Besides Halley’s and Lexel’s comets, ten or twelve others are now known to form part of the solar system; that is to say, they return to the sun at stated periods. Six of them have periods of less than eight years. That generally called Encke’s comet, or the comet of the short period, was first seen by MM. Messier and Mechain in 1786, again by Miss Herschel in 1805, and its returns, in the years 1805 and 1819, were observed by other astronomers, under the impression that all four were different bodies. However, Professor Encke not only proved their identity, but determined the circumstances of the comet’s motion. Its reappearance in the years 1825, 1828, and 1832, accorded with the orbit assigned by M. Encke, who thus established the length of its period to be 1204 days, nearly. This comet is very small, of feeble light, and invisible to the naked eye, except under very favourable circumstances, and in particular positions. It has no tail, it revolves in an ellipse of great excentricity inclined at an angle of 13° 22ʹ to the plane of the ecliptic, and is subject to considerable perturbations from the attraction of the planets, which occasion variations in its periodic time. Among the many perturbations to which the planets are liable, their mean motions, and therefore the major axes of their orbits, experience no change; while, on the contrary, the mean motion of the moon is accelerated from age to age—a circumstance at first attributed to the resistance of an ethereal medium pervading space, but subsequently proved to arise from the secular diminution of the excentricity of the terrestrial orbit. Although the resistance of such a medium has not hitherto been perceived in the motions of such dense bodies as the planets and satellites, its effects on the revolutions of the comets leave no doubt of its existence. From the numerous observations that have been made on each return of the comet of the short period, the elements have been computed with great accuracy on the hypothesis of its moving in vacuo. Its perturbations occasioned by the disturbing action of the planets have been determined; and, after everything that could influence its motion had been duly considered, M. Encke found that an acceleration of about two days in each revolution has taken place in its mean motion, precisely similar to that which would be occasioned by the resistance of an ethereal medium. And, as it cannot be attributed to a cause like that which produces the acceleration of the moon, it must be concluded that the celestial bodies do not perform their revolutions in an absolute void, and that, although the medium be too rare to have a sensible effect on the masses of the planets and satellites, it nevertheless has a considerable influence on so rare a body as a comet. Contradictory as it may seem that the motion of a body should be accelerated by the resistance of an ethereal medium, the truth becomes evident if it be considered that both planets and comets are retained in their orbits by two forces which exactly balance one another; namely, the centrifugal force producing the velocity in the tangent, and the attraction of the gravitating force directed to the centre of the sun. If one of these forces be diminished by any cause, the other will be proportionally increased. Now, the necessary effect of a resisting medium is to diminish the tangential velocity, so that the balance is destroyed, gravity preponderates, the body descends towards the sun till equilibrium is again restored between the two forces; and, as it then describes a smaller orbit, it moves with increased velocity. Thus, the resistance of an ethereal medium actually accelerates the motion of a body; but, as the resisting force is confined to the plane of the orbit, it has no influence whatever on the inclination of the orbit, or on the place of the nodes. In computing its effect, M. Encke assumed the increase to be inversely as the square of the distance, and that its resistance acts as a tangential force proportional to the squares of the comet’s actual velocity in each point of its orbit. Another comet belonging to our system, which returns to its perihelion after a period of 63⁄4 years, has been accelerated in its motion by a whole day during one revolution, which puts the existence of ether beyond a doubt, and confirms the undulatory theory of light. Since this comet, which revolves nearly between the orbits of the earth and Jupiter, is only accelerated one day at each revolution, while Encke’s, revolving nearly between the orbits of Mercury and Pallas, is accelerated two, the ethereal medium must increase in density towards the sun. The comet in question was discovered by M. Biela at Josephstadt on the 27th of February, 1826, and ten days afterwards it was seen by M. Gambart at Marseilles, who computed its parabolic elements, and found that they agreed with those of the comets which had appeared in the years 1789 and 1795, whence he concluded them to be the same body moving in an ellipse, and accomplishing its revolution in 2460 days. The perturbations of this comet were computed by M. Damoiseau, who predicted that it would cross the plane of the ecliptic on the 29th of October, 1832, a little before midnight, at a point nearly 18,484 miles within the earth’s orbit; and as M. Olbers of Bremen, in 1805, had determined the radius of the comet’s head to be about 21,136 miles, it was evident that its nebulosity would envelop a portion of the earth’s orbit,—a circumstance which caused some alarm in France, from the notion that, if any disturbing cause had delayed the arrival of the comet for one month, the earth must have passed through its head. M. Arago dispelled these fears by his excellent treatise on comets, in the Annuaire of 1832, where he proves that, as the earth would never be nearer the comet than 18,000,000 British leagues, there could be no danger of collision. The earth is in more danger from these two small comets than from any other. Encke’s crosses the terrestrial orbit sixty times in a century, and may ultimately come into collision, but both are so extremely rare, that little injury is to be apprehended.
