_SATURN._
Apparent lustre.—Grand spectacle afforded by the Rings.—Period
&c.—“Square-shouldered” aspect.—Early Observations.—Belts and
Spots on the Planet.—Rotation-Period.—The Rings.—Divisions
in the outer Ring.—The transparent or Crape-ring.—Discordant
Observations.—Eccentric position of the Rings.—Aspect.—Further
Observations required.—Occultations of Saturn.—The
Satellites.—Occultations of Stars by Saturn.
“Muse, raise thy voice, mysterious truth to sing,
How o’er the copious orb a lucid ring,
Opaque and broad, is seen its arch to spread
Round the big globe, at stated periods led.”
This planet shines brighter than an ordinary first-magnitude star, and
is a pretty conspicuous object, though less luminous than either Venus,
Jupiter, or Mars. He emits a dull yellowish light, steadier than the
sparkling lustre of Mercury or Venus.
The globe of Saturn is surrounded by a system of highly reflective
rings, giving to the planet a character of form which finds no parallel
among the other orbs of our system. His peculiar construction is well
calculated to be attractive in the highest degree to all those who
take delight in viewing the wonders of the heavens. Saturn is justly
considered one of the most charming pictures which the telescope
unfolds. A person who for the first time beholds the planet, encircled
in his rings and surrounded by his moons, can hardly subdue an
exclamation of surprise and wonder at a spectacle as unique as it is
magnificent. Even old observers, who again and again return to the
contemplation of this remarkable orb, confess they do so unwearyingly,
because they find no parallel elsewhere; the beautifully curving
outline of the symmetrical image always retains its interest, and
refreshes them with thoughts of the Divine Architect who framed it!
The luminous system of rings attending this planet not only gratifies
the eye but gives rise to entertaining speculations as to its origin,
character, and purposes with regard to the globe of Saturn. Why, it has
been asked, was this planet alone endowed with so novel an appendage?
and what particular design does it fulfil in the economy of Saturn? It
cannot be regarded as simply an ornament in the firmament, but must
subserve important ends, though these may not yet have been revealed to
the eye of our understanding.
_Period &c._—Saturn revolves round the Sun in 10,759 days 5 hrs. 16
min., which is equal to nearly 29½ years. His mean distance from the
Sun is 886,000,000 miles, but this interval varies from 841 to 931
millions, owing to the eccentricity of his orbit. When in opposition
his apparent diameter reaches 20″·7, and declines to 15″ at the time
of conjunction. The planet’s actual diameter is 75,000 miles, and his
polar compression very considerable, viz. about 1/10, which exceeds
that of any other planet. His synodic period is equal to 378 days; so
that he comes into opposition with the Sun thirteen days later every
year. The oblate figure of his disk is very noticeable when the rings
are turned edgeways to the Earth and practically invisible; but when
they are inclined the complete contour of the globe is lost, and the
polar flattening becomes scarcely obvious.
_“Square-shouldered” Aspect._—Sir W. Herschel, from observations in
April 1805, said:—“There is a singularity which distinguishes the
figure of Saturn from that of all the other planets.” On April 19 of
the year named he described the planet as “like a parallelogram with
the four corners rounded off deeply, but not so much as to bring it to
a spheroid.” This gave the globe a “square-shouldered” aspect. But this
curious figure appears to have been very rarely observed in subsequent
years; and accurate measures with the micrometer were adduced in
1833-48 in proof that no such anomaly had a real existence. Dr.
Kitchiner, commenting on Herschel’s remarks, said:—“I have occasionally
observed this planet during thirty years, and I do not remember to have
seen the body of it of this singular form except for a few months about
September 1818.” But there is no doubt that occasionally the planet
_does_ assume an _apparent_ form similar to that attributed to it by
Herschel. In the autumn of 1880 I studied the visible appearance of
Saturn by means of a 10-inch reflector, and recorded as follows:—“The
S. pole, over which the dark belts lay, seemed compressed in the most
remarkable manner; but where a bright belt intervened, in about lat.
45°, the contrary effect was produced. Here the limbs were apparently
raised (by irradiation) above the spherical contour; so that the
distorted image gave the planet that distinctly ‘square-shouldered’
aspect sometimes mentioned in text-books.” The explanation appears to
me very simple. The singular figure is due to the contrasting effects
of the belts. While the bright belt in lat. 45° causes a very evident
shouldering-out of the limbs at its extremities, the dark belts nearer
the pole and the equator act with opposite effect, for they apparently
compress the disk where they meet the limbs, and thus the eye discerns
a figure to all appearance distorted into the “square-shouldered” form.
