irregular motions, the unexpected manner in which they often burst
upon us, and the imposing magnitudes which they occasionally assume,
have in all ages rendered them objects of astonishment, not unmixed
with superstitious dread to the uninstructed, and an enigma to those
most conversant with the wonders of creation and the operations
of natural causes. Even now, that we have ceased to regard their
movements as irregular, or as governed by other laws than those
which retain the planets in their orbits, their intimate nature,
and the offices they perform in the economy of our system, are as
much unknown as ever. No distinct and satisfactory account has yet
been rendered of those immensely voluminous appendages which they
bear about with them, and which are known by the name of their tails
(though improperly, since they often precede them in their motions),
any more than of several other singularities which they present.
The number of comets which have been astronomically observed, or of
which notices have been recorded in history, is very great, amounting
to several hundreds, and when we consider that in the earlier ages
of astronomy, and indeed in more recent times, before the invention
of the telescope, only large and conspicuous ones were noticed; and
that, since due attention has been paid to the subject, scarcely
a year has passed without the observation of one or two of these
bodies, and that sometimes two and even three have appeared at once;
it will be easily supposed that their actual number must be at least
many thousands. Multitudes, indeed, must escape all observation, by
reason of their paths traversing only that part of the heavens which
is above the horizon in the daytime. Comets so circumstanced can
only become visible by the rare coincidence of a total eclipse of the
sun—a coincidence which happened, as related by Seneca, sixty-two
years before Christ, when a large comet was actually observed very
near the sun. Several, however, stand on record as having been bright
enough to be seen with the naked eye in the daytime, even at noon
and in bright sunshine. Such were the comets of 1402, 1532 and 1843,
and that of 43 B. C. which appeared during the games celebrated by
Augustus in honor of Venus shortly after the death of Cæsar, and
which the flattery of poets declared to be the soul of that hero
taking its place among the divinities.
Comets consist for the most part of a large and more or less splendid
but ill-defined nebulous mass of light called the head, which is
usually much brighter toward its centre, and offers the appearance
of a vivid _nucleus_, like a star or planet. From the head, and in a
direction _opposite to that in which the sun is situated_ from the
comet, appear to diverge two streams of light, which grow broader and
more diffused at a distance from the head, and which most commonly
close in and unite at a little distance behind it, but sometimes
continue distinct for a great part of their course; producing an
effect like that of the trains left by some bright meteors, or
like the diverging fire of a sky-rocket (only without sparks or
perceptible motion). This is the tail. This magnificent appendage
attains occasionally an immense apparent length. Aristotle relates
of the tail of the comet of 371 B. C., that it occupied a third of
the hemisphere, or 60°; that of A. D. 1618 is stated to have been
attended by a train no less than 104° in length. The comet of 1680,
the most celebrated of modern times, and on many accounts the most
remarkable of all, with a head not exceeding in brightness a star
of the second magnitude, covered with its tail an extent of more
than 70° of the heavens, or, as some accounts state, 90°; that of
the comet of 1769 extended 97°, and that of the comet of 1843 was
estimated at about 65° when longest.
The tail is, however, by no means an invariable appendage of comets.
Many of the brightest have been observed to have short and feeble
tails, and a few great comets have been entirely without them. Those
of 1585 and 1763 offered no vestige of a tail; and Cassini describes
the comets of 1665 and 1682 as being as round and as well defined
as Jupiter. On the other hand, instances are not wanting of comets
furnished with many tails or streams of diverging light. That of 1744
had no less than six, spread out like an immense fan, extending to a
distance of nearly 30° in length. The small comet of 1823 had two,
making an angle of about 160°, the brighter turned as usual from the
sun, the fainter toward it, or nearly so. The tails of comets, too,
are often somewhat curved, bending, in general, toward the region
which the comet has left, as if moving somewhat more slowly, or as if
resisted in their course.
