ill-regulated (if the expression be permissible), is distinctly
of the solar type. It is itself a “semi-sun,” showing no trace of
a solid surface, but a continual succession of cloud-like masses
belched forth from within. Jupiter’s low mean density, considered
apart from every other circumstance, suffices to demonstrate the
primitive nature of his state. In a sun-like body, the circulation is
bodily and vertical. That the processes going on in Jupiter are of
this kind is beyond question. Exchanges of hot and colder substances
are effected, not by surface-flows, but by up and down rushes. The
parallelism of his belts to his equator makes this visible to the
eye. An occasional oblique streak betokens a current in latitude, but
it is exceptional, and might be called out of character.
Jupiter’s true atmosphere encompasses the disturbed shell of vapors
observed telescopically. Its general absorptive action upon light
is betrayed by the darkening of the planet’s limb—another point of
resemblance to the sun; while its special, or selective, absorption
can only be detected with the spectroscope.
The actinic power of Jupiter’s light is very remarkable. It surpasses
that of moonlight nine times, and that of Mars twenty-four times. Dr.
Lohse further ascertained that the Southern Hemisphere is twice as
chemically effective as the Northern. This superiority is doubtless
connected with the greater physical agitation of the same region. A
series of photographs of Jupiter, taken in 1891 with the great Lick
refractor, were the first of any value for purposes of investigation.
Jupiter’s satellites were the first trophies of telescopic
observation. They are, indeed, bright enough for naked-eye
perception, could they be removed from the disk which obscures
them with its excessive splendor; and the first and third have
actually been seen, in despite of the glare, by a few persons with
phenomenally good eyesight. The mythological titles of the Galilean
group—Io, Europa, Ganymede, and Calypso (proceeding from within
outward)—have been superseded by prosaic numbers.
The Jovian family presents an animated and attractive spectacle. The
smallest of its original members (No. II) is almost exactly the size
of our moon; the largest (No. III), with its diameter of 3,550 miles,
considerably exceeds the modest proportions of Mercury. Satellite I
revolves in 42½ hours at the same average distance from Jupiter’s
surface that our moon does from that of the earth. No. II has a
period of 3 days 13 hours, and its distance from Jupiter’s centre is
415,000 miles. Both these orbits are sensibly circular; and Nos. III
and IV travel in ellipses of very small eccentricity, the one at a
mean distance of 664,000, the other at 1,167,000 miles, in periods
respectively of 7 days 4 hours, and 16 days 16½ hours. All four
revolve strictly in the plane of Jupiter’s equator.
They constitute a system bound together by peculiar dynamical
relations, in consequence of which they can never be all either
eclipsed or seen aligned at one side of their primary at the same
time. They can all, however, be simultaneously hidden behind it, or
in its shadow; although this moonless condition is looked out for as
a telescopic rarity.
The transits of the satellites across the Jovian disk present many
curious appearances, due to complicated and changeable effects of
light and shade both upon the planetary background and upon the
little circular objects self-compared with it. These, in the ordinary
course, show bright while near the dusky limb, then vanish during the
central passage, and re-emerge again bright at the opposite side.
But instead of duly vanishing, they now and then darken even to the
point of becoming indistinguishable from their own shadows, by which
they are preceded or followed. This difference of behavior can not
be attributed wholly to varieties of lustre in the sections of the
disk transited; otherwise it could be predicted. But this has never
been attempted; “black transits” come when least expected. The third
and fourth satellites are those chiefly subject to these phases; the
second has never been known to exhibit them; and they but slightly
affect the first. Indeed, all the satellites, except, perhaps, No.
II, are striped or spotted; and this leads to seeming deformations
in their shape, as well as fluctuations in their brightness, the
markings being evidently of atmospheric origin, and hence changeable.
Their distinct and accurate perception has been made possible by the
excellence of the Lick 36-inch refractor.
