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COMETS AND METEORS.

CHAPTER I.
A GENERAL VIEW OF THE SOLAR SYSTEM.

A descriptive treatise on Comets and Meteors may properly be preceded by a brief general view of the planetary system to which these bodies are related, and by which their motions, in direction and extent, are largely influenced.

The Solar System consists of the sun, together with the planets, comets, and meteors which revolve around it as the centre of their motions. The sun is the great controlling orb of this system, and the source of light and heat to its various members. Its magnitude is one million three hundred thousand times greater than that of the earth, and it contains more than seven hundred times as much matter as all the planets put together.

Mercury is the nearest planet to the sun; its mean distance being about 35,400,000 miles. Its diameter is 3000 miles, and it completes its orbital revolution in 88 days.

Venus, the next member of the system, is sometimes our morning and sometimes our evening star. Its magnitude is almost exactly the same as that of the earth. It revolves round the sun in 225 days.

The earth is the third planet from the sun in the order of distance; the radius of its orbit being about 92,000,000 miles. It is attended by one satellite,—the moon,—the diameter of which is 2160 miles.

Mars is the first planet exterior to the earth's orbit. It is considerably smaller than the earth, and has no satellite. It revolves round the sun in 687 days.

The Asteroids.—Since the commencement of the present century a remarkable zone of telescopic planets has been discovered immediately exterior to the orbit of Mars. These bodies are extremely small; some of them probably containing less matter than the largest mountains on the earth's surface. 131 members of the group are known at present, and the number is annually increasing.

Jupiter, the first planet exterior to the asteroids, is nearly 500,000,000 miles from the sun, and revolves round it in a little less than 12 years. This planet is 86,000 miles in diameter, and contains more than twice as much matter as all the other planets, primary and secondary, put together. Jupiter is attended by four moons or satellites.

Saturn is the sixth of the principal planets in the order of distance. Its orbit is about 400,000,000 miles beyond that of Jupiter. This planet is attended by eight satellites, and is surrounded by three broad flat rings. Saturn is 73,000 miles in diameter, and its mass or quantity of matter is more than that of all the other planets except Jupiter.

Uranus is at double the distance of Saturn, or nineteen times that of the earth. Its diameter is about 34,000 miles, and its period of revolution 84 years. It is attended by at least four satellites.

Neptune is the most remote known member of the system; its distance being 2,800,000,000 miles. It is somewhat larger than Uranus; has certainly one satellite, and probably several more. Its period is about 165 years. A cannon-ball flying outward from the sun at the uniform velocity of 500 miles per hour would not reach the orbit of Neptune in less than 639 years.

These planets all move round the sun in the same direction,—from west to east. Their motions are nearly circular, and also nearly in the same plane. Their orbits, except that of Neptune, are represented in the frontispiece. It is proper to remark, however, that all representations of the solar system by maps and planetariums must give an exceedingly erroneous view either of the magnitudes or distances of its various members. If the earth, for instance, be denoted by a ball half an inch in diameter, the diameter of the sun, according to the same scale (16,000 miles to the inch), will be between four and five feet; that of the earth's orbit, about 1000 feet; while that of Neptune's orbit will be nearly six miles. To give an accurate representation of the solar system at a single view is therefore plainly impracticable.

The Zodiacal Light.—This term was first applied by Dominic Cassini, in 1683, to a faint nebulous aurora, somewhat resembling the milky way, apparently of a conical or lenticular form, having its base toward the sun and its axis nearly in the direction of the ecliptic. The most favorable time for observing it is when its axis is most nearly perpendicular to the horizon. This, in our latitudes, occurs in March, for the evening, and in October, for the morning. The angular distance of its vertex from the sun is frequently seventy or eighty degrees, while sometimes, though rarely (except within the tropics), it exceeds even one hundred degrees. It was noticed in the latter part of the 16th century by Tycho Brahe. The first accurate description of the phenomenon was given, however, by Cassini. This astronomer supposed the appearance to be produced by the blended light of innumerable bodies too small to be separately observed,—a theory still very generally accepted. In other words, the zodiacal light is probably a swarm of infinitesimal planets; the greater part of the cluster being interior to Mercury's orbit.

The distances between the different members of our planetary system, vast as they may seem, sink into insignificance when compared with the intervals which separate us from the so-called fixed stars. Alpha Centauri, the nearest of those twinkling luminaries, is 7000 times more distant than Neptune from the sun. Even light itself, which moves 185,000 miles in a second, is more than three years in traversing the mighty interval.

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CHAPTER II.
COMETS.

The term comet—which signifies literally a hairy star—may be applied to all bodies that revolve about the sun in very eccentric orbits. The sudden appearance, vast dimensions, and extraordinary aspect of these celestial wanderers, together with their rapid and continually varying motions, have never failed to excite the attention and wonder of all observers. Nor is it surprising that in former times, when the nature of their orbits was wholly unknown, they should have been looked upon as omens of impending evil, or messengers of an angry Deity. Even now, although modern science has reduced their motions to the domain of law, determined approximately their orbits, and assigned in a number of instances their periods, the interest awakened by their appearance is in some respects still unabated.

The special points of dissimilarity between planets and comets are the following:—The former are dense, and, so far as we know, solid bodies; the latter are many thousand times rarer than the earth's atmosphere. The planets all move from west to east; many comets revolve in the opposite direction. The planetary orbits are but slightly inclined to the plane of the ecliptic; those of comets may have any inclination whatever. The planets are observed in all parts of their orbits; comets, only in those parts nearest the sun.

The larger comets are attended by a tail, or train of varying dimensions, extending generally in a direction opposite to that of the sun. The more condensed part, from which the tail proceeds, is called the nucleus; and the nebulous envelope immediately surrounding the nucleus is sometimes termed the coma. These different parts are seen in Fig. 2, which represents the great comet of 1811.

