THE EARTH HAS three moons, two of them very different from the familiar one. They are cosmic dust clouds first reported in 1961 by Kazimierz Kordylewksi, the Polish astronomer. The clouds are found at two of the five points in the earth‐moon system where a small mass is expected to be in dynamic equilibrium with the massive earth and moon revolving about their common center of mass. The existence of such points is most simply understood when viewed from the rotating coordinate system of the two heavy masses. Then the five equilibrium positions, known as libration points, are the places at which the gravitational forces just balance the centrifugal force on the small mass. We have been studying the two clouds for nearly three years to determine experimentally the dynamic behavior of matter at the libration points. Recently we have successfully developed techniques for photographing the libration clouds.

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