_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.
In recent years the rapidly moving bright spots have usually appeared
in the equatoreal side of the great S. dark belt. The polar side of the
great N. belt also exhibits bright spots, but these rotate in a period
only a few seconds less than that of the red spot. Bright spots are
also observed to the S. of the latter object and on other portions of
the disk.
As to the belts, they are usually straight; but cases are recorded
of slant-belts, in which the direction has been very oblique. One of
these was noticed in the planet’s N. hemisphere in Mar.-April 1860, and
another was seen in the S. hemisphere in Jan. 1872. I observed one near
the N. polar shading in Dec. 1881.
_New Belts._—The formation of the dark belts seems to be brought
about gradually, and they appear to be sustained in certain cases by
eruptions of dark matter, which gradually spread out into streams. On
Oct. 17, 1880, two dark spots, separated by 20° of longitude, broke
out on a belt some 25° N. of the equator. Other spots quickly formed
on each side of the pair alluded to, and distended themselves along
the belt so that by Dec. 30 they covered three fourths of its entire
circumference. At the middle of January the spots formed a complete
girdle round the planet; but they became much fainter, and were soon
eradicated by combination with the belt on which they had appeared.
_Changes on Jupiter._—Prof. Hough, of Chicago, is adverse to the
opinion that rapid changes occur on Jupiter, and mentions the
stability of the red spot and other markings in support of his views.
He believes that the erroneous statements about sudden changes made by
both ancient and modern astronomers are largely due to differences in
the telescopic images due to atmospheric variations. No doubt such an
explanation will suffice to meet some instances, and the swift rotation
of the planet may also have been the unsuspected cause of some of the
extraordinary changes described; but there are real variations as well.
These are very frequent in the planet’s equatoreal zone.
_Further Observations required._—Drawings of Jupiter obtained under
the highest powers that may be employed with advantage, and with a
cautious regard to faithful delineation, will probably throw much
light on the phenomena occurring in this planet’s atmosphere. And it
is most desirable to pursue the various markings year after year with
unflagging perseverance; for it is only by such means that we can hope
to unravel the extraordinary problem which their visible behaviour
offers for solution. Too much stress cannot possibly be laid on the
necessity of observers being as precise as possible in their records.
The times when an object comes to the central meridian should be
invariably noted; for this affords a clue to its longitude, and a means
of determining its velocity. Its position, N. or S. of the equator,
should be either measured or estimated; and alterations in tone,
figure, or tint described, with a view to ascertain its real character.
The climate of England is very ill-adapted to an investigation of
this sort, where the most needful point consists in frequency of
observation. If the markings on Jupiter could be re-examined every
night, and traced through their changes, an explanation of certain
phenomena exhibited by them would soon be forthcoming. The interrupted
character of previous observations destroys much of their value.
Closely consecutive results are necessary to remove doubts as to the
identity of the objects observed; so that, in such a research, natural
advantages of position are more desirable than instrumental advantages,
for the latter are impotent in a cloudy atmosphere.
The red spot must be watched as long as any vestiges of it remain.
Its variations of speed may ultimately yield indications of
periodicity[33]; so may its alterations of tint. The belts in the
vicinity of the spot demand an equal share of attention; for it may be
possible to divine from their changes whether there are any links of
association between them and the red spot. In recent years the latter
has apparently repulsed the belts on its N. side, though suffering
encroachments from those on its S. side.
The equatoreal spots also deserve continued vigilance on the part
of observers. It has already been stated that the bright spots vary
rapidly; their motions are not uniform in rate, and what is now wanted
is a large number of new observations. Does accelerated velocity occur
with increased brilliancy of these objects? Are their alternating
disappearances and revivals uniform in period? and are they really due
to transitory obscurations of the same durable forms? Are the dark
spots which frequently border the white spots implicated in effacing
the latter? Many other questions like these are suggested by the
curious behaviour of the markings, and the discriminating observer will
know how to gather the materials likely to aid in answering them. The
rotation-period has been already found in regard to many features; but
this element may be re-investigated with profit, for the velocity of
the spots offers a very complex problem for solution. Do the markings
generally exhibit a retardation of speed as long as they subsist?
