Strategic Eclipse

Solar eclipses were once innocent festivals of science, observed for the blameless information that could be extracted from them. That age has passed. Last week the Air Force Cambridge Research Center told how the eclipse of next June 30 will be organized as elaborately as a major bombing raid.

The actual observing will be done by civilian scientists, U.S. and foreign, coordinated by the American Geographical Society, but the Air Force will finance the costly campaign. It will airlift the scientists to inaccessible sites and supply them with intricate gear and radio time signals. In return, the Air Force hopes to get more accurate information about the shape of the earth and about distances between widely separated points on its surface. Both of these items would be of value in dispatching aircraft or guided missiles to global targets.

When the great, round shadow of the moon sweeps across the earth next June from Nebraska to India, its speed will be known accurately from astronomical data. It will be like a railroad train traveling at known speed past stations whose distances apart are not known as accurately. By pinpointing the train's time of arrival at each station, the distances between stations can be computed.

The "stations"' on the path of the solar eclipse will be ten well-equipped observatories strung out from Canada to Iran (see map). The biggest gap will be the U.S.S.R., where the Russians presumably have stations of their own. As the shadow sweeps past, each observatory will determine the instant of totality, i.e., the time when the moon is centered in front of the sun. This can be done by 1) taking high-speed motion pictures of the eclipse, 2) watching photoelectrically for the moment when the light from the sun is weakest, 3) photographing the sun's spectrum, which changes character sharply during the event. Use of all three methods, the scientists hope, will give the instant of totality to one hundredth or even one thousandth of a second. Thus, the distance between North America and Europe may be computed with an error of only 150 ft.

No such accuracy would be possible without consideration of the moon's rugged topography. So the Naval Observatory (as interested in practical geodesy as the Air Force is) has supplied a new and accurate map of lunar mountains and plains that will show on June 30 at the edge of the moon's disk. Shafts of sunlight slipping through lunar passes can be allowed for in figuring totality. If such calculations were not made, the error of fitting the North Atlantic to missile warfare would increase about ten times.

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