CHAPTER III.COMETS, OLD AND NEW.
As if moved by some unconscious presentiment of his
future destiny, Professor Palmyrin Rosette had always
evidenced a strong predilection for the study of comets.
He had based his opinions on the best authorities, and was
never more in his element than when he was expatiating
on his favourite theme as he presided at some astronomical
conference.
“Comets, gentlemen,” he would say, “are nebulous
bodies which occasionally appear in the heavens, consisting
ordinarily of a bright central light called the nucleus and
in the more conspicuous cases accompanied by a long trail
of light called the tail. Owing to the great eccentricity of
their orbits, they are visible to the earth during only a
portion of their course.”
The professor never failed to point out the two characteristics
by which they were to be distinguished from other
heavenly bodies:
“Although these comets, gentlemen, may be deficient
either with respect to the luminous tail or to the nebulous
coma, the progressive motion with which they are endued
prevents them from ever being mistaken for fixed stars,
while the extreme length of the ellipses which they
describe makes it impossible to confound them with
planets,”
During the long years of the astronomer's application
to his fascinating study, he had composed an elaborate
treatise, exhibiting the results of all his investigations, and
when, after the sudden convulsion, he found himself
actually upon the surface of one of the very bodies the
properties of which had engrossed so much of his interest,
it was necessarily a disappointment to feel that, alone upon
Formentera, he had no audience to whom he could address
himself.
The treatise which Rosette had compiled had been
arranged under four distinct heads:
1. The number of comets.
2. Periodic and non-periodic comets.
3. The probability of collision between a comet and
the earth.
4. The consequences of such a collision.
First: with respect to the number of comets, the
professor had recorded that, according to Arago, who
grounded his estimate on the number that revolve between
Mercury and the sun, there are at least 17,000,000 of these
luminous bodies in our solar system; whilst Lambert asserts
that within the orbit of Saturn, that is, within a radius of
872,135,000 miles, there are no less then 500,000,000.
According to Kepler, two hundred years previously, the
number of comets can only be compared to the fishes in
the sea, and in following out his simile he declares that an
angler throwing out his line from the surface of the sun
could not fail to touch several of them; and now in recent
times a computation has been made that their aggregate
reaches a total of 74,000,000,000 distinct comets. The
truth seems to be that their number really sets all calculation
at defiance; so erratic, moreover, are their movements,
that they sometimes pass from system to system, and
whilst some, entirely escaping the influence of the sun,
vanish, to find a new centre of attraction, others never before
observed make their appearance upon the terrestrial
horizon.
Even the comets which belong exclusively to our own
system are by no means exempt from strange irregularities;
the orbits of several, ceasing to be ellipses, have become
parabolas or hyperbolas; and the planets, Jupiter in particular,
have been observed to exercise a large disturbing
action upon their paths.
Secondly: under the head of periodic and non-periodic
comets, Professor Rosette had stated that as many as 500
or 600 comets have been made objects of careful astronomical
investigation; those being called “periodic” of which
the return at fixed intervals has been established as a
certainty; those, on the other hand, being classed as “non-periodic”
which recede to such immeasurable distances
from the sun that it cannot be determined whether they
will return or not.
Of the periodic comets there are not more than forty
of which the times of their revolution have been ascertained
with exact precision; but of these there are ten, generally
known as the “short-period comets,” the movements of
which have been established with the nicest accuracy.
The short-period comets are respectively called by the
names of their discoverers, and are commonly distinguished
as Halley's comet, Enckes, Gambart's or Biela's, Faye's,
Brörsen's, D'Arrest's, Tattle's, Winnecke's, De Vico's, and
Tempel's.
Subjoined is a brief account of each of these in detail.
Halley's comet is that which has been the longest
known. It is supposed to be identical with the one which
was observed in the years 134 and 52 B.C., and afterwards
in the years 400, 855, 930, 1006, 1230, 1305, 1380, 1456,
1531, 1607, 1682, 1759, and 1835 A.D. It revolves from
east to west, in a direction contrary to the planets. The
intervals between its consecutive appearances vary from 75
to 76 years, according as its course is less or more disturbed
by the attraction of Jupiter and Saturn, which
sometimes influence its course to such an extent as to
make a difference of 200 days in the period of its arrival.
The last appearance of this comet was in 1835, when Sir
John Herschel, at the Cape of Good Hope, a more favourable
station for observation than any in the northern
hemisphere, was able to watch it until the end of March
1836, after which its distance from the earth rendered it
invisible. At its aphelion it is 3,200,000,000 miles from
the sun, that is to say, it is beyond the orbit of Neptune,
but at its perihelion it is less than 57,000,000 miles from
the sun, and consequently is nearer than the planet Venus.
Little did the professor dream, at the time when he
drew up his treatise, that his own Gallia would transport
him to a still closer proximity to the great luminary.