Abnormal spots, such as those which made their apparition in the autumn
of 1880, should be traced through any vagaries they may present; and
peculiar shape or direction in the belts will also merit study, as
possibly supplying facts of consequence. It will be important to learn
whether objects in a certain latitude have a common rotation-period, or
whether different forms give different times. The rate of motion shown
by certain features may depend upon their character, and not so much
upon their position in latitude.
The altitudes of the various markings affords another promising line of
research. The appearances and changes of closely contiguous features
may be expected to furnish useful data in this connection. Owing to
their proper motions they apparently overlap each other at times, and
in their alterations of aspect the observer may discover the clue to
their relative heights. The subject is discussed in a practical and
interesting way by Mr. Green (Memoirs R. A. S. vol. xlix. p. 264) and
by Mr. Stanley Williams (‘Zenographic Fragments,’ i. p. 112), and these
works should be consulted by everyone engaged in the study of Jovian
phenomena.
It is unfortunate that the observer, in delineating this object, must
perforce adopt an extremely hurried method of representing what he sees
at the telescope. The planet turns so quickly upon his axis that forms
near the central meridian become sensibly displaced in a few minutes;
indeed, it has been stated that an interval of two minutes only is
sufficient to introduce a change obvious to simple eye-estimation.
In order, therefore, to complete a sketch, the utmost dispatch is
requisite; for this object cannot be depicted from the combined outcome
of several evenings of observation. The proper motions of the different
features prevent this. With Mars, or any orb exhibiting markings
relatively constant, collective results are extremely valuable, and
more trustworthy than pictures depending upon an isolated observation.
Amateurs, in entering upon these observations, should be prepared for
rapid changes in the apparent aspect of Jupiter caused by his rotation,
and not hastily infer them to be real. They should also hesitate before
placing confidence in any anomalous results obtained under indifferent
seeing; for bad images have been directly responsible for many
misleading announcements.
[Illustration: Fig. 35.
Occultation of Jupiter, Aug. 7, 1889.]
_Occultations of Jupiter by the Moon._—Phenomena of this kind are
always awaited with keen interest by the possessors of telescopes;
but it is rarely that all the circumstances are favourable. The first
recorded instance appears to have been in A.D. 847. In 1792, on April
7, Schröter observed an occultation of this planet, with a desire to
verify his suspicion of a lunar atmosphere. He saw that “some of the
satellites became indistinct at the limb of the Moon, while others
did not suffer any change of colour. The belts and spots of Jupiter
appeared perfectly distinct when close to the limb of the Moon.” On
Jan. 2, 1857, an occultation took place under conditions which rendered
it visible to many observers in this country, and the most interesting
fact elicited was that at emersion a dark border was seen attached to
the arc of the Moon projected on the planet. Mr. Lassell described
this dark border as “a shadowy line, in character, magnitude, and
intensity extremely like Saturn’s obscure ring projected on the ball.”
During the thirty years following 1859 only two occultations visible
in England occurred, and the last of these, on August 7, 1889, was
widely observed. On this occasion Capt. Noble and others redetected the
shadowed edge of the Moon seen by Lassell in 1857. “It was a strongly
marked shading, following the outline of the Moon’s limb.” At Bristol I
recorded that, at the disappearance, the outer margin of our satellite
was fringed with light where it crossed the planet; but at the
reappearance this effect had vanished, and the appearance was perfectly
normal. The disk of Jupiter, where it met the edge of the lunar disk,
looked dusky by the effects of contrast; but I saw no marked shading
with a sharply terminating boundary, such as appears to have been
remarked elsewhere. As the planet emerged definition was superb, the
belts were lividly distinct, and the spectacle was one of the prettiest
that could be imagined. The red spot was going off the W. limb, and the
disk was covered with belts; many of them near the poles were extremely
narrow, like fine lines drawn with a sharp lead pencil. I used a 4-inch
refractor, powers 65 and 145, with this instrument the foregoing sketch
was made. The exceptional distinctness of the Jovian markings on this
occasion shows that the proximity of the Moon has certainly no tendency
to efface planetary details, but rather to intensify them[34].
On Sept. 3, 1889, an occultation of Jupiter was visible in America, and
observed by Mr. Brooks at Geneva, N.Y., with a 10-1/8-inch equatoreal.
His drawing, made from a photograph and eye-observations, shows nothing
of a dark fringe bordering the Moon’s limb.
