METEORS OR SHOOTING STARS
Any one who happens to gaze at the sky for a short time on a clear night
is pretty certain to be rewarded with a view of what is popularly known
as a "shooting star." Such an object, however, is not a star at all, but
has received its appellation from an analogy; for the phenomenon gives
to the inexperienced in these matters an impression as if one of the
many points of light, which glitter in the vaulted heaven, had suddenly
become loosened from its place, and was falling towards the earth. In
its passage across the sky the moving object leaves behind a trail of
light which usually lasts for a few moments. Shooting stars, or meteors,
as they are technically termed, are for the most part very small bodies,
perhaps no larger than peas or pebbles, which, dashing towards our earth
from space beyond, are heated to a white heat, and reduced to powder by
the friction resulting from their rapid passage into our atmosphere.
This they enter at various degrees of speed, in some cases so great as
45 miles a second. The speed, of course, will depend greatly upon
whether the earth and the meteors are rushing towards each other, or
whether the latter are merely overtaking the earth. In the first of
these cases the meteors will naturally collide with the atmosphere with
great force; in the other case they will plainly come into it with much
less rapidity. As has been already stated, it is from observations of
such bodies that we are enabled to estimate, though very imperfectly,
the height at which the air around our globe practically ceases, and
this height is imagined to be somewhere about 100 miles. Fortunate,
indeed, is it for us that there is a goodly layer of atmosphere over our
heads, for, were this not so, these visitors from space would strike
upon the surface of our earth night and day, and render existence still
more unendurable than many persons choose to consider it. To what a
bombardment must the moon be continually subject, destitute as she is of
such an atmospheric shield!
It is only in the moment of their dissolution that we really learn
anything about meteors, for these bodies are much too small to be seen
before they enter our atmosphere. The débris arising from their
destruction is wafted over the earth, and, settling down eventually upon
its surface, goes to augment the accumulation of that humble domestic
commodity which men call dust. This continual addition of material
tends, of course, to increase the mass of the earth, though the effect
thus produced will be on an exceedingly small scale.
The total number of meteors moving about in space must be practically
countless. The number which actually dash into the earth's atmosphere
during each year is, indeed, very great. Professor Simon Newcomb, the
well-known American astronomer, has estimated that, of the latter, those
large enough to be seen with the naked eye cannot be in all less than
146,000,000,000 per annum. Ten times more numerous still are thought to
be those insignificant ones which are seen to pass like mere sparks of
light across the field of an observer's telescope.
Until comparatively recent times, perhaps up to about a hundred years
ago, it was thought that meteors were purely terrestrial phenomena which
had their origin in the upper regions of the air. It, however, began to
be noticed that at certain periods of the year these moving objects
appeared to come from definite areas of the sky. Considerations,
therefore, respecting their observed velocities, directions, and
altitudes, gave rise to the theory that they are swarms of small bodies
travelling around the sun in elongated elliptical orbits, all along the
length of which they are scattered, and that the earth, in its annual
revolution, rushing through the midst of such swarms at the same epoch
each year, naturally entangles many of them in its atmospheric net.
The dates at which the earth is expected to pass through the principal
meteor-swarms are now pretty well known. These swarms are distinguished
from one another by the direction of the sky from which the meteors seem
to arrive. Many of the swarms are so wide that the earth takes days, and
even weeks, to pass through them. In some of these swarms, or streams,
as they are also called, the meteors are distributed with fair evenness
along the entire length of their orbits, so that the earth is greeted
with a somewhat similar shower at each yearly encounter. In others, the
chief portions are bunched together, so that, in certain years, the
display is exceptional (see Fig. 20, p. 269). That part of the heavens
from which a shower of meteors is seen to emanate is called the
"radiant," or radiant point, because the foreshortened view we get of
the streaks of light makes it appear as if they radiated outwards from
this point. In observations of these bodies the attention of astronomers
is directed to registering the path and speed of each meteor, and to
ascertaining the position of the radiant. It is from data such as these
that computations concerning the swarms and their orbits are made.
[Illustration: FIG. 20.--Passage of the Earth through the thickest
portion of a Meteor Swarm. The Earth and the Meteors are here
represented as approaching each other from opposite directions.]
For the present state of knowledge concerning meteors, astronomy is
largely indebted to the researches of Mr. W.F. Denning, of Bristol, and
of the late Professor A.S. Herschel.
During the course of each year the earth encounters a goodly number of
meteor-swarms. Three of these, giving rise to fine displays, are very
well known--the "Perseids," or August Meteors, and the "Leonids" and
"Bielids," which appear in November.
