SECT. XXXV. COMET OF 1770. 361 



great, since the similarity extends to no less than four elements, 

 every one of which is capable of an infinity of variations. But, 

 even if the orbit be determined with all the accuracy the case 

 admits of, it may be difficult, or even impossible, to recognize a 

 comet on its return, because its orbit would be very much 

 changed if it passed near any of the large planets of this or of 

 any other system, in consequence of their disturbing energy, 

 which would be very great on bodies of so rare a nature. 



By far the most curious and interesting instance of the disturb- 

 ing action of the great bodies of our system is found in the comet 

 of 1770. The elements of its orbit, determined by Messier, did 

 not agree with those of any comet that had hitherto been com- 

 puted, yet Lexel ascertained that it described an ellipse about 

 the sun, whose major axis was only equal to three times the 

 length of the diameter of the terrestrial orbit, and consequently 

 that it must return to the sun at intervals of five years and a half. 

 This result was confirmed by numerous observations, as the comet 

 was visible through an arc of 170 ; yet this comet had never 

 been observed before the year 1770, nor has it ever again been 

 seen till 1843, though very brilliant. The disturbing action of 

 the larger planets affords a solution of this anomaly, as Lexel 

 ascertained that in 1767 the comet must have passed Jupiter at 

 a distance less than the fifty-eighth part of its distance from the 

 sun, and that in 1779 it would be 500 times nearer Jupiter than 

 the sun ; consequently the action of the sun on the comet would 

 not be the fiftieth part of what it would experience from Jupiter, 

 so that Jupiter became the primum mobile. Assuming the orbit 

 to be such as Lexel had determined in 1770, La Place found that 

 the action of Jupiter, previous to the year 1770, had so completely 

 changed the form of it, that the comet which had been invisible 

 to us before 1770 was then brought into view, and that the 

 action of the same planet, producing a contrary effect, has subse- 

 quently to that year removed it from our sight, since it was 

 computed to be revolving in an orbit whose perihelion was 

 beyond the orbit of Ceres. However, the action of Jupiter during 

 the summer of 1840 must have been so great, from his proximity 

 to that singular body, that he seems to have brought it back to 

 its former path as he had done in 1767, for the elements of the 

 orbit of a cornet which was discovered in November 1843, by M. 

 Faye, agree so nearly with those of the orbit of Lexel's comet 



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