The earth would fall to the sun in 641⁄2 days, if it were struck by a comet with sufficient impetus to destroy its centrifugal force. What the earth’s primitive velocity may have been it is impossible to say. Therefore a comet may have given it a shock without changing the axis of rotation, but only destroying part of its tangential velocity, so as to diminish the size of the orbit—a thing by no means impossible, though highly improbable. At all events, there is no proof of this having occurred; and it is manifest that the axis of the earth’s rotation has not been changed, because, as the ether offers no sensible resistance to so dense a body as the earth, the libration would to this day be evident in the variation it must have occasioned in the terrestrial latitudes. Supposing the nucleus of a comet to have a diameter only equal to the fourth part of that of the earth, and that its perihelion is nearer to the sun than we are ourselves, its orbit being otherwise unknown, M. Arago has computed that the probability of the earth receiving a shock from it is only one in 281 millions, and that the chance of our coming in contact with its nebulosity is about ten or twelve times greater. Only comets with retrograde motions can come into direct collision with the earth, and if the momentum were great the event might be fatal; but in general the substance of comets is so rare, that it is likely they would not do much harm if they were to impinge; and even then the mischief would probably be local, and the equilibrium soon restored, provided the nucleus were gaseous, or very small. It is, however, more probable that the earth would only be deflected a little from its course by the approach of a comet, without being touched by it. The comets that have come nearest to the earth were that of the year 837, which remained four days within less than 1,240,000 leagues from our orbit: that of 1770, which approached within about six times the distance of the moon. The celebrated comet of 1680 also came very near to us; and the comet whose period is 63⁄4 years was ten times nearer the earth in 1805 than in 1832, when it caused so much alarm.
Encke’s and Biela’s comets are at present far removed from the influence of Jupiter, but they will not always remain so, because, the aphelia and nodes of the orbits of these two comets being the points which approach nearest to the orbit of Jupiter at each meeting of the planet and comets, the major axis of Encke’s comet will be increased and that of Biela’s diminished, till in the course of time, when the proximity has increased sufficiently, the orbits will be completely changed, as that of Lexel’s was in 1770. Every twenty-third year, or after seven revolutions of Encke’s comet, its greatest proximity to Jupiter takes place, and at that time his attraction increases the period of its revolution by nine days—a circumstance which took place in the end of the years 1820 and 1843. But from the position of the bodies there is a diminution of three days in the six following revolutions, which reduces the increase to six days in seven revolutions. Thus, before the year 1819, the periodic time of Encke’s comet was 1204 days, and it was 1219 days in accomplishing the revolution that ended in 1845. By this progressive increase the orbit of the comet will reach that of Jupiter in seven or eight centuries, and then by the very near approach of the two bodies it will be completely changed.
At present the Earth and Mercury have the most powerful influence on the motions of Encke’s and Biela’s comets; and have had for so long a time that, according to the computation of Mr. Airy, the present orbit of the latter was formed by the attraction of the Earth, and that of Encke’s by the action of Mercury. With regard to the latter comet, that event must have taken place in February 1776. In 1786 Encke’s comet had both a tail and a nucleus, now it has neither; a singular instance of the possibility of their disappearance. It was in perihelio in 1855.
In 1846 Biela’s comet was divided into two distinct bodies, by what strange accident is altogether a mystery. The nuclei of the two comets were separated by about 150,000 miles, and they travelled together with their tails parallel, and an arch of light over their heads. Till that time Biela’s comet never had been seen with a tail. The new head was dull at first, but increased in size and brightness till it surpassed its companion in both; besides, it had a bright flashing diamond-like point in its centre—gradually it resumed its dull appearance, and its period was computed to be eight days longer than that of the original head. They had separated to a greater distance from one another in 1853, but were still travelling together, one having become smaller than the other.
