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--

[11] Danville Quarterly Review, December, 1861.

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]

[12] Others, it was supposed, might have originated within the system,--a view which the writer has not wholly abandoned.

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.

[13] "Quæst. Nat.," lib. vii., cap. xvi.

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]

[14] Chambers' "Descr. Astr.," p. 374.

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

[15] Ibid., p. 383.

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.

[16] Ibid., p. 388.

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]

[17] Hevelius, "Cometographia," p. 341. See also Grant's "Hist. of Phys. Astr.," p. 302.

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:

Comet of 1532. Comet of 1661.

Longitude of perihelion 111° 48´ 115° 16´ Longitude of ascending node 87 23 81 54 Inclination 32 36 33 1 Perihelion distance 0.5192 0.4427 Motion Direct. Direct.

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.

[18] "Cometographia," p. 417.

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.

[19] Williams' "Chinese Observations of Comets," p. 73.

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.

[20] One of the parts was seen at Madras, India, on the mornings of December 2 and 3, 1872.

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 2-1/2°. 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_.

II.

METEORS.