COMETS.
I suppose that it is the experience of all those who happen to be in any
sense, however humble, specialists in a certain branch of science, that
from time to time, they are beset with questions on the part of their
friends respecting those particular matters which it is known that they
have specially studied. There is no fault to be found with this thirst
for information, always supposing that it is kept within due bounds; but
my motive for alluding to it here, is to see whether any well-marked
conclusion can be drawn from it, within my own knowledge as regards
astronomical facts or events. Now in the case of the science of
astronomy (for which in this connection I, for the moment, will venture
to speak), there is certainly no one department which so unfailingly, at
all times and in all places, seems to evoke such popular sympathy and
interest as the department which deals with Comets.
Sun-spots may come and go; bright planets may shine more brightly; the
Sun or Moon may be obscured by eclipses; temporary stars may burst
forth,—all these things are within the ken of the general public by
means of newspapers or almanacs, but it is a comet which evokes more
questionings and conversations than all the other matters just referred
to put together. When a new and bright comet appears, or even when any
comet not very bright gets talked about, the old question is still fresh
and verdant—“Is there any danger to the Earth to be apprehended from
collision with a Comet?” followed by “What is a Comet?” “What is it made
of?” “Has it ever appeared before?” “Will it come back again?” and so
on. Questions in this strain have more often than I can tell of been put
to me. They seem the stock questions of all who will condescend to
replace for five minutes in the day the newest novel or the pending
parliamentary election.
It may be taken as a fact (though in no proper sense a rule) that a
bright and conspicuous comet comes about once in 10 years, and a very
remarkable comet every 30 years. Thus we have had during the present
century bright comets in 1811, 1825, 1835, 1843, 1858, 1861, 1874 and
1882, whereof those of 1811, 1843, and 1858 were specially celebrated.
Tested then by either standard of words “bright and conspicuous,” or
“specially celebrated,” it may be affirmed that a good comet is now due,
so let us prepare for it by getting up the subject in advance.
I will not attempt to answer in regular order or in any set form the
questions which I have just mentioned as being stock questions, but they
will be answered in substance as we go along. There is one matter in
connection with comets which has deeply impressed itself upon the public
mind, and that is the presence or absence of a “tail.” It is not too
much to say that the generality of people regard the tail of a comet as
_the_ comet; and that though an object may be a true comet from an
astronomer’s point of view, yet if it has no tail its claims go for
nought with the mass of mankind. We have here probably a remnant of
ancient thought, especially of that line of thought which in bygone
times associated Comets universally with the idea that they were
especially sent to be portents of national disasters of one kind or
another. This is brought out by numberless ancient authors, and by none
more forcibly than Shakespeare. Hence we have such passages as the
following in _Julius Cæsar_ (Act ii., sc. 2):—
“When beggars die there are no comets seen,
The Heavens themselves blaze forth the death of princes.”
In _Henry VI._ (Part I., Act i., sc. 1) we find the well-known passage:—
“Comets importing change of times and states
Brandish your crystal tresses in the sky,
And with them scourge the bad revolting stars
That have consented unto Henry’s death.”
There are in point of fact two distinct ideas evolved here: (1) that
comets are prophetic of evil, and (2) stars potential for evil.
There is another passage in _Henry VI_. (Part I., Act iii., sc. 3) even
more pronounced:—
“Now shine it like a Comet of revenge,
A prophet to the fall of all our foes.”
Again; in _Hamlet_ (Act i., sc. 1) we find:—
“As stars with trains of fire, and dews of blood,
Disasters in the Sun.”
Once more; in the _Taming of the Shrew_ (Act iii., sc. 2) we have the
more general, but still emphatic enough, idea expressed by the simple
words of reference to—
“Some Comet or unusual prodigy.”
Shakespeare may be said to have lived at the epoch when astrology was in
high favour, and it may be that he only gave utterance to the current
opinion prevalent among all classes in those still somewhat “Dark Ages”
(so called). This, however, can hardly be said of the author of my next
quotation—John Milton (_Paradise Lost_, bk. II.):—
“Satan stood
Unterrified, and like a Comet burned,
That fires the length of Ophiuchus huge
In th’ Arctic sky, and from its horrid hair
Shakes pestilence and war.”
Jumping over a century we find the ancient theory still in vogue, or
Thomson (_Seasons_, Summer) would never have written:—
“Amid the radiant orbs
That more than deck, that animate the sky,
The life-infusing suns of other worlds;
Lo! from the dread immensity of space,
Returning with accelerated course,
The rushing comet to the sun descends;
And, as he sinks below the shading earth,
With awful train projected o’er the heavens,
The guilty nations tremble.”
Even Napoleon I. had servile flatterers who, as late as 1808, tried to
extract astrological influence out of a comet by way of bolstering up
“Old Bony.” But enough of poetry and fiction, let us hasten back to
prosaic fact.
[Illustration: Fig. 19.—Telescope Comet with a nucleus.]
Comets as objects to look at may be classed under three forms, though
the same comet may undergo such changes as will at different epochs in
its career cause it to put on each variety of form in succession. Thus
the comet of 1825 seen during that year as a brilliant naked-eye object,
after being lost in the sun’s rays, was again found on April 2, 1826 by
Pons. Lamentable were his cries at the miserable plight it was in. He
described it as totally destroyed: without tail, beard, coma or nucleus,
a mere spectre. The simplest form of comet is a mere nebulous mass,
almost always circular, or perhaps a little oval, in outline. It may
maintain this appearance throughout its visibility; or, growing brighter
may become a comet of the second class, with a central condensation,
which developing becomes a “nucleus” or head. It may retain this feature
for the rest of its career, or may pass into the third class and throw
out a “coma” or beard, which will perhaps develop into a tail or tails.
Doing this it will not unfrequently grow bright enough and large enough
to become visible to the naked eye. In exceptional cases the nucleus
will become as bright as a 2nd or even 1st magnitude star, and the tail
may acquire a length of several or many degrees. In the last named case
of all the comet becomes, _par excellence_ according to the popular
sentiment, “a comet.” It will now be readily inferred that the
astronomer in his observatory has to do with many comets which the
public at large never hear of, or if they do hear of, treat with
contempt, because they are destitute of tails.