Jupiter’s moons seem to resemble him in constitution. The first three
possess the same high reflective power. No. II is as bright as the
planet’s brightest parts, so that its albedo can not fall short of
0.70. And even No. IV (formerly designated “Calypso” in reference
to its frequent obscurations) exactly matches, during its darkest
phases, the blue-gray polar hoods of its primary. On an average, too,
the satellites seem to be of about the same mean density as Jupiter,
No. I being considerably the lightest for its bulk; and their
spectra, according to Vogel’s observations in 1873, are composed of
solar rays modified in precisely the same way as those reflected by
the planet.
The discovery, September 9, 1892, of Jupiter’s “fifth satellite”
was one of the keenest astronomical surprises on record. Professor
Barnard seized the opportunity, lent by the specially favorable
opposition of 1892, to rummage the system for novelties. Keeping
the telescopic field dark by means of a metallic bar placed so as
to occult the gorgeous planetary round, he sought, night after
night, for what might appear. At length, on September 9, he caught
the glimmer he wanted, and made sure, September 10, that it truly
intimated the presence of a new satellite.
This small body revolves in a period of 11 hours, 57 minutes, 23
seconds at a mean distance of 112,160 miles from Jupiter’s centre, or
67,000 from his bulged equatorial surface. Hence, it should by right
be called “No. I” instead of “No. V.” The major axis of the ellipse
in which it circulates advances so rapidly, owing to the disturbance
caused by Jupiter’s spheroidal figure, as to complete a revolution in
five months. The implied eccentricity of its orbit, as M. Tisserand
has shown, very slightly exceeds that of the orbit of Venus, yet it
has been made obvious by Barnard’s observations of the differences
between its east and west elongations. Its orbital velocity of 16½
miles a second far surpasses that of any other satellite in the
solar system. Close vicinity to a mass so vast as Jupiter’s demands
counterbalancing swiftness. Its period of revolution being, however,
longer by one hour than Jupiter’s period of rotation, it so far
conducts itself normally as to rise in the east and set in the west.
On the other hand, since its progress over the sphere is measured by
the difference between the two periods, it spends five Jovian days in
journeying from one horizon to the other, running, in the meantime,
four times through all its phases. Yet it never appears full.
Jupiter’s voluminous shadow cuts off sunlight from it during nearly
one-fifth of each circuit.
SATURN.—AGNES M. CLERKE
Nearly twice as far from the sun as Jupiter revolves a planet,
the spacious orbit of which was, until 1781, supposed to mark the
uttermost boundary of the Solar System. The mean radius of that orbit
is 886 millions of miles; but in consequence of its eccentricity,
the sun is displaced from its middle point to the extent of 50
million miles, and Saturn is accordingly 100 million miles nearer
to him at perihelion than at aphelion. The immense round assigned
to the “saturnine” planet is traversed in 29½ years, at the tardy
pace of six miles a second. His seasons are thus twenty-nine times
more protracted than ours, and are nominally more accentuated,
since his axis of rotation deviates from the vertical by 27°. But
solar heat, however distributed, plays an insignificant part in his
internal economy. In the first place, its amount is only 1/91st its
amount on the earth; in the second, Saturn, like Jupiter—even more
than Jupiter—is thermally self-supporting. The bulk of his globe
comparatively to its mass suffices in itself to make this certain.
The mean diameter of Saturn is 71,000 miles, or nine times (very
nearly) that of the earth; if of equal density, its mass should then
be nine cubed, or 729 times the same unit. The actual proportion,
however, is 95; hence the planet has a mean density of only
95/729th, or between 1/7th and 1/8th the terrestrial, and being thus
composed of matter as light as cork, would float in water. Professor
G. H. Darwin has, moreover, demonstrated, from the movements of its
largest satellite, that its density gains markedly with descent into
the interior, so that its surface-materials must be lighter than any
known solid or liquid.
