always maintained that the planet found by observation was not the one
whose existence had been predicted by Leverrier and Adams, though its
action would completely explain all the irregularities in the motion of
Uranus. His last statement on this point is as follows: "My position is,
that there were _two possible planets_, either of which might have
caused the observed irregular motions of Uranus. Each planet excluded
the other; so that, if one was, the other was not. They coincided in
direction from the earth at certain epochs, once in six hundred and
fifty years. It was at one of these epochs that the prediction was made,
and at no other time for six centuries could the prediction of the one
planet have revealed the other. The observed planet was not the
predicted one."
282. _Bode's Law Disproved._--The following table gives the distances of
the planets according to Bode's law, their actual distances, and the
error of the law in each case:--
Planet. Numbers of Actual Errors.
Bode. Distances.
Mercury 0 + 4 = 4 3.9 0.1
Venus 3 + 4 = 7 7.2 0.2
Earth 6 + 4 = 10 10.0 0.0
Mars 12 + 4 = 16 15.2 0.8
Minor 24 + 4 = 28 20 to 35
planets
Jupiter 48 + 4 = 52 52.0 0.0
Saturn 96 + 4 = 100 95.4 4.6
Uranus 192 + 4 = 196 191.9 4.1
Neptune 384 + 4 = 388 300.6 87.4
It will be seen, that, before the discovery of Neptune, the agreement
was so close as to indicate that this was an actual law of the
distances; but the discovery of this planet completely disproved its
existence.
[Illustration: Fig. 304.]
283. _The Satellite of Neptune._--Neptune is accompanied by at least one
moon, whose orbit is shown in Fig. 304. The orbit of this satellite is
inclined about thirty degrees to the plane of the ecliptic, and the
motion of the satellite is retrograde, or from east to west.
VII. COMETS AND METEORS.
I. COMETS.
General Phenomena of Comets.
284. _General Appearance of a Bright Comet._--Comets bright enough to be
seen with the naked eye are composed of three parts, which run into each
other by insensible gradations. These are the _nucleus_, the _coma_, and
the _tail_.
The _nucleus_ is the bright centre of the comet, and appears to the eye
as a star or planet.
The _coma_ is a nebulous mass surrounding the nucleus on all sides.
Close to the nucleus it is almost as bright as the nucleus itself; but
it gradually shades off in every direction. The nucleus and coma
combined appear like a star shining through a small patch of fog; and
these two together form what is called the _head_ of the comet.
The _tail_ is a continuation of the coma, and consists of a stream of
milky light, growing wider and fainter as it recedes from the head, till
the eye is unable to trace it.
[Illustration: Fig. 305.]
The general appearance of one of the smaller of the brilliant comets is
shown in Fig. 305.
[Illustration: Fig. 306.]
[Illustration: Fig. 307.]
285. _General Appearance of a Telescopic Comet._--The great majority of
comets are too faint to be visible with the naked eye, and are called
_telescopic_ comets. In these comets there seems to be a development of
coma at the expense of nucleus and tail. In some cases the telescope
fails to reveal any nucleus at all in one of these comets; at other
times the nucleus is so faint and ill-defined as to be barely
distinguishable. Fig. 306 shows a telescopic comet without any nucleus
at all, and another with a slight condensation at the centre. In these
comets it is generally impossible to distinguish the coma from the tail,
the latter being either entirely invisible, as in Fig. 306, or else only
an elongation of the coma, as shown in Fig. 307. Many comets appear
simply as patches of foggy light of more or less irregular form.
[Illustration: Fig. 308.]
286. _The Development of Telescopic Comets on their Approach to the
Sun._--As a rule, all comets look nearly alike when they first come
within the reach of the telescope. They appear at first as little foggy
patches, without any tail, and often without any visible nucleus. As
they approach the sun their peculiarities are rapidly developed. Fig.
308 shows such a comet as first seen, and the gradual development of its
nucleus, head, and tail, as it approaches the sun.
[Illustration: Fig. 309.]
[Illustration: Fig. 310.]
[Illustration: Fig. 311.]
If the comet is only a small one, the tail developed is small; but these
small appendages have a great variety of form in different comets. Fig.
309 shows the singular form into which _Encke's_ comet was developed in
1871. Figs. 310 and 311 show other peculiar developments of telescopic
comets.