[Illustration: Fig. 20.—Wells’s Comet of 1882, seen in full daylight
near the Sun on Sept. 18.]
[Illustration: Fig. 21.—Quenisset’s Comet, July 9, 1893 (Quenisset).]
The tails of comets exhibit very great varieties not only of size but of
form; some are long and slender; some are long and much spread out
towards their ends, like quill pens, for instance; some are short and
stumpy, mere tufts or excrescences rather than tails. Not unfrequently a
tail seems to consist of two parallel rays with no cometary matter, or
it may be only a very slight amount of cometary matter traceable in the
interspace; some have one main tail consisting of a pair of rays such as
just described, together with one or more subsidiary or off-shoot tails.
The comet of 1825 had five tails and the comet of 1744 had six tails. It
might be inferred from all this that the tails of comets are so
exceedingly irregular, uncertain and casual as to be amenable to no
laws. This was long considered to be the case; but a Russian observer
named Bredichin, as the result of much study and research, has arrived
at the conclusion that all comet tails may be brought under one or other
of three types; and that each type is indicative of certain distinct
differences of origin and condition which he considers himself able to
define. The first type comprises tails which are long and straight;
“they are formed” (to quote Young’s statement of Bredichin’s views) “of
matter upon which the Sun’s repulsive action is from twelve to fifteen
times as great as the gravitational attraction, so that the particles
leave the comet with a relative velocity of at least four or five miles
a second; and this velocity is continually increased as they recede,
until at last it becomes enormous, the particles travelling several
millions of miles in a day. The straight rays which are seen in the
figure of the tail of Donati’s Comet, tangential to the tail, are
streamers of this first type; as also was the enormous tail of the comet
of 1861. The second type is the curved plume-like train, like the
principal tail of Donati’s Comet. In this type the repulsive force
varies from 2.2 times gravity (for the particles on the convex edge of
the tail) to half that amount for those which form the inner edge. This
is by far the most common type of cometary train. A few comets show
tails of the third type—short, stubby, brushes violently curved, and due
to matter of which the repulsive force is only a fraction of
gravity—from 1/10 to ½.”
Bredichin wishes it to be inferred that the tails of the 1st type are
probably composed of hydrogen; those of the 2nd type of some
hydro-carbon gas; and those of the 3rd of the vapour of iron, probably
with some admixture of sodium and other substances. Bredichin, as a
reason for these conclusions, supposes that the force which generates
the tails of comets is a repulsive force, with a surface action the same
for equal surfaces of any kind of matter; the effective accelerating
force therefore measured by the velocity which it would produce would
depend upon the ratio of surface to mass in the particles acted upon,
and so, in his view, should be inversely proportional to their molecular
weights. Now it happens that the molecular weights of hydrogen, of
hydro-carbon gases, and of the vapour of iron bear to each other just
about the required proportion.
I am here stating the views and opinions of others without definitely
professing to be satisfied with them, but as they have met with some
acceptance, it is proper to chronicle them, though we know nothing of
the nature of the repulsive force here talked about. It might be
electric, it might be anything. The spectroscope certainly lends some
countenance to Bredichin’s views, but we need far more knowledge and
study of comets before we shall be justly entitled to dogmatise on the
subject.
[Illustration: Fig. 22.—Holmes’s Comet, Nov. 9, 1892 (Denning).]
[Illustration: Fig. 23.—Holmes’s Comet, Nov. 16, 1892 (Denning).]
This has been rather a digression. I go back now to prosaic matters of
fact, of which a vast and interesting array present themselves for
consideration in connection with comets. Let us consider a little in
detail what they are, to look at. We have seen that a well-developed
comet of the normal type usually comprises a nucleus, a head or coma,
and a tail. Comets which have no tails generally exhibit heads of very
simple structure; and if there is a nucleus, the nucleus is little else
than a stellar point of light. But in the case of the larger comets,
which are almost or quite visible to the naked eye, the head often
exhibits a very complex structure, which in not a few cases seems to
convey very definite indications of the operations going on at the time.
Figs. 22 and 23 may be taken as samples of a complex cometary head,
though no two comets resemble one another exactly in details. Fig. 24
forcibly conveys the idea that we are looking at a process of
development analogous to an uprush of water from a fountain, or perhaps
I might better say, from a burst waterpipe. There is a distinct idea of
a jet. This self-same idea, in another form, presents itself in the case
of those comets which exhibit what astronomers are in the habit of
calling “luminous envelopes.” The jet in this case is not strictly a jet
because it is not a continuous outflow, or overflow, of matter; the idea
rather suggests itself of an intermittent overflow resulting in
accumulated layers, or strata, of matter becoming visible. But with this
we come to a standstill; we cannot tell where the matter comes from, and
still less, where it goes to; we can only record what our eyes, assisted
by telescopes, tell us. There can, however, I think, be no doubt that
the matter of a comet becomes displayed to our senses as the result of a
process of expulsion, or repulsion, from the nucleus; and then, having
become launched into space, it comes under the influence, also
repulsive, of the Sun. All these things are visible facts. As to causes,
we suggest little, because we know so little. Anyone who has seen a
comet and has watched the displays of jets and luminous envelopes, such
as I have endeavoured to set forth, will realise at once how impossible
it is to describe these things in words. They must be seen either in
actual being or in picture. Some further allusions to this branch of the
subject may perhaps be more advantageously made after we have considered
the movements and orbits of comets.
[Illustration: Fig. 24.—Comet III. of 1862, on Aug 22, showing jet of
luminous matter (Challis).]
There is often a slight general resemblance between a planet and a
comet, as regards the path which each class of body pursues. Probably
the least reflective person likely to be following me here understands
the bare fact, that all the planets revolve round the Sun, and are held
to defined orbits by the Sun’s influence, or attraction, as it is
called. Perhaps, it is not equally realised, that in a somewhat similar,
but not quite the same way, comets are influenced and controlled by the
Sun.
Comets must be considered as regards their motions to be divisible into
two classes:—(1) Those which belong to the Solar System; and (2) those
which do not. Each of these two classes must again be sub-divided, if we
would really obtain a just conception of how things stand.
