Popular Science Monthly/Volume 16/December 1879/Expected Meteoric Display
By RICHARD A. PROCTOR.
IT is expected, by nearly all astronomers who have given attention to the subject, that there will be a display of falling stars on or about November 27th next, though the night of the shower may perhaps fall earlier or later, within a week or so either way. The display, should it occur, will possess far more interest than any ordinary shower of shooting stars, or even than the displays which have been witnessed on the night of November 13th-14th, in 1799, 1833, 1866, and other years. For, though we now know that when these showers of Leonides (as the meteors of November 14th-15th are called) occur, the earth is passing through the track of a comet which is followed by uncounted millions of meteors, and the like when on the nights of August 10th, 11th, and 12th the meteors called Perseids are seen, yet the comets corresponding to these longer-known meteoric showers are less interesting to astronomers than the comet along whose track those bodies travel which produced the shower of falling stars seen on the night of November 27, 1872, and which are expected to produce a similar display this year. It was well remarked by M. O. Struve, at the last meeting of the German astronomical Society, that no object has thrown more light on the general nature of cometic bodies than the comet known as Biela's. I propose now to give a brief sketch of the history of this interesting body, and then to consider the reasons why astronomers expect that during the last week of November, 1879, there will be a display of shooting stars as the earth passes through the comet's track.
In the year 1826 Biela discovered a comet, the path of which was calculated by Gambart, a French astronomer, insomuch that, according to the usual rule in such cases, the comet should be called Gambart's, not Biela's. It was found to revolve around the sun in a period of about six and two thirds years. It was not a conspicuous body—in fact, it has seldom been much more than barely visible under the most favorable conditions by the naked eye. Yet it differed from most telescopic comets in showing not only a nucleus and a coma, but a tail also. In 1832, 1839, and 1846, this comet returned to the earth's neighborhood, and on two of these occasions it was well seen. In 1839 it was so situated as to be lost in the sun's rays. In fact, at every third return astronomers knew that it would be hopeless to search for the comet. Thus, it was discovered in 1826, and well seen in 1832, but not seen and not even looked for in 1839. So, again, it was seen in 1846 in its calculated place, and again in 1852, but it was not looked for in 1859. In 1866 and 1872 it should have been visible, but, as will presently be explained more fully, it was not seen. In this present year, 1879, supposing all had gone on as in the forty preceding years, the comet would not have been visible, passing too near the sun's place in the sky. Astronomers have been set to search for it this year (but quite fruitlessly), because there were reasons to believe that, if seen at all, the comet would not be seen on its former track. But we must not pass to this part of the comet's history until the strange circumstances connected with former returns and with former expected returns of the comet to visibility have been briefly considered.
In the year 1846, when Biela's comet was well seen, it divided—or rather, after having apparently been single, it was seen to be divided—into two distinct comets, each having coma, nucleus, and a short tail of its own. These two comets traveled along side by side until they passed out of view; but in 1852 both returned into view, though the distance between them was then greatly increased. Whether in 1859 the companion comets would have been seen had the earth been more favorably situated, is not known. The comet was not even looked for in that year, so hopeless did the search seem for so faint an object, close as the comet then was to the sun's apparent place in the sky. But in 1866 the comet should have been seen as favorably as in 1846. The superintendent of the "Nautical Almanac" published an ephermis of the comet's motions—in other words, he stated where the comet was to be looked for day after day, and a number of the most skillful practical observers in Europe searched carefully for it, but it was not seen. "There was not the slightest room," I wrote in 1872 (and, despite the opinions which have been since expressed by several astronomers, I see no reason for changing my opinion), "for questioning the accuracy of the calculations by which its path had been predicted. Astronomers were certain that, if undestroyed or undissipated, the comet would follow the assigned path—as certain as a station-master would be that a train would enter a station along the line of rails assigned to it, unless some accident or mistake should occur. But comets do not make mistakes, though, as we now see, they are not free from accidents. This comet had already met with an accident, being broken by some mischance into two parts, under the very eyes of astronomers. Possibly in 1859 it met with further misadventures. At any rate, something had happened to the comet since its retreat in 1852. 'It is now,' Sir J. Herschel wrote in February, 1866, 'overdue. Its orbit has been recomputed, and an ephemeris calculated. Astronomers have been eagerly looking out for its reappearance for the last two months, when, according to all former experience, it ought to have been conspicuously visible, but as yet without success—giving rise to the strangest theories. At all events, it seems to have fairly disappeared, and that without any such excuse as in the case of Lexell's, viz., the preponderant attraction of some great planet. Can it have come into contact or exceedingly close approach to some asteroid as yet undiscovered? or, peradventure, plunged into, and got bewildered among, the ring of meteorites, which astronomers more than suspect?'"
