frequently recorded. One of the most remarkable meteoric showers of the
eighteenth century occurred on the night of November 13th, 1799, and was
observed throughout North and South America and Europe. On this
memorable night thousands of falling stars were seen traversing the sky
between midnight and morning. Humboldt and Boupland, then traveling in
South America, observed the phenomena at Cumana, between two and five
o'clock in the morning. They saw an innumerable number of shooting-stars
going from north to south, appearing like brilliant fire-works. Several
of these meteors left long phosphorescent trails in the sky, and had
nuclei whose apparent diameter, in some cases, surpassed that of the
Moon.
The shower of November 13th, 1833, was still more remarkable for the
great number of meteors which traversed the heavens, and was visible
over the whole of North and South America. On that occasion the falling
stars were far too numerous to be counted, and they fell so thickly that
Prof. Olmsted, of New Haven, who observed them carefully, compared their
number at the moment of their maximum fall to half that of the flakes of
snow falling during a heavy storm. This observer estimated at 240,000
the number of meteors which must have traversed the heavens above the
horizon during the seven hours while the display was visible.
[PLATE XII.--THE NOVEMBER METEORS.
As observed between midnight and 5 o'clock A.M. on the night of November
13-14 1868.]
In the years 1866, 1867 and 1868, there were also extraordinary meteoric
displays on the night of November 13th. It was on the last mentioned
date that I had the opportunity to observe the remarkable shower of
shooting-stars of which I have attempted to represent all the
characteristic points in Plate XII. My observations were begun a little
after midnight, and continued without interruption till sunrise. Over
three thousand meteors were observed during this interval of time in the
part of the sky visible from a northern window of my house. The maximum
fall occurred between four and five o'clock, when they appeared at a
mean rate of 15 in a minute.
In general, the falling stars were quite large, many being superior to
Jupiter in brightness and apparent size, while a few even surpassed
Venus, and were so brilliant that opaque objects cast a strong shadow
during their flight. A great many left behind them a luminous train,
which remained visible for more or less time after the nucleus had
vanished. In general, these meteors appeared to move either in straight
or slightly curved orbits; but quite a number among them exhibited very
extraordinary motions, and followed very complicated paths, some of
which were quite incomprehensible.
While some moved either in wavy or zig-zag lines, strongly accentuated,
others, after moving for a time in a straight line, gradually changed
their course, curving upward or downward, thus moving in a new
direction. Several among them, which were apparently moving in a
straight line with great rapidity, suddenly altered their course,
starting at an abrupt angle in another direction, with no apparent
slackening in their motion. One of them, which was a very conspicuous
object, was moving slowly in a straight course, when of a sudden it made
a sharp turn and continued to travel in a straight line, at an acute
angle with the first, retreating, and almost going back towards the
regions from which it originally came. As nearly all the meteors which
exhibited these extraordinary motions left the trace of their passage in
the sky by a luminous trail, it was easily ascertained that these
appearances were not deceptive. On one occasion I noticed that the
change of direction in the orbit corresponded with the brightening up of
the meteor thus disturbed in its progress.
Among these meteors, some traveled very slowly, and a few seemed to
advance as if by jerks, but in general they moved very rapidly. One of
the meteors thus appearing to move by jerks left a luminous trail, upon
which the various jerks seemed to be left impressed by a succession of
bright and faint spaces along the train. Some of the largest meteors
appeared to rotate upon an axis as they advanced, and most of these
revolving meteors, as also a great number of the others, seemed to
explode just before they disappeared, sending bright fiery sparks of
different colors in all directions, although no sound was at any time
heard. The largest and most brilliant meteor observed on that night
appeared at 5h. 30m., a little before sunrise. It was very bright, and
appeared considerably larger than Venus, having quite a distinct disk.
This meteor moved very slowly, leaving behind a large phosphorescent
trail, which seemed to issue from the inside of the nucleus as it
advanced. For a moment the train increased in size and brightness close
to the nucleus, which then appeared as an empty transparent sphere,
sprinkled all over with minute fiery sparks; the nucleus then suddenly
burst out into luminous particles, which immediately vanished, only the
luminous trail of considerable dimensions being left.
Many of the trails thus left by the meteors retained their luminosity
for several minutes, and sometimes for over a quarter of an hour. These
trails slowly changed their form and position; but it is perhaps
remarkable that almost all those which I observed on that night assumed
the same general form--that of an open, irregular ring, or horse-shoe,
somewhat resembling the letter C. This ring form was subsequently
transformed into an irregular, roundish cumulus-like cloud. The trail
left by a very large meteor, which I observed on the evening of
September 5th, 1880, also exhibited the same general character of
transformation.