By the Comets which belong to the Sun, I mean those which revolve round
the Sun in closed orbits;[5] and are, or may be, seen again and again at
recurring intervals. There are 2 or 3 dozen comets which present
themselves to our gaze at stated intervals, varying from about 3 to 70
years. There are again other comets which without any doubt
(mathematically) are revolving round the Sun in closed orbits, but in
orbits so large and with periods of revolution so long (often many
centuries), that though they will return again to the sight of the
inhabitants of the earth some day, yet no second return having been
actually recorded, the astronomer’s prediction that they will return,
remains at present a prediction based on mathematics but nothing more.
There is another class of Comet of which we see examples from time to
time, and having seen them once shall never see again. This is because
these Comets move in orbits which are not closed, and which are known as
parabolic or hyperbolic orbits respectively, because derived from those
sections of a cone which are called the Parabola and the Hyperbola. It
must be understood that what I am now referring to is purely a matter of
orbit, and that no relationship subsists between the size and physical
features of a Comet and the path it pursues in space. The only sort of
reservation, perhaps, to be made to this statement is, that the comets
celebrated for their size and brilliancy, are often found to be
revolving in elliptic orbits of great eccentricity, which means that
their periods may amount to many centuries.
It may be well to say something now as to what is the ordinary career of
a comet, so far as visibility to us, the inhabitants of the Earth, is
concerned. Though this might be illustrated by reference to the history
of many comets, perhaps there is no one more suitable for the purpose
than Donati’s Comet of 1858. In former times, when telescopes were few
or non-existent, brilliant comets often appeared very suddenly, just as
a carriage or a man does, as you turn the corner of a street. Such
things even happen still: for instance, the great comet of 1861 burst
upon us all at once at a day’s notice. Usually, however, now in
consequence of the large size of the telescopes in use, and the great
number of observers who are incessantly on the watch, comets are
discovered when they are very small, because remote both from the Earth
and Sun, and many weeks, or even months, it may be, before they shine
forth in their ultimate splendour. Now, let us see how these statements
are supported by the history of Donati’s comet in 1858. On June 2 in
that year, it was first seen by Donati at Florence, as a faint
nebulosity, slowly journeying northwards. June passed away, and July,
and August, the comet all the while remaining invisible to the naked
eye; that is to say, it first became perceptible to the naked eye on
August 29, having put forth a faint tail about August 20. After the
beginning of September its brilliancy rapidly increased. On September
17, the head equalled in brightness a 2nd magnitude star, the tail being
4° long. Passing its point of nearest approach to the Sun on September
29, it came nearest to the Earth on October 10; though, perhaps, its
appearance a few days previously, namely on October 5, is the thing best
remembered by those who saw it, because it was on that night that the
comet passed over the 1st magnitude star Arcturus. For several days
about this time, the comet was an object of striking beauty in the
Western Heavens, during the hours immediately after sun-set. After
October 10, it rapidly passed away to the Southern hemisphere,
diminishing in brightness, as it did so, because receding from the Earth
and the Sun. It continued its career through the winter; became
invisible to the naked eye; and finally invisible altogether in March
1859. It remained in view, therefore, for more than nine months, not to
return again till about the year 3158 A.D., for its period of revolution
was found to be about 2000 years.
I have been particular in sketching somewhat fully the history of this
comet so far as we are concerned, because, as I have already said, it is
typical of the visible career of many comets. Halley’s comet in 1835 and
1836, went through a somewhat similar series of changes. This comet—a
well-known periodical one of great historic interest and brilliancy—may
be commended to the younger members of the rising generation, because it
is due to return again to these parts of space a few years hence, or in
1910.
[Illustration: Fig. 25.—Sawerthal’s Comet, June 4, 1888 (Charlois).]
What is a comet made of? Men of Science equally with the general public
would like to be able to answer this question, but they cannot do so
with satisfactory certainty. A great many years ago Sir John Herschel
wrote thus:—“It seems impossible to avoid the following conclusion, 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.” This passage was written of course
before the spectroscope had been brought to bear on the observations of
comets, but so far as Sir John Herschel’s remark implies the presence of
vapour, that is gas, in a comet, the surmise has been amply borne out by
later discoveries. The fact that as a comet approaches the Sun some
forces, no doubt of solar origin, come into operation to vaporise and
therefore expand the matter composing the comet is sufficiently shown by
the great developement which takes place as we have seen in the tails of
comets, but in regard to the heads of comets we are face to face with a
strange enigma. Though the tails expand the heads contract as the comet
approaches its position of greatest proximity to the Sun. Having passed
this point the head expands again. This curious circumstance, first
pointed out by Kepler in 1618, has often been noticed since, and noticed
indeed not as the result of mere eye impressions, but after careful
micrometrical measurement with suitable instruments. I think the
confession must be made that we are hopelessly ignorant of the nature of
comet’s except that gases are largely concerned in their constitution.
It seems impossible to doubt that some tails of comets are hollow
cylinders or hollow cones. Such a theory would account for the fact, so
often noticed, that single tails are usually much brighter at their two
edges than at the centre. This is the natural effect of looking
transversely at any translucent cylinder of measureable thickness.
It was long a moot point whether comets are self-luminous, or whether
they shine by reflected light; but it is now generally admitted that
whilst a part of the light of a comet may be derived by reflection from
the Sun yet as a rule they must be regarded as shining by their own
intrinsic light.
It should be stated here by way of caution that the observations on this
subject are not so consistent as one could wish, and it seems necessary
to assume that all comets are not constituted alike, and that therefore
what is true of one does not necessarily apply to another.
To those who possess telescopes (not necessarily large ones)
opportunities for the study of comets have much multiplied during the
last few years, for we are now acquainted with a group of small comets
which are constantly coming into view at short intervals of time. The
comets have now become so numerous that seldom a year passes without one
or more of them coming into view. Whilst that known as Encke’s revolves
round the Sun in 3¼ years, Tuttle’s doing the same in 13½ years, there
are four others whose periods average about 5½ years, 5 which average 6½
years, together with one of 7½ years and one of 8 years. It is thus
evident that there is a constant succession of these objects available
for study, and that very few months can ever elapse that some one or
more of them are not on view. They bear the names of the astronomers who
either discovered them originally, or who, by studying their orbits,
discovered their periodicity. The names run as follows, beginning with
the shortest in period and ending with the longest:—
Encke’s. Winnecke’s.
