William F. Denning.

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naked eye. This proved to be Vesta, though the identity of the object
was not known at first, and it formed the subject of two Dun Echt
circulars.

Formerly, hazy indefinite outlines were attributed to some of the
planetoids; but the appearance probably arose from instrumental defects.

The search for these bodies is not a work likely to engage amateurs.
Professional observers are best able to grapple with the difficulties
attending this kind of observation, where large telescopes, means of
exact measurement, and ample data, such as star-charts and ephemerides
of the planetoids previously discovered, are requisite. The ‘Nautical
Almanack’ annually contains ephemerides of Ceres, Pallas, Juno, and
Vesta; and observers wishing to pick up any one of them may readily
ascertain positions by reference to this work.

_Occultation of Vesta._—An occultation of Vesta occurred on Dec. 30,
1871, and it was observed by Mr. C. G. Talmage at Leyton with a 10-inch
refractor, power 80. He says the planet was exceedingly bright right up
to the Moon’s limb.




CHAPTER XI.

_JUPITER._

An interesting Object.—Brightness and Position.—Period &c.—Belts and
Spots on the Planet.—Observations of Hooke, Cassini, and others.—The
“Ellipse” of 1869-70.—The Red Spot, its appearance, dimensions,
and rotation-period.—Bright equatoreal Spots.—Dark Spots in N.
hemisphere.—Rotation-period.—Nature of the Red Spot and of the bright
and dark equatoreal Spots.—New Belts.—Changes on Jupiter.—Further
Observations required.—Occultations of Jupiter.—The four Satellites,
and their phenomena.—Occultation of a star by Jupiter.

“Beyond the sphere of Mars, in distant skies,
Revolves the mighty magnitude of Jove,
With kingly state, the rival of the Sun.”


Of all the planets, Jupiter is the most interesting for study by the
amateur. It is true that Saturn forms an exquisite object, and that
his wonderful ring-system is well calculated to incite admiration as a
feature unique in the solar system. But when the two planets come to
be repeatedly observed, and the charm of first impressions has worn
away, the observer must admit that Jupiter, with his broad disk and
constantly changing markings, affords the materials for prolonged study
and sustained interest. With Saturn the case is different. His features
are apparently quiescent; usually there are no definite spots upon the
belts or rings. There is a _sameness_ in the telescopic views; and this
ultimately leads to a feeling of monotony, which causes the object to
be neglected in favour of another where active changes are in visible
progress.

_Brightness and Position._—Jupiter is a brilliant object in the
heavens, his lustre exceeding that of Mars or Saturn, though not equal
to that of Venus. I have occasionally seen the planet with the naked
eye in the daytime, about half an hour after sunrise; and it has been
frequently observed by Bond, in America, with the Sun at a considerable
altitude. Humboldt and Bonpland, at Cumana, 10° N. lat., saw Jupiter
distinctly with the naked eye, 18 minutes after the Sun had appeared in
the horizon, on Sept. 26, 1830. The planet is favourably visible for a
considerable time every year, and is only beyond reach near the times
of his conjunctions with the Sun, when he usually evades observation
for about three months. As regards his altitude, Jupiter becomes
exceptionally well placed at intervals of 12 years; thus in 1859,
1870-1, and 1882 his declination was 22° or 23° N., and his height
therefore very great when passing the meridian. In 1894 he will occupy
a similarly auspicious region to observers in the N. hemisphere. In
1865, 1877, and 1889 his declination was 23° S., and he was favourably
presented to southern astronomers.

The image of Jupiter as seen in a telescope is involved in a slight
yellow tinge, and with the naked eye the same colour is often apparent.
But when observed through a very pure transparent atmosphere, his light
nearly approaches the silvery lustre of Venus or the Moon. The planet
shines with unusual splendour, considering his great distance from
the Sun, and his atmosphere must be highly reflective and possibly
intensified by inherent light from the planet himself. The central
parts of Jupiter’s disk are usually the brightest, as there is a faint
shading-off and indefiniteness at the limbs. These and other facts
support the view that Jupiter is still incalescent and sufficiently
self-luminous to emit a small amount of light.

