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 assigned 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.
[Illustration: 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 clusters. 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 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. 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 assassination of Julius Cæsar. The
Romans called it the _Julium Sidus_, and regarded it as a celestial
chariot sent to convey the soul of Cæsar 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 sunshine. 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 passed 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
glass at one o'clock in the afternoon,--only five hours after its
perihelion passage. 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 spaces 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 passed 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 orbit. 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 passage. That
discovered by Klinkerfues on the 10th of June, 1853, and which passed
its perihelion on the 1st of September, was seen at Olmutz in the
daytime, August 31, when only twelve degrees from the sun. After passing
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
circumstances. 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 passed 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 passed 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 pass off to wander indefinitely in the sidereal spaces.
_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 passed 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 retard
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 pass its perihelion about the 13th of April,
1759. It was stated, however, that, being pressed for want of time, they
had neglected certain quantities which might somewhat affect the result.
The comet, in fact, passed 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 Uranus, 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 masses of Jupiter
and Saturn had been determined with greater accuracy, and Uranus had
been added to the known members of the planetary system. A nearer
approximation to the exact time of the comet's perihelion passage 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 Institute--for the most perfect discussion of its motions. That
of the former was awarded to Damoiseau,--that of the latter to
Pontecoulant. The times assigned by these distinguished mathematicians
for the comet's perihelion passage were very nearly the same, and
differed but a few days from the true time. Had the present received
mass 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.
The visit of 1531 was the earliest that Halley succeeded in determining
with any degree of certainty. Peter Apian, by whom it was at that time
observed, was the first European to ascertain the fact that, as a
general thing, the tails of comets are turned from the sun.[3] To
confirm this discovery, he carefully followed the body in its progress
through the constellations. By means of his recorded observations Halley
was enabled to identify this comet with that of 1607 and 1682. The great
comet of 1456 he _conjectured_ to be the same, from the date of its
appearance. Pingré subsequently confirmed this suspicion by a careful
examination of the few trustworthy records that could be collected from
the writers of that period.
[3] The Chinese, however, as appears from Biot's researches, had
observed the same fact 700 years earlier. See Humboldt's Cosmos,
vol. iv. (Bohn's ed.), p. 544.
From the earlier descriptions of this comet we infer that its brilliancy
is gradually diminishing. In 1456 its tail, which was slightly curved
like a sword or sabre, extended two-thirds of the distance from the
horizon to the zenith. The appearance of such an object, in a grossly
superstitious age, excited throughout Europe the utmost consternation.
The Moslems had just taken Constantinople, and were threatening to
advance westward into Europe. Pope Calixtus III., regarding the comet as
confederate with the Turk, ordered prayers to be offered three times a
day for deliverance from both. The alarm, however, was of short
duration. Within ten days of its appearance the comet reached its
perihelion. Receding from the sun, the sword-like form began to diminish
in brilliancy and extent; and finally, to the great relief of Europe, it
entirely disappeared.
The perihelion passage of 1456 was, until recently, the earliest known.
It was shown by Laugier, however, in 1843, that among the notices of
comets extracted by Edward Biot from the Chinese records, were
observations of a body in 1378, which was undoubtedly the comet of
Halley. Further researches among these annals enabled the same
astronomer to recognize two ancient returns, one in 760, the other in
451. Still more recently the distinguished English astronomer, Mr. Hind,
has traced back the returns to the year 11 B.C. He remarks, however,
that previous to that epoch, "the Chinese descriptions of comets are too
vague to aid us in tracing any more ancient appearances," and that
"European writers of these remote times render us no assistance." Let us
now inquire whether the comet had probably made any former approach to
the sun in an orbit nearly identical with the present. It is well known
that the modern period of this body is considerably less than the
ancient. Thus, the mean period since A.D. 1456 has been 75.88 years;
while from 11 B.C. to 1456 A.D. it was 77.27 years. In determining the
approximate dates of former returns, the ancient period should evidently
be employed. Now, it is a remarkable fact that of more than 70
comets,[4] or objects supposed to be comets, whose appearance was
recorded during the six centuries immediately preceding the year 11
B.C., but one--that of 166 B.C.--was observed at a date corresponding
nearly to that of a former return of Halley's comet. Of this object it
is merely recorded that "a torch was seen in the heavens." Whether this
was a comet or some other phenomenon, it is impossible to determine. But
as the comet of Halley was more brilliant in ancient than in modern
times, it seems highly improbable that seven _consecutive_ returns of so
conspicuous an object should have been unrecorded, especially as twelve
comets per century[5] were observed during the same period. It would
appear, therefore, that the perihelion passage of 11 B.C. was in fact
the first ever made by the comet, or at least the first in an orbit
nearly the same as the present.