Zeno, Democritus, and other Greek philosophers held that comets were produced by the collection of several stars into clusters. Aristotle taught that they were formed by exhalations, which, rising from the earth's surface, ignited in the upper regions of the atmosphere. This hypothesis, through the great influence of its author, was generally received for almost two thousand years. Juster views, however, were entertained by the celebrated Seneca, who maintained that comets ought to be ranked among the permanent works of nature, and that their disappearance was not an extinction, but simply a passing beyond the reach of our vision. The observations of Tycho Brahe first established the fact that comets move through the planetary spaces far beyond the limits of our atmosphere. The illustrious Dane, however, supposed them to move in circular orbits. Kepler, on the other hand, was no less in error in considering their paths to be rectilinear. James Bernoulli supposed comets to be the satellites of a very remote planet, invisible on account of its great distance,—such satellites being seen only in the parts of their orbits nearest the earth. Still more extravagant was the hypothesis of Descartes, who held that they were originally fixed stars, which, having gradually lost their light, could no longer retain their positions, but were involved in the vortices of the neighboring stars, when such as were thus brought within the sphere of the sun's illuminating power again became visible.

Comets visible in the daytime.

Comets of extraordinary brilliancy have sometimes been seen during the daytime. At least thirteen authentic instances of this phenomenon have been recorded in history. The first was the comet which appeared about the year 43 B.C., just after the assassination of Julius CÆsar. The Romans called it the Julium Sidus, and regarded it as a celestial chariot sent to convey the soul of CÆsar to the skies. It was seen two or three hours before sunset, and continued visible for eight successive days. The great comet of 1106, described as an object of terrific splendor, was seen simultaneously with the sun, and in close proximity to it. Dr. Halley supposed this and the Julian comet to have been previous visits of the great comet of 1680. In the year 1402 two comets appeared,—one about the middle of February, the other in June,—both of which were visible while the sun was above the horizon. One was of such magnitude and brilliancy that the nucleus and even the tail could be seen at midday. The comet of 1472, one of the most splendid recorded in history, was visible in full daylight, when nearest the earth, on the 21st of January. This comet, according to Laugier, moves very nearly in the plane of the ecliptic, its inclination being less than two degrees. Its least distance from our globe was only 3,300,000 miles. The comet of 1532, supposed by some to be identical with that of 1661, was also visible in full sunshine. The apparent magnitude of its nucleus was three times greater than that of Jupiter. The comet of 1577 was seen with the naked eye by Tycho Brahe before sunset. It was by observations on this body that Aristotle's doctrine in regard to the origin, nature, and distance of comets was proved to be erroneous. It was simultaneously observed by Tycho at Oranienberg, and Thaddeus Hagecius at Prague; the points of observation being more than 400 miles apart, and nearly on the same meridian. The comet was found to have no sensible diurnal parallax; in other words, its apparent place in the heavens was the same to each observer, which could not have been the case had the comet been less distant than the moon. The comet which passed its perihelion on the 8th of November, 1618, was distinctly seen by Marsilius when the sun was above the horizon. The great comet of 1744 was seen without the aid of a glass at one o'clock in the afternoon,—only five hours after its perihelion passage. The diameter of this body was nearly equal to that of Jupiter. It had six tails, the greatest length of which was about 30,000,000 miles, or nearly one-third of the distance of the earth from the sun. The spaces between the tails were as dark as the rest of the heavens, while the tails themselves were bordered with a luminous edging of great beauty.

The great comet of 1843 was distinctly visible to the naked eye, at noon, on the 28th of February. It appeared as a brilliant body, within less than two degrees from the sun. This comet passed its perihelion on the 27th of February, at which time its distance from the sun's surface was only about one-fourth of the moon's distance from the earth. This is the nearest approach to the sun ever made by any known comet. The velocity of the body in perihelion was about 1,280,000 miles an hour, or nearly nineteen times that of the earth in its orbit. The apparent length of its tail was sixty-five degrees, and its true length 150,000,000 miles. The first comet of 1847, discovered by Mr. Hind, was also seen near the sun on the day of its perihelion passage. That discovered by Klinkerfues on the 10th of June, 1853, and which passed its perihelion on the 1st of September, was seen at Olmutz in the daytime, August 31, when only twelve degrees from the sun. After passing its perihelion, it was again observed, at noon, on the 2d, 3d, and 4th of September. Finally, the great comet of 1861 was seen before sunset, on Monday evening, July 1, by Rev. Henry W. Ballantine, of Bloomington, Indiana. It was again detected on the following evening just as the sun was in the horizon.

Besides the thirteen comets which we have enumerated, at least four others have been seen in the daytime; all, however, under peculiar circumstances. Seneca relates that during a great solar eclipse, 63 years before our era, a large comet was observed not far from the sun. "Philostorgius says that on the 19th of July, A.D. 418, when the sun was eclipsed and stars were visible, a great comet, in the form of a cone, was discovered near that luminary, and was afterwards observed during the nights."[2] The comet which passed its perihelion on the 18th of November, 1826, was observed by both Gambart and Flaugergues to transit the solar disk,—the least distance of the nucleus from the sun's surface being about 2,000,000 miles. The second comet of 1819 and the comet of 1823 are both known in like manner to have passed between the sun and the earth. Unfortunately, however, the transits were not observed.

A few cometary orbits are hyperbolas, more ellipses, and a still greater number parabolas. Comets moving in ellipses remain permanently within the limits of solar influence. Others, however, visit our system but once, and then pass off to wander indefinitely in the sidereal spaces.

Comets of known periodicity.

