COMETS AND METEORS
There is one type of celestial object which seldom fails to stir up
the mind of even the most sluggishly unastronomical member of the
community and to inspire him with an interest in the science--an
interest which is usually conspicuous for a picturesque inaccuracy
in the details which it accumulates, for a pathetic faith in the most
extraordinary fibs which may be told in the name of science, and for
a subsidence which is as rapid as the changes in the object which gave
the inspiration. The sun may go on shining, a perpetual mystery and
miracle, without attracting any attention, save when a wet spring
brings on the usual talk of sun-spots and the weather; Jupiter
and Venus excite only sufficient interest to suggest an occasional
question as to whether that bright star is the Star of Bethlehem; but
when a great comet spreads its fiery tail across the skies everybody
turns astronomer for the nonce, and normally slumber-loving people
are found willing, or at least able, to desert their beds at the
most unholy hours to catch a glimpse of the strange and mysterious
visitant. And, when the comet eventually withdraws from view again, as
much inaccurate information has been disseminated among the public as
would fill an encyclopædia, and require another to correct.
Comets are, however, really among the most interesting of celestial
objects. Though we no longer imagine them to foretell wars, famines,
and plagues, or complacently to indicate the approbation of heaven
upon some illustrious person deceased or about to decease, and have
almost ceased to shiver at the possibilities of a collision between a
comet and the earth, they have within the last half century taken on a
new and growing interest of a more legitimate kind, and there are few
departments of science in which the advance of knowledge has been more
rapid or which promise more in the immediate future, given material to
work upon.
The popular idea of a comet is that it is a kind of bright wandering
star with a long tail. Indeed, the star part of the conception is
quite subsidiary to the tail part. The tail is _the_ thing, and a
comet without a tail is not worthy of attention, if it is not rather
guilty of claiming notice on false pretences. As a matter of fact, the
tail is absent in many comets and quite inconspicuous in many more;
and a comet may be a body with any degree of resemblance or want of
resemblance to the popular idea, from the faint globular stain of
haze, scarcely perceptible in the telescopic field against the dark
background of the sky, up to a magnificent object, which, like the
dragon in the Revelation, seems to draw the third part of the stars
of heaven after it--an object like the Donati comet of 1858, with a
nucleus brighter than a first-magnitude star, and a tail like a great
feathery plume of light fifty millions of miles in length. It seems
as impossible to set limits to the variety of form of which comets are
capable as it is to set limits to their number.
Generally speaking, however, a comet consists of three parts: The
nucleus--which appears as a more or less clearly defined star-like
point, and is the only part of the comet which will bear any
magnification to speak of--the coma, and the tail. In many telescopic
comets the nucleus is entirely absent, and, in the comets in which
it is present, it is of very varied size, and often presents curious
irregularities in shape, and even occasionally the appearance of
internal motions. It frequently changes very much in size during the
period of the comet's visibility. The nucleus is the only part of a
comet's structure which has even the most distant claim to solidity;
but even so the evidence which has been gradually accumulated all goes
to show that while it may be solid in the sense of being composed of
particles which have some substance, it is not solid in the sense of
being one coherent mass, but rather consists of something like a swarm
of small meteoric bodies. Surrounding the nucleus is the coma, from
which the comet derives its name. This is a sort of misty cloud
through which the nucleus seems to shine like a star in a nebula or
a gas-lamp in a fog. Its boundaries are difficult to trace, as it
appears to fade away gradually on every side into the background; but
generally its appearance is more or less of a globular shape except
where the tail streams away from it behind. Sometimes the coma is of
enormous extent--the Great Comet of 1811 showed a nucleus of 428 miles
diameter, enclosed within a nebulous globe 127,000 miles across, which
in its turn was wrapped in a luminous atmosphere of four times
greater diameter, with an outside envelope covering all, and extending
backwards to form the tail. But it is also of the most extraordinary
tenuity, the light of the very faintest stars having been frequently
observed to shine undimmed through several millions of miles of coma.
Finally, there is the tail, which may be so short as to be barely
distinguishable; or may extend, as in the case of Comet 1811 (ii.), to
130,000,000 miles; or, as in that of Comet 1843 (i.), to 200,000,000.