287. _Development of Brilliant Comets on their Approach to the
Sun._--Brilliant comets, as well as telescopic comets, appear nearly
alike when they come into the view of the telescope; and it is only on
their approach to the sun that their distinctive features are developed.
Not only do these comets, when they first come into view, resemble each
other, but they also bear a close resemblance to telescopic comets.
As the comet approaches the sun, bright vaporous jets, two or three in
number, are emitted from the nucleus on the side of the sun and in the
direction of the sun. These jets, though directed towards the sun, are
soon more or less carried backward, as if repelled by the sun. Fig. 312
shows a succession of views of these jets as they were developed in the
case of _Halley's_ comet in 1835.
[Illustration: Fig. 312.]
The jets in this case seemed to have an oscillatory motion. At 1 and 2
they seemed to be attracted towards the sun, and in 3 to be repelled by
him. In 4 and 5 they seemed to be again attracted, and in 6 to be
repelled, but in a reverse direction to that in 3. In 7 they appeared to
be again attracted. Bessel likened this oscillation of the jets to the
vibration of a magnetic needle when presented to the pole of a magnet.
In the case of larger comets these luminous jets are surrounded by one
or more envelops, which are thrown off in succession as the comet
approaches the sun. The formation of these envelops was a conspicuous
feature of _Donati's_ comet of 1858. A rough view of the jets and the
surrounding envelops is given in Fig. 313. Fig. 314 gives a view of the
envelops without the jets.
[Illustration: Fig. 313.]
[Illustration: Fig. 314.]
288. _The Tails of Comets._--The _tails_ of brilliant comets are rapidly
formed as the comet approaches the sun, their increase in length often
being at the rate of several million miles a day. These appendages seem
to be formed entirely out of the matter which is emitted from the
nucleus in the luminous jets which are at first directed towards the
sun. The tails of comets are, however, always directed away from the
sun, as shown in Fig. 315.
[Illustration: Fig. 315.]
It will be seen that the comet, as it approaches the sun, travels head
foremost; but as it leaves the sun it goes tail foremost.
The apparent length of the tail of a comet depends partly upon its real
length, partly upon the distance of the comet, and partly upon the
direction of the axis of the tail with reference to the line of vision.
The longer the tail, the nearer the comet; and the more nearly at right
angles to the line of vision is the axis of the tail, the greater is the
apparent length of the tail. In the majority of cases the tails of
comets measure only a few degrees; but, in the case of many comets
recorded in history, the tail has extended half way across the heavens.
The tail of a comet, when seen at all, is usually several million miles
in length; and in some instances the tail is long enough to reach across
the orbit of the earth, or twice as far as from the earth to the sun.
The tails of comets are apparently hollow, and are sometimes a million
of miles in diameter. So great, however, is the tenuity of the matter in
them, that the faintest stars are seen through it without any apparent
obscuration. See Fig. 316, which is a view of the great comet of 1264.
[Illustration: Fig. 316.]
[Illustration: Fig. 317.]
[Illustration: Fig. 318.]
[Illustration: Fig. 319.]
[Illustration: Fig. 320.]
The tails of comets are sometimes straight, as in Fig. 316, but usually
more or less curved, as in Fig. 317, which is a view of _Donati's_ comet
as it appeared at one time. The tail of a comet is occasionally divided
into a number of streamers, as in Figs. 318 and 319. Fig. 318 is a view
of the great comet of 1744, and Fig. 319 of the great comet of 1861. No.
1, in Fig. 320, is a view of the comet of 1577; No. 2, of the comet of
1680; and No. 3, of the comet of 1769.
[Illustration: Fig. 321.]
Fig. 321 shows some of the forms which the imagination of a
superstitious age saw depicted in comets, when these heavenly visitants
were thought to be the forerunners of wars, pestilence, famine, and
other dire calamities.
289. _Visibility of Comets._--Even the brightest comets are visible only
a short time near their perihelion passage. When near the sun, they
sometimes become very brilliant, and on rare occasions have been visible
even at mid-day. It is seldom that a comet can be seen, even with a
powerful telescope, during its perihelion passage, unless its perihelion
is either inside of the earth's orbit, or but little outside of it.
Motion and Origin of Comets.
