Oil Encke's Comet. 277 



Encke used only the German observations. After various 

 trials he found that he could not obtain any parabola that would 

 represent them within three minutes, and the error was much 

 greater in the early observations made at Marseilles, being 

 above thirty minutes in right ascension, and six minutes in 

 declination ; a sure indication that the orbit varied much from a 

 parabola, and that it was necessary to notice its elliptical form. 



After some preliminar)' calcidations, Prof. Encke assumed 

 the following elliptical elements of the orbit in 1819, using 

 always the viean time for the meridian of Seeberg, and the 

 mean equinox for the year in which the comet was observed. 



Time of passing the perihelion 1819 Jan. 27.275 



Longitude of the perihelion - - 



Longitude of the ascending node 



Inclmation of the orbit to the ecliptic 



Eccentricity (comet's dist. from sun=l' 



Mean distance from the sun 



Periodical revolution, about 1203 days. 



Motion direct. 



These elements represent the place of the comet, during its 

 appearance, to a great degree of accuracy. For, out of forty- 

 two observations of the right ascension and declination, thirty- 

 live are given to less than half a minute, and no error exceeds 

 1' 33": so that there can be no doubt that the true figure of 

 the orbit in 1819 is very nearly given by the above elements. 



When this result had been obtained, it became an interest- 

 ing object of inquiry to find whether the comet had been be- 

 fore observed by astronomers. Upon looking over the general 

 table of the orbits. Professor Encke selected \h&Jirst comet of 

 1805 (marked 107 in Delambre's table) as a former appear- 

 ance, with an interval of four revolutions. It was near the con- 

 stellation Ursa Major, when first discovered by Mr. Pons, Oct. 

 20, 1 805, and appeared as a star of the fourth magnitude, w ith 

 a nucleus and a very faint tail, 2i° in length. It continued 

 visible the 15th of November. The parabolic orbit had been 

 computed by several persons, with considei'able difference in 

 the results, which had been imputed, in great measure, to the 

 incompleteness of the observations. In tliese different calcula- 

 tions the time of passing the perihelion varied from November 

 17th to 18th; place of the perihelion 147° to 149°; of the 

 node 340° to 345°; inclination I5i° to 17A°; perihelion di- 

 stance 0.34G to 0.379. The parabolic element*, published by 

 the accurate and indefatigable Professor Bessel, [Monalliche 

 Corrc'spondenz^ b. xiii. xiv.) did not re}irescnt the right ascen- 

 sions and declinations without errors of 26', 11', 8', 7', 6', &c. 



Instead 