The most tenuous substances with which we are acquainted seem to be
solidity itself compared with the material of a comet's tail. It is
'such stuff as dreams are made of.'
Comets fall into two classes. There are those whose orbits follow
curves that are not closed, like the circle or the ellipse, but appear
to extend indefinitely into space. A comet following such an orbit
(parabolic or hyperbolic) seems to come wandering in from the depths
of space, passes round the sun, and then gradually recedes into the
space from which it came, never again to be seen of human eye. It is
now becoming questionable, however, whether any comet can really
be said to come in from infinite space; and the view is being more
generally held that orbits which to us appear portions of unclosed
curves may in reality be only portions of immensely elongated
ellipses, and that all comets are really members of the solar system,
travelling away, indeed, to distances that are immense compared with
even the largest planetary orbit, but yet infinitely small compared
with the distances of the fixed stars.
Second, there are those comets whose orbits form ellipses with a
greater or less departure from the circular form. Such comets must
always return again, sooner or later, to the neighbourhood of the sun,
which occupies one of the foci of the ellipse, and they are known as
Periodic Comets. The orbits which they follow may have any degree of
departure from the circular form, from one which does not differ very
notably from that of such a planet as Eros, up to one which may be
scarcely distinguishable from a parabola. Thus we have Periodic Comets
again divided into comets of short and comets of long period. In the
former class, the period ranges from that of Encke's comet which never
travels beyond the orbit of Jupiter, and only takes 3·29 years to
complete its journey, up to that of the famous comet whose periodicity
was first discovered by Halley, whose extreme distance from the sun
is upwards of 3,200,000,000 miles, and whose period is 76·78 years.
Comets of long period range from bodies which only require a paltry
two or three centuries to complete their revolution, up to others
whose journey has to be timed by thousands of years. In the case of
these latter bodies, there is scarcely any distinction to be made
between them and those comets which are not supposed to be periodic;
the ellipse of a comet which takes three or four thousand years to
complete its orbit is scarcely to be distinguished, in the small
portion of it that can be traced, from a parabola.
Several comets have been found to be short period bodies, which,
though bright enough to have been easily seen, have yet never been
noticed at any previous appearance. It is known that some at least of
these owe their present orbits to the fact that having come near to
one or other of the planets they have been, so to speak, captured, and
diverted from the track which they formerly pursued. Several of the
planets have more or less numerous flocks of comets associated with
them which they have thus captured and introduced into a short period
career. Jupiter has more than a score in his group, while Saturn,
Uranus and Neptune have smaller retinues. There can be no question
that a comet of first-class splendour, such as that of 1811, that of
1858, or that of 1861, is one of the most impressive spectacles that
the heavens have to offer. Unfortunately it is one which the present
generation, at least in the northern hemisphere, has had but little
opportunity of witnessing. Chambers notices 'that it may be taken as
a fact that a bright and conspicuous comet comes about once in ten
years, and a very remarkable comet once every thirty years;' and adds,
'tested then by either standard of words "bright and conspicuous," or
"specially celebrated," it may be affirmed that a good comet is now
due.' It is eleven years since that hopeful anticipation was penned,
and we are still waiting, not only for the 'specially celebrated,' but
even for the 'bright and conspicuous' comet; so that on the whole we
may be said to have a grievance. Still, there is no saying when the
grievance may be removed, as comets have a knack of being unexpected
in their developments; and it may be that some unconsidered little
patch of haze is even now drawing in from the depths which may yet
develop into a portent as wonderful as those that astonished the
generation before us in 1858 and 1861.
The multitude of comets is, in all probability, enormous. Between
the beginning of the Christian era and 1888 the number recorded was,
according to Chambers, 850; but the real number for that period must
have been indefinitely greater, as, for upwards of 1600 out of the
1888 years, only those comets which were visible to the naked eye
could have been recorded--a very small proportion of the whole. The
period 1801 to 1888 shows 270, so that in less than one century
there has been recorded almost one-third of the total for nineteen
centuries. At present no year goes by without the discovery of several
comets; but very few of them become at all conspicuous. For example,
in 1904, six comets were seen--three of these being returns of comets
previously observed, and three new discoveries; but none of these
proved at all notable objects in the ordinary sense, though Comet 1904
(_a_), discovered by Brooks, was pretty generally observed.
