COMETS AND METEORS
158. VISITORS IN THE SOLAR SYSTEM.--All of the objects--sun, moon,
planets, stars--which we have thus far had to consider, are permanent
citizens of the sky, and we have no reason to suppose that their present
appearance differs appreciably from what it was 1,000 years or 10,000
years ago. But there is another class of objects--comets, meteors--which
appear unexpectedly, are visible for a time, and then vanish and are
seen no more. On account of this temporary character the astronomers of
ancient and mediæval times for the most part refused to regard them as
celestial bodies but classed them along with clouds, fogs,
Jack-o'-lanterns, and fireflies, as exhalations from the swamps or the
volcano; admitting them to be indeed important as harbingers of evil to
mankind, but having no especial significance for the astronomer.
The comet of 1618 A. D. inspired the lines--
"Eight things there be a Comet brings,
When it on high doth horrid range:
Wind, Famine, Plague, and Death to Kings,
War, Earthquakes, Floods, and Direful Change,"
which, according to White (History of the Doctrine of Comets), were to
be taught in all seriousness to peasants and school children.
It was by slow degrees, and only after direct measurements of parallax
had shown some of them to be more distant than the moon, that the tide
of old opinion was turned and comets were transferred from the sublunary
to the celestial sphere, and in more recent times meteors also have
been recognized as coming to us from outside the earth. A meteor, or
shooting star as it is often called, is one of the commonest of
phenomena, and one can hardly watch the sky for an hour on any clear and
moonless night without seeing several of those quick flashes of light
which look as if some star had suddenly left its place, dashed swiftly
across a portion of the sky and then vanished. It is this misleading
appearance that probably is responsible for the name shooting star.
[Illustration: FIG. 101.--Donati's comet.--BOND.]
159. COMETS.--Comets are less common and much longer-lived than meteors,
lasting usually for several weeks, and may be visible night after night
for many months, but never for many years, at a time. During the last
decade there is no year in which less than three comets have appeared,
and 1898 is distinguished by the discovery of ten of these bodies, the
largest number ever found in one year. On the average, we may expect a
new comet to be found about once in every ten weeks, but for the most
part they are small affairs, visible only in the telescope, and a fine
large one, like Donati's comet of 1858 (Fig. 101), or the Great Comet of
September, 1882, which was visible in broad daylight close beside the
sun, is a rare spectacle, and as striking and impressive as it is rare.
[Illustration: FIG. 102.--Some famous comets.]
Note in Fig. 102 the great variety of aspect presented by some of the
more famous comets, which are here represented upon a very small scale.
Fig. 103 is from a photograph of one of the faint comets of the year
1893, which appears here as a rather feeble streak of light amid the
stars which are scattered over the background of the picture. An
apparently detached portion of this comet is shown at the extreme left
of the picture, looking almost like another independent comet. The
clean, straight line running diagonally across the picture is the flash
of a bright meteor that chanced to pass within the range of the camera
while the comet was being photographed.
A more striking representation of a moderately bright telescopic comet
is contained in Figs. 104 and 105, which present two different views of
the same comet, showing a considerable change in its appearance. A
striking feature of Fig. 105 is the star images, which are here drawn
out into short lines all parallel with each other. During the exposure
of 2h. 20m. required to imprint this picture upon the photographic
plate, the comet was continually changing its position among the stars
on account of its orbital motion, and the plate was therefore moved
from time to time, so as to follow the comet and make its image always
fall at the same place. Hence the plate was continually shifted relative
to the stars whose images, drawn out into lines, show the direction in
which the plate was moved--i. e., the direction in which the comet was
moving across the sky. The same effect is shown in the other
photographs, but less conspicuously than here on account of their
shorter exposure times.
These pictures all show that one end of the comet is brighter and
apparently more dense than the other, and it is customary to call this
bright part the _head_ of the comet, while the brushlike appendage that
streams away from it is called the comet's _tail_.
[Illustration: FIG. 103.--Brooks's comet, November 13, 1893. BARNARD.]
160. THE PARTS OF A COMET.--It is not every comet that has a tail,
though all the large ones do, and in Fig. 103 the detached piece of
cometary matter at the left of the picture represents very well the
appearance of a tailless comet, a rather large but not very bright star
of a fuzzy or hairy appearance. The word comet means long-haired or
hairy star. Something of this vagueness of outline is found in all
comets, whose exact boundaries are hard to define, instead of being
sharp and clean-cut like those of a planet or satellite. Often,
however, there is found in the head of a comet a much more solid
appearing part, like the round white ball at the center of Fig. 106,
which is called the nucleus of the comet, and appears to be in some sort
the center from which its activities radiate. As shown in Figs. 106 and
107, the nucleus is sometimes surrounded by what are called envelopes,
which have the appearance of successive wrappings or halos placed about
it, and odd, spurlike projections, called jets, are sometimes found in
connection with the envelopes or in place of them. These figures also
show what is quite a common characteristic of large comets, a dark
streak running down the axis of the tail, showing that the tail is
hollow, a mere shell surrounding empty space.
[Illustration: FIG. 104.--Swift's comet, April 17, 1892.--BARNARD.]
The amount of detail shown in Figs. 106 and 107 is, however, quite
exceptional, and the ordinary comet is much more like Fig. 103 or 104.
Even a great comet when it first appears is not unlike the detached
fragment in Fig. 103, a faint and roundish patch of foggy light which
grows through successive stages to its maximum estate, developing a
tail, nucleus, envelopes, etc., only to lose them again as it shrinks
and finally disappears.
[Illustration: FIG. 105.--Swift's comet, April 24, 1892.--BARNARD.]
161. THE ORBITS OF COMETS.--It will be remembered that Newton found, as
a theoretical consequence of the law of gravitation, that a body moving
under the influence of the sun's attraction might have as its orbit any
one of the conic sections, ellipse, parabola, or hyperbola, and among
the 400 and more comet orbits which have been determined every one of
these orbit forms appears, but curiously enough there is not a hyperbola
among them which, if drawn upon paper, could be distinguished by the
unaided eye from a parabola, and the ellipses are all so long and
narrow, not one of them being so nearly round as is the most eccentric
planet orbit, that astronomers are accustomed to look upon the parabola
as being the normal type of comet orbit, and to regard a comet whose
motion differs much from a parabola as being abnormal and calling for
some special explanation.
