God's glory in the heavens/The History Of Comets

From Wikisource
Jump to navigation Jump to search

Head of Comet of 1811.

XI.
THE HISTORY OF COMETS.

We are induced to return to the subject of comets, chiefly on account of the sudden appearance (1860) of one of these bodies in the northern horizon, which is suspected by some to be one of much historical interest, and whose return has been long expected. At the date of our writing, nothing is positively known about it except its physical aspect. Sufficient time has not elapsed to determine its orbit, and establish its identity with one of a previous epoch.

The manner of its discovery has been a little tantalising to professional observers. No observatory claims the honour; no amateur, even, has been the first favoured discoverer of it. There are many whose sole occupation is to dredge the depths of space for comets. In astronomical language, they "sweep" the heavens with their comet-seeker, and perseverance is almost always rewarded with success. In sweeping for asteroids, there is a zone of the heavens to which observations are limited; and the astronomer selects this lucky region, just as the fisherman lets down his net over some bank which he knows, by experience, is likely to reward his toil. The dredger for comets must, however, cast his net at random, for these erratic bodies affect no special locality. They come, as moths, from out the darkness, and are visible only when they circle round the sun, as moths flutter about the lamp; but they come in all directions, and the telescope must sweep over the whole heavens to detect the next comer.

The discovery of the comet is tantalising, as it was made by one who was not at all in search of it. M. le Baron de Marguerit, while attending to his military duties in the camp at Chalons, was the fortunate man. In his letter to the Abbé Moigno, announcing the discovery, he tells with great naïveté the manner in which it was made. Disgusted with the slow life of the camp, and the desolate plains of Champagne, he turned his eyes in dreamy mood towards the heavens. He thought he saw something unusual, and on taking up his opera-glass, he found it was a comet with a well-marked tail, the full extent of which could not be detected, on account of the strong twilight in the northern horizon. It has been seen by many since as a brilliant object to the naked eye. The stronger twilight, in the higher latitudes of Scotland, has as yet deprived us of this interesting spectacle. The shape is much like that of the comet of 1858, but its lustre is much greater than that of the latter, when it first became visible to the naked eye. Leverrier apologises for the nodding of his staff of observers, by pleading the badness of the weather. The comet was allowed to approach, and gather brilliancy behind the murky atmosphere, and when, at last, the clouds broke up, the unaided eye, on account of the brilliancy of the object, had as good a chance as the most powerful telescope.

It is difficult to understand the rationale on which science distributes her honours. If a man, innocent of all scientific knowledge, only happen to catch the first glimpse of a comet, or pick up some new fossil, he receives the award of immortality, by having his name associated with the discovery. The comet or the fossil is called after him for all future time. When, however, the discovery is of real merit, such as that of a planet, all attempts to affix to them the names of mortals are in vain. It was attempted to give the name of Leverrier to the planet Neptune; but the scientific world would not recognise the designation. Galileo, as a compliment to the family of the Medici, named the satellites of Jupiter after them, but the name is now obsolete. Uranus, for a time, bore the name of the discoverer, and then of his royal patron, but it is now never known by the planet of Herschel, or the Georgium Sidus. How differently are cometary honours meted out, when the new comet will, in future, be known as that of the Baron de Marguerit, his sole merit consisting in looking up to the sky as he sauntered forth in the evening to while away the time.

Discoverers are losing their hold even of comets, as they are more frequently designated by the year of the discovery and the numerical order in which they have been discovered during the year. At present an attempt is made by Leverrier to abandon the present system of naming the asteroids after heathen deities, and to adopt, instead, the name of the discoverer, with the number of the asteroid in the order of discovery. This innovation is strongly resisted by most astronomers, and, at present, there is one. No. 59, without a name, simply because the discoverer, M. Chacornac, thinks with Leverrier, and persists in refusing to give it a name.

