Through the Telescope

by James Baikie

COMETS AND METEORS

There is one type of celestial object which seldom fails to stir up the mind of even the most sluggishly unastronomical member of the community and to inspire him with an interest in the science—an interest which is usually conspicuous for a picturesque inaccuracy in the details which it accumulates, for a pathetic faith in the most extraordinary fibs which may be told in the name of science, and for a subsidence which is as rapid as the changes in the object which gave the inspiration. The sun may go on shining, a perpetual mystery and miracle, without attracting any attention, save when a wet spring brings on the usual talk of sun-spots and the weather; Jupiter and Venus excite only sufficient interest to suggest an occasional question as to whether that bright star is the Star of Bethlehem; but when a great comet spreads its fiery tail across the skies everybody turns astronomer for the nonce, and normally slumber-loving people are found willing, or at least able, to desert their beds at the most unholy hours to catch a glimpse of the strange and mysterious visitant. And, when the comet eventually withdraws from view again, as much inaccurate information has been disseminated among the public as would fill an encyclopædia, and require another to correct.

Comets are, however, really among the most interesting of celestial objects. Though we no longer imagine them to foretell wars, famines, and plagues, or complacently to indicate the approbation of heaven upon some illustrious person deceased or about to decease, and have almost ceased to shiver at the possibilities of a collision between a comet and the earth, they have within the last half century taken on a new and growing interest of a more legitimate kind, and there are few departments of science in which the advance of knowledge has been more rapid or which promise more in the immediate future, given material to work upon.

The popular idea of a comet is that it is a kind of bright wandering star with a long tail. Indeed, the star part of the conception is quite subsidiary to the tail part. The tail is the thing, and a comet without a tail is not worthy of attention, if it is not rather guilty of claiming notice on false pretences. As a matter of fact, the tail is absent in many comets and quite inconspicuous in many more; and a comet may be a body with any degree of resemblance or want of resemblance to the popular idea, from the faint globular stain of haze, scarcely perceptible in the telescopic field against the dark background of the sky, up to a magnificent object, which, like the dragon in the Revelation, seems to draw the third part of the stars of heaven after it—an object like the Donati comet of 1858, with a nucleus brighter than a first-magnitude star, and a tail like a great feathery plume of light fifty millions of miles in length. It seems as impossible to set limits to the variety of form of which comets are capable as it is to set limits to their number.

Generally speaking, however, a comet consists of three parts: The nucleus—which appears as a more or less clearly defined star-like point, and is the only part of the comet which will bear any magnification to speak of—the coma, and the tail. In many telescopic comets the nucleus is entirely absent, and, in the comets in which it is present, it is of very varied size, and often presents curious irregularities in shape, and even occasionally the appearance of internal motions. It frequently changes very much in size during the period of the comet's visibility. The nucleus is the only part of a comet's structure which has even the most distant claim to solidity; but even so the evidence which has been gradually accumulated all goes to show that while it may be solid in the sense of being composed of particles which have some substance, it is not solid in the sense of being one coherent mass, but rather consists of something like a swarm of small meteoric bodies. Surrounding the nucleus is the coma, from which the comet derives its name. This is a sort of misty cloud through which the nucleus seems to shine like a star in a nebula or a gas-lamp in a fog. Its boundaries are difficult to trace, as it appears to fade away gradually on every side into the background; but generally its appearance is more or less of a globular shape except where the tail streams away from it behind. Sometimes the coma is of enormous extent—the Great Comet of 1811 showed a nucleus of 428 miles diameter, enclosed within a nebulous globe 127,000 miles across, which in its turn was wrapped in a luminous atmosphere of four times greater diameter, with an outside envelope covering all, and extending backwards to form the tail. But it is also of the most extraordinary tenuity, the light of the very faintest stars having been frequently observed to shine undimmed through several millions of miles of coma. Finally, there is the tail, which may be so short as to be barely distinguishable; or may extend, as in the case of Comet 1811 (ii.), to 130,000,000 miles; or, as in that of Comet 1843 (i.), to 200,000,000. The most tenuous substances with which we are acquainted seem to be solidity itself compared with the material of a comet's tail. It is 'such stuff as dreams are made of.'