The fact that comet orbits are parabolas, or differ but little from
them, explains at once the temporary character and speedy disappearance
of these bodies. They are visitors to the solar system and visible for
only a short time, because the parabola in which they travel is not a
closed curve, and the comet, having passed once along that portion of it
near the earth and the sun, moves off along a path which ever thereafter
takes it farther and farther away, beyond the limit of visibility. The
development of the comet during the time it is visible, the growth and
disappearance of tail, nucleus, etc., depend upon its changing distance
from the sun, the highest development and most complex structure being
presented when it is nearest to the sun.
[Illustration: FIG. 106.--Head of Coggia's comet, July 13,
1874.--TROUVELOT.]
Fig. 108 shows the path of the Great Comet of 1882 during the period
in which it was seen, from September 3, 1882, to May 26, 1883. These
dates--IX, 3, and V, 26--are marked in the figure opposite the parts
of the orbit in which the comet stood at those times. Similarly, the
positions of the earth in its orbit at the beginning of September,
October, November, etc., are marked by the Roman numerals IX, X, XI,
etc. The line _S V_ shows the direction from the sun to the vernal
equinox, and _S_ [Ô] is the line along which the plane of the comet's
orbit intersects the plane of the earth's orbit--i. e., it is the line
of nodes of the comet orbit. Since the comet approached the sun from the
south side of the ecliptic, all of its orbit, save the little segment
which falls to the left of _S_ [Ô], lies below (south) of the plane of
the earth's orbit, and the part which would be hidden if this plane were
opaque is represented by a broken line.
[Illustration: FIG. 107.--Head of Donati's comet, September 30, October
2, 1858.--BOND.]
162. ELEMENTS OF A COMET'S ORBIT.--There is a theorem of geometry to the
effect that through any three points not in the same straight line one
circle, and only one, can be drawn. Corresponding to this there is a
theorem of celestial mechanics, that through any three positions of a
comet one conic section, and only one, can be passed along which the
comet can move in accordance with the law of gravitation. This conic
section is, of course, its orbit, and at the discovery of a comet
astronomers always hasten to observe its position in the sky on
different nights in order to obtain the three positions (right
ascensions and declinations) necessary for determining the particular
orbit in which it moves. The circle, to which reference was made above,
is completely ascertained and defined when we know its radius and the
position of its center. A parabola is not so simply defined, and five
numbers, called the _elements_ of its orbit, are required to fix
accurately a comet's path around the sun. Two of these relate to the
position of the line of nodes and the angle which the orbit plane makes
with the plane of the ecliptic; a third fixes the direction of the axis
of the orbit in its plane, and the remaining two, which are of more
interest to us, are the date at which the comet makes its nearest
approach to the sun (_perihelion passage_) and its distance from the sun
at that date (_perihelion distance_). The date, September 17th, placed
near the center of Fig. 108, is the former of these elements, while the
latter, which is too small to be accurately measured here, may be found
from Fig. 109 to be 0.82 of the sun's diameter, or, in terms of the
earth's distance from the sun, 0.008.
[Illustration: FIG. 108.--Orbits of the earth and the Great Comet of
1882.]
Fig. 109 shows on a large scale the shape of that part of the orbit near
the sun and gives the successive positions of the comet, at intervals of
2/10 of a day, on September 16th and 17th, showing that in less than 10
hours--17.0 to 17.4--the comet swung around the sun through an angle of
more than 240°. When at its perihelion it was moving with a velocity of
300 miles per second! This very unusual velocity was due to the comet's
extraordinarily close approach to the sun. The earth's velocity in its
orbit is only 19 miles per second, and the velocity of any comet at any
distance from the sun, provided its orbit is a parabola, may be found by
dividing this number by the square root of half the comet's
distance--e. g., 300 miles per second equals 19 ÷ 0.004^{1/2}.
[Illustration: FIG. 109.--Motion of the Great Comet of 1883 in passing
around the sun.]
Most of the visible comets have their perihelion distances included
between 1/3 and 4/3 of the earth's distance from the sun, but
occasionally one is found, like the second comet of 1885, whose nearest
approach to the sun lies far outside the earth's orbit, in this case
halfway out to the orbit of Jupiter; but such a comet must be a very
large one in order to be seen at all from the earth. There is, however,
some reason for believing that the number of comets which move around
the sun without ever coming inside the orbit of Jupiter, or even that of
Saturn, is much larger than the number of those which come close enough
to be discovered from the earth. In any case we are reminded of Kepler's
saying, that comets in the sky are as plentiful as fishes in the sea,
which seems to be very little exaggerated when we consider that,
according to Kleiber, out of all the comets which enter the solar system
probably not more than 2 or 3 per cent are ever discovered.
[Illustration: FIG. 110.--The Great Comet of 1843.]
163. DIMENSIONS OF COMETS.--The comet whose orbit is shown in Figs. 108
and 109 is the finest and largest that has appeared in recent years. Its
tail, which at its maximum extent would have more than bridged the space
between sun and earth (100,000,000 miles), is made very much too short
in Fig. 109, but when at its best was probably not inferior to that of
the Great Comet of 1843, shown in Fig. 110. As we shall see later,
there is a peculiar and special relationship between these two comets.
The head of the comet of 1882 was not especially large--about twice the
diameter of the ball of Saturn--but its nucleus, according to an
estimate made by Dr. Elkin when it was very near perihelion, was as
large as the moon. The head of the comet shown in Fig. 107 was too large
to be put in the space between the earth and the moon, and the Great
Comet of 1811 had a head considerably larger than the sun itself. From
these colossal sizes down to the smallest shred just visible in the
telescope, comets of all dimensions may be found, but the smaller the
comet the less the chance of its being discovered, and a comet as small
as the earth would probably go unobserved unless it approached very
close to us.