Of the above three the _Leonid_ display is by far the most important,
and the high degree of attention paid to it has laid the foundation of
meteoric astronomy in much the same way that the study of the
fascinating corona has given such an impetus to our knowledge of the
sun. The history of this shower of meteors may be traced back as far as
A.D. 902, which was known as the "Year of the Stars." It is related that
in that year, on the night of October 12th--the shower now comes about a
month later--whilst the Moorish King, Ibrahim Ben Ahmed, lay dying
before Cosenza, in Calabria, "a multitude of falling stars scattered
themselves across the sky like rain," and the beholders shuddered at
what they considered a dread celestial portent. We have, however, little
knowledge of the subsequent history of the Leonids until 1698, since
which time the maximum shower has appeared with considerable regularity
at intervals of about thirty-three years. But it was not until 1799 that
they sprang into especial notice. On the 11th November in that year a
splendid display was witnessed at Cumana, in South America, by the
celebrated travellers, Humboldt and Bonpland. Finer still, and
surpassing all displays of the kind ever seen, was that of November 12,
1833, when the meteors fell thick as snowflakes, 240,000 being estimated
to have appeared during seven hours. Some of them were even so bright as
to be seen in full daylight. The radiant from which the meteors seem to
diverge was ascertained to be situated in the head of the constellation
of the Lion, or "Sickle of Leo," as it is popularly termed, whence
their name--Leonids. It was from a discussion of the observations then
made that the American astronomer, Olmsted, concluded that these meteors
sprang upon us from interplanetary space, and were not, as had been
hitherto thought, born of our atmosphere. Later on, in 1837, Olbers
formulated the theory that the bodies in question travelled around the
sun in an elliptical orbit, and at the same time he established the
periodicity of the maximum shower.
The periodic time of recurrence of this maximum, namely, about
thirty-three years, led to eager expectancy as 1866 drew near. Hopes
were then fulfilled, and another splendid display took place, of which
Sir Robert Ball, who observed it, has given a graphic description in his
_Story of the Heavens_. The display was repeated upon a smaller scale in
the two following years. The Leonids were henceforth deemed to hold an
anomalous position among meteor swarms. According to theory the earth
cut through their orbit at about the same date each year, and so a
certain number were then seen to issue from the radiant. But, in
addition, after intervals of thirty-three years, as has been seen, an
exceptional display always took place; and this state of things was not
limited to one year alone, but was repeated at each meeting for about
three years running. The further assumption was, therefore, made that
the swarm was much denser in one portion of the orbit than
elsewhere,[27] and that this congested part was drawn out to such an
extent that the earth could pass through the crossing place during
several annual meetings, and still find it going by like a long
procession (see Fig. 20, p. 269).
In accordance with this ascertained period of thirty-three years, the
recurrence of the great Leonid shower was timed to take place on the
15th of November 1899. But there was disappointment then, and the
displays which occurred during the few years following were not of much
importance. A good deal of comment was made at the time, and theories
were accordingly put forward to account for the failure of the great
shower. The most probable explanation seems to be, that the attraction
of one of the larger planets--Jupiter perhaps--has diverted the orbit
somewhat from its old position, and the earth does not in consequence
cut through the swarm in the same manner as it used to do.
The other November display alluded to takes place between the 23rd and
27th of that month. It is called the _Andromedid_ Shower, because the
meteors appear to issue from the direction of the constellation of
Andromeda, which at that period of the year is well overhead during the
early hours of the night. These meteors are also known by the name of
_Bielids_, from a connection which the orbit assigned to them appears to
have with that of the well-known comet of Biela.
M. Egenitis, Director of the Observatory of Athens, accords to the
Bielids a high antiquity. He traces the shower back to the days of the
Emperor Justinian. Theophanes, the Chronicler of that epoch, writing of
the famous revolt of Nika in the year A.D. 532, says:--"During the same
year a great fall of stars came from the evening till the dawn." M.
Egenitis notes another early reference to these meteors in A.D. 752,
during the reign of the Eastern Emperor, Constantine Copronymous.
Writing of that year, Nicephorus, a Patriarch of Constantinople, has as
follows:--"All the stars appeared to be detached from the sky, and to
fall upon the earth."
The Bielids, however, do not seem to have attracted particular notice
until the nineteenth century. Attention first began to be riveted upon
them on account of their suspected connection with Biela's comet. It
appeared that the same orbit was shared both by that comet and the
Bielid swarm. It will be remembered that the comet in question was not
seen after its appearance in 1852. Since that date, however, the Bielid
shower has shown an increased activity; which was further noticed to be
especially great in those years in which the comet, had it still
existed, would be due to pass near the earth.
The third of these great showers to which allusion has above been made,
namely, the _Perseids_, strikes the earth about the 10th of August; for
which reason it is known on the Continent under the name of the "tears
of St. Lawrence," the day in question being sacred to that Saint. This
shower is traceable back many centuries, even as far as the year A.D.
811. The name given to these meteors, "Perseids," arises from the fact
that their radiant point is situated in the constellation of Perseus.
This shower is, however, not by any means limited to the particular
night of August 10th, for meteors belonging to the swarm may be observed
to fall in more or less varying quantities from about July 8th to August
22nd. The Perseid meteors sometimes fall at the rate of about sixty per
hour. They are noted for their great rapidity of motion, and their
trails besides often persist for a minute or two before being
disseminated. Unlike the other well-known showers, the radiants of which
are stationary, that of the Perseids shifts each night a little in an
easterly direction.