I. Halley's Comet.

As comets are subject to great changes of appearance, one can never be identified by any description of its magnitude, brilliancy, etc., at the time of a previous return. This can be done only by a comparison of orbits. If, for example, we find the elements of an orbit very nearly corresponding in every particular with those of a former comet, there is a degree of probability, amounting almost to certainty, that the two are identical. Sir Isaac Newton, in his Principia, published shortly after the appearance of the comet of 1682, explained how the periods of those mysterious visitors might thus be ascertained, thus directing the attention of astronomers to the subject. Dr. Halley soon after undertook a thorough discussion of all the recorded cometary observations within his reach. In the course of his investigations he discovered that the path of the comet observed by Kepler in 1607 coincided almost exactly with that of the one which passed its perihelion in 1682. Hence he concluded that they were the same. He found also that the comet of 1531, whose course had been particularly observed by Apian, moved in the same path. The interval between the consecutive appearances being nearly 76 years, Halley announced this as the time of the comet's revolution, and boldly predicted its return in 1758 or 1759. The law of universal gravitation had at this time just been discovered and announced. But although its application to the determination of planetary and cometary perturbations had not been developed, Halley was well aware that the attractive influence of Jupiter and Saturn might accelerate or retard the motion of the comet, so as to produce a considerable variation in its period. During the interval from 1682 to 1759, the application of the higher mathematics to problems in physical astronomy had been studied with eminent success. The disturbing effect of the two large planets, Jupiter and Saturn, was computed with almost incredible labor by Clairaut, Lalande, and Madame Lepaute. The result as announced by Clairaut to the Academy of Sciences in November, 1758, was that the period must be 618 days longer than that immediately preceding, and that the comet accordingly would pass its perihelion about the 13th of April, 1759. It was stated, however, that, being pressed for want of time, they had neglected certain quantities which might somewhat affect the result. The comet, in fact, passed its perihelion in March, within less than a month of the predicted time. When it is considered that the attraction of the earth was not taken into the account, and that Uranus, whose influence must have been sensible, had not then been discovered, this must certainly be regarded as a remarkable approximation.

But during the next interval of 76 years the theory of planetary perturbations had been more perfectly developed. The masses of Jupiter and Saturn had been determined with greater accuracy, and Uranus had been added to the known members of the planetary system. A nearer approximation to the exact time of the comet's perihelion passage in 1835 was therefore to be expected. Prizes were offered by two of the learned societies of Europe—the Academy of Sciences at Turin, and the French Institute—for the most perfect discussion of its motions. That of the former was awarded to Damoiseau,—that of the latter to Pontecoulant. The times assigned by these distinguished mathematicians for the comet's perihelion passage were very nearly the same, and differed but a few days from the true time. Had the present received mass of Jupiter been used in the calculations, Pontecoulant, it is believed, would not have been in error as much as 24 hours. It may be proper to remark that, during the entire period from 1759 to 1835, the position of Neptune was such that it could produce no considerable effect on the motion of the comet.

This interesting object will again return about 1911.

The visit of 1531 was the earliest that Halley succeeded in determining with any degree of certainty. Peter Apian, by whom it was at that time observed, was the first European to ascertain the fact that, as a general thing, the tails of comets are turned from the sun.[3] To confirm this discovery, he carefully followed the body in its progress through the constellations. By means of his recorded observations Halley was enabled to identify this comet with that of 1607 and 1682. The great comet of 1456 he conjectured to be the same, from the date of its appearance. PingrÉ subsequently confirmed this suspicion by a careful examination of the few trustworthy records that could be collected from the writers of that period.

From the earlier descriptions of this comet we infer that its brilliancy is gradually diminishing. In 1456 its tail, which was slightly curved like a sword or sabre, extended two-thirds of the distance from the horizon to the zenith. The appearance of such an object, in a grossly superstitious age, excited throughout Europe the utmost consternation. The Moslems had just taken Constantinople, and were threatening to advance westward into Europe. Pope Calixtus III., regarding the comet as confederate with the Turk, ordered prayers to be offered three times a day for deliverance from both. The alarm, however, was of short duration. Within ten days of its appearance the comet reached its perihelion. Receding from the sun, the sword-like form began to diminish in brilliancy and extent; and finally, to the great relief of Europe, it entirely disappeared.

The perihelion passage of 1456 was, until recently, the earliest known. It was shown by Laugier, however, in 1843, that among the notices of comets extracted by Edward Biot from the Chinese records, were observations of a body in 1378, which was undoubtedly the comet of Halley. Further researches among these annals enabled the same astronomer to recognize two ancient returns, one in 760, the other in 451. Still more recently the distinguished English astronomer, Mr. Hind, has traced back the returns to the year 11 B.C. He remarks, however, that previous to that epoch, "the Chinese descriptions of comets are too vague to aid us in tracing any more ancient appearances," and that "European writers of these remote times render us no assistance." Let us now inquire whether the comet had probably made any former approach to the sun in an orbit nearly identical with the present. It is well known that the modern period of this body is considerably less than the ancient. Thus, the mean period since A.D. 1456 has been 75.88 years; while from 11 B.C. to 1456 A.D. it was 77.27 years. In determining the approximate dates of former returns, the ancient period should evidently be employed. Now, it is a remarkable fact that of more than 70 comets,[4] or objects supposed to be comets, whose appearance was recorded during the six centuries immediately preceding the year 11 B.C., but one—that of 166 B.C.—was observed at a date corresponding nearly to that of a former return of Halley's comet. Of this object it is merely recorded that "a torch was seen in the heavens." Whether this was a comet or some other phenomenon, it is impossible to determine. But as the comet of Halley was more brilliant in ancient than in modern times, it seems highly improbable that seven consecutive returns of so conspicuous an object should have been unrecorded, especially as twelve comets per century[5] were observed during the same period. It would appear, therefore, that the perihelion passage of 11 B.C. was in fact the first ever made by the comet, or at least the first in an orbit nearly the same as the present.

The motion of Halley's comet is retrograde. The point of its nearest approach to the sun is situated within the orbit of Venus. Its greatest distance from the centre of the system is nearly twice that of Uranus, or 36 times that of the earth. The comet is, consequently, subject to great changes of temperature. When nearest the sun its light and heat are almost four times greater than the earth's; when most remote, they are 1200 times less. In the former position, the sun would appear much larger than to us; in the latter, his apparent diameter would not greatly exceed that of Jupiter, as viewed from the earth. It would be difficult to conjecture what the consequences might be, were our planet transported to either of these extremes of the cometary path. In the perihelion, the waters of the ocean would undoubtedly be reduced to a state of vapor; in the aphelion, they would be solidified by congelation.

