THE COMETARY SYSTEM.

New Zealand Colonist and Port Nicholson Advertiser, Volume I, Issue 67, 21 March 1843, Page 4

THE COMETARY SYSTEM.

(Continued.) Now the peculiar circumstances out of which the stability of the planetary system arises, are precisely those in which we have described it, as distinguished from the cometary system. They are the great excess of the attraction of the sun upon any of the bodies which compose it, as compared with that of any other body. The uniform direction of the revolutions of the planets in their respective orbits, towards the east, the small eccentricities of their orbits, and their small inclinations to one another. From these provisions in our system, it arises, that it is stable, and if any one of them were wanting, it would be unstable. Now t , in the cometary system, not one of these obtains; it is therefore unstable, and in a state of continual and rapid change, and thence arises the great difficulty of calculating the motions of the comets.

The masses of such comets as have been observed are all exceedingly small, indeed it would seem infinitely small,* as compared with those of the planets of our system; so . that although they exercise no perceptible influence on the motions of the planets, however near they approach them, yet do the planets exercise a very sensible control over theirs. A comet was discovered in 1770, and its orbit was calculated by Lexel, to he described in 5£ years. At the expiration of that period, it was however looked for in vain, and it was called Lexel’s lost comet. Years afterwards, it was shown by Laplace, that this comet, when returning to keep its appointment with Lexel, had passed so near to Jupiter, that the attraction of the planet upon it, had become 200 times as great as the attraction of the sun, and the result was, that the form of its orbit had been so completely altered, that from 5-§- years, it came to be an orbit described in 30 years. The attraction of Jupiter upon this erratic comet, actually brought it between that planet and his satellites, and yet, so small was its mass.f so wonderful its tenuity, that it produced not the slightest alteration in the motions of any of them.

Every comet when it enters our system has its orbit more or less changed by the influence of the planets, and in some cases, that influence is felt throughout the whole of the comet’s course. Thus, the comet of 1835 never, throughout the whole of its course, extending three billions of miles from the sun, escaped the sensible attraction of Jupiter. Thus, then, it appears, that those changes which, in respect to the orbits of the planets, are necessarily, and must ever be gradual, and almost imperceptible, are, as regards the orbits of the comets, not only perceptible, but remarkable, and moreover that, whereas the changes of the planetary orbits must return in certain cyclical periods for ever, those of the cometary orbits will not; so that what the cometary system is at any given time, it can never (that is, it cannot, except by an infinite improbability) be again; but, to what this perpetual series of change tends, or in what it will terminate, no one has, probably, been bold enough to make the subject of his speculations. Enough has been said, to show that the calculation of the motions of a comet is no easy matter. The attractions of five bodies, all of which, except one, are continually moving, upon another, which is itself also perpetually in motion —these attractions, each of them varying, with each change of distance, their effects in accelerating or retarding the attracted body, or in altering the path which it has described —effects to be considered and allowed for, during a period, not of some few weeks or months, but through seventy-six long years ; this is a task, about which are accumulated difficulties of no common order. It is a work of infinite complication, learning, ingenuity, and labour; nevertheless it was undertaken and accomplished in respect of the comet of 1835.

The comet of 1535 was, in its last revolution, influenced appreciably by the attractions of the four planets, Jupiter, Saturn, Uranus, and the Earth, and of course by the attraction of the Sun ; and MM. Damoiseau and Pontecoulant severally and independently undertook the task of calculating their amount, and separately completed it. M. Pontecoulant found that the action of Jupiter would, as compared with the last revolution of the comet, on the whole accelerate it 135.34 days; that of Saturn, retard it 51.53 days; that of Uranus, retard it 6.07 days; and that of the Earth 11.7 days. The principal portion of the influence of the Earth on its motions, dating as far back as the year 1759, or the very beginning of its