Temple’s Second (1873, II.) Brorsen’s.
Temple’s First (1867, II.)
Swift’s (1880, V.) Wolf’s (1884, III.)
Barnard’s (1884, II.) Faye’s.
D’Arrest’s. Denning’s.
Finlay’s. Tuttle’s.
I cannot stay to dwell upon either the history or description of these
comets separately, but must content myself by saying generally that
whilst as a rule they are not visible to the naked eye, yet several of
them may occasionally become so visible when they return to perihelion
under circumstances which bring them more near than usual to the earth.
Several other comets are on record which it was supposed at one time
would certainly have been entitled to a place in the above list, but
three of them in particular have, under very mysterious circumstances,
entirely disappeared from the Heavens.
Chief amongst the mysterious comets must be ranked that which goes by
the name of Biela. This comet, first seen in 1772, was afterwards found
to have a period of about 6¾ years, and on numerous occasions it
reappeared at intervals of that length down to 1845, when the mysterious
part of its career seems to have commenced. In December of that year
this comet threw off a fragment of nearly the same shape as itself, and
the two portions travelled together side by side for four months, the
distance between the fragments slowly increasing. At the end of the four
months in question the comet passed out of sight owing to the distance
from the earth to which it had attained. The comet returned again to
perihelion in 1852, remaining visible for three weeks. The two portions
of the comet noticed in 1846 retained their individuality in 1852, but
the distance between them had increased to about eight times the
greatest distance noticed in 1846. As a comet Biela’s Comet has never
been seen since 1852, and it must now be regarded as having permanently
disappeared. But what seems to have happened is this, that Biela’s Comet
has become broken up into a mass of meteors. On November 27, 1872, and
again in November 1885, when the earth in travelling along its own orbit
reached a certain point where its orbit intersected the former orbit of
Biela’s Comet the Earth encountered, instead of the comet which ought to
have been there, a wonderful mass of meteors; and it is now generally
accepted that these meteors, which apparently are keeping more or less
together as a fairly compact swarm, are nought else than the
disintegrated materials of what once was Biela’s Comet.
[Illustration: Fig. 26.—Biela’s Comet, February 19, 1846.]
It is extremely probable that as time goes on we shall be able to say
that an intimate connection subsists between particular comets which
have been and particular meteoric swarms. We already possess proof that
other comets which once came within our view were at that time revolving
round the Sun in orbits so comparatively small that they should have
reappeared at intervals of half-a-dozen or so years, yet they have not
reappeared. The question therefore suggests itself, Have they been
subject to some great internal disaster which has led to their
disintegration? It may be said without doubt that this is in the highest
degree probable; but short of this, that is short of total
disintegration into small fragments, we have several cases on record of
what I may, for the moment, call ordinary comets breaking up into two or
three fragments. For a long while astronomers were naturally loath to
believe that this was possible, and therefore they discredited the
statements to that effect which had been made. Though it would occupy
too much space to give the particulars of these comets in full it may
yet be worth while just to mention the names of some of them, presumed
to be of short period, which seemed nevertheless to have eluded our
grasp. I would here specially mention Liais’s Comet of 1860 and the
second comet of 1881 as seemingly having undergone some sort of
disruption akin to what happened in the case of Biela’s Comet.
There is another group of periodical comets to be mentioned. These are
six in number and seem to have periods of 70 years or a little more. Of
these three have not yet given us the chance of seeing them again; two
have paid us a second visit, and therefore their periods are not open to
doubt; whilst the most famous of this group, “Halley’s,” has been
recorded to have shown itself to the Earth no less than 25 times,
beginning with the year 11 B. C. It was Halley’s comet which shone over
Europe in April 1066, and was considered the forerunner of the conquest
of England by William of Normandy. It figures in the famous Bayeux
tapestry as a hairy star of strange shape.
It would seem that there exists in some inscrutable manner a connection
between each of the three great exterior planets and certain groups of
comets. In the case of Jupiter the association is so very pronounced as
long ago to have attracted notice; but the French astronomer,
Flammarion, has brought forward some suggestions that Saturn has one
comet (and perhaps two) with which it is associated; Uranus, two (and
perhaps three); and Neptune, six; whilst farther off than Neptune the
fact that there are two comets, supposed periodical, without a known
planet to run with them has inspired Flammarion to look with a friendly
eye on the idea (often mooted) that outside of Neptune there exists
another undiscovered planet revolving round the sun in a period of about
300 years.
The Jupiter group of comets deserves a few additional words. There are
certainly nine, and perhaps twelve comets revolving round the Sun in
orbits of such dimensions that they either reach up to or slightly
overreach the orbit of Jupiter. The effect of this condition of things
is that on occasions Jupiter and each of the comets may come into such
proximity that the superior mass of Jupiter may exercise a very
seriously disturbing influence over a flimsy and ethereal body like a
comet. There is reason to suppose that some of the comets now belonging
to the Jupiter group have not done so for any great length of time, but
having been wandering about, either in elliptic orbits of great extent,
or even in parabolic orbits, have accidentally come within reach of
Jupiter, and so have been, as it were, captured by him. Hence, the
origin of the term, the “capture theory,” as applied to these family
groups of comets which I have just stated to exist, each presided over,
as it were, by a great planet. It may be that at some future time this
theory will help us to a clue to the fact that besides the comets of
Lexell of 1770, Blainpain of 1819, and Di Vico of 1844, short period
comets unaccountably missing, there are several others presumed to have
been revolving in short period orbits when discovered, and as to which
it is very strange that they should not have been seen before their only
recorded visit to us, and equally strange that they should never have
been seen since.