Encke's comet has the shortest period of any, its
revolution being accomplished in about 1205 days, or less
than three years and a half. Unlike Halley's, it moves
as the planets, from west to east. It was observed on the
26th of November, 1818, and a calculation of its elements
proved it to be identical with the comet of 1805. According
to prediction, it was seen again in 1822, and since that
time has never failed in making its appearance at regular
intervals. Its orbit lies within that of Jupiter, and it never
recedes more than 387,000,000 miles from the sun, its
perihelion distance being only 32,000,000 miles, or less
than that of Mercury.
One important observation that has been made with
regard to Encke's comet, places it beyond doubt that the
axis major of its elliptical orbit is gradually diminishing,
and consequently its average distance from the sun is
growing continuously less and less, so that the probability
arises that unless it is previously volatilized by the solar
heat, it may be ultimately absorbed in the sun itself.
Gambart's comet (otherwise known as Biela's) was
noticed in 1772, 1789, 1795, and 1805; but it was not
until the 28th of February, 1826, that its elements were
satisfactorily determined. Its motion is direct, and its
period of revolution 2410 days, or about seven years. At
perihelion it passes 82,000,000 miles from the sun, rather
nearer than the earth; at aphelion it is beyond the orbit
of Jupiter.
A singular phenomenon with regard to Biela's comet
was first observed in the year 1846: it appeared like a
double star, in two distinct fragments, doubtless sundered
by the action of some internal force; these fragments
travelled together at an interval of about 160,000 miles
apart, but at the next appearance in 1852 this interval was
found to be largely increased.
Faye's comet was discovered by him for the first time
on the 22nd of November, 1843. The elements of its
orbit were calculated, and it was predicted that it would
return again in 1851, after a period of 2718 days, or in
about seven years and a half. The prediction was realized;
the comet was visible at the time announced, and has
subsequently appeared at similar intervals. Its motion is
direct At perihelion it is 192,000,000 miles from the sun,
never approaching so near as Mars; at aphelion it is
distant 603,000,000 miles, so that it recedes, like Biela's
comet, beyond the pathway of Jupiter.
Brörsen's comet was discovered on the 26th of February,
1846. Its movement is from west to east; it accomplishes
its revolution in about 2042 days; its perihelion distance
is 64,000,000 miles, its aphelion 537,000,000 miles.
Of the other short-period comets, D'Arrest's, which in
1862 passed within 30,000,000 miles of the planet Jupiter,
completes its revolution in rather more than six years and
a half; Tuttle's revolves in thirteen years and eight
months; Winnecke's and Temple's in about five years and
a half; whilst that of De Vico, after being computed to
revolve in a period of rather more than five years, seems
to have wandered away altogether into space.
Then follows a short enumeration of some of the “long-period”
comets.
The comet of 1556, commonly called the comet of
Charles-Quint, was expected again in 1860, but did not
re-appear.
The comet of 1680 furnished the data for Newton's
cometary theories, and, according to Whiston, was the cause
of the deluge, on account of its close approximation to the
earth. Its revolution takes about 575 years, so that it was
visible in 1106 and 531, as well as in 43 B.C. and probably
in 619 B.C. At its perihelion it passes so near the sun that
it receives 28,000 times more heat than the earth, that
is, it is 2000 times hotter than molten iron.
The comet of 1744 was by far the most brilliant of the
eighteenth century; it was seen on the 1st of March in
full daylight, and had six tails, spread out like a fan across
a large space in the heavens.
The great comet of 1811, which has caused the year of
its appearance to be familiarly recognised as “the comet-year,”
had a nucleus 2637 miles in diameter; its head
was 1,270,000 miles in diameter, and its tail 100,000,000
miles in length.
The comet of 1843, observed by Cassini, has been supposed
to be identical with that of 1668, 1494, and 1317,
but astronomers are not agreed upon the period of its
revolution. At its perihelion it passes nearer to the sun
than any other comet recorded in history, travelling at a
rate of more than 40,000 miles a second. The heat that it
thus receives is equal to that which 47,000 suns would
communicate to the earth, and to such a degree does this
prodigious temperature increase its density, that at its last
appearance its tail was visible in broad daylight.
Donati's comet, which in 1858 shone with such brilliancy
amongst the northern constellations, has a mass that has
been estimated at .07 of that of the earth.
The comet of 1862 was adorned with luminous tufts
or aigrettes, and resembled some fantastic mollusk.
The list is completed by the comet of 1868, the revolution
of which occupies a period of no less than 2800
centuries, so that it may practically be considered as
having vanished in infinite space.
Thirdly: the next section of the professor's dissertation
was devoted to the probability of a collision between any
one of these numerous comets and the earth.
