VOL. LV.3 PHILOSOPHICAL TRANSACTIONS. 2flS 



XXXllI. On the History of the Return of the famous Comet o/" l682, with 

 Observations of the same, made at Paris, at the Marine Observatory, in Jan. 

 Feb., March, Jpril, May, and the beginning of June, I759. By M. 

 Messier, Astronomer, Keeper of the Journals, Plans, and Maps, belonging 

 to the Marine of France, and F. R. S. &c. Translatedfrom the French by M. 

 Maty, M.D. Sec. R.S. p. 294. 



Dr. Halley, who was first aware of the unequal returns of this comet in its 

 former appearances, which he found to have been*lternately of 75 and 76 years, 

 was likewise the first who assigned their true cause<- He ascribed it to the nearer 

 or more distant approaches of the planets of our system ; and having observed 

 that this comet came very near Jupiter in the summer of 1 681, above a year 

 before its last appearance, and remained several months in the neighbourhood of 

 that planet, he judged that circumstance alone sufficient to have considerably 

 retarded its motion, and prolonged the duration of its revolution. Hence he 

 concluded that its return was not to be expected till the latter end of 1758, or 

 the beginning of the next year. Dr. Halley observes, in confirmation of this 

 opinion, that the action of Jupiter on Saturn is alone sufficient to alter the du- 

 ration of Saturn's period one whole month; and he adds, how much greater ir- 

 regularities must not a comet be liable to, which at its remotest distance gets 

 near 4 times farther from the sun than Saturn, and whose velocity in drawing 

 near the sun needs but a very small increase to change its elliptic into a parabolic 

 curve. 



Dr. Halley does not determine more exactly the time of the return of the 

 comet of l682; neither could he do it but by determining exactly the effect of 

 the neighbourhood of Jupiter, which must very sensibly affect the velocity with 

 which the comet was moving towards the sun. Besides, regard must be had, 

 not only to this approach to Jupiter in 1681, but also to the other approaches 

 to this and all the other planets, which act more or less on the comet, as they 

 do on each other. In short, it'was necessary to consider all the different situa- 

 tions and distances of all the planets with regard to the comet, during the whole 

 of its last revolution, and even during the former ones, when the returns had 

 been found to be unequal. 



M. Clairaut undertook this calculation, and his results differed but one month 

 from the observation. No small degree of exactness this, considering the im- 

 mensity of the object. In November 1758, he published his conclusion, which 

 allowed about 618 days more for the period that was to end in 1759 than for the 

 former; whence he inferred that the comet must be in its perihelion towards the 

 middle of April. The comet however reached its perihelion on the 13th of 

 March in the morning. M. Clairaut has since published the methods and cal- 

 culations, by which he has arrived at this conclusion. 