When at its nearest to the earth, Saturn is as large as a sixpence
held up at a distance of 210 yards. But instead of being round like
a sixpence, it is strongly compressed—more compressed even than
Jupiter. The spectra of the two planets are almost identical. Both
are impressed with traces of aqueous absorption, and include the “red
star line.”
Saturn resembles to the eye a large, dull star; its rays are entirely
devoid of the sparkling quality which distinguishes those of Jupiter.
But it shows telescopically an analogous surface-structure. Its most
conspicuous markings are tropical dark belts of a grayish or greenish
hue; the equatorial region is light yellow, diversified by vague
white spots; while the poles carry extensive pale blue canopies. The
apparent tranquillity of the disk may be attributed in part to the
vast distance from which it is viewed; yet not wholly.
From measures executed by Barnard in 1895, it appears that the
equatorial diameter of Saturn is 76,470, its polar diameter 69,770
miles, giving a mean diameter of 74,240, and a compression of about
1/12th. Gravity, at its surface, is only 1/5th more powerful than on
the earth.
Thus, Saturn not only belongs to the same celestial species as
Jupiter, but is a closely related individual of that species. There
is no probability that either is to any extent solid. Both exhibit
the same type of markings; both betray internal tumults by eruptions
of spots which, by their varying movements, supply a measure for the
profundity of their origin; both possess identically constituted
atmospheres, and are darkened marginally by atmospheric absorption.
Saturn is, however, distinguished by the possession of a unique set
of appendages. Nothing like them is to be seen elsewhere in the
heavens; and when well opened they form, with the globe they inclose,
and the retinue of satellites in waiting outside, a strange and
wonderful telescopic object. The rings, since they lie in the plane
of Saturn’s equator, are inclined 27° to the Saturnian orbit, and
28° to the ecliptic. The earth is, however, comparatively to Saturn,
so near the sun, that their variations in aspect, as viewed from it,
may in a rough way be considered the same as if seen from the sun.
They correspond exactly with the Saturnian seasons. At the Saturnian
equinoxes, the rings are illuminated edgewise, and disappear, totally
or approximately; at the Saturnian solstices, sunlight strikes them
nearly at the full angle of 27°, first from _below_, then from
_above_. At these epochs, we perceive the appendage expanded into
an ellipse about half as wide as it is long. Two concentric rings
(generally called A and B) are then very plainly distinguishable, the
inner being the brighter. The black fissure which separates them is
called “Cassini’s division,” because that eminent observer was, in
1675, the first to perceive it. A chasm known as “Encke’s division,”
in the outer ring (A), is a thinning-out rather than an empty space;
and temporary gaps frequently appear in A, while B is entirely
exempt from them. There are then two definite and permanent bright
rings, and no more; but with them is associated the dusky formation
discovered by W. C. Bond, November 15, 1850, and described by Lassell
as “something like a crape veil covering a part of the sky within
the inner ring.” It is semi-transparent, the limb of Saturn showing
distinctly through it.
The exterior diameter of the ring-system is 172,800, while its
breadth is 42,300 miles. The rings A and C are each 11,000 miles
wide; while B measures 18,000, Cassini’s division 2,270, and the
clear interval between C and the planetary surface somewhat less than
6,000 miles. Each ring, C included, is brightest at its outer edge;
but there is no gap between the shining and the dusky structures,
B shading by insensible gradations up to C, yet maintaining
distinctness from it. The earliest exact determinations of the former
were made by Bradley in 1719, since when they have been affected by
no appreciable change. The theoretically inevitable subversion of the
system is progressing with extreme slowness.
