The Ephemeral Ghosts of the Cosmos

“ Most comets appear to have bright centres—nuclei, as they are called; but these nuclei are not solid bodies, for as soon as they are viewed by powerful telescopes, they become as diffused and transparent as the fainter cometic substance. Comets are properly atmospheres without contained spheres; enormous clouds rushing along in space, and bathed with its sunshine, for they have no light excepting sunlight. They become brighter and brighter as they get deeper within the solar glare, and dimmer and paler as they float outwards from the same. ”

“ Comets may almost be regarded as the ghosts of heavenly bodies. The heads, in some cases, may consist of separate solid fragments, though on this astronomers are by no means agreed, but the tails at any rate are in fact of almost inconceivable tenuity. We know that a cloud a few hundred feet thick is sufficient to hide, not only the stars, but even the Sun himself. A Comet is thousands of miles in thickness, and yet even extremely minute stars can be seen through it with no appreciable diminution of brightness. ”

“ The mean density of a comet is very small. Young states that it is nothing more than a sandbank—a swarm of solid particles of unknown size and widely separated. This light and airy object darts through space at a most hectic speed, often at the rate of a number of hundred miles per second, when close to the sun. It is of such flimsy construction that its light does not even obscure the stars although there have been comets so bright that they were visible in the daytime. ”

“ There is considerable difference of opinion as to whether the bead of the comet is solid matter or inflammable gas.
"There is nearly always a point of superior brilliancy perceptible in the comet's head, which is termed its nucleus, and it is necessarily a matter of pressing interest to determine what this bright nucleus is; whether it is really a kernel of hard, solid substance, or merely a whiff of somewhat more condensed vapor. Newton, from the first, maintained that the comet is made partly of solid substance, and partly of an investment of thin, elastic vapors. ”

“ The question whether there is any central solid of considerable mass is still undecided; it can only be said that if so, it is probably small relative to cosmic masses in general—more likely less than greater than 100 m. in diameter. The light of the comet therefore proceeds from two sources: one the incandescence of gases, the other the sunlight reflected from the solid parts. No estimate can be formed of the ratio between these two kinds of light until a bright comet shall be spectroscopically observed during an entire apparition. ”

“ The possession of the spectroscope has enabled astronomers during later years to study the chemical composition of comets by analyzing their light. At first the only substances thus discovered in them were hydro-carbon compounds, due evidently to the gaseous envelopes in which some combination of hydrogen with carbon existed. Behind this gaseous spectrum was found a faint continuous spectrum ascribed to the nucleus, which apparently both reflects the sunlight and gives forth the light of a glowing solid or liquid. Subsequently sodium and iron lines were found in cometary spectra. ”

“ After a voyage through space the comet at length begins to draw in towards the central parts of our system, and as it approaches the sun, its pace becomes gradually greater and greater; in fact, as the body sweeps round the sun the speed is sometimes 20,000 times faster than that of an express train. It is sometimes more than 1000 times as fast as the swiftest of rifle bullets, occasionally attaining the rate of 200 miles a second. The closer the comet goes to the sun, the faster it moves ”

“ But not unscathed; like the moth, the Comet has been singed. The fierce light of the Sun has beaten upon it, and spread out its particles and scattered them along its path.”
As a comet swings toward and away from the Sun, it travels at a tremendous rate of speed—over a million miles an hour. The distance covered from one end of the orbit to the other is 3,370,000,000 miles.
The great majority of Comets appear to travel in parabolas, open curves leading from infinite space to and around the Sun, and thence back into infinite space to some other fixed star invisible to us. ”

“ As, however, it is the comet's nature to guard its ornamental appendages with jealous care, it may be conceived that this tail of 170 million miles might prove a somewhat troublesome travelling companion in so rapid a journey. Comets always turn their tails prudentially out of harm's way as they whisk through the neighbourhood of the solar blaze. In whatever [pg 103] direction these bodies may be moving, they are always seen to project their caudal beams directly from the sun. ”