164. THE MASS OF A COMET.--There is no known case in which the mass of a
comet has ever been measured, yet nothing about them is more sure than
that they are bodies with mass which is attracted by the sun and the
planets, and which in its turn attracts both sun and planets and
produces perturbations in their motion. These perturbations are,
however, too small to be measured, although the corresponding
perturbations in the comet's motion are sometimes enormous, and since
these mutual perturbations are proportional to the masses of comet and
planet, we are forced to say that, by comparison with even such small
bodies as the moon or Mercury, the mass of a comet is utterly
insignificant, certainly not as great as a ten-thousandth part of the
mass of the earth. In the case of the Great Comet of 1882, if we leave
its hundred million miles of tail out of account and suppose the entire
mass condensed into its head, we find by a little computation that the
average density of the head under these circumstances must have been
less than 1/1500 of the density of air. In ordinary laboratory practice
this would be called a pretty good vacuum. A striking observation made
on September 17, 1882, goes to confirm the very small density of this
comet. It is shown in Fig. 109 that early on that day the comet crossed
the line joining earth and sun, and therefore passed in transit over the
sun's disk. Two observers at the Cape of Good Hope saw the comet
approach the sun, and followed it with their telescopes until the
nucleus actually reached the edge of the sun and disappeared, behind it
as they supposed, for no trace of the comet, not even its nucleus, could
be seen against the sun, although it was carefully looked for. Now, the
figure shows that the comet passed between the earth and sun, and its
densest parts were therefore too attenuated to cut off any perceptible
fraction of the sun's rays. In other cases stars have been seen through
the head of a comet, shining apparently with undimmed luster, although
in some cases they seem to have been slightly refracted out of their
true positions.
165. METEORS.--Before proceeding further with the study of comets it is
well to turn aside and consider their humbler relatives, the shooting
stars. On some clear evening, when the moon is absent from the sky,
watch the heavens for an hour and count the meteors visible during that
time. Note their paths, the part of the sky where they appear and where
they disappear, their brightness, and whether they all move with equal
swiftness. Out of such simple observations with the unaided eye there
has grown a large and important branch of astronomical science, some
parts of which we shall briefly summarize here.
A particular meteor is a local phenomenon seen over only a small part of
the earth's surface, although occasionally a very big and bright one may
travel and be visible over a considerable territory. Such a one in
December, 1876, swept over the United States from Kansas to
Pennsylvania, and was seen from eleven different States. But the
ordinary shooting star is much less conspicuous, and, as we know from
simultaneous observations made at neighboring places, it makes its
appearance at a height of some 75 miles above the earth's surface,
occupies something like a second in moving over its path, and then
disappears at a height of about 50 miles or more, although occasionally
a big one comes down to the very surface of the earth with force
sufficient to bury itself in the ground, from which it may be dug up,
handled, weighed, and turned over to the chemist to be analyzed. The
pieces thus found show that the big meteors, at least, are masses of
stone or mineral; iron is quite commonly found in them, as are a
considerable number of other terrestrial substances combined in rather
peculiar ways. But no chemical element not found on the earth has ever
been discovered in a meteor.
166. NATURE OF METEORS.--The swiftness with which the meteors sweep down
shows that they must come from outside the earth, for even half their
velocity, if given to them by some terrestrial volcano or other
explosive agent, would send them completely away from the earth never to
return. We must therefore look upon them as so many projectiles,
bullets, fired against the earth from some outside source and arrested
in their motion by the earth's atmosphere, which serves as a cushion to
protect the ground from the bombardment which would otherwise prove in
the highest degree dangerous to both property and life. The speed of the
meteor is checked by the resistance which the atmosphere offers to its
motion, and the energy represented by that speed is transformed into
heat, which in less than a second raises the meteor and the surrounding
air to incandescence, melts the meteor either wholly or in part, and
usually destroys its identity, leaving only an impalpable dust, which
cools off as it settles slowly through the lower atmosphere to the
ground. The heating effect of the air's resistance is proportional to
the square of the meteor's velocity, and even at such a moderate speed
as 1 mile per second the effect upon the meteor is the same as if it
stood still in a bath of red-hot air. Now, the actual velocity of
meteors through the air is often 30 or 40 times as great as this, and
the corresponding effect of the air in raising its temperature is more
than 1,000 times that of red heat. Small wonder that the meteor is
brought to lively incandescence and consumed even in a fraction of a
second.
167. THE NUMBER OF METEORS.--A single observer may expect to see in the
evening hours about one meteor every 10 minutes on the average,
although, of course, in this respect much irregularity may occur. Later
in the night they become more frequent, and after 2 A. M. there are
about three times as many to be seen as in the evening hours. But no one
person can keep a watch upon the whole sky, high and low, in front and
behind, and experience shows that by increasing the number of observers
and assigning to each a particular part of the sky, the total number of
meteors counted may be increased about five-fold. So, too, the observers
at any one place can keep an effective watch upon only those meteors
which come into the earth's atmosphere within some moderate distance of
their station, say 50 or 100 miles, and to watch every part of that
atmosphere would require a large number of stations, estimated at
something more than 10,000, scattered systematically over the whole face
of the earth. If we piece together the several numbers above considered,
taking 14 as a fair average of the hourly number of meteors to be seen
by a single observer at all hours of the night, we shall find for
the total number of meteors encountered by the earth in 24 hours,
14 × 5 × 10,000 × 24 = 16,800,000. Without laying too much stress upon
this particular number, we may fairly say that the meteors picked up by
the earth every day are to be reckoned by millions, and since they come
at all seasons of the year, we shall have to admit that the region
through which the earth moves, instead of being empty space, is really a
dust cloud, each individual particle of dust being a prospective meteor.
On the average these individual particles are very small and very far
apart; a cloud of silver dimes each about 250 miles from its nearest
neighbor is perhaps a fair representation of their average mass and
distance from each other, but, of course, great variations are to be
expected both in the size and in the frequency of the particles. There
must be great numbers of them that are too small to make shooting stars
visible to the naked eye, and such are occasionally seen darting by
chance across the field of view of a telescope.
168. THE ZODIACAL LIGHT is an effect probably due to the reflection of
sunlight from the myriads of these tiny meteors which occupy the space
inside the earth's orbit. It is a faint and diffuse stream of light,
something like the Milky Way, which may be seen in the early evening or
morning stretching up from the sunrise or sunset point of the horizon
along the ecliptic and following its course for many degrees, possibly
around the entire circumference of the sky. It may be seen at any season
of the year, although it shows to the best advantage in spring evenings
and autumn mornings. Look for it.
