The importance of the meteors in the planetary system will be recognised when we remember that the November group alone extends along its oval course in one complete system of meteors for a length of more than 1,700 millions of miles, with an average thickness of about a million miles (determined by noting the average time occupied by the earth in passing through the system on November 13-14), and an unknown cross breadth which probably does not fall short of three or four millions of miles. Other systems are, no doubt, far more important, for it has been found that meteors follow in the track of comets. Now the November meteors follow in the track of a comet (Tempel's comet of 1866), which was so small when last favourably placed for observation that it escaped detection by the naked eye. If so small a comet as this is followed by so large a meteoric system, in which also meteors are strewn so richly that during the passage of the earth through it, tens of thousands of meteors have been counted, how vast must be the numbers and how large probably the individual bodies following in the track of such splendid comets as Newton's, Donati's (1858), the comets of 1811, 1847, 1861, and others! For it should be remembered that we become cognisant of the existence of a meteoric system only when the earth threads its way through one, when those which she encounters may become visible as falling stars if it so chances that she encounters them on the dark or night half of her surface. But the earth is far smaller compared with a system like the November meteor-flight than a rifle-ball compared with the largest flight of birds ever yet seen. Such a ball fired into a very dense and widely extending flock of birds might encounter here and there along its course some five or six birds—not one in 10,000, perhaps, of the entire flight; and if the flock continued flying with unchanging course, a hundred rifle balls might be fired through it without seemingly reducing its numbers. Our earth has passed hundreds of times through the November meteor system, yet its meteoric wealth has scarcely been reduced at all, so exceedingly minute is the track of the earth through the meteor system compared with the extension of the system itself. The region through which the earth has passed is less than a billionth part of the entire region occupied by the system. But the November system is but one among several hundreds through which the earth passes—in other words, the systems which chance to be traversed by that mere thread-like ring in space traversed each year by the earth, are not a millionth, not a billionth, of the total number of such systems. It will be conceived, therefore, that the total amount of meteoric matter, travelling on orbits of all degrees of eccentricity and extension from the sun and inclined at all angles to the general plane of the solar system, must be enormously great. The idea once advanced by an eminent astronomer that the total quantity of unattached matter, so to speak, existing within the solar domain must be estimated rather by pounds than by tons is now altogether exploded. It would be truer to say that the totality of matter thus freely travelling around the sun must be estimated by billions of tons rather than by millions.
Whether it is likely that there will be a display of meteors to-night (or, rather, to-morrow morning), is a question to which most astronomers would be disposed, we believe, to reply definitely in the negative. The display of November 13-14, 1866, was very brilliant; that of 1867 (best seen in the United States) was almost equally so; but successive showers steadily diminished. In other words, the part of the system crossed by the earth in 1866 and 1867 was very rich, but the part which she crossed afterwards (the rich part having passed far on towards the remote aphelion of the system outside the orbit of Uranus) was less rich. For the last few years very few November meteors have been seen, though the few stragglers which have been seen, and have been identified as belonging to the family by their paths athwart the star-depths, have been almost as interesting to astronomers as the showers of such bodies seen in 1799, 1833, 1866, and 1867. But it is not altogether impossible that in the small hours 'ayont the twal' to-morrow morning a shower of meteors may be seen. For Schiaparelli (the Italian astronomer who first started the ideas which led when properly followed up to the discovery of the relations existing between meteors and comets) asserts that it has happened before now that the November meteors have appeared in great numbers in years lying midway between the times of maximum display. These times are separated on the average by about 33¼ years. Thus, in 1799, there was a great display of November meteors, a shower rendered specially celebrated by Humboldt's description. In 1833 there was another, the display which so terrified the negroes of South Carolina, but more interesting scientifically as described by Arago. In 1866 the shower again attained its maximum splendour, though the display of 1867 was little inferior. It will not be till 1899 that another great shower of November meteors may be confidently looked for. But if Schiaparelli be right, it is quite possible that there may be a shower this year, due to some scattered flight of the November meteors which, delayed accidentally (through some special perturbation) many hundreds of years ago, has come in the course of ages to travel nearly half a circuit behind the richest part of the system, the 'gem of the meteor-ring,' as it has been poetically called. Even, however, though no display of November meteors should be seen, yet the recognition of even a few scattered stragglers would be exceedingly interesting to astronomers. A single meteor seen to-night which could be regarded as certainly belonging to the November system would suffice to show the possibility that a whole flight of the November meteors might travel at a similar distance behind the main body. It would be more easy, however, to identify two such meteors than one, six than two, and a score than half-a-dozen. The only way in which a meteor can be questioned, so to speak, respecting the family it belongs to, is by noting its path across the sky. If this path tends directly from the constellation Leo (however remote Leo may be from the part of the heavens traversed by the meteor), the chances are that the meteor is a Leonid, or one of the November family. If the path tends from that particular part of the constellation Leo (near the end of the curved blade of the so-called Sickle in Leo), the probability of the meteor being a Leonid is increased. If two or more meteors are seen to-morrow morning (after 12.30) which both tend from the Sickle in Leo, even though they seem to tend in opposite directions, the chances are yet greater that they are travelling in parallel paths along the track of the November meteors, but some 2,000 million miles behind the main body. Should the number mount up to a score or so, the conclusion would be, to all intents and purposes, certain; and the possible occurrence of even a shower of Leonids at a time midway between the customary maxima of the meteoric displays would be placed beyond question.