Is there any reason to fear the results of a collision between a comet
and the Earth? None whatever. However vague may be, and in a certain
sense must be, our answer to the question, “What is a comet?” certain is
it that every comet is a very imponderable body—a sort of airy nothing,
a mass of gas or vapour.[6] At the same time it always has been and
perhaps still is difficult to persuade the public that whatever might be
the effect on a comet if it were to strike the Earth, the effect on the
Earth, were it to be struck by a comet, would be _nil_. This is not
altogether a matter of speculation, for according to a calculation by
Hind, on June 30, 1861, the Earth passed into and through the tail of
the great comet of that year at about two-thirds of its distance from
the nucleus. Assuredly there was no dynamical result; but it seems,
however, not unlikely that there was an optical result; at any rate,
traces of something of this sort were noted. Hind himself, in Middlesex,
observed a peculiar phosphorescence or illumination of the sky which he
attributed at the time to an auroral glare. Lowe, in Nottinghamshire
confirmed Hind’s statement of the appearance of the heavens on the same
day. The sky had a yellow auroral glare-like look, and the Sun, though
shining, gave but feeble light. The comet was plainly visible at 7.45 p.
m. (during sunshine), and had a much more hazy appearance than on any
subsequent evening. Lowe adds that his Vicar had the pulpit candles
lighted in the Parish Church at 7 o’clock (it was a Sunday), though only
five days had elapsed since Midsummer day, which itself proves that some
sensation of darkness was felt even while the Sun was shining.
So far as I remember there has been no such thing as a comet panic
during the present generation, at any rate in civilised countries, but
it is on record that there was a very considerable panic in 1832 in
connection with the return of Biela’s Comet in the winter of that year.
Olbers as the result of a careful study in advance of the comet’s
movements found that the comet’s centre would pass only 20,000 miles
within the Earth’s orbit, and that as the nebulosity of the comet had in
1805 been more than 20,000 miles in diameter, it was certain, unless its
dimensions had diminished in the 27 years, that some of the comet’s
matter would overlap the Earth’s orbit; in other words would envelop the
Earth itself, if the Earth happened to be there. This conclusion when it
became public was quite enough to create a panic and make people talk
about the forthcoming destruction of our globe. It was nothing to the
point (in the public mind) that astronomers were able to predict that
the Earth would not reach the place where the comet would cross the
Earth’s orbit until four weeks after the comet had come and gone.
However, we now know that nothing happened, and I am justified in adding
that even if there had been contact, Earth meeting comet face to face,
nothing (serious) would have occurred so far as the Earth was concerned.
This seems a convenient place for referring to a matter which when it
was first broached excited a great deal of interest, but about which one
does not hear much now-a-days. The period of the small comet known as
Encke’s (which, revolving as it does round the Sun in a little more than
three years, has the shortest period of any of the periodical comets)
was found many years ago to be diminishing at each successive return.
That is to say, it always attained its nearest distance from the Sun at
each apparition 2½ hours sooner than it ought to have done. In order to
account for this gradual diminution in the comet’s period Encke
conjectured the existence of a thin ethereal medium sufficiently dense
to affect a light flimsy body like a comet, but incapable of obstructing
a planet. It has been remarked by Hind that “this contraction of the
orbit must be continually progressing, if we suppose the existence of
such a medium; and we are naturally led to inquire, What will be the
final consequence of this resistance? Though the catastrophe may be
averted for many ages by the powerful attraction of the larger planets,
especially Jupiter, will not the comet be at last precipitated on the
Sun? The question is full of interest, though altogether open to
conjecture.”
Astronomers are not altogether agreed as to the propriety of this
explanation. One argument against it is that with _perhaps_ one
exception none of the other short-period comets (all of them small and
presumably deficient in density) seem affected as Encke’s is. On the
other hand Sir John Herschel favoured the explanation just given, as
also does Hind who is the highest living authority on comets. A German
mathematician, Von Asten, who devoted immense labour to the study of the
orbit of Encke’s Comet, thought there should be no hesitation in
accepting the idea of a resisting medium, subject to the limitation that
it does not extend beyond the orbit of Mercury. Von Asten’s allusion to
Mercury touches a subject which belongs more directly to the question of
Mercury’s orbit and to that other very interesting question, “Are there
any planets, not at present known, revolving round the Sun within the
orbit of Mercury.”
Which is the largest and most magnificent comet recorded in history? It
is virtually impossible to answer this question, because of the
extravagant and inflated language made use of by ancient and medieval (I
had almost added, and modern) writers. There is no doubt that the comet
of 1680, studied by Sir I. Newton, the tail of which was curved, and
from 70° to 90° long, must have been one of the finest on record, as it
was also the one which came nearest to the Sun, for it almost grazed the
Sun’s surface.
The comet of 1744, visible as it was in broad daylight, was, no doubt,
the finest comet of the 18th century, though in size it has been
surpassed; yet its six tails must have made it a most remarkable object.
So far as the 19th century is concerned, our choice lies between the
comets of 1811, 1843, 1858, and 1861. The comet of 1811 is spoken of by
Hind as “perhaps the most famous of modern times. Independently of its
great magnitude, the position of the orbit and epoch of perihelion
passage, were such as to render it a very splendid circumpolar object
for some months.” The tail as regards its length was not so very
remarkable, for at its best, in October 1811, it was only about 25°
long, its breadth, however, was very considerable; at one time 6°, the
real length of the tail, about the middle of October, was more than
100,000,000 of miles, and its breadth about 15,000,000 of miles. The
visibility of this comet was coincident with those events which proved
to be the turning-point in the career of Napoleon I., and there were not
wanting those who regarded the comet as a presage of his disastrous
failure in Russia. Owing to the long period (17 months), during which
this comet was visible, it was possible to determine its orbit with
unusual precision. Argelander found its period to be 3065 years, with no
greater uncertainty than 43 years. The great dimensions of its orbit
will be realised when it is stated that this comet recedes from the Sun
to a distance of 14 times that of the planet Neptune.
[Illustration: Fig. 27.—The Great Comet of 1811.]
Donati’s comet of 1858, has already received a good deal of notice at my
hands, but the question remains, what are its claims, to be regarded as
the comet of the century, compared with that of 1843? It is not a little
strange that though there must have been many persons who saw both, yet
it was only quite recently that I came across, for the first time, a
description of both these comets from the same pen. It ought, however,
to be mentioned by way of explanation, that the inhabitants of Europe
only saw the comet of 1843, when its brilliancy and the extent of its
tail had materially diminished, about a fortnight after it was at its
best.
