forms itself into concentric luminous arcs, separated by comparatively
dark intervals. These luminous semi-circles vary in number, but
sometimes there are as many as four or five at a time. All great comets
show these concentric curves more or less, but sometimes only a portion
is visible, the rest of the coma having a different structure. When
great comets approach near the Sun, their coma is generally composed of
two distinct parts, an inner and an outer coma, the inner one being due
to the luminous jets issuing from the nucleus, which, never extending
very far, form a distinct, bright zone within the fainter exterior coma.
The tails of comets, which are in fact a prolongation of the coma, are
likewise extremely variable in form. They are sometimes straight like a
rod; again, are curved like a sabre, or even crooked like an S, as was
that of the comet of 1769. They are also fan-shaped, pointed, or of the
same width throughout. Many of these appendages appear longitudinally
divided through their middle by a narrow, darkish rift, extending from
the nucleus to the extremity. This peculiarity appears in the comet
shown on Plate XI. Sometimes the dark rift does not commence near the
nucleus, but at some distance from it, as I observed in the case of
comet 1881, III., on June 26th. This dark rift is not a permanent
feature of a comet's tail, but may be visible one day and not at all the
next. Comet 1881, III., which had shown a dark rift towards the end of
June, did not exhibit any such rift during July and August, when, on the
contrary, its tail appeared brighter in the middle. Coggia's comet,
which showed so prominent a dark rift in July, 1874, had none on June
10th. On the contrary, the tail was on that date very bright along its
middle, as also along each of its edges.
The tail of a comet does not invariably point directly away from the
Sun, as above mentioned, and sometimes the deviation is considerable;
for instance, the tail of the comet of 1577 deviated 21° from the point
opposite to the Sun.
In general, the tail inclines its extremity towards the regions of space
which it has just left, always presenting its convex border to the
regions towards which it is moving. It is also a remarkable fact that
this convex border, moving first in space, always appears brighter and
sharper than the opposite one, which is often diffused. From these
peculiarities it would seem that in moving about the Sun the comets
encounter some resistance to their motion, from the medium through which
they pass, and that this resistance is sufficient to curve their tails
away from the course in which they move, and to crowd their particles
together on the forward side. It is especially when they approach their
perihelion, and move more rapidly on a curve of a shorter radius, that
the comets' tails show the greatest curvature, unless their position in
regard to the observer prevents their being advantageously seen. The
tail of Donati's comet presented a fair illustration of this
peculiarity, its curvature having augmented with the velocity of the
comet's motion about the Sun. But possibly this phenomenon has another
cause, and may be found rather in the solar repulsion which acts on
comets and is not instantaneously propagated throughout their mass.
Although, in general, comets have but one tail, it is not very rare to
see them with multiple tails. The comets of 1807 and 1843 had each a
double tail; Donati's comet, in 1858, showed several narrow, long
rectilinear rays, issuing from its abruptly curved tail. The comet of
1825 had five branches, while that of 1744 exhibited no less than six
distinct tails diverging from the coma at various angles. In general
character the multiple and single tails are similar. When a comet has
two tails, it is not rare for the second to extend in the general
direction of the Sun, as was the case with the great comet of 1881,
III., represented on Plate XI. From July 14th to the 21st it exhibited
quite an extended conical tail, starting obliquely downwards from the
right side of the coma, and directed towards the Sun. From the 24th of
July to the 2d of August this secondary tail was exactly opposite in its
direction from that of the primary tail, and gave to the head a very
elongated appearance. Comet 1881, IV., also exhibited a secondary
appendage, not directed towards the Sun, but making an angle of about
45° with the main tail.
