Hector Servadac (Frewer translation)/Part 2 Chapter III
CHAPTER III.
COMETS, OLD AND NEW.[edit]
As if moved by some unconscious presentiment of his future destiny, Professor Palmyrin Rosette had always evidenced a strong predilection for the study of comets. He had based his opinions on the best authorities, and was never more in his element than when he was expatiating on his favourite theme as he presided at some astronomical conference.
“Comets, gentlemen,” he would say, “are nebulous bodies which occasionally appear in the heavens, consisting ordinarily of a bright central light called the nucleus and in the more conspicuous cases accompanied by a long trail of light called the tail. Owing to the great eccentricity of their orbits, they are visible to the earth during only a portion of their course.”
The professor never failed to point out the two characteristics by which they were to be distinguished from other heavenly bodies:
“Although these comets, gentlemen, may be deficient either with respect to the luminous tail or to the nebulous coma, the progressive motion with which they are endued prevents them from ever being mistaken for fixed stars, while the extreme length of the ellipses which they describe makes it impossible to confound them with planets,”
During the long years of the astronomer's application to his fascinating study, he had composed an elaborate treatise, exhibiting the results of all his investigations, and when, after the sudden convulsion, he found himself actually upon the surface of one of the very bodies the properties of which had engrossed so much of his interest, it was necessarily a disappointment to feel that, alone upon Formentera, he had no audience to whom he could address himself.
The treatise which Rosette had compiled had been arranged under four distinct heads:
1. The number of comets.
2. Periodic and non-periodic comets.
3. The probability of collision between a comet and the earth.
4. The consequences of such a collision.
First: with respect to the number of comets, the professor had recorded that, according to Arago, who grounded his estimate on the number that revolve between Mercury and the sun, there are at least 17,000,000 of these luminous bodies in our solar system; whilst Lambert asserts that within the orbit of Saturn, that is, within a radius of 872,135,000 miles, there are no less then 500,000,000. According to Kepler, two hundred years previously, the number of comets can only be compared to the fishes in the sea, and in following out his simile he declares that an angler throwing out his line from the surface of the sun could not fail to touch several of them; and now in recent times a computation has been made that their aggregate reaches a total of 74,000,000,000 distinct comets. The truth seems to be that their number really sets all calculation at defiance; so erratic, moreover, are their movements, that they sometimes pass from system to system, and whilst some, entirely escaping the influence of the sun, vanish, to find a new centre of attraction, others never before observed make their appearance upon the terrestrial horizon.
Even the comets which belong exclusively to our own system are by no means exempt from strange irregularities; the orbits of several, ceasing to be ellipses, have become parabolas or hyperbolas; and the planets, Jupiter in particular, have been observed to exercise a large disturbing action upon their paths.
Secondly: under the head of periodic and non-periodic comets, Professor Rosette had stated that as many as 500 or 600 comets have been made objects of careful astronomical investigation; those being called “periodic” of which the return at fixed intervals has been established as a certainty; those, on the other hand, being classed as “non-periodic” which recede to such immeasurable distances from the sun that it cannot be determined whether they will return or not.
Of the periodic comets there are not more than forty of which the times of their revolution have been ascertained with exact precision; but of these there are ten, generally known as the “short-period comets,” the movements of which have been established with the nicest accuracy.
The short-period comets are respectively called by the names of their discoverers, and are commonly distinguished as Halley's comet, Enckes, Gambart's or Biela's, Faye's, Brörsen's, D'Arrest's, Tattle's, Winnecke's, De Vico's, and Tempel's.
Subjoined is a brief account of each of these in detail.
Halley's comet is that which has been the longest known. It is supposed to be identical with the one which was observed in the years 134 and 52 B.C., and afterwards in the years 400, 855, 930, 1006, 1230, 1305, 1380, 1456, 1531, 1607, 1682, 1759, and 1835 A.D. It revolves from east to west, in a direction contrary to the planets. The intervals between its consecutive appearances vary from 75 to 76 years, according as its course is less or more disturbed by the attraction of Jupiter and Saturn, which sometimes influence its course to such an extent as to make a difference of 200 days in the period of its arrival. The last appearance of this comet was in 1835, when Sir John Herschel, at the Cape of Good Hope, a more favourable station for observation than any in the northern hemisphere, was able to watch it until the end of March 1836, after which its distance from the earth rendered it invisible. At its aphelion it is 3,200,000,000 miles from the sun, that is to say, it is beyond the orbit of Neptune, but at its perihelion it is less than 57,000,000 miles from the sun, and consequently is nearer than the planet Venus.