Comets fall into two classes. There are those whose orbits follow curves that are not closed, like the circle or the ellipse, but appear to extend indefinitely into space. A comet following such an orbit (parabolic or hyperbolic) seems to come wandering in from the depths of space, passes round the sun, and then gradually recedes into the space from which it came, never again to be seen of human eye. It is now becoming questionable, however, whether any comet can really be said to come in from infinite space; and the view is being more generally held that orbits which to us appear portions of unclosed curves may in reality be only portions of immensely elongated ellipses, and that all comets are really members of the solar system, travelling away, indeed, to distances that are immense compared with even the largest planetary orbit, but yet infinitely small compared with the distances of the fixed stars.

Second, there are those comets whose orbits form ellipses with a greater or less departure from the circular form. Such comets must always return again, sooner or later, to the neighbourhood of the sun, which occupies one of the foci of the ellipse, and they are known as Periodic Comets. The orbits which they follow may have any degree of departure from the circular form, from one which does not differ very notably from that of such a planet as Eros, up to one which may be scarcely distinguishable from a parabola. Thus we have Periodic Comets again divided into comets of short and comets of long period. In the former class, the period ranges from that of Encke's comet which never travels beyond the orbit of Jupiter, and only takes 3·29 years to complete its journey, up to that of the famous comet whose periodicity was first discovered by Halley, whose extreme distance from the sun is upwards of 3,200,000,000 miles, and whose period is 76·78 years. Comets of long period range from bodies which only require a paltry two or three centuries to complete their revolution, up to others whose journey has to be timed by thousands of years. In the case of these latter bodies, there is scarcely any distinction to be made between them and those comets which are not supposed to be periodic; the ellipse of a comet which takes three or four thousand years to complete its orbit is scarcely to be distinguished, in the small portion of it that can be traced, from a parabola.

Several comets have been found to be short period bodies, which, though bright enough to have been easily seen, have yet never been noticed at any previous appearance. It is known that some at least of these owe their present orbits to the fact that having come near to one or other of the planets they have been, so to speak, captured, and diverted from the track which they formerly pursued. Several of the planets have more or less numerous flocks of comets associated with them which they have thus captured and introduced into a short period career. Jupiter has more than a score in his group, while Saturn, Uranus and Neptune have smaller retinues. There can be no question that a comet of first-class splendour, such as that of 1811, that of 1858, or that of 1861, is one of the most impressive spectacles that the heavens have to offer. Unfortunately it is one which the present generation, at least in the northern hemisphere, has had but little opportunity of witnessing. Chambers notices 'that it may be taken as a fact that a bright and conspicuous comet comes about once in ten years, and a very remarkable comet once every thirty years;' and adds, 'tested then by either standard of words "bright and conspicuous," or "specially celebrated," it may be affirmed that a good comet is now due.' It is eleven years since that hopeful anticipation was penned, and we are still waiting, not only for the 'specially celebrated,' but even for the 'bright and conspicuous' comet; so that on the whole we may be said to have a grievance. Still, there is no saying when the grievance may be removed, as comets have a knack of being unexpected in their developments; and it may be that some unconsidered little patch of haze is even now drawing in from the depths which may yet develop into a portent as wonderful as those that astonished the generation before us in 1858 and 1861.

The multitude of comets is, in all probability, enormous. Between the beginning of the Christian era and 1888 the number recorded was, according to Chambers, 850; but the real number for that period must have been indefinitely greater, as, for upwards of 1600 out of the 1888 years, only those comets which were visible to the naked eye could have been recorded—a very small proportion of the whole. The period 1801 to 1888 shows 270, so that in less than one century there has been recorded almost one-third of the total for nineteen centuries. At present no year goes by without the discovery of several comets; but very few of them become at all conspicuous. For example, in 1904, six comets were seen—three of these being returns of comets previously observed, and three new discoveries; but none of these proved at all notable objects in the ordinary sense, though Comet 1904 (a), discovered by Brooks, was pretty generally observed.

It would serve no useful purpose to repeat here the stories of any of the great comets. These may be found in considerable detail in such volumes as Chambers's 'Handbook of Astronomy,' vol. i., or Miss Agnes Clerke's 'History of Astronomy.' Attention must rather be turned to the question, 'What are comets?' It is a question to which no answer of a satisfactory character could be given till within the last fifty years. Even the great comet of 1858, the Donati, which made so deep an impression on the public mind, and was so closely followed and studied by astronomers, was not the medium of any great advance in the knowledge of cometary nature. The many memoirs which it elicited disclosed nothing fundamentally new, and broke out no new lines of inquiry. Two things have since then revolutionized the study of the subject—the application of the spectroscope to the various comets that have appeared in the closing years of the nineteenth century, and the discovery of the intimate connection between comets and meteors.