Curiosities of the Sky

by Garrett Putman Serviss

Then the facts were made plain that comets are subject to the law of gravitation equally with the planets; that there are many which regularly return to the neighborhood of the sun (perihelion); and that these travel in orbits differing from those of the planets only in their greater eccentricity, although they have the peculiarity that they do not, like the planets, all go round the sun in the same direction, and do not keep within the general plane of the planetary system, but traverse it sometimes from above and sometimes from below. Other comets, including most of the “great” ones, appear to travel in parabolic or, in a few cases, hyperbolic orbits, which, not being closed curves, never bring them back again. But it is not certain that these orbits may not be extremely eccentric ellipses, and that after the lapse of hundreds, or thousands, of years the comets that follow them may not reappear. The question is an interesting one, because if all orbits are really ellipses, then all comets must be permanent members of the solar system, while in the contrary case many of them are simply visitors, seen once and never to be seen again. The hypothesis that comets are originally interlopers might seem to derive some support from the fact that the certainly periodic ones are associated, in groups, with the great outer planets, whose attraction appears to have served as a trap for them by turning them into elliptical orbits and thus making them prisoners in the solar system. Jupiter, owing to his great mass and his commanding situation in the system, is the chief “comet-catcher;” but he catches them not for himself, but for the sun. Yet if comets do come originally from without the borders of the planetary system, it does not, by any means, follow that they were wanderers at large in space before they yielded to the overmastering attraction of the sun. Investigation of the known cometary orbits, combined with theoretical considerations, has led some astronomers to the conclusion that as the sun travels onward through space he “picks up en route” cometary masses which, without belonging strictly to his empire, are borne along in the same vast “cosmical current” that carries the solar system.

But while no intelligent person any longer thinks that the appearance of a great comet is a token from the heavenly powers of the approaching death of a mighty ruler, or the outbreak of a devastating war, or the infliction of a terrible plague upon wicked mankind, science itself has discovered mysteries about comets which are not less fascinating because they are more intellectual than the irrational fancies that they have displaced. To bring the subject properly before the mind, let us see what the principal phenomena connected with a comet are.

At the present day comets are ordinarily “picked up” with the telescope or the photographic plate before any one except their discoverer is aware of their existence, and usually they remain so insignificant in appearance that only astronomers ever see them. Yet so great is the prestige of the word “comet” that the discovery of one of these inconspicuous wanderers, and its subsequent movements, become items of the day’s news which everybody reads with the feeling, perhaps, that at least he knows what is going on in the universe even if he doesn’t understand it. But a truly great comet presents quite a different proposition. It, too, is apt to be detected coming out of the depths of space before the world at large can get a glimpse of it, but as it approaches the sun its aspect undergoes a marvelous change. Agitated apparently by solar influence, it throws out a long streaming tail of nebulous light, directed away from the sun and looking as if blown out like a pennon by a powerful wind. Whatever may be the position of the comet with regard to the sun, as it circles round him it continually keeps its tail on the off side. This, as we shall soon see, is a fact of capital importance in relation to the probable nature of comets’ tails. Almost at the same time that the formation of the tail is observed a remarkable change takes place in the comet’s head, which, by the way, is invariably and not merely occasionally its most important part. On approaching the sun the head usually contracts. Coincidently with this contraction a nucleus generally makes its appearance. This is a bright, star-like point in the head, and it probably represents the totality of solid matter that the comet possesses. But it is regarded as extremely unlikely that even the nucleus consists of a uniformly solid mass. If it were such, comets would be far more formidable visitors when they pass near the planets than they have been found to be. The diameter of the nucleus may vary from a few hundred up to several thousand miles; the heads, on the average, are from twenty-five thousand to one hundred thousand miles in diameter, although a few have greatly exceeded these dimensions; that of the comet of 1811, one of the most stupendous ever seen, was a million and a quarter miles in diameter! As to the tails, not withstanding their enormous length—some have been more than a hundred million miles long—there is reason to believe that they are of extreme tenuity, “as rare as vacuum.” The smallest stars have been seen shining through their most brilliant portions with undiminished luster.

