eclipses of the Sun. During a period equivalent to ten of our
terrestrial years, such eclipses continually succeed each other with but
very short periods of interruption; and even during a long series of
rotations of Saturn, the Sun remains completely invisible in those
regions where the apparent arcs which it describes coincide with the
arcs of the rings. In neighboring latitudes, the eclipses of the Sun,
although still frequent, would have a shorter and shorter duration as
the observer should travel north or south. These eclipses of the Sun
must produce a partial darkness of the regions involved in the shadow of
the rings, which may be compared to the darkness produced on our globe
by a total eclipse of the Sun. The frequent recurrence of these
eclipses, and their comparatively long duration in some regions, must
still further reduce the duration of the short Saturnian days.
The globe of Saturn, as already shown, casts a shadow on the rings,
which, according to the position of the planet in its orbit, either
extends across their whole breadth, or covers only a part of their
surface. The shadow on the rings rising in the east after sunset,
ascends to the culminating point of their arcs in the sky, in 2h. 34m.,
and as rapidly descends on the western horizon, to disappear with
sunrise. This shadow, when projected on the rings in the sky, must be
hardly distinguishable from the dark background of the heavens, except
from the absence of stars in the regions which it occupies. It must
appear as a large dark gap, separating the rings into two parts, and
constantly moving from east to west. Possibly the refraction of the
solar rays, in passing through Saturn's atmosphere, may cast some
colored light on the rings, similar to that observed on the Moon during
its eclipses.
An observer on the rings would behold phenomena still more curious, a
long day of 14¾ years being followed by a long night of 14¾ years. The
long days of Saturn's rings are, however, diversified by numerous
eclipses of the Sun, which regularly occur every 10¼ hours; the
phenomenon being due to the interposition of the globe of Saturn between
the rings and the Sun. These eclipses produce partial obscurations of
their surface, lasting from 1½ to 2 hours at a time. Although the
surface of the rings never receives direct sunlight during their long
nights, yet they are not plunged all the time in total darkness, as they
receive some reflected light from that part of the globe of Saturn which
is illuminated by the Sun. To the supposed observer on the rings, during
every 10¼ hours, the immense globe would exhibit continually changing
phases. At first he would see a point of light rapidly ascending from
the horizon, and appearing under the form of a half crescent of
considerable radius; 5⅛ hours later, the crescent having gradually
increased, would appear as a half circle, covering ⅛ of the visible
heavens, its surface being more than 20,000 times as large as the
surface of the Moon. Upon this brilliantly illuminated semi-circle would
be projected the shadows of the rings, appearing as black belts
separated by a narrow luminous band.
It is very difficult for one to conceive how such a delicate structure,
as the system of rings appears to be, can keep together in equilibrium
and avoid destruction from the powerful attraction of the planet on one
side and the disturbing influence of the satellites on the other. To
explain it, several hypotheses have been advanced. The rings were first
supposed to be solid, and upon this supposition Laplace determined the
necessary conditions for their equilibrium; the most important of which
require that the cross section of the rings should be an ellipse of
irregular curvature, and having its major axis directed towards the
centre of the planet, and also that the system should rotate upon an
axis perpendicular to the plane of the rings. This theory was superseded
by another, which supposed the rings to be fluid. This one was soon
rejected for a third, assuming the system to be composed of vapors or
gases; and more recently, all these theories were considered untenable,
and replaced by a fourth, which supposes the system of rings to be made
up of a congregation of innumerable small, independent bodies, revolving
around Saturn in concentric zones. Naturally, such a divergence of
opinion can only result from our comparative ignorance of the subject,
and sufficiently indicates our inability to explain the phenomena; and
it must be admitted that, so far, nothing is certainly known about this
strange system. We shall probably remain in the same uncertainty until
the rotation of the rings is ascertained by direct observations. It is
pretty certain, however, that none of these theories account for the
observed phenomena in their details, although a partial explanation may
be obtained by borrowing something from each hypothesis.
It has been conjectured, and a theory has been advanced, that the
breadth of the whole ring system is gradually increasing inwards, and
that it will come in contact with the planet in about 2,150 years; but
the question seems to have been settled in the negative by the elaborate
measurements of the English observers. It is likely that the increase is
only in the defining power of the instruments.
COMETS
PLATE XI
Among the celestial phenomena, none are more interesting than those
mysterious apparitions from the depths which unexpectedly display their
strange forms in our familiar constellations, through which they wander
for a time, until they disappear like phantoms.
A comet, with its luminous diffused head, whence proceeds a long vapory
appendage gradually fading away in the sky, presents an extraordinary
aspect, which may well astonish and deeply impress the observer.
Although these visitors from infinite space do not now inspire dread, as
in by-gone times, yet, owing to the mystery in which the phenomenon is
still involved, the apparition of a large comet, even in our days, never
fails to create a profound sensation, and in some cases that unconscious
fear which results from the unknown.
