comet, was one of the most magnificent of modern times. When at its
brightest it was only about fifty million miles from the earth. Its tail
was then more than fifty million miles long. Had the comet at this time
been directly between the earth and sun, the earth must have passed
through its tail; but this did not occur. The orbit of this comet was
found to be decidedly elliptic, with a period of about two thousand
years. This comet is especially celebrated on account of the careful
telescopic observations of its nucleus and coma at the time of its
perihelion passage. Attention has already been called (287) to the
changes it underwent at that time. Its tail was curved, and of a curious
feather-like form, as shown in Fig. 335. At times it developed lateral
streamers, as shown in Fig. 336. Fig. 337 shows the head of the comet as
it was seen by Bond of the Harvard Observatory, whose delineations of
this comet have been justly celebrated.
[Illustration: Fig. 335.]
[Illustration: Fig. 336.]
[Illustration: Fig. 337.]
302. _The Comet of 1861._--The great comet of 1861 is remarkable for its
great brilliancy, for its peculiar fan-shaped tail, and for the probable
passage of the earth through its tail. Sir John Herschel declared that
it far exceeded in brilliancy any comet he had ever seen, not excepting
those of 1811 and 1858. Secchi found its tail to be a hundred and
eighteen degrees in length, the largest but one on record. Fig. 338
shows this comet as it appeared at one time. Fig. 339 shows the position
of the earth at _E_, in the tail of this comet, on the 30th of June,
1861. Fig. 340 shows the probable passage of the earth through the tail
of the comet on that date. As the tail of a comet doubtless consists of
something much less dense than our atmosphere, it is not surprising that
no noticeable effect was produced upon us by the encounter, if it
occurred.
[Illustration: Fig. 338.]
[Illustration: Fig. 339.]
[Illustration: Fig. 340.]
303. _Coggia's Comet._--This comet, which appeared in 1874, looked very
large, because it came very near the earth. It was not at all brilliant.
Its nucleus was carefully studied, and was found to develop a series of
envelops similar to those of Donati's comet. Figs. 341 and 342 are two
views of the head of this comet. Fig. 343 shows the system of envelops
that were developed during its perihelion passage.
[Illustration: Fig. 341.]
[Illustration: Fig. 342.]
[Illustration: Fig. 343.]
304. _The Comet of June, 1881._--This comet, though far from being one
of the largest of modern times, was still very brilliant. It will ever
be memorable as the first brilliant comet which has admitted of careful
examination with the spectroscope.
Connection between Meteors and Comets.
305. _Shooting-Stars._--On watching the heavens any clear night, we
frequently see an appearance as of a star shooting rapidly through a
short space in the sky, and then suddenly disappearing. Three or four
such _shooting-stars_ may, on the average, be observed in the course of
an hour. They are usually seen only a second or two; but they sometimes
move slowly, and are visible much longer. These stars begin to be
visible at an average height of about seventy-five miles, and they
disappear at an average height of about fifty miles. They are
occasionally seen as high as a hundred and fifty miles, and continue to
be visible till within thirty miles of the earth. Their visible paths
vary from ten to a hundred miles in length, though they are occasionally
two hundred or three hundred miles long. Their average velocity,
relatively to the earth's surface, varies from ten to forty-five miles a
second.
The average number of shooting-stars visible to the naked eye at any one
place is estimated at about _a thousand an hour_; and the average number
large enough to be visible to the naked eye, that traverse the
atmosphere daily, is estimated at _over eight millions_. The number of
telescopic shooting-stars would of course be much greater.
Occasionally, shooting-stars leave behind them a trail of light which
lasts for several seconds. These trails are sometimes straight, as shown
in Fig. 344, and sometimes curved, as in Figs. 345 and 346. They often
disappear like trails of smoke, as shown in Fig. 347.
[Illustration: Fig. 344.]
[Illustration: Fig. 345.]
[Illustration: Fig. 346.]
[Illustration: Fig. 347.]
Shooting-stars are seen to move in all directions through the heavens.