_Period &c._—This planet revolves round the Sun in 4332d 14h 2m, which
is equal to more than 11-3/4 years. His orbit is somewhat eccentric, so
that his distance from the Sun varies from 506,500,000 to 460,000,000
miles, and the mean is 483,300,000 miles. His apparent diameter ranges
from a max. of 50″ at a good opposition to 30″·4 in conjunction. The
planet’s diameter measured along the equator is 88,000 miles, and the
polar compression is very marked, amounting to 1/16, or, more exactly,
to 1/15·82, according to Engelmann, from a mean derived from eleven
observers. When Jupiter is in quadrature there is a slight phase
evident in the shading-off of the limb furthest from the Sun.

_Belts and Spots on the Planet._—From the time that the telescope
became available as a means of astronomical research, it may be readily
surmised that an object coming so well within the reach of ordinary
appliances, and one displaying so many prominent and variable features,
should absorb a large share of attention, and that many facts of
interest should have been gleaned as to his physical peculiarities. But
it must be confessed that, though something has been learned as to the
visible behaviour of the markings, there is much that is perplexing
in their curious vagaries. No doubt the vast changes affecting the
Jovian envelope, the diversity of the markings, and their proper
motions result from the operations of a peculiarly variable atmosphere,
affected probably by a heated and active globe beneath it, and by the
very rapid movement of rotation to which it is subject.

The telescope, on being turned towards Jupiter, reveals at once an
array of dark and light stripes or belts stretching across the disk in
a direction parallel to one another and to the equator of the planet.
These belts are supposed to have been first detected by Zucchi in 1630.
Usually there are two broad and prominent dark belts, one on either
side of the equator; while towards the poles other belts appear, some
of them very narrow, partly by the effects of foreshortening. The
equatoreal zone of the planet is of a lighter tint, and variegated
with white and dark spots and streaks, liable to rapid changes, and
indicating that this region is in a highly disturbed condition.

_Observations of Hooke, Cassini, and others._—Hooke and Cassini were
amongst the first to find definite spots on the surface of Jupiter.
From 1664 to 1667 a particularly large and distinct spot was frequently
seen in the planet’s S. hemisphere. This object disappeared in the
latter year, but returned in 1672, and was seen until the close of
1674, when it again temporarily vanished, to reappear at subsequent
epochs. Cassini was enabled to determine the rotation-period from
this spot. He found that the markings in the immediate vicinity
of the equator moved with greater celerity than those in higher
latitudes, the difference in their rotation-periods being nearly 6
minutes. A century later Sir W. Herschel confirmed these results: he
saw a bright spot which completed a rotation in nearly 5 minutes less
time than several dark spots. Schröter also made many observations,
and noted frequent changes in the spots and differences in their
rotation-periods. He watched a bright object near the equator which had
a period more than 5 minutes less than some dark spots. In later years
Mädler and others followed up the investigation of these markings,
and with nearly similar results. The various spots were undoubtedly
affected by proper motions, enabling them to yield discordant
rotation-periods. Bright forms near the equator moved with great
rapidity and effected a rotation in about 9^h 50^m, while dark spots on
either side of it occupied between 9^h 55^m and 9^h 56^m. The markings
were evidently controlled by currents of different velocities in the
planet’s atmosphere.

Dawes, in 1849 and following years, noted luminous spots, like
satellites in transit, on a belt in the planet’s S. hemisphere. In
October 1857 he observed a group of eleven of these objects; and in
1858 Lassell saw many similar appearances in a bright belt near the
equator.

_The Ellipse of 1869-70._—In 1869 and 1870 Gledhill, of Halifax, and
Prof. Mayer, of the Lehigh University, saw a remarkable formation just
south of the great belt lying on the S. side of the equator. It was
in the form of a perfect ellipse, ruddy in colour, and very distinct
in outline. Its major axis was parallel with the belts. It was first
observed on Nov. 14, 1869, and had disappeared in July 1870, though
on Dec. 1, 1871, a similar elliptic ring was seen resting on the S.
equatoreal dark band.

_The Red Spot._—In July 1878 a large spot, of oval form and intense
red colour, appeared in about the same latitude as the ellipse seen by
Gledhill and Mayer in 1869-70. It was first announced by Dennett of
Southampton, though it appears to have been seen a few weeks earlier
by Prof. Pritchett, of Missouri, U.S.A. The object alluded to soon
attracted general notice; and as it continued visible during the
oppositions of 1879, 1880, and 1881 under the same striking aspect, it
created a considerable stir among telescopists, and the “great red spot
on Jupiter” became familiarly known both in appearance and in title.

