William F. Denning.

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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.




CHAPTER XIII.

_URANUS AND NEPTUNE._

Discovery of Uranus.—Mistaken for a Comet.—True character
revealed.—Period &c.—Observations.—Belts on Uranus.—Further
Observations required.—The Satellites.—Discovery of Neptune.—The
planet observed in 1795.—Period &c.—Observations.—Supposed
Ring.—Satellite.—A trans-Neptunian Planet.—Planetary Conjunctions.


_Discovery._—While Sir W. Herschel was a musician at Bath he formed
the design of making a telescopic survey of the heavens. When engaged
in this he accidentally effected a discovery of great importance, for
on the night of March 13, 1781, an object entered the field of his
6·3-inch reflector which ultimately proved to be a new major planet
of our system. The acute eye of Herschel, directly it alighted upon
the strange body, recognized it as one of unusual character, for it
had a perceptible disk, and could be neither fixed star nor nebula. He
afterwards found the object to be in motion, and its appearance being
“hazy and ill-defined” with very high powers he was led to regard it
as a comet, and communicated his discovery to the Royal Society at its
meeting on April 26, 1781. His paper begins as follows:—

“On Tuesday, March 13, 1781, between 10 and 11 in the evening, while I
was examining the small stars in the neighbourhood of H Geminorum, I
perceived one that appeared visibly larger than the rest. Being struck
with its uncommon magnitude, I compared it to H Geminorum and the small
star in the quartile between Auriga and Gemini, and finding it so much
larger than either of them suspected it to be a comet.... The power I
had on when I first saw the comet was 227.”

The supposed “comet” soon came under the observation of others,
including Maskelyne the Astronomer Royal, and Messier, the “Comet
Ferret” of Paris. The latter, in a letter to Herschel, said:—“Nothing
was more difficult than to catch it, and I cannot conceive how you
could have hit this star or comet several times, for it was absolutely
necessary for me to observe it for several days in succession before I
could perceive that it was in motion.”

_True character revealed._—As observations began to accumulate it was
seen that a parabolic orbit failed to accommodate them. Ultimately the
secret was revealed. The only orbit to represent the motion of the
new body was found to be an approximately circular one situated far
outside the path of Saturn, and the inference became irresistible that
the supposed “comet” must in reality be a new primary planet revolving
on the outskirts of the solar system. This conclusion was justified by
facts of a convincing nature, and its announcement created no small
excitement in the scientific world. Every telescope was directed to
that part of the firmament which contained the new orb, and its pale
blue disk, wrapped in tiny proportions, was viewed again and again
with all the delight that so great a novelty could inspire. From the
earliest period of ancient history, no discovery of the same kind had
been effected. The Chaldæans were acquainted with five major planets,
in addition to the Earth, and the number had remained constant until
the vigilant eye of Herschel enlarged our knowledge, and Saturn was
relieved as the sentinel planet going his rounds on the distant
frontiers of our system.

When the elements of the new body had been computed a search was
instituted amongst the records of previous observers, and it was
found that Herschel’s planet had been seen on many occasions, but it
had invariably been mistaken for a fixed star. Flamsteed observed it
on six occasions between 1690 and 1715, while Le Monnier saw it on
12 nights in the years from 1750 to 1771, and it seems to have been
pure carelessness on the part of the latter which prevented him from
anticipating Herschel in one of the greatest discoveries of modern
times.

The name Uranus was applied to the new planet, though the discoverer
himself called it the _Georgium Sidus_, and there were others who
termed it “Herschel,” in honour of the man through whose sagacity it
had been revealed.

_Period &c._—Uranus revolves round the Sun in 30,687 days, which very
slightly exceeds 84 terrestrial years. His mean distance from the Sun
is 1,782,000,000 miles, but the interval varies between 1,699 and 1,865
millions of miles. The apparent diameter of the planet undergoes little
variation; the mean is 3″·6, but observers differ. His real diameter
is approximately 31,000 miles, and the polar compression about 1/13,
though this value is not that found by all authorities.

_Observations._—The planet near opposition shines like a star of
the 6th magnitude, and is observable with the naked eye. He emits a
bluish light. While engaged in meteoric observations, I have sometimes
followed the planet with the naked eye during several months, and noted
the changes in his position relatively to the stars near. It is clear
from this that Uranus admitted of detection before the invention of the
telescope.

A luminous ring, similar to that of Saturn, was at first supposed to
surround Uranus, and Herschel suspected the existence of such a feature
on several occasions; but it scarcely survived his later researches,
and modern observations have finally disposed of it.

Lassell, when working with his 2-foot speculum at Malta, thought he
saw a spot near the centre of the planet’s disk, but he considered
this might possibly be due to an optical illusion. In 1862, Jan. 29,
he said:—“I received an impression which I am unable to render certain
of an equatoreal dark belt.” In the early months of 1870, Mr. Buffham,
using a 9-inch “With” mirror, powers 212 and 320, saw bright spots and
zones on the planet, and inferred a rotation-period of about 12 hours.
On Jan. 16, 1873, when definition was very good, no traces of any
markings were visible in Lord Rosse’s 6-foot reflector. In May and June
1883 Prof. Young, having the advantage of the fine 23-inch refractor
at the Princeton Observatory, observed two faint belts, one on each
side of the equator, and much like the belts of Saturn. On March 18,
1884, Messrs. Thollon and Perrotin, with the 14-inch equatoreal at
Nice, remarked dark spots similar to those on Mars, towards the centre
of the disk, and a white spot was seen on the limb. Two different
tints were perceived, the colour of the N.W. hemisphere being dark,
and that of the S.E. a bluish-white colour. In April observations were
continued, and the white spot was seen “rather as a luminous band
than a simple spot,” but it was most conspicuous near the limb. The
observers thought the appearances indicated a rotation-period of about
10 hours. The brothers Henry at Paris, in 1884, invariably noticed two
belts lying parallel to each other, and including between them the
brighter equatoreal zone of the planet. Their results apparently show
that the angle between the plane of the Uranian equator and that of the



Online LibraryWilliam F. DenningTelescopic Work for Starlight Evenings → online text (page 18 of 32)