It would serve no useful purpose to repeat here the stories of any of
the great comets. These may be found in considerable detail in such
volumes as Chambers's 'Handbook of Astronomy,' vol. i., or Miss Agnes
Clerke's 'History of Astronomy.' Attention must rather be turned to
the question, 'What are comets?' It is a question to which no answer
of a satisfactory character could be given till within the last fifty
years. Even the great comet of 1858, the Donati, which made so deep an
impression on the public mind, and was so closely followed and
studied by astronomers, was not the medium of any great advance in
the knowledge of cometary nature. The many memoirs which it elicited
disclosed nothing fundamentally new, and broke out no new lines of
inquiry. Two things have since then revolutionized the study of the
subject--the application of the spectroscope to the various comets
that have appeared in the closing years of the nineteenth century, and
the discovery of the intimate connection between comets and meteors.
It was in 1864, a year further made memorable astronomically by Sir
William Huggins's discovery of the gaseous nature of some of the
nebulæ, that the spectroscope was first applied to the study of
a comet. The celestial visitor thus put to the question, a comet
discovered by Tempel, was in nowise a distinguished object, appearing
like a star of the second magnitude, or less, with a feeble though
fairly long tail. When analyzed by Donati, it was found to yield a
spectrum consisting of three bright bands, yellow, green, and blue,
separated by dark spaces. This observation at once modified ideas as
to cometary structure. Hitherto it had been supposed that comets shone
by reflected light; but Donati's observation revealed beyond question
that the light of the 1864 comet at all events was inherent, and that,
so far as the observation went, the comet consisted of glowing gas.
[Illustration:
PLATE XXV.
Great Comet. Photographed May 5, 1901, with the 13-inch Astrographic
Refractor of the Royal Observatory, Cape of Good Hope.]
In 1868 Sir William Huggins carried the matter one step further by
showing that the spectrum of Winnecke's comet of that year agreed
with that of olefiant gas rendered luminous by electricity; and the
presence of the hydrocarbon spectrum has since been detected in
a large number of comets. The first really brilliant comet to be
analyzed by the spectroscope was Coggia's (1874), and it presented not
only the three bright bands that had been already seen, but the whole
range of five bands characteristic of the hydrocarbon spectrum. In
certain cases, however--notably, that of Holmes's comet of 1892 and
that of the great southern comet of 1901 (Plate XXV.)--the spectrum
has not exhibited the usual bright band type, but has instead shown
merely a continuous ribbon of colour. From these analyses certain
facts emerge. First, that the gaseous surroundings of comets consist
mainly of hydrogen and carbon, and that in all probability their
luminosity is due, not to mere solar heat, but to the effect of some
electric process acting upon them during their approach to the sun;
and second, that, along with these indications of the presence
of luminous hydrocarbon compounds, there is also evidence of the
existence of solid particles, mainly in the nucleus, but also to some
extent in the rest of the comet, which shine by reflected sunlight. It
is further almost certain, from the observation by Elkin and Finlay
of the beginning of the transit of Comet 1882 (iii.) across the sun's
face, that this solid matter is not in any sense a solid mass. The
comet referred to disappeared absolutely as soon as it began to pass
the sun's edge. Had it been a solid mass or even a closely compacted
collection of small bodies it would have appeared as a black spot upon
the solar surface. The conclusion, then, is obvious that the solid
matter must be very thinly and widely spread, while its individual
particles may have any size from that of grains of sand up to that of
the large meteoric bodies which sometimes reach our earth.