169. GREAT METEORS.--But there are other meteors, veritable fireballs in
appearance, far more conspicuous and imposing than the ordinary shooting
star. Such a one exploded over the city of Madrid, Spain, on the morning
of February 10, 1896, giving in broad sunlight "a brilliant flash which
was followed ninety seconds later by a succession of terrific noises
like the discharge of a battery of artillery." Fig. 111 shows a large
meteor which was seen in California in the early evening of July 27,
1894, and which left behind it a luminous trail or cloud visible for
more than half an hour.
Not infrequently large meteors are found traveling together, two or
three or more in company, making their appearance simultaneously as did
the California meteor of October 22, 1896, which is described as triple,
the trio following one another like a train of cars, and Arago cites an
instance, from the year 1830, where within a short space of time some
forty brilliant meteors crossed the sky, all moving in the same
direction with a whistling noise and displaying in their flight all the
colors of the rainbow.
The mass of great meteors such as these must be measured in hundreds if
not thousands of pounds, and stories are current, although not very well
authenticated, of even larger ones, many tons in weight, having been
found partially buried in the ground. Of meteors which have been
actually seen to fall from the sky, the largest single fragment
recovered weighs about 500 pounds, but it is only a fragment of the
original meteor, which must have been much more massive before it was
broken up by collision with the atmosphere.
[Illustration: FIG. 111.--The California meteor of July 27, 1894.]
170. THE VELOCITY OF METEORS.--Every meteor, big or little, is subject
to the law of gravitation, and before it encounters the earth must be
moving in some kind of orbit having the sun at its focus, the particular
species of orbit--ellipse, parabola, hyperbola--depending upon the
velocity and direction of its motion. Now, the direction in which a
meteor is moving can be determined without serious difficulty from
observations of its apparent path across the sky made by two or more
observers, but the velocity can not be so readily found, since the
meteors go too fast for any ordinary process of timing. But by
photographing one of them two or three times on the same plate, with an
interval of only a tenth of a second between exposures, Dr. Elkin has
succeeded in showing, in a few cases, that their velocities varied from
20 to 25 miles per second, and must have been considerably greater than
this before the meteors encountered the earth's atmosphere. This is a
greater velocity than that of the earth in its orbit, 19 miles per
second, as might have been anticipated, since the mere fact that meteors
can be seen at all in the evening hours shows that some of them at least
must travel considerably faster than the earth, for, counting in the
direction of the earth's motion, the region of sunset and evening is
always on the rear side of the earth, and meteors in order to strike
this region must overtake it by their swifter motion. We have here, in
fact, the reason why meteors are especially abundant in the morning
hours; at this time the observer is on the front side of the earth which
catches swift and slow meteors alike, while the rear is pelted only by
the swifter ones which follow it.
A comparison of the relative number of morning and evening meteors makes
it probable that the average meteor moves, relative to the sun, with a
velocity of about 26 miles per second, which is very approximately the
average velocity of comets when they are at the earth's distance from
the sun. Astronomers, therefore, consider meteors as well as comets to
have the parabola and the elongated ellipse as their characteristic
orbits.
171. METEOR SHOWERS--THE RADIANT.--There is evident among meteors a
distinct tendency for individuals, to the number of hundreds or even
hundreds of millions, to travel together in flocks or swarms, all going
the same way in orbits almost exactly alike. This gregarious tendency is
made manifest not only by the fact that from time to time there are
unusually abundant meteoric displays, but also by a striking peculiarity
of their behavior at such times. The meteors all seem to come from a
particular part of the heavens, as if here were a hole in the sky
through which they were introduced, and from which they flow away in
every direction, even those which do not visibly start from this place
having paths among the stars which, if prolonged backward, would pass
through it. The cause of this appearance may be understood from Fig.
112, which represents a group of meteors moving together along parallel
paths toward an observer at _D_. Traveling unseen above the earth until
they encounter the upper strata of its atmosphere, they here become
incandescent and speed on in parallel paths, _1_, _2_, _3_, _4_, _5_,
_6_, which, as seen by the observer, are projected back against the sky
into luminous streaks that, as is shown by the arrowheads, _b_, _c_,
_d_, all seem to radiate from the point _a_--i. e., from the point in
the sky whose direction from the observer is parallel to the paths of
the meteors.
[Illustration: FIG. 112.--Explanation of the radiant of a meteoric
shower.--DENNING.]
Such a display is called a meteor shower, and the point _a_ is called
its radiant. Note how those meteors which appear near the radiant all
have short paths, while those remote from it in the sky have longer
ones. Query: As the night wears on and the stars shift toward the west,
will the radiant share in their motion or will it be left behind? Would
the luminous part of the path of any of these meteors pass across the
radiant from one side to the other? Is such a crossing of the radiant
possible under any circumstances? Fig. 113 shows how the meteor paths
are grouped around the radiant of a strongly marked shower. Select from
it the meteors which do not belong to this shower.
[Illustration: FIG. 113.--The radiant of a meteoric shower, showing
also the paths of three meteors which do not belong to this
shower.--DENNING.]
Many hundreds of these radiants have been observed in the sky, each of
which represents an orbit along which a group of meteors moves, and the
relation of one of these orbits to that of the earth is shown in Fig.
114. The orbit of the meteors is an ellipse extending out beyond the
orbit of Uranus, but so eccentric that a part of it comes inside the
orbit of the earth, and the figure shows only that part of it which lies
nearest the sun. The Roman numerals which are placed along the earth's
orbit show the position of the earth at the beginning of the tenth
month, eleventh month, etc. The meteors flow along their orbit in a long
procession, whose direction of motion is indicated by the arrow heads,
and the earth, coming in the opposite direction, plunges into this
stream and receives the meteor shower when it reaches the intersection
of the two orbits. The long arrow at the left of the figure represents
the direction of motion of another meteor shower which encounters the
earth at this point.
[Illustration: FIG. 114.--The orbits of the earth and the November
meteors.]