II. Encke's Comet.

It was formerly supposed that all comets have their aphelia far beyond the limits of the planetary system. In 1818, however, a small comet was discovered by Pons, the orbit of which was subsequently found to be wholly interior to that of Jupiter. Its elements were presented by Bouvard, in 1819, to the Board of Longitude at Paris. The form and position of the orbit were immediately found to correspond with those of a comet observed by several astronomers in 1805. The different appearances were consequently regarded as returns of the same body. Its elliptic orbit was calculated by Encke, who found its period to be only about three years and four months. Its perihelion is within the orbit of Mercury; its aphelion, between the asteroids and the orbit of Jupiter.

Encke's comet is invisible to the naked eye, except in very favorable circumstances; it has no tail; its motion, like that of the planets, is from west to east; and its orbit is inclined about 13° to the ecliptic.

A comparison of the successive periods of this interesting object has led to the discovery that its time of revolution is gradually diminishing; a fact regarded by Encke and other astronomers as indicating the existence of an ethereal medium.

III. Biela's Comet.

The discovery of Encke's comet of short period was followed, in 1826, by that of another, whose revolution is completed in about six years and eight months. It was observed on the 27th of February, by M. Biela, an Austrian officer; accordingly it has since been known as Biela's comet. On computing its elements and comparing them with those of former comets, it was found to have been observed in 1772 and 1805. Damoiseau having calculated the dimensions of the comet's elliptic path and the time of its return, announced as the result of his computations the remarkable fact that the orbits of the earth and comet intersect each other, and that the comet would cross the earth's path on the 29th of October, 1832. This produced no little alarm among the uneducated, especially in France. Even some journalists are said to have predicted the destruction of our globe by a collision with the comet. When the latter, however, passed the point of intersection at the predicted time, the earth was at a distance of 50,000,000 miles.

At the return of 1845-6, Biela's comet exhibited a most remarkable appearance. Instead of a single comet, it appeared as two distinct bodies moving together side by side, at a distance from each other somewhat less than that of the moon from the earth. Astronomers, anxious to determine whether the cometary fragments had continued separate during an entire revolution, awaited the next return with no ordinary interest. The two bodies appeared at the predicted time (September, 1852); their distance apart having increased to 1,250,000 miles. In 1859 the comet, on account of its proximity to the sun, entirely escaped detection. At the return in 1865-6 the position of the object was quite favorable for observation, yet the search of astronomers was again unsuccessful. In 1872 the body escaped detection both in Europe and America. One fragment was seen, however, at Madras, India, on the mornings of the 2d and 3d of December,—several weeks after its perihelion passage. The comet's non-appearance in 1866 and its greatly diminished magnitude in 1872 leave no room to doubt its progressive dissolution. This subject will again be referred to in discussing the phenomena of meteoric showers.

IV. Faye's Comet.

On the 22d of November, 1843, M. Faye, of the Paris Observatory, discovered a comet, which was shown by Dr. Goldschmidt to revolve in an elliptic orbit, the perihelion of which is exterior to the orbit of Mars, and the aphelion immediately beyond that of Jupiter. The eccentricity is, therefore, less than that of any other comet previously discovered. Its period is about 7 years and 5 months.

It is possible that a comet moving in a parabola or hyperbola, with the sun in the focus, may be thrown into an elliptic orbit by the disturbing influence of Jupiter or one of the other large planets. The celebrated Leverrier undertook to determine whether the comet of Faye had in this manner been recently fixed as a permanent member of the solar system. He found that it could not have been so introduced since 1747, and, consequently, that it must have completed at least thirteen revolutions before its discovery.

This comet has been observed at each return from 1843 to the present time.

V. De Vico's Comet.

On the 22d of August, 1844, De Vico, of Rome, discovered a comet whose orbit is included between those of the earth and Jupiter. Its period is 1996 days, or about 5½ years. This body, from some cause,—perhaps a gradual dissolution,—has not been observed at any subsequent return.

VI. Brorsen's Comet.

On the 26th of February, 1846, Mr. Brorsen, of Kiel, discovered a faint comet, the mean distance and period of which are almost identical with those of De Vico's. This comet was not observed during the perihelion passage of 1851, on account of its unfavorable position with respect to the sun. It has, however, been subsequently detected.

VII. D'Arrest's Comet.

Dr. D'Arrest discovered a comet on the 27th of June, 1851, which was soon found to move in an elliptic orbit, with a period of about 6½ years. It entirely escaped observation, both in Europe and America, during its perihelion passage in 1857. It was observed, however, at the Cape of Good Hope. Its invisibility in 1864 was due to its unfavorable position. At its return in 1870, it was first seen on the 31st of August, by Dr. Winnecke, of Carlsruhe.

VIII. Tuttle's Comet.

A faint telescopic comet was discovered at the Observatory of Harvard College, on the evening of January 4, 1858, by Mr. H. P. Tuttle. The same body was independently found one week later by Dr. Bruhns, of Berlin. From observations made at Cambridge, Massachusetts, and Ann Arbor, Michigan, its elements were soon computed by different astronomers; the result in each case coinciding so closely with the elements of the second comet of 1790, as to place its identity wholly beyond doubt. Its period is nearly 13 years and 8 months. It had returned, therefore, without detection, in the years 1803, 1817, 1831, and 1844. On its approach to perihelion in 1871, it was first detected by M. Borelly, of Marseilles.

IX. Winnecke's Comet.

The second comet of 1858 was discovered on the 8th of March, by Dr. Winnecke, of Bonn. This proved to be identical with the third comet of 1819, whose period was computed by Encke to be about 5½ years. It had therefore returned unperceived no less than six times between 1819 and 1858. At its return in 1863 it again escaped detection. The perihelion passage of 1869 was made on the 30th of June. The comet was seen as early as April 13, and, after passing the sun, as late as October 11. "SchÖnfeld states that in part of April and May it appeared to have not one, but several, centres of condensation, and Vogel says that, in the beginning of June, it had a much greater resemblance to a star-cluster than to a nebula." This phenomenon, it may be remarked, bore a striking resemblance to the appearances observed in the comets of 389, 1618, and 1661.