[Illustration: Fig. 36.
Jupiter and satellites seen in a small glass.]
_The four Satellites._—When Galilei directed his telescope to Jupiter
on the evening of Jan. 7, 1610, he saw three small star-like points
near the planet; so:—
[Illustration]
On Jan. 13 he discovered a fourth; thus:—
[Illustration]
and ascertaining that these bodies followed Jupiter in his course,
concluded them to be moons in attendance upon him. At first the
discovery was discredited by others; but it soon had to be accepted
as an incontestable fact of observation. These satellites are usually
among the very first objects which the amateur views in his telescope,
and they form, in combination with their primary, an exquisite picture,
the impression of which is not soon forgotten. The periods, distances,
&c. of the satellites are as follows:—
+—————————————+—————————————————————+————————+————————-+————————-+
| | Mean Distance. | | | |
|No. and Name.+———————————+————————-+Sidereal|Mean |Real |
| |Diameters | Miles. |Period. |Apparent |Diameter,|
| |of Jupiter.| | |Diameter.|in miles.|
+—————————————+—————————————————————+————————+————————-+————————-+
| | | | h m s| ″ | |
| I. Io | 3·03 | 267,000| 1 18 29| 1·02 | 2390 |
| II. Europa | 4·72 | 425,000| 3 13 18| 0·91 | 2120 |
|III. Ganymede| 7·71 | 678,000| 7 4 0| 1·49 | 3480 |
| IV. Callisto| 13·55 |1,193,000|16 18 5| 1·27 | 2970 |
+—————————————+—————————————————————+————————+————————-+————————-+
The third satellite is much the largest, and its brightness is about
equal to that of a star of the 6th mag. The other three may be rated
as generally 7th mag., though their brightness is variable, especially
that of the fourth satellite, which has been seen exceedingly faint.
It is customary to distinguish these objects, not by their names, as
in the case of the moons of Mars, Saturn, and Uranus, but by the Roman
numbers affixed to them progressively according to their distances from
Jupiter.
The satellites are just visible to the naked eye when the conditions
favour their detection; but they are so much involved in the rays
of the planet, and often so near to him, that it may be regarded as
an exceptional feat to discern them without telescopic aid. When
III. and IV. are near their max. elongation and on the same side of
the planet, they have been occasionally observed separately. I. and
II., though much closer to Jupiter and more within the influence of
his glare and rays, have been similarly seen. When attempting such
observations it is best to hide the bare disk of the planet behind
some terrestrial object, as this will cut off the obnoxious rays and
prevent the brilliant light from dazzling the eye. An opera-glass, or
any small portable telescope, reveals the whole retinue of satellites,
and enables them to be traced through their revolutions. The ‘Nautical
Almanack’ gives diagrams of their diurnal positions, and with this work
as a reference observers will find no difficulty in identifying them
apart.
Sir W. Herschel, in the years 1794 to 1796, found that the satellites
revolve on their axes in the same time that they revolve about the
planet. He was led to this conclusion by a study of the variations in
the light emitted by the satellites in different parts of their orbits,
and described I. as “of a very intense bright, white, and shining
light,—brighter than II. or IV. (not larger). IV. inclines to red, and
nearly as bright as II. The latter is of a dull ash-colour. III. is
very white.” Modern observers have selected II. as relatively the most
highly reflective, while IV. is the least. Spots exist on the surfaces
of these objects, and probably occasion many of the differences
observed.
The eclipses, occultations, and transits of the satellites afford a
very fertile and attractive series of phenomena for telescopic review.
The exact times of occurrence are tabulated in the ‘Nautical Almanack’
and asterisks are affixed to such as are visible in this country. Prior
to the date of opposition of Jupiter the eclipses occur of course on
the W. side of the disk, while after opposition they take place on the
E. side. The durations are as follow for the several satellites:—I. =
2^h 20^m, II. = 2^h 56^m, III. = 3^h 43^m, IV. = 4^h 56^m. In reference
to III. and IV. the entire phenomenon may be generally observed; but
this is not so in regard to II., as the emersions are frequently
effected behind the planet. Only the immersions of I. are visible
before opposition, from the same cause; for the satellite enters the
cone of shadow close to the planet’s limb, and only comes out of it
when the globe of the planet is interposed in the line of sight. In
such cases the satellite emerges soon after from the limb of Jupiter;
so that its obscuration has been compounded of two separate phenomena,
viz. an eclipse and an occultation. After opposition this satellite
is first occulted and then eclipsed. IV. sometimes escapes eclipse
altogether, by passing above or below the shadow.
