Art. LX.—On the Constitution of Comets.
[Read before the Southland Institute, 10th October, 1882.]
Kepler, with the prescience of genius, supposed that comets throng in space as fish in the sea. It is true that only some 650 comets are recorded as seen during the Christian era, and that a considerable number of these have been reappearances of periodic visitors. But of these 650, about 120 have been seen in the Nineteenth Century,—mostly telescopic, however. This large number is owing to improved methods of observing. As many as eight have been seen in one year, and we have in our morning sky the fourth for the year 1882. Now, it is certain that not half the comets that approach the sun, within the range of the telescope, are seen by man. It may also be regarded as highly probable that vast numbers of comets have their perihelia at distances which preclude their discovery.
A comet crossing the solar frontier with the momentum of a few miles per annum, along a line forming an angle of 45° with the radius from the sun's centre to the comet's centre, at the moment of crossing, would secure a perihelion distance of many millions of miles. If, however, the momentum amounted to miles per hour, the comet would be carried far too wide of the sun to be observed by the inhabitants of the earth.
As, therefore, it is probable that comets enter the sun's domain with a great variety of momenta; and as, in the case of any particular comet, the probabilities are many millions to one against its momentum being in the direction of the little point represented by the sun and the planetary orbits, it is extremely probable that the great majority of visitors from interstellar space never come within human ken.
Let us suppose that, for every comet seen, nine others make their perihelion passage unobserved. Of 200 comets, the elements of whose orbits have been ascertained, twenty per cent. belong to the solar system, and eighty per cent. were visitors for the first and last time. If, therefore, astronomers record on an average one comet per annum, and nine others pass unrecorded, and if of these ten, two belong to the solar system, and eight are strangers, how great must be the wealth of cometic matter in the universe! For as these strangers are supposed to be some 5,000,000 years in the sun's dominions before making their perihelion sweep, it is evident that at any moment the sun must have, under his control, a supply of foreign comets for 5,000,000 years, at the rate of eight per annum! That is 40,000,000; besides vast multitudes of comparatively domestic comets, to whom he saith “Go,” and they go; “Come,” and they come.
If, then, each sun of the midnight sky, and of the astronomer's “optic tube,” can boast such a following of comets, it shall come very near to be thought that the objects so long beheld with terror, on account of their rarity, are, indeed, the most numerous family of bodies in the universe.
Comets are among the very few celestial objects that are waiting to be explained, and although the question of their constitution, like the secret of the Pole, is of no practical importance, it is yet of absorbing interest. The solution, however, of many of the questions that may perplex us to-night may already be in the hands of those fortunate scientific men whose position has enabled them to analyze our present brilliant visitor with first-class spectroscopes.
For ages comets were regarded as vapours, and exhalations, more or less pestilential, floating in the atmosphere. Tycho Brahé was the first to rise to the conception that comets were beyond the moon.
Kepler's theory was wonderfully acute, considering the information at his disposal. He considered comets to be wholly or principally gaseous, and that the tail consisted of gaseous material, highly rarefied by the sun's heat, and then carried away by the repulsive force of the sun's rays. Others supposed the tail to be a column of vapour lighter than the medium in which the comet moved, and therefore raised, as smoke is raised in the column of heated air from a chimney. Newton supposed comets to have
considerable solid nuclei, calculated the heat to which the body of his comet (1680) was raised, and how long it would take to cool, on the supposition of its being as large as the moon. Another astronomer has calculated the effect upon the earth's orbit of a comet, with three times the mass of the earth, passing within 40,000 miles of her. Maupertuis sought to relieve the popular dread of collision by the suggestion that it might only destroy a part of the terrestrial surface, and that those who survived the shock might find the debris of the comet to consist largely of gold, diamonds, and the like. Boyle, however, the celebrated French philosopher, published a treatise in 1680 setting forth 239 elaborate reasons why comets could neither do or presage evil to the earth.