Thus the state of the case as regards the constitution of comets is,
roughly speaking, this: They consist of a nucleus of solid
matter, held together, but with a very slack bond, by the power of
gravitation. From this nucleus, as the comet approaches perihelion,
the electric action of the sun, working in a manner at present
unknown, drives off volumes of luminous gas, which form the tail; and
in some comets the waves of this vapour have been actually seen rising
slowly in successive pulses from the nucleus, and then being driven
backwards much as the smoke of a steamer is driven. It has been found
also by investigation of Comet Wells 1882 and the Great Comet of 1882
that in some at least of these bodies sodium and iron are present.
The question next arises, What becomes of comets in the end? Kepler
long ago asserted his belief that they perished, as silkworms perish
by spinning their own thread, exhausting themselves by the very
efforts of tail-production which render them sometimes so brilliant to
observation; and this seems to be pretty much the case. Thus Halley's
comet, which was once so brilliant and excited so much attention,
was at its last visit a very inconspicuous object indeed. At its
apparition in 1845-1846 Biela's comet was found to have split into
two separate bodies, which were found at their return in 1852 to have
parted company widely. Since that year it has never been observed
again in the form of a comet, though, as we shall see, it has
presented itself in a different guise. The same fate has overtaken the
comets of De Vico (1844), and Brorsen (1846). The former should have
returned in 1850, but failed to keep its appointment; and the latter,
after having established a character for regularity by returning to
observation on four occasions, failed to appear in 1890, and has never
since been seen.
The mystery of such disappearances has been at least partially
dispelled by the discovery, due to Schiaparelli and other workers in
the same field, that various prominent meteor-showers travel in orbits
precisely the same as those of certain comets. Thus the shower of
meteors which takes place with greater or less brilliancy every year
from a point in the constellation Perseus has been proved to follow
the orbit of the bright comet of 1862; while the great periodic shower
of the Leonids follows the track of the comet of 1866; the orbit of
the star-shower of April 20--the Lyrids--corresponds with that of a
comet seen in 1861; and the disappearance of Biela's comet appears to
be accounted for by the other November shower whose radiant point is
in the constellation Andromeda. In fact, the state of the matter is
well summed up by Kirkwood's question: 'May not our periodic meteors
be the débris of ancient but now disintegrated comets, whose matter
has become distributed round their orbits?' The loosely compacted mass
which forms the nucleus of the comet appears to gradually lose its
cohesion under the force of solar tidal action, and its fragments
come to revolve independently in their orbit, for a time in a loosely
gathered swarm, and then gradually, as the laggards drop behind, in
the form of a complete ring of meteoric bodies, which are distributed
over the whole orbit. The Leonid shower is in the first condition, or,
rather, was when it was last seen, for it seems to be now lost to us;
the Perseid shower is in the second. The shower of the Andromedes has
since confirmed its identity with the lost comet of Biela by displays
in 1872, 1885, and 1892, at the seasons when that comet should
have returned to the neighbourhood of the sun. It appears to be
experiencing the usual fate of such showers, and becoming more
widely distributed round its orbit, and the return in 1905 was very
disappointing, the reason apparently being that the dense group in
close attendance on the comet has suffered disturbance from Jupiter
and Saturn, and now passes more than a million miles outside the
earth's orbit.
In 1843 there appeared one of the most remarkable of recorded comets.
It was not only of conspicuous brilliancy and size, though its tail
at one stage reached the enormous length of 200,000,000 miles, but was
remarkable for the extraordinarily close approach which it made to the
sun. Its centre came as near to the sun as 78,000 miles, leaving no
more than 32,000 miles between the surfaces of the two bodies; it
must, therefore, have passed clear through the corona, and very
probably through some of the prominences. Its enormous tail was
whirled, or rather appeared to be whirled, right round the sun in a
little over two hours, thus affording conclusive proof that the
tail of a comet cannot possibly be an appendage, but must consist of
perpetually renewed emanations from the nucleus. But in addition to
these wonders, the comet of 1843 proved the precursor of a series of
fine comets travelling in orbits which were practically identical. The
great southern comet of 1880 proved, when its orbit had been computed,
to follow a path almost exactly the same as that of its predecessor
of thirty-seven years before. It seemed inconceivable that a body so
remarkable as the 1843 comet should have a period of only thirty-seven
years, and yet never previously have attracted attention. Before
the question had been fairly discussed, it was accentuated by
the discovery, in 1881, of a comet whose orbit was almost
indistinguishable from that of the comet of 1807. But the 1807 comet
was not due to return till A.D. 3346. Further, the comet of 1881
proved to have a period, not of seventy-four years, as would have been
the case had it been a return of that of 1807, but of 2,429 years.
