POPULAR ASTRONOMY. 



327 



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The Heavens in February 



BY PROF S. ALFRED MITCHFXL, OF COLUM- 

 BIA UNIVERSITY. 



The year 1910 has centered in it Hal- 

 ley's comet, and beyond a few com- 

 paratively uninteresting- eclipses of sun 

 and moon, nothing- else looms up in 

 the astronomical horizon. The ques- 

 tion of where the comet is needs no 

 further comment, but what it is brings 

 us to another story. The most impor- 

 tant observations to be made will not 

 be those which give us the position of 

 the comet in the sky by visual meas- 

 urements, nor yet, those obtained by 

 the sensitive plate and pnotographic 

 camera. Such observations will be of 

 enormous value, but those of greatest 

 service to the scientist will be some 

 which will not appeal very much to 

 the public mind, as they will be little 

 understood. The spectroscope is to be 

 the important instrument of investiga- 

 tion, for Halley's comet promises to 

 be the very first bright comet that has 

 visited the earth since the modern spec- 

 troscope has been devised. By its 

 means we will probably be able to 

 decipher the enigma of the comet's tail, 

 the puzzle to astronomers of all ages, 

 why it always points away from the 

 sun. How science changes its point 

 of view and adopts new hypotheses 

 as occasion demands is splendidly il- 

 lustrated by the story of these comets 

 and their tails. Since the first explana- 

 tions given three hundred years ago, 

 even before the foundation of the law 

 of gravitation, there have been plenty 

 of theories propounded, and earlier 

 ideas have been gradually discarded on 

 becoming untenable through improved 

 knowledge of the laws of matter. 



The life history of a comet, indeed, 

 seemed to contain a riddle which no 

 astronomer could fully read, and as 

 such, it was regarded as one of the 

 "problems of astronomy." Newton's 



law of universal gravitation itself ap- 

 peared to be set at naught by comets, 

 for instead of pointing toward the sun, 

 as being attracted in this direction by 

 gravity, their tails pointed in diametri- 

 cally the opposite direction, just as if 

 under the action of a solar repulsion. 

 Could this be a case which showed 

 that gravity was not universal? Or, 

 if gravity did act, what was the na- 

 ture of the force centered in the sun, 

 which actually repelled matter so as 

 to form the tail? Various theories 

 of comets' tails have resulted from at- 

 tempting to find the nature of this 

 force. 



After investigating planetary mo- 

 tions and giving to the world the 

 three great laws of motion bearing 

 his name, Kepler turned his atten- 

 tion to comets. After carefully ob- 

 serving in 1607 the comet, which 

 proved eventually to be Halley's, he 

 announced that to the best of his 

 knowledge the head of a comet be- 

 comes vaporized by the heat of the 

 sun, and that particles are driven 

 therefrom to form the tail by a force 

 of repulsion that was explained easily 

 enough by the then accepted theory 

 of light. According to this theory the 

 sun is continually emitting particles 

 of matter which travel through 

 space at enormous velocities. On 

 reaching the comet a portion of the 

 energy of these corpuscles becomes 

 imparted to the cometary material, 

 and there results a tail pointed away 

 from the sun. 



Newton, the great founder of the 

 law of gravitation in 1687, did not 

 entirely accept this explanation. While 

 believing implicilty in the emission 

 theory of light, he tried to prove that 

 gravity was universal, and consequent- 

 ly that all celestial motions must be 

 the result of gravitation, and, there- 

 fore, he thought, the repulsions pro- 