Little did the professor dream, at the time when he drew up his treatise, that his own Gallia would transport him to a still closer proximity to the great luminary.
Encke's comet has the shortest period of any, its revolution being accomplished in about 1205 days, or less than three years and a half. Unlike Halley's, it moves as the planets, from west to east. It was observed on the 26th of November, 1818, and a calculation of its elements proved it to be identical with the comet of 1805. According to prediction, it was seen again in 1822, and since that time has never failed in making its appearance at regular intervals. Its orbit lies within that of Jupiter, and it never recedes more than 387,000,000 miles from the sun, its perihelion distance being only 32,000,000 miles, or less than that of Mercury.
One important observation that has been made with regard to Encke's comet, places it beyond doubt that the axis major of its elliptical orbit is gradually diminishing, and consequently its average distance from the sun is growing continuously less and less, so that the probability arises that unless it is previously volatilized by the solar heat, it may be ultimately absorbed in the sun itself.
Gambart's comet (otherwise known as Biela's) was noticed in 1772, 1789, 1795, and 1805; but it was not until the 28th of February, 1826, that its elements were satisfactorily determined. Its motion is direct, and its period of revolution 2410 days, or about seven years. At perihelion it passes 82,000,000 miles from the sun, rather nearer than the earth; at aphelion it is beyond the orbit of Jupiter.
A singular phenomenon with regard to Biela's comet was first observed in the year 1846: it appeared like a double star, in two distinct fragments, doubtless sundered by the action of some internal force; these fragments travelled together at an interval of about 160,000 miles apart, but at the next appearance in 1852 this interval was found to be largely increased.
Faye's comet was discovered by him for the first time on the 22nd of November, 1843. The elements of its orbit were calculated, and it was predicted that it would return again in 1851, after a period of 2718 days, or in about seven years and a half. The prediction was realized; the comet was visible at the time announced, and has subsequently appeared at similar intervals. Its motion is direct At perihelion it is 192,000,000 miles from the sun, never approaching so near as Mars; at aphelion it is distant 603,000,000 miles, so that it recedes, like Biela's comet, beyond the pathway of Jupiter.
Brörsen's comet was discovered on the 26th of February, 1846. Its movement is from west to east; it accomplishes its revolution in about 2042 days; its perihelion distance is 64,000,000 miles, its aphelion 537,000,000 miles.
Of the other short-period comets, D'Arrest's, which in 1862 passed within 30,000,000 miles of the planet Jupiter, completes its revolution in rather more than six years and a half; Tuttle's revolves in thirteen years and eight months; Winnecke's and Temple's in about five years and a half; whilst that of De Vico, after being computed to revolve in a period of rather more than five years, seems to have wandered away altogether into space.
Then follows a short enumeration of some of the “long-period” comets.
The comet of 1556, commonly called the comet of Charles-Quint, was expected again in 1860, but did not re-appear.
The comet of 1680 furnished the data for Newton's cometary theories, and, according to Whiston, was the cause of the deluge, on account of its close approximation to the earth. Its revolution takes about 575 years, so that it was visible in 1106 and 531, as well as in 43 B.C. and probably in 619 B.C. At its perihelion it passes so near the sun that it receives 28,000 times more heat than the earth, that is, it is 2000 times hotter than molten iron.
The comet of 1744 was by far the most brilliant of the eighteenth century; it was seen on the 1st of March in full daylight, and had six tails, spread out like a fan across a large space in the heavens.
The great comet of 1811, which has caused the year of its appearance to be familiarly recognised as “the comet-year,” had a nucleus 2637 miles in diameter; its head was 1,270,000 miles in diameter, and its tail 100,000,000 miles in length.
The comet of 1843, observed by Cassini, has been supposed to be identical with that of 1668, 1494, and 1317, but astronomers are not agreed upon the period of its revolution. At its perihelion it passes nearer to the sun than any other comet recorded in history, travelling at a rate of more than 40,000 miles a second. The heat that it thus receives is equal to that which 47,000 suns would communicate to the earth, and to such a degree does this prodigious temperature increase its density, that at its last appearance its tail was visible in broad daylight.
Donati's comet, which in 1858 shone with such brilliancy amongst the northern constellations, has a mass that has been estimated at .07 of that of the earth.
The comet of 1862 was adorned with luminous tufts or aigrettes, and resembled some fantastic mollusk.
The list is completed by the comet of 1868, the revolution of which occupies a period of no less than 2800 centuries, so that it may practically be considered as having vanished in infinite space.
Thirdly: the next section of the professor's dissertation was devoted to the probability of a collision between any one of these numerous comets and the earth.
