the _middle_, or _intermediary_, which are separated by a wide and
continuous black line, called the _principal division_. This line, and
indeed all the features of the surface of the rings are better seen, and
appear more prominent on that part of the ring on either side called the
_ansa_, or handle. Besides these two conspicuous rings, a third, of very
dark bluish or purplish color, lies between this middle ring, to which
it is contiguous, and the planet. This inner ring, which is quite wide,
is called the _gauze_ or _dusky ring_. Closer examination shows that the
outer ring is itself divided by a narrow, faint, grayish line called the
pencil line, which, from its extreme faintness, is only visible on the
ansæ. Moreover, the middle ring is composed of three concentric zones,
or belts, which, although not apparently divided by any interval of
space, are distinguished by the different shadings of the materials
composing them. The outer zone of this compound middle ring is, by far,
the brightest of all the system of rings and belts, especially close to
its external border, where, on favorable occasions, I have seen it
appear on the ansæ as if mottled over, and covered throughout with
strongly luminous cloud-like masses. On the ansæ of the double outer
ring, similar cloudy forms have also been seen at different times. The
second zone of the middle ring is darker than the first, the innermost
being darker still. All the characteristic points which have thus been
described, are shown in Plate X.
Although suspected in 1838, the dusky ring was not recognized before
1850, when G. P. Bond discovered it with the 15-inch refractor of the
Cambridge Observatory. It was also independently discovered the same
year in England by Dawes and Lassell. The dusky ring differs widely in
appearance and in constitution from the other rings, inasmuch as these
last are opaque, and either white or grayish, while the former is very
dark, and yet so transparent that the limb of the planet is plainly seen
through its substance. On particularly favorable occasions, the
appearance of this ring resembles that of the fine particles of dust
floating in a ray of light traversing a dark chamber. Whatever may be
the material of which this ring is composed, it must be quite rarefied,
especially towards its inner border, which appears as if composed of
distinct and minute particles of matter feebly reflecting the solar
light. That the inner part of the dusky ring is composed of separate
particles, is proved by the fact that the part of the ring which is seen
in front of the globe of Saturn has its inner border abruptly deflected
and curved inward on entering upon the disk, causing it to appear
considerably narrower than it must be in reality, a peculiarity which is
shown in the Plate. This phenomenon may be attributed to an effect of
irradiation, due to the strong light reflected by the central parts of
the ball, which so reduces the apparent diameter of the individual
particles that they become invisible to us, especially those near the
inner border, which are more scattered and less numerous than elsewhere.
The dusky ring, which was described by Bond, Lassell and other
astronomers as being equally transparent throughout all its width, has
not been found so by me in later years. The limb of the planet, seen by
these observers through the whole width of the dusky ring in 1850, could
not be traced through its outer half by myself in 1872 and 1874, and
this with the very same instrument used by Bond in his observations of
1848 and 1850. Moreover, I have plainly seen that its transparency was
not everywhere equal, but greatest on the inner border, from which it
gradually decreases, until it becomes opaque, as proved by the gradual
loss of distinctness of the limb, which vanishes at about the middle of
the dusky ring. These facts, which have been well ascertained, prove
that the particles composing this ring are not permanently located, and
are undergoing changes of relative position. It will be shown that the
surface of the other rings is also subject to changes, which are
sometimes very rapid.
The globe of Saturn is not self-luminous, but opaque. It shines by the
solar light, as is proved by the shadow it casts opposite the Sun upon
the ring. Although receiving its light from the Sun, Saturn does not
exhibit any traces of phases, like the other planets nearer to the Sun,
owing to its great distance from the Earth. When near its quadratures,
however, the limb opposite to the Sun appears much darker, and shows
traces of twilight. As far as can be ascertained, the rings, with the
exception of the inner one, are opaque, as proved by the strong shadow
which they cast on the globe of Saturn.
