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.