By RICHARD A. PROCTOR.
During the last two years several comets—some telescopic, others visible to the naked eye, and even conspicuous objects in the heavens—have been observed, not only by the older methods, but by some which have only been available within recent years. It is naturally expected, therefore, by the general public that some new light should be thrown on these mysterious objects, whose phenomena still remain among the unexplained, seemingly the inexplicable problems of the celestial depths.
We propose to consider here what has thus been learned, and what also (unfortunately it is much more) remains still to be learned, respecting comets. But first it will be well to show what are the special phenomena which present themselves for explanation.
A comet apparently comes out from the remote depths of space in a condition of comparative calm. It appears as a small round nebulous object, looking like a tiny cloud of extreme tenuity—the idea of tenuity being suggested by the exceeding faintness of the comet’s light. This cloud appears somewhat condensed toward the middle. As the comet draws nearer to the sun, it usually grows somewhat long in the direction of the sun; and before long a portion within the part nearest the sun is seen to be brighter than the rest, and to have a more or less defined outline. This is the nucleus—sometimes seen as a dull disc of nearly uniform brightness, at others as a mere bright point not unlike a star. The fainter light around this is the coma, or hair, which resembles a luminous fog round the nucleus, usually brighter on the side toward the sun, and on the other side growing fainter and fainter till it can no longer be seen. Later this lengthening of the comet in directions toward and from the sun becomes more marked, until at length the comet may fairly be said to have a head directed toward the sun and a tail directed from him. Nucleus, coma, and tail may be very different in appearance in different comets, and in particular the tail may be more or less complicated in structure, being sometimes a mere straight streak, at others twofold, multiple, curved, with thwart streaks, and so forth—no two comets, in fine, having tails resembling each other except in general details.
Dr. Huggins, in a rather disappointing article on comets, recently communicated to a contemporary, remarks that the nucleus, though an apparently insignificant speck, “is truly the heart and kernel of the whole thing—potentially it is the comet.” This has scarcely yet been proved, though it appears exceedingly probable. It is true, however, as he adds, that this part only of the comet conforms rigorously to the laws of gravitation, and moves strictly in its orbit. “If we could see a great comet,” he proceeds, “during its distant wandering, when it has put off the gala trappings of perihelion excitement, it would appear as a very sober object, and consist of little more than nucleus alone.” This again seems probable, though it has never yet been proved, and the division of some comets into two or more parts, each having coma, nucleus, and tail of its own, shows that the nucleus cannot be, in every case, what Dr. Huggins seems here to suggest. Dr. Huggins has done well in saying (though scarcely with sufficient emphasis, considering how often the mistake is repeated) that “though many telescopic comets are of extremely small mass, nucleus included—so small, indeed, that they are unable to perturb such small bodies as Jupiter’s satellites—yet we should mistake greatly if we were to suppose that all comets are ‘airy nothings.’ In some large comets the nucleus may be a few hundred miles in diameter, or even very much larger, and may consist of solid matter. It is not necessary to say that the collision of a cometary nucleus of this order with the earth would produce destruction on a wide scale.”
It is even more necessary to correct the widely-spread misapprehension as to the relations between meteors and comets. We hear it stated that the nucleus of a comet is made up of meteoric stones (Professor P. G. Tait says—for unknown reasons—that they resemble “paving stones or even bricks”) as confidently as though the earth had at some time passed through the nucleus of a comet, and some of our streets were now paved with stones which had fallen to earth on such an occasion. As a matter of fact, all that has yet been proved is that meteoric bodies follow in the track (which is very different from the tail) of some known comets, and that probably all comets are followed by trains of meteors. These may have come out of the head or nucleus in some way as yet unexplained; but it is by no means certain that they have done so, and it is by many astronomers regarded as more than doubtful.
The most important points to be noticed in the behavior of large comets, as they approach the sun, is that usually the side of the coma which lies toward the sun is the scene of intense disturbance. Streams of luminous matter seem to rise continually toward the sun, attaining a certain distance from the head, when, assuming a cloud-like appearance, they seem to form an envelope around the nucleus. This envelope gradually increases its distance from the sun, growing fainter and larger, while within it the process is repeated, and a new envelope is formed. This in turn ascends from the nucleus, expanding as it does so, while within it a new envelope is formed. Meanwhile, the one first formed has grown fainter, perhaps has disappeared. But sometimes the process goes on so rapidly (a day or two sufficing for the formation of a complete new envelope) that several envelopes will be seen at the same time, the outermost faintest, the innermost most irregular in shape and most varied in brightness, while the envelope or envelopes between are the best developed and most regular.
