JUPITER
Passing outwards from the zone of the minor planets, we come to the
greatest and most magnificent member of the solar system, the giant
planet Jupiter. To most observers, Jupiter will probably appear not
only the largest, but also the most interesting telescopic object
which our system affords. Some, no doubt, will put in a claim for
Mars, and some will share Sir Robert Ball's predilection for Saturn;
but the interest attaching to Mars is of quite a different character
from that which belongs to Jupiter, and while Saturn affords a picture
of unsurpassed beauty, there is not that interest of variety and
change in his exquisite system which is to be found in that of his
neighbour planet. Jupiter is constantly attractive by reason of the
hope, or rather the certainty, that he will always provide something
fresh to observe; and the perpetual state of flux in which the details
of his surface present themselves to the student offers to us the
only instance which can be conveniently inspected of the process of
world-formation. Jupiter is at the very opposite end of the scale
from such a body, for example, as our own moon. On the latter it would
appear as though all things were approaching the fixity of death; such
changes as are suspected are scarcely more than suspected, and,
even if established, are comparatively so small as to tax the utmost
resources of observation. On the former, such a thing as fixity or
stability appears to be unknown, and changes are constantly occurring
on a scale so gigantic as not to be beyond the reach of small
instruments, at least in their broader outlines.
The main facts relating to the planet may be briefly given before
we go on to consider the physical features revealed to us by the
telescope. Jupiter then travels round the sun in a period of 11
years, 314·9 days, at an average distance of almost 483,000,000 miles.
According to Barnard's measures, his polar diameter is 84,570, and his
equatorial diameter 90,190 miles. He is thus compressed at the poles
to the extent of 1/16th, and there is no planet which so conspicuously
exhibits to the eye the actual effect of this polar flattening, though
the compression of Saturn is really greater still. In volume he is
equal to more than 1,300 earths, but his density is so small that only
316 of our worlds would be needed to balance him. This low density,
not much greater than that of water, is quite in accordance with all
the other features which are revealed by observation, and appears to
be common to all the members of that group of large exterior planets
of which Jupiter is at once the first and the chief.
The brilliancy of the great planet is exceedingly remarkable, far
exceeding that of Mars or Saturn, and only yielding to that of Venus.
In 1892 his lustre was double that of Sirius, which is by far the
brightest of all the fixed stars; and he has been repeatedly seen by
the unaided eye even when the sun was above the horizon. According to
one determination he reflects practically the same amount of light as
newly fallen snow; and even if this be rejected as impossibly high,
Zöllner's more moderate estimate, which puts his reflective power
at 62 per cent. of the light received, makes him almost as bright as
white paper. Yet to the eye it is very evident that his light has
a distinct golden tinge, and in the telescopic view this remains
conspicuous, and is further emphasized by the presence on his disc of
a considerable variety of colouring.
Under favourable circumstances Jupiter presents to us a disc which
measures as much as 50″ in diameter. The very low magnifying power of
50 will therefore present him to the eye with a diameter of 2,500″,
which is somewhat greater than the apparent diameter of the moon. In
practice it is somewhat difficult to realize that this is the case,
probably owing to the want of any other object in the telescopic field
with which to compare the planet. But while there may be a little
disappointment at the seeming smallness of the disc even with a power
double that suggested, this will quickly be superseded by a growing
interest in the remarkable picture which is revealed to view.
[Illustration: FIG. 25.
JUPITER, October 9, 1891, 9.30 p.m.; 3-7/8-inch, power 120.]
Some idea of the ordinary appearance of the planet may be gained from
Fig. 25, which reproduces a sketch made with a small telescope on
October 9, 1891. The first feature that strikes the eye on even the
most casual glance is the polar compression. The outline of the disc
is manifestly not circular but elliptical, and this is emphasized by
the fact that nearly all the markings which are visible run parallel
to one another in the direction of the longest diameter of the oval.