290. _Recognition of a Telescopic Comet._--It is impossible to
distinguish telescopic comets by their appearance from another class of
heavenly bodies known as _nebulæ_. Such comets can be recognized only by
their motion. Thus, in Fig. 322, the upper and lower bodies look exactly
alike; but the upper one is found to remain stationary, while the lower
one moves across the field of view. The upper one is thus shown to be a
nebula, and the lower one a comet.
[Illustration: Fig. 322.]
291. _Orbits of Comets._--All comets are found to move in _very
eccentric ellipses_, in _parabolas_, or in _hyperbolas_.
Since an ellipse is a _closed_ curve (48), all comets that move in
ellipses, no matter how eccentric, are permanent members of the solar
system, and will return to the sun at intervals of greater or less
length, according to the size of the ellipses and the rate of the
comet's motion.
Parabolas and hyperbolas being _open_ curves (48), comets that move in
either of these orbits are only temporary members of our solar system.
After passing the sun, they move off into space, never to return, unless
deflected hither by the action of some heavenly body which they pass in
their journey.
[Illustration: Fig. 323.]
Since a comet is visible only while it is near the sun, it is
impossible to tell, by the form of the portion of the orbit which it
describes during the period of its visibility, whether it is a part
of a very elongated ellipse, a parabola, or a hyperbola. Thus in
Fig. 323 are shown two orbits, one of which is a very elongated
ellipse, and the other a parabola. The part _ab_, in each case, is
the portion of the orbit described by the comet during its
visibility. While describing the dotted portions of the orbit, the
comet is invisible. Now it is impossible to distinguish the form of
the visible portion in the two orbits. The same would be true were
one of the orbits a hyperbola.
Whether a comet will describe an ellipse, a parabola, or a
hyperbola, can be determined only by its _velocity_, taken in
connection with its _distance from the sun_. Were a comet ninety-two
and a half million miles from the sun, moving away from the sun at
the rate of twenty-six miles a second, it would have just the
velocity necessary to describe a _parabola_. Were it moving with a
greater velocity, it would necessarily describe a _hyperbola_, and,
with a less velocity, an _ellipse_. So, at any distance from the
sun, there is a certain velocity which would cause a comet to
describe a parabola; while a greater velocity would cause it to
describe a hyperbola, and a less velocity to describe an ellipse. If
the comet is moving in an ellipse, the less its velocity, the less
the eccentricity of its orbit: hence, in order to determine the form
of the orbit of any comet, it is only necessary to ascertain its
distance from the sun, and its velocity at any given time.
Comets move in every direction in their orbits, and these orbits
have every conceivable inclination to the ecliptic.
292. _Periodic Comets._--There are quite a number of comets which are
known to be _periodic_, returning to the sun at regular intervals in
elliptic orbits. Some of these have been observed at several returns, so
that their period has been determined with great certainty. In the case
of others the periodicity is inferred from the fact that the velocity
fell so far short of the parabolic limit that the comet must move in an
ellipse. The number of known periodic comets is increasing every year,
three having been added to the list in 1881.
The velocity of most comets is so near the parabolic limit that it is
not possible to decide, from observations, whether it falls short of it,
or exceeds it. In the case of a few comets the observations indicate a
minute excess of velocity; but this cannot be confidently asserted. It
is not, therefore, absolutely certain that any known comet revolves in a
hyperbolic orbit; and thus it is possible that all comets belong to our
system, and will ultimately return to it. It is, however, certain, that,
in the majority of cases, the return will be delayed for many centuries,
and perhaps for many thousand years.
293. _Origin of Comets._--It is now generally believed that the original
home of the comets is in the stellar spaces outside of our solar system,
and that they are drawn towards the sun, one by one, in the long lapse
of ages. Were the sun unaccompanied by planets, or were the planets
immovable, a comet thus drawn in would whirl around the sun in a
parabolic orbit, and leave it again never to return, unless its path
were again deflected by its approach to some star. But, when a comet is
moving in a parabola, the slightest _retardation_ would change its orbit
to an ellipse, and the slightest _acceleration_ into a hyperbola. Owing
to the motion of the several planets in their orbits, the velocity of a
comet would be changed on passing each of them. Whether its velocity
would be accelerated or retarded, would depend upon the way in which it
passed. Were the comet accelerated by the action of the planets, on its
passage through our system, more than it was retarded by them, it would
leave the system with a more than parabolic orbit, and would therefore
move in a hyperbola. Were it, on the contrary, retarded more than
accelerated by the action of the planets, its velocity would be reduced,
so that the comet would move in a more or less elongated ellipse, and
thus become a permanent member of the solar system.