Can you determine from the figure answers to the following questions? On
what day of the year will the earth meet each of these showers? Will the
radiant points of the showers lie above or below the plane of the
earth's orbit? Will these meteors strike the front or the rear of the
earth? Can they be seen in the evening hours?
From many of the radiants year after year, upon the same day or week in
each year, there comes a swarm of shooting stars, showing that there
must be a continuous procession of meteors moving along this orbit, so
that some are always ready to strike the earth whenever it reaches the
intersection of its orbit with theirs. Such is the explanation of the
shower which appears each year in the first half of August, and whose
meteors are sometimes called Perseids, because their radiant lies in the
constellation Perseus, and a similar explanation holds for all the star
showers which are repeated year after year.
172. THE LEONIDS.--There is, however, a kind of star shower, of which
the Leonids (radiant in Leo) is the most conspicuous type, in which the
shower, although repeated from year to year, is much more striking in
some years than in others. Thus, to quote from the historian: "In 1833
the shower was well observed along the whole eastern coast of North
America from the Gulf of Mexico to Halifax. The meteors were most
numerous at about 5 A. M. on November 13th, and the rising sun could not
blot out all traces of the phenomena, for large meteors were seen now
and then in full daylight. Within the scope that the eye could contain,
more than twenty could be seen at a time shooting in every direction.
Not a cloud obscured the broad expanse, and millions of meteors sped
their way across in every point of the compass. Their coruscations were
bright, gleaming, and incessant, and they fell thick as the flakes in
the early snows of December." But, so far as is known, none of them
reached the ground. An illiterate man on the following day remarked:
"The stars continued to fall until none were left. I am anxious to see
how the heavens will appear this evening, for I believe we shall see no
more stars."
An eyewitness in the Southern States thus describes the effect of this
shower upon the plantation negroes: "Upward of a hundred lay prostrate
upon the ground, some speechless and some with the bitterest cries, but
with their hands upraised, imploring God to save the world and them. The
scene was truly awful, for never did rain fall much thicker than the
meteors fell toward the earth--east, west, north, and south it was the
same." In the preceding year a similar but feebler shower from the same
radiant created much alarm in France, and through the old historic
records its repetitions may be traced back at intervals of 33 or 34
years, although with many interruptions, to October 12, 902, O. S., when
"an immense number of falling stars were seen to spread themselves over
the face of the sky like rain."
Such a star shower differs from the one repeated every year chiefly in
the fact that its meteors, instead of being drawn out into a long
procession, are mainly clustered in a single flock which may be long
enough to require two or three or four years to pass a given point of
its orbit, but which is far from extending entirely around it, so that
meteors from this source are abundant only in those years in which the
flock is at or near the intersection of its orbit with that of the
earth. The fact that the Leonid shower is repeated at intervals of 33 or
34 years (it appeared in 1799, 1832-'33, 1866-'67) shows that this is
the "periodic time" in its orbit, which latter must of course be an
ellipse, and presumably a long and narrow one. It is this orbit which is
shown in Fig. 114, and the student should note in this figure that if
the meteor stream at the point where it cuts through the plane of the
earth's orbit were either nearer to or farther from the sun than is the
earth there could be no shower; the earth and the meteors would pass by
without a collision. Now, the meteors in their motion are subject to
perturbations, particularly by the large planets Jupiter, Saturn, and
Uranus, which slightly change the meteor orbit, and it seems certain
that the changes thus produced will sometimes thrust the swarm inside or
outside the orbit of the earth, and thus cause a failure of the shower
at times when it is expected. The meteors were due at the crossing of
the orbits in November, 1899 and 1900, and, although a few were then
seen, the shower was far from being a brilliant one, and its failure was
doubtless caused by the outer planets, which switched the meteors aside
from the path in which they had been moving for a century. Whether they
will be again switched back so as to produce future showers is at the
present time uncertain.
173. CAPTURE OF THE LEONIDS.--But a far more striking effect of
perturbations is to be found in Fig. 115, which shows the relation of
the Leonid orbit to those of the principal planets, and illustrates a
curious chapter in the history of the meteor swarm that has been worked
out by mathematical analysis, and is probably a pretty good account of
what actually befell them. Early in the second century of the Christian
era this flock of meteors came down toward the sun from outer space,
moving along a parabolic orbit which would have carried it just inside
the orbit of Jupiter, and then have sent it off to return no more. But
such was not to be its fate. As it approached the orbit of Uranus, in
the year 126 A. D., that planet chanced to be very near at hand and
perturbed the motion of the meteors to such an extent that the character
of their orbit was completely changed into the ellipse shown in the
figure, and in this new orbit they have moved from that time to this,
permanent instead of transient members of the solar system. The
perturbations, however, did not end with the year in which the meteors
were captured and annexed to the solar system, but ever since that time
Jupiter, Saturn, and Uranus have been pulling together upon the orbit,
and have gradually turned it around into its present position as shown
in the figure, and it is chiefly this shifting of the orbit's position
in the thousand years that have elapsed since 902 A. D. that makes the
meteor shower now come in November instead of in October as it did
then.
[Illustration: FIG. 115.--Supposed capture of the November meteors by
Uranus.]
174. BREAKING UP A METEOR SWARM.--How closely packed together these
meteors were at the time of their annexation to the solar system is
unknown, but it is certain that ever since that time the sun has been
exerting upon them a tidal influence tending to break up the swarm and
distribute its particles around the orbit, as the Perseids are
distributed, and, given sufficient time, it will accomplish this, but up
to the present the work is only partly done. A certain number of the
meteors have gained so much over the slower moving ones as to have made
an extra circuit of the orbit and overtaken the rear of the procession,
so that there is a thin stream of them extending entirely around the
orbit and furnishing in every November a Leonid shower; but by far the
larger part of the meteors still cling together, although drawn out into
a stream or ribbon, which, though very thin, is so long that it takes
some three years to pass through the perihelion of its orbit. It is only
when the earth plunges through this ribbon, as it should in 1899, 1900,
1901, that brilliant Leonid showers can be expected.