But, as I pointed out at the time, there was a convincing objection against the first of these theories in the circumstance that, the two comets into which Biela had separated being more than a million miles apart when they passed out of view in 1852, it was not in the least likely that both would be so far perturbed by asteroidal perturbations as to remain thenceforward undiscoverable. "It would be a singular chance," I said (this was before November 27, 1872, when fresh light, presently to be noted, was thrown on this object), "which should bring one of these objects into collision with a minor planet, or so near as to occasion an important disturbance. But, supposing this to happen, then the fellow comet, not traveling in the wake of the first, but side by side, would certainly have escaped. For it must be remembered that, although 1,250,000 miles is a very small distance indeed by comparison with the dimensions of the solar system, it is an enormous distance compared with the dimensions of the minor planets, some of which have a surface not much greater than that of an English county. The minor planet occasioning the comet's disturbance would presumably be one of the smallest, since it has not yet been detected, and the newly-discovered planets are on the average much smaller than those first detected. Now, the earth herself would have no very marked influence on a comet or meteor passing her at a distance of 1,250,000 miles; for it is to be remembered that the comet as well as the earth would have an enormously rapid motion, and the disturbing power of the earth would therefore only act for a short time. But a minor planet—even the largest of the family—would not have the twenty-thousandth part of the earth's power to disturb a passing comet. At a distance of 200,000 miles a comet would pass such an asteroid without any marked disturbance of its motions," and at a distance of 1,250,000 miles there would practically be no disturbance at all. "It is, of course, not absolutely impossible that one of the comets of the pair should have been encountered by one minor planet, and the other by another, but the probability against such a contingency is so great, that we need scarcely entertain the idea even as a bare possibility."
On the other hand, the supposition that the comet was destroyed or dissipated by meteor-streams, though not altogether untenable, seems little likely to be correct. I was disposed, when I wrote the article from which I have quoted the above passages, to think otherwise. "The comet," I said, "had been seen to divide into two parts in a portion of the solar system where certainly no bodies but meteorites can be supposed to travel. It seems reasonable to suppose that on that occasion the head of the comet had come upon some group of meteors, and so had divided, as a stream of water divides against a rock. Assuming this, we find reason for believing that the track of this comet crosses a rich meteor region. The particular group which had caused the division of the comet would of course pass away, and would not probably come again in the comet's way for many years or even centuries; but another group belonging to the same system might in its turn encounter the comet, and complete the process of dissipation which the former had commenced. On this theory the distance between the companion comets would introduce no difficulty. For not only is it quite a common circumstance for meteoric systems to have a range of several millions of miles, but—a much more important consideration—both the comets would be bound to return to the scene of the former encounter. It was there that each had been sent off on a new track, but each new track started from there, and therefore each new track must pass through there." The reasoning here is correct enough as far as it goes, but it does not duly take into consideration the extreme sparsity of meteoric distribution and the extreme tenuity of the heads and even of the nuclei of comets. As I pointed out in an essay which appeared in the "Popular Science Review" two months only after the essay from which I have quoted above had appeared in the "St. Paul's Magazine" (if I remember rightly), the meteors of even one of those comparatively rich clusters which produce an important display are strewed so sparsely that each occupies on the average a space corresponding in volume to a cube one hundred miles in length, breadth, and height. The largest meteor in the solid form is probably not many inches in diameter (I am speaking, be it remembered, of the meteors producing displays of ordinary shooting stars or falling stars, not of those masses which thrust their way through the upper regions of the air, and, exploding, cast their fragments often over many square miles of the earth's surface). It will be understood how small is the chance that a flight of bodies so minute compared with the average space occupied by each could cause the dispersion of a mass so rare, and therefore so free to pass through a meteor-flight without disintegration or disturbance, as a comet.
