POPULAR ASTRONOMY 



375 



THE HEAVENS I> JAM A I? Y. 



BY GARRETT P. SERVISS, BROOKLYN, N. Y. 



The keynote of astronomy in the pres- 

 ent year is furnished by the approaching 

 return of the great comet of Halley. 

 That comet will not be in perihelion, i. e., 

 at its nearest point with regard to the 

 sun, until May, 19 10, but during this 

 year it will .come into sight, and every 

 observatory in the world will be busy 

 studying it. The preparations for this 

 great astronomical event have already, 

 for a long time, been going on, because 

 it is an unparalleled thing of its kind. 

 For this reason we give in a corner of 

 this month's chart a little diagram show- 

 ing the orbit of Halley's comet and the 

 position which the comet occupies at 

 the present moment. It is now about as 

 far from the sun as the planet Jupiter, 

 and if it were a large solid body like 

 Jupiter it would already be plainly visible 

 to the naked eye. But a comet is not a 

 solid body, although it is probably made 

 up of solid particles, and onlv when it 

 gets near the sun does it begin to dis- 

 play its characteristic features. Then 

 only is the wonderful tail unfolded to 

 amaze all beholders. The tail of a comet 

 is believed to consist of immeasurably 

 minute particles which are shot off from 

 the head, or nucleus, by some repellant 

 force emanating from the sun. Recent 

 investigations seem to prove that this 

 force is electric in its nature. It has 

 also been thought that the "push" of 

 the light waves from the sun may play 

 a part in the phenomenon for strange as 

 it may seem, it is now known that the 

 waves of light are able to drive before 

 them excessively small particles of matter 

 which they may encounter. The idea is 

 that the heat and perhaps the electro- 

 magnet action of the sun on the ap- 

 proaching comet have the effect of 

 r eleasing infinitesimal particles which 



are then driven away by the flowing 

 light waves (which travel 185,000 miles 

 per second) like dust before a high wind. 

 The certainty that the sun exerts a re- 

 pellent force of some kind is sufficiently 

 established by the well-known fact that 

 the tail of a comet always points away 

 from the sun, changing its direction as 

 the head of the comet passes around the 

 sun so that the tail always remains on 

 the off side. The reason why the return 

 of Halley's comet excites so much atten- 

 tion from astronomers is because it is 

 absolutely the only really great comet 

 the period of whose revolution and the 

 date of whose returns are known. It 

 comes back once about every seventy-six 

 years. It was seen in the time of Sir 

 Isaac Newton, and then Halley, who in 

 this respect outstripped Newton, detected 

 the fact that it was a periodic comet and 

 predicted its return in 1758. Halley was 

 dead long before the comet got back, but 

 back it came as he had predicted. The 

 next return was in 1835. The present 

 return is the third since the days of New- 

 ton. Just at what moment it will be 

 caught sight of nobody can tell. That 

 fact gives special zest to the search. 

 There is some hope that photography 

 may reveal it some time this winter, for 

 very faint objects are frequently caught 

 in that way before they become apparent 

 to the telescope. As soon as it becomes 

 visible its course through the sky will 

 be represented on our monthly charts. 

 Halley's comet during some of its returns 

 in the past was a very startling object, 

 spreading terror all over the world. It 

 was visible at the time of the Norman 

 conquest of England, and it has recently 

 been shown that its appearance on the 

 present occasion will be strikingly similar, 

 as far as the conditions of visibility are 

 concerned, to that which it presented in 

 1066. Its period varies to the extent of 

 several years on account of the disturbing 