The description of these two comets to which I have alluded, will be
found in General J. A. Ewart’s “_Story of a Soldier’s Life_,” published
in 1881. Writing first of all of the comet of 1843, General Ewart says:—
“It was during our passage from the Cape of Good Hope to the Equator,
and when not far from St. Helena, that we first came in sight of the
great comet of 1843. In the first instance a small portion of the tail
only was visible, at right angles to the horizon; but night after night
as we sailed along, it gradually became larger and larger, till at last
up came the head, or nucleus, as I ought properly to call it. It was a
grand and wonderful sight, for the comet now extended the extraordinary
distance of one-third of the heavens, the nucleus being, perhaps, about
the size of the planet Venus.”—(Vol. i., p. 75.)
[Illustration: Fig. 28.—The Great Comet of 1882, on October 19 (Artus).]
As regards Donati’s comet of 1858, what the General says is:—
“A very large comet made its appearance about this time, and continued
for several weeks to be a magnificent object at night; it was, however,
_nothing to the one I had seen in the year 1843_, when on the other side
of the equator.”—(Vol. ii., p. 205.)
Passing over the great comet of 1861, on which I have already said a
good deal, I must quit the subject of famous comets by a few words about
that of 1882, which, though by no means one of the largest, was, in some
respects, one of the most remarkable of modern times. It was visible for
the long period of nine months, and was conspicuously prominent to the
naked eye during September, but these facts, though note-worthy, would
not have called for particular remark, had not the comet exhibited some
special peculiarities which distinguished it from all others. In the
first place, it seems to have undergone certain disruptive changes, in
virtue of which the nucleus became split up into four independent
nuclei. Then the tail may have been tubular, its extremity being not
only bifid, but totally unsymmetrical with respect to the main part. The
tubular character of the tail was suggested by Tempel. To other
observers, this feature gave the idea of the comet, properly so-called,
being enclosed in a cylindrical envelope, which completely surrounded
the comet, and overlapped it for a considerable distance at both ends.
Finally (and in this resembling Biela’s comet) the comet of 1882 seems
to have thrown off a fragment which became an independent body.
What has gone before, will, I think, have served abundantly to establish
the position with which I started, namely, that comets occupy (and
deservedly so) the front rank amongst those astronomical objects in
which, on occasions, the general public takes an interest.
I have thus completed, so far as the space at my disposal has permitted,
a popular descriptive Survey of the Solar System. Those who have perused
the preceding pages, however slight may have been their previous
acquaintance with the Science of Astronomy taken as a whole, will have
no difficulty in realising that what I have said bears but a small
proportion to what I have left unsaid. They will equally, I hope, be
able to see, without indeed the necessity of a suggestion, that all
those wondrous orbs which we call the planets could neither have come
into existence nor have been maintained in their allotted places for so
many thousands of years, except as the result of Design emanating from
an All-powerful Creator.
FOOTNOTES.
[1]The remark in the text applies to all the major planets and to a
large number of the minor planets, but certain of the minor planets
travel in orbits which are considerably inclined to the ecliptic,
and therefore to all the other planets.
[2]Given in full in my _Handbook of Astronomy_, 4th ed., vol. i., p. 26.
[3]“_Recollections of Past Life_,” 2nd ed., p. 305.
[4]For some information respecting these Secchi “Types” of Stars, see my
“_Story of the Stars_,” 2nd ed., p. 140.
[5]The circle and the ellipse are what are called “closed” curves.
[6]It is not a little singular that the Chinese in bygone centuries have
often alluded to comets under the name of vapours; _e.g._, the comet
of 1618 is recorded as having been “a white vapour 20 cubits long.”
APPENDIX.—TABLES OF THE SOLAR SYSTEM.
THE SUN AND PLANETS.
Name. Sidereal Mean Diameter. Surface. Volume. Mass. Density. Axial Force of Velocity
period. distance Miles. Earth=1. Earth=1. Earth=1. Earth=1. Rotation. gravity. in orbit.
from d. h. m. Fall: Miles per
Sun. Ft. in 1 hour
Millions sec.
of
miles.
SUN ..... ..... 866,200 11,946 1,305,000 332,000 0.25 25 7 48 461
h. m. s.
MERCURY 88 days. 36 3,008 0.144 0.055 0.066 1.26 24 5 30 7 107,000
VENUS 225 ″ 67 7,480 0.891 0.841 0.782 0.92 23 21 23 14 78,000
EARTH 365 ″ 93 7,926 1.000 1.000 1.000 1.00 23 56 4 16 66,000
MARS 687 ″ 141 5,000 0.398 0.251 0.107 0.45 24 37 23 4 53,000
Minor Planets
Eros (433) 1.76 ..... 18 Eros is the nearest to the Sun of the Minor Planets, part
years. of its orbit falling between the Earth and Mars.
Vesta (4) 3.6 ″ 219 214 Vesta is the largest of the Minor Planets.
Thule (279) 8.8 ″ 396 ..... Thule is the most distant from the Sun of the Minor Planets.
JUPITER 11.8 ″ 483 88,439 124 1,389 317 0.23 9 55 21 41 29,000
SATURN 29.4 ″ 886 75,036 89 848 94 0.11 10 29 17 18 21,000
URANUS 84.0 ″ 1782 30,875 15 59 14 0.25 ? 13 15,000
NEPTUNE 164.6 ″ 2792 37,205 21 103 17 0.17 ? 12 12,000
SATELLITE OF THE EARTH.
Name. Sidereal Distance Diameter Surface. Volume. Mass. Density. Axial Force of Velocity
period. from Miles. Earth=1. Earth=1. Earth=1. Earth=1. Rotation. gravity. in orbit.
d. h. m. Earth. d. h. m. Fall: Miles per
Miles. Feet in hour.
1 sec.
MOON 27 7 43 237,300 2,160 0.074 0.02034 0.0128 0.63 27 7 43 2.48 2,273
SATELLITES OF MARS.
Name. Discoverer. Mean Sidereal Diameter. Maximum Apparent
distance period. Miles. elongation. star
from d. h. m. ″ magnitude.
Mars.
Miles.
1. PHOBOS A. Hall, August 6,000 0 7 39 7 12 11½
17, 1877.
2. DEIMOS A. Hall, August 15,000 1 6 18 6 32 13½
11, 1877.
SATELLITES OF JUPITER.