These cometary appendages sometimes attain prodigious dimensions. The
comets of 1680 and 1769 had tails so extended that, after their heads
had set under the horizon, the extremities of these immense appendages
were still seen as far up as the zenith. In a single day the tail of the
comet of 1843 extended 100°, and it was thrust from the comet "as a
dart of light" to the enormous distance of 48,500,000 miles, and yet of
this immense appendage nothing was left on the following day. The tail
of Donati's comet, in 1858, attained a real length of 42,000,000 miles,
while that of the great comet of 1843 had the enormous length of
200,000,000 miles. If this last comet had occupied the position of the
Sun, which it approached very nearly for a moment, the extremity of its
tail would have extended 60,000,000 miles beyond the orbit of Mars.
In some cases the tails of comets have been seen undulating and
vibrating in a manner similar to the undulations and coruscations of
light characteristic of some auroras. Many observers report having seen
such phenomena. The comet of 1769 was traversed by luminous waves and
pulsations, comparable to those seen in the aurora borealis. I myself
observed these curious undulations in Coggia's comet in 1874, while the
head of this object was below the horizon. For an hour the undulations
rapidly succeeded each other, and ran along the whole length of the
tail.
Some of the brightest comets have shone with such splendor that they
could be observed easily in full sunshine. Many comets, such as those of
1577 and 1744, have equaled Sirius and Venus in brilliancy. The great
comet of 1843, which suddenly appeared in our sky, was so brilliant that
it was seen by many observers at noon time, within a few degrees from
the Sun. I remember that I myself saw this remarkable object in the day
time, with a number of persons, who were gazing at the wonderful
apparition. So brilliant was this comet, that besides its nucleus and
head, a portion of its tail was also visible in the day time, provided
the observer screened his eyes from the full sunlight by standing in the
shadow of some building.
Of all the bodies revolving around the Sun, none have been known to
approach so near its surface as did the comet of 1843. When it arrived
at perihelion, the distance from the centre of its nucleus to the
surface of the Sun's photosphere was only 96,000 miles, while the
distance from surface to surface was less than 60,000 miles. This comet,
then, went through the solar atmosphere, and in traversing it with its
tremendous velocity of 366 miles per second, may very possibly have
swept through some solar protuberances, many of which attain much higher
elevations than that at which the comet passed. The comet of 1680 also
approached quite near the surface of the Sun, and near enough to
encounter some of the high solar protuberances, its distance at
perihelion being about two-thirds of the Moon's distance from the Earth.
The rapidity of motion of the comet of 1843 was such, when it approached
the Sun, that it swept through all that part of its orbit which is
situated north of the plane of the ecliptic in a little more than two
hours, moving in this short time from one node to the other, or 1800.
But if some comets have a very short perihelion distance, that of others
is considerable. Such a comet was that of 1729, whose perihelion
distance was 383,000,000 miles, the perihelion point being situated
between the orbits of Mars and Jupiter.
While some comets come near enough to the Sun at perihelion to be
volatilized by its intense heat, others recede so far from it at
aphelion that they may be said to be frozen. The shortest cometary
aphelion distance known is that of Encke's comet, whose greatest
distance from the sun is 388,000,000 miles. But that of the comet of
1844 is 406,000,000,000 miles from the Sun. The comets of 1863 and 1864
are so remote in space when they reach their aphelion points that light,
with its velocity of 185,500 miles a second, would require 171 days in
the first case, and 230 in the last, to pass from them to the Earth.
The period of revolution of different comets also varies immensely.
While that of Encke's comet is only 3½ years, that of comet 1864, II.,
is 280,000 years.
Among the periodic comets of short period, some have exhibited highly
interesting phenomena. Encke's comet, discovered in 1818, is remarkable
for the fact that its period of revolution diminishes at each of its
successive returns, and consequently this comet, with each revolution,
approaches nearer and nearer to the Sun. The decrease of the period is
about 2½ hours at each return. Although the decrease is small, if it go
on in future as it does at present, the inevitable consequence will be
that this comet will finally fall into the Sun. This curious phenomenon
of retardation has been attributed by astronomers to the existence of a
resisting medium filling space, but so rare and ethereal that it does
not produce any sensible effect on the movements of the planets. But
some other causes may retard this comet, as similar retardations have
not been observed in the case of other periodic comets of short period.