The only possible conclusion was that here were two comets which were
really fragments of one great comet which had suffered disruption,
as Biela's comet visibly did, and that one fragment followed in the
other's wake with an interval of seventy-four years.
Meanwhile, the question of the 1843 and 1880 comets was still
unsettled, and it received a fresh complication by the appearance
of the remarkable comet of 1882, whose transit of the sun has been
already alluded to, for the orbit of this new body proved to be a
reproduction, almost, but not quite exact, of those of the previous
two. Astronomers were at a greater loss than ever, for if this were a
return of the 1880 comet, then the conclusion followed that something
was so influencing its orbit as to have shortened its period from
thirty-seven to two years. The idea of the existence of some medium
round the sun, capable of resisting bodies which passed through it,
and thus causing them to draw closer to their centre of attraction and
shortening their periods, was now revived, and it seemed as though,
at its next return, this wonderful visitant must make the final plunge
into the photosphere, with what consequences none could foretell.
These forebodings proved to be quite baseless. The comet passed so
close to the sun (within 300,000 miles of his surface), that it must
have been sensibly retarded at its passage by the resisting medium,
had such a thing existed; but not the slightest retardation was
discernible. The comet suffered no check in its plunge through the
solar surroundings, and consequently the theory of the resisting
medium may be said to have received its quietus.
Computation showed that the 1882 comet followed nearly the same orbit
as its predecessors; and thus we are faced by the fact of families
of comets, travelling in orbits that are practically identical, and
succeeding one another at longer or shorter intervals. The idea that
these families have each sprung from the disruption of some much
larger body seems to be most probable, and it appears to be confirmed
by the fact that in the 1882 comet the process of further disruption
was actually witnessed. Schmidt of Athens detected one small offshoot
of the great comet, which remained visible for several days. Barnard
a few days later saw at least six small nebulous bodies close to their
parent, and a little later Brooks observed another. 'Thus,' as Miss
Agnes Clerke remarks, 'space appeared to be strewn with the filmy
débris of this beautiful but fragile structure all along the track of
its retreat from the sun.'
The state of our knowledge with regard to comets may be roughly summed
up. We have extreme tenuity in the whole body, even the nucleus
being apparently not solid, but a comparatively loose swarm of
solid particles. The nucleus, in all likelihood, shines by reflected
sunlight--in part, at all events. The nebulous surroundings and tail
are produced by solar action upon the matter of which the comet is
composed, this action being almost certainly electrical, though heat
may play some part in it. The nebulous matter appears to proceed in
waves from the nucleus, and to be swept backward along the comet's
track by some repellent force, probably electrical, exerted by
the sun. This part of the comet's structure consists mainly of
self-luminous gases, generally of the hydrocarbon type, though sodium
and iron have also been traced. Comets, certainly in many cases,
probably in all, suffer gradual degradation into swarms of meteors.
The existence of groups of comets, each group probably the outcome of
the disruption of a much larger body, is demonstrated by the fact of
successive comets travelling in almost identically similar orbits.
Finally, comets are all connected with the solar system, so far, at
least, that they accompany that system in its journey of 400,000,000
miles a year through space. Our system does not, as it were, pick up
the comets as it sweeps along upon its great journey; it carries them
along with it.
A few words may be added as to cometary observation. It is scarcely
likely that any very great number of amateur observers will ever
be attracted by the branch of comet-hunting. The work is somewhat
monotonous and laborious, and seems to require special aptitudes,
and, above all, an enormous endowment of patience. Probably the true
comet-hunter, like the poet, is born, not made; and it is not likely
that there are, nor desirable that there should be, many individuals
of the type of Messier, the 'comet-ferret.' 'Messier,' writes a
contemporary, 'is at all events a very good man, and simple as a
child. He lost his wife some years ago, and his attendance upon her
death-bed prevented his being the discoverer of a comet for which he
had been lying in wait, and which was snatched from him by Montaigne
de Limoges. This made him desperate. A visitor began to offer him
consolation for his recent bereavement, when Messier, thinking only of
the comet, answered, "I had discovered twelve; alas! to be robbed of
the thirteenth by that Montaigne!" and his eyes filled with tears.