The effect of such a spectacle largely depends upon its rarity; but
since the telescope has been applied to the sounding of the heavens, it
has been found that the appearance of comets is by no means an unusual
occurrence. If so few comets, comparatively, are seen, it is because
most of them are telescopic objects, and are therefore invisible to the
naked eye. Most of the telescopic comets are not only too faint to be
perceived by the unaided eye, but are insignificant objects, even when
observed through the largest telescopes.
It was Kepler's opinion that comets are as numerous in the sky as fishes
are in the ocean. Undoubtedly the number of these bodies must be great,
considering that we can only see them when they come into the
neighborhood of the Earth, and that many even here remain invisible, or
at least pass unperceived. That many of them have passed unperceived
heretofore, is proved by the fact that the number of those observed
becomes greater every year, with the increase of the number of
instruments used in their search. The number of comets observed with the
naked eye during historic times is nearly 600, and that of telescopic
comets, which, of course, all belong to the last few centuries, is more
than 200, so that we have a total number of about 800 comets of which
records have been kept. From theoretical considerations, Lambert and
Arago estimated their entire number at several millions, but such
speculations have generally no real value, since they cannot be
established on a firm basis.
Comets remain visible for more or less time, according to their size and
the nature and position of their orbits, but in general, the large ones
can be followed with the telescope for several months after they have
become invisible to the naked eye. The comet of 1861, for example,
remained telescopically visible for a year, and that of 1811, for 17
months after disappearing from ordinary sight.
While a comet remains visible, it appears to revolve daily about us like
the stars in general; but it also moves among the constellations, and
from this movement its orbit may be computed like that of a planet. From
the apparent diurnal motion of a comet with the heavens, result the
changes of position which it seems to undergo in the course of a night.
The direction of the head and tail of a comet, of course, has only
changed in regard to the horizon, but not in regard to the sky, in which
they occupy very nearly the same position throughout a given night, and
even for many nights in succession.
The movements of the comets in their orbits are, like those of the
planets, in accordance with Kepler's laws, the Sun occupying one of the
foci of the orbit they describe; but the orbits of comets differ,
however, in several points from those of the planets. Their eccentricity
is always great, being sometimes apparently infinite, in which case the
orbit is said to be parabolic, or hyperbolic; but the smallness of the
portion of a cometary orbit which can ordinarily be observed, makes it
difficult to determine this with certainty. Again, while the planetary
orbits are usually near the plane of the ecliptic, those of comets
frequently have great inclinations to that plane, and even when the
inclination is less than 90°, the comet may have a retrograde movement,
or, in other words, a movement contrary to the course in which all the
planets revolve about the Sun.
Notwithstanding these differences between the elements of the orbits of
the comets and those of the planets, the fact that each has the Sun in
one focus indicates that the body moving in it is a member of the solar
system, either for the time, or permanently, according to the nature of
its orbit.
A distinction may accordingly be made between the comets which are
permanent members of our solar system and those which are only
accidental or temporary visitors. Those moving in elliptical orbits
around the Sun, like the planets, and therefore having a determinate
period of revolution, from which the time of their successive returns
may be predicted, are permanent members of our system, and are called
periodic comets. All comets moving in parabolical or hyperbolical
curves, are only temporary members of the solar system, being apparently
strangers who have been diverted from their courses by some disturbing
influence. No comet is classed as periodical which does not follow a
perceptibly elliptical orbit. Any comet passing around the Sun at the
mean distance of the Earth from this body, with a velocity of 26 miles
per second, will fly off into infinite space, to return to us no more.
The time of revolution of the different periodic comets thus far
observed varies greatly, as do also the distances to which they recede
from the Sun at aphelion. Whilst the period of revolution of Encke's
comet, the shortest thus far known, is only 3½ years, that of the comet
of 1844, II., is 102,000 years; and whilst the orbit of the first is
comprised within the orbit of Jupiter, that of the last extends to a
distance equal to 147 times the distance of Neptune from the Sun. But so
vast an orbit cannot be accurately determined from the imperfect data at
our disposal.
The periodic comets are usually divided into two classes. The comets
whose orbits are within the orbit of Neptune are called interior comets,
while those whose orbits extend beyond that of Neptune are called
exterior comets. The known interior periodic comets are twelve in
number, while, including all the cases in which there is some slight
evidence of elliptic motion, the number of exterior comets observed is
six or seven times as great. The periodic comets of short period are
very interesting objects, inasmuch as by their successive returns they
afford an opportunity to calculate their motions and to observe the
physical changes which they undergo in their intervals of absence.
From observation of the periodic comets, it has been learned that the
same comet never presents twice the same physical appearance at its
different returns, its size, shape and brilliancy varying so greatly
that a comet can never be identified by its physical characters alone.