175. RELATION OF COMETS AND METEORS.--It appears from the foregoing that
meteors and comets move in similar orbits, and we have now to push the
analogy a little further and note that in some instances at least they
move in identically the same orbit, or at least in orbits so like that
an appreciable difference between them is hardly to be found. Thus a
comet which was discovered and observed early in the year 1866, moves in
the same orbit with the Leonid meteors, passing its perihelion about ten
months ahead of the main body of the meteors. If it were set back in its
orbit by ten months' motion, _it would be a part of the meteor swarm_.
Similarly, the Perseid meteors have a comet moving in their orbit
actually immersed in the stream of meteor particles, and several other
of the more conspicuous star showers have comets attending them.
Perhaps the most remarkable case of this character is that of a shower
which comes in the latter part of November from the constellation
Andromeda, and which from its association with the comet called Biela
(after the name of its discoverer) is frequently referred to as the
Bielid shower. This comet, an inconspicuous one moving in an unusually
small elliptical orbit, had been observed at various times from 1772
down to 1846 without presenting anything remarkable in its appearance;
but about the beginning of the latter year, with very little warning, it
broke in two, and for three months the pieces were watched by
astronomers moving off, side by side, something more than half as far
apart as are the earth and moon. It disappeared, made the circuit of its
orbit, and six years later came back, with the fragments nearly ten
times as far apart as before, and after a short stay near the earth once
more disappeared in the distance, never to be seen again, although the
fragments should have returned to perihelion at least half a dozen times
since then. In one respect the orbit of the comet was remarkable: it
passed through the place in which the earth stands on November 27th of
each year, so that if the comet were at that particular part of its
orbit on any November 27th, a collision between it and the earth would
be inevitable. So far as is known, no such collision with the comet has
ever occurred, but the Bielid meteors which are strung along its orbit
do encounter the earth on that date, in greater or less abundance in
different years, and are watched with much interest by the astronomers
who look upon them as the final appearance of the _débris_ of a worn-out
comet.
176. PERIODIC COMETS.--The Biela comet is a specimen of the type which
astronomers call periodic comets--i. e., those which move in small
ellipses and have correspondingly short periodic times, so that they
return frequently and regularly to perihelion. The comets which
accompany the other meteor swarms--Leonids, Perseids, etc.--also belong
to this class as do some 30 or 40 others which have periodic times less
than a century. As has been already indicated, these deviations from the
normal parabolic orbit call for some special explanation, and the
substance of that explanation is contained in the account of the Leonid
meteors and their capture by Uranus. Any comet may be thus captured by
the attraction of a planet near which it passes. It is only necessary
that the perturbing action of the planet should result in a diminution
of the comet's velocity, for we have already learned that it is this
velocity which determines the character of the orbit, and anything less
than the velocity appropriate to a parabola must produce an
ellipse--i. e., a closed orbit around which the body will revolve time
after time in endless succession. We note in Fig. 115 that when the
Leonid swarm encountered Uranus it passed _in front of_ the planet and
had its velocity diminished and its orbit changed into an ellipse
thereby. It might have passed behind Uranus, it would have passed behind
had it come a little later, and the effect would then have been just the
opposite. Its velocity would have been increased, its orbit changed to a
hyperbola, and it would have left the solar system more rapidly than it
came into it, thrust out instead of held in by the disturbing planet. Of
such cases we can expect no record to remain, but the captured comet is
its own witness to what has happened, and bears imprinted upon its orbit
the brand of the planet which slowed down its motion. Thus in Fig. 115
the changed orbit of the meteors has its _aphelion_ (part remotest from
the sun) quite close to the orbit of Uranus, and one of its nodes,
[mho], the point in which it cuts through the plane of the ecliptic from
north to south side, is also very near to the same orbit. It is these
two marks, aphelion and node, which by their position identify Uranus as
the planet instrumental in capturing the meteor swarm, and the date of
the capture is found by working back with their respective periodic
times to an epoch at which planet and comet were simultaneously near
this node.
Jupiter, by reason of his great mass, is an especially efficient
capturer of comets, and Fig. 116 shows his group of captives, his
family of comets as they are sometimes called. The several orbits are
marked with the names commonly given to the comets. Frequently this is
the name of their discoverer, but often a different system is
followed--e. g., the name 1886, IV, means the fourth comet to pass
through perihelion in the year 1886. The other great planets--Saturn,
Uranus, Neptune--have also their families of captured comets, and
according to Schulhof, who does not entirely agree with the common
opinion about captured comets, the earth has caught no less than nine of
these bodies.
[Illustration: FIG. 116.--Jupiter's family of comets.]
177. COMET GROUPS.--But there is another kind of comet family, or comet
group as it is called, which deserves some notice, and which is best
exemplified by the Great Comet of 1882 and its relatives. No less than
four other comets are known to be traveling in substantially the same
orbit with this one, the group consisting of comets 1668, I; 1843, I;
1880, I; 1882, II; 1887, I. The orbit itself is not quite a parabola,
but a very elongated ellipse, whose major axis and corresponding
periodic time can not be very accurately determined from the available
data, but it certainly extends far beyond the orbit of Neptune, and
requires not less than 500 years for the comet to complete a revolution
in it. It was for a time supposed that some one of the recent comets of
this group of five might be a return of the comet of 1668 brought back
ahead of time by unknown perturbations. There is still a possibility of
this, but it is quite out of the question to suppose that the last four
members of the group are anything other than separate and distinct
comets moving in practically the same orbit. This common orbit suggests
a common origin for the comets, but leaves us to conjecture how they
became separated.
The observed orbits of these five comets present some slight
discordances among themselves, but if we suppose each comet to move in
the average of the observed paths it is a simple matter to fix their
several positions at the present time. They have all receded from the
sun nearly on line toward the bright star Sirius, and were all of them,
at the beginning of the year 1900, standing nearly motionless inside of
a space not bigger than the sun and distant from the sun about 150 radii
of the earth's orbit. The great rapidity with which they swept through
that part of their orbit near the sun (see § 162) is being compensated
by the present extreme slowness of their motions, so that the comets of
1668 and 1882, whose passages through the solar system were separated by
an interval of more than two centuries, now stand together near the
aphelion of their orbits, separated by a distance only 50 per cent
greater than the diameter of the moon's orbit, and they will continue
substantially in this position for some two or three centuries to come.