X. Tempel's Comet.

On the 19th of December, 1865, M. Tempel, of Marseilles, discovered a small comet, which continued visible four weeks, passing its perihelion January 11, 1866. Dr. Oppolzer, of Vienna, after a careful determination of its elements, announced the interesting fact that its orbit very nearly intersects those of the earth and Uranus; the perihelion being situated immediately within the former, and the aphelion a short distance exterior to the latter. The period, according to the same astronomer, is 33 years and 65 days. The identity of this comet with that of 1366 was suggested by Professor H. A. Newton soon after its appearance,—a suggestion which subsequent research has strongly corroborated. It is also highly probable that the comet observed in China, September 29, 1133, was a former return of the same body. In 1366 it was conspicuous to the naked eye, while in 1866 it was wholly invisible without a telescope,—a fact indicative of its gradual dissolution. The connection of this comet with the meteors of November 14 will be elsewhere considered.

XI. The Second Comet of 1867.

Another comet of short period was discovered by M. Tempel on the 3d of April, 1867. Its orbit is the least eccentric of all known comets: the perihelion being exterior to the orbit of Mars; the aphelion interior to that of Jupiter. Its motion is direct, and it completes a revolution in 5 years and 8 months.

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CHAPTER III.
COMETS WHOSE ELEMENTS INDICATE PERIODICITY, BUT WHOSE RETURNS HAVE NOT BEEN RECOGNISED.

I. The Group whose periods are nearly equal to that of Uranus.

Since the commencement of the present century five comets have been discovered, which form, with Halley's, an interesting and remarkable group. The first of these was detected by Pons, on the 20th of July, 1812; the second by Olbers, on the 6th of March, 1815; the third by De Vico, on the 28th of February, 1846; the fourth by Brorsen, on the 20th of July, 1847; and the last by Westphal, on the 27th of June, 1852. The periods of these bodies are all nearly equal, ranging from 68 to 76 years; their eccentricities are not greatly different; the motions of all, except that of Halley's, are direct; and the distances of their aphelia are somewhat greater than Neptune's distance from the sun. Of this group, the comets of 1812 and 1846 seem worthy of special notice. The former became visible to the naked eye shortly after its discovery, and each continued visible about ten weeks. Their elements are as follows:

The wonderful similarity of these elements, except in the longitude of the ascending node, is at once apparent. It will also be noticed that the longitude of the descending node of the latter is very nearly coincident with that of the ascending node of the former. These remarkable coincidences are presented to the eye in the following diagram, where the dotted ellipse represents the orbit of the comet of 1812, and the continuous curve that of the comet of 1846.

It is infinitely improbable that these coincidences should be accidental; they point undoubtedly to a common origin of the two bodies.

According to the theory now generally accepted, comets enter the solar system ab extra, move in parabolas or hyperbolas around the sun, and, if undisturbed by the planets, pass off beyond the limits of the sun's attraction, to be seen no more. If in their motion, however, they approach very near any of the larger planets, their direction is changed by planetary perturbation,—their orbits being sometimes transformed into ellipses. The new orbits of such bodies would pass very nearly through the points at which their greatest perturbation occurred; and accordingly we find that the aphelia of a large proportion of the periodic comets are near the orbits of the major planets. "I admit," says M. Hoek, "that the orbits of comets are by nature parabolas or hyperbolas, and that in the cases when elliptical orbits are met with, these are occasioned by planetary attractions, or derive their character from the uncertainty of our observations. To allow the contrary would be to admit some comets as permanent members of our planetary system, to which they ought to have belonged since its origin, and so to assert the simultaneous birth of that system and of these comets. As for me, I attribute to these a primitive wandering character. Traveling through space, they move from one star to another in order to leave it again, provided they do not meet any obstacle that may force them to remain in its vicinity. Such an obstacle was Jupiter, in the neighborhood of our sun, for the comets of Lexell and Brorsen, and probably for the greater part of periodical comets; the other part of which may be indebted for their elliptical orbits to the attractions of Saturn and the remaining planets.

"Generally, then, comets come to us from some star or other. The attraction of our sun modifies their orbit, as had been done already by each star through whose sphere of attraction they had passed. We can put the question if they come as single bodies or united in systems."

The conclusion of this astronomer's interesting discussion is that—

"There are systems of comets in space that are broken up by the attraction of our sun, and whose members attain, as isolated bodies, the vicinity of the earth during a course of several years."[6]

In the researches here referred to, it is shown by Professor Hoek that the comets of 1860 III., 1863 I., and 1863 IV. formed a group in space previous to their entrance into our system. The same fact has also been demonstrated in regard to other comets which need not here be specified. Now, the comets of 1812 and 1846 IV. have their aphelia near the orbit of Neptune, and hence the original parabolas in which they moved were probably transformed into ellipses by the perturbations of that planet. Before entering the solar domain, they were doubtless members of a cometary system. Passing Neptune near the same time, and at some distance from each other, their different relative positions with regard to the disturbing body may account for the slight differences in the elements of their orbits.

Comets of the Jovian Group.

Besides the eight comets enumerated in Chapter II. whose aphelia are in the vicinity of Jupiter's orbit, five others have been observed which belong apparently to the same cluster. These are the comets of 1585, 1743 I., 1766 II., 1783, and 1819 IV. "The fact that these comets have not been re-observed on their successive returns through perihelion may be explained either by the difficulty of observing them, owing to their unfavorable positions, and to the circumstances of observers not expecting their reappearance, their periodic character not being then suspected, or because they may have been thrown by the disturbing action of the larger planets into orbits such as to keep them continually out of the range of view of terrestrial observers."[7]

Lexell's comet of 1770 is the most remarkable instance known of the change produced in the orbits of these bodies by planetary attraction. This comet passed so near Jupiter in 1779 that the attraction of the latter was 200 times greater than that of the sun. The consequence was that the comet, whose mean distance corresponded to a period of 5½ years, was thrown into an orbit so entirely different that it has never since been visible.