In the majority of cases the retardation would be so slight that it
could not be detected by the most delicate observation, and the comet
would return to the sun only after the expiration of tens or hundreds of
thousands of years; but, were the comet to pass very near one of the
larger planets, the retardation might be sufficient to cause the comet
to revolve in an elliptical orbit of quite a short period. The orbit of
a comet thus captured by a planet would have its aphelion point near the
orbit of the planet which captured it. Now, it happens that each of the
larger planets has a family of comets whose aphelia are about its own
distance from the sun. It is therefore probable that these comets have
been captured by the action of these planets. As might be expected from
the gigantic size of Jupiter, the Jovian family of comets is the
largest. The orbits of several of the comets of this group are shown in
Fig. 324.
[Illustration: Fig. 324.]
294. _Number of Comets._--The number of comets recorded as visible to
the naked eye since the birth of Christ is about five hundred, while
about two hundred telescopic comets have been observed since the
invention of the telescope. The total number of comets observed since
the Christian era is therefore about seven hundred. It is certain,
however, that only an insignificant fraction of all existing comets have
ever been observed. Since they can be seen only when near their
perihelion, and since it is probable that the period of most of those
which have been observed is reckoned by thousands of years (if, indeed,
they ever return at all), our observations must be continued for many
thousand years before we have seen all which come within range of our
telescopes. Besides, as already stated (289), a comet can seldom be seen
unless its perihelion is either inside the orbit of the earth, or but
little outside of it; and it is probable that the perihelia of the great
majority of comets are beyond this limit of visibility.
Remarkable Comets.
295. _The Comet of 1680._--The great comet of 1680, shown in Fig. 320,
is one of the most celebrated on record. It was by his study of its
motions that Newton proved the orbit of a comet to be one of the conic
sections, and therefore that these bodies move under the influence of
gravity. This comet descended almost in a direct line to the sun,
passing nearer to that luminary than any comet before known. Newton
estimated, that, at its perihelion point, it was exposed to a
temperature two thousand times that of red-hot iron. During its
perihelion passage it was exceedingly brilliant. Halley suspected that
this comet had a period of five hundred and seventy-five years, and that
its first recorded appearance was in 43 B.C., its third in 1106, and its
fourth in 1680. If this is its real period, it will return in 2255. The
comet of 43 B.C. made its appearance just after the assassination of
Julius Cæsar. The Romans called it the _Julian Star_, and regarded it as
a celestial chariot sent to convey the soul of Cæsar to the skies. It
was seen two or three hours before sunset, and continued visible for
eight successive days. The great comet of 1106 was described as an
object of terrific splendor, and was visible in close proximity to the
sun. The comet of 1680 has become celebrated, not only on account of its
great brilliance, and on account of Newton's investigation of its orbit,
but also on account of the speculation of the theologian Whiston in
regard to it. He accepted five hundred and seventy-five years as its
period, and calculated that one of its earlier apparitions must have
occurred at the date of the flood, which he supposed to have been caused
by its near approach to the earth; and he imagined that the earth is
doomed to be destroyed by fire on some future encounter with this comet.
[Illustration: Fig. 325.]
296. _The Comet of 1811._--The great comet of 1811, a view of which is
given in Fig. 325, is, perhaps, the most remarkable comet on record. It
was visible for nearly seventeen months, and was very brilliant,
although at its perihelion passage it was over a hundred million miles
from the sun. Its tail was a hundred and twenty million miles in length,
and several million miles through. It has been calculated that its
aphelion point is about two hundred times as far from the sun as its
perihelion point, or some seven times the distance of Neptune from the
sun. Its period is estimated at about three thousand years. It was an
object of superstitious terror, especially in the East. The Russians
regarded it as presaging Napoleon's great and fatal war with Russia.
[Illustration: Fig. 326.]
[Illustration: Fig. 327.]