The appearance of the present comet naturally suggests the inquiry, Is it the great comet of 1556, the return of which has been confidently expected about this time? After various approximations, Mr Hind has finally fixed the limits of 1857 and 1861 within which we may reasonably expect this return. In establishing the identity of a comet, little reliance can be placed on the physical aspect. Comets that, at one period, shone with great splendour, have, at another, come back again, shorn of all their glory. They have not the stable constitution of the planets, which possess an identity altogether independent of their orbit. They may be more fitly compared to a cloud floating in the atmosphere. At sunset we see, in the western horizon, well-defined clouds gleaming with golden glory, and when we look again, the glory is gone, and we fail to recognise the object, its shape being now entirely altered. A comet fluctuates in like manner—its substance being of a much rarer consistency, and subject to influences which are constantly inducing changes in shape and lustre. The comet differs from a cloud in this, that it has a well-defined path by which its identity is established. The periodic revolution of comets cannot in general, however, be fixed with such accuracy as that of the planets. We can observe only one end of the long ovals in which comets move, but the ends of different ovals may be very much alike, and yet they may difier very much in length; consequently, an error made in determining the shape of the ellipse may entail a great error in its length, and the comet's period of revolution. Besides, comets move across the orbits of the planets, and are jostled about as a man moving through a crowd. One planet pulls this way and another that, so that the comet's real course may differ much from the undisturbed orbit. In predicting the return of the great comet of 1556, another formidable source of error lies in the circumstance, that the observation of its appearance in 1264 was very imperfectly recorded. The most reliable accounts are those of the Chinese, who were at that period far in advance of European astronomers; still, tbey are not given with sufficient accuracy to enable us to predict the return within a single year. Assuming that the above dates are correct, the period is 292 years, and the comet, if it moved in an undisturbed orbit, should return in 1848. But allowing for the disturbance of planets, the probable period is about ten years longer. As we are now approaching the latest limit assigned by competent astronomers, it is natural that the first inquiry should be, on the discovery of a new comet, Is it the expected great comet? The comet of 1556 is named after Charles V., from the circumstance that its dreaded aspect is said to have led that monarch to cede the imperial crown to his son Ferdinand.

We are saved the necessity of any further speculation as to whether the comet Marguerit is the comet of Charles V., by learning, while we write, that Leverrier has authoritatively decided the matter. He has made observations on three different days, and has thus been able to determine all the elements of the orbit of the former. The result is, that it is not the expected comet. A year of grace is still given, and astronomers have yet a short period of anxiety and suspense before them. But even though the great comet should not appear in 1861, it is not to be inferred that astronomical observations are fallacious. It may return, but it may be so dimmed in its lustre, or so overpowered by the sun's rays, that it may escape detection. The return would certainly establish the identity, but the non-appearance would not necessitate the conclusion that astronomy has been at fault.

For about a week, it was doubtful whether Marguerit was to retain his laurels, as it was maintained by some observers, that his comet was merely one that had been discovered on the 17th of April by M. Eumker of the Observatory at Hamburgh. The observations at the Paris Observatory have conclusively settled the matter, by shewing that the orbits are quite different. The new comet will therefore stand in the history of astronomy, as the comet of Marguerit, or Comet III. of the year 1860.

It may at first appear singular that comets should so long retain their prophetic character, seeing that the event must have so often falsified the prediction. But in this, as in many other cases, the belief of the prophecy often brought about the event. It was so in the case of Charles Y., whose resignation of his royal honours was due to his belief, that this was the true interpretation of the celestial prodigy. There are, however, fulfilments that cannot be explained so readily. A man, for example, believes that the appearance of a comet, or some other sign which he considers supernatural, announces his death on a particular day, and he dies accordingly on the very day. Numerous cases of this nature might be cited, which cannot be accounted for merely by accident. The explanation is a psychological one. The strong belief itself produces the physical effect. The phenomena of mesmerism, or, as it has been called, biology, sufficiently illustrate this. The operator works upon the mind of a weak or susceptible subject, and impresses strongly the belief, that some effect such as catalepsy must ensue, and, accordingly, the subject becomes cataleptic. Cases too are on record, where, by concert, some credulous person is made to believe, on the testimony of various parties, that he is labouring under a disease which must necessarily be fatal, and death has actually ensued. The belief in the prophetic character of comets, however, has chiefly been sustained by the fallacy of human judgment, which consists in giving more weight to a few coincidences than to many failures. The moon is a familiar instance. Even at the present day, there is a very general belief among intelligent people that its various phases influence the weather; and men of sound judgment in other matters, will speak of the change of the moon, and its virtue, as beyond dispute, though accurate observation has shewn that these changes influence the weather as little as the comet does the fate of nations or individuals.