Then, recollecting that it was necessary to deplore his wife, he
exclaimed, "Ah! cette pauvre femme!" and again wept for his comet.' In
addition to the fact that few have reached such a degree of scientific
detachment as to put a higher value upon a comet than upon the nearest
of relatives, there is the further fact that the future of cometary
discovery, and of the record of cometary change seems to lie almost
entirely with photography, which is wonderfully adapted for the work
(Plate XXVI.).
[Illustration:
PLATE XXVI.
1 2
Photographs of Swift's Comet. By Professor E. E. Barnard.
1. Taken April 4, 1892; exposure 1 hour. 2. Taken April 6, 1892;
exposure 1 hour 5 minutes.]
Anyone who desires to become a comet-hunter must, in addition to
the possession of the supreme requisites, patience and perseverance,
provide himself with an instrument of at least 4 inches aperture,
together with a good and comprehensive set of star-charts and the New
General Catalogue of nebulæ with the additions which have been made to
it. The reason for this latter item of equipment is the fact that many
telescopic comets are scarcely to be distinguished from nebulæ, and
that an accurate knowledge of the nebulous objects in the regions to
be searched for comets, or at least a means of quickly identifying
such objects, is therefore indispensable. The portions of the heavens
which afford the most likely fields for discovery will naturally be
those in the vicinity of where the sun has set at evening, or where he
is about to rise in the early morning, all comets having of necessity
to approach the sun more or less closely at their perihelion passage.
Other parts of the heavens should not be neglected; but these are the
most likely neighbourhoods.
Most of us, however, will be content to discover our comets in the
columns of the daily newspaper, or by means of a post-card from
some obliging friend. The intimation, in whatever way received, will
generally contain the position of the comet at a certain date, given
in right ascension and declination, and either a statement of its
apparent daily motion, or else a provisional set of places for several
days ahead. Having either of these, the comet's position must be
marked down on the star-map, and the course which it is likely to
pursue must be traced out in pencil by means of the data--a perfectly
simple matter of marking down the position for each day by its
celestial longitude and latitude as given. The observer will next note
carefully the alignment of the comet with the most conspicuous stars
in the neighbourhood of the particular position for the day of his
observation; and, guiding his telescope by means of these, will point
it as nearly as possible to that position. He may be lucky enough
to hit upon his object at once, especially if it be a comparatively
bright one. More probably, he will have to 'sweep' for it. In this
case the telescope must be pointed some little distance below and to
one side of the probable position of the comet, and moved slowly and
gently along, careful watch being kept upon the objects which pass
through the field, until a similar distance on the opposite side of
the position has been reached. Then raise the instrument by not more
than half a field's breadth, estimating this by the stars in the
field, and repeat the process in the opposite direction, going on
until the comet appears in the field, or until it is obvious that it
has been missed. A low power should be used at first, which may be
changed for a somewhat higher one when the object has been found. But
in no case will the use of really high magnifiers be found advisable.
It is, of course, simply impossible with the tail, for which the
naked eye is the best instrument, nor can the coma bear any degree
of magnification, though occasionally the nucleus may be sufficiently
sharply defined to bear moderate powers. The structure of the latter
should be carefully observed, with particular attention to the
question of whether any change can be seen in it, or whether there
seem any tendency to such a multiplication of nuclei as characterized
the great comet of 1882. It is possible that the pulses of vapour
sunwards from the nucleus may also be observed.
Appearance of motion, wavy or otherwise, in the tail, should also be
looked for, and carefully watched if seen. Beyond this there is not
very much that the ordinary observer can do; the determination of
positions requires more elaborate appliances, and the spectroscope is
necessary for any study of cometary constitution. It only remains
to express a wish for the speedy advent of a worthy subject for
operations.