As represented in plane diagrams, the orbits of planetary and cometary bodies appear continually to be intersecting one another; but in free space of three dimensions this is by no means necessarily the case; the planes of the orbits being inclined at various angles to the ecliptic, which is the plane of the terrestrial orbit. Nevertheless, out of the large number of comets, is it impossible that one of them should come in contact with the earth?
In conducting this investigation, it had to be recollected that as the earth never leaves the plane of the ecliptic, three conditions must be fulfilled in order to bring about the result of impact: first, the comet must meet the earth in the ecliptic; secondly, the earth and the comet must arrive at the point of intersection of their orbits at the same moment; and thirdly, the distance between the centres of the bodies themselves must be less than the sum of their radii. The problem, therefore, resolved itself into an inquiry whether these three conditions could occur simultaneously.
Laplace did not reject the possibility of such an encounter, and in his “Exposition du Système du Monde” has at some length detailed the consequences. Arago, when asked his opinion on the subject, replied that by calculation there were 280,000,000 chances to 1 against a collision. The illustrious astronomer, however, based his estimate upon two conditions that are only fulfilled with the greatest uncertainty; in the first place, that at perihelion the comet should be nearer the sun than the earth is; and in the next, that the diameter of the comet should be equal to one-fourth of that of the earth. On the other hand, he only reckoned for the earth coming in contact with the actual nucleus, whilst if the whole extent of the nebulosity were to be taken into account, the chances of collision would be increased tenfold.
In enunciating his problem, Arago adds:
“If we take it for granted that the result of a comet running foul of the earth would be the total annihilation of the human race, then the risk of death which each individual incurs from the probability of such a catastrophe is just what would be his chance of drawing, at the first draw, the only white ball out of an urn containing 280,000,000 coloured ones.” So remote appear the chances of collision.
All astronomers, moreover, concur in distinctly denying that any such collision has ever happened Arago asserts that if it had happened, the consequences would have been an immediate alteration in the earth s axis of rotation, and a general disturbance of terrestrial latitudes; but he alleges no evidence in proof of his assertion. He speaks, however, much more to the purpose when he declares that “the theory held by some, that the depression of the Caspian Sea 300 feet below the level of the ocean is to be attributed to the shock of a comet, is utterly untenable.”
But the matter under consideration was not whether collision had ever occurred, but whether it ever could occur.
Now in 1832, at the re-appearance of Gambart's comet, the world was thrown Into some alarm because it was announced as the result of astronomical calculations, that at the time of the passage of the comet through its descending node on the 29th of October, the earth would be travelling precisely in the same region. Contact seemed not only probable but inevitable, if Olbers' observation was correct, that the radius of the comet was five times as large as that of the earth. Happily, however, the earth did not arrive at that point of the ecliptic until the 30th of November, by which time the comet was more than 50,000,000 miles away. But supposing that the earth had reached that place of intersection of the two orbits a month sooner, or the comet a month later, it is hard to say what could have obviated the likelihood of collision. At the very least, some singular perturbations must have ensued. In 1805 Indeed, this identical comet had passed within 6,000,000 miles of the earth, ten times closer than in 1832, but as its proximity was unknown, the fact did not excite any panic.
Again in 1843 there seemed reasonable ground for fear that the atmosphere of the earth would be vitiated by passing through the nebulous tail of a comet 150,000,000 miles in length.
Altogether, therefore, from the entire evidence, it appeared a necessary inference that collision between the earth and a comet was by no means impossible.
Fourthly, then, Professor Rosette had to discuss the remaining question to bring his treatise to a close; as to the probable consequences of such a collision.
These consequences would manifestly vary according as the comet had or had not a nucleus. As some fruits have no kernel, so some comets have no nucleus, and such is the tenuity of their substance, that stars of the tenth magnitude have been seen through them without any sensible diminution of light. It is a property that must make their external form very susceptible of change, and tends in a degree to make them difficult of recognition. The same transparency characterises the tail, the development of which is apparently due entirely to the evaporation of the coma under the action of solar heat; in proof of which it is notified that no tail, either single or multiple, has ever been found attached to a comet until that comet has arrived within 80,000,000 miles of the sun; whilst it has been observed that some comets, presumably composed of denser structure, have emitted no tail at all.
In the case of the earth coming into contact with a comet destitute of a nucleus, there would be no violent collision; strictly speaking, there would be no shock at all. The astronomer Faye asserts that a cannon ball would find more resistance in a cobweb than in the nebulous parts of a comet; and for the nebulous matter to be injurious, it must either be incandescent, in which case it would scorch up the surface of the earth, or it must be impregnated with noxious elements, in which case it might be fatally destructive to life. This latter contingency, however, is unlikely to arise; for, according to Babinet, the earth's atmosphere possesses sufficient density of its own to resist the penetration of any cometary vapours, of which the tenuity is so slight, that Newton has calculated that if a comet, without a nucleus, 1,000,000,000 miles in radius, were reduced to the density of the air at the earth's surface, it might all be contained in a thimble less than an inch in diameter.
