MEMOIR OF ENCKE. 195 



Enclce immecHately thereafter addressed himself to a similar yet more trouble- 

 some inquiry. The editors of the Zeifschrift fur Astronomie had chosen as the 

 thesis for the prize otJ'ered by Cotta, the computation of the orbit of the comet of 

 1680. Tliis comet, on account of its luminosity, the length of its tail, which 

 comprised 80 deg-rees, as well as the duration of its visibility, had presented one 

 of the o-iandest phenomena of which tlie history of the heavens makes mention. 

 It had been extensively observed by the astronomers of the period, es])ecially 

 by Flarasteed, Xewton, and Cassini, thouiih the observations had been limited 

 to the measin-ement of the distances of the fixed stars, and the comets' ])lacehad 

 been in part only estimated, inasmuch as the oliservers had been content to sup- 

 pose the body bisected by two great circles drawn through four neighboring 

 stars, which circles only approximately touched it. 



Encke wrote toBessel that, at the special request of Lindenau, he would attempt 

 the solution of the problem, and with this view requested the communication f)f 

 a number of stellar positions from the catalogue of Bradley. With this request 

 Bessel at once complied, but expressed the apprehension that the result wotihl 

 scarcely justify the expenditure of tiiBe, ashe deemed the observations too uncertain. 

 And, indeed, of so large a number of measurements there were but few that 

 were avaihible. In many cases it was even uncertain from what fixed stars the 

 distances were measured. Nevertheless it was found, on critical examination, tliat 

 Flamsteed's observations disclosed a high degree of accuracy, since their probable 

 en-ors amounted to but 15 seconds of arc spherical. With due regard to planetary 

 disturbances this orbit was also shown to be elliptical, and the period of revolu- 

 tion was found to be 8,813 years. 



Gauss cordially congratulated Encke '' on this admirable prize essay, to which 

 he Avas indebted for so much pleasure." Bessel expressed himself in more 

 specific terms : " It is without example that the more ancient observatittns have 

 been reduced to so small a probable error. We learn from this, that to a good 

 astronomical result there is indispensable, besides a tolerably good instrument and 

 a capable observer, an able calculator also. If the last be wanting, the rest is 

 little worth." 



Together wnth these great labors Encke occupied himself in many ways witli 

 the incidental calculation of cometary orbits. As often as one was discovered, 

 a"!ul had been for some time observed, he was accustomed to publish, not only 

 its orbital elements, but, to facilitate further observations, its ephemeris as well. 

 The constant practice and fine perception which guided him in the choice and 

 grouping of the observations enabled him to arrive with wonderful certainty at 

 a correct result. A remarkable example of this was furnished by the third comet 

 of 1819, for which Encke, from some scanty Marseillese and a few Milanese 

 observations, so acciuately calculated the elements of an elliptic orbit and a 

 revolution of 5^ years, that on the re-discovery of the comet at the Bonn obser- 

 vatory after seven revolutions (in the year 1858) no important qualifications were 

 admissible. This practical knowledge and perspicacity led, no doitbt, to the 

 discovery through which Encke's name is most generally known. 



A comet was discovered November 26, 1818, by Pons at Marseilles, the path 

 of which, as Gibers soon remarked, Avas nearly coincident with that of the comet 

 of 1795 and 1805. It had been already surmised that the orbit was elliptical 

 and the period of revolution short l)eyond example. The elucidation of this 

 reniarkalde circumstance was probably undertaken by many astronomers, thougli 

 it was Encke who first sitcceeded in shedding complete light upon it. The time 

 of revolution amounted to 1,207 days, or nearly 3:} years, and it was this comet 

 respecting which Bessel had expressed a conjectural opinion that it moved in an 

 ellii)tical orbit.* 



lu a communication on this comet Olbefs extols " the skill, the care, and tho 



• In tho Berliner Aslronomiches Jahrbuch for 1822, which was published in 1819. 