“ The air that we breathe is a dense blanket in comparison. Several hundred cubic miles of the matter composing that [Pg 105] wonderful luminous plume would not outweigh a jarful of air. By reason of its fairy lightness, it is possible for a tail occupying a volume thousands of times greater than the sun to sweep through our solar system without causing any perturbations in planetary movements.
No celestial phenomenon has caused more perplexity than the ghostly sheaf of light we call a Comet’s tail. In a day, in a few hours even, the form of that wonderful gossamer may change. ”

“ Many features of this theory of cometary tails are borne out by examination of their light with the spectroscope, although the investigation is as yet fragmentary. It is evident that the tail of a comet is formed at the expense of the substance of the nucleus and head; so that the matter repelled is forever dissipated through the regions of space which the comet has traveled. Comets must lose much of their original substance every time they return to perihelion. ”

“ Thus the sun at one and the same time is drawing the comet toward itself and driving off from the comet in an opposite direction minute particles or atoms which, instead of obeying the gravitational force, are plainly compelled to disobey it. That this energy, which the sun exercises against its own gravitation, is electrical in its nature, hardly anybody will doubt. The head of the comet being comparatively heavy and massive, falls on toward the sun, despite the electrical repulsion. ”

“ If this process went on indefinitely every comet would, in the course of ages, be entirely dissipated. This result has actually happened in the case of some known comets, the best established example of which is that of Biela, in which the process of disintegration was clearly followed. As the amount of matter lost by a comet at any one return cannot be estimated, and may be very small, it is impossible to set any limit to the period during which its life may continue. ”

“ But, for physical astronomers, the greatest interest attaches to the reduction of radius vector of Encke’s comet, the splitting of Biela’s comet into two comets in 1846, and the somewhat similar behaviour of other comets. It must be noted, however, that comets have a sensible size, that all their parts cannot travel in exactly the same orbit under the sun’s gravitation, and that their mass is not sufficient to retain the parts together very forcibly; also that the inevitable collision of particles, or else fluid friction, is absorbing energy, and so reducing the comet’s velocity. ”

“ Hence the ease with which the comet may be dismembered and its particles strung out into streaming swarms.
So thickly is the space we traverse strewn with this cometary dust that the earth sweeps up, according to Professor Newcomb's estimate, a million tons of it each day. Each individual particle, perhaps no larger than a millet seed, becomes a shooting-star, or meteor, as it burns to vapor in the earth's upper atmosphere. And if one tiny planet sweeps up such masses of this cosmic matter, the amount of it in the entire stretch of our system must be beyond all estimate. ”

“ 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. ”

“ They approach the sun from all directions, and their motions, in relation to the planets, are as often retrograde as direct. Not only are their orbits excessively elliptical, but they are also inclined to the ecliptic at all angles from 0° to 90°. It is evidently impossible to account satisfactorily for the origin of comets if we assume them all to have been evolved out of the solar nebula, although this has been attempted by M. Faye and others. Comets are evidently, as Laplace and Professor A. Winchell both conclude, strangers to our system, and have come from distant regions of space. ”

“ The possibility that some straggling fragments from the ruins of one world might become the satellites of another, depends on a principle which has been long applied in tracing the origin of the cometary members of the solar family. It has been maintained by Laplace that comets were at first strangers in our system, and that many of them, coming from remote interstellar regions and entering the sphere of the sun's attraction, obtained a permanent domicile in his extensive domain; their orbits being changed from hyperboles to ellipses chiefly by some planetary disturbances. ”

“ The certainty of rapid disintegration of the periodic comets—extremely rapid in comparison with astronomical time-intervals—is all but equivalent to saying that the periodic comets have been recently captured by our planets; for the periodic comets which we are still observing could not have been following their present orbits during many centuries, except at the price of disintegration to the point of total disappearance. ”

“ When they flit through our skies, they shew themselves in all possible positions, and move along all possible directions. They sometimes, however, yield too much to temptation, and have to suffer the penalty of a short imprisonment in consequence. Lexell's Comet, for instance, rushed in its hyperbolic path too near to Jupiter, and was caught in the attraction of its mass, and made to dance attendance on the sun through two successive elliptical revolutions. ”