The thickness of the rings is quite inconsiderable. They are flat
sheets, without (so to speak) a third dimension. For this reason,
they disappear utterly in most telescopes, when their plane passes
through the earth, as it does twice in each Saturnian year. Only
under exceptional conditions, a narrow, knotted, often nebulous,
streak survives as an index to their whereabouts. On October 26,
1891, Professor Barnard, armed with the Lick refractor, found it
impossible to see them projected upon the sky, notwithstanding that
their shadow lay heavily on the planet. It was not until three
days later that “slender threads of light” came into view. The
corresponding thickness of the formation was estimated at less than
fifty miles. The phenomenon of ring disappearance will not recur
until July 29, 1907.
The constitution of this marvelous structure is no longer doubtful.
It represents what might be called the fixed form of a revolving
multitude of diminutive bodies. This was demonstrated by Clerk
Maxwell in the Adams Prize Essay of 1857. His conclusion proved
irreversible. The pulverulent composition of Saturn’s rings is one
of the acquired truths of science. An incalculable number of tiny
satellites revolving independently in distinct orbits, in the precise
periods prescribed by their several distances from the planet, are
aggregated into the unmatched appendages of Galileo’s _tergeminus
planeta_. The local differences in their brightness depend upon the
distribution of the component satelloids. Where they are closely
packed, as in the outer margins of rings A and B, sunlight is
copiously reflected; where the interspaces are wide, the blackness
of the sky is barely veiled by the scanty rays thrown back from the
thinly scattered cosmic dust. The appearance of the crape ring as
a _dark_ stripe on the planet results—as M. Seeliger has pointed
out—not from the transits of the objects themselves, but from the
flitting of their shadows in continual procession across the disk.
The albedo of these particles is so high as to render it improbable
that they are of an earthy or rocky nature, such as the meteorites
which penetrate our atmosphere. The rings they form are, on the
whole, more lustrous than Saturn’s globe; but this superiority is
held to be due to the absence of atmospheric absorption. Their
spectrum is that of unmodified sunlight.
An eclipse of Japetus, the eighth Saturnian moon, by the globe and
rings, November 1, 1889, was highly instructive as to the nature of
the dusky appendage. The satellite was never lost sight of during
its passage behind it; but became more and more deeply obscured as
it traveled outward; then, at the moment of ingress into the shadow
of ring B, suddenly disappeared. Certainty was thus acquired that
the particles forming the crape ring are most sparsely strewn at its
inner edge—which is, nevertheless, perfectly definite—and gradually
reach a maximum of density at its outer edge. Yet, while there is not
the smallest clear interval, a sharp line of demarcation separates
it from the contiguous bright ring. Professor Barnard was the only
observer of these curious appearances. The distribution of the
ring-constituents, like that of the asteroids, was governed by the
law of commensurable periods, Saturn’s moons replacing Jupiter as the
perturbing and regulating power.
The “satellite-theory” of Saturn’s rings has received confirmation
from apparently the least promising quarters. Professor Seeliger
of Munich showed, from photometric experiments in 1888, that their
constant lustre under angles of illumination ranging from 0° to
30° was proof positive of their composition out of discrete small
bodies. And Professor Keeler of Alleghany, by a beautiful and refined
application of the spectroscopic method, arrived at the same result
in April, 1895. “Under the two different hypotheses,” he remarked,
“that the ring is a rigid body, and that it is a swarm of satellites,
the relative motion of its parts would be essentially different.”
The former would necessarily involve increasing velocity _outward_,
the latter, increase of velocity _inward_, just for the same reason
that Mercury moves more swiftly than the earth, and the earth than
Saturn; while the sections of a solid body, which could have but one
period of rotation, should move faster, _in miles per second_, the
further they were from the centre of attraction. The line of sight
test is then theoretically available; but it was an arduous task to
render it practically so. The difficulties were, however, one by one
overcome; and a successful photograph of the spectra of Saturn and
its rings gave the required information in unmistakable shape. From
measurements of the inclinations of five dusky rays contained in it
with reference to a standard horizontal line, rates of movement were
derived of 12½ miles per second for the inner edge of ring B, and
of 10 miles for the outer edge of ring A. The agreement with theory
was, as nearly as possible, exact; the components of the rings were
experimentally demonstrated to be moving, each independently of every
other, under the dominion of Kepler’s laws.