These, however, are not so near to the Sun, and perhaps our luminary may
be surrounded by matter of extreme tenuity, which does not exist at a
greater distance from it.
Another of the periodic comets which has exhibited a very remarkable
phenomenon of transformation is Biela's comet, which divided into two
distinct parts, moving together in the same direction. When this comet
was first detected at its return in 1845, it presented nothing unusual,
but in the early part of 1846 it was noticed by several astronomers to
be divided into two parts of unequal brightness, forming thus a twin
comet. At its next return in 1852, the two sister comets were still
traveling in company, but their distance apart, which in 1846 was
157,000 miles, had increased to 1,500,000 miles. At the two next returns
in 1859 and 1865, their position not being very favorably situated for
observation, the comets were not seen. In 1872 the position should have
been favorable for observation, and they were consequently searched for,
but in vain; neither comet was found. An astronomer in the southern
hemisphere, however, found a comet on the track of Biela's, but
calculation has shown that the two objects are probably not identical,
since this comet was two months behind the computed position for
Biela's. It will be shown in the following chapter that our globe
probably crossed the orbit of Biela's comet on November 27th, 1872, and
the phenomena resulting from this passage will be there described.
It is seen from these observations that comets may be lost or dissipated
in space by causes entirely unknown to us. Biela's comet is not the only
one which has been thus disintegrated. Ancient historians speak of the
separation of large comets into two or more parts. In 1661 Hevelius
observed the apparent division of the comet of that year and its
reduction to fragments. The return of this comet, calculated for 1790,
was vainly waited for; the comet was not seen.
Other comets, whose periods of revolution were well known, have
disappeared, probably never to return. Such is Lexell's comet, whose
period was 5⁶⁄₁₀ years; also De Vico's comet, both of which are
now lost. It is supposed that Lexell's comet, which passed twice very
near the giant planet Jupiter, had its orbit changed from an ellipse to
a parabola, by the powerful disturbing influence of this planet, and was
thus lost from our system. Several other comets, in traveling over their
different orbits, have approached near enough to Saturn, Jupiter and the
Earth to have their orbits decidedly altered by the powerful attraction
of these bodies.
But since comets are liable to pass near the planets, and several have
orbits which approach that of the Earth, it becomes important for us to
know whether an encounter of such a body with our globe is possible, and
what would then be the result for us. Although that knowledge would not
enable us to modify the possibilities of an encounter, yet it is better
to know the dangers of our navigation through space than to ignore them.
This question of a collision of the Earth with a comet has been answered
in different ways, according to the ideas entertained in regard to the
mass of these bodies. While some have predicted calamities of all kinds,
such as deluges, conflagrations, or the reduction of the Earth to
incandescent gases, others have asserted that it would produce no more
effect than does a fly on encountering a railroad train. In our days
astronomers entertain very little fears from such an encounter, because
the probabilities of danger from an occurrence of this sort are very
slight, the mass of an ordinary comet being so small compared with that
of our globe. We know with certainty that the Earth has never had an
encounter with a comet _by which it has been transformed into gases_, at
least within the several millions of years during which animal and
vegetable life have left their marks upon the stony pages of its
history, otherwise these marks would not now be seen. If, then, such an
accident has not happened during this long period, the chances for its
occurring must be very small, so small indeed that they might almost be
left out of the question. It is true that our globe shows signs of great
perturbations of its surface, but we have not the slightest proofs that
they resulted from an encounter with a celestial body. It seems very
probable that our globe passed through the tail of the comet of 1861,
before it was first seen on June 29th; but nothing unusual was observed,
except perhaps some phosphorescent light in the atmosphere, which was
afterwards attributed to this cause.