The orbit of the Perseids cuts that of the earth almost perpendicularly.
The bodies are generally supposed to be the result of the disintegration
of an ancient comet which travelled in the same orbit. Tuttle's Comet,
which passed close to the earth in 1862, also belongs to this orbit; and
its period of revolution is calculated to be 131 years. The Perseids
appear to be disseminated all along this great orbit, for we meet them
in considerable quantities each year. The bodies in question are in
general particularly small. The swarm has, however, like most others, a
somewhat denser portion, and through this the earth passed in 1848. The
_aphelion_, or point where the far end of the orbit turns back again
towards the sun, is situated right away beyond the path of Neptune, at a
distance of forty-eight times that of the earth from the sun. The comet
of 1532 also belongs to the Perseid orbit. It revisited the
neighbourhood of the earth in 1661, and should have returned in 1789.
But we have no record of it in that year; for which omission the then
politically disturbed state of Europe may account. If not already
disintegrated, this comet is due to return in 1919.
This supposed connection between comets and meteor-swarms must be also
extended to the case of the Leonids. These meteors appear to travel
along the same track as Tempel's Comet of 1866.
It is considered that the attractions of the various bodies of the
solar system upon a meteor swarm must eventually result in breaking up
the "bunched" portion, so that in time the individual meteors should
become distributed along the whole length of the orbit. Upon this
assumption the Perseid swarm, in which the meteors are fairly well
scattered along its path, should be of greater age than the Leonid. As
to the Leonid swarm itself, Le Verrier held that it was first brought
into the solar system in A.D. 126, having been captured from outer space
by the gravitative action of the planet Uranus.
The acknowledged theory of meteor swarms has naturally given rise to an
idea, that the sunlight shining upon such a large collection of
particles ought to render a swarm visible before its collision with the
earth's atmosphere. Several attempts have therefore been made to search
for approaching swarms by photography, but, so far, it appears without
success. It has also been proposed, by Mr. W.H.S. Monck, that the stars
in those regions from which swarms are due, should be carefully watched,
to see if their light exhibits such temporary diminutions as would be
likely to arise from the momentary interposition of a cloud of moving
particles.
Between ten and fifteen years ago it happened that several well-known
observers, employed in telescopic examination of the sun and moon,
reported that from time to time they had seen small dark bodies,
sometimes singly, sometimes in numbers, in passage across the discs of
the luminaries. It was concluded that these were meteors moving in space
beyond the atmosphere of the earth. The bodies were called "dark
meteors," to emphasise the fact that they were seen in their natural
condition, and not in that momentary one in which they had hitherto been
always seen; _i.e._ when heated to white heat, and rapidly vaporised, in
the course of their passage through the upper regions of our air. This
"discovery" gave promise of such assistance to meteor theories, that
calculations were made from the directions in which they had been seen
to travel, and the speeds at which they had moved, in the hope that some
information concerning their orbits might be revealed. But after a while
some doubt began to be thrown upon their being really meteors, and
eventually an Australian observer solved the mystery. He found that they
were merely tiny particles of dust, or of the black coating on the inner
part of the tube of the telescope, becoming detached from the sides of
the eye-piece and falling across the field of view. He was led to this
conclusion by having noted that a gentle tapping of his instrument
produced the "dark" bodies in great numbers! Thus the opportunity of
observing meteors beyond our atmosphere had once more failed.
_Meteorites_, also known as ærolites and fireballs, are usually placed
in quite a separate category from meteors. They greatly exceed the
latter in size, are comparatively rare, and do not appear in any way
connected with the various showers of meteors. The friction of their
passage through the atmosphere causes them to shine with a great light;
and if not shattered to pieces by internal explosions, they reach the
ground to bury themselves deep in it with a great rushing and noise.
When found by uncivilised peoples, or savages, they are, on account of
their celestial origin, usually regarded as objects of wonder and of
worship, and thus have arisen many mythological legends and deifications
of blackened stones. On the other hand, when they get into the
possession of the civilised, they are subjected to careful examinations
and tests in chemical laboratories. The bodies are, as a rule, composed
of stone, in conjunction with iron, nickel, and such elements as exist
in abundance upon our earth; though occasionally specimens are found
which are practically pure metal. In the museums of the great capitals
of both Continents are to be seen some fine collections of meteorites.
Several countries--Greenland and Mexico, for instance--contain in the
soil much meteoric iron, often in masses so large as to baffle all
attempts at removal. Blocks of this kind have been known to furnish the
natives in their vicinity for many years with sources of workable iron.
The largest meteorite in the world is one known as the Anighito
meteorite. It was brought to the United States by the explorer Peary,
who found it at Cape York in Greenland. He estimates its weight at from
90 to 100 tons. One found in Mexico, called the Bacubirito, comes next,
with an estimated weight of 27-1/2 tons. The third in size is the
Willamette meteorite, found at Willamette in Oregon in 1902. It measures
10 × 6-1/2 × 4-1/2 feet, and weighs about 15-1/2 tons.
[27] The "gem" of the meteor ring, as it has been termed.