William F. Denning.

Telescopic Work for Starlight Evenings online

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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



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