The density and mass of comets must be comparatively very small. Their
tails consist of matter of such extreme tenuity that it affects but very
little the light of the small stars over which they pass. The coma and
nucleus, however, are not quite so transparent, and may have greater
masses. On several occasions I have seen the light of stars reduced by
the interposition of cometary matter, comet 1881, III., presenting
remarkable cases of this sort. On July 8th, at 10h. 50m., several small
stars were involved in this comet, one of which passed quite near the
nucleus through the bright inner coma. At that time the comet was
greatly disturbed, its nucleus was contracting and enlarging rapidly,
and becoming bright and again faint in an instant. Every time that the
nucleus grew larger, the star became invisible, but reappeared the
moment the nucleus was reduced in size. This phenomenon could not be
attributed to an atmospheric effect, since, while the nucleus was
enlarging, a very small inner nucleus was visible within the large
diffused one, the matter of which had apparently spread over the part of
the coma in which the star was involved, making it invisible.
That the mass of comets is small, is proved by the fact that they have
sometimes passed near the planets without disturbing them in any
sensible manner. Lexell's comet, which in 1770 remained four months very
near Jupiter, did not affect in the least the orbits, or the motions of
its satellites. The same comet also came within less than 1,500,000
miles from the Earth, and on this occasion it was calculated that its
mass could not have been the ¹⁄₅₀₀₀ part of that of our
globe, since otherwise the perturbations which it would have caused in
the elements of the Earth's orbit would have been sensible. There was,
however, no change. If this comet's mass had been equal to that of our
globe, the length of our year would have been increased by 2h. 47m. The
comet of 1837 remained four days within 3,500,000 miles of the Earth,
with no sensible effect.
It seems quite difficult to admit that the denser part of a comet
forming the nucleus is solid, as supposed by some physicists, since it
is so rapidly contracted and dilated by the solar forces, while the
comet is yet at a too great distance from the Sun to allow these effects
to be attributed to solar heat alone. This part of a comet, as indeed
the other parts, seems rather to be in the gaseous than in the solid
state; the changes observed in the intensity of its light and in its
structure may be conceived as due to some solar action partaking of the
nature of electricity.
It has been a question whether comets are self-luminous, or whether they
simply reflect the solar light. When their light is analyzed by the
spectroscope, it is found that the nucleus of a comet generally gives a
continuous spectrum, while the coma and tail give a spectrum consisting
of several bright diffused bands. The spectrum given by the nucleus is
rarely bright enough to allow the dark lines of the solar spectrum to be
discerned upon it; but such lines were reported in the spectrum of comet
1881, III., a fact proving that this nucleus at least reflected some
solar light. The nucleus of a comet may be partly self-luminous, and
either solid, liquid, or composed of incandescent gases submitted to a
great pressure. As to the coma and tail, they are evidently gaseous, and
partly, if not entirely, self-luminous, as is proved by the band
spectrum which they give. The position of these bands, moreover,
indicates that the luminous gases of which they are composed contain
carbon. The phenomena of polarization, however, seem to prove that these
parts of comets also reflect some solar light.
No theory so far proposed, to explain comets and the strange phenomena
they exhibit, seems to have been successful in its attempts, and the
mystery in which these bodies have been involved from the beginning of
their apparition, seems to be now nearly as great as ever. It has been
supposed that their tails have no real existence, but are due to an
optical illusion. Prof. Tyndall has endeavored to explain cometary
phenomena by supposing these bodies to be composed of vapors subject to
decomposition by the solar radiations, and thus made visible, the head
and tail being an actinic cloud due to such decompositions. According to
this view, the tails of comets would not consist of matter projected
into spacer but simply of matter precipitated by the solar rays in
traversing the cometary nebulosity. The endeavor has also been made to
explain the various phenomena presented by comets by an electrical
action of the Sun on the gases composing these objects. Theories taking
this as a base seem to us to be more likely to lead to valuable results.
M. Faye, who has devoted much time and learning to this subject, assumes
a real repulsive force of the Sun, acting inversely to the square of the
distance and proportionally to the surface, and not to the mass as
attraction does. He supposes, however, that this repulsive force is
generated by the solar heat, and not by electricity. Prof. Wm. Harkness
says that many circumstances seem to indicate that the comets' tails are
due, in a great measure, to electrical phenomena.