The matter raised up in these envelopes seems to have undergone a certain change of character, causing it no longer to obey the sun’s attractive influence, but to experience a strong repulsive action from him, whereby it is apparently swept away with great rapidity to form the tail. “It flows past the nucleus,” says Dr. Huggins, “on all sides, still ever expanding and shooting backward until a tail is formed in a direction opposite to the sun. This tail is usually curved, though sometimes rays or extra tails sensibly straight are also seen.” The description is, however, incomplete in one important respect. The matter raised from the nucleus to form the envelopes may be, and probably is, carried past the nucleus on all sides; but the appearance presented by the tail just behind the nucleus is not exactly in accordance with our ideas as to what should result from the flowing past “on all sides.” There is a dark space immediately behind the nucleus, that is, where the nucleus, if solid, would throw its shadow, if there were matter to receive the light all around so that the shadow could be seen. Now it may be thought at first that this corresponds exactly with what should be seen: when we look just behind the nucleus there is no light, or very little; when we look on either side of that dark space there is the luminous matter which has been driven back from the envelopes in front of the nucleus. But if the luminous matter flows past the nucleus on all sides, it must flow past the nucleus on the side nearest to the observer, and also on the side farthest away; and it is just where the line of the sight passes through these two regions of brightness that a dark streak is seen just behind the nucleus. Let the reader draw two concentric circles—one an inch in diameter, the other two inches—and let him then draw two parallel tangents to the inner circle on opposite sides of it. Supposing now the space between the two circles to represent in section the luminous matter which flows all round the nucleus, while the surface of the inner circle represents the unilluminated part behind the nucleus, the two tangent lines will represent the lines of sight on either side of the dark region, where as we might expect, we get plenty of light; and we can also understand very well why outside of that the line of sight through the luminous matter (or the chords to our outer circle), getting shorter and shorter, the light of the luminous streaks bounding this part of the tail gets fainter and fainter; but if just inside either of the two tangents, chords are drawn parallel to them, crossing the inner circle, the parts of these chords which lie between the two circles are very nearly equal in length to the tangent lines themselves; and even a common diameter to both circles has, lying between them, two portions together equal to the radius of the outer. Hence, since the line of sight even across the middle of the space behind the nucleus, passes through a considerable range of luminous matter, while a line within but near the outskirts of that space passes through nearly as great a range of luminous matter as one just outside that space, there should be plenty of light where yet to the eye there seems to be something like absolute darkness. Either then the eye is greatly deceived, or else we must find some explanation of darkness existing where considerable brightness might be expected.[D]
The matter which forms the tail, seems, as I have said, to be swept off from the envelopes raised by the sun’s action on the nucleus. It seems as though the matter thus raised had undergone in some way a change of character, which caused it no longer to obey the law of gravity as it had done when forming part of the nucleus, but instead of yielding to the sun’s attraction, to submit rather to an intense repulsive action, carrying it at a much greater rate from the sun than, under the action of gravity—starting from rest and free from all perturbing influences—it could have been drawn toward him. Dr. Huggins thus words his account of what seems to happen: “Now is seen to take place a change which is most puzzling—namely, these envelopes of light appear to give up their substance under the influence of a strong repulsive force exerted from the sun, and to be forced backwards.” Sir John Herschel, after his long and careful study of the comet of 1830 (Halley’s at its second return) came to the conclusion that repulsive action exerted by the sun on the matter raised in these envelopes had been distinctly proved.
Yet here, where we seem to have our first firm ground for hypothesis respecting these mysterious objects—comets’ tails—we meet with stupendous difficulties. Consider, for instance, the phenomena presented by Newton’s comet. That comet had traversed the last ninety millions of miles of its approach toward the sun in four weeks. At the end of that time it passed out of view for a few days, having then a tail ninety millions of miles, at least, in length. Four days passed, and it reappeared on the other side of the sun—having in the interval traversed nearly a semi-circle—in reality, of course, the perihelion end of its long oval path. At its reappearance, it had a tail still ninety millions of miles in length, but the tail with which it reappeared had, of course, a direction entirely different from that of the tail which had been seen before—the two directions were inclined about one hundred and sixty degrees to each other. Now, as Sir John Herschel remarks, we can not look on the tail of a comet as something whirled round like a stick, as the comet circles round its perihelion sweep. The tail with which the comet reappeared must have been an entirely new formation. Nor can we doubt that if the comet had been watched as it swept around the sun, the changes in the tail’s position which had been observed to the time of disappearance, would have been observed to progress continuously, the tail passing by a uniform motion from the position it then had to that which it was observed to have at the time of reappearance. So that we may fairly suppose the tail with which the comet reappeared to have been formed in much less than the time during which the comet had been out of sight. Probably its farthest part had been formed in much less than a day, the part near the head being, of course, formed later. But if the matter repelled from the head was thus driven over a distance of ninety million miles in twenty-four hours, at the outside, the average velocity of its motion was about a thousand miles per second, or nearly three times as great as the greatest velocity which the sun can communicate by his attractive energy to matter approaching him from without, even though such matter come to him from an almost infinite distance, and in a perfectly straight line—the conditions most favorable for giving a high rate of final velocity. Such velocity as the sun can thus give by his attractive energy is only given to matter which has been exposed a long time to his influence; but here, in the tail of the great comet of 1680, matter seems to have acquired almost instantaneously a velocity sufficing to carry it over ninety million miles with an average speed three times as great as the sun can thus, after long effort, communicate by means of his attractive power!