THE RETURN OF ENCICE'S CO MET.
A RECENT telegram from America announces the .4-1- detection of Encke's comet, on its twenty-fifth return since the year 1818, when the little comet discovered by Pons at Marseilles was found by Encke to be moving in an orbit which, brings it back to the neighbourhood of the son once in three and a half years. By long-established custom a new comet should bear the name of its discoverer. This one would have brought him but little glory, for from the spec- tacular point of view it was of no account; it might have passed into the limbo of those recorded but almost unremem- bered visitors to our system which have spent a few weeks or months with us unregarded by any save a few diligent astrono- mers who make it a point of honour to observe everything that comes into their field of view. It had no advantages of bright starry head and flaunting tail to compel the terror of the ignorant and the admiration of the rather more in- structed, and the man who found it wandering in the field of his telescope regarded it in all likelihood as no very great catch, so soon as it was plain that it never .would become all object of magnificence. It remained for Encke to demonstrate the true claim which this humble and unpretentious little body has. on our regard. He showed that it. was no casual 'visitor, compelled by the attraction of the sun to pay us a flying visit on its solitary journey through space, but a member of our sun's own family, of at least many years' standing ; for several times in the preceding century it had been observed as it passed within range of the earth in the regular course of its orbit, a very elongated ellipse of which the end nearer the sun lies just within the orbit of Mercury, and the remoter end beyond the orbit of Pallas, the second discovered of the host of small planets which move between the orbits of Mars and Jupiter. At its previous apparitions it had passed each time for a new body. It was EnckOs good fortune, after a thorough deter- mination of the nature of the path of the little comet which Pons had found, to be able to trace it back through the century before, and to show that those seeming new bodies were but one and the same with it. Pons could no longer claim to be its godfather,. for Mechain had found it in 1786, Caroline Herschel in 1795, and Pons himself in 1805. By strict right of discovery it should have been Mechain's ; by right of full recognition and appreciation it was Encke's, and by his name it has ever since been known.
Nowadayst.we can claim quite a large family of small comets as ,...al-cstablished members of the solar system, and the discovery -of One more is hailed with but a mom enthusiasm. For comet-seeking is a very sporting cee4a. tion. If after sweeping the sky night after night, blunderinz across nebula which on an hour's watching proclaim thee:. selves plainly to be only nebuha by their °ffxity among .the stars, the chance which in the long run comes most oft% to those whose perseverance has deserved most brings into jaar sight a faint patch of light which is really a comet, there tray yet be disappointment in store. You may have been heat" by a few hours. by a rival hunter. You may but hive clamed upon one of the old members of the sun's family which pun in an .appearance every five. years or so. There is a distil,. guiahed American astronomer who in his early days was a famous comet-hunter; and he had a rival. Twice the fatter patiently hunted down a comet, only to find that the quarry had been secured a few hours earlier by.his friend.. The third time he was confident that. the prize was really his, and the news of the discovery -of..a new comet was duly circulated, ".I think," wrote his friend, "you_ will find it is my comet of six years ago come back." So it was; and the disgusted reply came by wire, " Why don't you keep your :amen chained up ?"
But a: hundred years ago there were none of these familiar domesticated. Comets to spoil the sport; they still roamed uncaptured and unknown. One indeed—texell'a-a-founcl in 1770, had given every promise of returning in five and_a ball years; but it was never seen again ; most likely it was over powered by the attraction of Jupiter which it met soon alter. wards., Halley's great comet was a welbrecogriised member of the sun's family ; it still, stands alone as the only regularly returning comet which is worthy, in popular. estimation, of the name comet at all,- fine bright body with a con. spicuous tail. But its path reaches far beyond the orbit of Neptune, and lt.can. appear but four times in three centuries, notoften enougla to be of perennial interest in the short span of a lifetime. :When, therefore,. Encke_ announeedtbathis little comet completed its course round tia4 sun in less than three and a - half years, and that were the circumstances favourable it would. appear just so often, this unpretentious body, scarcely visible to the . unaided eye in the most favour- able conditions, was -raised at once to a celebrity 'whieliluas been enhanced by oft-reneWed acquaintance.' There Wati no other comet knoWn then—nor is there --noW—with-a period so short. Thirty times in a century it returns to-the neighbour- hood of the stin, and only on a- few occasions, since its first recOgnition has its path been so nnfavoura.bly placed in the daylight sky that it has escaped reobservation.