Mr. J. L. McCance confirmed these remarks by independent observations
at the same period with a 10-inch reflector by Calver (‘Monthly
Notices,’ vol. xli. pp. 84, 282).
_Early Observations._—The appearance of Saturn offered a considerable
difficulty to observers soon after the invention of the telescope.
Galilei became greatly perplexed. He saw the planet, not as a circular
globe like Jupiter, but distinctly elongated in shape, and conceived
the appearance to be due to a central globe with smaller spheres
hanging on the sides! He continued his observations, without, however,
arriving at the solution of the mystery, until the malformation began
to disappear; and in 1612 he was astonished to find the disk spherical.
In his surprise, he asked—“Were the appearances indeed illusion and
fraud, with which the glasses have so long deceived me, as well as many
others to whom I have shown them?... The shortness of the time, the
unexpected nature of the event, the weakness of my understanding, and
the fear of being mistaken, have greatly confounded me.” Gassendi, in
1633, also announced that Saturn appeared to him to be closely attended
by two globes of the same colour as the planet. Riccioli alleged
that the planet was surrounded by a thin, plain, elliptic ring,
connected with the sphere by two arms. None of Galilei’s contemporaries
possessed the instrumental means to extricate him from his doubts;
and it remained for Huygens, in 1654 (twelve years after the death
of Galilei), to discover that Saturn “is surrounded by a slender
flat ring, which in no part coheres with the body of the planet, and
is inclined to the ecliptic”[36]. The same observer showed that the
disappearance which had so puzzled Galilei arose from the varying
inclination in the ring: at times it would become invisible, when
presenting its narrow edge to the Earth, and this actually occurred
again in 1671, as Huygens had predicted. In 1676 Cassini detected a
belt upon the planet, and also a dark division in the ring. Dr. Smith’s
‘Optics’ (1738) thus alludes to these discoveries:—
[Illustration: Fig. 38.
Saturn, as observed by Cassini in August 1676.]
“In the year 1676, after Saturn had emerged from the Sun’s rays, Sig.
Cassini saw him in the morning twilight with a darkish belt upon his
globe, parallel to the long axis of his ring as usual. But what was
most remarkable, the broad side of the ring was bisected right round
by a dark elliptical line, dividing it, as it were, into two rings, of
which the inner ring appeared brighter than the outer one, with nearly
the like difference in brightness as between that of silver polished
and unpolished—which, though never observed before, was seen many times
after with tubes of 34 and 20 feet, and more evidently in the twilight
or moonlight than in a darker sky.”
From the time when Galilei’s inadequate glass revealed the “threefold”
aspect of Saturn, and led up to Huygens’s solution of the mystery in
1654, this planet has been successively interrogated with the improved
telescopes which every generation has produced. Cassini, W. Herschel,
Encke, Bond, Lassell, Dawes, and Hall are names familiar to us as
having materially advanced our knowledge of this unique orb, both as to
his surface-configuration and as to his numerous retinue of satellites.
_Belts and Spots on the Planet._—Parallel belts are seen on the surface
of Saturn, but they are much fainter than those on Jupiter, and they
seldom display the spots and other irregularities interspersed with
the belts of the latter planet. Well-bounded spots have rarely been
distinguished on the disk of Saturn; the belts normally appear equal
in tone, without breaks, condensations, abrupt curves, or branches,
so that the rotation-period has only been accurately determined by
Herschel and Hall. And in these cases the markings were certainly
atmospheric, and probably affected by proper motions similar to those
operating on Jupiter.
Cassini and Fatio remarked two bright streaks on the planet as early
as 1683. Sir W. Hershel, in 1790, observed a very dark spot near the
margin of the limb, and a few modern observers have been successful
in distinguishing either bright or dark spots or patches, though no
continuous and useful observations appear to have been secured. In the
winter of 1793 Herschel noticed a very distinct quintuple belt, which
consisted of three dusky and two intervening light zones. The dark
belts presented a dusky yellow hue, while the spaces separating them
were white. He recognized the evidences of rotation in the quintuple
belt; for on the same nights, after a few hours’ interval, it exhibited
considerable variation. Though seen with great precision at first, it
became indistinct at a later hour, and the individual belts were placed
at unequal distances.