While I was observing a long brilliant trail left by a meteor on the
night of November 13th, 1868, it was suddenly crossed by another bright
shooting-star. The latter apparently went through the luminous substance
forming the trail, which was suddenly altered in form, and considerably
diminished in brightness simultaneously with this passage, although
electrical action at some distance might perhaps as well explain the
sudden change observed.
In the majority of cases the meteors appeared white; but many,
especially the largest, exhibited a variety of brilliant colors, among
which the red, blue, green, yellow and purple were the most common. In
general the trails exhibited about the same color as the nucleus, but
much fainter, and they were usually pervaded by a greenish tint. In some
instances the trails were of quite a different color from the nucleus.
The luminous cloud observed at 5h. 30m. on the morning of November 14th,
1868, after having passed through the series of transformations above
described, remained visible for a long while after sunrise, appearing
then as a small cirrus cloud, exactly similar in appearance to the
hundreds of small cirrus clouds then visible in the sky, which had
probably the same meteoric origin. For over three hours after sunrise,
these cirrus clouds remained visible in the sky, moving all together
with the wind in the high regions of the atmosphere.
Although Plate XII. is intended to represent all the characteristics
exhibited by the meteors observed on that night, every form represented
having been obtained by direct observation, yet the number is much
greater than it was at any single moment during the particular shower of
1868. As regards number, the intention was to give an idea of a great
meteoric shower, such as that of 1833, for instance. Although many of
the falling stars seem to be close to the Earth's surface, yet this is
only an effect of perspective due to their great distance, very few of
these meteors ever coming into the lower regions of our atmosphere at
all.
The phenomena exhibited during other great meteoric showers have been
similar to those presented by the shower just described, the only
differences consisting in variations of size and brightness in the
meteors, and also in the trails, which sometimes are not so numerous as
they were in 1868.
While some shooting-stars move so rapidly that they can hardly be
followed in their orbits, others move so slowly that the sight can
easily follow them, and even remark the peculiarities of their
movements, some remaining visible for half a minute. Some of the falling
stars move at the rapid rate of 100 miles a second, but others only 10
miles a second, and even less. In general, they move about half as fast
again as the Earth in its orbit. The arcs described by the meteors in
the sky are variable. While some extend 80° and even 100°, others are
hardly half a degree in length. While some shooting-stars are so faint
that they can hardly be seen through the largest telescopes, others are
so large and brilliant that they can be seen in the day-time. In
general, a shooting-star of average brightness resembles a star of the
third or fourth magnitude.
Whatever may be the origin of the shooting-stars, they are, when we see
them, not in the celestial spaces, like the planets, the comets, or the
stars, but in our atmosphere, through which they travel as long as they
remain visible. The height at which they appear and disappear is
variable, but in general they are about 80 miles above the surface of
our globe when they are first seen, and at about 55 miles when they
disappear. In many cases, however, they have been observed at greater
elevations, as also at smaller. A meteor simultaneously observed at two
different stations first appeared at the height of 285 miles, and was
last seen at 192 miles above the Earth's surface; but in rare cases the
falling stars have been seen below a layer of clouds completely covering
the sky. I myself saw one such shooting-star a few years since. The fact
that the meteors are visible at so great elevations, proves that our
atmosphere extends much farther than was formerly supposed, although at
these great heights it must be extremely rarefied, and very different
from what it is in its lower regions.
There is a remarkable difference between the sporadic meteors seen in
the sky on every night, and the meteoric showers observed only at
comparatively rare intervals. While the first appear from different
points in the sky and travel in all directions, being perfectly
independent, the meteors of a shower all come from the same point of the
heavens, from which they apparently diverge in all directions. This
point of divergence of the meteors is called the _radiant point_ of the
shower. Although the meteors seem to diverge in all directions from the
radiant point, yet they all move in approximately parallel lines, the
divergence being an effect of perspective.
Whatever may be the position of the radiant point in the constellations,
it remains as fixed in the sky as the stars themselves, and participates
with them in the apparent motion which they undergo by the effect of the
diurnal motion, and thus rises and sets with the constellation to which
it belongs. This fact is sufficient to prove that the orbits of these
meteors are independent of the Earth's motion, and that consequently
they do not originate in our atmosphere. It has been shown by Encke that
the radiant point of the meteoric shower of November 13th is precisely
the point towards which our globe moves in space on November 13th; a
tangent to the Earth's orbit would pass through this radiant point.
The meteoric showers are particularly remarkable, not merely because of
the large number of meteors which are visible and the fact that they all
follow a common orbit, but chiefly because they have a periodic return,
either after an interval of a year, or after a lapse of several years.