We turn now to those bodies which, as has been pointed out, appear to
be the débris of comets which have exhausted their cometary destiny,
and ceased to have a corporate existence. Everyone is familiar with
the phenomenon known as a meteor, or shooting-star, and there are few
clear nights on which an observer who is much in the open will not see
one or more of these bodies. Generally they become visible in the form
of a bright point of light which traverses in a straight line a longer
or shorter path across the heavens, and then vanishes, sometimes
leaving behind it for a second or two a faintly luminous train. The
shooting-stars are of all degrees of brightness, from the extremely
faint streaks which sometimes flash across the field of the telescope,
up to brilliant objects, brighter than any of the planets or fixed
stars, and sometimes lighting up the whole landscape with a light like
that of the full moon.
The prevailing opinion, down to a comparatively late date, was that
shooting-stars were mere exhalations in the earth's atmosphere,
arising as one author expressed it, 'from the fermentation of acid
and alkaline bodies, which float in the atmosphere'; and it was
also suggested by eminent astronomers that they were the products
of terrestrial volcanoes, returning, after long wanderings, to their
native home.
The true study of meteoric astronomy may be said to date from the year
1833, when a shower of most extraordinary splendour was witnessed.
The magnificence of this display was the means of turning greater
attention to the subject; and it was observed as a fact, though the
importance of the observation was scarcely realized, that the meteors
all appeared to come from nearly the one point in the constellation
Leo. The fact of there being a single radiant point implied that
the meteors were all moving in parallel lines, and had entered
our atmosphere from a vast distance. Humboldt, who had witnessed a
previous appearance of this shower in 1799, suggested that it might be
a periodic phenomenon; and his suggestion was amply confirmed when
in 1866 the shower made its appearance again in scarcely diminished
splendour. Gradually other showers came to be recognised, and their
radiant points fixed; and meteoric astronomy began to be established
upon a scientific basis.
In 1866 Schiaparelli announced that the shower which radiates in
August from the constellation Perseus follows the same track as that
of Swift's comet (1862 iii.); and in the following year the great
November shower from Leo, already alluded to, was proved to have a
similar connection with Tempel's comet (1866 i.). The shower which
comes from the constellation Lyra, about April 20, describes the
same orbit as that of Comet 1861 i.; while, as already mentioned,
the mysterious disappearance of Biela's comet received a reasonable
explanation by its association with the other great November
shower--that which radiates from the constellation Andromeda. With
regard to the last-named shower, it has not only been shown that
the meteors are associated with Biela's comet, but also that they
separated from it subsequent to 1841, in which year the comet's orbit
was modified by perturbations from Jupiter. The Andromeda meteors
follow the modified orbit, and hence must have been in close
association with the comet when the perturbation was exercised.
The four outstanding meteor radiants are those named, but there are
very many others. Mr. Denning, to whom this branch of science owes so
much, estimates the number of distinct radiants known at about 4,400;
and it seems likely that every one of these showers, some of them, of
course very feeble, represents some comet deceased. The history of a
meteor shower would appear to be something like this: When the comet,
whose executor it is, has but recently deceased, it will appear as a
very brilliant periodic shower, occurring on only one or two nights
exactly at the point where the comet in its journeying would have
crossed the earth's track, and appearing only at the time when the
comet itself would have been there. Gradually the meteors get more and
more tailed out along the orbit, as runners of unequal staying powers
get strung out over a track in a long race, until the displays may
be repeated, with somewhat diminished splendour, year after year for
several years before and after the time when the parent comet is
due. At last they get thinly spread out over the whole orbit, and
the shower becomes an annual one, happening each year when the earth
crosses the orbit of the comet. This has already happened to the
Perseid shower; at least 500,000,000 miles of the orbit of Biela's
comet are studded with representatives of the Andromedes; and the
Leonid shower had already begun to show symptoms of the same process
at its appearance in 1866. Readers will remember the disappointment
caused by the failure of the Leonid shower to come up to time in 1899,
and it seems probable that the action of some perturbing cause has so
altered the orbit of this shower that it now passes almost clear
of the earth's path, so that we shall not have the opportunity of
witnessing another great display of the Leonid meteors.