_Rotation-Period._—Prof. A. Hall, at Washington, discovered on Dec. 7,
1876, a well-defined white spot, 2″ or 3″ in diameter, and situated
just below the ring of Saturn. He watched this object till Jan. 2
following, when it had become faint and indistinct, and the planet
being low and the weather unfavourable no further observations were
made. The spot had fortunately been seen at four other observatories
in the United States, Prof. Hall having notified its existence to
them; and on discussing the results, a rotation-period was found not
differing largely from Herschel’s value derived from the quintuple belt
in 1793. These are, in fact, the only two determinations on which we
may place confidence. They are as below:—
h m s Probable error.
1793. Sir W. Herschel 10 16 0·4 2 min.
1877. Prof. A. Hall 10 14 23·8 2·3 sec.
Schröter, from different spots, computed periods of 11^h 40^m 30^s,
11^h 51^m, and more than 12^h; but these are probably excessive. The
difference of 1^m 37^s between the values of Herschel and Hall is
relatively a trivial one, as the markings observed were doubtless
atmospheric and subject to irregularities of motion. As to the rotation
of the ring, Herschel, in 1789, detected some bright marks on it, and
deduced the period as 10^h 32^m 15^s·4[37]. Many astronomical works
give the rotation-period of Saturn as 10^h 29^m 16^s·8; and this is
adopted in Chambers’s ‘Descriptive Astronomy,’ 4th edit. vol. i. p.
653. The mistake has its origin in Laplace’s _Système du Monde_, where
it is stated that Saturn rotates in 0·428 of a day, and the ring in
0·437, which, reduced to hours, minutes, and seconds, give 10^h 16^m
17^s·2 and 10^h 29^m 16^s·8.
The equator of Saturn is usually the brightest part of the disk. On its
S. side, in recent years, it has been bounded by a very dark narrow
belt. Further S. the whole disk seems involved in a faint shading, of
a decidedly yellowish hue. Sometimes a considerable number of belts
are visible; but they are evidently liable to changes, so that the same
number and arrangement are not preserved from year to year.
_The Rings._—As to the luminous rings, the extreme diameter of the
outer one is about 40″, or more than 170,000 miles; and the black
division, separating it from the inner one, is 0″·4, or 1700 miles.
The outer ring has a breadth of 2″·4, or 10,000 miles; while the inner
one measures 3″·9, or 17,000 miles. The outer ring is less luminous
than the inner; the latter, round its outer edges, is extremely
brilliant, and has sometimes been described as the brightest part of
the Saturnian system. The inner part of this ring is much shaded-off,
and offers a strong contrast to the silvery whiteness of the other
portion.
[Illustration: Fig. 39.
Saturn, 1885, Dec. 23, 7^h 54^m. (10-inch reflector, power 252.)
]
_Divisions in the Outer Ring._—In the middle of the eighteenth century
Short, the optician, using one of his excellent reflectors, thought he
saw the outer ring divided by several dark lines; but no other observer
confirmed his suspicion. In the third decade of the present century
Quetelet and Capt. Kater appear to have observed Short’s divisions, but
Sir J. Herschel and Struve looked for them in vain. In 1837 Encke fully
satisfied himself, by several observations and measurements, as to the
objective existence of the divided outer ring. The division was not
central, cutting the ring into equal parts, but situated in the inner
part of the ring, so that the wider part was outermost. In subsequent
years this division has been sometimes seen and placed nearest the
outer edge of the ring. Certain observers, provided with ample means,
have seen nothing of it; others regard the division as variable. It is
sometimes described as a narrow black line; while others refer to it
as a faint pencil~like shading, and not a real division at all. One
observer occasionally sees it with considerable distinctness at the
very same time that another observer, with a more powerful telescope,
cannot glimpse it though looking specially for such an appearance! It
is difficult to reconcile such discordant experiences, and unsafe to
accept results of such a contradictory nature.
_The “Crape”-Ring._—A far more certain feature was discovered in the
autumn of 1850[38], and one in reference to which there is unanimity of
testimony. On Nov. 11 G. P. Bond, in America, and Dawes, in England, on
Nov. 25, saw a nebulosity or faint luminous appearance like twilight,
fringing the interior margin of the inner ring. Later observations
showed this to be occasioned by a transparent ring situated immediately
within the inner luminous ring. Dawes considered the new ring to
be divided into two parts; but Lassell, with his large reflector,
subsequently negatived this supposition. Both limbs of Saturn may be
readily perceived through the transparent ring where it crosses the
globe of the planet. Some irregularities have been suspected in it at
different times by various observers. In 1887 dark condensations were
reported to disturb its normal aspect; but these were not seen at many
observatories where such features, if real, could hardly have escaped
detection.