Their apparent paths are, however, generally inclined downward, though
sometimes upward; and after midnight they come in the greatest numbers
from that quarter of the heavens toward which the earth is moving in its
journey around the sun.
306. _Meteors._--Occasionally these bodies are brilliant enough to
illuminate the whole heavens. They are then called _meteors_, although
this term is equally applicable to ordinary shooting-stars. Such a
meteor is shown in Fig. 348.
[Illustration: Fig. 348.]
Sometimes these brilliant meteors are seen to explode, as shown in Fig.
349; and the explosion is accompanied with a loud detonation, like the
discharge of cannon.
[Illustration: Fig. 349.]
Ordinary shooting-stars are not accompanied by any audible sound, though
they are sometimes seen to break in pieces. Meteors which explode with
an audible sound are called _detonating meteors_.
307. _Aerolites._--There is no certain evidence that any deposit from
ordinary shooting-stars ever reaches the surface of the earth; though a
peculiar dust has been found in certain localities, which has been
supposed to be of meteoric origin, and which has been called _meteoric
dust_. But solid bodies occasionally descend to the earth from beyond
our atmosphere. These generally penetrate a foot or more into the earth,
and, if picked up soon after their fall, are found to be warm, and
sometimes even hot. These bodies are called _aerolites_. When they have
a stony appearance, and contain but little iron, they are called
_meteoric stones_; when they have a metallic appearance, and are
composed largely of iron, they are called _meteoric iron_.
There are eighteen well-authenticated cases in which aerolites have
fallen in the United States during the last sixty years, and their
aggregate weight is twelve hundred and fifty pounds. The entire number
of known aerolites the date of whose fall is well determined is two
hundred and sixty-one. There are also on record seventy-four cases of
which the date is more or less uncertain. There have also been found
eighty-six masses, which, from their peculiar composition, are believed
to be aerolites, though their fall was not seen. The weight of these
masses varies from a few pounds to several tons. The entire number of
aerolites of which we have any knowledge is therefore about four hundred
and twenty.
Aerolites are composed of the same elementary substances as occur in
terrestrial minerals, not a single new element having been found in
their analysis. Of the sixty or more elements now recognized by
chemists, about twenty have been found in aerolites.
While aerolites contain no new elements, their appearance is quite
peculiar; and the compounds found in them are so peculiar as to enable
us by chemical analysis to distinguish an aerolite from any terrestrial
substance.
Iron ores are very abundant in nature, but iron in the metallic state is
exceedingly rare. Now, aerolites invariably contain metallic iron,
sometimes from ninety to ninety-six per cent. This iron is malleable,
and may be readily worked into cutting instruments. It always contains
eight or ten per cent of nickel, together with small quantities of
cobalt, copper, tin, and chromium. This composition _has never been
found in any terrestrial mineral_. Aerolites also contain, usually in
small amount, a compound of iron, nickel, and phosphorus, which has
never been found elsewhere.
Meteorites often present the appearance of having been fused on the
surface to a slight depth, and meteoric iron is found to have a peculiar
crystalline structure. The external appearance of a piece of meteoric
iron found near Lockport, N.Y., is shown in Fig. 350. Fig. 351 shows the
peculiar internal structure of meteoric iron.
[Illustration: Fig. 350.]
[Illustration: Fig. 351.]
308. _Meteoroids._--Astronomers now universally hold that
shooting-stars, meteors, and aerolites are all minute bodies, revolving,
like the comets, about the sun. They are moving in every possible
direction through the celestial spaces. They may not average more than
one in a million of cubic miles, and yet their total number exceeds all
calculation. Of the nature of the minuter bodies of this class nothing
is certainly known. The earth is continually encountering them in its
journey around the sun. They are burned by passing through the upper
regions of our atmosphere, and the shooting-star is simply the light of
that burning. These bodies, which are invisible till they plunge into
the earth's atmosphere, are called _meteoroids_.