So far as is known, no member of one of these great showers has
ever fallen to the earth. There are two possible exceptions to this
statement, as in 1095 a meteor fell to the ground during the progress
of a shower of Lyrids, and in 1885 another fell during a display of
the Andromedes. In neither case, however, was the radiant point noted,
and unless it was the same as that of the shower the fall of the
meteor was a mere coincidence. It seems probable that this is the
case, and the absence of any evidence that a specimen from a cometary
shower has reached the earth points to the extreme smallness of the
various members of the shower and also to the fine division of the
matter of the original comet.
In addition to the meteors originating from systematic showers, there
are also to be noted frequent and sometimes very brilliant single
meteors. Specimens of these have in many instances been obtained. They
fall into three classes--'Those in which iron is found in considerable
amount are termed siderites; those containing an admixture of iron and
stone, siderolites; and those consisting almost entirely of stone are
known as aerolites' (Denning). The mass of some of these bodies is
very considerable. Swords have been forged out of their iron, one of
which is in the possession of President Diaz of Mexico, while diamonds
have been found in meteoric irons which fell in Arizona. It may
be interesting to know that, according to a grave decision of the
American courts, a meteor is 'real estate,' and belongs to the person
on whose ground it has fallen; the alternative--that it is 'wild
game,' and the property of its captor--having been rejected by the
court. So far as I am aware, the legal status of these interesting
flying creatures has not yet been determined in Britain.
The department of meteoric astronomy is one in which useful work can
be done with the minimum of appliances. The chief requisites are a
good set of star-maps, a sound knowledge of the constellations, a
straight wand, and, above all, patience. The student must make himself
familiar with the constellations (a pleasant task, which should be
part of everyone's education), so that when a meteor crosses his
field of view he may be able to identify at once with an approach to
accuracy its points of appearance and disappearance. It is here that
the straight wand comes into play. Mr. Denning advises the use of it
as a means of guiding the eye. It is held so as to coincide with the
path of the meteor just seen, and will thus help the eye to estimate
the position and slope of the track relatively to the stars of the
constellations which it has crossed. This track should be marked as
quickly as possible on the charts. Mere descriptions of the appearance
of meteors, however beautiful, are quite valueless. It is very
interesting to be told that a meteor when first seen was 'of the size
and colour of an orange,' but later 'of the apparent size of the
full moon, and surrounded by a mass of glowing vapour which further
increased its size to that of the head of a flour-barrel'; but the
description is scarcely marked by sufficient precision of statement
for scientific purposes. The observer must note certain definite
points, of which the following is a summary: (1) Date, hour, and
minute of appearance. (2) Brightness, compared with some well-known
star, planet, or, if exceptionally bright, with the moon. (3) Right
ascension and declination of point of first appearance. (4) The
same of point of disappearance. (5) Length of track. (6) Duration of
visibility. (7) Colour, presence of streak or train, and any other
notable features. (8) Radiant point.
When these have been given with a reasonable approach to accuracy,
the observer has done his best to provide a real, though small,
contribution to the sum of human knowledge; nor is the determination
of these points so difficult as would at first appear from their
number. The fixing of the points of appearance and disappearance and
of the radiant will present a little difficulty to start with; but in
this, as in all other matters, practice will bring efficiency. It may
be mentioned that the efforts of those who take up this subject would
be greatly increased in usefulness by their establishing a connection
with the Meteor Section of the British Astronomical Association.
One curious anomaly has been established by Mr. Denning's patient
labour--the existence, namely, of what are termed 'stationary
radiants.' It is obvious that if meteors have the cometary connection
already indicated, their radiant point should never remain fixed; as
the showers move onwards in their orbit they should leave the original
radiant behind. Mr. Denning has conclusively proved, however, that
there are showers which do not follow the rule in this respect, but
proceed from a radiant which remains the same night after night, some
feeble showers maintaining the same radiant for several months. It is
not easy to see how this fact is to be reconciled with the theory of
cometary origin; but the fact itself is undeniable.