METEORS.
There is no more interesting chapter in the history of astronomy than
that relating to meteors. A hundred years ago shooting-stars were not
considered to be astronomical phenomena. They were supposed to be merely
inflammable vapours which caught fire in the upper regions of our
atmosphere, although both Halley and the scientist _Ernst Chladni_
(1756-1827) had notions of their celestial origin. For thirty-three
years after the beginning of the century, however, nothing was heard of
meteoric astronomy, nor was the subject considered as part of the
astronomer’s labours.
A great meteoric shower took place on the night of November 12 and
morning of November 13, 1833. The shower was probably the grandest ever
witnessed, the shooting-stars being literally innumerable. The display
was best observed in America, and was attentively watched by _Denison
Olmsted_ (1791-1859), Professor of Mathematics at Yale, and by the
American physicist, _A. C. Twining_ (1801-1884). These investigators
discovered that all the meteors which fell during the great shower
seemed to come from the same part of the celestial vault. In other
words, their paths, when traced back, were found to converge to a point
near the star γ Leonis. This observation gave the death-blow to the
theory of their terrestrial origin. The point known as the “radiant” was
clearly a point independent of the Earth. Olmsted also recognised the
fact that the shower had taken place in the previous year, and he
regarded it as produced by a swarm of particles moving round the Sun in
a period of 182 days. Soon after this it was noticed that the phenomenon
took place in 1834 and subsequent years with gradually decreasing
intensity. It was then remembered that Humboldt had observed in November
1799 a very brilliant shower, and accordingly Olbers suggested that
another shower might be seen in 1867.
The falling stars of August were next proved by _Adolphe Quetelet_
(1791-1874) to form another meteoric system; and accordingly the theory
of Olmsted that the November meteors moved round the Sun in 182 days had
to be abandoned, for, says Miss Clerke, “If it would be a violation of
probability to attribute to _one_ such agglomeration a period of an
exact year or sub-multiple of a year, it would be plainly absurd to
suppose the movements of _two_ or more regulated by such highly
artificial conditions.” Accordingly Erman suggested in 1839 the theory
that meteors revolved in closed rings, intersecting the terrestrial
orbit; and that when the Earth crossed through the point of
intersection, it met some members of the swarm. The subject now remained
in abeyance for thirty-four years, if we except some wonderful ideas put
forward in 1861 by _Daniel Kirkwood_ (1813-1896), an American
astronomer, who stated his belief in the disintegration of comets into
meteors; but little attention was paid to his opinions. In 1864 the
subject was taken up by _Hubert Anson Newton_ (1830-1896), Professor at
Yale, who undertook a search through ancient records for the
thirty-three-year period of the Leonids or November meteors. His search
was highly successful, and having demonstrated the existence of the
period, Newton set himself to determine the orbit. He indicated five
possible orbits for the swarm, ranging from 33 years to 354½ days.
Newton was unable to solve the question mathematically; but here Adams,
the discoverer of Neptune, came to the rescue, and demonstrated that the
period of 33¼ years was alone possible, and that the others were
untenable. These investigations, completed in March 1867, proved the
existence of a great meteoric orbit extending to the orbit of Uranus.
Meanwhile Newton had predicted a meteoric shower on the evening of
November 13 and morning of November 14, 1866. His prediction was
fulfilled. The shower was inferior to that of 1833, but was still a
magnificent spectacle. Sir Robert Ball, then employed at Lord Rosse’s
Observatory, observed the shower, and records the impossibility of
counting the meteors. This great shower attracted the attention of
astronomers all over the world to the study of meteors. Meanwhile
Schiaparelli had been working at the subject for some time, and in four
letters addressed to Secchi, towards the end of 1866, he showed that
meteors were members of the Solar System, possessed of a greater
velocity than that of the Earth, and travelling in orbits resembling
those of comets, in the fact that they moved in no particular plane, and
that their motion was both direct and retrograde. Schiaparelli computed
the orbit of the Perseids or August meteors, and was astonished to find
it identical with the comet of August 1862. This was a proof of the
connection between these two apparently widely different types of
celestial bodies. Early in 1867 Schiaparelli found that Le Verrier’s
elements for the orbit of the Leonids were identical with those of the
comet of 1866, discovered by _Ernst Tempel_ (1821-1889). Peters of
Altona had meanwhile reached the same conclusion; while _Edmund Weiss_
(born 1837) of Vienna pointed out the similarity of the orbit of a
star-shower on April 20 and that of the comet of 1861. He also drew
attention, independently of Galle and D’Arrest, to the close connection
between the orbits of the lost Biela’s comet and the Andromedid meteors
of November.