[Illustration: Fig. 40.
Saturn, as observed by F. Terby, February 1887.]
It is strange to reflect that this transparent ring avoided discovery
for so long a period. It forms a feature distinctly to be recognized
in relatively small telescopes—in fact, Grover has seen it, where it
crosses the globe of Saturn, with only 2 inches of aperture. Yet,
though ever on the alert to detect new formations, and exercising
constant vigilance in their pursuit, Sir W. Herschel, Schröter, and
many others allowed this ring to escape them! There is no reason to
suppose that it is variable, and that it was not so plain a century ago
as now. It affords another instance of how easily an unknown object
may elude recognition, though everyone sees it readily enough when
attention is called to it.
In March 1889 a white spot was detected on the rings by Dr. Terby, at
Louvain, and it was seen by other observers with comparatively small
instruments. The spot was stationary, and placed near the apparent
junction of the globe and rings, in the E. ansa. But with large
telescopes nothing of this object could be detected: it was shown to be
an optical effect.
_Discordant Observations._—It is curious that the details of Saturn
have occasioned more dissension amongst observers than those of any
other planet. This may have partly arisen from the great distance of
Saturn, the comparative feebleness of his light, and complexity of his
structure. The planet is usually better defined than either Mars or
Jupiter; but with tolerably high powers on small instruments the image
is faint, and the features so diluted that the impressions received
cannot always be depended upon, especially when the air is unsteady.
A fluttering condition of the object is sufficient in itself to cause
deception. Prof. Hall, in speaking of the work done by the 25·8-inch
refractor at Washington in 1883, says:—“Saturn’s ring has been
observed, but many of the strange phenomena noted by other observers
have not been seen even on the best nights.” The evidence afforded by
this large instrument may not always be conclusive, but in this case
there can be no doubt it properly failed to show “phenomena” which had
no existence.
_Eccentric Position of the Rings._—The rings are slightly eccentric
with regard to the ball; in other words, the ball is not situated in
the centre of the rings. Differences have been observed denoting this,
though the observations are not altogether satisfactory. It has been
shown theoretically that the eccentricity referred to is necessary
to maintain the stable equilibrium of the system; for were the rings
perfectly concentric with the planet, they must coalesce with the ball.
The preservation of so complicated a structure must evidently require
judicious and nicely balanced conditions.
With the great 23-inch refractor at Princeton, U.S.A., the ball of
Saturn was seen through the division in the ring in November 1883—an
observation which had previously been made by Lassell in 1852.
_Aspect of the Rings._—In different years the rings present a varying
outline, owing to the fact of their inclination (28° 10′) and to
changes in the relative positions of the Earth and Saturn. At intervals
of about fifteen years the rings are widely open, as they were in 1855,
1869, and 1885, and will be in 1899. At similar intervals they are
rendered invisible, being turned edgeways to the Earth, as in 1848,
1862, 1877, and 1891. Since 1877 the S. side of the rings has been
presented to terrestrial observers; but in 1893 the N. side will come
under inspection, and remain in view until 1907. The S. side of the
rings is obviously more favourably visible to observers in England and
other N. latitudes, because the planet is always above the equator
and attains a fair altitude when it is presented. The N. side of the
rings is exposed when Saturn is in S. declination, and therefore more
liable to our atmospheric disturbances owing to his comparatively
low altitude. The extreme narrowness of the rings is apparent at the
periods when the planet crosses the node and they are situated in the
plane of the line of sight. In small telescopes they become invisible,
and the finest instruments only exhibit them as thread-like extensions
from the equator of the planet. Sir J. Herschel says that on April 29,
1833, the disappearance of the ring was _complete_ when observed with a
reflector of 18 inches aperture and 20 feet focal length. It remained
visible in 1862 as a broken line of light. At such times the satellites
are seen as bright beads, threading their way along the narrow wavering
line of the belts. Inequalities have been observed at such times; for
the line of light into which the rings are then resolved is not uniform
in breadth, but appears broken and undulatory, as though indicating a
very rugged character of surface.