309. _Origin of the Light of Meteors._--When one of these meteoroids
enters our atmosphere, the resistance of the air arrests its motion to
some extent, and so converts a portion of its energy of motion into that
of heat. The heat thus developed is sufficient to raise the meteoroid
and the air around it to incandescence, and in most cases either to
cause the meteoroid to burn up, or to dissipate it as vapor. The
luminous vapor thus formed constitutes the luminous train which
occasionally accompanies a meteor, and often disappears as a puff of
smoke. When a meteoroid is large enough and refractory enough to resist
the heat to which it is exposed, its motion is sufficiently arrested, on
entering the lower layers of our atmosphere, to cause it to fall to the
earth. We then have an _aerolite_. A brilliant meteor differs from a
shooting-star simply in magnitude.
310. _The Intensity of the Heat to which a Meteoroid is Exposed._--It
has been ascertained by experiment that a body moving through the
atmosphere at the rate of a hundred and twenty-five feet a second raises
the temperature of the air immediately in front of it one degree, and
that the temperature increases as the square of the velocity of the
moving body; that is to say, that, with a velocity of two hundred and
fifty feet, the temperature in front of the body would be raised four
degrees; with a velocity of five hundred feet, sixteen degrees; and so
on. To find, therefore, the temperature to which a meteoroid would be
exposed in passing through our atmosphere, we have merely to divide its
velocity in feet per second by a hundred and twenty-five, and square the
quotient. With a velocity of forty-four miles a second in our
atmosphere, a meteoroid would therefore be exposed to a temperature of
between three and four million degrees. The air acts upon the body as if
it were raised to this intense heat. At such a temperature small masses
of the most refractory or incombustible substances known to us would
flash into vapor with the evolution of intense light and heat.
If one of these meteoric bodies is large enough to pass through the
atmosphere and reach the earth, without being volatilized by the heat,
we have an aerolite. As it is only a few seconds in making the passage,
the heat has not time to penetrate far into its interior, but is
expended in melting and vaporizing the outer portions. The resistance of
the denser strata of the atmosphere to the motion of the aerolite
sometimes becomes so enormous that the body is suddenly rent to pieces
with a loud detonation. It seems like an explosion produced by some
disruptive action within the mass; but there can be little doubt that it
is due to the velocity--perhaps ten, twenty, or thirty miles a
second--with which the body strikes the air.
If, on the other hand, the meteoroid is so small as to be burned up or
volatilized in the upper regions of the atmosphere, we have a common
shooting-star, or a meteor of greater or less brilliancy.
[Illustration: Fig. 352.]
311. _Meteoric Showers._--On ordinary nights only four or five
shooting-stars are seen in an hour, and these move in every direction.
Their orbits lie in all possible positions, and are seemingly scattered
at random. Such meteors are called _sporadic_ meteors. On occasional
nights, shooting-stars are more numerous, and all move in a common
direction. Such a display is called a _meteoric shower_. These showers
differ greatly in brilliancy; but during any one shower the meteors all
appear to radiate from some one point in the heavens. If we mark on a
celestial globe the apparent paths of the meteors which fall during a
shower, or if we trace them back on the celestial sphere, we shall find
that they all meet in the same point, as shown in Fig. 352. This point
is called the _radiant point_. It always appears in the same position,
wherever the observer is situated, and does not partake of the diurnal
motion of the earth. As the stars move towards the west, the radiant
point moves with them. The point in question is purely an effect of
perspective, being the "vanishing point" of the parallel lines in which
the meteors are actually moving. These lines are seen, not in their real
direction in space, but as projected on the celestial sphere. If we look
upwards, and watch snow falling through a calm atmosphere, the flakes
which fall directly towards us do not seem to move at all, while the
surrounding flakes seem to diverge from them on all sides. So, in a
meteoric shower, a meteor coming directly towards the observer does not
seem to move at all, and marks the point from which all the others seem
to radiate.
312. _The August Meteors._--A meteoric shower of no great brilliancy
occurs annually about the 10th of August. The radiant point of this
shower is in the constellation _Perseus_, and hence these meteors are
often called the _Perseids_. The orbit of these meteoroids has been
pretty accurately determined, and is shown in Fig. 353.
[Illustration: Fig. 353.]