The slowness with which these bodies move when far from the sun is
strikingly illustrated by an equation of celestial mechanics which for
parabolic orbits takes the place of Kepler's Third Law--viz.:
r^3 / T^2 = 178,
where _T_ is the time, in years, required for the comet to move from its
perihelion to any remote part of the orbit, whose distance from the sun
is represented, in radii of the earth's orbit, by _r_. If the comet of
1668 had moved in a parabola instead of the ellipse supposed above, how
many years would have been required to reach its present distance from
the sun?
178. RELATION OF COMETS TO THE SOLAR SYSTEM.--The orbits of these comets
illustrate a tendency which is becoming ever more strongly marked.
Because comet orbits are nearly parabolas, it used to be assumed that
they were exactly parabolic, and this carried with it the conclusion
that comets have their origin outside the solar system. It may be so,
and this view is in some degree supported by the fact that these nearly
parabolic orbits of both comets and meteors are tipped at all possible
angles to the plane of the ecliptic instead of lying near it as do the
orbits of the planets; and by the further fact that, unlike the planets,
the comets show no marked tendency to move around their orbits in the
direction in which the sun rotates upon his axis. There is, in fact, the
utmost confusion among them in this respect, some going one way and some
another. The law of the solar system (gravitation) is impressed upon
their movements, but its order is not.
But as observations grow more numerous and more precise, and comet
orbits are determined with increasing accuracy, there is a steady gain
in the number of elliptic orbits at the expense of the parabolic ones,
and if comets are of extraneous origin we must admit that a very
considerable percentage of them have their velocities slowed down
within the solar system, perhaps not so much by the attraction of the
planets as by the resistance offered to their motion by meteor particles
and swarms along their paths. A striking instance of what may befall a
comet in this way is shown in Fig. 117, where the tail of a comet
appears sadly distorted and broken by what is presumed to have been a
collision with a meteor swarm. A more famous case of impeded motion is
offered by the comet which bears the name of Encke. This has a periodic
time less than that of any other known comet, and at intervals of forty
months comes back to perihelion, each time moving in a little smaller
orbit than before, unquestionably on account of some resistance which it
has suffered.
[Illustration: FIG. 117.--Brooks's comet, October 21, 1893.--BARNARD.]
179. THE DEVELOPMENT OF A COMET.--We saw in § 174 that the sun's action
upon a meteor swarm tends to break it up into a long stream, and the
same tendency to break up is true of comets whose attenuated substance
presents scant resistance to this force. According to the mathematical
analysis of Roche, if the comet stood still the sun's tidal force would
tend first to draw it out on line with the sun, just as the earth's
tidal force pulled the moon out of shape (§ 42), and then it would cause
the lighter part of the comet's substance to flow away from both ends of
this long diameter. This destructive action of the sun is not limited to
comets and meteor streams, for it tends to tear the earth and moon to
pieces as well; but the densities and the resulting mutual attractions
of their parts are far too great to permit this to be accomplished.
As a curiosity of mathematical analysis we may note that a spherical
cloud of meteors, or dust particles weighing a gramme each, and placed
at the earth's distance from the sun, will be broken up and dissipated
by the sun's tidal action if the average distance between the particles
exceeds two yards. Now, the earth is far more dense than such a cloud,
whose extreme tenuity, however, suggests what we have already learned of
the small density of comets, and prepares us in their case for an
outflow of particles at both ends of the diameter directed toward the
sun. Something of this kind actually occurs, for the tail of a comet
streams out on the side opposite to the sun, and in general points away
from the sun, as is shown in Fig. 109, and the envelopes and jets rise
up toward the sun; but an inspection of Fig. 106 will show that the tail
and the envelope are too unlike to be produced by one and the same set
of forces.
It was long ago suggested that the sun possibly exerts upon a comet's
substance a repelling force in addition to the attracting force which we
call gravity. We think naturally in this connection of the repelling
force which a charge of electricity exerts upon a similar charge placed
on a neighboring body, and we note that if both sun and comet carried a
considerable store of electricity upon their surfaces this would furnish
just such a repelling force as seems indicated by the phenomena of
comets' tails; for the force of gravity would operate between the
substance of sun and comet, and on the whole would be the controlling
force, while the electric charges would produce a repulsion, relatively
feeble for the big particles and strong for the little ones, since an
electric charge lies wholly on the surface, while gravity permeates the
whole mass of a body, and the ratio of volume (gravity) to surface
(electric charge) increases rapidly with increasing size. The repelling
force would thrust back toward the comet those particles which flowed
out toward the sun, while it would urge forward those which flowed away
from it, thus producing the difference in appearance between tail and
envelopes, the latter being regarded from this standpoint as stunted
tails strongly curved backward. In recent years the Russian astronomer
Bredichin has made a careful study of the shape and positions of comets'
tails and finds that they fit with mathematical precision to the
theories of electric repulsion.
180. COMET TAILS.--According to Bredichin, a comet's tail is formed by
something like the following process: In the head of the comet itself a
certain part of its matter is broken up into fine bits, single molecules
perhaps, which, as they no longer cling together, may be described as in
the condition of vapor. By the repellent action of both sun and comet
these molecules are cast out from the head of the comet and stream away
in the direction opposite to the sun with different velocities, the
heavy ones slowly and the light ones faster, much as particles of smoke
stream away from a smokestack, making for the comet a tail which like a
trail of smoke is composed of constantly changing particles. The result
of this process is shown in Fig. 118, where the positions of the comet
in its orbit on successive days are marked by the Roman numerals, and
the broken lines represent the paths of molecules _m^{I}_, _m^{II}_,
_m^{III}_, etc., expelled from it on their several dates and traveling
thereafter in orbits determined by the combined effect of the sun's
attraction, the sun's repulsion, and the comet's repulsion. The comet's
attraction (gravity) is too small to be taken into account. The line
drawn upward from _VI_ represents the positions of these molecules on
the sixth day, and shows that all of them are arranged in a tail
pointing nearly away from the sun. A similar construction for the other
dates gives the corresponding positions of the tail, always pointing
away from the sun.
[Illustration: FIG. 118.--Formation of a comet's tail.]