The popular idea of a comet, is a star with a tail! But a tail is only a temporary appendage to those that have it, while multitudes exhibit no tail. A constant characteristic of comets must be sought in the path pursued, rather than in any appearance presented to the eye. Some comets have been seen that could at times only be distinguished from stars by their course. Others present a star-like nucleus, surrounded by a coma, or a vast nebulous atmosphere. Donati's comet (1858) appeared to have a nucleus, or pellet of light, 1,600 miles in diameter. This was surrounded by two envelopes, one 7,000 and the other over 12,000 miles, high. The whole diameter of the head was 26,400 miles. Other comets, again, present only a nebulous mass, somewhat condensed at the centre, owing, probably, to the greater depth of matter.
Whether the nucleus is solid opaque matter or not, is, perhaps, an open question. Some observers, in the last century, supposed that they saw phases in certain nuclei, similar to those of the moon. But this was, perhaps, the result of earnest expectation and of devotion to a theory. More recent observers have not obtained similar results, and the spectroscope, as applied to several faint comets, seems to show that the nucleus does not shine by reflected sunlight, but has some apparently native luminosity. The tail, however, and the outer envelope of the head shine by reflected light. Some observers declare that they have seen stars through the nucleus, while others say they have seen stars occulted by the nucleus. It must be observed here that a bright nucleus would obliterate a small star by its very lustre, whether transparent or not. Through comets with purely nebulous heads, stars have certainly been seen. Sir J. Herschell declares that he has seen stars of the sixteenth or seventeenth magnitude, that a breath would obscure, through fully 50,000 miles of cometic matter. Compared with this extreme tenuity, this almost spiritual subtilty, the highest and most feathery cirrhus of our atmosphere may be regarded as dense and solid.
It is also worthy of note, that stars seen through comets have neither been displaced nor distorted, showing that the cometic matter has no refracting power.
Another argument against the solidity of comets is, that two at least have been placed in the scales, and found to be practically imponderable—weighed in the balance, in fact, and found wanting. In 1770, Lexall's comet plunged through the system of Jupiter, passing between the planet and one of its moons. The comet was whirled into a new orbit, but the circular motion of Jupiter's moons was not in the least degree interfered with. As if to make assurance upon this point doubly sure by an experiment with the smallest planet, as well as with the largest, Encke's comet passed very near to Mercury some years ago, and was deflected from its course, affording a new and accurate measure of the mass of the planet. But Mercury himself was not in the least degree perturbed. This proves that the mass of a comet is quite insignificant.
It may also be noted that in 1846 Biela's comet divided into two, a greater and a lesser. These gradually parted from each other during thirteen years, in which period they made the circuit of their orbit twice. No perturbations were observed, to indicate that they sensibly attracted each other, and to give a clue to respective masses. With regard, then, to these three comets at least, the more general dictum of a philosopher of no low degree seems to be probable, “that the solid matter of a comet might be put into a gentleman's snuff-box.”
If there be any resisting medium in the inter-planetary spaces, it is evident that vast, and almost imponderable bodies, like comets, would be the first to show in their motions the effects of it, just as a light object thrown through the air will show the resistance of the air far more clearly than a dense and heavy object. Encke's comet is one of the several short-period comets now known. It was discovered in 1789, and has now made twenty-seven revolutions since its discovery. Each one has shown itself to occupy on the average about two hours and a half less than its predecessor, so that the comet now comes to its perihelion some five or six weeks earlier than it would have done according to the period it observed at the time of its discovery. The fact that a number of eminent astronomers have attributed this effect to a resisting medium in space, is proof that no other sufficient cause is known to science. I am not aware that any other cause, with any degree of probability in it, has been assigned by any authority.
Should it be proved that the comet is really describing a spiral course, the vortex of which is the sun, it will simply suggest to the mind how, in unimaginable years, the solid planet must sink into that vortex too. At the same time be it noted that, if a comet should ultimately prove to us
that there is a resisting medium in all space, comets do now prove far more clearly, by the astounding velocity that their attenuated substances attain, how nearly absolutely empty space must be to admit of their motion.
