December, 1902.] 



KNOWLEDGE. 



273 



THE COMETS OF 1903. 



By J. B. D.u-E, iM.A. 



In the year 190:3 the returns of eight perioilical comets 

 are due. It is very improbable, liowever, that more than 

 two or three out of their number will be seen. Some of 

 them will ivmaiu invisible on account of their unfavour- 

 able situation with re.spect to the earth at the time of 

 pas.sinfi; their perihelia; ami in the ease of two at least, 

 planetaiT perturbations will have disturbed the orbits to 

 such a degree that the dates of their reappearances must 

 be regarded as very uncertain. 



The following list is based upon the elements obtained 

 from the positions observed at last apparition : — 



Nnnilior of 



Periodic Approxinmto Dat»;s of times 



Coinet. Time in T.:i.st A]tlielion Next Perihelion previously 



Years. P;issai.'e. Pass:it'e. observed. 



(1) Tempel-Swift 5-53-i 3 M;ir., 1900 9 Hoc, 1902 4 



(2) Perrinc ... 6411 10 Feb., 1900 5 May, 1903 1 



(3) Giaoobini ... 6 5-19 3 Feb., 1900 15 MaV, 1903 1 



(4) Spitaler ... 6402 3 June, 1900 16 Aug., 1903 1 



(5) Faye 7 566 IJan., 1900 13 Oct., 1903 8 



(6) Winnecke ... 5832 18 Feb., 1901 30 Dec, 1903 7 



(7) Brook.'* ... 7097 25 Mar, 1900 12 Dee., 1903 2 



(8) D'.\rrcst . 6675 24 Sept., 1900 25 Jan., 19at 6 



The first-named comet was discovered in 18150, and has 

 been observed since at intervals of eleven years. During 

 last aphelion passage it w-as in the neighbourhood of 

 Jupiter, and in consequence of the attraction of this planet 

 its return to perihelion will be somewhat later than stated 

 in the table. 



The comets bearing the names of Perrine and Giacobini 

 were both discovered in 1896; Spitaler's was discovered 

 six years earlier, and has already made one unobserved 

 return. On the present occasion its distance from the 

 earth will never be much less than twice the radius of the 

 earth's orbit, so that very likely it will again escape 

 detection. Apparent proximity to the sun will obscure 

 Perrine's comet, but Giacobini's will be more favourably 

 situated, and possibly may be seen after passing j)erihelion. 



Three of the comets expected may Ix- considered to be 

 we!l-esta.blished members of the solar system. Faye's 

 was discovered in 1843; in 1819 Pons discovered the comet 

 that now bears the name of Winnecke, while D' Arrest's 

 was discovered in 1851. 



Unfortunately there is much cause for fear that the 

 good character for regularity in its reappearances hitherto 

 borne by Faye's comet will lie found to have been lost 

 since its last return. Its orbit lies close to that of Perrine's, 

 and at the beginning of 1900 both comets were at their 

 aphelia and near to Jupiter. Both bodies must have 

 experienced consideral)le perturbations, but exact com- 

 putations will alone show whether, in their new orbits, 

 the comets will come within range of observation. 



Neither Winnecke's nor D'Arrest's comets have suffered 

 much perturbation, but, like Perrine's, they will at peri- 

 helion be on the opposite side of the sun to that on which 

 the earth will be. 



The remaining comet on the list, Brooks', is by no means 

 so favourably situated as it was in 1889 and 1896 ; still its 

 detection should not be regarded as hopeless. At the 

 same time we must bear in mind that it has already 

 shown signs of disintegration, and as yet we are so igno- 

 rant of all the conditions which determine the visibility 

 of coniets that it is presumption to speak with certainty 

 regarding any of them. 



AH the above comets are members of a family closely 

 connected with Jupiter. On the accompanying diagram 

 are shown the major axes of the cometary orbits pro- 

 jected on the plane of the ecliptic, also the orbits of 



Jupiter and of the earth. The numbers jilaced at the ex- 

 tremities of the axes indicate the corresponding comets 

 in the table, the numbered points on the orbit of Jupiter 

 mark the positions of that planet at the aphelia of the 

 comets, and the similarly numbered points on the 

 earth's orbit its positions at the approaching perihelion 

 passages. 



It is noticeable that the ajihelia points all lie dose to 

 the path of Jupiter. In addition to this, the inclinations 

 of the planes of the cometary orbits to that of Jupiter are 

 small, none exceeding 18", while in every case the direction 

 of revolution is the same as that of the general planetary 

 motions. These facts indicate that whatever may have 

 been the previous history of the comets, their present 

 position in the system has been chiefly determined hy the 

 action of Jupiter, secondary of course to that of the sun. 



This group of comets is of special importance in view of 

 their comparatively short periodical times. They come 

 frequently under observat ion, and every return adds, to the 

 material for writing their life-history. 



In the triple system presented by any one of them in 

 conjunction with Jupiter and the sun, we have one of the 

 simplest cases of the problem of three bodies, a case upon 

 which Hill, Poincarc, and Darwin have bestowed much 

 labour, but whose complete solution is yet to be sought, 

 if indeed it is attainable. 



Perhaps the most striking circumstance which the study 

 of these bodies has brought more prominently into notice, 

 is that bodies moving under their mutual gravitation do' 

 not always tend towards aggregation and mutual coherence, 

 but that disintegration is a frequent result. Jupiter not 

 only captures comets, but also releases them, causing tliem 

 to move in paths which may take them far beyond the 

 limits of the solar system. Moreover, the condensation 

 of a swarm of meteorites into a planet has yet to be 

 explained, while it is not difiicult to prove that the action 

 of the sun, or of one of the larger planets, must under 

 certain conditions bring about the disintegration of bodies 

 passing near to them. Biela's comet is the classical 

 example of such breaking up of cometary masses ; but 

 similar phenomena have been observed in comets Tempel- 

 Swift and Brooks. For this reason their return may be 

 looked forward to with interest. 