* If Halley’s comet had been the 20,000 th part of the mass of Jupiter, Laplace has calculated, that it would have produced an effect on the motions of that planet, which would have been in 1682, distinctly perceptible with our instruments, and in 1835, it would have been perceptible even had the mass of the comet been much less. If the comet of 1770 had been the 5,000 th part of the mass of the Earth, it would have perceptibly lengthened our year. f A distinction must be made by the reader between mass and dimensions : mass has reference only to the quantity of matter; and thus a body may have a very small mass, and yet very great dimensions : this is the case with the comets.

revolution, at which time it passed very near the Earth. Allowance being made for these, the whole period of the comet’s last revolution was brought to 27,937 days, and counting from the 13th of March, 1759, when it last passed through its perihelion, or nearest extremity of its orbit to the sun, this brought its next perihelion passage to the 13th of November, 1835 * At the same time, M. Ponfocaulant expressly stated, that there might be an error of a few days in his time, and assigned as a proximate cause of such an error, a possible incorrectness in the assumed masses of some of the planets. His words are, “ We must here once more repeat, that it is not pretended that the time announced for the comet’s return to its perihilion may not be in error some days." Elsewhere he says, “ Thus then it is conclusive, that about the middle of November, 1835, the passage of the comet through its perihelion will take place." The determination of the time when the comet would first appear, was altogether another and, a much less important matter : it depended upon the time when it would enter that portion of the heavens then visible at night, at that particular place; it depended upon the intensity of its light, as compared with that twilight there might be when it first appeared ; it depended upon the state of the atmosphere. All these were variable elements, and none of them, except one, could be calculated upon with any certainty. It would have been madness to have mentioned a particular day when it should first be seen; nevertheless, both M. Pontecoulant and M. Damoiseau ventured to announce it as probable that it would appear during the first days of the month of August. The facts which astronomy reveals are so stupendous, her results so far beyond the range of ordinary thought, the steps which she takes through time and space so rapid, leaving even imagination far behind, that of all sciences, she would find least credit with the world, were it not for the predictions to which she appeals, and which everybody may verify. It is this prophetic power which constitutes her strength, her whole strength, or her weakness with the vulgar. Driven from every other test, there were people disposed to cavil at this science, (as there are always people disposed to cavil at what they do not understand), who had fixed their criterion in the predicted return of the comet of 1759. , Now what was the result ? It had been announced that the conlet would probably be visible daring the first.days of August. It was seen on the sth of August at Rome, f by MM. Dumouchel and Vico, its light being then exceedingly feeble. But more than .this, the precise place in the heavens which the comet would occupy on every day whilst it should be visible, had been calculated and announced beforehand, and it was when they directed their telescope to that point in the heavens which had been so predicted for the sth of August, that MM. Dumouchel and Vico saw it. It had been foretold that it would pass its perihelion on the 13th of November, that there might be an error of a few days, but that, nevertheless, it certainly would pass it about the middle of November. It passed its perihelion on the 16fA of November. It had been assigned by M. Pontecoulant, as a reason for the uncertainty which he thus felt in respect to the time of the perihelion passage, amounting, however, only to a few days, that the masses usually assigned to some of the planets by astronomers, and used by him in his calculations, might require correction. Of all the planets, Jupiter exercised the greatest influence over the motions of this comet. Any error in the mass which had been assigned to Jupiter, would, therefore, most affect the result. Now the mass he had assigned to Jupiter, was such, that 1054 such masses would equal the mass of the sun. Recent observations have shown, that the mass of Jupiter repeated 1049 times, would equal the mass of the sun; and that it has been ascertained, that if M. Pontecoulant had usepl in his calculation this corrected measurement of the mass of Jupiter, instead of that which he did use, it would have protracted the predicted time of the perihelion passage three days, and brought it to the 1 5th, and to within six hours of the time when it actually took place—an error of six hours in a period of seventy-six years / * M. Damoiseau fixed its perihelion passage to the sth of November. 1* The reader need not be reminded how pure and clear is the atmosphere of Rome. (To be continued.)