At the beginning of the present century only two meteoric showers were
known, those of August 10th and of November 13th, and their periodicity
had not yet been recognized, although it had begun to be suspected. It
was only in 1836 that Quetelet and Olbers ventured to predict the
reappearance of the November meteors in the year 1867. Having made
further investigations, Prof. Newton, of Yale College, announced their
return in the year 1866. In both of these years, as also in 1868, the
meteors were very numerous, and were observed in Europe and in America
on the night of November 13th. The predictions having thus been
fulfilled, the periodicity of the meteors was established. Since then,
other periodic showers have been recognized, although they are much less
important in regard to number than those of August and November, except
that of November 27th, which exhibited so brilliant a display in Europe
in 1872. These successive appearances have established the main fact
that meteoric showers are more or less visible every year when the Earth
occupies certain positions in its orbit.
The meteoric shower of the 10th of August has its radiant point situated
in the vicinity of the variable star Algol, in the constellation
Perseus, from which its meteors have received the name of Perseids.
Although varying in splendor, this meteoric swarm never fails to make
its appearance every year. The Perseids move through our atmosphere at
the rate of 37 miles per second. The shower usually lasts about six
hours.
The meteoric shower of November 13th has its radiant point situated in
the vicinity of the star Gamma, in the constellation Leo, from which its
meteors have been called Leonids. But while the August meteors recur
regularly every year, with slight variations, the shower of November
does not occur with the same regularity. During several years it is
hardly noticeable, and is even totally absent, while in other years it
is very remarkable. Every 33 years an extraordinary meteoric shower
occurs on the 13th of November, and the phenomenon is repeated on the
two succeeding years at the same date, but with a diminution in its
splendor at each successive return. The Leonids move in an opposite
direction to that of the Earth, and travel in our atmosphere with an
apparent velocity of 45 miles per second, this being about the maximum
velocity observed in falling stars. But when the motion of our globe is
taken into account, and a deduction is made of the 18 miles which it
travels per second, it is found that these meteors move at an actual
mean rate of 27 miles a second.
In a meteoric shower the stars do not fall uniformly throughout the
night, there being a time when they appear in greater numbers. Usually
it is towards morning, between 4 and 6 o'clock, that the maximum occurs.
The probable cause of this phenomenon will be explained in its place
hereafter.
The orbits of the meteoric showers are not all approximately in the same
plane, like those of the planets, but rather resemble those of comets,
and have all possible inclinations to the ecliptic. Like the comets,
too, the different meteoric showers have either direct or retrograde
motion.
The shooting-stars were formerly considered as atmospheric meteors,
caused by the combustion of inflammable gases generated at the surface
of the Earth, and transported to the high regions of our atmosphere by
their low specific gravity. But the considerable height at which they
usually appear, the great velocity of their motion, the common orbit
followed by the meteors of the same shower, and the periodicity of their
recurrence, do not permit us now to entertain these ideas, or to doubt
their cosmical origin. But what is their nature?
It is now generally admitted that innumerable minute bodies, moving in
various directions around the Sun, are scattered in the interplanetary
spaces through which our globe travels. It has been supposed that
congregations of such minute bodies form elliptical rings, within which
they are all moving in close parallel orbits around the Sun. On the
supposition that such rings intersect the orbit of the Earth at the
proper places, it was practicable to account for the shooting-stars by
the passage through our atmosphere of the numerous minute cosmical
bodies composing the rings, and the Leonid and Perseid showers were so
explained. But when the elements of the orbits of these two last swarms
came to be better known, and were compared with those of other celestial
bodies, it was found necessary to alter this theory.
It had for a long while been suspected that some kind of relation
existed between the shooting-stars and the comets. This idea, vaguely
formulated by Kepler more than two centuries ago, more clearly expressed
by Chladni, and still more by Mr. Grey, before the British Association,
at Liverpool, in 1855, has recently received a brilliant confirmation by
the researches of Professor Schiaparelli, Director of the Observatory of
Milan. A thorough investigation of the orbits of the August and November
meteors led Schiaparelli to the discovery of a remarkable relation
between meteoric and cometary orbits. By comparing the elements of these
meteoric orbits with those of comets, he found a very close resemblance
between the orbit of the August meteors and that of the comet 1862,
III., and again between the orbit of the November meteors and that of
Tempel's comet, 1866, I. These resemblances were too striking to be the
result of mere chance, and demonstrated the identity of these cometary
orbits with those of the Perseid and Leonid showers. In accordance with
these new facts, it is now admitted that the meteoric showers result
from the passage of our globe through swarms of meteoric particles
following the orbits of comets, which intersect the orbit of the Earth.
Professor Schiaparelli has attempted to show how these meteoric swarms
were originally scattered along the orbits of comets, by supposing these
bodies to originate from nebulous masses, which, in entering the sphere
of attraction of the Sun, are gradually scattered along their orbits,
and finally form comets followed by long trails of meteoric particles.
