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Page:Popular Science Monthly Volume 90.djvu/922

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906

��Popular Science Monthly

���A group of women at the Harvard College Observatory, whose duty it is to study the photographs and record them

telescope an instrument is created which is almost the exact optical equivalent of the human eye.

Thanks to this combination of the photo- graphic plate and the telescope, the field of astronomical exploration has been almost immeasurably broadened. It is often literally possible to discover things without actually seeing them.

Among the planets there is a group of very tiny faint bodies which are known as asteroids. Be- cause of their very great number, only the approxi- mate orbits have been com- puted for most of them. This fact alone makes it difficult to find them on an ordinary star chart. Here, the photographic plate has proved itself to be of immense value.

Two photographic methods of searching for asteroids are in use — methods which are the reverse of each other. Both are illustrated in the accompanying photographs. The first, employed for many years, is based on the fact that asteroids can be detected by their movements among the stars. Ordinarily, when a telescope is used for photo- graphing the heavens, it is connected with clockwork which carries it just as fast as the skies revolve, so that the stars remain fixed in the

��field of view and appear on the photo- graphic plate as round, sharp images. If, on the other hand, there is present a body which is moving among the stars, it will produce a trail on the plate instead of a round dot. An asteroid is just such a moving body. Therefore it can be recognized by the trail which it leaves and be made an object of further study. Many of the smaller ones, how- ever, make trails which are too faint to be detected in this way. An ingenious method of over- coming the difficulty has been devised by Rev. Joel Metcalf, of Winchester, Mass. He computed in advance what the average motion would be of an asteroid in the particular part of the sky opposite to the sun at any desired time, and gave to his telescope a motion equal in amount but opposite in direction. As a result, the asteroid produced a round dot on the plate, while the stars left trails.

��Discovering the Events of Yesterday

Because the photographic plate is not only extraordinarily sensitive, but also

���Early photographs of Halley's Comet which were taken at the Yerkes Observatory in 1909. It appears on the photographic plate as a star of the seventeenth magnitude

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