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NATURE 



[November 29, 1894 



HISTORY OF E SOKE'S COMET. 



HISTORICALLY, Encke's comet, which has recently 

 come into view again, stands next in interest to 

 Halley's. The history of the latter can be carried back 

 much farther than that of any other comet. It was in- 

 deed conjectured that the one (the first telescopically 

 discovered comet, by Kiich) which made so near an 

 approach to the sun in i6So, and in reference to which 

 Newton first applied his principle of universal gravita- 

 tion to the motions of these bodies, was identical with 

 comets seen at intervals of about 575 years before, and 

 Gibbon (not exactly an astronomical authority, who 

 recommends others to study Newton and Halley on the 

 question) devotes a section of his forty-third chapter to 

 the supposed history of these early appearances (two of 

 them in mythical times', concluding with the remark 

 that the calculations with regard to it might perhaps 

 in the year 2355 "be verified by the astronomers of 

 some future capital in the Siberian or American wilder- 

 ness,"little thinking how many splendid telescopes would 

 be employed on the study of the heavens in the " far 

 west," before a century had elapsed from his own death. 

 There was then, in 1794, no observatory on any part 

 of the American continent. But it is now known that 

 the period of the comet of 16S0 amounts not only to 

 hundreds, but to thousands of years ; and one of the 

 supposed previous appearances, that in the reign of the 

 Emperor Justinian, was in all probability a return of the 

 smaller or less conspicuous comet which appeared in 

 l6S2,and at the next return in 1758-9 acquired the name 

 of Halley's comet, because that eminent astronomer had 

 confidently predicted its return at that date, calling upon 

 posterity to notice that the prediction had been made by 

 an Englishman. He recognised its identity with comets 

 observed in 1531 and 1607, by a comparison of the orbits 

 calculated for each, and considered from the similarity of 

 period, that the fine comet of 1456 was also probably 

 an earlier appearance of the same. Later investigations, 

 and the accessibility of Chinese records, have shown 

 since his time that successive appearances of this body 

 can be traced with very great probability to a date before 

 the Christian era, our distinguished countryman, Dr. 

 Hind, having taken a leading part in these calculations. 

 Of the subsequent observations of this comet in 1S35, 

 this is not the place to speak, nor of the full expectation 

 astronomers then living will entertain of seeing it again 

 in 1910, and applying the new methods of analysis to it, 

 thereby obtaining information respecting its consti- 

 tion, which was beyond the wildest flights of imagination 

 at its last appearance. Tor our present subject is a comet 

 which acquired the name by which it is now universally 

 known as a fitting meed of honour to an astronomer who 

 worthily presided over the then new observatory at 

 Berlin within our own recollection. .Many comets have 

 since that time returned according to prediction ; but 

 when Encke announced that the small one discovered by 

 Pons at .Marseilles on November 26, 1818, was identical 

 with the discovery of Mcchain in 1786, of Miss Herschel 

 in 1795, ■''"d °f Thulis in 1S05, no predicted return of ! 

 any comet but Halley's had ever taken place, though two 

 predictions had been made, by himself and by Bessel re- 

 spectively, of the returns of comets observed in 18 12 and \ 

 1815, which duly came to pass in 1S83 and 1887, the 

 periods of these being nearly as long as that of Halley's. 

 The remarkable point about Encke's comet was the ex- 

 treme shortness of its period, amounting only to 1212 

 days, or three years and about four months. It was 

 therefore concluded that it would reappear in 1822 ; true 

 to prediction it did then appear, but from its situation in 

 the heavens was visible only in the southern hemisphere, 

 which then possessed only one observatory, that estab- 

 lished (but which has long ceased to exist) by Sir Thomas 

 Brisbane at Paramatta, New South Wales, wherj the 



NO. 1309, VOL. 51] 



comet was rediscovered by Riimker on June 2. The 

 next appearance took place in the autumn of 1825, when 

 the comet was observed in this hemisphere, and since 

 that time it has never failed to be observed at the calcu- 

 lated epochs. Encke did not desert it after he had deter- 

 mined its period in 1S19, but. following up its motions 

 with accuracy, was led to notice a remarkable continuous 

 shortening of the period by a fraction of a day at each 

 return. The question had before his time been started 

 whether a medium might be diffused through the solar 

 system which, though insufficient to atTect the motions 

 of the planets, would produce appreciable effects upon 

 those of comets, composed as they must be of matter in a 

 state of great rarity. Here was a case which seemed to 

 settle the question in the affirmative. Encke's calculations 

 showing that the diminution in the observed length of the 

 period was such as might well be caused by the action 

 of such a resisting medium checking the onward motion 

 of the comet, which would bring it a little nearer to the 

 sun at each return, and thus shorten both the orbit of 

 revolution and the period of time in which it was accom- 

 plished. The difficulty remained how to explain the fact 

 that no such effect was perceptible in the motions of any 

 other comet ; a difficulty which the lapse of time has not 

 removed, for though in one other case (that of a comet 

 known as Winnecke's) a similar effect was for a while 

 thought to be noticed, further investigation showed that 

 this view could not be sustained. Encke, however, to 

 the end of his life (he died in 1S65) was able to trace the 

 above continuous effect in the motion of his own comet, 

 the period of which was then i2io"2 days, or i'6 days 

 shorter than it had been in 1S19. But, strangely enough, 

 soon afterwards, the amount of retardation was reduced 

 by about one half, at which it has remained from 1868 

 to the present time. Must the resisting-medium theory 

 be modified, or must it be altogether abandoned and 

 some other cause be sought for the retardation in 

 question.' Prof. Young suggests a regularly-recurring 

 encounter with a cloud of meteoric matter. 



When nearest the sun, Encke's comet is at very nearly 

 the same distance from him as the planet Mercury. 

 When farthest from him, it is in the zone of small planets 

 (nearly four hundred of which are now known), revolving 

 between the orbits of Mars and Jupiter. May the at- 

 traction of some of these have something to do with the 

 effect above referred to .' Small as is the mass of most 

 of the tiny bodies in question, it is possible that at cer- 

 tain times some of them may act together and produce a 

 cumulative and appreciable effect. Of great value to 

 astronomy has been the position of Encke's comet at the 

 other extremity of its orbit, in perihelion. Before its dis- 

 covery, the mass of Mercury had been rather a matter of 

 conjectural inference than of actual calculation, that planet 

 having no satellite the motionsof which would beaffected 

 by its attraction. But at certain returns, the comet of 

 which we are treating, made very near approaches to the 

 planet, and the effects pro luced on these occasions have 

 enabled astronomers to obtain determinations of the 

 mass of the planet as accurate, or nearly so, as those 

 determined for the larger planets which have satellites. 

 The first of these near approaches since the comet's dis- 

 covery took place in 1S35 ; the last at the most recent 

 return, in 1891. 



We now come to the physical appearance of 

 Encke's comet. It has on some occasions, when nearest 

 the i:arlh,been just visible to the naked eye, particularly 

 in the autumns of 1S28 and 184S. After Miss Herschel 

 had detected it (supposed to be a new comet), at its 

 return in 1795, her brother. Sir William Herschel, 

 observed it on November K, and noticed it the following 

 day pass centrally over a star of the twelfth magnitude 

 without obscuring it, whence he concluded that the 

 comet " is evidently nothing but what maybe called a 

 collection of vapours." Maskelyne, who observed it at 