It has been shown that in approaching the Sun the comets become
considerably elongated, their particles being disseminated over immense
distances by the solar repulsion. It seems probable that, owing to its
feeble attractive power, the nucleus is incompetent to recall the
scattered cometary particles and retain them in its grasp when they are
relieved from the solar repulsion, so that they remain free from the
nucleus, although they continue to move along its orbit. It is
supposable that these cometary particles will scatter more and more in
course of time. Forming at first an elongated meteoric cloud, they will
finally spread along the whole orbit, and thus form a ring of meteoric
particles. Since our globe constantly moves in its orbit and daily
occupies a different position, it follows that at any point where such a
cometary orbit happens to cross that of the Earth, our globe will
necessarily encounter the cometary particles as a shower of meteors.
This encounter will take place at a certain time of the year, either
yearly, if they form a continuous ring, or after a succession of years,
if they simply form an elongated cloud. Such meteoric clouds or rings
would not be visible in ordinary circumstances, even through the largest
telescopes, except on penetrating the upper regions of our atmosphere,
when they would appear as showers of falling stars. It is supposed that
in penetrating our atmosphere, even in its most rarefied regions, these
meteors are heated by the resistance offered by the air to their motion,
first becoming luminous and then being finally vaporized and burnt
before they can reach the surface of the Earth.
The orbit of the comet of 1862, III., which so closely corresponds with
that of the Perseid meteors, is much more extended than that of Tempel's
comet corresponding with that of the Leonids. While the first extends
far beyond the orbit of Neptune, the latter only goes a little beyond
that of Uranus. The former orbit makes a considerable angle with the
plane of the Earth's orbit, but the latter is much nearer to parallelism
with it. The period of revolution of the first is 108 years, and that of
the last about 33¼ years.
From the fact that the Perseid shower occurs yearly on the 10th of
August, when the Earth crosses the orbit of the comet of 1862, III., it
is supposed that the cometary particles producing this shower are
disseminated along the whole orbit, and form a ring encircling the Sun
and Earth. To explain the yearly variations in the number of the
shooting-stars observed, these particles are supposed to be unequally
distributed over the orbit, being more crowded at one place than they
are at another. In order to explain the meteoric shower of Leonids,
which appears in all its splendor every 33 years, and then with
diminished intensity for two successive years, after which it is without
importance, it is supposed that the cometary particles of the comet of
1866, I., have not as yet spread all along the orbit, a sufficient time
not having been allowed, but form an elongated meteoric cloud, more
dense in its front than in its rear part. From these considerations it
has been supposed also that the comet of 1866, I., is of a more recent
date than that of 1862, III. While Tempel's comet makes its revolution
around the Sun in about 33 years, this meteoric cloud, which has the
same period and returns to the same point of its orbit every 33 years,
encounters our globe for three successive years. The first year we are
passing through its densest parts, and the two following years in less
and less crowded parts, from which result the observed phenomena. An
idea of the extent of this meteoric cloud may be formed from the fact
that, with its cometary velocity of motion, it takes this cloud three
years at least to cross the Earth's orbit. From recent researches it
would appear that the Leonid cloud is not single, but that at least two
others of smaller importance exist, and have periods of 33¼ years.
Biela's comet, which was divided into two parts in 1846, is another of
the few comets whose orbit approaches that of the Earth. Possessing this
knowledge, and knowing then the close connection existing between
meteors and comets, astronomers supposed that there were sufficient
reasons to expect a meteoric shower when this comet was passing near the
Earth. They consequently expected a meteoric display in 1872, when our
globe was to cross its orbit. Their anticipation was plainly fulfilled,
and on the night of November 27th, 1872, a splendid meteoric display,
having its radiant point in the constellation Andromeda, was observed in
Europe, and also in America, but the meteors seen here were not so
numerous as in Europe. Other meteoric showers of less importance, such
as that of April 20th, for instance, have also been identified with
cometary orbits, so that now no doubt seems to remain as to the identity
of cometary particles and shooting-stars.
The fact that the maximum number of meteors is always observed in the
morning hours, supports the hypothesis of the cosmic origin of the
shooting-stars, since the regions of the Earth where it is morning are
precisely those fronting the regions towards which our globe is moving
in space, and accordingly encounter more directly the meteors moving in
their orbit. The greater abundance of falling stars at that time may
thus be accounted for.
The number of meteors penetrating our atmosphere must be very great;
there is not an hour and probably not a minute during which none fall.
From various considerations, some astronomers have estimated at from
65,000,000,000 to 146,000,000,000 the total number of shooting-stars
yearly penetrating in our atmosphere. The actual number is undoubtedly
great, yet the fact that the meteors are rarely seen through the
telescope while employed in observing various celestial objects, does
not indicate that they are so numerous as these figures imply. It is
only occasionally that one is seen traversing the field of the
instrument. Even when the sky is observed with a low power eye-piece for
several hours in succession, many nights may pass without disclosing
one, although an observer, sweeping the sky more freely with the naked
eye, may often perceive four or five during an ordinary night.