How Biela's comet came actually to be divided into two distinct bodies, and later to be so far dissipated as to be no longer visible even in the most powerful telescopes and under the most favorable circumstances, will probably be understood when we know the nature of those processes of repulsion which lead to the formation of comets' tails. For our present purpose it is only necessary to observe that these processes of repulsion do most obviously carry away parts of the substance of a comet's head to enormous distances, and that, in some way, Biela's comet was divided, even as it were under the eyes of astronomers, into two distinct comets; for we thus learn to recognize the further disintegration of the comet as part of a process undoubtedly commenced in 1846 and undoubtedly competent to effect the dissipation of the comet's substance. As the comet was searched for in vain in 1866 in the region which unquestionably it would have traversed had it remained unchanged, there can be no reason for doubting that it had thus been thoroughly dissipated and disintegrated. If anything could have made this more certain, it would have been the circumstance that in 1872, also, the comet was searched for in vain. Remembering that the observations made during the first few weeks after the comet's discovery in 1826 gave astronomers such a mastery over its motions that they could successfully predict its return in 1832, and show precisely where it would appear, nay, even calculate back its path and recognize its identity with a comet discovered by Montaigne in 1772, and rediscovered (though not recognized as the same) by Pons in 1805, it is obvious that in 1866, after several carefully observed returns and nearly a century after its first discovery, the comet's motions must have been much more thoroughly understood. It would have been much more easily detected that year than in 1846 and 1852, even as Halley's comet was much more easily detected at its return in 1835 than at its return in 1759.
If the comet had been like most of its fellows, astronomers must have given up all idea of obtaining further information respecting it. But in one important respect it differed from them. It is one of the few known comets whose paths cross, or at least pass very close to, the track of the earth. Already in 1832 attention had been called to this circumstance. Indeed, fears had been excited among those unfamiliar with astronomical relations by the announcement that the comet would cross the earth's path in that year, although it was explained that the comet would pass a month before the earth reached that point of her path. "We escaped that time," Sir John Herschel wrote in 1866. "Had a meeting taken place, from what we know of comets, it is probable that no harm would have happened, and that nobody would have known anything about it." But from what we have since learned we have reason to believe that we should have known a great deal about the encounter, though it remains altogether probable that no harm would have happened. For we have learned that as a rule the tracks of comets are followed by millions of meteoric bodies, which, as the earth passes through the flight, produce displays of falling stars, each meteor in its rush through the earth's atmosphere producing a trail or streak of light; and doubtless in the head itself of a comet meteoric bodies are much more richly strewed, so that an encounter with the head would produce an unusually splendid display of falling stars. It is, however, very noteworthy, as will presently appear more clearly, that no display of meteors is recorded as having occurred in the last week of November, 1832, though the comet had crossed the earth's track less than a month before. Yet in 1872 astronomers were led to expect somewhat confidently that, as the earth passed the track of Biela's comet, which had gone that way only some ten or twelve weeks before, there would be a shower of falling stars produced by the bodies following in the comet's path.
I may pause here, by the way, to remark on the clear way in which this expectation, and what was actually observed, should show every one who has clear mathematical conceptions that it is the train, and not the tail, of a comet, which is followed by meteoric attendants. Professor Tait, of Edinburgh, who is a master of mathematical analysis, but apparently wanting in the power of clearly conceiving geometrical relations, has based on the mistaken idea that comets' tails are made up of meteors a wild theory of the phenomena presented by these appendages, a theory which could not be accepted even if it had been proved that comets' tails are formed of meteor-flights. For he explains the appearance of a long cometic tail as due to the circumstance that at the time the earth is in the plane of a vast meteoric stratum attending on the comet, though it is certain that not one of the known long-tailed comets can have kept its stratified meteoric tail (assuming always that it had one) directed with its plane earthward during half the time of the tail's actual visibility. But so far as real evidence is concerned, the probability is that there are no meteors in or near the tail of a comet. For, on the one hand, on the only occasion when the earth is known to have passed through the tail of a comet—namely, when she passed through the tail of the splendid comet of 1861—no meteors were seen which could have belonged to that appendage; and on the other, in every single case in which meteors have been associated with a comet, those meteors have not been in or anywhere near the comet's tail. As I have said, Biela's comet is a case in point, and so obviously in point that it is difficult to understand how any mathematician could follow the history of the case without at once recognizing the error which nevertheless has misled and still misleads Professor Tait. That double comet, with its tails projecting from the sun, crossed the earth's path in or about the first week of September, 1872, traveling on a path slanted to the plane of the earth's orbit at an angle of twelve and a half degrees, and with a velocity considerably exceeding that of the earth in her orbit. Moving at this rate, and with this inclination, the companion comets would of course attain in ten weeks a position many millions of miles south of the plane of the earth's orbit. Thus a line from the sun to either comet would not, where prolonged into the tail, approach within many millions of miles of the earth's orbit—that is, of any position which the earth can possibly occupy. Both comets were even farther away from the actual position occupied by the earth at the time when, nevertheless, astronomers predicted a star-shower, and when, as they predicted, such a shower occurred. For the comets had left that place ten or twelve weeks before, and nearly the whole of the comets' motion had carried them away from that place, whereas only a small part of their motion had carried them away from the plane of the earth's orbit. In fact, no one who had studied with any attention the circumstances of any predicted meteor-display could have fallen into the mistake made by Professor Tait, a mistake actually so elaborated as to be made the basis of an entirely novel, and for other reasons utterly impossible, theory of comets' tails.