The fact that the tails of comets are better defined and brighter on the
forward side, associated with the other fact that they curve the most
when their motion is most rapid, sufficiently indicates that these
appendages are material, and that they either encounter some resistance
from the medium in which they move, or from a solar repulsion. The
phenomena of condensation and extension, which I have observed in the
comets of 1874 and 1881, added to the curious behavior exhibited by the
jets issuing from the nucleus, seem to indicate the action of electrical
forces rather than of heat. The main difficulty encountered in the
framing of a theory of comets consists in explaining how so delicate and
extended objects as their tails seem to be, can be transported and
whirled around the Sun at their perihelion with such an enormous
velocity, always keeping opposite to the Sun, and, as expressed by Sir
John Herschel, "in defiance of the law of gravitation, nay, even of the
received laws of motion."
To consider the direction of the comets' tails as an indirect effect of
attraction, seems out of the question; the phenomenon of repulsion so
plainly exhibited by these objects seems to point to a positive solar
repulsion, as alone competent to produce these great changes. The
repulsive action of the Sun on comets' tails might be conceived, for
instance, as acting in a manner similar to that of a powerful current of
wind starting from the Sun, and constantly changing in direction, but
always keeping on a line with the comet. Such a current, acting on a
comet's tail as if it were a pennant, would drive it behind the nucleus
just as observed. If it could once be ascertained that the great
disturbances on comets correspond with the magnetic disturbances on our
globe and with the display of the auroral light, the electric nature of
the forces acting so strangely on the comets would be substantially
demonstrated. I have shown that some of the great disturbances observed
in the comets of 1874 and 1881 have coincided with auroral displays, and
it will be shown hereafter that similar displays have also coincided
with the passage of meteoric showers through our atmosphere. Whether
these simultaneous phenomena were simple coincidences having no
connection, or whether they are the result of a common cause, can only
be ascertained by long continued future observations.
SHOOTING-STARS AND METEORS
PLATE XII
While contemplating the heavens on a clear moonless night, we
occasionally witness the sudden blazing forth of a star-like meteor,
which glides swiftly and silently across some of the constellations, and
as suddenly disappears, leaving sometimes along its track a
phosphorescent trail, which remains visible for a while and gradually
vanishes. These strange apparitions of the night are called _Falling_ or
_Shooting-stars_.
There is certainly no clear night throughout the year during which some
of these meteors do not make their appearance, but their number is quite
variable. In ordinary nights only four or five will be observed by a
single person in the course of an hour; but on others they are so
numerous that it becomes impossible to count them. When the falling
stars are only a few in number, and appear scattered in the sky, they
are called _Sporadic Meteors_, and when they appear in great numbers
they constitute _Meteoric Showers_ or _Swarms_.
Probably there is no celestial phenomenon more impressive than are these
wonderful pyrotechnic displays, during which the heavens seem to break
open and give passage to fiery showers, whose luminous drops describe
fantastic hieroglyphics in the sky. While observing them, one can fully
realize the terror with which they have sometimes filled beholders, to
whom it seemed that the stability of the universe had come to an end,
and that all the stars of the firmament were pouring down upon the Earth
in deluges of fire.
The ancients have left record of many great meteoric displays, and the
manner in which they describe them sufficiently indicates the fear
caused by these mysterious objects. Among the many meteoric showers
recorded by ancient historians may be mentioned one observed in
Constantinople, in the month of November, 472, when all the sky appeared
as if on fire with meteors. In the year 599, meteors were seen on a
certain night flying in all directions like fiery grasshoppers, and
giving much alarm to the people. In March, 763, "the stars fell
suddenly, and in such crowded number that people were much frightened,
and believed the end of the world had come." On April 10th, 1095, the
stars fell in such enormous quantity from midnight till morning that
they were as crowded as are the hail stones during a severe storm.