[4] See the Catalogues of Chambers and Williams.
[5] The average number.
The motion of Halley's comet is retrograde. The point of its nearest
approach to the sun is situated within the orbit of Venus. Its greatest
distance from the centre of the system is nearly twice that of Uranus,
or 36 times that of the earth. The comet is, consequently, subject to
great changes of temperature. When nearest the sun its light and heat
are almost four times greater than the earth's; when most remote, they
are 1200 times less. In the former position, the sun would appear much
larger than to us; in the latter, his apparent diameter would not
greatly exceed that of Jupiter, as viewed from the earth. It would be
difficult to conjecture what the consequences might be, were our planet
transported to either of these extremes of the cometary path. In the
perihelion, the waters of the ocean would undoubtedly be reduced to a
state of vapor; in the aphelion, they would be solidified by
congelation.
II. Encke's Comet.
It was formerly supposed that all comets have their aphelia far beyond
the limits of the planetary system. In 1818, however, a small comet was
discovered by Pons, the orbit of which was subsequently found to be
wholly interior to that of Jupiter. Its elements were presented by
Bouvard, in 1819, to the Board of Longitude at Paris. The form and
position of the orbit were immediately found to correspond with those of
a comet observed by several astronomers in 1805. The different
appearances were consequently regarded as returns of the same body. Its
elliptic orbit was calculated by Encke, who found its period to be only
about three years and four months. Its perihelion is within the orbit of
Mercury; its aphelion, between the asteroids and the orbit of Jupiter.
Encke's comet is invisible to the naked eye, except in very favorable
circumstances; it has no tail; its motion, like that of the planets, is
from west to east; and its orbit is inclined about 13° to the ecliptic.
A comparison of the successive periods of this interesting object has
led to the discovery that its time of revolution is gradually
diminishing; a fact regarded by Encke and other astronomers as
indicating the existence of an ethereal medium.
III. Biela's Comet.
The discovery of Encke's comet of short period was followed, in 1826, by
that of another, whose revolution is completed in about six years and
eight months. It was observed on the 27th of February, by M. Biela, an
Austrian officer; accordingly it has since been known as _Biela's
comet_. On computing its elements and comparing them with those of
former comets, it was found to have been observed in 1772 and 1805.
Damoiseau having calculated the dimensions of the comet's elliptic path
and the time of its return, announced as the result of his computations
the remarkable fact that the orbits of the earth and comet intersect
each other, and that the comet would cross the earth's path on the 29th
of October, 1832. This produced no little alarm among the uneducated,
especially in France. Even some journalists are said to have predicted
the destruction of our globe by a collision with the comet. When the
latter, however, passed the point of intersection at the predicted time,
the earth was at a distance of 50,000,000 miles.
At the return of 1845-6, Biela's comet exhibited a most remarkable
appearance. Instead of a single comet, it appeared as two distinct
bodies moving together side by side, at a distance from each other
somewhat less than that of the moon from the earth. Astronomers,
anxious to determine whether the cometary fragments had continued
separate during an entire revolution, awaited the next return with no
ordinary interest. The _two_ bodies appeared at the predicted time
(September, 1852); their distance apart having increased to 1,250,000
miles. In 1859 the comet, on account of its proximity to the sun,
entirely escaped detection. At the return in 1865-6 the position of the
object was quite favorable for observation, yet the search of
astronomers was again unsuccessful. In 1872 the body escaped detection
both in Europe and America. One fragment was seen, however, at Madras,
India, on the mornings of the 2d and 3d of December,--several weeks
after its perihelion passage. The comet's non-appearance in 1866 and its
greatly diminished magnitude in 1872 leave no room to doubt its
progressive dissolution. This subject will again be referred to in
discussing the phenomena of meteoric showers.
IV. Faye's Comet.
On the 22d of November, 1843, M. Faye, of the Paris Observatory,
discovered a comet, which was shown by Dr. Goldschmidt to revolve in an
elliptic orbit, the perihelion of which is exterior to the orbit of
Mars, and the aphelion immediately beyond that of Jupiter. The
eccentricity is, therefore, less than that of any other comet previously
discovered. Its period is about 7 years and 5 months.