A little attention will reveal these markings as a series of dark
shadowy bands, of various breadths and various tones, which stretch
from side to side of the disc, fading a little in intensity as they
approach its margin, and giving the whole planet the appearance of
being girdled by a number of cloudy belts. The belts may be seen with
very low powers indeed, the presence of the more conspicuous ones
having repeatedly been evident to the writer with the rudimentary
telescope mentioned in Chapter II., consisting of a non-achromatic
double convex lens of 1-1/2-inch aperture, and a single lens eye-piece
giving a power of 36. Anything larger and more perfect than this will
bring them out with clearness, and an achromatic of from 2 to 3 inches
aperture will give views of the highest beauty and interest, and
will even enable its possessor to detect some of the more prominent
evidences of the changes which are constantly taking place.
The number of belts visible varies very considerably. As many as
thirty have sometimes been counted; but normally the number is much
smaller than this. Speaking generally, two belts, one on either side
of a bright equatorial zone, will be found to be conspicuous, while
fainter rulings may be traced further north and south, and the dusky
hoods which cover the poles will be almost as easily seen as the
two main belts. It will further become apparent that this system
of markings is characterized by great variety of colouring. In this
respect no planet approaches Jupiter, and when seen under favourable
circumstances and with a good instrument, preferably a reflector,
some of the colour effects are most exquisite. Webb remarks: 'There is
often "something rich and strange" in the colouring of the disc.
Lord Rosse describes yellow, brick-red, bluish, and even full-blue
markings; Hirst, a belt edged with crimson lake; Miss Hirst, a small
sea-green patch near one of the poles.' The following notes of colour
were made on December 26, 1905: The south equatorial belt distinct
reddish-brown; the equatorial zone very pale yellow, almost white,
with faint slaty-blue shades in the northern portion; the north polar
regions a decided reddish-orange; while the south polar hood was of a
much colder greyish tone. But the colours are subject to considerable
change, and the variations of the two great equatorial belts appear,
according to Stanley Williams, to be periodic, maxima and minima of
redness being separated by a period of about twelve years, and the
maximum of the one belt coinciding with the minimum of the other.
[Illustration:
PLATE XXII.
Jupiter, January 6, 1906--8 hours 20 minutes. Instrument, 9-1/4-inch
Reflector.
[lambda] = 238° (System 1); [lambda] = 55° (System 2).
Rev. T. E. R. Phillips.]
These changes in colouring bring us to the fact that the whole system
of the Jovian markings is liable to constant and often very rapid
change. Anyone who compares drawings made a few years ago with those
made at the present time, such as Plates XXII. and XXIII., cannot fail
to notice that while there is a general similarity, the details have
changed so much that there is scarcely one individual feature which
has not undergone some modification. Indeed, this process of change is
sometimes so rapid that it can be actually watched in its occurrence.
Thus Mr. Denning remarks that 'on October 17, 1880, two dark spots,
separated by 20° of longitude, broke out on a belt some 25° north
of the equator. Other spots quickly formed on each side of the pair
alluded to, and distributed themselves along the belt, so that by
December 30 they covered three-fourths of its entire circumference.'
The dark belts, according to his observations, 'appear to be sustained
in certain cases by eruptions of dark matter, which gradually spread
out into streams.'
Even the great equatorial belts are not exempt from the continual flux
which affects all the markings of the great planet, and the details
of their structure will be found to vary to a considerable extent at
different periods. At present the southern belt is by far the most
conspicuous feature of the surface, over-powering all other details by
its prominence, while its northern rival has shrunk in visibility to
a mere shadow of what it appears in drawings made in the seventies.
Through all the changes of the last thirty years, however, one very
remarkable feature of the planet has remained permanent at least in
form, though varying much in visibility. With the exception of the
canals of Mars, no feature of any of the planets has excited so
much interest as the great red spot on Jupiter. The history of this
extraordinary phenomenon as a subject of general study begins in 1878,
though records exist as far back as 1869 of a feature which almost
certainly was the same, and it has been suggested that it was observed
by Cassini two centuries ago. In 1878 it began to attract general
attention, which it well deserved. In appearance it was an enormous
ellipse of a full brick-red colour, measuring some 30,000 miles in
length by 7,000 in breadth, and lying immediately south of the south
equatorial belt. With this belt it appears to have some mysterious
connection. It is not actually joined to it, but seems, as Miss
Clerke observes, to be 'jammed down upon it'; at least, in the south
equatorial belt, just below where the spot lies, there has been formed
an enormous bay, bounded on the following side (_i.e._, the right
hand as the planet moves through the field), by a sharply upstanding
shoulder or cape. The whole appearance of this bay irresistibly
suggests to the observer that it has somehow or other been hollowed
out to make room for the spot, which floats, as it were, within it,
surrounded generally by a margin of bright material, which divides it
from the brown matter of the belt. The red spot, with its accompanying
bay and cape, is shown in Fig. 25 and in Plate XXII., which represents
the planet as seen by the Rev. T. E. R. Phillips on January 6, 1906.