“ That there never is a moment when there is not some comet in the sky. Virtually there is no year in which several new comets are not discovered, so plentiful are they. Luminous fleas on a vast black dog—in popular impressions, there is no realization of the extent to which this solar system is flea-bitten.
If a comet have not the orbit that astronomers have predicted—perturbed. If—like Halley's comet—it be late—even a year late—perturbed. When a train is an hour late, we have small opinion of the predictions of timetables. ”

“ The knowledge we have of comets is very imperfect, as they afford few observations on which to ground conjecture. They are generally supposed to be planetary bodies, forming a part of our system: for, like the planets, they revolve about the sun, but in different directions, and in extremely long elliptic curves, being sometimes near the sun, at others staying far beyond the orbit of the outermost planet; whereas the orbits of the planets are nearly circular. ”

“ It also assailed the dogma of the crystalline spheres, because the motion of a superlunar comet would send it crashing through the spheres. It was hard for superstitious man to give up the "signs of the heavens" that had so long misguided him. As late as the latter part of the seventeenth century a book was published by Father De Angelis, of the Clementine College, Rome, in defense of the old cometary faith. He claimed that comets originate in our atmosphere below the moon. Everything heavenly is eternal. We see the beginning and ending of comets, hence they are not heavenly bodies. ”

“ Sun-spots may come and go; bright planets may shine more brightly; the Sun or Moon may be obscured by eclipses; temporary stars may burst forth,—all these things are within the ken of the general public by means of newspapers or almanacs, but it is a comet which evokes more questionings and conversations than all the other matters just referred to put together. When a new and bright comet appears, or even when any comet not very bright gets talked about, the old question is still fresh and verdant—“Is there any danger to the Earth to be apprehended from collision with a Comet?” ”

“ The passage of comets has never sensibly disturbed the stability of the solar system; their nucleus is rare, and their transit so rapid, that the time has not been long enough to admit of a sufficient accumulation of impetus to produce a perceptible effect. The comet of 1770 passed within 80000 miles of the earth without even affecting our tides, and swept through the midst of Jupiter's satellites without deranging the motions of those little moons. ”

“ How is it that the system has been able to tolerate the vagaries of comets for so many ages? Solely because the comets, though capable of suffering from perturbations, are practically incapable of producing any perturbations on the planets. The efficiency of a body in producing perturbations depends upon the mass of the body. Now all we have hitherto seen with regard to comets tends to show that the masses of comets are extremely small. ”

“ Experience has, however, taught us that collisions between the earth and comets do not lead to any serious consequence. The earth has several times passed through the tails of comets—for instance, in 1819 and 1861—and it was only the calculating astronomer who became aware of the fact. Once on such an occasion we have thought that we observed a glow like that of an aurora in the sky. When the earth was drawing near the denser parts of the comet, particles fell on the earth in the shape of showers of shooting-stars, without doing any appreciable damage. ”

“ It was computed by Olbers that if a comet having a mass of 1⁄2000th of the earth’s mass—which would form a globe of about 520 miles in diameter and of the density of granite—collided with the earth, with a velocity of 40 miles a second, our globe would be shattered into fragments. [200] But that any comet has a solid nucleus of this size seems very doubtful; and we may further say that the collision of the earth with any comet is highly improbable.
It seems to be a common idea that harvests are affected by comets, and even “comet wines” are sometimes spoken of. ”

“ If in some respects the fears inspired by comets have been reduced by modern scientific discoveries respecting these bodies, yet in other respects the very confidence engendered by the exactness of modern astronomical computations has proved a source of terror. There is nothing more remarkable, for instance, in the whole history of cometary superstition, than the panic which spread over France in the year 1773, in consequence of a rumour that the mathematician Lalande had predicted the occurrence of a collision between a comet and the earth, and that disastrous effects would inevitably follow. ”