All doubt as to the connection of comets and meteors was removed by the
great shower on November 27, 1872. Biela’s lost comet was due at
perihelion in 1872, and although searched for was not observed; but when
the Earth crossed its orbit, a great meteoric shower took place. “It
became evident,” says Miss Clerke, “that Biela’s comet was shedding over
us the pulverised products of its disintegration.” The shower was little
inferior to that of 1866. Meanwhile _Ernst Klinkerfues_ (1827-1884),
Professor at Göttingen, believing that Biela’s comet itself had
encountered the Earth, telegraphed to _Norman Robert Pogson_
(1829-1891), Government astronomer at Madras, to search for the comet in
the opposite region of the sky. Pogson did observe a comet, but
certainly not Biela’s, although probably another fragment of the missing
body.
The theory of the actual disintegration of comets was enunciated by
Schiaparelli in 1873, and developed in his work ‘Le Stelle Cadenti.’ He
was led to regard comets as cosmical clouds formed in space by “the
local concentration of celestial matter.” He then remarks that a
cosmical cloud seldom penetrates to the interior of the Solar System,
“unless it has been transformed into a parabolic current,” which may
occupy years, or centuries, in passing its perihelion, “forming in space
a river, whose transverse dimensions are very small with respect to its
length: of such currents, those which are encountered by the earth in
its annual motion are rendered visible to us under the form of showers
of meteors diverging from a certain radiant.”
Schiaparelli next pointed out that when the current of meteors
encounters a planet, the resulting perturbations cause some of the
meteoric bodies to move in separate orbits, forming the bolides and
aerolites which fall from the sky at intervals. “The term _falling
stars_” he says, “expresses simply and precisely the truth respecting
them. These bodies have the same relation to comets that the small
planets between Mars and Jupiter have to the larger planets.” In the
third chapter of his ‘Le Stelle Cadenti’ he explicitly states that “the
meteoric currents are the products of the dissolution of comets, and
consist of minute particles which certain comets have abandoned along
their orbits, by reason of the disintegrating force which the Sun and
planets exert on the rare materials of which they are composed.”
In 1878 _Alexander Stewart Herschel_ (born 1836), son of Sir John
Herschel, and a famous meteoric observer, published a list of known or
suspected coincidences of meteoric and cometary orbits, amounting to
seventy-six. Meanwhile much progress has since been made in the
observation of meteoric showers and the determination of their radiant
points. In this branch of astronomy, by far the greatest name is that of
William Frederick Denning, the self-made English astronomer. Born at
Redpost, in Somerset, in 1848, his career of meteoric observation
commenced in 1866. For the past forty years he has attentively devoted
himself to the observation of meteors. From 1872 to 1903 he determined
the radiant points of no fewer than 1179 meteoric showers. In addition
to this, he published, in 1899, a catalogue of meteoric radiants,
containing 4367; and he has carefully studied the remarkable objects
known as fireballs or “sporadic meteors.” He has occasionally been able
to trace a connection between fireballs and weak meteoric showers, but
he concludes that they “must either be merely single sporadic bodies, or
else the survivors of some meteor group, nearly exhausted by the waste
of its material during many past ages.” All of Denning’s meteoric work
has been done in his spare time, for it must be borne in mind that he
pursues the profession of accountant in Bristol, and that only his
leisure hours have been devoted to the science of astronomy. His
researches have been entirely conducted with the unaided eye. His only
instrument is a perfectly straight wand, which he uses as a help and
corrective to the eye in ascribing the paths of the meteors. Thanks to
the laborious work of this able English astronomer, the observation of
meteors is now a _scientific_ branch of astronomy. In the words of
Maunder, “for six thousand years men stared at meteors and learned
nothing, for sixty years they have studied them and learned much, and
half of what we know has been taught us in half that time by the efforts
of a single observer.”
