REDISCOVERY OF BRORSEN'S COMET.
THERE are so few comets which revolve in short periods around the sun, and these few are subjected to so many dangers,— existing seemingly under the continual risk of dissolution,—that a certain interest attaches to the search for periodical comets at their successive approach to perihelion. Of nearly twenty comets which are recognized members of this class, only eight have been known to return at the predicted seasons, and one of these has lately failed to put in an appearance at the appointed time, although all the leading observers in Europe swept diligently with their telescopes over the region along which it was expected to travel. No one knows what has happened to this particular comet. Its path had not brought it near to any of the large planets. But a certain suspicion had already fallen on its cha- racter for consistency since, at a former visit, it had separated into two distinct comets under the very telescopes of observers. Another comet of the same class was watched during two revolu- tions and then vanished. But astronomers had an inkling in this case of the cause of this catastrophe, since it was known that Jupiter had introduced this stranger within the solar system, and it was recognized that that giant comet-compeller 0E9EX47EQira Zits/0 was competent also to dismiss the comet from the neighbour- hood of the sun. The majority of these comets, however, have disappeared from our ken without any assignable cause.
The comet whose rediscovery has just been announced was detected by M. Brorsen, at Kiel, on February 26, 1846. It is not to be confounded with another comet also named after this astro- nomer, but which revolves in a much longer period. The comet we are now dealing with was found to have an elliptical path, and a period of revolution of about five and a half years. But it was not so favourably situated for observation as to enable astro- nomers to calculate its orbit with any great precision. It was not seen at its return to perihelion in 1851, being hidden in the sun's light during the greater part of its path near perihelion. How- ever, on its next visit, in 1857, M. Bruhns, at Berlin, rediscovered it and recalculated it motions.
Again, in 1862, Brorsen's comet came and went undetected by astronomers. But during the last few weeks several of the leading Continental observers were on the look-out for the wanderer. Almost simultaneously Bruhns at Berlin, Schmidt at Athens, and Tempel at Marseilles announced its discovery, and before many days had elapsed Bruhns published an ephemeris of its path on the celestial sphere. From this ephemeris it appears that the comet has already entered the constellation Lynx, at the end of May it will cross the Greater Bear's forepaws, in the middle of June it will traverse his hind feet, on June 26 and 27 it will traverse the conspicuous cluster of stars which form the Hair of Berenice, and during July and August it will pass slowly away across the feet of the Herdsman. During all this time it will be favourably situated for observation.
The comet has already been subjected to spectroscopic obser- vation, though hitherto, we understand, its light has not been strong enough to afford very satisfactory indications of its struc- ture. It seems clear, however, that it shines by its own and not by reflected light.
There is one peculiarity about Brorsen's comet which renders it comparatively safe from disturbance. Its path is largely inclined to that particular plane in space near which all the planets travel. Thus there are only two points of its orbit at which it runs great risk of disturbance ; one of these is the point at which it passes from south to north of the ecliptic plane, the other is the point at which it passes from north to south. The former point lies within the orbit of Venus, but far enough from that orbit to render the comet tolerably safe from the planet's influence. The other point lies outside the path of Jupiter, and awkwardly near to his orbit. Its absolute distance from the orbit is consider- able enough, and, indeed, far greater than the distance of the former point from the orbit of Venus. But then Jupiter is such a giant compared with Venus, and has been the ruin of so many comets, that if ever Brorsen's comet should pass this critical point when Jupiter is near, it is more than probable that there will be a catastrophe. There have been sonie, indeed, who have supposed that Brorsen's comet is no other than Lexell's, which had been hurled out of the solar system by Jupiter in 1779. That Jupiter might, after playing with Lexell's comet for sixty-seven years, have sent it again within the earth's orbit is, indeed, possible ; but Leverrier has examined the subject too carefully for us to be permitted to doubt the jus- tice of his conclusion that Brorsen's comet is not identical with Lexell's.
Indeed, it is a singular circumstance that there are other comets besides Brorsen's which have been mistaken for Lexell's. Amongst these are Faye's and De Vico's. Leverrier has shown, however, that none of them can be identified with Lexell's comet.
It remains only to be mentioned that there is a singular family likeness between the comets of short period. They all travel around the sun in the same direction, and the paths of all but Brorsen's are but little inclined to the ecliptic. Even Brorsen's travels in a path less inclined to the ecliptic than that of the asteroid Pallas. Then, again, the ovals in which they travel present a marked similarity of figure, the smaller axis being in nearly every case about five-eighths (roughly) of the greater. Quite a large proportion also of these comets have a period of about five and a half years and an orbit just reaching beyond that of Jupiter. These peculiarities are far too marked (as any one will see who examines a diagram of the orbits) to. be accidental. And singularly enough, there is another family of comets which presents a corresponding series of resemblances. This is a set of periodic comets whose orbits all extend a little beyond that of the planet Neptune. They all have periods of about seventy years, all are considerably inclined to the ecliptic, and all have oval orbits so closely resembling each other in shape, though wholly different from the orbits of the inner family of comets, that it is not easy to distinguish one orbit from another by this feature alone. It is certainly a singular circumstance that there should thus exist two families of comets, each presenting a distinct type, the members of one family travelling at their greatest excursion just beyond the orbit of the outermost major planet, those of the other family extending their excursions just beyond the path og the innermost major planet.
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