Sir J. Herschel estimated the thickness of the rings as 250 miles, but
Bond thought it far less—about 40 miles. There are great obstacles in
the way of ascertaining the exact proportions of a structure so distant
and offering such an extremely slender form to our view.
_Further Observations required._—The globe and rings of Saturn
offer an encouraging prospect for additional discoveries. Though
the more prominent details have already been descried, there remain
other features, probably of more delicate outline and intermittent
visibility, which will be glimpsed in future years. Small instruments
will scarcely be competent to deal efficiently with this object:
observers who can command at least a moderate grasp of light may,
however, enter upon the work with every assurance of interesting
results. In this, as in other sections of observational astronomy, the
student will realize that in oft-repeated observation and comparison
of records and drawings he acquires a familiarity with the appearance
of the object which will enable him to discern more and more of its
configuration, until ultimately he feels confident he has progressed as
far as the utmost capacity of his instrument will permit. It is in the
sedulous application of his powers that the observer will find the key
to success. Partial devotion to a subject offers a prospect far less
encouraging; for observations of a disconnected character are seldom
valuable.
Changes are unquestionably occurring both in connection with the
ball and rings of Saturn[39]. Some of the discrepancies between the
observations published from time to time are only to be explained on
this assumption. It should therefore be the aim of observers to obtain
further evidence of such variations, and this may be best accomplished
by assiduously watching the lineaments of the planet during the most
favourable periods of each opposition. The collection of a number of
reliable materials through a series of years would undoubtedly possess
weight in removing some of the anomalies of past observation, and
afford us a more thorough knowledge of the delicate markings.
The rotation-period of Saturn is probably not much different from
that given by the atmospheric markings seen by Herschel and Hall.
But additional determinations are very desirable for many reasons.
The spots which are so plentiful on Mars and Jupiter have furnished
observers with a valid and concise means of ascertaining the rate
of axial motion of those planets. Saturn, however, has far more
sparingly provided the data for such an investigation; for if we
disregard Schröter’s uncertain figures, we have but two values for
the rotation-period. These were fortunately effected by observers
of exceptional ability, and the periods may be accepted without
reservation; but other independent determinations are much required.
By multiplying results of this nature, we have a prolific source of
comparison; and comparisons, apart from being interesting, are of
importance in denoting erratic results and indicating those entitled
to credence. Moreover, a reliable mean value may be sometimes deduced
from multiple records; hence it becomes advisable to secure as many as
possible.
The planet should be frequently examined during every opposition with
the highest powers that are consistent with a perfectly distinct image;
and the observer should closely scan the various parts of the disk,
with an endeavour to trace spots, breaks, or other irregularities
in the belts. Certain inequalities of tone have been occasionally
apparent in past years, and they will doubtless reappear. The recovery
of these features will form a welcome addition to our knowledge, and,
if adequately observed, will enable the rotation-period of the planet
to be rediscussed. In an enquiry of this kind many observations are
needful, and the longer the interval over which they extend the more
accurate the results derived from them are likely to be. If a broken
belt should appear on Saturn, the time of its passing the planet’s
central meridian should be recorded, either by measurement or careful
estimation, and an ephemeris computed based on a rotation-period of
10-1/4^h, which is equal to a daily rate of nearly 843°. Then it should
be carefully looked for on subsequent evenings at the times given in
the ephemeris, and on every occasion when re-observed its time of
transit should be noted as at first. As long as the break continues
visible, so long ought it to be kept in view and the times of its
central passages tabulated. It would be advisable in such a case to
secure cooperation from other observers, as more numerous observations
would be sure to accrue, so that, on the appearance of a marking
such as that alluded to, the discoverer will do well to announce it
immediately to other amateurs who are engaged upon planetary work and
most likely to assist him. A white or dark spot, or any condensation
on the belts, would of course serve the same purpose as a broken belt.
The nature of the object is not necessarily to be considered, the main
requirement being that it is one of which the longitude admits of
determination. Markings on the belts, if they are ever discernible,
must be watched with corresponding assiduity for traces of motion; and
if such motion should betray itself, the object of the observer will be
to ascertain its rate.
