CHAP. III., 5.] 



ASTRONOMY. M. ENCKE GAMBART. 



59 



(268.) 

 Cometary 

 orbits de- 

 rived from 

 observa- 

 tion. 





(269.) 

 VI. Encke's 

 esearches 

 >n the 

 :omet of 

 819 pe- 

 iod 3J 



to the orbit of Jupiter), and made the subject by 

 him of a series of investigations altogether peculiar. 

 On this account it bears the name of Encke, instead 

 of that of its discoverer Pons, although M. Encke 

 himself, with unaffected modesty, always describes it 

 as the Comet of Pons. 



Newton gave, in the Principia, his celebrated so- 

 lution of the problem of determining a Comet's orbit 

 assumed to be parabolic, from three geocentric places. 

 This solution has been simplified and improved by 

 Lagrange and Boscovich, and also by Olbers. La- 

 place gave a method for an elliptic orbit, which may 

 represent any number of observations. Gauss, in 

 his Theoria Motus Corporum Celestium, treated the 

 subject with great skill and generality. I am unable 

 to state who first attempted to discriminate an ellip- 

 tic from a parabolic cometary orbit, or to determine 

 the period in the former from observations at one ap- 

 parition only. It is evident that such outstanding 

 differences as are irreconcilable with a parabolic 

 orbit, will be most perceptible in the case of comets 

 whose orbits have a tolerably short major axis, or 

 whose period is not very great, and will be mate- 

 rially increased by watching a comet through a con- 

 siderable part of its orbit, which the assiduous appli- 

 cation of telescopes to every part of the heavens has 

 of late years rendered much more frequent than for- 

 merly. Amongst others, Bessel, who has signalized 

 himself by a capital performance in this, as in every 

 other department of Astronomy, applied rigorous me- 

 thods to determine the orbits of the comets of 18 07 and 

 1815 ; the latter of which will very probably return 

 to its perihelion in 1887. It is undeniable, however, 

 that expert calculators have often been deceived in 

 assigning orbits, even when believed to be of short 

 period, founded upon a single apparition. 



M. Encke, however, was more fortunate in the case 

 of the first comet of 1819. Using the methods of 

 Gauss, he showed that an Elliptic Orbit of about 3 

 years must be admitted, and that the comet had pro- 

 bably been already observed in 1786, by Mechain, in 

 1795 by Miss Herschel, 1 and in 1805 by Pons. He 

 investigated with great labour the effects of the planet- 

 ary perturbations on this body, which, in the case of 

 Jupiter, are occasionally very large, if that planet be 

 in the part of its orbit near the aphelion position of 

 the comet, when it approaches the orbit of Jupiter. 

 The careful calculations of M. Encke for the next re- 

 turn in 1822 were verified by the observations of Sir 

 Thomas Brisbane, at that time fortunately governor 

 of New South Wales, where he had, with character- 

 istic liberality, founded an Observatory. Since then, 

 this body, insignificant in its physical appearance 

 (being to all appearance a small cloud of vapour 

 without a solid nucleus), has been detected in one or 



another part of the world at every revolution : 

 namely, in 1825, 1828, 1832, 1835, 1838, 1842, Its appa- 

 1845, 1848, and 1852 ; so that the Comet has been rition8 ' 

 observed a^. fourteen (not all consecutive) returns. 



The complete establishment of the existence and pe- ( 270 -) 

 riodicity of a comet, quite in the interior of the planet- J^^Jj 

 ary system (its greatest distance from the Sun being tion 

 four times the Earth's distance, and its least distance 

 but one-third of the Earth's), was a discovery in itself 

 highly interesting. But something yet remained be- 

 hind. Professor Encke, in comparing the earlier with 

 the later apparitions of the Comet, detected a gradual 

 acceleration of its movement, which amounted be- 

 tween 1786 and 1838, to 1-8 days,on a period of about 

 1211 days; being about 2J- hours per revolution. 

 Whatever may be the cause of this, the fact is undis- 

 puted, even by Bessel, who was indisposed to accept 

 M. Encke's explanation. This fact, it will be observed, 

 is unique in Astronomy. The major axis and periodic 

 times of the planets and satellites have, as we have 

 seen in the chapter on Physical Astronomy, no secu- 

 lar variation. The moon's apparent acceleration has 

 been otherwise accounted for. M. Encke at once, attributed 

 and at an early period, attributed the acceleration of* a resist * 

 the Comet's mean motion to the effect of a slightly dhLn! 6 " 

 resisting medium, insensible in the case of the planets, 

 partly owing to their incomparably greater density (for 

 this Comet appears to be one of the most loosely ag- 

 gregated bodies known, being transparent to its very 

 centre); and also, to the circumstance, that the density 

 of the ether or resisting medium is assumed to dimi- 

 nish rapidly at a distance from the Sun. M. Encke 

 supposes it to decrease in density with the square of 

 the distance, and only to affect the Comet sensibly 

 within 25 days preceding or following its perihelion. 

 That the effect of resistance is to accelerate the return 

 of the Comet is evident, by considering that the pro- 

 jectile force becoming gradually extinguished, the 

 Sun's attraction must be more available to pull the 

 body inwards at each revolution, thus shortening the 

 major axis of the ellipse, and diminishing the time. 



In order satisfactorily to arrive at any such con- (271.) 

 elusion, it was of course necessary to estimate with P rturb i a - 

 great accuracy the perturbing effects of the planets Encke's 

 on the Comet's motion; and it is not a little curious comet ap- 

 and satisfactory, that the movements of this insigni- P hed to 

 ficant erratic body should have occasioned a m ate- mas g e { of 8 

 rial rectification of the masses of two of the Planets. Mercury 

 M. Encke very early suspected that the received and Ju pi- 

 mass of Jupiter was too small, a fact clearly esta- er ' 

 Wished afterwards by Mr Airy ; and in 1838 M. Encke 

 showed that the mass of Mercury (which, not having 

 a satellite, was little more than guessed at previously) 

 had been assumed nearly three times too great by La- 

 grange. The perihelion of the Comet approaches much 



lisa Caro- 

 ine Her- 

 chel. 



1 Caroline Lucretia Herschel, sister of Sir William and aunt of Sir John Herschel, deserves a passing notice, not only as the 

 independent discoverer of eight comets (of which five were first seen by her), but as the indefatigable and intelligent assistant of 

 Sir William Herschel during the busiest years of his life. For this service King George III., carrying out his judicious liberality 

 to her brother, granted her a small pension. She died at Hanover 9th January 1848, aged 97. 