It is only when its elements have been calculated, and are found to
agree with those of a cometary orbit previously known, that the two
comets can be identified one with the other. There are reasons to
believe that, in general, comets decrease in brightness and size at each
of their successive returns, and that they are also continually losing
some of their matter as they traverse their orbits.
When very far away from us, all comets appear nearly alike, consisting
of a faint nebulosity, of varying dimensions. When a comet first appears
in the depths of space, and travels towards the Sun, it generally
resembles a faint, uniformly luminous nebulosity, either circular or
slightly elongated in form. As it approaches nearer to the Sun, a slight
condensation of light appears towards its centre, and as it draws still
nearer, it becomes brighter and brighter, and in condensing forms a kind
of diffused luminous nucleus. At the same time that the comet acquires
this concentration of light, the nebulosity gradually becomes elongated
in the direction of the Sun. These effects generally go on increasing so
long as the comet is approaching the Sun; the condensation of light
sometimes forms a bright nucleus, comparable to a very brilliant star,
while the elongation becomes an immense appendage or tail. When the
comet has passed its perihelion and recedes from the Sun, the inverse
phenomena are observed; the comet, decreasing in brightness, gradually
loses its nucleus and tail, resumes its nebulous aspect, and finally
vanishes in space, to appear again in due course, if it chance to be a
periodic comet. While all comets become brighter in approaching the Sun,
they do not all, however, develop a large tail, some of them showing
only a slight elongation.
When a comet is first discovered with the telescope at a great distance
from the Sun, it is difficult to predict whether it will become visible
to the naked eye, or will remain a telescopic object, as it is only in
approaching the Sun that these singular bodies acquire their full
development. Thus, Donati's comet, whose tail became so conspicuous an
object at its full appearance in 1858, remained two months after its
discovery by the telescope without any indication of a tail. The comet
of Halley, which before and after its return in 1759, remained five
years inside of the orbit of Saturn, showed not the least trace of its
presence during the greater part of this time. Nothing but calculation
could then indicate the position in the sky of this invisible object,
which was so prominent when it approached the Sun.
Another curious phenomenon exhibited by comets, and first noticed by
Valz, is that in approaching the Sun the nebulosity composing these
bodies contracts, instead of dilating, as would be naturally supposed
from the greater amount of solar heat which they must then receive. In
receding from the Sun, on the contrary, they expand gradually. As comets
approach the Sun, the tail and nucleus are developed, while the
nebulosity originally constituting these comets contracts, as if its
material had been partly consumed in this development. In a certain
sense it may be said that the comets are partly created by the Sun; in
more exact terms, the changes of form which they undergo are induced by
the Sun's action upon them at different distances and under varying
conditions. Moreover, they are rendered visible by its influence,
without which they would pass unperceived in our sky. When a comet
disappears from view, it is not because its apparent diameter is so much
reduced by the distance that it vanishes, but rather on account of the
diminution of its light, both that which it receives from the Sun, and
its own light; these bodies being in some degree self-luminous, as will
be shown below.
The large comets, such as can be seen with the naked eye, always show
the following characteristics, on examination with the telescope. A
condensation of light resembling a diffused star forms the brightest
part of the comet, this condensation being situated towards the
extremity the nearest to the Sun. It is this starlike object which is
called the _nucleus_. The nucleus seems to be entirely enclosed in a
luminous vapory envelope of the same general texture, called the _coma_.
This envelope, which is quite variable in brightness and form, is
brightest next to the nucleus, and gradually fades away as it recedes
from it. The _nucleus_ and the _coma_, considered as a whole, constitute
the _head_ of a comet. From the head of a comet proceeds a long trail of
pale nebulous light, which usually grows wider, but fainter, as it
recedes from the nucleus, and insensibly vanishes in the sky. This
delicate appendage, or tail, as it is commonly called, varies very much
in size and shape, not only in different comets, but in the very same
comet, at different times. Its direction is generally opposite to that
of the Sun from the head of the comet.
The nuclei vary very much in brightness, in size and in shape; and while
in some telescopic comets they are either absent or barely
distinguishable as a small condensation of light, in bright comets they
may become plainly visible to the naked eye, and they sometimes even
surpass in brightness the most brilliant stars of the heavens. But
whatever may be the size of cometary nuclei, they are subject to sudden
and rapid changes, and vary from day to day. Sometimes they appear
exceedingly brilliant and sharply outlined, while at other times they
are so dim and diffused that they are hardly distinguishable from the
coma of which they seem then to form a part.
[PLATE XI.--THE GREAT COMET OF 1881.
Observed on the night of June 25-26 at 1h. 30m. A.M.]