No planetary marking in modern times has enlisted half the amount of
attention that has been devoted to this object. It has endured amid
all the turmoils of the Jovian atmosphere for twelve years, and has
preserved an integrity of form and size which prove it to have been
singularly capable of withstanding disruption. But its tint has varied
greatly; so that at times the oval outline of the spot could hardly
be discerned amongst the contiguous belts. In the winter of 1881 the
interior of the ellipse began to lose tone, and in 1882 it faded
rapidly, so that the central region of the spot assumed nearly the same
light tint as the outlying bright belts. Apparently the spot had either
been filled up with luminous cloudy material or had been partially
obscured by the interposition of matter situated higher in the Jovian
atmosphere. The elliptical contour of the object was still intact,
however, though it had quite lost its bold and prominent character.
Only the skeleton of its former self remained, and its entire
disappearance seemed imminent. But further decadence was fortunately
averted by influences unknown to us, and the spot has continued visible
to this day, though shorn of the attributes which roused so much
enthusiasm amongst observers more than ten years ago.

From measures at Chicago, in the years from 1879 to 1884, Prof. Hough
found the mean dimensions of the spot to be:—Length 11″·75, breadth
3″·71. These figures represent a real length of 25,900 miles and a
diameter of 8200 miles. The latitude of the spot was 6″·97 S.

This object has served an important end in attracting wide-spread
observation, not only to itself, but to the general phenomena occurring
on the surface of Jupiter. Observers, in studying the red spot, were
also led to study the bright equatoreal spots and other features so
plentifully distributed over the disk. It was most important this
should be done; for since the time of Herschel and Schröter not much
progress had been made in elucidating the proper motions of the spots
and finding an accurate rotation-period for the planet. Dawes, Lassell,
and many others had, it is true, secured some interesting observations
and drawings, but not of the special kind required, and thus no fresh
light had been thrown upon the vagaries in the behaviour of the spots,
as described by the old observers. But a mass of new facts were now
to be realized. Schmidt at Athens, Prof. Hough at Chicago, A. Stanley
Williams at Brighton, and many others, including myself at Bristol,
began systematic observations of Jupiter, with a view to learn
something more of the periods, changes, and general characteristics
of the spots and other features. The results were of an interesting
nature, though too extensive for more than bare mention here. In 1879
the red spot gave a rotation-period of 9^h 55^m 34^s·2, but this
increased to 9^h 55^m 35^s·6 in 1880-1 and to 9^h 55^m 38^s·2 in
1881-2. During the ensuing three years the period was almost stationary
at 9^h 55^m 39^s·1, but in 1885-6 it further augmented to 9^h 55^m
41^s·1, since which year it has ranged between 9^h 55^m 40^s and 41^s.
From ten years’ observations, the mean period of the red spot is as
nearly as possible 9^h 55^m 39^s.

_Bright Equatoreal Spots._—The bright spots near the equator rotated
in 9^h 50^m 6^s in 1880; but in subsequent years the time slightly
increased, for in 1882 I found it 9^h 50^m 8^s·8, and in 1883 9^h 50^m
11^s·4. The bright spots therefore perform a rotation in 5½ minutes
less time than the red spot. The former move so much more swiftly than
the latter that they pass it at the rate of 260 miles per hour, and in
44½ days have effected a complete circuit of Jupiter relatively to it.
Thus a brilliant white spot, if noticed in the same longitude as the
red spot on one night, will, on subsequent nights, be observed to the
W. of it, and, after an interval of about 44½ days, the same objects
will again occupy coincident longitude.

_Dark Spots in N. hemisphere._—In the autumn of 1880 there was a
confluent outbreak of dark spots from a belt in about 25° N. latitude,
and these exhibited a rotation-period of only 9^h 48^m, so that they
travelled more rapidly than the white spots on the equator. Some short
dusky belts were also remarked slightly S. of the latitude of the red
spot, and these indicated a period of 9^h 55^m 18^s. It is clear from
these various results that the motion of the Jovian markings does not
decrease according to their distance from the equator.

