402 



COMETS. 



like what we now understand the comets to be, 

 intersecting and traversing those solid spheres of 

 crystal, was wholly inadmissible and irreconcilable 

 with the received notions, touching the heavenly 

 bodies ; a striking example of the tendency of 

 error to propagate itself. 



Comets being thus recognised as very distant 

 bodies like the planets, the next inquiry was to 

 ascertain the paths they described, and the laws 

 which govern their motions. The keen and pene- 

 trating eye of Newton was now directed to this 

 subject, and comets at once assumed the dignity of 

 planets, revolving round the same central body, 

 describing the same kind of curves, preserved in 

 their places by the same forces, subjected to the 

 same laws, and differing only, or principally in this: 

 that their orbits are more oval, or more elongated, 

 and lie in all manner of directions. This bold posi- 

 tion was put to the test, in the case of the remark- 

 able comet of 1680, which presented itself at this 

 critical juncture, as if to vindicate this class of 

 bodies of which it was so splendid a representative. 

 The genius of Newton triumphed in this as in all 

 his great enterprises ; and comets now began to be 

 regarded as an important part of the solar system. 

 To put this rational and sublime theory beyond all 

 question, and to convince the world of its truth, 

 it was only necessary to identify a comet as one 

 which had before appeared, or, in other words, to 

 foretell the return of one of these bodies, and de- 

 lineate beforehand its path through the heavens. 

 This nice and difficult task was undertaken and 

 accomplished by Halley. By comparing a comet 

 which appeared in 1682, and which he saw himself, 

 with one which was observed in 1607, they seemed 

 to describe one and the same orbit.* It was not 

 to be supposed that two comets would follow each 

 other in identically the same path round the sun. 

 It was fairly presumed, therefore, that these were 

 not two separate comets, but different appearances 

 of one and the same comet ; and that the interval 

 between 1607 and 1682, or about seventy-five years, 

 was the time employed to complete a revolution. 

 This conclusion was rendered still more probable 

 by going back about seventy-five years further, 

 iiamely, to 1531, when we find a comet described, 

 that, from the observations that have come down 

 to us, evidently pursued the same track through 

 the heavens, which was described by that of 1607, 

 and that of 1602. If any doubt remained of the 

 identity of these comets, it must certainly be re- 

 moved, when we are told further, that at another 

 interval of the same length, that is to say, in the 

 year 1456, there is a record of a comet whose path 

 seemed evidently to correspond with those already 

 referred to. 



With such evidence before him, Halley ventured 



* An orbit is recognised to be the same with a former one, 

 when it has in all respects the same position in the heavens, 

 that is, when there is an agreement in the following- particu- 

 lars. 1. The perihelion distance, or distance of the point of 

 nearest approach to the sun. 2. The position of this point or 

 longitude of the perihelion. 3. The inclination of the plane of 

 the orhit to the plane of the ecliptic. 4. The position of the 

 intersection of these planes, or the longitude of the node. 5. 

 The direction of the motion, whether from west to east like 

 the planets or the re verse. The comet of 1G82, for instance, 

 had its orbit determined as follows : 



Inclinatioi 

 17=42' 



Longitude 



of the nude 



50 4S' 



Direction. 

 Retrograde. 



Perihelion Longilndo . 

 distance the prinrli< 

 0. J8 Son'l Ml* If 



diMence from the 

 eartb>. being I. 



The following are the corresponding particulars for the 

 comet of 1607. 



O.JS 302" Iff 170 2- SU21' Ketrr.gride. 



to infer that the comet of 1682 revolved round the 

 sun in about seventy-five or seventy-six years, and 

 that consequently it might be expected to appear 

 again, after the lapse of another such period. He 

 accordingly had the courage and the good fortune 

 to predict that this same comet would return to- 

 wards the close of the year 1758, or the beginning 

 of 1759. The time of a revolution seemed to hi; 

 liable to considerable variation, which might be 

 attributed, with great probability, to the attraction 

 of the planets. To fix the time of the predicted 

 return, as precisely as the nature of the case would 

 admit, Clairaut undertook to calculate the effect 

 which the larger planets, Jupiter and Saturn, would 

 have, in accelerating or retarding the motion of 

 the comet, the result of which was, that the return 

 of the comet to its perihelion, would be delayed 

 by these two planets, about one year and eight 

 months. The time fixed by Clairaut for the comet's 

 reaching its point of nearest approach to the sun, 

 was the 4th of April. He observed, at the same 

 time, that he might err a month, by neglecting 

 small quantities in the calculation. The comet 

 was first seen near Dresden, by a peasant, on the 

 25th of December, 1758, thus verifying the predic- 

 tion of Halley; and it passed its point of nearest 

 approach to the sun, the 13th of March ITo'J, 

 within twenty-three days of the time fixed by 

 Clairaut, for the perihelion passage, and accordingly 

 falling within the assigned limits. 



It again appeared in the autumn of 1835, form- 

 ing a period of about seventy-six years from the 

 preceding appearances. There is reason to believe 

 that an appearance of this comet can be traced 

 back as far as 130 years B.C.; and that it is tlie 

 same comet that was seen in the years 323 and 399, 

 when it was decribed as being of vast magnitude 

 and horrible aspect. This is also supposed to be 

 the comet which appeared in 550, 855, 930, and 

 1006 ; in which latter appearance, its nucleus is re- 

 ported to have been four times as large as the 

 planet Venus. It was seen in 1230, 1305, and 

 1380, and again in 1456: at this last visit it was 

 accompanied by a tail, which extended over sixty 

 degrees of the heavens, and gave rise to so much 

 terror and consternation, that Pope Calixtus ordered 

 prayers to be offered up from all parts of Christen- 

 dom, for the removal of the comet and its malign 

 influences. The Turks were engaged at this time 

 in a successful war against Greece ; and when 

 Europe was fearing nothing less than subjugation 

 at the hands of the victorious Turks, this comet, 

 having a tail in the form of a Turkish sabre, was re- 

 garded by the tribes of the West as the emblem of 

 divine vengeance. After this, its appearances and 

 progress are regularly recorded to the present times. 

 In the sixteenth century it is said to have displayed 

 a fine gold colour. Halley, in 1682, describes it as 

 being as large and bright as Jupiter, with a tail of 

 30. The inclination of its orbit to the ecliptic is 

 about 7T. In 1835, the length of its tail to the 

 naked eye was about 6 ; with a telescope it ap- 

 peared larger. 



The fulfilment of Halley 's memorable prediction, 

 Placed the theory of comets upon a sure basis. It 

 has not only confirmed those who had entered into 

 all the a priori reasoning upon the subject, but it 

 was of a nature to carry conviction to the unlearned 

 and the sceptical, to convert the mass of man- 

 kind, and make them firm believers, not only in 

 the doctrine respecting comets, but in the whole 

 science of modern astronomy. A comet is no 