We must, however, admit that it seems less likely there will be anything like a display of Leonids to-night than that patient observers may be able to identify a few of these bodies, and thus—though by observations of a less attractive kind—to advance our knowledge of this interesting system. Far more likely is it that towards the end of the month there will be a display of meteors belonging to another and an entirely distinct family, a family scarcely less worthy to be called November meteors par excellence, but actually rejoicing in the classically unsatisfactory name of Andromeds.
(From the Times of November 25, 1878.)
It is probable that during the next three nights some light may be thrown on one of the most perplexing yet most interesting of all the problems recently suggested to the study of astronomers. It is confidently expected that many of those November meteors called Andromeds will be seen on one or other of those nights, if not on all three. No meteor systems, not even the famous systems of August and November, are more remarkable than this singular family. To explain why astronomers regard the Andromeds with so much interest, it will be necessary to speak of an object which at first sight seems in no way connected with them—an object, in fact, which, so long as it was actually known to astronomers, was never supposed to be connected with any family of meteors—the celebrated lost comet called Biela's (or, by Frenchmen, Gambart's comet). In February, 1826, Biela discovered in the constellation Aries a comet which was found to be travelling in an oval path round the sun, in a period of about six years seven and a half months. Tracing its course backwards, astronomers found that it had been seen in 1772 by Montaigne, and observed for two or three weeks in that year by Messier, the great comet hunter. Nothing very remarkable was recognised regarding this comet in 1826, except the fact that its path nearly intersects that of our own earth; so that if ever the earth is to encounter a comet, here seemed to be the comet she had to fear. Great terror was, indeed, excited by the announcement that in 1832 the comet would cross the earth's track only four or five weeks before the earth came to the place of danger. But no harm happened. In that year, and again in 1839, the comet returned quietly enough, though in 1839 it was not observed, being so placed that it was lost in the splendour of the solar rays. In February, 1846, the comet was again seen, this being the third return since its discovery in 1826, or rather, since its recognition as a member of the solar system, the eleventh since it was first seen by Montaigne. At this time everything seemed to suggest that this comet, unless our earth at some future time should absorb it, would remain for a long time a steady member of the sun's comet family. But only a few days after its detection in February, 1846, the comet was found to have divided into two, which travelled side by side until both vanished from view with increasing distance. In 1852 the companion comets reappeared, and again both continued in view till their motion carried them beyond telescopic range. As the distance between the coupled comets had increased from about 160,000 miles in 1846 to about 1,250,000 miles in 1852, astronomers anticipated a most interesting series of observations at the successive returns of the double comet to the earth's neighbourhood. Unfortunately, in 1859 the comet's course carried it athwart a part of the sky illuminated by the sun's rays, so that astronomers could not then expect to see it. But in 1866 it was looked for hopefully. Its orbit had now been most carefully computed, and many observers, armed with excellent telescopes, were on the watch for it, with very accurate knowledge of the course along which it might be expected to travel, and even of its position from day to day and from hour to hour. But it was not seen. Nor, again, was it seen in 1872, when fresh computations had been made, and observations were extended over a wider range, to make sure, as was hopefully thought, that this time it should not escape recognition. Could it have come, asked Herschel in 1866—and in 1872 the same question might still more pertinently be asked—into contact or exceedingly close approach to some asteroid as yet undiscovered? or, peradventure, had it plunged into and got bewildered among the rings of meteorolites, which astronomers more than suspected?