Peters' Comet.

A telescopic comet was discovered by Dr. Peters on the 26th of June, 1846, which continued to be observed till the 21st of July. Its period, according to the discoverer, is about 13 years, and its aphelion, like that of Tuttle's comet, is in the vicinity of Saturn's orbit. It was expected to return in 1859, and again in 1872, but each time escaped detection, owing probably to the fact that its position was unfavorable for observation.

Stephan's Comet (1867 I.).

In January, 1867, M. Stephan, of Marseilles, discovered a new comet, the elements of which, after two months' observations, were computed by Mr. G. M. Searle, of Cambridge, Massachusetts. The perihelion of this body is near the orbit of Mars; its aphelion near that of Uranus,—the least distance of the orbits being about 2,000,000 miles. The present form of the cometary path is doubtless due to the disturbing action of Uranus. The comet completes its revolution in 33.62 years; consequently (as has been pointed out by Mr. J. R. Hind) five of its periods are almost exactly equal to two periods of Uranus. The next approximate appulse of the two bodies will occur in 1985, when the form of the comet's orbit may be sensibly modified.

Elliptic Comets whose Aphelia are at a much Greater Distance than Neptune's Orbit.

In October, 1097, a comet was seen both in Europe and China, which was noted for the fact of its having two distinct tails, making with each other an angle of about 40°. From a discussion of the Chinese observations (which extended through a longer period than the European), Laugier concluded that this body is identical with the third comet of 1840, which was discovered by Galle on the 6th of March. If, therefore, it has made no intermediate return without being observed, it must have a period of about 743 years. It is also highly probable, from the similarity of elements, that the comet which passed its perihelion on the 5th of June, 1845, was a reappearance of the comet of 1596,—the period of revolution being 249 years. The elements of the great comet of 1843 are somewhat uncertain. There is a probability, however, of the identity of this body with the comet of 1668. This would make the period 175 years. The third comet of 1862 is especially interesting from its connection with the August meteors. Its period, according to Dr. Oppolzer, is 121½ years.

The Great Comet of 1858

was one of the most remarkable in the nineteenth century. It was discovered on the 2d of June, by Donati, of Florence, and first became visible to the naked eye about the last of August. The comet attained its greatest brilliancy about the 10th of October, when its distance from the earth was 50,000,000 miles. The length of its tail somewhat exceeded this distance. If, therefore, the comet had been at that time directly between the sun and the earth, the latter must have been enveloped for a number of hours in the cometic matter.

The observations of this comet during a period of five months enabled astronomers to determine the elements of its orbit within small limits of error. It completes a revolution, according to Newcomb, in 1854 years, in an orbit somewhat more eccentric than that of Halley's comet. It will not return before the 38th century, and will only reach its aphelion about the year 2800. Its motion per second when nearest the sun is 36 miles; when most remote, only 234 yards.

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CHAPTER IV.
OTHER REMARKABLE COMETS.

It remains to describe some of the most remarkable comets of which we have any record, but of which we have no means of determining with certainty whether they move in ellipses, parabolas, or hyperbolas.

In the year 466 B.C., a large comet appeared simultaneously with the famous fall of meteoric stones near Ægospotamos. The former was supposed by the ancients to have had some agency in producing the latter phenomenon. Another of extraordinary magnitude appeared in the year 373 B.C. This comet was so bright as to throw shadows, and its tail extended one-third of the distance from the horizon to the zenith. The years 156, 136, 130, and 48, before our era, were also signalized by the appearance of very large comets. The apparent magnitude of the first of these is said to have equaled that of the sun itself; while its light was sufficient to diminish sensibly the darkness of the night. The second is said to have filled a fourth part of the celestial hemisphere. The comet of 130 B.C., sometimes called the comet of Mithridates, because of its appearance about the time of his birth, is said to have rivaled the sun in splendor.

In A.D. 178 a large comet was visible during a period of nearly three months. Its nucleus had a remarkably red or fiery appearance, and the greatest length of its tail exceeded 60°. The most brilliant comets of the sixth century were probably those of 531 and 582. The train of the latter, as seen in the west soon after sunset, presented the appearance of a distant conflagration.

Great comets appeared in the years 975, 1264, and 1556. Of these, the comet of 1264 had the greatest apparent magnitude. It was first seen early in July, and attained its greatest brilliancy in the latter part of August, when its tail was 100° in length. It disappeared on the 3d of October, about the time of the death of Pope Urban IV., of which event the comet, in consequence of this coincidence, was considered the precursor. These comets, on account of the similarity of their elements, were believed by many astronomers to be the same, and to have a period of about 300 years. In the case of identity, however, another reappearance should have occurred soon after the middle of the nineteenth century. As no such return was observed, we may conclude that the comets were not the same, and that their periods are wholly unknown.

The comet discovered on the 10th of November, 1618, was one of the largest in modern times; its tail having attained the extraordinary length of 104°. The comet of 1652, so carefully observed by Hevelius, almost equaled the moon in apparent magnitude. It shone, however, with a lurid, dismal light. The tail of the comet of 1680 was 90° in length. This body is also remarkable for its near approach to the sun; its least distance from the solar surface having been only 147,000 miles. It will always be especially memorable, however, for having furnished Newton the data by means of which he first showed that comets in their orbital motions are governed by the same principle that regulates the planetary revolutions.

Of all the comets which appeared during the eighteenth century, that which passed its perihelion on the 7th of October, 1769, had the greatest apparent magnitude. It was discovered by Messier on the 8th of August, and continued to be observed till the 1st of December. On the 11th of September the length of its tail was 97°. The comet discovered on the 26th of March, 1811, is in some respects the most remarkable on record. It was observed during a period of 16 months and 22 days,—the longest period of visibility known. On account of its situation with respect to the earth, the apparent length of its tail was much less than that of some other comets; its true length, however, was at one time 120,000,000 miles; and Sir William Herschel found that on the 12th of October the greatest circular section of the tail was 15,000,000 miles in diameter. The same astronomer found the diameter of the head of the comet to be 127,000 miles, and that of the envelope at least 643,000. As a general thing, the length of a comet-train increases very rapidly as the body approaches the sun. But the perihelion distance of the comet of 1811 was considerably greater than the distance of the earth from the sun; while its nearest approach to the earth was 110,000,000 miles. Its true magnitude, therefore, has probably not been surpassed by any other observed; and had its perihelion been very near the sun, it must have exhibited an appearance of terrific grandeur. This comet has an elliptic orbit, and its period, according to Argelander, is 3065 years.