Concluding thus that from comets purely nebulous there was a minimum of danger to be apprehended, the professor proceeded to inquire what would be the result of concussion if the comet consisted of a solid nucleus.
First of all, however, rises the preliminary question whether in any case the nucleus of a comet is really solid, There can be no doubt that if a comet can attain a degree of concentration sufficient to pass out of its gaseous condition, it will, if interposed between the earth and a star, make an occupation of that star. No sound reliance is to be placed on testimony such as that of Anaxagoras, who, living in the time of Xerxes, about the year 480 B.C., recorded that the sun was eclipsed by a comet; nor on that of Dion, who maintains that a similar eclipse occurred a few days before the death of Augustus, which could not be occasioned by the moon, then in direct opposition. Modern science has, with more than sufficient justice, entirely repudiated the accuracy of these statements; but the indisputable testimony of recent observation all goes to establish the certainty of the existence of comets with a solid nucleus. The comets of 1774 and of 1828 are known to have caused the occupation of stars of the eighth magnitude; it is admitted on all hands that the comets of 1402, 1532, and 1744 were solid masses; whilst, as for the comet of 1843, the fact is patent to the world that the body could be seen close to the sun, in broad daylight, by the naked eye.
Not only, therefore, do they exist, but in some cases these solid nuclei have been actually measured. Their diameters vary considerably in length; that of Gambart's comet being only 30 or 40 miles, that of the comet of 1845 being 8800 miles, considerably longer than the diameter of the earth, so that in the event of a collision between the two bodies, the preponderance would have been on the side of the comet. The nebulous surroundings have also, in a variety of instances, been measured, and found to vary from 200,000 to 1,000,000 miles in diameter.
Upon the whole, modern investigation bears out the general statement of M. Arago that there are three kind? of comets; that is to say, comets without any nucleus; comets with a transparent nucleus; and comets with a nucleus both solid and opaque.
It had to be borne in mind that without any actual shock by collision, the mere proximity of a comet to the earth might entail some very singular phenomena. Not that from a comet of inferior mass any serious consequences could be expected, for the comet of 1770, which approached within 1,600,000 miles of the earth, did not affect the length of the terrestrial year a single second, although the action of the earth retarded the period of the comet's revolution by three whole days. But if the mass of the two bodies were equal, and if the comet passed within 150,000 miles of the earth, the result would be that the terrestrial year would be prolonged by sixteen hours and five minutes, and the obliquity of the ecliptic altered by two degrees, to say nothing of the chance that the comet might capture the moon in its passage.
What, finally, would happen in the event of the one body actually impinging on the other? The consequences, manifestly, would be far more considerable. Either the comet, in grazing the earth's surface, would leave behind it a fragment detached from itself, or it would carry off with itself a fragment detached from the earth. If, instead of being oblique, the impact should be direct, there would at least be a rupture of continents, even if the globe were not shivered into pieces.
In any case, the tangential velocity of the earth must receive a sudden check or a sudden impulse; trees, houses, living creatures, would be precipitated backwards or forwards with increased momentum; the seas, dashed from their natural basins, would overwhelm all that lay in the path of their projection; the central forces of the globe, still in their normal state of fusion, would be propelled to the surface; the terrestrial axis would undergo a change in its direction, so that a new equator would be established, and as the conditions of equilibrium would be disturbed, there might be nothing properly to counter-balance the attraction of the sun, the consequence of which, by the law of gravity, would be that the earth, drawn perpetually on in a straight line, in the space of sixty-four days and a half would be absorbed into the elements of the great central luminary of the system.
One speculation there was which to the last remained doubtful; whether, according to Tyndall's theory that heat is only a form of motion, the velocity of the earth would not, under the sudden elevation of the temperature, mechanically transform itself into heat so intense, that through its action, the earth itself, in the course of a few seconds, would be completely volatilized.
Such were the deductions of Palmyrin Rosette's treatise, which he brought to a conclusion by a repetition of the philosopher's comforting assurance, that the chances were as 280,000,000 to 1 against the occurrence of any collision.
How little could the professor, as he tabulated his scientific notes, anticipate his experiences in the future, with regard to his own Gallia!
How little could he foresee, that at some future séance, he would be in the position to say:
“You see, gentlemen, that we have drawn the one white ball from the urn!”