CHAPTER V.

COMETS OF SHORT PERIOD-THE COMET OF ENCKE.

THE most remarkable of the comets of short period is that known as ENCKE'S; it is a body of the highest interest to the astronomer, and we shall here recapitulate briefly the principal points in its history.

On the 17th of January, 1786, M. Mechain, of Paris, discovered a telescopic comet, near the star ß, Aquarii. It was pretty large and bright, with a sensible nucleus surrounded by nebulosity, but without any appearance of a tail. Cloudy weather prevented M. Mechain from observing it, except on January 17th and 19th; so that no orbit could be deduced from observation.

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Miss Caroline Herschel, sister of the celebrated Sir W. Herschel, detected a comet not far from the star marked in Cygnus, on the 7th of November, 1795. It was just visible to the naked eye in the telescope no nucleus was perceptible, but merely a condensation of the nebulous matter near the centre. The comet was observed at Berlin, by Professor Bode, on the 11th of November, an amateur astronomer named Carl having announced its visibility; and on the 14th, M. Bouvard saw it at Paris. The outline of the nebulosity is

described as nearly if not quite circular, the greatest apparent diameter amounting to about five minutes of space on the day of discovery. The elements were investigated, on the assumption of parabolic motion, by Dr. Olbers, M. Bouvard, and Baron de Zach.

On the evening of the 20th of October, 1805, a comet was found, almost simultaneously, by M. Pons at Marseilles, Professor Huth at Frankfort, and M. Bouvard at Paris. It was situate in Ursa Major. The perihelion passage occurred about a month after the discovery, and the comet was observed until within a week from this epoch. The parabolic orbit was calculated by MM. Bouvard and Legendre, and Professors Bessel and Gauss.

M. Pons, of Marseilles, a most industrious observer, detected a telescopic comet on the 26th of November, 1818, which remained visible till the 12th of January following. It was speedily found that no parabolic orbit could be made to represent the observations within their probable limits of error, and this circumstance induced the celebrated calculator, Professor Encke, to undertake the rigorous investigation of the elements. On treating the observations agreeably to the methods of Professor Gauss (but little practised at that time), a most important result was obtained. Professor Encke showed that the real path of the comet must be an ellipse, with a period of about three years and a quarter; and on looking over the catalogue of comets already observed, it appeared highly probable that the one in question had been previously seen, and was, in fact, identical with the comets of 1786, 1795, and 1805, those years agreeing

well with the presumed time of revolution of the comet of 1819. The question could only be definitely settled by calculating the planetary perturbations backward to the respective epochs; and this enormous labour was entered upon by Professor Encke, and completed in the most masterly manner, after close application for about six weeks. It was found that the comet had arrived at perihelion on January 30th, 1786; May 19th, 1789; September 4th, 1792; December 21st, 1795; April 11th, 1799; August 2d, 1802; November 22d, 1805; March 12th, 1809; June 26th, 1812; October 13th, 1815; and again on January 27th, 1819. In 1789, 1792, 1799, 1802, 1809, 1812, and 1815, it passed through these parts of space without being detected by any observer. The average time of revolution appeared to be about 1208 days; but on allowing for the perturbations of Jupiter, the period still exhibited indications of a gradual diminution since the year 1786, unaccounted for by the disturbances produced by other planets, and too large to be attributed to the effect of any quantities omitted in the calculation. On this interesting point we shall have more to say very shortly.

After having thus brought to light the past history of the comet, at the expense of a vast amount of labour, Professor Encke turned his attention to the next return to perihelion, and again undertook the determination of the perturbations produced by planetary attraction on the movements of the comet. He announced, as the principal result of his investigations, that the comet would arrive at perihelion on the 24th of May, 1822, after undergoing a retardation of rather

more than nine days from the influence of the planet Jupiter. So complete were these calculations that astronomers universally attached the name of ENCKE to the comet of 1819, not only as an acknowledgment of his diligence and success in the performance of some of the most intricate and laborious computations that occur in practical astronomy, but also to mark the epoch of the first detection of a comet of short period, one of no ordinary importance in this department of the science.

Owing to the position of the comet in the heavens about the time of perihelion passage in 1822, it was not seen at all in Europe, but there fortunately existed an observatory in full activity at Paramatta, New South Wales, founded and maintained by the munificence of a private individual, Sir Thomas Brisbane, then Governor of the colony. Mr. Rümker, the present director of the observatory at Hamburg, detected the comet at Paramatta on the 2nd of June, and followed it until the 23rd of that month. observations enabled Professor Encke to rectify the elements, and predict the time of its next arrival at perihelion with the greater accuracy. He found it should occur on the 16th of September, 1825, and that the apparent path of the comet in the heavens would allow of observations at the European observatories.

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Professor Harding, the discoverer of Juno, obtained the first glimpse of the wanderer on the 26th of July, 1825, in a position differing from the predicted one by less than three minutes of space. The comet was observed at nearly all the astronomical

establishments of Europe until about ten days before the perihelion, when it was lost in the sun's rays. It appeared round and very bright towards the centre, and it was remarked by Professor Argelander in the strong twilight as a small planetary disk, with scarcely any signs of the surrounding nebulosity.

The next return of the comet, in 1828, excited great interest, as, for the first time since the discovery of its periodicity, the apparent track amongst the stars would be peculiarly favourable to observation in Europe. The epoch of perihelion passage was fixed by Encke on the morning of the 10th of January, 1829. Professor Struve, employing the magnificent telescope then newly erected at Dorpat by the Emperor of Russia, saw the comet as a very faint nebulosity on the 16th of September, 1828. At Berlin it was seen by MM. Kunowsky and Encke with a much smaller instrument on the 7th, but it was not generally observed until the end of the month.

On November 30th it was visible to the naked eye as a conspicuous star of the sixth magnitude, and a week afterwards was still brighter, so as to be rated with stars of the fifth class. The outline of the nebulosity was not circular, but somewhat elliptical, the major axis, on one occasion, being nearly at right angles to the line joining the sun and comet, which is the usual direction of the tails of these bodies. The angle of position of the longer axis of the elliptic comet in respect to the circle of declination, and counted from the north towards the west, was measured on four evenings by Professor Struve, and on comparing it with the direction of the line joining the sun and