The motion of light was discovered, and its velocity determined, by
means of the eclipses of Jupiter’s satellites. These phenomena are also
useful in ascertaining longitudes. A spectator on Jupiter himself would
see a vast number of solar and lunar eclipses—about 4500 of each—during
the Jovian year of 4332·6 days, because the three inner satellites
exhibit these phenomena at every revolution, their orbits being very
slightly inclined to Jupiter’s equator, and the latter being but little
inclined to the plane of the ecliptic.
The occultations of the satellites are comparatively frequent, and
may be well observed in a good telescope. A tolerably high magnifier
is required to witness these occurrences with the best effect, the
disks of the satellites being small and not clearly traceable through
the various stages of their disappearances unless much amplified.
With considerable telescopic power the disks are well seen, and it
then becomes feasible to watch the satellites, first as they come
into contact with the limb, then as the globe of the planet overlaps
more and more of their diminutive forms, and finally as they reach
last contact and withdraw their narrow unobscured segments behind the
expansive sphere of their primary. Both the beginning and end phase
of these occultations is generally observable in regard to Sat. IV.,
and frequently also in the case of III. But with reference to II. and
I. it often happens that only the disappearance or reappearance can
be witnessed. These occultations have furnished some singular and
unexplained facts of observation. On meeting the limb of Jupiter Sats.
I. and II. have not always disappeared in a normal way. On April 26,
1863, Wray, with an 8-inch objective, saw II. distinctly projected
within the limb for nearly 20^s. Other similar cases are recorded. The
satellites have been seen apparently “through the edge of the disk.”
One observer mentions that II. appeared and disappeared several times
before occultation. The explanation appears to be that there is so much
irradiation round the disk of Jupiter that it produces a false limb,
and it is through this the satellites have been seen. A very tremulous
image, in bad air, may also be responsible for some of the anomalies
recorded.
The transits of the Jovian moons offer the most attractive phenomena
of all, and they come well within the reach of small telescopes.
On entering upon the planet they are visible as bright round spots
projected on the dusky limb, and subsequently present some eccentric
features. II. is invisible, except on the limbs; I. is often seen
as a grey spot threading along the belts; III. appears as a large
dark spot[35], nearly as black as its shadow; IV. seems to be black,
and scarcely to be recognized from its shadow. The appearances are
certainly to some extent variable. Mr. Stanley Williams has seen
III. as a _brilliant_ disk at mid-transit. I. sometimes crosses the
whole disk as a white spot; at certain other times it is invisible;
at others, again, it is seen as a faint grey spot. IV. is not always
black, its aspect depending upon the chord it traverses. Thus, on the
evening of Sept. 12, 1889, Mr. Williams, Mr. G. T. Davis of Reading,
and myself were observing Jupiter when IV. was in transit on a belt in
the N. hemisphere, but not a vestige of the satellite was seen by any
of us. On the morning of May 23, 1890, at 3^h 30^m A.M., however, while
observing the red spot on Jupiter, I noticed a black circular spot on
the great N. equatoreal belt; and this proved to be IV. in transit.
These peculiarities have been accounted for as partly due to contrast
and partly to dusky spots on the surfaces of the satellites. Dr.
Spitta has made a number of experiments to elucidate this subject, and
concludes that “the perpetual whiteness of the second satellite, and
the darkened tints of the others during transit, are due to differences
in their relative albedo [reflective power] as compared with that of
Jupiter, and are not dependent upon the relative quantity of light
reflected by one or the other, or upon any physical peculiarities of
the Jovian system.”
The shadows of the satellites transit the disk as dark spots larger
than the satellites themselves, owing to the penumbral fringes. Before
opposition these shadows precede the satellites; after opposition the
latter come first. The shadow of II. appears to be much lighter than
the others, and is usually of a pale chocolate-colour; and I saw it
thus at the opening of the year 1885:—
[Illustration: Fig. 37.
Shadows of Jupiter’s Satellites II. and III. near an equatoreal white
spot (Jan. 1, 1885, 7^h 20^m A.M.).]