With reference to the narrow division in the outer ring, usually
termed “Encke’s division,” astronomers would regard it as a gratifying
advance could the doubts overhanging this feature be removed. Is it
a real division in the ring, or simply a pencil-line of shading on
the flat surface? Is it constant in place and appearance, or does it
frequently exhibit changes both as to intensity and position? Judging
from prior experiences, this particular object would appear to be
extremely fugitive, and incapable of being assigned either a definite
place or aspect. Yet the more pronounced and well-attested details of
Saturn show no such vagaries: Cassini’s division seems invariable. Are
we therefore to surmise that the curious behaviour of Encke’s division
is to be referred to errors of observation arising from the effects of
unsteady air upon a very delicate object? It is for future observers
to answer these questions, and this will entail no ordinary effort,
for the same impediments will be encountered in the future as in the
past. But fortunately our science is rapidly progressive, and there is
no doubt the mystery of Encke’s division will find-its solution before
long. A powerful telescope, and a keen and continuous study of the
outer ring, will enable some discriminating observer to tell us the
true story of its phenomena.
Many other points in the Saturnian system require renewed attention,
but some of them appear to be so doubtful as to scarcely deserve
mention. Possibly the student had better commence his review of the
planet without any of the bias or prejudice which former observations
might occasion. But it is as well to know the true state of the case;
for the judgment of a careful observer is not likely to be warped by
preconception, and of course some of the doubtful observations may be
amply verified at a future time. Several of these have already been
briefly referred to, and a few others may here be noted. The form
of the shadow thrown on the rings from the ball has been observed
of a curious shape, and M. Trouvelot supposes it to be variable and
occasioned by changes on the level surface of the rings. The same
observer has noticed transverse notches in the edges of the inner
bright ring. Evidence of variation is not entirely wanting in regard
to the chief division, and observers should notice whether it appears
uniformly black, as it has been suggested that a gauze ring fills
the interval. Exterior to the outer ring a faint luminosity has
also been suspected, as though the phenomenon of the inner ring had
its counterpart here. The colour of the belts on the ball should be
ascribed by careful estimates, as many such observations may give an
insight into the variations occurring. Some observers have alleged that
the transparent ring of Bond and Dawes is subject to very perceptible
alterations. It must be remembered, however, that the visible aspect of
this exceedingly delicate structure is much affected by the condition
of the atmosphere, and that the inclination of the Saturnian system
must obviously introduce changes. When the inclination is considerable,
the globe of the planet may be discerned through this ring with greater
effect than at other times, because we have to look through a thinner
stratum of its material.
The observer, in seeking to elucidate some of the anomalies of former
researches, will possibly himself gain a knowledge of features not
hitherto recognized. Of the real existence of these he should assure
himself by many critical observations before venturing to announce them.
We have hinted that further discoveries upon Saturn may be considered
as practically beyond the reach of small telescopes; but the gratifying
fact remains that some of the more noteworthy of the known features are
visible in glasses of little pretention as regards size. With a 2-inch
refractor, power about 90, not only are the rings splendidly visible,
but Cassini’s division is readily glimpsed, as well as the narrow dark
belt on the body of the planet. This sufficiently proves that a very
small and portable instrument is capable of affording some excellent
views of one of the most wonderful objects in the heavens. Grover has
seen, with an aperture similar to that named, not only the belts and
the shadow of the ball on the rings, but two of the satellites as well;
and others may be equally successful.
_Occultations of Saturn by the Moon._—Phenomena of this kind were well
observed in England on May 8, 1859, April 20 and Sept. 30, 1870. Those
of 1859 and Sept. 30, 1870, were observed by the Rev. S. J. Johnson,
who noted that “the dull hue of the planet contrasted strikingly with
the brilliant yellow of the Moon.” Dawes witnessed the occultation
in 1859, and saw the opaque edge of our satellite sharply defined on
the ball and rings of Saturn, without the slightest distortion of
form. No dark shading was remarked by him contiguous to the Moon’s
bright edge at the reappearance, such as he and others had observed on
Jupiter on the occasion of his occultation, Jan. 2, 1857. Saturn was
described as of a pale greenish hue, and offered a strong contrast to
the brilliant yellow lustre of the Moon. On the early morning of April
20, 1870, several observers were on the _qui vive_ for this interesting
occurrence; and their experiences are reported in the ‘Monthly Notices
R. A. S.’ vol. xxx. p. 175 _et seq._, from which the following are
brief extracts:—
Mr. Ellis:—“The light of the planet, by contrast with the Moon, was
very faint.” Mr. Carpenter:—“There was not the least alteration in the
planet’s form.” Capt. Noble:—“Saturn appeared of a richly-greenish
yellow when compared with the brilliant white light of the Moon.”
