HISTORY OP PHYSICAL ASTRONOMY.
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cupying about 151 years. The only objection which offered itself to theopinion of their being the same comet arose from the irregularities of theperiods, and the difference in the inclinations of the orbits. The intervalcomprised between the comets of 1531 and 1607 amounted to 76 years,62 days ; that comprised between the comets of 1607 and 1082 wasequal only to 75 years, less 42 day's. The difference between the two in-tervals was therefore more than fifteen months. The inclination of theorbit was also found to vary nearly to the extent of a degree on the occa-sion of each successive appearance of the comet. These discordanceswould have offered an insuperable difficulty to many inquirers in attempt-ing to establish the identity of the comets; but lialley was too well ac-quainted with the principles of the Newtonian theory not to perceive thatthey might be occasioned by the disturbing action of the planets. Heremarked that the period of the plauet Saturn sometimes varied to theextent of 1 3 days from the action of Jupiter , and that, under certain cir-cumstances, the alteration of the time of revolution might even amount toa month. “ How much more liable to derangement then,” says he, “ is acomet whose excursion into space is four times greater than that of Saturn ,and whose orbit is so eccentric, that, if the velocity were increased by T J ^tlipart of its value, the ellipse described by the comet would be changed intoa parabola.”
He remarked, that in the summer of 108J, when the comet was approaching its perihelion, it passed so close to Jupiter that the force exertedby that planet on it amounted to T^th of the sun’s force; and, as it con-tinued for several months exposed in this manner to the powerful in-fluence of the plauet, he inferred that the periodic time must have been inconsequence affected to a very considerable extent. The question thereforewhich now suggested itself to him was to ascertain the precise time whenthe comet should again return. The period of revolution, as determinedby the appearances of 1531 and 1007, would assign the month of November,1758, for its next return; on the other hand, if the period derived fromthe appearances of 1607 and 1682 were adopted, the comet ought to re-turn in the month of August, 1757. No means existed in Halley ’s timeof computing the derangement of the comet’s motion caused by the pla-nets ; hut he very sagaciously concluded that the action of Jupiter wouldhave the effect of retarding its arrival, and that in consequence it wouldnot be visible before the end of 1758, or the beginning of 1759. “ Where-
fore,” says this illustrious astronomer, “ if it should return according to ourprediction about the year 1758, impartial posterity will not refuse to acknowledge that this was first discovered by an Englishman.’’*
As the time fixed by Halley for the return of the comet drew nigh, anintense interest was awakened in the minds of astronomers by the expectedevent, but no one had the courage to calculate the precise time when thecomet should be visible by taking into account the perturbing action of theplanets. At length, in 1757, Clairaut , who had already distinguished him-self by his researches in physical astronomy, undertook the examination ofthis difficult question. By applying his solution of the problem of three
* “ Quoeirca si secundum predicta nostra redierit iterum circa annum 1758, hoc primumab homine Anglo inventum fuisse non inficiabitur aequa posteritas.” Synopsis AstronomicCometicae. This important little treatise was first published in the volume of the Philo-sophical Transactions for the year 1705. It was afterwards considerably enlarged by theauthor, but it was not published in this form until 1749, when it appeared along with histables of the planets. The words above cited are not contained in the earlier edition.