It is curious to mark, how the superstitions of a dark age, when banished by the progress of science, take refuge under the wing of science itself. The incantation, magic, and witchcraft of other days have reappeared in our day under the forms of clairvoyance and spirit-rapping, and, with the sanction of the quasi science of mesmerism, intelligent people believe in and practise them. When science banished cometary superstitions, it still afforded a refuge for the love of the marvellous and dreadful. Halley and other astronomers, who were the first to indicate the true nature of comets, loved to describe the terrible consequences that might ensue from a collision with the earth. The comet played a most important part, too, in the rude geological theories of those days. Comets are in much disrepute among geologists at the present day, as accounting for the various convulsions and cataclysms indicated by the earth's crust. They find it more convenient to draw their hypothetical forces from the unknown interior of the earth. In the days, of Whiston, however, the tails of comets were all-important, as they could tilt up the axis of the earth, and produce deluges at will, to account for the various geological phenomena. By this means, the popular dread of comets was kept up long after superstition had departed. At various periods, great alarm has been created in the popular mind. To soothe this alarm, mathematicians have calculated the chances of a collision, and have shewn the improbability of such a catastrophe. Still, they have not been able to shew the impossibility; and the popular mind is sometimes as much alarmed at the possible as at the probable. Astronomers may give an assurance that a comet will come in collision with the earth only once in 280,000,000 of years, but then they can give no positive assurance that that one time may not be in our day, and this possibility is, to many minds, a sufficient ground of alarm. Our great security is, that the comets are not confined to one plane, as the planets are. If they were, they would, in crossing our path, subject us constantly to the danger of collision. There is, however, one known comet, Biela's, thus awkwardly circumstanced. Its orbit may be compared to a level-crossing over a railway. The danger of such crossings is well known, and hence the stringency of the law in requiring, generally, that the public road should pass above or under the line, and not on the same level. All other comets, as far as is known, either pass above or under the path of the earth, but this one passes so nearly on a level, that it is always a question of time every seven years whether we are to come into collision. Much apprehension was created in 1832, when it was thought the case of our earth was very critical. We, , succeeded in giving a wide berth to our dangerous neighbour, there being the margin of a whole month. In 1805, we had a much narrower escape; we then saved our distance by only three days. Fortunately the danger was not known till it was over. It was not, till afterwards, that the comet was known to be a periodic one. It can, however, be shewn from our knowledge of its orbit, that, in this generation at least, there is not only no probability, but no possibility of a collision.

Some astronomers, on the subject of comets, treat the popular mind as they would a spoiled child, by administering soothing and comforting assurances of all kinds. One of these is, that though we did go right through a comet, we would be none the worse of it. Judging from their tone, they would rather like the thing as a pleasant experiment. We would, however, require to know something about our suspicious neighbours, before venturing on such an experiment with anything like confidence. There would be little comfort in the assurance, that the tenuity of the substance is such that it would do neither good nor harm. We know that the most deadly miasmata are so subtle, that it is impossible to detect them by any chemical test, and a very homoeopathic dose of a comet, in addition to the elements of our atmosphere, might produce the most fatal effects.

It is hardly possible, at the present time, to comprehend the terror which a great comet usually inspired in former days. Astrology had worked up the cometary terrors into a regular system. Comets were divided into seven species, according to the number and nature of the seven planets. The colour served as the chief discriminating mark. Each species had its own special prognostication; but not only so, the house or constellation in which the comet was visible, had a modifying influence, and the complexity was so great, that the astrologer was never baffled in making the event coincide with the prediction. The Church of the middle ages, profiting by the superstitions of the times, reaped occasional advantages from the visitation of a comet. In the year 837, a great comet shone forth in the southern part of the heavens. All Europe was filled with consternation; but the comet took chief effect on the conscience of Louis I., King of France, who, to avert the impending judgment, founded many conventual establishments, which exist at the present day. The ringing of cathedral bells at noon, in Koman Catholic countries, can be traced to the influence of a great comet, viz., that of 1456, now known as Halley's. Pope Calixtus III., who then wore the triple crown, imagining that the comet was some demon come to shake his throne, ordered all the bells in Christendom to be rung—the ringing of bells being always considered a sovereign remedy against evil spirits. How strong a hold must that comet have taken of the popular mind, when four centuries have not rung off its terrors! The ejaculation, "God bless you," when a person sneezes, can, it is said, be traced to the comet of the year 590. It was universally believed that the comet caused the epidemic of that year. The most marked character of the disease was the fit of sneezing, in which the patient frequently died. When the bystanders heard a sneeze, they bestowed their benedictions by exclaiming, "God bless you." And through the long centuries following, this custom has been kept up.