It will be seen that the perihelion point of this orbit is at about the
distance of the earth from the sun; so that the earth encounters the
meteors once a year, and this takes place in the month of August. The
orbit is a very eccentric ellipse, reaching far beyond Neptune. As the
meteoric display is about equally brilliant every year, it seems
probable that the meteoroids form a stream quite uniformly distributed
throughout the whole orbit. It probably takes one of the meteoroids
about a hundred and twenty-four years to pass around this orbit.
[Illustration: Fig. 354.]
313. _The November Meteors._--A somewhat brilliant meteoric shower also
occurs annually, about the 13th of November. The radiant point of these
meteors is in the constellation _Leo_, and hence they are often called
the _Leonids_. Their orbit has been determined with great accuracy, and
is shown in Fig. 354. While the November meteors are not usually very
numerous or bright, a remarkably brilliant display of them has been seen
once in about thirty-three or thirty-four years: hence we infer, that,
while there are some meteoroids scattered throughout the whole extent of
the orbit, the great majority are massed in a group which traverses the
orbit in a little over thirty-three years. A conjectural form of this
condensed group is shown in Fig. 355. The group is so large that it
takes it two or three years to pass the perihelion point: hence there
may be a brilliant meteoric display two or three years in succession.
[Illustration: Fig. 355.]
The last brilliant display of these meteors was in the years 1866 and
1867. The display was visible in this country only a short time before
sunrise, and therefore did not attract general attention. The display of
1833 was remarkably brilliant in this country, and caused great
consternation among the ignorant and superstitious.
[Illustration: Fig. 356.]
314. _Connection between Meteors and Comets._--It has been found that a
comet which appeared in 1866, and which is designated as 1866, I., has
exactly the same orbit and period as the November meteors, and that
another comet, known as the 1862, III., has the same orbit as the August
meteors. It has also been ascertained that a third comet, 1861, I., has
the same orbit as a stream of meteors which the earth encounters in
April. Furthermore, it was found, in 1872, that there was a small stream
of meteors following in the train of the lost comet of Biela. These
various orbits of comets and meteoric streams are shown in Fig. 356. The
coincidence of the orbits of comets and of meteoric streams indicates
that these two classes of bodies are very closely related. They
undoubtedly have a common origin. The fact that there is a stream of
meteors in the train of Biela's comet has led to the supposition that
comets may become gradually disintegrated into meteoroids.
Physical and Chemical Constitution of Comets.
315. _Physical Constitution of Telescopic Comets._--We have no certain
knowledge of the physical constitution of telescopic comets. They are
usually tens of thousands of miles in diameter, and yet of such tenuity
that the smallest stars can readily be seen through them. It would seem
that they must shine in part by reflected light; yet the spectroscope
shows that their spectrum is composed of bright bands, which would
indicate that they are composed, in part at least, of incandescent
gases. It is, however, difficult to conceive how these gases become
sufficiently heated to be luminous; and at the same time such gases
would reflect no sunlight.
It seems probable that these comets are really made up of a combination
of small, solid particles in the form of minute meteoroids, and of gases
which are, perhaps, rendered luminous by electric discharges of slight
intensity.
316. _Physical Constitution of Large Comets._--In the case of large
comets the nucleus is either a dense mass of solid matter several
hundred miles in diameter, or a dense group of meteoroids. Professor
Peirce estimated that the density of the nucleus is at least equal to
that of iron. As such a comet approaches the sun, the nucleus is, to a
slight extent, vaporized, and out of this vapor is formed the coma and
the tail.
That some evaporating process is going on from the nucleus of the comet
is proved by the movements of the tail. It is evident that the tail
cannot be an appendage carried along with the comet, as it seems to be.
It is impossible that there should be any cohesion in matter of such
tenuity that the smallest stars could be seen through a million of miles
of it, and which is, moreover, continually changing its form. Then,
again, as a comet is passing its perihelion, the tail appears to be
whirled from one side of the sun to another with a rapidity which would
tear it to pieces if the movement were real. The tail seems to be, not
something attached to the comet, and carried along with it, but a stream
of vapor issuing from it, like smoke from a chimney. The matter of which
it is composed is continually streaming outwards, and continually being
replaced by fresh vapor from the nucleus.