The spot has varied very much in colour and in visibility, but on the
whole its story has been one of gradual decline; its tint has paled,
and its outline has become less distinct, as though it were being
obscured by an outflow of lighter-coloured matter, though there have
been occasional recoveries both of colour and distinctness. In 1891 it
was a perfectly easy object with 3-7/8 inches; at the present time
the writer has never found it anything but difficult with an 18-inch
aperture, though some observers have been able to see it steadily in
1905 and 1906 with much smaller telescopes. The continued existence
of the bay already referred to seems to indicate that it is only the
colour of the spot that has temporarily paled, and that observers may
in course of time witness a fresh development of this most interesting
Jovian feature.
The nature of the red spot remains an enigma. It may possibly
represent an opening in the upper strata of Jupiter's dense
cloud-envelope, through which lower strata, or even the real body of
the planet, may be seen. The suggestion has also been made that it is
the glow of some volcanic fire on the body of the planet, seen through
the cloud-screen as the light of a lamp is seen through ground-glass.
But, after all, such ideas are only conjectures, and it is impossible
to say as yet even whether the spot is higher or lower than the
average level of the surface round it. A curious phenomenon which
was witnessed in 1891 suggested at first a hope that this question of
relative height would at least be determined. This phenomenon was the
overtaking of the red spot by a dark spot which had been travelling
after it on the same parallel, but with greater speed, for some
months. It appeared to be quite certain that the dark spot must either
transit the face of the red spot or else pass behind it; and in either
case interesting information as to the relative elevations of the two
features in question would have been obtained. The dark spot, however,
disappointed expectation by drifting round the south margin of the red
one, much as the current of a river is turned aside by the buttress of
a bridge. In fact, it would almost appear as though the red spot had
the power of resisting any pressure from other parts of the planet's
surface; yet in itself it has no fixity, for its period of rotation
steadily lengthened for several years until 1899, since when it has
begun to shorten again, so that it would appear to float upon the
surface of currents of variable speed rather than to be an established
landmark of the globe itself. The rotation period derived from it was,
in 1902, 9 hours 55 minutes 39·3 seconds.
The mention of the changing period of rotation of the red spot lends
emphasis to the fact that no single period of rotation can be assigned
to Jupiter as a whole. It is impossible to say of the great planet
that he rotates in such and such a period: the utmost that can be said
is that certain spots upon his surface have certain rotation periods;
but these periods are almost all different from one another, and even
the period of an individual marking is subject, as already seen, to
variation. In fact, as Mr. Stanley Williams has shown, no fewer than
nine different periods of rotation are found to coexist upon the
surface; and though the differences in the periods seem small when
expressed in time, amounting in the extreme cases only to eight and
half minutes, yet their significance is very great indeed. In the case
of Mr. Williams's Zones II. and III., the difference in speed of these
two surface currents amounts to 400 miles per hour. Certain bright
spots near the equator have been found to move so much more rapidly
than the great red spot as to pass it at a speed of 260 miles an hour,
and to 'lap' it in forty-four and a half days, completing in that time
one whole rotation more than their more imposing neighbour. It cannot,
therefore, be said that Jupiter's rotation period is known; but the
average period of his surface markings is somewhere about nine hours
fifty-two minutes.
Thus the rotation period adds its evidence to that already afforded
by the variations in colour and in form of the planet's markings that
here we are dealing with a body in a very different condition from
that of any of the other members of our system hitherto met with. We
have here no globe whose actual surface we can scrutinize, as we can
in the case of Mars and the moon, but one whose solid nucleus, if
it has such a thing, is perpetually veiled from us by a mantle which
seems more akin to the photosphere of the sun than to anything else
that we are acquainted with. The obvious resemblances may, and very
probably do, mask quite as important differences. The mere difference
in scale between the two bodies concerned must be a very important
factor, to say nothing of other causes which may be operative in
producing unlikeness. Still, there is a considerable and suggestive
general resemblance.