As represented in plane diagrams, the orbits of planetary
and cometary bodies appear continually to be intersecting
one another; but in free space of three dimensions
this is by no means necessarily the case; the planes of the
orbits being inclined at various angles to the ecliptic, which
is the plane of the terrestrial orbit. Nevertheless, out of the
large number of comets, is it impossible that one of them
should come in contact with the earth?
In conducting this investigation, it had to be recollected
that as the earth never leaves the plane of the ecliptic,
three conditions must be fulfilled in order to bring about
the result of impact: first, the comet must meet the earth
in the ecliptic; secondly, the earth and the comet must
arrive at the point of intersection of their orbits at the
same moment; and thirdly, the distance between the
centres of the bodies themselves must be less than the sum
of their radii. The problem, therefore, resolved itself into
an inquiry whether these three conditions could occur
simultaneously.
Laplace did not reject the possibility of such an encounter,
and in his “Exposition du Système du Monde”
has at some length detailed the consequences. Arago,
when asked his opinion on the subject, replied that by
calculation there were 280,000,000 chances to 1 against
a collision. The illustrious astronomer, however, based his
estimate upon two conditions that are only fulfilled with
the greatest uncertainty; in the first place, that at perihelion
the comet should be nearer the sun than the earth
is; and in the next, that the diameter of the comet should
be equal to one-fourth of that of the earth. On the other
hand, he only reckoned for the earth coming in contact
with the actual nucleus, whilst if the whole extent of the
nebulosity were to be taken into account, the chances of
collision would be increased tenfold.
In enunciating his problem, Arago adds:
“If we take it for granted that the result of a comet
running foul of the earth would be the total annihilation of
the human race, then the risk of death which each individual
incurs from the probability of such a catastrophe
is just what would be his chance of drawing, at the first
draw, the only white ball out of an urn containing
280,000,000 coloured ones.” So remote appear the chances
of collision.
All astronomers, moreover, concur in distinctly denying
that any such collision has ever happened Arago asserts
that if it had happened, the consequences would have been
an immediate alteration in the earth s axis of rotation,
and a general disturbance of terrestrial latitudes; but he
alleges no evidence in proof of his assertion. He speaks,
however, much more to the purpose when he declares that
“the theory held by some, that the depression of the
Caspian Sea 300 feet below the level of the ocean is to be
attributed to the shock of a comet, is utterly untenable.”
But the matter under consideration was not whether
collision had ever occurred, but whether it ever could
occur.
Now in 1832, at the re-appearance of Gambart's comet,
the world was thrown Into some alarm because it was
announced as the result of astronomical calculations, that
at the time of the passage of the comet through its descending
node on the 29th of October, the earth would be
travelling precisely in the same region. Contact seemed
not only probable but inevitable, if Olbers' observation was
correct, that the radius of the comet was five times as large
as that of the earth. Happily, however, the earth did not
arrive at that point of the ecliptic until the 30th of
November, by which time the comet was more than
50,000,000 miles away. But supposing that the earth had
reached that place of intersection of the two orbits a
month sooner, or the comet a month later, it is hard to
say what could have obviated the likelihood of collision.
At the very least, some singular perturbations must have
ensued. In 1805 Indeed, this identical comet had passed
within 6,000,000 miles of the earth, ten times closer than
in 1832, but as its proximity was unknown, the fact did
not excite any panic.
Again in 1843 there seemed reasonable ground for fear
that the atmosphere of the earth would be vitiated by
passing through the nebulous tail of a comet 150,000,000
miles in length.
Altogether, therefore, from the entire evidence, it appeared
a necessary inference that collision between the
earth and a comet was by no means impossible.
Fourthly, then, Professor Rosette had to discuss the
remaining question to bring his treatise to a close; as to
the probable consequences of such a collision.
These consequences would manifestly vary according
as the comet had or had not a nucleus. As some fruits
have no kernel, so some comets have no nucleus, and such
is the tenuity of their substance, that stars of the tenth
magnitude have been seen through them without any
sensible diminution of light. It is a property that must
make their external form very susceptible of change, and
tends in a degree to make them difficult of recognition.
The same transparency characterises the tail, the development
of which is apparently due entirely to the evaporation
of the coma under the action of solar heat; in proof
of which it is notified that no tail, either single or multiple,
has ever been found attached to a comet until that comet
has arrived within 80,000,000 miles of the sun; whilst it
has been observed that some comets, presumably composed
of denser structure, have emitted no tail at all.
In the case of the earth coming into contact with a
comet destitute of a nucleus, there would be no violent
collision; strictly speaking, there would be no shock at
all. The astronomer Faye asserts that a cannon ball
would find more resistance in a cobweb than in the
nebulous parts of a comet; and for the nebulous matter to
be injurious, it must either be incandescent, in which case
it would scorch up the surface of the earth, or it must be
impregnated with noxious elements, in which case it might
be fatally destructive to life. This latter contingency,
however, is unlikely to arise; for, according to Babinet,
the earth's atmosphere possesses sufficient density of its
own to resist the penetration of any cometary vapours, of
which the tenuity is so slight, that Newton has calculated
that if a comet, without a nucleus, 1,000,000,000 miles in
radius, were reduced to the density of the air at the earth's
surface, it might all be contained in a thimble less than an
inch in diameter.