Only the lightest kind of molecules--e. g., hydrogen--could drift away
from the comet so rapidly as is here shown. The heavier ones, such as
carbon and iron, would be repelled as strongly by the electric forces,
but they would be more strongly pulled back by the gravitative forces,
thus producing a much slower separation between them and the head of the
comet. Construct a figure such as the above, in which the molecules
shall recede from the comet only one eighth as fast as in Fig. 118, and
note what a different position it gives to the comet's tail. Instead of
pointing directly away from the sun, it will be bent strongly to one
side, as is the large plume-shaped tail of the Donati comet shown in
Fig. 101. But observe that this comet has also a nearly straight tail,
like the theoretical one of Fig. 118. We have here two distinct types of
comet tails, and according to Bredichin there is still another but
unusual type, even more strongly bent to one side of the line joining
comet and sun, and appearing quite short and stubby. The existence of
these three types, and their peculiarities of shape and position, are
all satisfactorily accounted for by the supposition that they are made
of different materials. The relative molecular weights of hydrogen, some
of the hydrocarbons, and iron, are such that tails composed of these
molecules would behave just as do the actual tails observed and
classified into these three types. The spectroscope shows that these
materials--hydrogen, hydrocarbons, and iron--are present in comets, and
leaves little room for doubt of the essential soundness of Bredichin's
theory.
181. DISINTEGRATION OF COMETS.--We must regard the tail as waste matter
cast off from the comet's head, and although the amount of this matter
is very small, it must in some measure diminish the comet's mass. This
process is, of course, most active at the time of perihelion passage,
and if the comet returns to perihelion time after time, as the periodic
ones which move in elliptic orbits must do, this waste of material may
become a serious matter, leading ultimately to the comet's destruction.
It is significant in this connection that the periodic comets are all
small and inconspicuous, not one of them showing a tail of any
considerable dimensions, and it appears probable that they are far
advanced along the road which, in the case of Biela's comet, led to its
disintegration. Their fragments are in part strewn through the solar
system, making some small fraction of its cloud of cosmic dust, and in
part they have been carried away from the sun and scattered throughout
the universe along hyperbolic orbits impressed upon them at the time
they left the comet.
But it is not through the tail only that the disintegrating process is
worked out. While Biela's comet is perhaps the most striking instance in
which the head has broken up, it is by no means the only one. The Great
Comet of 1882 cast off a considerable number of fragments which moved
away as independent though small comets and other more recent comets
have been seen to do the same. An even more striking phenomenon was the
gradual breaking up of the nucleus of the same comet, 1882, II, into a
half dozen nuclei arranged in line like beads upon a string, and
pointing along the axis of the tail. See Fig. 119, which shows the
series of changes observed in the head of this comet.
182. COMETS AND THE SPECTROSCOPE.--The spectrum presented by comets was
long a puzzle, and still retains something of that character, although
much progress has been made toward an understanding of it. In general it
consists of two quite distinct parts--first, a faint background of
continuous spectrum due to ordinary sunlight reflected from the comet;
and, second, superposed upon this, three bright bands like the carbon
band shown at the middle of Fig. 48, only not so sharply defined. These
bands make a discontinuous spectrum quite similar to that given off by
compounds of hydrogen and carbon, and of course indicate that a part of
the comet's light originates in the body itself, which must therefore be
incandescent, or at least must contain some incandescent portions.
[Illustration: FIG. 119.--The head of the Great Comet of
1882.--WINLOCK.]
By heating hydrocarbons in our laboratories until they become
incandescent, something like the comet spectrum may be artificially
produced, but the best approximation to it is obtained by passing a
disruptive electrical discharge through a tube in which fragments of
meteors have been placed. A flash of lightning is a disruptive
electrical discharge upon a grand scale. Now, meteors and electric
phenomena have been independently brought to our notice in connection
with comets, and with this suggestion it is easy to frame a general idea
of the physical condition of these objects--for example, a cloud of
meteors of different sizes so loosely clustered that the average density
of the swarm is very low indeed; the several particles in motion
relative to each other, as well as to the sun, and disturbed in that
motion by the sun's tidal action. Each particle carries its own electric
charge, which may be of higher or lower tension than that of its
neighbor, and is ready to leap across the intervening gap whenever two
particles approach each other. To these conditions add the inductive
effect of the sun's electric charge, which tends to produce a particular
and artificial distribution of electricity among the comet's particles,
and we may expect to find an endless succession of sparks, tiny
lightning flashes, springing from one particle to another, most frequent
and most vivid when the comet is near the sun, but never strong enough
to be separately visible. Their number is, however, great enough to make
the comet in part self-luminous with three kinds of light--i. e., the
three bright bands of its spectrum, whose wave lengths show in the comet
the same elements and compounds of the elements--carbon, hydrogen, and
oxygen--which chemical analysis finds in the fallen meteor. It is not to
be supposed that these are the only chemical elements in the comet, as
they certainly are not the only ones in the meteor. They are the easy
ones to detect under ordinary circumstances, but in special cases, like
that of the Great Comet of 1882, whose near approach to the sun rendered
its whole substance incandescent, the spectrum glows with additional
bright lines of sodium, iron, etc.
183. COLLISIONS.--A question sometimes asked, What would be the effect
of a collision between the earth and a comet? finds its answer in the
results reached in the preceding sections. There would be a star
shower, more or less brilliant according to the number and size of the
pieces which made up the comet's head. If these were like the remains of
the Biela comet, the shower might even be a very tame one; but a
collision with a great comet would certainly produce a brilliant
meteoric display if its head came in contact with the earth. If the
comet were built of small pieces whose individual weights did not exceed
a few ounces or pounds, the earth's atmosphere would prove a perfect
shield against their attacks, reducing the pieces to harmless dust
before they could reach the ground, and leaving the earth uninjured by
the encounter, although the comet might suffer sadly from it. But big
stones in the comet, meteors too massive to be consumed in their flight
through the air, might work a very different effect, and by their
bombardment play sad havoc with parts of the earth's surface, although
any such result as the wrecking of the earth, or the destruction of all
life upon it, does not seem probable. The 40 meteors of § 169 may stand
for a collision with a small comet. Consult the Bible (Joshua x, 11) for
an example of what might happen with a larger one.