The vapor, as it emanates from the nucleus, is repelled by the sun with
a force often two or three times as great as the ordinary solar
attraction. The most probable explanation of this phenomenon is, that it
is a case of electrical repulsion, the sun and the particles of the
cometary mist being similarly electrified. With reference to this
electrical theory of the formation of comets' tails, Professor Peirce
makes the following observation: "In its approach to the sun, the
surface of the nucleus is rapidly heated: it is melted and vaporized,
and subjected to frequent explosions. The vapor rises in its atmosphere
with a well-defined upper surface, which is known to observers as an
_envelop_.... The electrification of the cometary mist is analogous to
that of our own thunder-clouds. Any portion of the coma which has
received the opposite kind of electricity to the sun and to the repelled
tail will be attracted. This gives a simple explanation of the negative
tails which have been sometimes seen directed towards the sun. In cases
of violent explosion, the whole nucleus might be broken to pieces, and
the coma dashed around so as to give varieties of tail, and even
multiple tails. There seems, indeed, to be no observed phenomenon of the
tail or the coma which is not consistent with a reasonable modification
of the theory." Professor Peirce regarded comets simply as the largest
of the meteoroids. They appear to shine partly by reflected sunlight,
and partly by their own proper light, which seems to be that of vapor
rendered luminous by an electric discharge of slight intensity.
[Illustration: Fig. 357.]
317. _Collision of a Comet and the Earth._--It sometimes happens that
the orbit of a comet intersects that of the earth, as is shown in Fig.
357, which shows a portion of the orbit of Biela's comet, with the
positions of the comet and of the earth in 1832. Of course, were a comet
and the earth both to reach the intersection of their orbits at the same
time, a collision of the two bodies would be inevitable. With reference
to the probable effect of such a collision, Professor Newcomb remarks,--
"The question is frequently asked, What would be the effect if a comet
should strike the earth? This would depend upon what sort of a comet it
was, and what part of the comet came in contact with our planet. The
latter might pass through the tail of the largest comet without the
slightest effect being produced; the tail being so thin and airy that a
million miles thickness of it looks only like gauze in the sunlight. It
is not at all unlikely that such a thing may have happened without ever
being noticed. A passage through a telescopic comet would be accompanied
by a brilliant meteoric shower, probably a far more brilliant one than
has ever been recorded. No more serious danger would be encountered than
that arising from a possible fall of meteorites; but a collision between
the nucleus of a large comet and the earth might be a serious matter.
If, as Professor Peirce supposes, the nucleus is a solid body of
metallic density, many miles in diameter, the effect where the comet
struck would be terrific beyond conception. At the first contact in the
upper regions of the atmosphere, the whole heavens would be illuminated
with a resplendence beyond that of a thousand suns, the sky radiating a
light which would blind every eye that beheld it, and a heat which would
melt the hardest rocks. A few seconds of this, while the huge body was
passing through the atmosphere, and the collision at the earth's surface
would in an instant reduce everything there existing to fiery vapor, and
bury it miles deep in the solid earth. Happily, the chances of such a
calamity are so minute that they need not cause the slightest
uneasiness. There is hardly a possible form of death which is not a
thousand times more probable than this. So small is the earth in
comparison with the celestial spaces, that if one should shut his eyes,
and fire a gun at random in the air, the chance of bringing down a bird
would be better than that of a comet of any kind striking the earth."
[Illustration: Fig. 358.]
[Illustration: Fig. 359.]
318. _The Chemical Constitution of Comets._--Fig. 358 shows the bands of
the spectrum of a telescopic comet of 1873, as seen by two different
observers. Fig. 359 shows the spectrum of the coma and tail of the comet
of 1874; and the spectrum of the bright comet of 1881 showed the same
three bands for the coma and tail. Now, these three bands are those of
certain hydrocarbon vapors: hence it would seem that the coma and tails
of comets are composed chiefly of such vapors (315).
II. THE ZODIACAL LIGHT.