atmos phere extends and where, at least approxi mately, it merges into the emptiness of outer space. Since we are 1,200 miles south of Professor Stormer's location, we undertook height meas urements to see how aurora heights vary with geographic latitude. We measure heights of auroras by taking photographs of the same display simultane ously from two points a number of miles apart. On the two pictures the same aurora will be in a slightly different position with respect to the stars in the background. Know ing how far apart the two photographs were taken, we can compute accurately the height of the aurora by measuring the difference in its position on the two photographs (pages 698, 699). Long-distance Phone Links Scientists Prof. C. L. Henshaw and I have been taking such photographs for several years at Cornell and Colgate Universities. My observatory is connected by telephone with the one high in the Memorial Chapel tower at Colgate, 53 miles away. On nights when a good aurora is on display, we use the telephone in arrang- 697
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