The mere bulk of the recorded observations of nelte's comet makes for it a history. larger than that. -of. any of its more Splendid brethren The pages of this .history might have been dull reading for any but the few abstruse people who pursue the tedious paths of what we may .term orbit- mongering. Save for the triumphant vindication they afford of the truth of Newton's laws of universal gravitation when the comet comes back-true time after time to its predicted place. the details of the computation of its pathare sufficiently unexciting, unless some sign of an irregularity is found which cannot for the moment be explained. And this is just what happened from the very first in the case of Encke's comet. Its case exhibited an abnormal symptom which persisted at every return until it became of most sinister import to the fate not only of the comet, but of the-whole solar 'system, our earth included. And here let us say. at once that.it was no probability of collision between-the comet and the earth that was to be feared. Such a catastrophe would in all' likelihood be terrible only in anticipation. Whenever there is an oppor- tunity of putting a comet in the scale against a planet it is found. that the comet, though in..bulk immense, has a mass almost .infinitesimally small. It may be. but a-flying cloud of widely-scattered-dust ; it may rise to the dignity of "a flight- of. brickbats," as some one has put it. The -former is the more probable, for at least there is not much doubt- that a comet gradually becomes disintegrated into .a swarm of meteors.. We should . all have been familiar with what happens when the earth runs into a meteor swarm; it was terrible only when it did not come off.
The symptom which -supervened in the case of trtekea cornet -was of a far graver character; it portended the decay and -ultimate-destruotion of our- whole system, s--1316* but sure and inevitable falling together of its component parts until all were engulfed and consumed by the sun. All ?sill was to be deduced from the simple fact, completely established, that the comet came back to perihelion at each iotarn two hours and a half too soon. Its period was shorten- ing.; it must consequently be drawing nearer and nearer to the sun, an effect to be explained—though the explanation is not quite obvious, and we must take it for granted here— only by the supposition that the space in which the comet moved was not quite void of something tangible that might sotard its motion. Here was to be seen a cause of peril to everything that moved round the sun. What was visibly happening to the tenuous comet, so little able to resist the slightest opposition,' must far more slowly but just as in- evitably happen to the heavy planets. The permanence of the system is proof against all the manifold disturbances which arise within it by the mutual attraction of planet upon planet; if there were in space some permanent resistance, ever so slight, to its motion • it would be doomed.
At its blackest the case was no immediate or even remote menace to the future of the human race. Long enough before it became of serious importance the earth would in all probability be cold and dead as the moon. But there is a pathetic interest in the prospect of destruction for the beautiful system of planets of whose permanence we were before so well assured. It comes as a relief to learn that the grave symptoms in the case of Encke's comet have in the last torty years become notably diminished. Since 180 the comet has come round more punctually. It is now little more than an hour late at each return, and since this change is a great deal harder to understand than the original fact, there is good reason for hope that perhaps the diagnosis was wrong from the first; that it may not after all be necessary to explain the comet's unpunctuality by the hypothesis of some resistance in space, 'dangerous 'to all bodies alike, both light and heavy. For the very existence of the alleged unpunctuality is rendered doubtful if we can suggest with any plausibility that the effects of the attractions of all the known bodies of the system have been not quite accurately evaluated. And there is here, it must be confessed, a loophole by which it may be possible some day to find the way out. A comet is but a frail body, pulled violently this way and that by the attractive power of the heavier bodies near which it may chance to wander. We might be quite cirtain that of these full account had been taken, were it not- for the fact that there are still some small outstanding irregu- larities in the movements of the planets themselves which continue to defy elucidation. So long as this is the case it is impossible to feel convinced that theory is perfect. It may be that the anomalous motion of Encke's comet, presenting itself so frequently to our scrutiny, may supply the key to the enigma which is still unsolved.
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