However great the tenuity of the substance of comets, there is a point in each one which rigidly obeys the law of gravitation. This point is the nucleus, or centre of the head. A considerable number of short-period comets have the time of their perihelion passage fixed almost as accurately as the time of an eclipse. Halley's comet, observed in 1682, was predicted to return in 77 years. Computists calculated the retarding influences of the known planets, and allowed 30 days for possible error in the time fixed by them for the perihelion sweep. Neither Uranus nor Neptune were then known, yet the comet was in perihelio in 1756, within 29 days of the time fixed. For its return in 1835—the planet Neptune being still unknown—the perihelion passage was fixed by Rosenberger between the 11th and the 16th of November. It took place on the 15th.
It is evident, therefore, that however small the mass of the huge volume of a comet, it yields the same obedience to law as the densest planet.
But a question naturally rises in our minds as to how the great body of a comet is held together, when its own power of gravity is known to be so small. If the difference of the distances of the centre and the surface of the minute and dense earth from the sun suffices to raise a considerable tide, we might naturally expect to find a comet, with its tremendous diameter, its small power of cohesion, and its proximity to the sun, rent into several sections, to be thrown into somewhat different orbits. The fact remains, however, that no such disruption takes place in the majority of cases; though there are several records of comets parting into two or more fragments.
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But a curious phenomenon is observed upon the approach of a comet to the sun; it is that the nucleus appears to shrink in a wonderful degree. M. Struve, in observing Encke's comet in 1828, found that on December 24th it only occupied 1/16000 part of the space it had occupied on the 28th of October. When it begins to recede from the sun, the comet begins also to recover its former volume. In this situation Halley's comet was observed by Sir J. Herschel, in 1835, to increase forty-fold in apparent size in a single week.
Several explanations of this have been given. A recent one is that of M. Valz, that the shrinking is due to the pressure of the sun's atmosphere. This theory, however, assumes an enormously extended atmosphere for the sun which can scarcely be granted; it also seems to require the comet to be enclosed in an envelope, to prevent it from mingling, like vapour from an engine, with the supposed atmosphere; it also supposes the comet to move
in a medium more dense than itself, which is utterly out of the question. The most probable theory is that which ascribes the change in apparent bulk to the powers of the sun's rays to rarefy, distend, and render invisible a large portion of the comet, which portion, upon reaching a cooler region, begins again to condense like an evening cloud.
Indeed, the extremes of heat and cold to which comets are subjected, render it easy to conceive of almost any change in their appearance. Even in the case of Halley's comet, the heat and light of perihelion are to the heat and light of aphelion as 3,000 to 1. But that comet does not pass very near the sun (54,000,000 of miles), nor recede much beyond the orbit of Neptune. But the comet of 1843, like that of 1882, passed within 30,000 miles of the sun's surface. Newton supposed his comet was subjected to a heat 2,000 times greater than that of red-hot iron. But the heat endured by the body mentioned must have exceeded this by about twenty-fold (for Newton's was 130,000 miles from the solar surface). This is a heat at which, of course, any terrestrial substance would be volatilized. On the other hand this same body will wander to regions where, under similar conditions, it would only receive one four hundredth part of the light and heat enjoyed by the earth.
May it not be that such extremes of heat and cold, together with the almost total absence of pressure, produce conditions of matter unknown and unknowable to us? And may not these unknown and unknowable conditions lie at the bottom of some of the problems that are so perplexing to the human mind?
Yet, something is certainly known of the constitutional elements of a few comets. The spectroscope has shown the nucleus, or central part of the head of one, to be luminous gas; while the outer part of the coma shone by reflected light. Another comet was found by Dr. Huggins to consist of volatilized (not burning) carbon,—the lines in the cometic spectrum agreeing exactly with the lines due to carbon in the spectrum of olefiant gas. This discovery, however, can only be regarded as adding another to the many problems connected with comets; for carbon is notable for its fixity at moderate temperatures, and the comet in question was in a temperate region of space. Comets examined since 1868 show hydrogen and other elements associated with carbon.