The smaller comets, such as are visible only in telescopes, or with
difficulty by the naked eye, and which are by far the most numerous,
offer very frequently no appearance of a tail, and appear only
as round or somewhat oval vaporous masses, more dense toward the
centre, where, however, they appear to have no distinct nucleus,
or anything which seems entitled to be considered as a solid body.
This was shown in a very remarkable manner in the case of the comet
discovered by Miss Mitchell in 1847, which on the 5th of October in
that year passed _centrally_ over a star of the fifth magnitude:
_so_ centrally that with a magnifying power of 100 it was impossible
to determine in which direction the extent of the nebulosity was
greatest. The star’s light seemed in no degree enfeebled; yet such
a star would be completely obliterated by a moderate fog, extending
only a few yards from the surface of the earth. And since it is an
observed fact that even those larger comets which have presented the
appearance of a nucleus have yet exhibited _no phases_, though we can
not doubt that they shine by the reflected solar light, it follows
that even these can only be regarded as great masses of thin vapor,
susceptible of being penetrated through their whole substance by
the sun-beams, and reflecting them alike from their interior parts
and from their surfaces. Nor will any one regard this explanation
as forced, or feel disposed to resort to a phosphorescent quality
in the comet itself, to account for the phenomena in question, when
we consider the enormous magnitude of the space thus illuminated,
and the extremely small _mass_ which there is ground to attribute to
these bodies. It will then be evident that the most unsubstantial
clouds which float in the highest regions of our atmosphere, and seem
at sunset to be drenched in light, and to glow throughout their
whole depth as if in actual ignition, without any shadow or dark
side, must be looked upon as dense and massive bodies compared with
the filmy and all but spiritual texture of a comet. Accordingly,
whenever powerful telescopes have been turned on these bodies, they
have not failed to dispel the illusion which attributes _solidity_ to
that more condensed part of the head which appears to the naked eye
as a nucleus; though it is true that in some a very minute stellar
point _has_ been seen, indicating the existence of something more
substantial.
[Illustration: Fig. 24.—Head of Comet]
That the luminous part of a comet is something in the nature of a
smoke, fog, or cloud, suspended in a transparent atmosphere, is
evident from a fact which has been often noticed, viz., that the
portion of the tail where it comes closest to and surrounds the
head is yet separated from it by an interval less luminous, as if
sustained and kept off from contact by a transparent stratum, as we
often see one layer of clouds over another with a considerable clear
space between. These, and most of the other facts observed in the
history of comets, appear to indicate that the structure of a comet,
as seen in section in the direction of its length, must be that of a
hollow envelope, of a parabolic form, inclosing near its vertex the
nucleus and head, something as represented in the preceding figure.
This would account for the apparent division of the tail into two
principal lateral branches, the envelope being oblique to the line of
sight at its borders, and therefore a greater depth of illuminated
matter being there exposed to the eye. In all probability, however,
they admit great varieties of structure, and among them may very
possibly be bodies of widely different physical constitution, and
there is no doubt that one and the same comet at different epochs
undergoes great changes, both in the disposition of its materials and
in their physical state.
We come now to speak of the motions of comets. These are apparently
most irregular and capricious. Sometimes they remain in sight only
for a few days, at others for many months; some move with extreme
slowness, others with extraordinary velocity; while not infrequently
the two extremes of apparent speed are exhibited by the same comet in
different parts of its course. The comet of 1472 described an arc of
the heavens of 40° of a great circle in a single day. Some pursue a
direct, some a retrograde, and others a tortuous and very irregular
course; nor do they confine themselves, like the planets, within any
certain region of the heavens, but traverse indifferently every part.
Their variations in apparent size, during the time they continue
visible, are no less remarkable than those of their velocity;
sometimes they make their first appearance as faint and slow-moving
objects, with little or no tail; but by degrees accelerate, enlarge,
and throw out from them this appendage, which increases in length
and brightness till (as always happens in such cases) they approach
the sun, and are lost in his beams. After a time they again emerge on
the other side, receding from the sun with a velocity at first rapid,
but gradually decaying. It is for the most part after thus passing
the sun that they shine forth in all their splendor, and that their
tails acquire their greatest length and development; thus indicating
plainly the action of the sun’s rays as the exciting cause of that
extraordinary emanation. As they continue to recede from the sun,
their motion diminishes and the tail dies away, or is absorbed into
the head, which itself grows continually feebler, and is at length
altogether lost sight of, in by far the greater number of cases never
to be seen more.