The Abbé Zantédeschi of Padua has recently contrived some experiments which strikingly illustrate the constitution of comets, and also, as he thinks, that of the corona of solar eclipses. His opinion is, and it is shared by M. Faye and Father Secchi of Rome, that the moon has an atmosphere of considerable height, and that the corona is simply this atmosphere illuminated by the sun hid behind the moon. The atmosphere is not gaseous like ours, but is composed of highly attenuated particles of matter derived from the surface of the moon. Such an atmosphere would not refract light, so as to be detected by the occultation of stars; but it would reflect light in such a way as to be made visible in total eclipses. The above physicists are therefore opposed to the general opinion that the corona belongs to the sun, and not to the moon. Zantédeschi, by using very fine dust to represent the lunar atmosphere, has produced in a dark chamber, by means of the sun's rays, a phenomenon precisely similar to the corona. These experiments are exceedingly interesting when viewed in connexion with the total solar eclipse of the 18th of July 1860. It is confidently expected that this debated point will finally be settled by the observations to be then made. The representation of comets by the same means is very interesting and striking. Observation confirms the supposition that comets are not composed of a continuous substance like a gas, but of a mass of discrete particles, like a cloud of dust. This is known by its not refracting light like our gaseous atmosphere. Some comets, when very near the sun, seem to lose their envelopes, though they should now expand to their largest dimensions. The explanation of Herschel is, that the heat converts the visible discontinuous particles into an invisible gas. A comet would thus correspond to a cloud which consists of watery particles, but which become invisible on the application of heat—the watery particles being converted into the gaseous form of invisible vapour. So attenuated is the matter of comets, that, if condensed into a solid body, the largest might probably be packed into a ship's hold. This is deduced from the fact that the largest comets do not, in the slightest degree, affect the motion of planets or satellites, however close they may approach; and we have the case of one comet which swept right through the system of Jupiter's moons without altering their motion.

The constitution of comets, viewed in connexion with the arrangement of the solar system, strikingly illustrates the wisdom of God. Were comets not composed of such attenuated matter, the stability of the solar system would be destroyed, and life would soon be impossible on our globe. The stability depends on all the principal bodies being confined nearly to one plane; but the comets move in every possible plane, and hence if they were possessed of a planetary density, they would fatally disturb the equilibrium of the system. Were the comets restricted to the same plane in which the planets move, the danger of collision would be very great, as the long ovals in which the comets move would be constantly crossing the more circular orbits of the planets. In a crowded assembly, as long as the throng promenade in the same direction round a suite of rooms, there is no danger of coming to a lock; but if some move in an opposite direction, or attempt to cross the stream, the circulation is in danger of being brought to a dead halt. The motion of comets would interfere similarly with the regular motion of the planets, did they move in the same plane; and when we consider the thousands of comets that are constantly sweeping across the solar system, the chance of collision would be by no means inconsiderable. This danger is avoided by making the comets move either on a higher or lower level, just as travellers are made to do at railway stations when they wish to cross the rails. One comet, Biela's, crosses the orbit of the earth, as if to remind us of the danger we would run were the crowd of comets allowed the same privilege.

Comets may be regarded as the scouts or pioneers of the solar system. The ancients supposed we were encased in crystal spheres; but the comets in their course shivered them to atoms. Descartes then encircled us with vortices, but the comets in the hands of Newton stopped their action for ever. They have told us also of a resisting medium, and hinted at the probable doom of the solar system; and, no doubt, there are other high functions assigned them. We may regard a comet as a plumb-line let down into the depths of space, to explore the nature of the currents, and the objects that may exist far beyond the reach of vision. The change in their constitution, and the disturbance of their orbits, may tell us of the existence of worlds which the telescope may never reach. As the lead of the mariner, with its adhesive surface, brings up unmistakable evidence of the nature of the bottom, so comets, when we understand them better, may bring interesting news of regions hitherto unexplored. Comets rush forth from the starting-post of the sun with the speed of lightning, but they soon slacken their pace as if to feel their way; and, by the time they reach the end of their journey, a child trundling a hoop would be more than a match for them in speed. It is by this cautious pace they are best fitted to gather news of the remote regions to which they travel, as they are then most liable to have their path changed by unknown bodies. But going at this sluggish pace, they are most apt to be captured and chained for ever to some other system. Yet their very loss would tell an interesting tale.