It is possible that a comet moving in a parabola or hyperbola, with the
sun in the focus, may be thrown into an elliptic orbit by the disturbing
influence of Jupiter or one of the other large planets. The celebrated
Leverrier undertook to determine whether the comet of Faye had in this
manner been recently fixed as a permanent member of the solar system. He
found that it could not have been so introduced since 1747, and,
consequently, that it must have completed at least thirteen revolutions
before its discovery.
This comet has been observed at each return from 1843 to the present
time.
V. De Vico's Comet.
On the 22d of August, 1844, De Vico, of Rome, discovered a comet whose
orbit is included between those of the earth and Jupiter. Its period is
1996 days, or about 5-1/2 years. This body, from some cause,--perhaps a
gradual dissolution,--has not been observed at any subsequent return.
VI. Brorsen's Comet.
On the 26th of February, 1846, Mr. Brorsen, of Kiel, discovered a faint
comet, the mean distance and period of which are almost identical with
those of De Vico's. This comet was not observed during the perihelion
passage of 1851, on account of its unfavorable position with respect to
the sun. It has, however, been subsequently detected.
VII. D'Arrest's Comet.
Dr. D'Arrest discovered a comet on the 27th of June, 1851, which was
soon found to move in an elliptic orbit, with a period of about 6-1/2
years. It entirely escaped observation, both in Europe and America,
during its perihelion passage in 1857. It was observed, however, at the
Cape of Good Hope. Its invisibility in 1864 was due to its unfavorable
position. At its return in 1870, it was first seen on the 31st of
August, by Dr. Winnecke, of Carlsruhe.
VIII. Tuttle's Comet.
A faint telescopic comet was discovered at the Observatory of Harvard
College, on the evening of January 4, 1858, by Mr. H. P. Tuttle. The
same body was independently found one week later by Dr. Bruhns, of
Berlin. From observations made at Cambridge, Massachusetts, and Ann
Arbor, Michigan, its elements were soon computed by different
astronomers; the result in each case coinciding so closely with the
elements of the second comet of 1790, as to place its identity wholly
beyond doubt. Its period is nearly 13 years and 8 months. It had
returned, therefore, without detection, in the years 1803, 1817, 1831,
and 1844. On its approach to perihelion in 1871, it was first detected
by M. Borelly, of Marseilles.
IX. Winnecke's Comet.
The second comet of 1858 was discovered on the 8th of March, by Dr.
Winnecke, of Bonn. This proved to be identical with the third comet of
1819, whose period was computed by Encke to be about 5-1/2 years. It had
therefore returned unperceived no less than six times between 1819 and
1858. At its return in 1863 it again escaped detection. The perihelion
passage of 1869 was made on the 30th of June. The comet was seen as
early as April 13, and, after passing the sun, as late as October 11.
"Schönfeld states that in part of April and May it appeared to have not
one, but several, centres of condensation, and Vogel says that, in the
beginning of June, it had a much greater resemblance to a star-cluster
than to a nebula." This phenomenon, it may be remarked, bore a striking
resemblance to the appearances observed in the comets of 389, 1618, and
1661.
X. Tempel's Comet.
On the 19th of December, 1865, M. Tempel, of Marseilles, discovered a
small comet, which continued visible four weeks, passing its perihelion
January 11, 1866. Dr. Oppolzer, of Vienna, after a careful determination
of its elements, announced the interesting fact that its orbit very
nearly intersects those of the earth and Uranus; the perihelion being
situated immediately within the former, and the aphelion a short
distance exterior to the latter. The period, according to the same
astronomer, is 33 years and 65 days. The identity of this comet with
that of 1366 was suggested by Professor H. A. Newton soon after its
appearance,--a suggestion which subsequent research has strongly
corroborated. It is also highly probable that the comet observed in
China, September 29, 1133, was a former return of the same body. In 1366
it was conspicuous to the naked eye, while in 1866 it was wholly
invisible without a telescope,--a fact indicative of its gradual
dissolution. The connection of this comet with the meteors of November
14 will be elsewhere considered.
XI. The Second Comet of 1867.
Another comet of short period was discovered by M. Tempel on the 3d of
April, 1867. Its orbit is the least eccentric of all known comets: the
perihelion being exterior to the orbit of Mars; the aphelion interior to
that of Jupiter. Its motion is direct, and it completes a revolution in
5 years and 8 months.