250 



THE GUIDE TO NATURE. 



rePU15\R.?KSrReN6MY 



The Heavens for December. 



BY PROF. S. ALFRED MITCHELL, OF COLUM- 

 BIA UNIVERSITY. 



Halley's Comet gives every promise 

 of being in many wa3 r s a remarkable 

 comet. That it should return accord- 

 ing to prediction was a foregone con- 

 clusion, for mathematical astronomy 

 admits of no mistakes. Since its re- 

 discovery on September n, astrono- 

 mers have been carefully measuring its 

 position in the sky, and again calcula- 

 ting its path in order to find out just 

 how near it will come to the earth, and 

 when it will pass closest to the sun. 

 Though these newer computations are 

 as yet but preliminary, they seem to 

 show that early in May the comet will 

 come very close to the earth, and conse- 

 quently it should present a magnifi- 

 cent spectacle in the sky, ' a brilliant 

 object with a long flowing tale possi- 

 bly 30 in length, as long as sixty full 

 moons, or stretching one-third of the 

 distance from the horizon to the zenith. 

 More than this, it seems altogether 

 likely that the comet will cross the 

 face of the sun, and that we on the 

 earth will pass through the tail of the 

 comet. This does not mean, of course, 

 that the world is coming to an end 

 (for there is not the remotest possi- 

 bility of it), but it will add to the in- 

 terest attaching to the comet; more- 

 over, it will require no great telescope 

 to bring this magnificent spectacle to 

 our ken, and in May 1910, the comet 

 will undoubtedly present a very won- 

 derful sight which one and all may 

 readily see. Just at the present time 

 the average person would be keenly 

 disappointed if a great telescope could 

 be placed at his disposal to view the 

 comet, which cannot now be seen in 

 a glass smaller than twelve inches in 

 diameter. If a professional astrono- 



mer could pick up the comet and set 

 the telescope on it, there would be 

 presented no long tail, but simply a 

 faint fuzzy star and it would probably 

 need all the persuasion of the astrono- 

 mer to convince the other person that 

 it was really the celebrated comet that 

 he was looking at. On October 19, 

 Professor Barnard, using the great 40- 

 inch Yerkes telescope, describes the 

 comet as "not fainter than the 13^ 

 magnitude, about fifteen seconds of 

 arc in diameter, indefinite brightening 

 in the middle, but with no elongation." 

 But the comet is rushing towards us 

 with enormous speed, and on the first 

 of December it is nearly half a million 

 miles nearer than it was twenty-four 

 hours previous. 



The great interest attaching to this 

 celebrated object caused many as- 

 tronomers to calculate its path about 

 the sun to predict its place in the sky. 

 As is well known, the comet goes 

 about the sun as its centre of motion 

 for the same reason that the earth jour- 

 neys once a year in its orbit, and the 

 reason is that it is compelled to do so 

 by the attraction of gravity. When 

 Dr. Halley first predicted the return 

 of the comet bearing his name little 

 was known of the art of calculating an 

 orbit, nor was the mathematics of the 

 day hardly sufficient to handle the 

 problem. In the two hundred years 

 since then wonderful advances have 

 been made in mathematical astronomy, 

 so that at the present time, if an as- 

 tronomer is furnished with the posi- 

 tions of the comet on but three sepra- 

 ate nights he is able to calculate its 

 orbit and predict its future path. If 

 the comet returns to visit the earth 

 its path is a closed curve, an ellipse, 

 with the sun placed at one focus. 



As may be readilv seen, if the three 