Without the clew furnished by the theory of gravitation, the enigma
of these seemingly irregular and capricious movements might have
remained forever unresolved. But Newton, having demonstrated the
possibility of any conic section whatever being described about the
sun, by a body revolving under the dominion of that law, immediately
perceived the applicability of the general proposition to the case
of cometary orbits; and the great comet of 1680, one of the most
remarkable on record, both for the immense length of its tail and for
the excessive closeness of its approach to the sun (within one-sixth
of the diameter of that luminary), afforded him an excellent
opportunity for the trial of his theory. The success of the attempt
was complete. From that time it became a received truth, that the
motions of comets are regulated by the same general laws as those of
the planets.
[Illustration: Fig. 25.—Orbit of Newton’s Comet (1680)]
Now calculations lead to the surprising fact, that the comets are by
far the most voluminous bodies in our system. The following are the
dimensions of some of those which have been made the subjects of such
inquiry.
The tail of the great comet of 1680, immediately after its perihelion
passage, was found by Newton to have been no less than 20,000,000 of
leagues in length, and to have occupied only two days in its emission
from the comet’s body! a decisive proof this of its being darted
forth by some active force, the origin of which, to judge from the
direction of the tail, must be sought in the sun itself. Its greatest
length amounted to 41,000,000 leagues, a length much exceeding the
whole interval between the sun and earth. The tail of the comet of
1769 extended 16,000,000 leagues, and that of the great comet of
1811, 36,000,000. The portion of the head of this last, comprised
within the transparent atmospheric envelope which separated it from
the tail, was 180,000 leagues in diameter. It is hardly conceivable
that matter once projected to such enormous distances should ever be
collected again by the feeble attraction of such a body as a comet—a
consideration which accounts for the surmised progressive diminution
of the tails of such as have been frequently observed.
The most remarkable of those comets which have been ascertained
to move in elliptic orbits is that of Halley, so called from the
celebrated Edmund Halley, who, on calculating its elements from its
perihelion passage in 1682, when it appeared in great splendor, with
a tail 30° in length, was led to conclude its identity with the great
comets of 1531 and 1607, whose elements he had also ascertained. The
intervals of these successive apparitions being 75 and 76 years,
Halley was encouraged to _predict_ its reappearance about the year
1759. So remarkable a prediction could not fail to attract the
attention of all astronomers, and, as the time approached, it became
extremely interesting to know whether the attractions of the larger
planets might not materially interfere with its orbital motion. The
computation of their influence from the Newtonian law of gravity, a
most difficult and intricate piece of calculation, was undertaken
and accomplished by Clairaut, who found that the action of Saturn
would retard its return by 100 days, and that of Jupiter by no less
than 518, making in all 618 days, by which the expected return would
happen later than on the supposition of its retaining an unaltered
period—and that, in short, the time of the expected perihelion
passage would take place within a month, one way or other, of the
middle of April, 1759. It actually happened on the 12th of March
in that year. Its next return was calculated by several eminent
geometers, and fixed successively for the 4th, the 7th, the 11th, and
the 26th of November, 1835; the two latter determinations appearing
entitled to the higher degree of confidence, owing partly to the more
complete discussion bestowed on the observations of 1682 and 1759,
and partly to the continually improving state of our knowledge of the
methods of estimating the disturbing effect of the several planets.
The last of these predictions, that of M. Lehmann, was published on
the 25th of July. On the 5th of August the comet first became visible
in the clear atmosphere of Rome as an exceedingly faint telescopic
nebula, within a degree of its place as predicted by M. Rosenberger
for that day. On or about the 20th of August it became generally
visible, and, pursuing very nearly its calculated path among the
stars, passed its perihelion on the 16th of November; after which,
its course carrying it south, it ceased to be visible in Europe,
though it continued to be conspicuously so in the Southern Hemisphere
throughout February, March, and April, 1836, disappearing finally on
the 5th of May.
