Page:Popular Science Monthly Volume 90.djvu/920

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��Popular Science Monthly

���The great comet discovered January, 1910. The photograph was taken at the Lowell Observatory, Flagstaff, Arizona, two weeks after its first appearance in the south

��time they rise just before it, or set just after it. In the former case they could be detected by an observer who looks toward the east in the early dawn; in the latter they must be sought for in the west soon after ^unset. Often use is made of a tele- scope especially designed for this purpose, which is called a "comet-seeker." It can be moved both in a vertical and a horizon- tal direction. It is set first at some con- venient low altitude, and swung slowly parallel to the horizon as far as the observer wishes to extend his search, say 45 degrees on either side of the sunset point. After he has swept carefully from end to end of this arc, he sets his instrument at a slightly dififerent angle and slowly swings it back. This slow swinging back and forth gives to the operation its name.

Brooks — Discoverer of Twenty- Seven Comets

Such a systematic search is sure to bring rewards in the end. Dr. William Brooks, Director of the Smith Observatory, Geneva, New York, and Professor of Astronomy at Hobart and William Smith Colleges, has to his credit twenty-seven comets. In a letter to the writer he says that this is only one less than the greatest number ascribed to any single astronomer. Pons, a French astronomer who died in 1831, discovered twenty-eight, and Dr. Brooks is ambitious to add two more to his list.

Most comets come from apparently no- where and leave us again, never to return. How do we know? An astronomer has only to note the position of a comet on three separate nights in order to obtain three points in the orbit of the comet. After having computed the orbit with the aid of these points he sees whether or not that orbit is an open curve, like a parabola or an hyperbola, or a closed curve like an ellipse. Only the comets that revolve around the sun in an ellipse are ever seen again. They are called "periodic comets" because their return can be accurately predicted.

Halley's Famous Comet

Most interesting of all periodic comets is that of Halley, named after the famous astronomer who was the first to compute its orbit. The last return of Halley's comet was such an important event among astronomers that a prize was offered for the best computation of its orbit. Because of the great number of varying factors

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