The predictions made in November, 1872, were not so precise as they would probably have been if the comet had been seen in 1866 and in 1872, as had been expected. Indeed, astronomers had very little experience as to the meteors of Biela's comet. They were in doubt what showers among those recorded by various observers of meteors as occurring during the last week of November and the first week of December could be associated with this particular meteor system. For until the astronomical significance of meteoric displays had been fully recognized, the observers of shooting stars, even when these were seen in showers, had been more careful to record the brightness and the number of the meteors than their course among the stars. So that the criterion which at present distinguishes one meteor system from another, even though both meteor systems may show falling stars on one and the same night or at one and the same time, is not applicable to most of the records of star-showers. That criterion, it need hardly be said, is the position of what is called the radiant point of the star-shower, the point from which all the meteor-tracks on the sky seem to tend. The reader must not fall into the mistake of supposing that every meteor-track absolutely extends from the so-called radiant. On the contrary, it may truly be said that not one such track does or can extend from that point. But each tends from the point in the sense that, if the course pursued by the meteor be supposed to be extended backward in a straight line (or, more correctly speaking, in a great circle of the heavenly sphere), the line would pass through the radiant point. The expression is used in the same general sense, and has, in fact, the same significance as the statement usually made about parallel lines and their vanishing point in perspective. Lines which are really parallel are so drawn in perspective that they all tend from one and the same point, but they do not extend from it. An artist might indeed draw them all in pencil from that point, but he would afterward rub out parts of the pencil-lines, leaving the rest all tending from the vanishing-point, but none of them extending actually from it.
Now, what is the radiant point of a meteor system? It is in reality that infinitely remote point from which all the meteors seem to be traveling—the point toward which all the parallel lines on which they are actually traveling seem to converge. No meteor, then, approaching the earth on the course thus indicated could possibly seem to move actually from the radiant point. If moving directly toward the observer, it would be visible at the radiant point, all the time, not seeming to move from it; if not moving directly toward the observer, but on a course parallel to that from the radiant point to the observer, it would be seen, from the beginning to the end of its flight, at points removed from the radiant, but all on a line tending from it. Thus the
actual path pursued by a meteor may be on one side of the heavenly sphere, while the radiant is on the other; precisely as any particular yard of a set of parallel railway lines and telegraph wires may be to the right or the left, or above or below, or may be behind an observer, while the point from which all these lines converge is in front of him. Yet two meteor-tracks, carefully observed, will suffice, unless absolutely coincident, to show the radiant point belonging to them, assuming of course that they belong to the same system. And when on any night many meteors of the same system are seen, the radiant point of the system, which indicates the direction from which with respect to the earth they all seem to travel, can be most accurately determined. In this way each meteor system is perfectly distinguishable from all others; and also, from the position of the radiant point of a system, the question whether the meteors are or are not bodies following in the track of any known comet, can be at once set at rest. The path of such bodies can be calculated with perfect exactness. The apparent path resulting from the combination of their motions with the motions of the earth can equally well be determined. This gives the radiant point of such bodies, if such bodies there are, as they appear in shooting-star displays in our skies. No scattered meteors, still less any meteor-shower, can be mistaken for attendants on such a comet—at least, if we set aside the bare possibility (for such it is) that bodies really traveling in a different course may appear to travel on the same course. This can happen; but it is so exceedingly unlikely, that if a meteor-flight appears at the time, and from the radiant point, corresponding to the attendants of a particular comet, it may be confidently assumed that they really are such attendants.
But, as I have said, on former occasions when displays of meteors occurred during the last week in November or the first week of December, which might therefore have indicated the earth's passage through the train of Biela's comet, no special observation was made of the tracks of individual meteors, so that it was not possible to ascertain afterward whether such showers might or might not be thus explained. Nor were any observations made for Biela meteors when the earth passed through the track of the comet in 1836, when, from what we now know, a display of such bodies might have been expected.