Mr. G. C. Talmage:—“The difference in colour between Saturn and the
Moon was most marked, the planet appearing of a yellow tint.” Mr. J.
Carpenter:—“At disappearance the planet was a very dull object when
in contact with the Moon; its light probably a twentieth as bright.
At reappearance the planet was rather tremulous; no distortion was
noticed.” On June 13, 1870, the Rev. J. Spear, of Bengal, watched the
Moon pass “steadily over the planet without causing any change of form
or giving any indication of the planet’s light passing through an
atmospheric medium. When near the Moon’s limb Saturn assumed a sickly
green hue.”
I observed the occultation of Sept. 30, 1870, at Bristol, with a
4-1/4-inch refractor; but the event offered no novel traits, the most
prominent feature being the difference of brightness in the Moon and
Saturn. Mr. C. L. Prince observed this event with a Tulley refractor of
6·8 inches aperture, power 250. He says there was not the slightest
distortion of either body, but he noticed that “the edge of the ring
lingered somewhat upon the Moon’s limb about the time of disappearance.”
Another occultation occurred soon after new Moon on April 9, 1883, and
one of the observers, Mr. Loomis, described the disappearance of the
rings as a spectacle of great interest, and said the impression was
forcibly conveyed to his mind that the Moon was very much nearer to the
eye than Saturn.
_The Satellites._—The discovery of the eight moons of this planet
ranged over the long period of 193 years. Five different observers
share the honours between them. Our knowledge of the Saturnian
satellites may almost be said to furnish us with a history of
improvements in the telescope; for they were severally detected at
epochs corresponding to instrumental advances. The following are the
periods, distances, &c. of the satellites:—
+——————————————+————————————————————+————————+——————+
| | Mean Distance. | | |
| +————————————————————+Sidereal| Real |
|No. and Name. |Diameters | |Period. | Diam.|
| |of Saturn.| Miles. | | |
+——————————————+————————————————————+————————+——————+
| | | | d h m|miles.|
|7th. Mimas | 1·53 | 115,000| 0 22 37| 1000 |
|6th. Enceladus| 1·97 | 148,000| 1 8 53| ... |
|5th. Tethys | 2·44 | 183,000| 1 21 18| 500 |
|4th. Dione | 3·12 | 234,000| 2 17 41| 500 |
|3rd. Rhea | 4·36 | 327,000| 4 12 25| 1200 |
|1st. Titan | 10·12 | 759,000|15 22 41| 3300 |
|8th. Hyperion | 12·23 | 917,000|21 7 7| ... |
|2nd. Iapetus | 29·61 |2,221,000|79 7 53| 1800 |
+——————————————+————————————————————+————————+——————+
+——————————————+——————————————-+———————————————-+
| | | |
| | Date of | |
|No. and Name. | Discovery. | Discoverer. |
| | | |
+——————————————+——————————————-+———————————————-+
| | | |
|7th. Mimas |1789, Sept. 17.|W. Herschel. |
|6th. Enceladus|1789, Aug. 28.|W. Herschel[40].|
|5th. Tethys |1684, Mar. 21.|J. D. Cassini. |
|4th. Dione |1684, Mar. 21.|J. D. Cassini. |
|3rd. Rhea |1672, Dec. 23.|J. D. Cassini. |
|1st. Titan |1655, Mar. 25.|C. Huygens. |
|8th. Hyperion |1848, Sept. 19.|Bond & Lassell. |
|2nd. Iapetus |1671, Oct. 25.|J. D. Cassini. |
+——————————————+——————————————-+———————————————-+
The numbers in the first column refer to the order of discovery.
[Illustration: Fig. 41.
Apparent Orbits of the Five Inner Satellites of Saturn, as seen in an
Inverting Telescope.
(The arrows in the diagram show the direction of the motion of the
satellites. The figures indicate the interval, in hours, from the time
of last East elongation.)]