_Rotation-Period._—Below are given the times of rotation ascertained by
some previous observers:—

h m s
1665. J. D. Cassini 9 55 58
1672. ” 9 55 50
1692. ” (A)9 50
1708. J. P. Maraldi 9 56 48
1713. ” 9 56
1773. Sylvabelle 9 56
1779. (B)W. Herschel 9 54 53}
to 9 55 40}
1779. ” (A)9 50 48 }
to 9 51 45·6}
1786. (C)J. H. Schröter 9 55 33·6
” ” 9 55 17·6
” ” (A)9 50 27
1835. J. H. Mädler 9 55 26·5
” G. B. Airy 9 55 21·3
1836. J. H. Mädler 9 55 23·5
1862. J. F. J. Schmidt 9 55 25·7
1866. ” 9 55 46·3
1873. O. Lohse 9 55 19·6
1880. J. F. J. Schmidt 9 55 34·4
” ” (A)9 50
1881. W. F. Denning 9 48
” ” 9 55 17·9
1883. ” (A)9 50 8·7
1885. G. W. Hough 9 55 37·4
” ” (A)9 50 9
1886. A. Marth 9 55 40·6
1887. A. S. Williams:—
Spots in 12° N. lat. 9 55 36·5
” 4° N. lat. (A)9 50 40·1
” 8° S. lat. (A)9 50 22·4
” 30° S. lat. 9 55 17·1
1890. (D)W. F. Denning 9 55 39

A: Bright spots near the equator of Jupiter.

B: Herschel’s observations embraced few rotations, and the periods
he derived differed considerably.

C: Schröter also alleges he saw spots return to the same part of
the disk in 7^h 7^m, 7^h 36^m, and 8^h 1^m!

D: From ten years’ observations of the red spot.

The foregoing list is by no means complete, for, owing to the large
number of recent determinations, I have thought it advisable to omit
some of them.

It should be mentioned here that the above times of rotation are
derived from atmospheric features more or less volatile in nature, and
that therefore the actual sphere of Jupiter rotates in a period which
we have not precisely discovered. No doubt the motion of the real
surface is not very different from that of the atmospheric markings
above it. There is reason to think that, whatever the character of the
planet’s crust may be, we have never yet obtained a glimpse of it. A
dense veil of impenetrable vapours appears to surround the globe on all
sides, and this is subject to violent derangement from the evolution
of heated material or gaseous fluids from the surface below. These
disturbances seem to be very durable in some instances as to their
observed effects. The atmosphere would, in fact, appear to possess a
singular capacity for retaining the impressions of its changes. The
permanency of certain spots can hardly be due to continued action from
those parts of the disk immediately underlying them; for their variable
motions soon transport them far from the places at which they were
first seen, and prove their existence to be quite independent of their
longitude.

_Nature of the Red Spot._—There is much in connection with the red
spot that remains in mystery. Its dimensions, form, and motion have
severally been ascertained within small limits of error, and the
alterations in its tint and degree of visibility have been recorded
with every care. But we can only conjecture as to the origin,
character, and end of this remarkable formation. What agency produced
it, and moulded the definite elliptical outline it has always
preserved—what forces control its oscillations of speed, and keep it
suspended so long in the aerial envelope of Jupiter—are matters of
pure theory. When, in July 1878, it first came under notice it was
a well-developed object, and though Russell in 1876, Lord Rosse and
others in 1873, and Gledhill and Mayer in 1869-70 had delineated forms
suspiciously like the red spot and situated in the same latitude,
yet the several features may not have been absolutely identical, for
nothing was seen of the spot in 1877 or in some other years. But there
is a strong probability that the red ellipse of 1869-70 must have been
the red spot in an incipient stage of its formation. The object may
have undergone temporary obscuration, similarly to Cassini’s spot two
centuries ago.

[Illustration: Fig. 33.

I. 1857, Nov. 27. (Dawes.) | II. 1859, Dec. 29. (Huggins.)
III. 1858, Mar. 2. (Huggins.) | IV. 1870, Jan. 23. (Gledhill.)
V. 1872, Feb. 2. (Gledhill.) | VI. 1885, Feb. 25. (Denning.)
]

My own opinion of the spot is that it represents an opening in the
atmosphere of Jupiter, through which, in 1878-82, we saw the dense red
vapours of his lower strata, if not his actual surface itself. Its
lighter tint in recent years is probably due to the filling-in of the
cavity by the encroachment of durable clouds in the vicinity. Parts of
some of the more prominent belts display an intense red hue like that
formerly shown by the red spot, and they may be due to the same causes.
Extensive fissures are probably formed in the atmosphere, and quickly
distended in longitude by the natural effect of the planet’s tremendous
velocity of rotation. It is curious, however, that these rents, after
a certain distention, assume a durable outline until they lose their
colouring and are temporarily if not finally obliterated.