From my observations upon the comets which have appeared since the year
1873, it is apparent that the changes in the nucleus, coma and tail, are
due to a solar action, which contracts or expands these objects in such
a manner that the nuclei become either bright and star-like, or dim and
diffused, in a very short time. I had excellent opportunity, especially
in the two large comets of 1881, to observe some of these curious
changes, a description of which will give an idea of their extent and
rapidity. On July 2d, 1881, at 9 o'clock, the nucleus of comet 1881,
III., which is represented on Plate XI., appeared sharply defined,
bright and considerably flattened crosswise; but half an hour later it
had considerably enlarged and had become so diffused that it could
hardly be distinguished from the coma, with which it gradually blended.
It is perhaps worth mention that, at the time this last observation was
made, an aurora borealis was visible. This comet 1881, III., underwent
other very important changes of its nucleus, coma and tail. On June
25th, the nucleus, which was bright and clearly defined, was ornamented
with four bright diverging conical wings of light, as shown on Plate XI.
On the 26th these luminous wings had gone, and the nucleus appeared
one-third smaller. On the 28th it had enlarged, but on the 29th its
shape was considerably altered, the nucleus extending in one direction
to three or four times its diameter on previous nights, and being
curved, so as to resemble a comma. On the 6th of July the nucleus of
this comet showed the greatest disturbances. The nucleus, which had
appeared perfectly round on the evening of the 5th, was found much
elongated at 10 o'clock on the 6th, forming then a straight, acute, and
well-defined wedge of light, inclined upwards to the left. The length of
the nucleus, at this time, was three or four times its ordinary
diameter. At the same time rapid changes occurred; the strangely shaped
nucleus soon became unsteady, extending and contracting alternately, and
varying greatly in brightness. At 10h. 45m., the elongated nucleus, then
gently curved, took the shape of a succession of luminous knots, which
at times became so brilliant and distinct that they seemed to be about
to divide and form separate nuclei; but such a separation did not
actually occur, at least while I was observing. While these important
changes were going on in the comet, a bright auroral arch appeared in
the north, which lasted only a short time. On July 7th, the sky being
cloudy, no observations were made, but on the 8th I observed the comet
again. The nucleus had then resumed its circular form, but it was yet
very unsteady, being sometimes small, bright and sharp, while a few
seconds later it appeared twice as large, but dim in outlines; and
sometimes an ill-defined secondary nucleus appeared at its centre. On
several occasions the nucleus appeared as if it were double, one nucleus
being apparently projected partly upon the other.
The nuclei of comets are sometimes very small, and in other cases very
large. Among those which have been measured, the nucleus of the comet of
1798, I., was only 28 miles in diameter, but that of Donati's comet, in
1858, was 5,600 miles, and that of the comet of 1845 was 8,000 miles in
diameter.
The coma of comets is found to be even more variable than the nucleus.
The changes observed in the coma are generally in close connection with
those of the nucleus and tail, the same perturbations affecting
simultaneously the whole comet. While the coma of the comet of 1847 was
only 18,000 miles in diameter, that of Halley's comet, in 1835, was
357,000 miles, and that of the comet of 1811 was 1,125,000 miles in
diameter. In general, as already stated, the coma of a comet decreases
in size in approaching the Sun. That of Encke's comet, which, on October
9th, 1838, had a diameter of 281,000 miles, gradually decreased at a
daily mean rate of 4,088 miles in going towards the Sun; so that, on
December 17th, when the distance of the comet from the Sun was more than
four times less than it was on the first date, its diameter was reduced
to 3,000 miles.
The form of the coma, in that part which is free from the tail, is in
general a portion of a circle, but is sometimes irregular, with its
border deformed. Thus, the border of the coma of Halley's comet was
depressed at one point towards the Sun. I observed a similar phenomenon
in Coggia's comet, with the great refractor of the Harvard College
Observatory, on July 13th, 1874, when its border appeared deeply
depressed on the side nearest to the Sun, as if repelled by this body.
The coma of comet 1881, III., showed also very singular outlines on the
nights of the 25th and 26th of June, when its border was so deeply
depressed that the coma appeared as if it were double. Luminous rays and
jets often radiate from the nucleus across the coma, and describe
graceful lateral curves, falling backwards and gradually fading away
into the tail, of which they then form a part. The rays and jets emitted
by the nucleus seem at first to obey the solar attraction and travel
towards the Sun; but they are soon repelled, and move backward towards
the tail. It is a mystery, as yet unexplained, how these cometary jets,
which at first seem to obey to the laws of attraction, are compelled to
retreat apparently by superior opposing forces. Among the forces of
nature, we know of no other than those of an electrical sort, which
would act in a similar manner; but this explanation would require us to
assume some direct electrical communication between the comet and the
Sun. Considering the distance between the two bodies, and the probable
absence or great tenuity of the gaseous material in interstellar space,
such an assumption is a difficult one.