It was otherwise in 1873, Biela's comet itself having been searched for fruitlessly, several astronomers called attention to the circumstance that in the last week of November the earth might be expected to pass through a train of meteors following in the track of the now disintegrated comet. They showed also how Biela meteors, if such existed, could be distinguished from other shooting stars; the radiant point corresponding to attendants on Biela's comet lying in the region where the constellation Andromeda borders on Cassiopeia, near the feet of the former of these celestial bodies. I myself wrote in the following terms, in a paper written in October, and which appeared in the "St. Paul's Magazine" for November, 1872: "There will probably be a display of meteors following the track of Biela's comet. At any rate, the skies should be carefully watched. The shower of meteors (should one occur) will fall in such a direction that shooting stars might be looked for at any hour of the night. Those belonging to Biela's comet could be very readily distinguished from others, because their tracks would seem to radiate from the constellation Cassiopeia. So that should any one observe, on any night between November 25th and December 5th, a shooting star following such a track, he will have the satisfaction of knowing that in all probability he has seen a fragment or portion of a comet which has divided into two if not three distinct comets, and has followed up that process of dissipation by dissolving altogether away."
The prediction thus made was abundantly fulfilled. On November 27, 1872, there was a display of shooting stars second only in magnificence among those seen since the middle of the present century to the shower observed in the early morning hours of November 14, 1866. In numbers, indeed, the shooting stars of November, 1872, fully equaled, if they did not exceed, the shooting stars of November, 1866. Professor Grant, of the Glasgow Observatory, counted no fewer than 10,579 meteors between 5h. 30m. p. m. and 11h. 50m. p. m. Four observers in Italy, who severally limited their observations to the four quarters of the heavens between the four cardinal points, counted in six and a half hours 33,400 shooting stars. It appears that the greatest number were seen between 7h. and 9h. p. m. Between 6h. 55m. and 6h. 56m. the whole of the sky around the radiant of the system seemed to be occupied by a meteoric cloud. This region lay, as predicted, near the feet of Andromeda. There remained no doubt that the earth on the night of November 27th had crossed a stream of meteorites, following in the track of Biela's comet.
But now followed what gave rise to considerable misapprehension, by which it would seem that even some mathematicians of considerable skill have been misled. A German astronomer, Klinkerfues, telegraphed to Pogson, the Government observer at Madras, "Biela touched earth November 27th; look for it near Theta Centauri": meaning, doubtless, what was in reality the case, that the earth had passed through the meteoric train of Biela, and that it might be worth while to look out for the retreating flight in the part of the heavens directly opposite the point from which the meteors had seemed to arrive. Whether Klinkerfues meant this, or whether, as some seem to suppose, he meant that possibly Biela's comet might have been delayed ten or twelve weeks, and so have certainly encountered the earth on November 27th, need not for the moment be considered. Suffice it that Pogson examined the heavens in the region indicated, and there, in the early morning hours of December 2, 1872, detected two cloud-like objects. These he saw again on the morning of December 3d—by which time their position on the star-vault had changed, so that it was clear they were not nebulae or star-clouds, but veritable attendants on the sun, though whether comets or meteor-flights was not clearly made out. It was, however, clearly shown that neither of these objects could possibly have been the meteor-flight crossed by the earth on the night of November 27, 1872. It was equally certain that neither the meteor-flight nor these two cometic objects could have been Biela's comet itself—though all three were traveling in such courses that they might be called attendants of that body.
There for the time the history of Biela's comet has closed, Nothing more has been seen of it, either as a comet or as a meteor-flight, though scattered meteors traveling in its train were seen toward the end of November, 1877, and more would probably have been seen at the same part of last year if the skies then had not been overclouded in nearly all European countries.
The next passage of the earth athwart the track of the comet is the first, since that of November 27, 1872, during which a meteor shower could be expected to occur. The comet crossed the earth's track, or passed very near to it, early in April last; and though the interval is considerably longer between then and the end of November than elapsed between the comet's passage in 1872 and the display of that year, yet it is most probable that many meteoric attendants of the comet will be seen on some nights (or perhaps on several nights) between November 25th and December 1st, and quite possible that a very fine shower may be seen. The meteors will be well worth looking for in any case, since, if they are carefully observed and counted hour by hour, astronomers will probably obtain some further insight into the nature of the processes which lead to the dissipation of a comet and cause its path to be occupied over a range of many millions of miles by scattered meteoric attendants. To others than astronomers, the meteors will be full of interest; and it is not at all unlikely that they will appear in such numbers as to produce an exceedingly beautiful display.—Belgravia.