In the sun and in Jupiter alike we have a view, not of the true
surface, but of an envelope which seems to represent the point of
condensation of currents of matter thrown up from depths below--an
envelope agitated in both cases, though more slowly in that of
Jupiter, by disturbances which bear witness to the operation of
stupendous forces beneath its veil. In both bodies there is a similar
small density: neither the sun nor Jupiter is much denser than water;
in both the determination of the rotation period is complicated by
the fact that the markings of the bright envelope by which the
determinations must be made move with entirely different speeds in
different latitudes. Here, however, there is a divergence, for while
in the case of the sun the period increases uniformly from the equator
to the poles, there is no such uniformity in the case of Jupiter. Thus
certain dark spots in 25° north latitude were found in 1880 to have a
shorter period than even the swift equatorial white markings.
One further circumstance remains to be noted in pursuance of these
resemblances. Not only does the disc of Jupiter shade away at its
edges in a manner somewhat similar to that of the sun, being much
more brilliant in the centre than at the limb, but his remarkable
brilliancy, already noticed, has given rise to the suggestion that to
some small extent he may shine by his own inherent light. There are
certain difficulties, however, in the way of such a suggestion. The
satellites, for example, disappear absolutely when they enter the
shadow of their great primary--a fact which is conclusive against the
latter being self-luminous to anything more than a very small extent,
as even a small emission of native light from Jupiter would suffice
to render them visible. But even supposing that the idea of
self-luminosity has to be abandoned, everything points to the fact
that in Jupiter we have a body which presents much stronger analogies
to the sun than to those planets of the solar system which we have
so far considered. The late Mr. R. A. Proctor's conclusion probably
represents the true state of the case with regard to the giant planet:
'It may be regarded as practically proved that Jupiter's condition
rather resembles that of a small sun which has nearly reached the dark
stage than that of a world which is within a measurable time-interval
from the stage of orb-life through which our own Earth is passing.'
Leaving the planet itself, we turn to the beautiful system of
satellites of which it is the centre. The four moons which, till 1892,
were thought to compose the complete retinue of Jupiter, were among
the first-fruits of Galileo's newly-invented telescope, and were
discovered in January, 1610. The names attached to them--Io, Europa,
Ganymede, and Callisto--have now been almost discarded in favour of
the more prosaic but more convenient numbers I., II., III., IV. The
question of their visibility to the unaided eye has been frequently
discussed, but with little result; nor is it a matter of much
importance whether or not some person exceptionally gifted with
keenness of sight may succeed in catching a momentary glimpse of one
which happens to be favourably placed. The smallest telescope or a
field-glass will show them quite clearly. They are, in fact, bodies
of considerable size, III., which is the largest, being 3,558 miles
in diameter, while IV. is only about 200 miles less; and a moderate
magnifying power will bring out their discs.
[Illustration: PLATE XXIII.
Jupiter, February 17, 1906. J. Baikie, 18-inch Reflector.]