The great comet of 1861 was discovered on the 13th of May, by Mr. John Tebbut, Jr., of New South Wales. In this country, as well as in Europe, it was first generally observed on the evening of June 30,—19 days after its perihelion passage. Sir John Herschel, who observed it in Kent, England, remarks that it far exceeded in brilliancy any comets he had ever seen, not excepting those of 1811 and 1858. According to Father Secchi, of the Collegio Romano, the length of its tail was 118°. This, with a single exception,[8] is the greatest on record. The computed orbit is elliptical; the period, 419 years.

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CHAPTER V.
THE POSITION AND ARRANGEMENT OF COMETARY ORBITS.

The cosmical masses from which comets are derived seem to traverse in great numbers the interstellar spaces. In consequence of the sun's progressive motion, these nebulous bodies are sometimes drawn toward the centre of our system. If, in this approach, they are not disturbed by any of the large planets, they again recede in parabolas or hyperbolas. When, however, as must sometimes be the case, they pass near Jupiter, Saturn, Uranus, or Neptune, their orbits may be transformed into elongated ellipses. The periodicity of many comets may thus be accounted for.

In the present chapter it is proposed to consider the probable consequences of the sun's motion through regions of space in which cometary matter is widely diffused; to compare our theoretical deductions with observed phenomena; and thus refer to their physical cause a variety of facts which have hitherto received no satisfactory explanation.[9]

1. As comets, at least in many instances, owe their periodicity to the disturbing action of the major planets, and as this planetary influence is sometimes sufficient, especially in the case of Jupiter and Saturn, to change the direction of cometary motion, the great majority of periodic comets should move in the same direction with the planets. Now, of the comets known to be elliptical, 70 per cent. have direct motion. In this respect, therefore, theory and observation are in striking harmony.

2. When the relative positions of a comet and the disturbing planet are such as to give the transformed orbit of the former a small perihelion distance, the comet must return to the point at which it received its greatest perturbation; in other words, to the orbit of the planet. The aphelia of the comets of short period ought therefore to be found, for the most part, in the vicinity of the orbits of the major planets. This, as already shown in Chapters II. and III., is strikingly the case. The actual distances of these aphelia, however, as compared with the respective distances of Jupiter, Saturn, Uranus and Neptune, are presented at one view in the following tables:

I. Comets whose Aphelion Distances are nearly Equal to 5.20,
the Radius of Jupiter's Orbit.

II. Comets whose Aphelion Distances are nearly Equal to 9.54,
the Radius of Saturn's Orbit.

III. Comets whose Aphelion Distances are nearly Equal to 19.18,
the Radius of Uranus's Orbit.

IV. Comets whose Aphelion Distances are nearly Equal to 30.04,
the Radius of Neptune's Orbit.

The coincidences here pointed out (some of which have been noticed by others) appear, then, to be necessary consequences of the motion of the solar system through spaces occupied by meteoric nebulÆ. Hence the observed facts receive an obvious explanation.

In regard to comets of long period we have only to remark that, for anything we know to the contrary, there may be causes of perturbation far exterior to the orbit of Neptune.

3. From what we observe in regard to the larger bodies of the universe—a clustering tendency being everywhere apparent,—it seems highly improbable that cometic matter should be uniformly distributed in the sidereal spaces. We would expect, on the contrary, to find it collected in groups or clusters. This view is also in remarkable harmony with the facts of observation. In 150 years, from 1600 to 1750, 16 comets were visible to the naked eye; of which 8 appeared in the 25 years from 1664 to 1689. Again, during 60 years, from 1750 to 1810, only 5 comets were visible to the naked eye, while in the next 50 years there were double that number. The probable cause of such variations is sufficiently obvious. As the sun in its progressive motion approaches a cometary group, the latter is drawn toward the centre of our system; the nearer members with greater velocity than the more remote. Those of the same cluster would enter the solar domain at periods not very distant from each other; the forms of their orbits depending upon their original relative positions with reference to the sun's course, and also on planetary perturbations. It is evident also that the passage of the solar system through a region of space comparatively destitute of cometic clusters would be indicated by a corresponding paucity of comets.

4. The line of apsides of a large proportion of comets will be approximately coincident with the solar orbit. The point towards which the sun is moving is in longitude about 260°. The quadrants bisected by this point and that directly opposite extend from 215° to 305°, and from 35° to 125°. The number of cometary perihelia found in these quadrants up to July, 1868 (periodic comets being counted but once) was 159, or 62 per cent.; in the other two quadrants, 98, or 38 per cent.

This tendency of the perihelia to crowd together in two opposite regions has been noticed by different writers.

5. Comets whose positions before entering our system were very remote from the solar orbit must have overtaken the sun in its progressive motion; hence their perihelia must fall, for the most part, in the vicinity of the point towards which the sun is moving; and they must in general have very small perihelion distances. Now, what are the observed facts in regard to the longitudes of the perihelia of the comets which have approached within the least distance of the sun's surface? But three have had a perihelion distance less than 0.01. All these, it will be seen by the following table, have their perihelia in close proximity to the point referred to:

I. Comets whose Perihelion Distances are Less than 0.01.

In Table II. all but the last have their perihelia in the same quadrant.

II. Comets whose Perihelion Distances are Greater than 0.01 and Less than 0.05.

The perihelion of the first comet in Table III. is remote from the direction of the sun's motion; that of the second is distant but 14°, and of the third 21°.