When the red spot was visible under its best conditions I frequently
examined it, hoping to detect some mark well in its interior which
might serve as a clue to the true rotation-period of the sphere of
Jupiter. For if the spot consisted of a clear patch in the planet’s
atmosphere, I thought it possible some real object on the surface might
be discerned through it, in which case the difference in its motion
and that of the red spot would enable the rate of motion of the globe
to be found. If the spot moves more slowly than the planet, then a
surface-marking must appear to pass from the E. to the W. side of the
spot; but no such evidence could be obtained, owing to the absence of
suitable markings. The red tint of the great spot seemed very general
over the entire area of the ellipse until its central regions paled in
1882. There were two dark specks, one at the E. and another at the W.
extremity of the spot; but these were unchangeable as regards position.

[Illustration: Fig. 34.

Jupiter, 1886, April 9^d 10^h 12^m. (10-inch reflector; power 252.)
]

The spot, though placed very near the border of the great S. belt, has
never been connected with it, though in Jan.-Feb. 1884, May 1885, and
March-April 1886 the spot became temporarily attached to a belt on its
S. side. There was some controversy as to this feature, Prof. Hough,
from observations with the 18½-inch refractor at Chicago, alleging that
at no time had the spot coalesced with or been joined to any belt in
its vicinity. But in 1886 many observers succeeded in detecting the
junction of the markings alluded to, and Prof. Young gave a drawing
of the appearance as seen with power 790 on the 23-inch objective at
Princeton (see ‘Sidereal Messenger,’ vol. v. p. 292). The spot and belt
were probably at different heights in the Jovian atmosphere, so that
there was no commingling of material, one object being simply projected
on the other, for the elliptical form of the red spot remained visible
all the time. The latter moves more slowly than the connecting belts,
and, when clear of them, is often seen with a white aureola fringing
its environs.

_Bright Equatoreal Spots._—These are affected by rapid changes of form,
brightness, and motion. Sometimes they are exceedingly bright; at other
periods they are quite invisible. This intermittency is not occasioned
(as I assured myself by many observations) by the total extinction
of spots and appearance of new ones, but is due to the temporary
obscuration of the same objects. The variations are irregular, and
probably depend upon phenomena also irregular. The motion of these
objects often shows great deviations from their average rate; they
are sometimes much in advance of or behind their computed positions.
One fine spot of this class was closely watched in 1880 and following
years. It was usually in the shape of a brilliant oval, well defined,
and occasionally quite as large as the third satellite of Jupiter; but
it was sometimes seen as a diffused white patch, apparently emerging
from the N. edge of the belt. Whenever the spot was very bright, there
was a trail of light or luminous matter running eastwards from it, as
though there were an eruption of shining material from the spot, which
the rapid rotation of the planet from W. to E. caused to drift in an
opposite direction.

_Dark Equatoreal Spots._—Closely contiguous to the white spots there
are almost invariably seen very dark spots, much deeper in tone than
the dark belt upon which they appear to be projected. It has been
suggested that these dark spots are shadows from the white spots,
which may be elevated formations protruding through the envelope of
Jupiter. This idea seems to me untenable; for the dark spots have been
distinguished under a vertical Sun, and sometimes they are found one
on each side of a white spot. Again, an intensely brilliant spot is
occasionally seen without any outlying condensation of dark matter.
But though they are not shadows, the dark equatoreal spots certainly
have an intimate relation with the brighter markings near them and move
with the same velocity.

It is proved from many observations that the longer an object is
observed the slower becomes its rate of rotation. Sir W. Herschel found
the converse. In discussing his results of 1778 and 1779, he said:—“By
a comparison of the different periods it appears that a spot gradually
performs its revolutions in less time than at first” (Phil. Trans.
1781, p. 126). But his periods were each based on less than fifty
rotations, so that no certain conclusions could be derived from them.



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