Comets and Meteors Part 1

Comets and Meteors.

by Daniel Kirkwood.

PREFACE.

The origin of meteoric astronomy, as a science, dates from the memorable star-shower of 1833. Soon after that brilliant display it was found that similar phenomena had been witnessed, at nearly regular intervals, in former times. This discovery led at once to another no less important, viz.: that the nebulous ma.s.ses from which such showers are derived revolve about the sun in paths intersecting the earth's...o...b..t. The theory that these meteor-clouds are but the scattered fragments of disintegrated comets was announced by several astronomers in 1867:--a theory confirmed in a remarkable manner by the shower of meteors from the _debris_ of Biela's comet on the 27th of November, 1872.

To gratify the interest awakened in the public mind by the discoveries here named, is the main design of the following work. Among the subjects considered are, cometary astronomy; aerolites, with the phenomena attending their fall; the most brilliant star-showers of all ages; and the origin of comets, aerolites, and falling stars.

It may be proper to remark that the language used by the writer in a volume[1] published several years since, and now nearly out of print, has been occasionally adopted in the following treatise.

BLOOMINGTON, INDIANA, April, 1873.

[1] Meteoric Astronomy.

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...o...b..tal 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...o...b..t 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...o...b..t. 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 princ.i.p.al planets in the order of distance.

Its...o...b..t 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 ma.s.s or quant.i.ty of matter is more than that of all the other planets except Jupiter.

Ura.n.u.s 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 Ura.n.u.s; 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...o...b..t, about 1000 feet; while that of Neptune's...o...b..t 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 Ca.s.sini, 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 lat.i.tudes, 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 Ca.s.sini. 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 cl.u.s.ter being interior to Mercury's...o...b..t.

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.

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 a.s.signed 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.

[Ill.u.s.tration: Fig. 2. THE GREAT COMET OF 1811.]

Zeno, Democritus, and other Greek philosophers held that comets were produced by the collection of several stars into cl.u.s.ters. 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 pa.s.sing beyond the reach of our vision. The observations of Tycho Brahe first established the fact that comets move through the planetary s.p.a.ces far beyond the limits of our atmosphere. The ill.u.s.trious 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 a.s.sa.s.sination of Julius Caesar. The Romans called it the _Julium Sidus_, and regarded it as a celestial chariot sent to convey the soul of Caesar 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 suns.h.i.+ne. 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 pa.s.sed 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 gla.s.s at one o'clock in the afternoon,--only five hours after its perihelion pa.s.sage. 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 s.p.a.ces 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 pa.s.sed 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...o...b..t. 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 pa.s.sage. That discovered by Klinkerfues on the 10th of June, 1853, and which pa.s.sed its perihelion on the 1st of September, was seen at Olmutz in the daytime, August 31, when only twelve degrees from the sun. After pa.s.sing 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 circ.u.mstances. 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 pa.s.sed 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 pa.s.sed between the sun and the earth. Unfortunately, however, the transits were not observed.

[2] Hind.

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 pa.s.s off to wander indefinitely in the sidereal s.p.a.ces.

_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 pa.s.sed 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 r.e.t.a.r.d 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 pa.s.s its perihelion about the 13th of April, 1759. It was stated, however, that, being pressed for want of time, they had neglected certain quant.i.ties which might somewhat affect the result.

The comet, in fact, pa.s.sed 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 Ura.n.u.s, 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 ma.s.ses of Jupiter and Saturn had been determined with greater accuracy, and Ura.n.u.s had been added to the known members of the planetary system. A nearer approximation to the exact time of the comet's perihelion pa.s.sage 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 Inst.i.tute--for the most perfect discussion of its motions. That of the former was awarded to Damoiseau,--that of the latter to Pontecoulant. The times a.s.signed by these distinguished mathematicians for the comet's perihelion pa.s.sage were very nearly the same, and differed but a few days from the true time. Had the present received ma.s.s 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.