Further meteoric showers from Biela’s comet were seen in 1885 and 1892.
The Leonid shower was confidently predicted for 1899, in accordance with
the thirty-three-year period, but the great display did not come off,
either in 1899 or 1900. In 1901 there was a certain weak shower observed
in America; and similar displays took place in 1903 and 1904. Many
explanations have been given as to the failure of the shower, the most
probable idea being that the attraction of Jupiter diverted the meteors
from their course.
Denning’s observations on meteors resulted, as early as 1877, in the
discovery of so-called “stationary radiants.” The radiant-point of a
long enduring shower usually exhibits an apparent motion, resulting from
the combined orbital motions of the Earth and the meteors; but Denning
found that in some cases the shower, though lasting for months,
persistently exhibited the same radiant-point, implying that the motion
of the Earth must be insignificant compared with that of the meteors,
computed by Ranyard at 880 miles per second. The difficulty of admitting
so great a velocity led the French astronomer, _François Felix
Tisserand_ (1845-1896), to doubt the existence of these stationary
radiants; but the fact of their existence cannot be doubted, although no
really satisfactory explanation has been offered.
Another type of meteors comprises the bodies termed respectively as
bolides, uranoliths, and aerolites,—stones which fall to the Earth from
the sky. In 1800 the French Academy declared the accounts of stones
having fallen from the heavens to be absolutely untrue. Three years
later an aerolite fell at Laigle, in the Department of Orne, on April
26, 1803, attended by a terrific explosion. In the words of Flammarion,
“Numerous witnesses affirmed that some minutes after the appearance of a
great bolide, moving from south-east to north-east, and which had been
perceived at Alençon, Caen, and Falaise, a fearful explosion, followed
by detonations like the report of cannon and the fire of musketry,
proceeded from an isolated black cloud in a very clear sky. A great
number of meteoric stones were then precipitated on the surface of the
ground, where they were collected, still smoking, over an extent of
country which measured no less than seven miles in length.”
Some aerolites, instead of being shattered into fragments, have been
observed to fall to the Earth intact, and bury themselves in the ground.
Numerous instances have been observed during the last century, and
masses of meteoric stones have been found in positions which clearly
indicate that they must have fallen from the sky. Chemists have made
analyses of the elements in these remarkable bodies, and have found them
to contain iron, magnesium, silicon, oxygen, nickel, cobalt, tin,
copper, &c. The spectrum of these aerolites, raised to incandescence,
has been studied by Vogel and by the Swedish observer, _Bernhardt
Hasselberg_ (born 1848), who detected the presence of hydrocarbons,
which are also present in cometary spectra.
When the existence of aerolites as celestial bodies was first
recognised, Laplace suggested that they had been ejected from volcanoes
on the Moon. This theory, although supported by Olbers and other
astronomers, was soon rejected. Next, it was suggested that they were
ejected from the Sun, and Proctor believed them to come from the giant
planets. A very detailed discussion of the subject is to be found in
Ball’s ‘Story of the Heavens’ (1886), in which he expresses views in
harmony with those of the Austrian physicist Tschermak. Ball
demonstrated that the meteors which fall to the Earth cannot have come
from any other planet, nor from the Sun. Accordingly, he concluded that
they were originally ejected by the volcanoes of the Earth many ages
ago, when they were active enough to throw up pieces of matter with a
velocity great enough to carry them away from the Earth altogether. Such
meteors would, however, intersect the terrestrial orbit at each
revolution.
The alternative theory to this, supported by Schiaparelli and Lockyer,
is that the aerolites are merely larger members of the meteor-swarms,
which have been deflected from their paths. The chief objection to this
theory is the absence of connection between the meteoric showers and the
falls of aerolites and bolides. Only on one occasion was a meteoric
stone observed to fall during a shower. On November 27, 1885, during the
shower of Andromedid meteors from Biela’s comet, a large bolide,
weighing more than eight pounds, fell at Mazapil, in Mexico. This,
however, was the only case hitherto observed; and it may have been
merely a coincidence.