Name. Discoverer. Mean Sidereal Diameter. Apparent Apparent
distance period. Miles. diameter star
from d. h. m. of Jupiter magnitude.
Jupiter. seen from
Miles. satellite.
° ″
5. Barnard ....... 0 11 49 ? ..... ....
1. IO ............... 267,000 1 18 29 2,390 19 49 7
2. EUROPA Galileo, January 425,000 3 13 13 2,120 12 25 7
7-13, 1610.
3. GANYMEDE ............... 678,000 7 4 0 3,980 7 47 6
4. CALLISTO ............... 1,192,000 16 18 5 2,970 4 25 7
SATELLITES OF SATURN.
Name. Discoverer. Mean Sidereal Diameter. Apparent Apparent
distance period. Miles. diameter star
from d. h. m. of Saturn magnitude.
Saturn. seen from
Miles. satellite.
°
1. MIMAS Sir W. Herschel, 115,000 0 22 37 1,000 33 17
Sept, 17, 1789
2. ENCELADUS ″ ″ Aug. 28, 1789 147,000 1 8 53 ? 26 15
3. TETHYS J. D. Cassini, 183,000 1 21 18 500 21 13
March, 1684
4. DIONE ″ ″ March, 1684 234,000 2 17 41 500 16 12
5. RHEA ″ ″ Dec. 23, 1672 327,000 4 12 25 1,200 12 10
6. TITAN Huygens, Mar. 25, 758,000 15 22 41 3,300 5 8
1653
7. HYPERION W. Bond & 916,000 21 7 7 ? 4 17
Lassell, Sept,
19, 1848
8. IAPETUS J. D. Cassini, 2,221,000 79 7 53 1,800 2 9
Oct. 25, 1671
SATELLITES OF URANUS.
Name. Discoverer. Mean distance Sidereal Maximum
from Uranus. period. elongation.
Miles. d. h. m. ″
1. ARIEL Lassell, September 124,000 2 12 28 12
14, 1847
2. UMBRIEL O. Struve, October 8, 173,000 4 3 27 15
1847
3. TITANIA Sir W. Herschel, 285,000 8 16 55 33
January 11, 1787
4. OBERON ″ ″ 381,000 13 11 6 44
SATELLITE OF NEPTUNE.
Discoverer. Mean distance Sidereal Maximum Apparent
from Neptune. period. elongation. star
Miles. d. h. m. ″ magnitude.
Lassell, October 223,000 5 21 8 18 14
10, 1846
GENERAL INDEX.
A.
Adams, J. C., 145.
Airy, Sir G. B., 145.
Anagram on Venus, 68;
on Saturn, 127.
Aphelion, 11, 102.
Apparent movements of the Planets, 12, 14.
Arago, D. J. F., 99, 124, 140.
Arctic regions of the Earth, 75.
Argelander, F. G. A., 178.
Aristarchus of Samos, 71.
Aristarchus (Lunar Mountain), 93, 94.
Asteroids, 110.
Aurora Borealis and spots on the Sun, 55;
and twinkling, 87.
Autumnal Equinox, 74.
Axial rotation of the Planets, 17, 183.
B.
Barnard. E. E., 120, 134.
Barnard’s Comet, 168.
Bayeux Tapestry, 171.
Beer and Mädler;
their map of the Moon, 96.
Belts on Jupiter, 115, 123;
on Saturn, 122.
Bergeron’s Experiment, 93.
Bessel, W., 124.
Biela’s Comet, 168.
Bode’s so-called “Law,” 110.
Bond, G. P., 132, 133.
Bouvard, A., 143.
Bradley, Rev. J., 144.
Bredechin’s theory of Tails of Comets, 157.
Brorsen’s Comet, 167.
C.
“Canals” on Mars, 106.
Carpenter, J., 134.
Carrington, R. C., 46, 49.
Cassini, J. D., 36, 118, 128, 130, 136.
Ceres (Minor Planet), 112.
Challis, Rev. J., 146.
Charts, Celestial, 112, 147.
Chinese observations, 25, 172.
Clouds influenced by the Moon, 98.
Coggia’s Comet, 1874, 151.
Comets, 150;
periodic, 162, 170;
remarkable, 176.
Comparative sizes of the Planets, 11.
Comparative size of the Sun from the Planets, 17.
Compression of the Planets, 70, 122, 137.
Conjunction of the Planets, 14, 15, 66.
Copernican System, 72.
Copernicus, 72, 73, 76.
D.
D’Arrest, 147.
D’Arrest’s Comet, 168.
Dawes, W. R., 106, 133.
Day and night, 12.
Day, length of, 74.
De La Rue, W., 35, 36, 49, 52.
Denning’s Comet, 168.
Density of the Planets, 17, 183.
Diameter of the Sun and Planets, 183.
Di Vico, 62, 63.
Di Vico’s Comet, 172.
Distance of the Planets from the Sun, 20, 183.
Donati’s Comet of 1858, 157, 163, 178.
Dufour on Twinkling, 84.
E.
Earth, annual motion of, 75;
figure of, 70.
Earthshine, 66, 93.
Eccentricity of Planetary orbits, 10, 60, 75.
Eclipses of the Sun, 56, 57;
of the Moon, 120.
Ecliptic, plane of, 9, 10, 74.
Egyptian System, 72.
Elliptic Orbits, 10.
Elongation of Planets, 14.
Encke, J. F., 147.
Encke’s Comet, 167, 174, 175.
Equinoxes, 74.
Ewart, Gen. J. A., 178.
F.
Faculæ on the Sun, 42, 43.
Faye’s Comet, 168.
Finlay’s Comet, 168.
Flammarion, C., 62, 66, 171.
Flamsteed, Rev. J., 140, 144.
G.
Galileo, 89, 101, 120, 125.
Galle, J. G., 147.
Gassendi, 76.
Geodesy, 70.
Georgium Sidus, 141.
Goldschmidt, H., 4.
Granules, Solar, 38.
Gravity on the Sun and Planets, 183.
H.
Hall, A., 109.
Halley’s Comet, 164, 171.
Heat rays of the Sun, 20;
of the Moon, 96.
Herschel, Sir W., 34, 40, 44, 58, 100, 103, 134, 137, 138, 142.
