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:
Perihelion Passage. 1812, Sept. 1846, Mar.
15_d._ 7_h._ 5_d._ 12_h._
Long. of Perih'n. 92° 51´ 90° 31´
Long. of A. Node. 253° 33´ 77° 37´
Incl. 73° 57´ 85° 6´
Peri'n Dist. 0.7771 0.6637
Eccentricity. 0.94454 0.96224
Period. 70.68_y_ 73.715
Direction. D D
Computer. Encke. Peirce.
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.
[Illustration: Fig. 3.]
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]
[6] Monthly Notices of the R. A. S., vol. xxv., p. 243.
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]
[7] Dr. Lardner.
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 5-1/2
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 121-1/2 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.