Concluding thus that from comets purely nebulous
there was a minimum of danger to be apprehended, the
professor proceeded to inquire what would be the result of
concussion if the comet consisted of a solid nucleus.
First of all, however, rises the preliminary question
whether in any case the nucleus of a comet is really solid,
There can be no doubt that if a comet can attain a degree
of concentration sufficient to pass out of its gaseous condition,
it will, if interposed between the earth and a star,
make an occupation of that star. No sound reliance is to
be placed on testimony such as that of Anaxagoras, who,
living in the time of Xerxes, about the year 480 B.C.,
recorded that the sun was eclipsed by a comet; nor on that
of Dion, who maintains that a similar eclipse occurred a
few days before the death of Augustus, which could not be
occasioned by the moon, then in direct opposition. Modern
science has, with more than sufficient justice, entirely
repudiated the accuracy of these statements; but the indisputable
testimony of recent observation all goes to
establish the certainty of the existence of comets with a
solid nucleus. The comets of 1774 and of 1828 are known
to have caused the occupation of stars of the eighth magnitude;
it is admitted on all hands that the comets of
1402, 1532, and 1744 were solid masses; whilst, as for the
comet of 1843, the fact is patent to the world that the
body could be seen close to the sun, in broad daylight, by
the naked eye.
Not only, therefore, do they exist, but in some cases
these solid nuclei have been actually measured. Their
diameters vary considerably in length; that of Gambart's
comet being only 30 or 40 miles, that of the comet of 1845
being 8800 miles, considerably longer than the diameter of
the earth, so that in the event of a collision between the
two bodies, the preponderance would have been on the
side of the comet. The nebulous surroundings have also,
in a variety of instances, been measured, and found to vary
from 200,000 to 1,000,000 miles in diameter.
Upon the whole, modern investigation bears out the
general statement of M. Arago that there are three kind?
of comets; that is to say, comets without any nucleus;
comets with a transparent nucleus; and comets with a
nucleus both solid and opaque.
It had to be borne in mind that without any actual
shock by collision, the mere proximity of a comet to the
earth might entail some very singular phenomena. Not
that from a comet of inferior mass any serious consequences
could be expected, for the comet of 1770, which approached
within 1,600,000 miles of the earth, did not affect
the length of the terrestrial year a single second, although
the action of the earth retarded the period of the comet's
revolution by three whole days. But if the mass of the two
bodies were equal, and if the comet passed within 150,000
miles of the earth, the result would be that the terrestrial
year would be prolonged by sixteen hours and five
minutes, and the obliquity of the ecliptic altered by two
degrees, to say nothing of the chance that the comet
might capture the moon in its passage.
What, finally, would happen in the event of the one
body actually impinging on the other? The consequences,
manifestly, would be far more considerable. Either the
comet, in grazing the earth's surface, would leave behind
it a fragment detached from itself, or it would carry off
with itself a fragment detached from the earth. If, instead
of being oblique, the impact should be direct, there would
at least be a rupture of continents, even if the globe were
not shivered into pieces.
In any case, the tangential velocity of the earth must
receive a sudden check or a sudden impulse; trees, houses,
living creatures, would be precipitated backwards or forwards
with increased momentum; the seas, dashed from
their natural basins, would overwhelm all that lay in the
path of their projection; the central forces of the globe,
still in their normal state of fusion, would be propelled to
the surface; the terrestrial axis would undergo a change
in its direction, so that a new equator would be established,
and as the conditions of equilibrium would be
disturbed, there might be nothing properly to counter-balance
the attraction of the sun, the consequence of
which, by the law of gravity, would be that the earth,
drawn perpetually on in a straight line, in the space of
sixty-four days and a half would be absorbed into the
elements of the great central luminary of the system.
One speculation there was which to the last remained
doubtful; whether, according to Tyndall's theory that
heat is only a form of motion, the velocity of the earth
would not, under the sudden elevation of the temperature,
mechanically transform itself into heat so intense, that
through its action, the earth itself, in the course of a few
seconds, would be completely volatilized.
Such were the deductions of Palmyrin Rosette's
treatise, which he brought to a conclusion by a repetition
of the philosopher's comforting assurance, that the chances
were as 280,000,000 to 1 against the occurrence of any
collision.
How little could the professor, as he tabulated his
scientific notes, anticipate his experiences in the future,
with regard to his own Gallia!
How little could he foresee, that at some future séance,
he would be in the position to say:
“You see, gentlemen, that we have drawn the one
white ball from the urn!”