The shadows cast by the planet on the ring, and by the ring on the
planet, are very interesting phenomena, inasmuch as they enable the
astronomer to recognize the form of the surface which receives them. The
shadow cast by the ring on the ball is not quite so interesting as the
other, although it has served to prove that the surface of this globe is
not smooth, as is likewise suggested by its mottled appearance. I have
sometimes found, as have also other observers, that the outline of this
shadow upon the ball was irregular and indented, an observation which
proves either that the surface of the ball is irregular, or that the
border of the ring casting the shadow was jagged. The shadow of the
globe on the rings has much more interest, as it enables us to get at
some knowledge of the form of the surface of the rings, which otherwise
is very difficult to discover, owing to the oblique position in which we
always see them.
In general, the shadow of the ball on the middle ring has its outline
concave towards the planet; while on the outer ring it is usually
slanting, and at a greater distance from the limb than on the middle,
and dusky rings. This form of the shadow evidently proves that the
middle ring stands at a higher level than the two others, especially
towards its outer margin. The system seems to increase gradually in
thickness from the inner border of the dusky ring to the vicinity of the
outer margin of the middle ring, after which it rapidly diminishes on
this border, while the surface of the outer ring is almost level.
But this surface is by no means fixed, as its form sometimes changes, as
proved by my observations and those of others. As may be noticed on
Plate X., the outline of the shadow of the planet on the rings is
strongly deviated towards the planet, near the outer margin of the
middle ring; the notch indicating an abrupt change of level, and a rise
of the surface at that point. Some observers have endeavored to explain
these deviations by the phenomena of irradiation, from which it would
follow that the maximum effect of deviation should be observed where the
ring is the brightest, which does not accord with observation; as the
deepest depression in the shadow is not to be found usually at the
brightest part, which is towards the outer border of the middle ring,
but occurs near its centre. From these observations it is undoubtedly
established that the surface of the rings is far from being flat
throughout, and is, besides, not permanent, but changes, as would, for
instance, the surface of a large mass of clouds seen from the top of a
high mountain. In general, the system is thickest not very far from the
outer border of the intermediary ring.
Some interesting phenomena which I had occasion to observe before and
after the passage of the Sun through the plane of the rings, on February
6th, 1878, conclusively show that the surface of this system cannot be
of a uniform level, but must be thicker towards the outer border of the
middle ring, thence gradually sloping towards the planet. Many of my
observations irresistibly lead to this conclusion. As it would, however,
be out of place to have them recorded here in detail, I will simply give
one of the most characteristic among them.
From December 18th, 1877, when the Sun was about 41' above the plane of
the rings, to February 6th, 1878, the day of its passage through their
plane, the illuminated surface of this system gradually decreased in
breadth with the lowering of the Sun, until it was lost sight of,
February 5th, on the eve of the passage of the Sun through their plane.
The phenomenon in question consisted in the gradual invasion of their
illuminated surface by what appeared to be a black shadow, apparently
cast by the front part of the outer portion of the middle ring the
nearest to the Sun. On January 25th, when the elevation of the Sun above
the plane of the rings was reduced to 15', the shadow thus cast had
extended so far on their surface that it reached the shadow cast by the
globe on the opposite part of the ring in the east, and accordingly the
remaining portion of the illuminated surface of the eastern ansa then
appeared entirely disconnected from the ball, by a large dark gap,
corresponding in breadth to that of the globe's shadow on the rings. On
February 4th, when the Sun was only 5' above the plane of the rings,
their illuminated and only visible surface was reduced to a mere thread
of light, which on the 5th appeared broken into separate points. It is
evident that the phenomenon was not caused by the obliquity of the ring
as seen from our globe, since the elevation of the Earth above the plane
of the rings--which on December 18th was 3° 20'--was still 1° 20' on
the 4th of February. In ordinary circumstances, when the Sun is a little
more elevated, and the rings seen at this last angle, they appear quite
broad and conspicuous, and even the dark open space separating the dusky
ring from the planet is perfectly visible on the ansæ, where the
Earth's elevation above their plane is reduced to 40'. It is also
evident that the phenomenon was not to be attributed to the reduction of
the light which they received from the Sun, although the illumination in
February might be expected to be comparatively feeble, since the Sun
then shone upon the rings so obliquely; yet (on the supposition that
their surface is flat) they should have been illuminated throughout, and
if not very brightly, sufficiently so, at least, to make them visible
and as bright as was the narrow thread of light observed on the 4th of
February. The phenomenon actually observed may be explained most readily
by assuming, as other phenomena also indicate, that the surface of the
ring is not flat, but more elevated towards, or in the vicinity of its
outer border, from which place it slopes inwardly towards the planet. On
this assumption, it is evident that the elevated part of the ring the
nearest to the Sun would cast a shadow, which, with the increasing
obliquity of the Sun, would gradually cover the whole surface comprised
within the elevated part, and thus become invisible to us. Several
observations made by Bond and other observers undoubtedly show the same
phenomenon, and do not seem to be intelligible on any other supposition.
