Page:The story of the comets.djvu/111

VI. 1857-8 and its return to its perihelion in 1864, M. Villarceau obtained the following results:—

(1) The longitude of the perihelion will have diminished 4° 35' to Aug. 1863, and will remain sensibly stationary for about a year from that epoch. (2) The longitude of the node will have continually diminished to the amount of 2° 8'. (3) The inclination will have increased 1° 49' to the middle of 1862, and will diminish 6' during a year, continuing stationary during the year following. (4) The eccentricity, after having increased to the middle of 1860, will diminish rather quickly, and will remain stationary from 1863-5 to 1864-6. "But of all these perturbations," says M. Villarceau, "the most considerable are those of the mean motion and the mean anomaly. After having increased from 5" to July 1860 the mean motion diminishes 9" in one year, and nearly 12" in the year following, remaining stationary in the last year, and with a value 15", 5" less than at its origin. The perturbations of the mean anomaly, after having gradually increased till 1860, will increase rapidly till 1861, when they will amount to 10° 28'; and setting out from this, they will increase 9', and in 1863 and 1864 they will have resumed the same value which they had in 1861."

The effect of the first of these perturbations will be to increase the time of the comet's revolution by about 69 days; and of the second, to hasten by 49 days the return of the comet to its perihelion in 1864. It will pass its perihelion on Feb. 26, whereas without the influence of these perturbations it would have passed it on April 15.

As was anticipated, the comet escaped notice at its return to perihelion in 1864, being unfavourably placed. But in 1870 it was found and followed for 4 months. In dealing with the observations of this return Winnecke pointed out that D' Arrest's Comet was undoubtedly the faintest of the known periodic comets, but probably that remark is no longer true. The comet was seen also in 1877, missed in 1884, and seen again in 1890, but its great southern declination limited the observations. Its light was reported to be feeble, and observations difficult even with large telescopes. Inasmuch as at its return in 1897 Perrine, at the Lick Observatory, saw the comet in a 3$1⁄4$-inch Finder it seems almost certain that Winnecke's remark just quoted no longer holds good. In 1903 the comet was very unfavourably placed and was not seen.

The history of the first discovery of this comet presents some novel points of interest. In the ordinary course of narrative we should say that Wolf, at Heidelberg on Sept. 17,