Daniel Kirkwood

“ It is found, however, that the reverse is true in regard to bolides, or fire-balls. These facts are susceptible of an obvious explanation. The fall of meteoric stones would be more likely to escape observation by night than by day, on account of the relatively small number of observers. But the days are shortest when the earth is in perihelion, and longest when in aphelion; the ratio of their lengths being nearly equal to that of the corresponding numbers of aerolitic falls. On the other hand, it is obvious that fire-balls, unless very large, would not be visible during the day. ”

“ And afterward they fell from the sky in such numbers, and so thickly together, that as they descended low in the air they seemed large and fiery, and the sky and the air seemed to be in flames, and even the earth appeared as if ready to take fire. ”

“ 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. ”

“ Approaching the sun, the comet undergoes great physical changes, which finally affect the stability of its structure: it can no longer hold together: parts of it take independent orbits around the sun, having great resemblance to the orbit of the parent comet. With periodical comets, this process is repeated at each successive approach to the sun. Gradually the products of disintegration are distributed along the comet's orbit, and if the earth's orbit cuts this, the phenomenon of shooting-stars is produced. ”

“ 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. ”

“ We have seen that at each return to perihelion the meteoric cluster is extended over a greater arc of its orbit. Now, Tuttle's comet and the August meteors undoubtedly constituted a single group previous to their entering the solar domain. It is evident, however, from the annual return of the shower during the last 90 years, that the ring is at present nearly if not quite continuous. That the meteoric mass had completed many revolutions before the ninth century of our era is manifest from the frequent showers observed between the years 811 and 841. ”

“ Why should we not adopt this view, since we may conjecture that those planetary bodies, like all the larger or smaller agglomerated masses revolving round the sun, have been thrown off from the once far more expanded solar atmosphere, and have been formed from vaporous rings describing their orbits round the central body? ”

“ 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 ob [Pg 42] served; 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. ”

“ Comets and meteors, according to his theory, are derived from cosmical clouds existing in great numbers in stellar space. These nebulæ, in consequence of their own motion or that of the sun, are drawn towards the centre of our system. By the unequal influence of the sun's attraction on different parts, such clouds are transformed into currents of great length before reaching the limits of the planetary system. Shooting-stars, fire-balls, aerolites, and comets being all of the same nature, differing merely in size, sometimes fall towards the sun as parts of the same current. ”

“ The celebrated Leverrier designated the year 126 of our era as the probable epoch of the comet's entrance into our system. This date, however, is incompatible with the period here adopted. It is worthy of remark, moreover, as bearing on this question, that the extension of the cluster in the tenth century, as indicated by the showers of 902, 931, and 934, was too great to have been effected in so short a period as 800 years. ”

“ 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. ”

“ It is worthy of remark that no new elements have been found in meteoric stones. Humboldt, in his "Cosmos," called attention to this interesting fact. "I would ask," he remarks, "why the elementary substances that compose one group of cosmical bodies, or one planetary system, may not in a great measure be identical? ”

“ 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 masses from which such showers are derived revolve about the sun in paths intersecting the earth's orbit. 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 débris of Biela's comet on the 27th of November, 1872. ”

“ As the earth is about five days in crossing the ring, its breadth in some parts cannot be less than 8,000,000 miles.
In 1866 Professor Schiaparelli, on computing the orbit of this meteoric stream, noticed the remarkable agreement of its elements with those of Swift's or Tuttle's comet [25] (1862, III.) , as computed by Dr. Oppolzer. These coincidences are exhibited in the following table:
It appears, therefore, that the third comet of 1862 is a part of the meteoric stream whose orbit is crossed by the earth on the 10th of August. ”

“ 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 [Pg 50]
5. That numerous facts—some observed in ancient and some in modern times—have been decidedly indicative of cometary disintegration. ”

“ 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 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 [Pg 14] 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 ”

“ 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 planet [Pg 54] ary perturbation. Portions of such streams as nearly intersect the earth's path sometimes penetrate the atmosphere. ”

“ The periodicity of the shower was thus, in a very short time, placed wholly beyond question. The facts in regard to the phenomena of November 13, 1833, were collected and discussed by Olmsted, Twining, and other astronomers. The inquiry, however, very naturally arose whether any trace of the same meteoric group could be found in ancient times. To determine this question many old historical records were ransacked by the indefatigable scientist, Edward C. Herrick, in our own country, and by Arago, Quetelet, and others, in Europe. ”

“ 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. ”

“ An analysis of any extensive table of meteorites and fire-balls proves that a greater number of aerolitic falls have been observed during the months of June and July, when the earth is near its aphelion, than in December and January, when near its perihelion. ”

“ 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. ”

“ In some parts of the country the inhabitants were completely terror-stricken by the magnificence of the display. In the afternoon of the day on which the shower occurred the writer met with an illiterate farmer who, after describing the phenomena as witnessed by himself, remarked that "the stars continued to fall till none were left," and added, "I am anxious to see how the heavens will appear this evening; I believe we shall see no more stars." ”

“ Professor Adams found that in this time the longitude of the node is increased 20 ́ by the action of Jupiter, 7 ́ by the action of Saturn, and 1 ́ by that of Uranus. The effect of the other planets is scarcely perceptible. The calculated motion in 331⁄4 years is therefore 28 ́. The observed motion in the same time, according to Professor Newton, as previously stated, is 29 ́. This remarkable accordance was at once accepted by astronomers as satisfactory evidence that the period is about 33.25 years. ”

“ 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 cer [Pg 19] tainty, that the two are identical. ”

“ The fact that comets and meteors, or at least a large proportion of such bodies, have entered the solar system from stellar space, is now admitted by all astronomers. The question, however, in regard to the origin and nature of these cosmical clouds still remains undecided. The theory that they consist of matter expelled with great velocity from the fixed stars appears to harmonize the greatest number of facts, and is accordingly entitled to respectful consideration. ”

“ From what we observe in regard to the larger bodies of the universe—a clustering tendency being [Pg 46] everywhere apparent,—it seems highly improbable that cometic matter should be uniformly distributed in the sidereal spaces. ”

“ The largest stone, which weighed 103 pounds, struck the earth at the foot of a large oak-tree, and, after cutting off two roots, one five inches in diameter, and grazing a third root, it descended two feet ten inches into hard clay. ”

“ D. Eckley Hunter counted 70 in 80 minutes. The numbers seen at New Haven were considerably greater. The fact that the display commenced before daylight had entirely closed seemed to indicate that only the termination of the shower had been observed in this country. Accordingly the display was soon found to have been witnessed from 60° E. to 90° W. of Greenwich, or through 150° of longitude. In England the first bolide of the swarm was seen by M. M. Brinkley, at 3 o'clock, P.M., in full daylight. The meteors were most numerous in the southern part of the continent, particularly in Italy. ”

“ The aphelion is consequently situated at a comparatively short distance beyond the orbit of Uranus. The orbit is represented in Fig. 4.
Fig. 4.
It was stated at the close of Chapter VI. that shooting-stars are the dissevered fragments of cometic matter, which, penetrating our atmosphere, are rendered luminous by the resistance so encountered. The discovery that comets and meteors are actually moving in the same orbits was first announced by Signor Schiaparelli in 1867. The [Pg 79] coincidence of the orbits of Tempel's comet [23] as computed by Dr. ”