From my observations made in 1881 it would appear, however, that the
opposite surfaces of the rings do not exactly correspond in form, but
this may not be a permanent feature, as the surface of this system is
subject to changes, as already shown.
The dimensions of the rings are great, the diameter of the outer one
being no less than 172,982 miles, the distance from the centre of the
globe to the outer border of the system being, therefore, 86,491 miles.
The breadth of the outer ring is 9,941 miles; that of the principal
division, 2,131 miles; that of the middle ring, 19,902 miles, and that
of the dusky ring, 8,772 miles. The breadth of all the rings taken
together is, therefore, 40,746 miles. The interval between the surface
of Saturn and the inner border of the dusky ring is 7,843 miles.
The thickness of the system of rings has been variously estimated by
astronomers, on account of the great difficulties attending its
determination. While Sir John Herschel estimated it at more than 250
miles, G. P. Bond reduces it to 40 miles. Both of these numbers are
evidently too small, as so slight a thickness cannot explain the
observed phenomenon of the shadow cast by a portion of the ring on its
own surface, when the Sun is very low in its horizon, as shown above.
The plane of the system of rings is inclined 27° to the planet's orbit,
and is parallel, or at least very nearly so, with the equator of the
planet, passing, therefore, through its centre, and dividing its globe
into northern and southern hemispheres. Seen from the Earth, a portion
of the ring always appears projected in front of the planet, thus
concealing a small part of its globe, while the opposite portion passes
behind the globe, which hides it from sight.
As the plane of the ring is not affected by the motion of the planet
around the Sun, but always remains parallel to itself, it follows that
as Saturn advances in its orbit the rings must successively present
themselves to us under various angles of inclination, appearing,
therefore, more or less elliptical, and presenting two maxima and two
minima of inclination in the course of one of its revolutions. As the
revolution of Saturn is accomplished in 29½ years, the maxima and the
minima must recur every 14 years and 9 months; the maxima being
separated from the minima by an interval of 7 years and 4½ months.
When Saturn arrives at the two opposite points of its orbit, where the
major axis of its ring is at right angles to the line joining its centre
to that of the Sun, the ring, which is then viewed at an inclination of
27°, the greatest angle at which it can ever be seen, has reached its
maximum opening, the smaller diameter of its ellipse being then about
half that of the larger. At this moment the outer ring projects north
and south beyond the globe, which is then completely enclosed in its
ellipse. The maximum opening of the northern surface of the ring takes
place, at present, when Saturn arrives in longitude 262°, in the
constellation Sagittarius, and that of the southern surface when it
arrives in longitude 82° in the constellation Taurus. When, on the
contrary, Saturn reaches the two opposite points of its orbit, where the
plane of its ring is parallel to the line joining its centre to that of
the Sun, the opening vanishes, as only the thin edge of the ring is then
presented to the Sun and receives its light, the rest being in darkness.
At this moment the ring disappears, except in the largest telescopes,
where it is seen as an exceedingly thin thread of light; and the
Saturnian globe, having apparently lost its ring, appears solitary in
the sky, like the other planets. The disappearance of the ring from this
cause occurs now when Saturn arrives at 90° from either of the
positions of maximum inclination, that is, in longitude 352° in the
constellation Pisces, and in longitude 172° in the constellation Leo.
When the planet is in any other position than one of these last two,
either the northern or the southern surface of the ring is illuminated
by the Sun, while the opposite surface is in the night, and does not
receive any direct sunlight. At the time of the passage of the plane of
the ring through the Sun's centre, a change takes place in the
illumination of the ring. If it is the northern surface which has
received the rays of the Sun during the previous half of the Saturnian
year, at the moment the plane has passed the centre of the Sun, the
southern surface, after having been buried in darkness for 14¾ years,
sees the dawn of its long day of the same length. Such a phenomenon will
not occur until 1892, when the passage of the Sun from the northern to
the southern side of the ring will close in twilight the day commenced
in 1878.