297. _Halley's Comet._--Halley's comet has become one of the most
celebrated of modern times. It is the first comet whose return was both
predicted and observed. It made its appearance in 1682. Halley computed
its orbit, and compared it with those of previous comets, whose orbits
he also computed from recorded observations. He found that it coincided
so exactly with that of the comet observed by Kepler in 1607, that there
could be no doubt of the identity of the two orbits. So close were they
together, that, were they both drawn in the heavens, the naked eye would
almost see them joined into one line. There could therefore be no doubt
that the comet of 1682 was the same that had appeared in 1607, and that
it moved in an elliptic orbit, with a period of about seventy-five
years. He found that this comet had previously appeared in 1531 and in
1456; and he predicted that it would return about 1758. Its actual
return was retarded somewhat by the action of the planets on it in its
passage through the solar system. It, however, appeared again in 1759,
and a third time in 1835. Its next appearance will be about 1911. The
orbit of this comet is shown in Fig. 326. Fig. 327 shows the comet as it
appeared to the naked eye, and in a telescope of moderate power, in
1835. This comet appears to be growing less brilliant. In 1456 it
appeared as a comet of great splendor; and coming as it did in a very
superstitious age, soon after the fall of Constantinople, and during the
threatened invasion of Europe by the Turks, it caused great alarm. Fig.
328 shows the changes undergone by the nucleus of this comet during its
perihelion passage in 1835.
[Illustration: Fig. 328.]
[Illustration: Fig. 329.]
[Illustration: Fig. 330.]
298. _Encke's Comet._--This telescopic comet, two views of which are
given in Figs. 329 and 330, appeared in 1818. Encke computed its orbit,
and found it to lie wholly within the orbit of Jupiter (Fig. 324), and
the period to be about three years and a third. By comparing the
intervals between the successive returns of this comet, it has been
ascertained that its orbit is continually growing smaller and smaller.
To account for the retardation of this comet, Olbers announced his
celebrated hypothesis, that the celestial spaces are filled with a
subtile _resisting medium_. This hypothesis was adopted by Encke, and
has been accepted by certain other astronomers; but it has by no means
gained universal assent.
299. _Biela's Comet._--This comet appeared in 1826, and was found to
have a period of about six years and two thirds. On its return in 1845,
it met with a singular, and as yet unexplained, accident, which has
rendered the otherwise rather insignificant comet famous. In November
and December of that year it was observed as usual, without any thing
remarkable about it; but, in January of the following year, it was found
to have been divided into two distinct parts, so as to appear as two
comets instead of one. The two parts were at first of very unequal
brightness; but, during the following month, the smaller of the two
increased in brilliancy until it equalled its companion; it then grew
fainter till it entirely disappeared, a month before its companion. The
two parts were about two hundred thousand miles apart. Fig. 331 shows
these two parts as they appeared on the 19th of February, and Fig. 332
as they appeared on the 21st of February. On its return in 1852, the
comets were found still to be double; but the two components were now
about a million and a half miles apart. They are shown in Fig. 333 as
they appeared at this time. Sometimes one of the parts appeared the
brighter, and sometimes the other; so that it was impossible to decide
which was really the principal comet. The two portions passed out of
view in September, and have not been seen since; although in 1872 the
position of the comet would have been especially favorable for
observation. The comet appears to have become completely broken up.
[Illustration: Fig. 331.]
[Illustration: Fig. 332.]
[Illustration: Fig. 333.]
[Illustration: Fig. 334.]
300. _The Comet of 1843._--The great comet of 1843, a view of which is
given in Fig. 334, was favorably situated for observation only in
southern latitudes. It was exceedingly brilliant, and was easily seen in
full daylight, in close proximity to the sun. The apparent length of its
tail was sixty-five degrees, and its real length a hundred and fifty
million miles, or nearly twice the distance from the earth to the sun.
This comet is especially remarkable on account of its near approach to
the sun. At the time of its perihelion passage the distance of the comet
from the photosphere of the sun was less than one-fourteenth of the
diameter of the sun. This distance was only one-half that of the comet
of 1680 when at its perihelion. When at perihelion, this comet was
plunging through the sun's outer atmosphere at the rate of one million,
two hundred and eighty thousand miles an hour. It passed half way round
the sun in the space of _two hours_, and its tail was whirled round
through a hundred and eighty degrees in that brief time. As the tail
extended almost double the earth's distance from the sun, the end of the
tail must have traversed in two hours a space nearly equal to the
circumference of the earth's orbit,--a distance which the earth, moving
at the rate of about twenty miles a second, is a _whole year_ in
passing. It is almost impossible to suppose that the matter forming this
tail remained the same throughout this tremendous sweep.