Herschel, Sir J. F. W., 24, 96, 99, 103, 132, 175.
Hind, J. R., 119, 173, 175.
Holden, E. S., 106.
Holland, Sir H., 78.
Holmes’s Comet, 1892, 159.
Homer’s _Iliad_, cited, 68.
Howlett, Rev. F., 47.
Huggins, W., 40.
Huygens, C., 127, 130.
Hyperbola, 162.
Hyperbolic Comets, 162.
I.
Inclination of planetary orbits, 9.
Inferior Planets, 9, 12.
J.
Janssen J., 47.
Juno (Minor Planet), 112.
Jupiter: its influence on Sun-spots doubtful, 51, 53;
on Comets, 171;
its light, 7.
K.
Kepler, 55, 125, 166.
Kew Observatory, 47.
L.
La Hire, 63.
Lalande, 34.
Lassell, W., 123, 142, 185.
Ledger, Rev. E., 84.
Le Monnier, 140, 144.
Le Verrier, U. J. J., 147.
Lexell’s Comet, 172.
Liais’s Comet, 170.
Logogriphs as to Venus, 68;
as to Saturn, 127.
M.
Mädler, J. H., 63, 96.
Magnetism, Terrestrial, 55.
Maps, Astronomical, 112, 147.
Mars, 100.
Maskelyne, Rev. N., 139.
Mass of the Sun, and of the Planets, 20, 183.
Medium, Resisting, 175.
Mercury, 57;
phases of, 15;
its influence on Sun-spots, 51;
its luminosity, 67.
Milton, J., _Paradise Lost_, cited, 153.
Minor Planets, 110.
Montigny on Twinkling, 84.
Moon, 89.
Moonlight, Brightness of, 96.
Motions of the Planets, 10.
Mountains of the Moon, 90.
N.
Napoleon I., 154, 176.
Nasmyth, J., 40.
Needle, Magnetic, 55.
Neptune, 143;
its influence on Uranus, 143.
Newton, Sir I., 176.
Nubian after-glow, 83.
Nucleus of a Sun-spot, 22.
O.
Obliquity of the ecliptic, 74.
Occultations of Jupiter’s Satellites, 121.
Olbers, W., 112, 174.
Opposition of Planets, 13.
Orbits of the Planets, 9, 10;
of Comets, 161.
P.
Pallas (Minor Planet), 112, 113, 114.
Parabola, 162.
Penumbra of a Sun-spot, 22.
Perihelion, 11, 102.
Periodic Comets, 162, 170.
Perturbations of Uranus by Neptune, 145.
Phases of an inferior Planet, 15;
of a superior Planet, 16;
of Mars, 100;
of Jupiter, 115.
Piazzi, G., 111.
Planets, classification of, 7;
comparative sizes of, 11;
movements of, 10.
Plutarch, 71.
Poles of Mars, snow at, 104.
Primary Planets, 7.
Ptolemaic System, 72.
Q.
Quadrature of a Planet, 16.
Quenisset’s Comet, 1893, 156.
R.
Rays of the Sun, 20.
Refraction, 77;
effect of, 77.
Red spot on Jupiter, 118.
Resisting Medium, 175.
Retrograde motion of a Planet, 15.
“Rice grains” on the Sun, 39, 40.
Rings of Saturn, 128.
Rosse, Earl of, 93.
Rotation of the Sun, 24, 28;
of the planets, 17, 183.
Rotundity of the Earth, 76.
S.
Satellites of Mars, 16, 184;
of Jupiter, 16, 184;
of Saturn, 16, 185;
of Uranus, 16, 185;
of Neptune, 16, 185.
Saturn, 122.
Sawerthal’s Comet, 165.
Schiaparelli, J. V., 59, 62, 63, 106.
Schmidt, J. F. F., 96.
Schröter, J. J., 58, 62, 63.
Schwabe’s observations of Sun spots, 46.
Seas, Lunar, 94.
Seasons on the Earth, 54, 74.
Secchi, A., 28, 35, 42, 50, 84, 86, 106.
Secondary Planets, 8.
Shakespeare, citations from, 152.
Smyth, C. P., 92.
Snow on Venus, 66;
on Mars, 104;
doubtful on Saturn, 123.
Solstices, 74.
Spherical form of the Earth, 70.
Spörer, 46, 50.
Spots on the Sun, 21, 118;
on Venus, 61;
on Jupiter, 117;
on Saturn, 123.
Stewart, B., 52, 56.
Struve, O., 134, 142.
Sun, 18, 183.
Superior Planets, 8;
movements of, 15.
Surfaces of the Sun and Planets, 183.
Swift’s Comet, 168.
Systems of the Universe, 72.
T.
Tables of the Major Planets, 183.
Tails of Comets, 154.
Temple’s Periodical Comets, 167.
Terminator of the Moon, 90.
Thomson’s _Seasons_ cited, 153.
Tides, 98.
Total Eclipses of the Sun, 57.
Transits of inferior Planets, 13, 25, 65.
Trouvelot, 133.
Turtle’s Periodical Comet, 167, 168.
Twilight, 80.
Twinkling, 83.
Tycho (Lunar Mountains), 94.
Tycho Brahe, 73.
Tychonic System, 73.
U.
Umbra of a Sun-spot, 22.
Uranus, 138;
the influence of Neptune on, 143.
V.
Velocity of the planets, 183.
Venus, 61;
phases of, 15;
its influence on Sun-spots, 51, 52, 53.
Vernal Equinox, 74.
Vesta (minor planet), 112, 113.
Virgil, citation from, 50.
Volume of the Sun, 20;
of the Planets, 183.
W.
Weather influences ascribed to the Sun, 44;
to the Moon, 98.
“Willow leaves” on the Sun, 39, 40.
Wilson’s theory of Sun-spots, 32.
Winnecke’s Periodical Comet, 167.
Wolf, R., observer of Sun-spots, 45, 48.
Wolf’s Periodical Comet, 168.
Y.
Young, C. A., 131.
Z.
Zodiac, movement of Planets in, 15.
THE END.
Transcriber’s Notes
--Some palpable typographical errors were corrected.
--Copyright and publisher’s information was included from the printed
copy: this eBook is public domain in the country of publication.