Titan is by far the largest satellite, being equal to a star of the
8th mag. and visible in any small telescope. Iapetus ranks next,
ordinarily about 9th mag., but there are variations at different parts
of the orbit similar to the variations which affect the satellites of
Jupiter; a variegated surface, and the effects of rotation, originate
the changes observed and give strong support to the inference that
this satellite rotates in the same period that it revolves round its
primary. Tethys, Dione, and Rhea are fainter, and the difficulty of
seeing them is intensified by their proximity to the planet; but a good
4-inch refractor will reveal them on a clear dark night. The others are
objects for powerful instruments and pellucid skies; but Enceladus is
sometimes seen with moderate aperture. The planet being usually much
inclined, his satellites are dispersed round about the rings, and are
not easy of identification. Minute stars lying near the path of Saturn
are very liable to be mistaken for them. But the ephemerides drawn up
by Mr. Marth, and published annually by the Royal Astronomical Society,
are of the utmost service to amateurs engaged in these observations. By
simple reference they may readily identify the individual satellites
on any night; and these ephemerides are additionally useful as giving
the times of conjunctions of some of the satellites with the ends of
the ring and N. and S. points of the ball.
When the thin side of the rings is presented to the Earth, transits and
other phenomena may be observed in connection with the Saturnian moons;
but they appear to have been rarely recorded. Sir W. Herschel describes
a “beautiful observation of the transit of the shadow of Titan over
the disk in 1789, November 2.” It was also seen in 1833 and 1862. The
late Mr. Capron re-observed it on Dec. 10, 1877, with a 8-1/4-inch
reflector, power 144, and made the following sketch:—
[Illustration: Fig. 42.]
These shadow-transits admit of easy observation with appliances of very
moderate capacity. Mr. Banks witnessed a phenomenon of the kind with a
refractor of only 2-7/8 inches, and says it was watched with the same
facility and ease as the shadow of Sat. I. on Jupiter.
In looking for Iapetus it must be remembered that it is commonly
situated at a great distance from the planet. Titan is relatively much
nearer, and will always be recognized without trouble. Enceladus,
Tethys, Dione, and Rhea hover near the outskirts of the ring; while
Mimas is extremely close to it.
Prof. Hall, with the great Washington refractor, has effected many
valuable measures of this system in recent years. He finds the orbits
of the five inner satellites are sensibly circular, and that they
are situated in the plane of the rings. Hyperion revolves in a very
eccentric orbit, and this satellite may approach very near to Titan. He
obtained an observation on March 25, 1885, which seems pertinent to the
question of variation in the light of the satellites. He says:—“Mimas
was remarkably bright, and could not be missed even when the full light
of the planet was admitted to the eye. Generally this satellite is a
difficult object, and from the ease with which it is occasionally seen
one might think it variable; but I think the difference is due to the
quality of the image.” There is no doubt that this is the main cause of
many assumed changes in celestial objects, and especially in regard to
those of a minute and delicate character.
_Occultations of Stars._—Stars are rarely observed to be occulted by
Saturn. Webb mentions that, in 1707 or 1708, Dr. Clark noticed a star
in the interval between the ball and rings; and Dawes once remarked a
star of 8·5 mag. disappear behind the outer edge of the exterior ring.
It would be extremely interesting to watch a tolerably conspicuous star
pass centrally behind the Saturnian system, and to trace it through
Cassini’s division and the transparent ring, noting any changes in
magnitude or appearance as they occurred.
FOOTNOTES:
[36] Huygens appears to have used a refractor of 2-1/3-inch aperture
and 23-feet focal length, with a power of 100, in effecting this
discovery.
[37] Schröter, Harding, Schwabe, and others have observed luminous
points on the rings, but they have remained stationary, so that the
period of rotation announced by Herschel has never been confirmed,
but rather disproved by counter-evidence. Herschel wrote, in November
1789:—“I formerly supposed the surface of the ring to be rough, owing
to luminous points like mountains seen on the ring, till one of these
supposed luminous points was kind enough to venture off the edge of the
ring and appear as a satellite. I have always found these appearances
to be due to satellites.”
[38] Galle, at Berlin, had, twelve years previously, made an
observation which, if it had been interpreted correctly, would have
given him priority. In June 1838 he remarked, on several nights, that
the inner boundary of the inner ring was very indistinct and “gradually
lost itself towards the body of the planet.” The space between the ring
and Saturn was half filled with a dim veil, extending inwards from the
ring. These observations failed to attract the notice their importance
deserved, and Galle himself did not appreciate their full significance
until the announcements of Bond and Dawes in 1850.
[39] Struve wrote, in 1883:—“That changes do take place in the
ring-system is sufficiently proved.” Trouvelot, Schiaparelli, and
others have also remarked variations of a sufficiently decided
character to be placed on record.
[40] Herschel remarks that he saw this satellite in his 20-foot
speculum two years before, viz. on Aug. 19, 1787, but he was then much
engaged in observations of the satellites of Uranus.