After the nucleus has been formed it begins to throw out bright jets directed toward the sun. A stream, and sometimes several streams, of light also project sunward from the nucleus, occasionally appearing like a stunted tail directed oppositely to the real tail. Symmetrical envelopes which, seen in section, appear as half circles or parabolas, rise sunward from the nucleus, forming a concentric series. The ends of these stream backward into the tail, to which they seem to supply material. Ordinarily the formation of these ejections and envelopes is attended by intense agitation of the nucleus, which twists and turns, swinging and gyrating with an appearance of the greatest violence. Sometimes the nucleus is seen to break up into several parts. The entire heads of some comets have been split asunder in passing close around the sun; The comet of 1882 retreated into space after its perihelion passage with five heads instead of the one that it had originally, and each of these heads had its own tail!

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. The presence of iron would seem to indicate that some of these bodies may be much more massive than observations on their attractive effects have indicated. In some recent comets, such as Morehouse’s, in 1908, several lines have been found, the origin of which is unknown.

Without going back of the nineteenth century we may find records of some of the most extraordinary comets that man has ever looked upon. In 1811, still spoken of as “the year of the comet,” because of the wonderful vintage ascribed to the skyey visitor, a comet shaped like a gigantic sword amazed the whole world, and, as it remained visible for seventeen months, was regarded by superstitious persons as a symbol of the fearful happenings of Napoleon’s Russian campaign. This comet, the extraordinary size of whose head, greatly exceeding that of the sun itself, has already been mentioned, was also remarkable for exhibiting so great a brilliancy without approaching even to the earth’s distance from the sun. But there was once a comet (and only once—in the year 1729) which never got nearer to the sun than four times the distance of the earth and yet appeared as a formidable object in the sky. As Professor Young has remarked, “it must have been an enormous comet to be visible from such a distance.” And we are to remember that there were no great telescopes in the year 1729. That comet affects the imagination like a phantom of space peering into the solar system, displaying its enormous train afar off (which, if it had approached as near as other comets, would probably have become the celestial wonder of all human memory), and then turning away and vanishing in the depths of immensity.

In 1843 a comet appeared which was so brilliant that it could be seen in broad day close beside the sun! This was the first authenticated instance of that kind, but the occurrence was to be repeated, as we shall see in a moment, less than forty years later.

The splendid comet of 1858, usually called Donati’s, is remembered by many persons yet living. It was, perhaps, both as seen by the naked eye and with the telescope, the most beautiful comet of which we have any record. It too marked a rich vintage year, still remembered in the vineyards of France, where there is a popular belief that a great comet ripens the grape and imparts to the wine a flavor not attainable by the mere skill of the cultivator. There are “comet wines,” carefully treasured in certain cellars, and brought forth only when their owner wishes to treat his guests to a sip from paradise.

The year 1861 saw another very remarkable comet, of an aspect strangely vast and diffuse, which is believed to have swept the earth with its immense tail when it passed between us and the sun on the night of June 30th, an event which produced no other known effect than the appearance of an unwonted amount of scattered light in the sky.

The next very notable comet was the “Great Southern Comet” of 1880, which was not seen from the northern hemisphere. It mimicked the aspect of the famous comet of 1843, and to the great surprise of astronomers appeared to be traveling in the same path. This proved to be the rising of the curtain for an astronomical sensation unparalleled in its kind; for two years later another brilliant comet appeared, first in the southern hemisphere, and it too followed the same track. The startling suggestion was now made that this comet was identical with those of 1843 and 1880, its return having been hastened by the resistance experienced in passing twice through the coronal envelope, and there were some who thought that it would now swing swiftly round and then plunge straight into the sun, with consequences that might be disastrous to us on account of the “flash of heat” that would be produced by the impact. Nervous people were frightened, but observation soon proved that the danger was imaginary, for although the comet almost grazed the sun, and must have rushed through two or three million miles of the coronal region, no retardation of its immense velocity was perceptible, and it finally passed away in a damaged condition, as before remarked, and has never since appeared.