A peculiar relation is known to exist between comets and meteor systems, but it is still involved in obscurity. No less an authority than J. C. Adams, the English discoverer of Neptune, has computed the orbit of the November meteors and shown that they pass beyond the planet Uranus, and have a period of 33¼ years. He assigned to them exactly the same path as that already assigned to Temple's comet! It is probable that the
whole of the vast orbit is peopled by flights of meteorites in endless chase, and that the comet, as the gem of the ring, moves round preceded and followed by a retinue of cosmic chips. That all comets are associated with meteors, though fairly probable, cannot be confidently asserted.
I must now come to the most difficult, but also the most fascinating, part of the subject—the so-called tail of the comet. When this appendage is shown at all, it is only while comparatively near the sun. A comet may develope a tail in approaching the sun and show none after perihelion; or it may only develope tail after perihelion; or it may show one both before and after; or neither before nor after.
The nucleus of a comet is separated from the coma by a dark ring, as the sunlit surface of a cloud might be separated from the earth by a band of invisible air. The nucleus is similarly parted from the tail, which appears to be an extension of the coma in the direction opposite to the sun. The nose of the comet is like the flame of a torch blown back by the wind, or like the apex of an upright jet of water from a fountain, where the liquid turns to fall back. The theory now very generally accepted by astronomers is that for some kinds of matter, or for matter in certain conditions, the sun has a repulsive force far more potent than his power of gravity; that, under the influence of intense heat, jets of volatilized matter are thrown out, perhaps in all directions, but certainly towards the sun; that presently the repulsive force overcomes their forward motion, turns them back, and sweeps them away into space until the particles are so widely dispersed as to be invisible.
It was at one time hoped that Mr. Crookes' radiometer was about to show us the repulsive force of the sun's rays at work in our very hands; but the dream vanished, and the repulsive force is still theory, although Sir J. Herschel declares it is proved beyond question by his own observations and those of others. This theory also explains some other phenomena, as the curvature of the tail, and the fact that the convex side of the tail is the brightest and least curved. The convexity of the tail is always towards the direction of the comet's motion. This has led to the gross idea that, like the smoke of a steamer, the tail was retarded by the medium in which the comet moved.
If we conceive of a comet being a rigid body, and that the tail is swung round as a walkingstick might be brandished by the handle, it will be evident that the end of the tail will have much further to travel than the head. But when matter is repelled from the nucleus, and from the sun, it has exactly the same forward momentum as the nucleus; as, therefore, it is driven further and further from the sun, and has a larger and larger orbit to describe, it of necessity falls behind, and cannot therefore be swung
round like a stick, but only like a jet of water from a hydrant. The curvature would, therefore, afford data for finding the velocity with which the repelled matter was driven off.
Again, the convex, or front side of the tail is brightest and straightest. In Donati's comet, 1858, small straight tails preceded the main tail.
One explanation serves for all these facts. The sun analyzes the matter of the comet. Some parts of it he can dart away at an incomparably higher velocity than others. This matter, most swiftly ejected, either makes separate straight tails, as in 1858, or somewhat straightens and brightens the convex, or front, side of the main tail.
There is much room for speculation and enquiry in connection with this repulsive force. Does it pursue the repelled matter, and drive it away with an ever-increasing motion, so that it will leave our system altogether, or does it give an initial impulse, and have done? Will the repelled matter change its condition by cooling, and cease to be liable to the persecution of the repelling force? Or has it lost its affinities and the power of changing its condition? Whatever may be its destiny it is certainly divorced from the comet for ever.
There is reason to suppose that all the matter of a comet is not susceptible to this repulsive force, and that a sufficient number of perihelion passages will sift all the susceptible matter out, and leave the comet incapable of producing a tail. Hence almost all the short-period comets that are in perihelio every few years are tailless; while visitors from the eternities of space, who can only be in that sifting position once in ten or twenty millions of years, frequently make a prodigious display of tail.