[Illustration: Fig. 26.—Forms of Cometary Orbits]
Its first appearance, while yet very remote from the sun, was that
of a small round or somewhat oval nebula, quite destitute of tail,
and having a minute point of more concentrated light eccentrically
situated within it. It was not before the 2d of October that the tail
began to be developed, and thenceforward increased pretty rapidly,
being already 4° or 5° long on the 5th. It attained its greatest
apparent length (about 20°) on the 15th of October. From that time,
though not yet arrived at its perihelion, it decreased with such
rapidity that already on the 29th it was only 3°, and on November
the 5th 2½° in length. There is every reason to believe that before
the perihelion, the tail had altogether disappeared, as, though
it continued to be observed at Pulkowa up to the very day of its
perihelion passage, no mention whatever is made of any tail being
then seen.
Reflecting on these phenomena, and carefully considering the evidence
afforded by the numerous and elaborately executed drawings which have
been placed on record by observers, it seems impossible to avoid the
following conclusions: 1st. That the matter of the nucleus of a comet
is powerfully excited and dilated into a vaporous state by the action
of the sun’s rays, escaping in streams and jets at those points of
its surface which oppose the least resistance, and in all probability
throwing that surface or the nucleus itself into irregular motions
by its reaction in the act of so escaping, and thus altering its
direction.
2. That this process chiefly takes place in that portion of the
nucleus which is turned toward the sun; the vapor escaping chiefly in
that direction.
3. That when so emitted, it is prevented from proceeding in the
direction originally impressed upon it by some force directed _from_
the sun, drifting it back and carrying it out to vast distances
behind the nucleus, forming the tail or so much of the tail as can
be considered as consisting of material substance.
4th. That this force, whatever its nature, acts unequally on the
materials of the comet, the greater portion remaining unvaporized,
and a considerable part of the vapor actually produced remaining in
its neighborhood, forming the head and coma.
5th. That the force thus acting on the materials of the tail can not
possibly be identical with the ordinary gravitation of matter, being
centrifugal or repulsive, as respects the sun, and of an energy very
far exceeding the gravitating force toward that luminary. This will
be evident if we consider the enormous velocity with which the matter
of the tail is carried backward, in opposition both to the motion
which it had as part of the nucleus and to that which it acquired in
the act of its emission, both which motions have to be destroyed in
the first instance, before any movement in the contrary direction can
be impressed.
6th. That unless the matter of the tail thus repelled from the sun
be retained by a peculiar and highly energetic attraction to the
nucleus, differing from and exceptional to the ordinary power of
gravitation, it must leave the nucleus altogether; being in effect
carried far beyond the coercive power of so feeble a gravitating
force as would correspond to the minute mass of the nucleus; and
it is therefore very conceivable that a comet may lose, at every
approach to the sun, a portion of that peculiar matter, whatever it
be, on which the production of its tail depends, the remainder being
of course less excitable by the solar action, and more impassive to
his rays, and therefore, _pro tanto_, more nearly approximating to
the nature of the planetary bodies.
7th. That, considering the immense distances to which at least some
portion of the matter of the tail is carried from the comet, and
the way in which it is dispersed through the system, it is quite
inconceivable that the whole of that matter should be reabsorbed—that
therefore it must lose during its perihelion passage some portion
of its matter, and if, as would seem far from improbable, that
matter should be of a nature to be repelled from, not attracted by,
the sun, the remainder will, by consequence, be, _pro quantitate
inertiæ_, more energetically attracted to the sun than the mean of
both. If then the orbit be elliptic, it will perform each successive
revolution in a shorter time than the preceding, until, at length,
the whole of the repulsive matter is got rid of.
