904 



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 