Sat. II. is probably involved in an atmosphere sufficiently dense to
enable it to present undue luminosity relatively to the others; and if
so, the feeble shadow it transmits on Jupiter may be partly explained
by the effects of refraction. On the day of opposition both satellites
and shadows are projected on the same part of the disk, and the latter
are occulted by the former. On Jan. 14, 1872, Mr. F. M. Newton saw I.
centrally placed on its shadow; so that the satellite was apparently
surrounded with a ring of shade. On May 13, 1876, Mr. G. D. Hirst saw
Sat. I. partly occulting its own shadow; a black crescent was seen in
the bright zone N. of the equator. On Feb. 18, 1885, Dr. R. Copeland,
at Dun Echt, saw the shadow of I. “almost totally occulted by the
satellite itself; as the satellite approached Jupiter’s limb it came
out quite bright and large, with a mere crescent of the shadow showing
on its southern edge.” This phenomenon was also observed at Bristol.
Occasionally all the satellites become invisible at the same time,
being either eclipsed, occulted, or in transit. An instance of this
kind was recorded by Molyneux on Nov. 2, 1681 (O.S.). Sir W. Herschel
observed a similar occurrence on May 23, 1802; also Wallis on April
15, 1826, and Dawes and others on Sept. 27, 1843, and Aug. 21, 1867.
A visible repetition of the event was narrowly avoided on the morning
of Oct. 15, 1883. On this occasion the planet should, according to
the ‘Nautical Almanack,’ have been denuded of his satellites for a
period of 19 minutes; but this disappearance did not occur, for at no
time were all the satellites included within the margin or shadow of
Jupiter. I observed that Sat. III. entered upon the disk just as IV.
released itself, and the two formed a curious configuration at 4^h
A.M., hanging close upon the planet’s limbs.
Spots have been seen on the satellites both in transit and while
shining on the dark sky. This particularly refers to III. and IV. II.
has never given indications of such markings on its bright uniformly
clear surface. Dawes, Lassell, and Secchi frequently observed and drew
the spots. Secchi described III. as similar in aspect to the mottled
disk of Mars as seen in a small telescope; his drawings exhibit no
analogy, however, to those by Dawes of the same object. III. has been
remarked of a curious shape, as if dark spots obliterated part of the
limbs. Sat. I. was observed in transit on Sept. 8, 1890 by Barnard and
Burnham, and it appeared to be _double_, being divided by a bright
interval or belt. They used a 12-inch refractor, powers 500 and 700,
and the seeing was very fine.
Many other curious points have been noticed in the various aspects and
phenomena of the Jovian satellites. Further observations will doubtless
throw new light on some of the puzzling records of the past.
_Occultation of a Star._—An occultation of the 7th mag. star 4
Geminorum by Jupiter took place on Nov. 7, 1882, and it was observed by
Prof. Pritchett, of Glasgow, Missouri, with a 12-1/4-inch equatoreal,
power 200. “The images of both planet and star were very steady. The
margin of Jupiter’s disk was very sharply defined. The immersion was
very near the N. border of the broad S. equatoreal belt. At 11^h 28^m
10^s·65 local mean time the star was _apparently_ within the dark
outline of the disk, apparent geometric contact having occurred at
11^h 20^m 24^s·49. For a moment the star seemed to disappear, but a
moment later was plainly seen, as if through a well-defined notch in
the otherwise _continuously even_ margin. This notch lasted 46^s·26,
and at 11^h 28^m 56^s·91 it _vanished_, and the light of the star
was _entirely extinguished_.” The emersion of the star could not be
observed, as clouds supervened.
FOOTNOTES:
[33] The question of periodicity is an extremely interesting one
as affecting the disposition, form, and colours of the markings
on Jupiter. Certain features visible in 1869-70 were unmistakably
reproduced in 1880, and it has been suspected that the cycle of these
changes accords with the length of the Jovian year. Future observations
must be compared with old drawings and records for the identification
of similar features if they are recurrent.
[34] On the morning of Dec. 5, 1887, I made a drawing of Saturn, the
image of the planet being remarkably well defined, though the Moon was
only 1° distant.
[35] Amongst the first observers of these dark transits were Cassini
(Sept. 2, 1665), Romer (1677), and Maraldi (1707).