[Illustration: Fig. 27.—Halley’s Comet]
Besides the comet of Halley, several other of the great comets
recorded in history have been surmised with more or less probability
to return periodically, and therefore to move in elongated ellipses
around the sun. Such is the great comet of 1680, whose period is
estimated at 575 years, and which has been considered, with at least
a high _prima facie_ probability, to be identical with a magnificent
comet observed at Constantinople and in Palestine, and referred by
contemporary historians, both European and Chinese, to the year A.
D. 1106; with that of A. D. 531, which was seen at noonday close to
the sun; with the comet of 43 B. C., already spoken of as having
appeared after the death of Cæsar, and which was also observed in
the daytime; and finally with two other comets, mention of which
occurs in the Sibylline Oracles, and in a passage of Homer, and
which are referred, as well as the obscurity of chronology and
the indications themselves will allow, to the years 618 and 1194
B. C. It is to the assumed near approach of this comet to the
earth, about the time of the Deluge, that Whiston ascribed that
overwhelming tide-wave to whose agency his wild fancy ascribed that
great catastrophe—a speculation, it is needless to remark, purely
visionary. These coincidences of time are certainly remarkable,
especially when it is considered how very rare are the appearances of
comets of this class. Professor Encke, however, has discussed, with
all possible care, the observations recorded of the comet of 1680,
taking into consideration the perturbations of the planets (which are
of trifling importance, by reason of the great inclination of its
orbit to the ecliptic), and his calculations show that no elliptic
orbit, with such a period as 575 years, is competent to represent
them within any probable or even possible limits of error, the most
probable period assigned by them being 8814 Julian years. Independent
of this consideration, there are circumstances recorded of the comet
of A. D. 1106 incompatible with its motion in any orbit identical
with that of the comet of 1680, so that the idea of referring all
these phenomena to one and the same comet, however seducing, must be
relinquished.
Another great comet, whose return about the year 1848 had been
considered by more than one eminent authority in this department of
astronomy highly probable, is that of 1556, to the terror of whose
aspect some historians have attributed the abdication of the Emperor
Charles V. This comet is supposed to be identical with that of 1264,
mentioned by many historians as a great comet, and observed also in
China.
In 1661, 1532, 1402, 1145, 891, and 243 great comets appeared—that
of 1402 being bright enough to be seen at noonday. A period of 129
years would conciliate all these appearances, and should have brought
back the comet in 1789 or 1790 (other circumstances agreeing).
That no such comet was observed about that time is no proof that
it did not return, since, owing to the situation of its orbit, had
the perihelion passage taken place in July it might have escaped
observation.
We come now, however, to a class of comets of short period,
respecting whose return there is no doubt, inasmuch as two at
least of them have been identified as having performed successive
revolutions round the sun; have had their return predicted already
several times; and have on each occasion scrupulously kept to their
appointments. The first of these is the comet of Encke, so called
from Professor Encke of Berlin, who first ascertained its periodical
return. It revolves in an ellipse of great eccentricity (though not
comparable to that of Halley’s), the plane of which is inclined at an
angle of about 13° 22′ to the plane of the ecliptic, and in the short
period of 1,211 days, or about 3⅓ years. This remarkable discovery
was made on the occasion of its fourth recorded appearance, in 1819.
From the ellipse then calculated by Encke, its return in 1822 was
predicted by him, and observed at Paramata, in New South Wales,
by M. Rümker, being invisible in Europe: since which it has been
repredicted and reobserved in all the principal observatories, both
in the Northern and Southern Hemispheres, as a phenomenon of regular
occurrence.
Another comet of short period is that of _Biela_, so called from
M. Biela of Josephstadt, who first arrived at this interesting
conclusion on the occasion of its appearance in 1826. It is
considered to be identical with comets which appeared in 1772, 1805,
etc., and describes its very eccentric ellipse about the sun in 2,410
days, or about 6¾ years; and in a plane inclined 12° 34′ to the
ecliptic. It appeared again, according to the prediction, in 1832 and
in 1846.