Aside from the periodic disappearance of the ring, resulting from the
passage of the Sun through its plane, the ring may also disappear from
other reasons. Just before or just after the time of the passage of the
Sun through the plane of the ring, the Earth and the Sun may occupy such
positions, that while the one is north of the plane of the ring, the
other is south of it, or vice versa, in which event the ring becomes
invisible, because its dark and non-illuminated surface is presented to
us. The ring may also become invisible to us when the Earth passes
through its plane.
Since the distance from Saturn to the Sun is to the distance of the
Earth from this last body as 9.54 is to 1; and since the circumference
of a circle increases in the same proportion as its radius, it follows
that the diameter of the Earth's orbit projected on the orbit of Saturn
would occupy only part of the latter, or about 12° 2', this being 6°
1' on either side of the nodes of the rings. To describe such an arc on
its orbit, it takes Saturn almost 360 days on an average, or almost a
complete year; the Earth describing therefore almost a whole revolution
around the Sun during the time it takes Saturn to advance 12° 2' on its
orbit. Then, when Saturn occupies a position comprised within an arc 6°
1' from either side of the nodes of its ring, the Earth, by its motion,
is liable to encounter the plane of the ring, when therefore it will
only present its thin edge to us, and becomes invisible. At least one
such encounter is unavoidable within the time during which Saturn
occupies either of these positions on its orbit; while three frequently
happen, and two are possible.
The natural impression received by looking at the rings, while seeing
the ponderous globe of Saturn enclosed in its interior, is that this
gigantic, but very delicate structure, in order to avoid destruction,
must be endowed with a swift movement of rotation on an axis
perpendicular to its plane, and that the centrifugal force thence
arising counterbalances the powerful attraction of the planet, and thus
keeps the system in equilibrium.
Theoretically, the rotation of the rings is admitted by every
astronomer, as being an essential condition to the existence of the
system, which otherwise, it is thought, would fall upon the planet.
Although the rotation of the rings seems so probable that it is
theoretically considered as certain, yet its existence has not been
satisfactorily demonstrated by direct observation, which alone can
establish it on a firm basis as a matter of scientific knowledge.
The determination of the period of rotation of the rings, which is
supposed to be 10h. 32m. 15s., rests only on the observations of W.
Herschel, made in 1790, from the apparent displacement of irregularities
on the ring; but his results have been contradicted by other
observations, and even by those of Herschel himself, made in later
years.
Although the system of rings is very nearly concentric with the globe of
Saturn, yet the coincidence is not considered as mathematically exact.
It seems to have been satisfactorily demonstrated by direct observations
that the centre of gravity of the system oscillates around that of the
planet, thus describing a minute orbit. This peculiarity is in
accordance with theory, which has shown it to be essential to the
stability of the system.
Besides its system of rings, which makes Saturn the most remarkable
planet of the solar system, this globe is attended by eight satellites,
moving in orbits whose planes very nearly coincide with the plane of the
rings, except that of the most distant one, which has an inclination of
about 12° 14'. In the order of their distance from the planets, the
satellites of Saturn are as follows: Mimas, Enceladus, Tethys, Dione,
Rhea, Titan, Hyperion and Iapetus. The three first satellites are nearer
to Saturn than the Moon is to the Earth; while Iapetus, the farthest, is
9½ times the distance of our satellite from us. All the satellites,
with the exception of the farthest, move more rapidly around Saturn than
the Moon moves around the Earth; while Iapetus, on the contrary, takes
almost three times as long to make one revolution.
The period of revolution of the four inner satellites is accomplished in
less than three days, that of Mimas being only a little more than 22
hours. From such swiftness of motion, it is easily understood how short
must be the intervals between the different phases of these satellites.
Mimas, for instance, passes from New Moon to First Quarter in less than
6 hours.