The beautiful symmetry of this miniature system was broken in 1892 by
Barnard's discovery of a fifth satellite--so small and so close to the
great planet that very few telescopes are of power sufficient to show
it. This was followed in 1904 by Perrine's discovery, from photographs
taken at the Lick Observatory with the Crossley reflector, of two
more members of the system, so that the train of Jupiter as at present
known numbers seven. The fifth, sixth, and seventh satellites are, of
course, far beyond the powers of any but the very finest instruments,
their diameters being estimated at 120, 100, and 30 miles
respectively. It will be a matter of interest, however, for the
observer to follow the four larger satellites, and to watch their
rapid relative changes of position; their occultations, when they pass
behind the globe of Jupiter; their eclipses, when they enter the great
cone of shadow which the giant planet casts behind him into space;
and, most beautiful of all, their transits. In occultations the
curious phenomenon is sometimes witnessed of an apparent flattening of
the planet's margin as the satellite approaches it at ingress or draws
away from it at egress. This strange optical illusion, which also
occurs occasionally in the case of transits, was witnessed by several
observers on various dates during the winter of 1905-1906. It is, of
course, merely an illusion, but it is curious why it should happen on
some occasions and not on others, when to all appearance the seeing is
of very much the same quality. The gradual fading away of the light of
the satellites as they enter into eclipse is also a very interesting
feature, but the transits are certainly the most beautiful objects of
all for a small instrument. The times of all these events are given in
such publications as the 'Nautical Almanac' or the 'Companion to the
Observatory'; but should the student not be possessed of either of
these most useful publications, he may notice that when a satellite is
seen steadily approaching Jupiter on the following side, a transit
is impending. The satellite will come up to the margin of the planet,
looking like a brilliant little bead of light as it joins itself to
it (a particularly exquisite sight), will glide across the margin, and
after a longer or shorter period will become invisible, being merged
in the greater brightness of the central portions of Jupiter's disc,
unless it should happen to traverse one of the dark belts, in which
case it may be visible throughout its entire journey. It will be
followed or preceded, according to the season, by its shadow, which
will generally appear as a dark circular dot. In transits which occur
before opposition the shadows precede the satellites; after opposition
they travel behind them. The transit of the satellite itself will
in most cases be a pretty sharp test of the performance of a 3-inch
telescope, or anything below that aperture; but the transit of the
shadow may be readily seen with a 2-1/2-inch, probably even with a
2-inch. There are certain anomalies in the behaviour of the shadows
which have never been satisfactorily explained. They have not always
been seen of a truly circular form, nor always of the same degree of
darkness, that of the second satellite being notably lighter in most
instances than those of the others. There are few more beautiful
celestial pictures than that presented by Jupiter with a satellite and
its shadow in transit. The swift rotation of the great planet and the
rapid motion of the shadow can be very readily observed, and the whole
affords a most picturesque illustration of celestial mechanics.
A few notes may be added with regard to observation. In drawing the
planet regard must first of all be paid to the fact that Jupiter's
disc is not circular, and should never be so represented. It is easy
for the student to prepare for himself a disc of convenient size, say
about 2-1/2 inches in diameter on the major axis, and compressed
to the proper extent (1/16), which may be used in outlining all
subsequent drawings. Within the outline thus sketched the details must
be drawn with as great rapidity as is consistent with accuracy. The
reason for rapidity will soon become obvious. Jupiter's period of
rotation is so short that the aspect of his disc will be found to
change materially even in half an hour. Indeed, twenty minutes
is perhaps as long as the observer should allow himself for any
individual drawing, and a little practice will convince him that it
is quite possible to represent a good deal of detail in that time, and
that, even with rapid work, the placing of the various markings may be
made pretty accurate. The darker and more conspicuous features should
be laid down first of all, and the more delicate details thereafter
filled in, care being taken to secure first those near the preceding
margin of the planet before they are carried out of view by rotation.
The colours of the various features should be carefully noted at
the sides of the original drawings, and for this work twilight
observations are to be preferred.
Different observers vary to some extent, as might be expected, in
their estimates of the planet's colouring, but on the whole there is
a broad general agreement. No planet presents such a fine opportunity
for colour-study as Jupiter, and on occasions of good seeing the
richness of the tones is perfectly astonishing. In showing the natural
colours of the planet the reflector has a great advantage over the
refractor, and observers using the reflecting type of instrument
should devote particular attention to this branch of the subject, as
there is no doubt that the colour of the various features is liable to
considerable, perhaps seasonal, variation, and systematic observation
of its changes may prove helpful in solving the mystery of Jupiter's
condition. The times of beginning and ending observation should be
carefully noted, and also the magnifying powers employed. These should
not be too high. Jupiter does not need, and will not stand, so much
enlargement as either Mars or Saturn. It is quite easy to secure
a very large disc, but over-magnifying is a great deal worse than
useless: it is a fertile source of mistakes and illusions. If the
student be content to make reasonable use of his means, and not to
overpress either his instrument or his imagination, he will find upon
Jupiter work full of absorbing interest, and may be able to make his
own contribution to the serious study of the great planet.