A comet discovered by M. Brorsen of Kiel, on the 26th of February, 1846, came, on the 20th of April following, nearly as close to Jupiter as his fourth satellite, when Jupiter’s attraction must have been ten times greater than that of the sun; so there is every reason to believe that the comet’s orbit will be as much altered as that of Lexel’s; and another discovered by Padre de Vico at Rome, on the 22nd of August, will, in all probability, be as much disturbed by the same cause. One of the comets found by that astronomer has a period which varies, according to different computations, from 55 to 99 years; it certainly has an elliptical orbit. That discovered at Naples by Mr. Peters revolves about the sun in 16 years; but Olbers’s comet of 1815 must go nearly the same distance into space with Halley’s, since its period is 74 years. Two discovered by M. Brorsen have periods, one of 500 and the other of 28 years; but of the latter there is some uncertainty.
The comet which appeared in 1596 and 1845 has a period of 249 years; and should M. Argelander’s computation be accurate, the orbit which has hitherto been assigned to the great comet of 1811 must be erroneous, since he has ascertained its period to be 3066 years.
The great comet of 1264, which had a tail that extended over 100° of the celestial vault, was observed and recorded by the Chinese, and was ascertained to be the same that had appeared in 1556, and of whose motions observations were taken at Vienna in the reign of the Emperor Charles V., but it was then less brilliant. In consequence of the discovery of the original observations of the comet of 1556, by Fabricius at Vienna, and by Heller at Nuremburg, Mr. Hind was induced to compute its orbit for that year; but after much labour, aided by all the improved methods of calculation, he found Heller’s observations so confused, and even erroneous, that he could not determine the curve described by the comet at that time with any precision, and therefore could only predict that the epoch of its return would be some time between 1848 and 1861. Before comets reach the sun they are rarely conspicuous; but if after passing their perihelion they come near the earth, then they have tails, and become brilliant in consequence of the sun’s action upon the matter of which they are formed. Now if the comet in question should pass its perihelion between the months of March and October, it possibly may be as remarkable as ever; but should it come nearest to the sun in winter, such is the position of its orbit with regard to the earth, that it may pass unnoticed—which is very unlikely, as search is being made for it at almost all the observatories in Europe and in the United States. Nearly the whole of its orbit lies below the plane of the ecliptic, and far from the paths of the larger planets, but it extends into space more than twice the distance of Neptune, or nearly six thousand millions of miles from the sun.
Comets in or near their perihelion move with prodigious velocity. That of 1680 appears to have gone half round the sun in ten hours and a half, moving at the rate of 880,000 miles an hour. If its enormous centrifugal force had ceased when passing its perihelion, it would have fallen to the sun in about three minutes, as it was then less than 147,000 miles from his surface. So near the sun, it would be exposed to a heat 27,500 times greater than that received by the earth; and as the sun’s heat is supposed to be in proportion to the intensity of his light, it is probable that a degree of heat so intense would be sufficient to convert into vapour every terrestrial substance with which we are acquainted. At the perihelion distance the sun’s diameter would be seen from the comet under an angle of 73°, so that the sun, viewed from the comet, would nearly cover the whole extent of the heavens from the horizon to the zenith. As this comet is presumed to have a period of 575 years, the major axis of its orbit must be so great, that at the aphelion the sun’s diameter would only subtend an angle of about fourteen seconds, which is not so great by half as the diameter of Mars appears to us when in opposition. The sun would consequently impart no heat, so that the comet would then be exposed to the temperature of the ethereal regions, which is 239° below the zero point of Fahrenheit. A body of such tenuity as the comet, moving with such velocity, must have met with great resistance from the dense atmosphere of the sun, while passing so near his surface at its perihelion. The centrifugal force must consequently have been diminished, and the sun’s attraction proportionally augmented, so that it must have come nearer to the sun in 1680 than in its preceding revolution, and would subsequently describe a smaller orbit. As this diminution of its orbit will be repeated at each revolution, the comet will infallibly end by falling on the surface of the sun, unless its course be changed by the disturbing influence of some large body in the unknown expanse of creation. Our ignorance of the actual density of the sun’s atmosphere, of the density of the comet, and of the period of its revolution, renders it impossible to form any idea of the number of centuries which must elapse before this event takes place.
The same cause may affect the motions of the planets, and ultimately be the means of destroying the solar system. But, as Sir John Herschel observes, they could hardly all revolve in the same direction round the sun for so many ages without impressing a corresponding motion on the ethereal medium, which may preserve them from the accumulated effects of its resistance. Should this material medium revolve about the sun like a vortex, it will accelerate the revolutions of such comets as have direct motions, and retard those that have retrograde motions.