The distance of the nearest satellite from the planet's surface is
84,000 miles, and its distance from the outer ring only 36,000 miles. It
is difficult to determine the diameter of objects so faint and distant
as are some of these satellites, but the diameter of Titan, the largest
of all, is pretty well known, and estimated to be ¹⁄₁₆ the
diameter of the planet, or more than half the diameter of our globe.
Iapetus is subject to considerable variations in brilliancy, and as the
maxima and minima always occur when this satellite occupies the same
parts of its orbit, it was conjectured by W. Herschel that, like our
Moon, it turns once upon its axis during each of its revolutions about
the planet. It has been shown by my observations, that Iapetus attains
its maximum brightness a little before it reaches its greatest western
elongation, and its minimum on the opposite side.
As the planes of the orbits of the satellites are inclined to the
planet's orbit, it follows that their transits, occultations and
eclipses, are only possible when Saturn is near its equinoxes. Passages
of the satellites and their shadows across the disk, although rare, have
been observed, and they somewhat resemble the phenomena exhibited by the
satellites of Jupiter in transit. When the Earth is very near the plane
of the rings, the satellites, except the farthest, appear to be in a
straight line nearly coincident with the plane of the rings, and are
seen occasionally moving along the thin edge of the rings, appearing as
luminous beads moving on a thread of light.
Owing to the considerable inclination of the axis of rotation of Saturn
to its orbit, the seasons of this planet must have greater extremes of
temperature than those of the Earth. As the year of Saturn consists of
25,217 Saturnian days, each season, on the average, is composed of 6,304
Saturnian days.
To an observer on Saturn, the immense arches formed by its rings would
appear as objects of great magnificence, spanning the sky like soft
colorless rainbows. Moreover, the eight moons, several of which are
always visible, would be of the highest interest, with their swift
motions and rapid phases. Mimas, traveling in its orbit at the rate of
16 of arc per minute of time, moves over a space equal to the apparent
diameter of our Moon in two minutes, or at the rate of 16° an hour.
Owing to the globular form of Saturn, the rings would be invisible in
latitudes situated above 65° from its equator, and their apparent form
and breadth would naturally vary with the latitude. At 63° only a very
small portion of the outer ring would be visible above the equatorial
horizon, where it would appear as a small segment of a circle. At 62°
the principal division would just graze the horizon. At 46° the outer
portion of the dusky ring would become visible, while at 35° its inner
edge would appear above the horizon. From 65° of latitude down to the
equator, the arches of the rings would be seen more and more elevated
above the equatorial horizon, but at the same time that they are seen
higher up, their apparent breadth gradually diminishes, owing to the
effect of foreshortening, and at the equator itself the system would
only present its thin edge to view.
During the summer seasons of either hemisphere of Saturn, the surface of
the rings turned towards such hemisphere, being fully illuminated by the
Sun, is visible from these regions. In the day time its light must be
feeble and similar to the light reflected by our Moon during sunshine;
but at night the system would display all its beauty, and the different
rings, with their divisions and their various reflective powers, must
present a magnificent sight.
During the nights of the long winter seasons on Saturn, on the contrary,
the surface of the rings turned towards the hemisphere undergoing
winter, receives no light from the Sun, and is invisible, or very nearly
so, except towards morning and evening, when it may be faintly
illuminated by the secondary light which it receives from the
illuminated globe of Saturn. Although dark and invisible, the rings may
make their form apparent at night by the absence of stars from the
region which they occupy in the sky. Again, in other seasons, the days
present very curious phenomena. In consequence of the diurnal rotation
of the planet, the Sun seems to move in circular arcs, which, owing to
the inclination of Saturn's axis, are more or less elevated above its
horizon, according to the position of the planet in its orbit. As such
arcs described by the Sun in the sky of Saturn are liable to encounter
the rings, the Sun in passing behind them becomes eclipsed. It must be a
magnificent spectacle to witness the gradual disappearance of the fiery
globe behind the outer ring, and its early reappearance, but for a
moment only, through the narrow gap of the principal division; to see it
vanish again behind the middle ring, to reappear a little later through
the semi-transparent dusky ring, but very faint and red colored at
first; and then, gradually brighten up, and finally emerge in all its
beauty from the inner edge of the dusky ring.