Between 1866, when Sir John Herschel thus wrote, and 1872, when again Biela's comet was sought in vain, a series of strange discoveries had been made respecting meteors, which led astronomers to believe that, even though the missing comet might never again be seen as a comet, we might still learn something respecting its present condition. It had been noticed that the remarkable comet of 1862 (comet 11 of that year) crossed the earth's track near the place where she is on August 10-11, the time of the August meteors, called the Tears of St. Lawrence in old times, but now known as the Perseids, because they seem to radiate from the constellation Perseus. Later the idea occurred to Schiaparelli, an Italian astronomer, that the August meteors may travel along the path of that comet. He could not prove this, but he advanced very strong evidence in favour of the opinion, for he found that bodies travelling along the path of the comet of 1862 would seem to radiate from Perseus as they traversed the earth's atmosphere. It was as if a person suspected that a steam-cloud seen on a distant railway track belonged to a particular train, and, though unable actually to prove this, was yet able to show that, with the wind and weather then prevailing, that train, travelling at its customary rate, would leave a steam-cloud behind it precisely of the apparent length and position of the observed steam-cloud. This cloud might have the observed position though otherwise produced, yet the evidence would be thought strongly to favour the supposition that it came from the train in question. In like manner the August meteors might be travelling on any one of a great number of tracks intersecting the earth's orbit in the place occupied by the earth on August 10-11; yet it was at least a striking coincidence that a flight of meteors travelling in the orbit of the chief comet of 1862 would seem to radiate from the constellation Perseus, precisely as the August meteors do.
While astronomers were still discussing the ideas of Schiaparelli, Professor Newton of Yale College, in America, called their attention to the great display of November meteors which might be expected on November 13-14, 1866. The fine shower of that year was well observed, and the part—we may almost say the point—of the constellation Leo from which the meteors radiated was correctly determined. And now a strange thing happened. Those who believed in Schiaparelli's account of the August meteors supposed of necessity that the bodies forming that system travel in an orbit of enormous extent, for the comet of 1862 travels on a path extending much further from the sun than the path of Neptune. There was, therefore, nothing to prevent them from believing that the Leonides travel in a track carrying them far away from the sun. The recurrence of great displays of these meteors at intervals of about 33 years might be readily explained on such an assumption, for if the Leonides have a period of about 33 years, their path must extend far beyond the path of Uranus. But hitherto astronomers had not been ready to admit such an explanation of the periodic recurrence of great displays of the November meteors. They preferred theories (for several were available) which accounted for the 33-year period, while assigning to the Leonides paths of much less extent. Now that the idea of vast meteoric orbits had been fairly broached, some astronomers thought it might at least be worth while to calculate the path of the November meteors on the assumption that their true period is about 33¼ years. This was perfectly easy, because the period of a body travelling round the sun determines the velocity at any given distance from the sun, and knowing thus (at least, on this assumption) the true velocity of the Leonides as they rush into our air, while their apparent path is known, their true course is as readily determined as the true course of the wind can be determined by a seaman from the apparent direction and velocity with which it reaches his ship. When the path of the November meteors had been determined (on the assumption mentioned), it was found to be identical with the path of a comet which had only been discovered a few months before-the comet called Tempel's. That a comet which is invisible to the naked eye should have been discovered in the very year when first astronomers made exact observations of the meteors which travel in its track—for it will presently be seen that the assumption above mentioned was a just one—cannot but be regarded as a very singular coincidence. It was a most fortunate coincidence for astronomers, since there can be but little doubt that but for it Schiaparelli's theory would very soon have been forgotten. As that theory was itself suggested by the fortuitous recognition of another comet (only visible at intervals of more than a century) at a time when attention had been specially directed to the August meteors, it may fairly be said that the theory which now associates meteors and comets in the most unmistakable manner was suggested by one accident and confirmed by another. Albeit such accidents happen only to the zealous student of nature's secrets. We shall presently see that the fortunate detection of Tempel's comet in 1866 was not the last of the series of coincidences by which the theory of meteors was established.