III. Comets whose Perihelion Distances are Greater than 0.05 and Less than 0.1.

With greater perihelion distances the tendency of the perihelia to crowd together round the point indicated is less distinctly marked.

6. Few comets of small perihelion distance should have their perihelia in the vicinity of longitude 80°, the point opposite that towards which the sun is moving. Accordingly we find, by examining a table of cometary elements, that with a perihelion distance less than 0.1 there is not a single perihelion between 35° and 125°; between 0.1 and 0.2 but 3; and between 0.2 and 0.3 only 1.

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CHAPTER VI.
THE DISINTEGRATION OF COMETS.

The fact that in several instances meteoric streams move in orbits identical with those of certain comets was first established by the researches of Signor Schiaparelli. The theory, however, of an intimate relationship between comets and meteors was advocated by the writer as long since as 1861,[11]—several years previous to the publication of Schiaparelli's memoirs. In the essay here referred to it was maintained—

1. That meteors and meteoric rings "are the dÉbris of ancient but now disintegrated comets whose matter has become distributed around their orbits."

2. That the separation of Biela's comet as it approached the sun in December, 1845, was but one in a series of similar processes which would probably continue until the individual fragments would become invisible.

3. That certain luminous meteors have entered the solar system from the interstellar spaces.[12]

4. That the orbits of some meteors and periodic comets have been transformed into ellipses by planetary perturbation; and

5. That numerous facts—some observed in ancient and some in modern times—have been decidedly indicative of cometary disintegration.

What was thus proposed as theory has been since confirmed as undoubted facts. When the hypothesis was originally advanced, the data required for its mathematical demonstration were entirely wanting. The evidence, however, by which it was sustained was sufficient to give it a high degree of probability.

The existence of a divellent force by which comets near their perihelia have been separated into parts is clearly shown by the following facts. Whether this force, as suggested by Schiaparelli, is simply the unequal attraction of the sun on different parts of the nebulous mass, or whether, in accordance with the views of other astronomers, it is to be regarded as a cosmical force of repulsion, is a question left for future discussion.

Historical Facts.

1. Seneca informs us that Ephoras, a Greek writer of the fourth century before Christ had recorded the singular fact of a comet's separation into two distinct parts.[13] This statement was deemed incredible by the Roman philosopher, inasmuch as the occurrence was then without a parallel. More recent observations of similar phenomena leave no room to question the historian's veracity.

2. The head of the great comet of A.D. 389, according to the writers of that period, was "composed of several small stars." (Hind's "Comets," p. 103.)

3. On June 27, A.D. 416, two comets appeared in the constellation Hercules, and pursued nearly the same apparent path. Probably at a former epoch the pair had constituted a single comet.[14]

4. On August 4, 813, "a comet was seen which resembled two moons joined together." They subsequently separated, the fragments assuming different forms.[15]

5. The Chinese annals record the appearance of three comets—one large and two smaller ones—at the same time, in the year 896 of our era. "They traveled together for three days. The little ones disappeared first, and then the large one."[16] The bodies were probably fragments of a large comet which, on approaching the sun, had been separated into parts a short time previous to the date of their discovery.

6. The third comet of 1618.—The great comet of 1618 exhibited decided symptoms of disintegration. When first observed (on November 30), its appearance was that of a lucid and nearly spherical mass. On the eighth day the process of division was distinctly noticed, and on the 20th of December it resembled a cluster of small stars.[17]

7. The comet of 1661.—The elements of the comets of 1532 and 1661 have a remarkable resemblance, and previous to the year 1790 astronomers regarded the bodies as identical. The similarity of the elements is seen at a glance in the following table:

The elements of the former are by Olbers; those of the latter by Mechain. The return of the comet about 1790, though generally expected, was looked for in vain. As a possible explanation of this fact, it is interesting to recur to an almost forgotten statement of Hevelius. This astronomer observed in the comet of 1661 an apparent breaking up of the body into separate fragments.[18] The case may be analogous to that of Biela's comet.

8. The identity of the comets of 1866 and 1366, first suggested by Professor H. A. Newton, is now unquestioned. The existence then of a meteoric swarm, moving in the same track, is not the only evidence of the original comet's partial dissolution. The comet of 1866 was invisible to the naked eye; that of 1366, seen under nearly similar circumstances, was a conspicuous object. The statement of the Chinese historian that "it appeared nearly as large as a tow measure,"[19] though somewhat indefinite, certainly justifies the conclusion that its magnitude has greatly diminished during the last 500 years. The meteors moving in the same orbit are doubtless the products of this gradual separation.

9. The repartition of Biela's comet in 1845, as well as the non-appearance of the two fragments in 1865 and 1872,[20] were referred to in a previous chapter.

The comet of Halley, if we may credit the descriptions given by ancient writers, has been decreasing in brilliancy from age to age. The same is true in regard to several others believed to be periodic. The comet of A.D. 1097 had a tail 50° long. At its return, in March, 1840, the length of its tail was only 5°. The third comet of 1790 and the first of 1825 are supposed, from the similarity of their elements, to be identical. Each perihelion passage occurred in May, yet the tail at the former appearance was 4° in length, at the latter but 2½°. Other instances might be specified of this apparent gradual dissolution. It would seem, indeed, extremely improbable that the particles driven off from comets in their approach to the sun, forming tails extending millions of miles from the principal mass, should again be collected around the same nuclei.

The fact, then, that meteors move in the same orbits with comets is but a consequence of that disruptive process so clearly indicated by the phenomena described. In this view of the subject, comets—even such as move in elliptic orbits—are not to be regarded as permanent members of the solar system. Their dÉbris becomes gradually scattered around the orbit. Some parts of the nebulous ring will be more disturbed than others by planetary perturbation. Portions of such streams as nearly intersect the earth's path sometimes penetrate the atmosphere. Their rapid motion renders them luminous. If very minute, they are burnt up or dissipated without leaving any solid deposit; we then have the phenomena of shooting-stars. When, however, as is sometimes the case, they contain a considerable quantity of solid matter, they reach the earth's surface as meteoric stones.

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