For the globe of Saturn, Professor Keeler obtained, by the same
exquisite method, a rotational period of 10 hours, 14 minutes, 24
seconds, in precise accordance with that indicated by the white
spot of 1876, which thus seems to have had no proper motion, but to
have floated on the ochreous equatorial surface as tranquilly as a
water-lily upon a stagnant pool. The result, so far as it goes, hints
that Saturn may be really, as well as apparently, less ebullient than
Jupiter.
Seers into the future of the heavenly bodies consider that the rings
of Saturn, like the gills of a tadpole, are symptomatic of an early
stage of development; and will be disposed of before he arrives
at maturity. They can not be regarded otherwise than as abnormal
excrescences. No other planet retains matter circulating round it in
such close relative vicinity. It was proved by Roche of Montpellier
that no secondary body of importance can exist within less than 2.44
mean radii of its primary; inside of that limit it would be rent
asunder by tidal strain. But the entire ring-system lies within the
assigned boundary; hence, being _where_ it is, it can only exist _as_
it is—in flights of discrete particles. Will it, however, always
remain where it is?
“Clerk Maxwell,” wrote Mr. Cowper Ranyard, “used to describe the
matter of the rings as a shower of brickbats, among which there would
inevitably be continual collisions. The theoretical results of such
impacts would be a spreading of the ring both inward and outward.
The outward spreading will in time carry the meteorites beyond
Roche’s limit, where, in all probability, they will, as Professor
Darwin suggests, slowly aggregate, and a minute satellite will be
formed. The inward spreading will in time carry the meteorites at
the inner edge of the ring into the atmosphere of the planet, where
they will become incandescent, and disappear as meteorites do in our
atmosphere.”
Yet it may be that collisions are infrequent in this conglomeration
of “brickbats.” There is the strongest presumption that they
all circulate in the same direction, in orbits nearly circular,
and scarcely deviating from the plane of the Saturnian equator.
Those pursuing markedly eccentric tracks must long ago have been
eliminated. Thus, encounters can only occur through gravitational
disturbances by Saturn’s moons, and they must be of a mild character,
depending upon very small differences of velocity. The first sign of
a “spreading outward” should be the formation of an exterior “crape
ring,” of which no faintest trace has yet been perceived.
Saturn’s rings are entirely invisible from its polar regions, but
occasion prolonged and complex eclipse-effects in its temperate
and equatorial zones. They have been fully treated of from the
geometrical point of view by Mr. Proctor in _Saturn and its System_.
Of this planet’s eight satellites,[28] the largest, Titan (No.
VI), was discovered first (by Huygens in 1655), and the smallest,
Hyperion (No. VII), last (by Lassell and Bond in 1848). The five
others were detected by J. D. Cassini and William Herschel. Titan,
alone of the entire group, equals our moon in size. It measures,
according to Professor Barnard, 2,720 miles across. Its period of
revolution is nearly sixteen days, its distance from Saturn’s centre,
771,000 miles. The orbit of Japetus (No. VIII) is the largest, and
its period the longest of any secondary body in the Solar System.
It circulates in 79⅓ days at a distance of 2,225,000 miles, equal
to 59½ of Saturn’s equatorial radii. Hence its path is of about
the same _proportional_ dimensions as that of our moon. Japetus is
remarkable for its variability in light. It is capable of tripling
or quadrupling its minimum lustre. Sir William Herschel noticed that
these maxima coincided with a position on the western side of the
planet, and inferred rotation of the lunar kind. “From the changes
in this body,” he argued in 1792, “we may conclude that some part of
its surface, and this by far the largest, reflects much less light
than the rest; and that neither the darkest nor the brightest side is
turned toward the planet, but partly one and partly the other, though
probably less of the bright side.”
This explanation, however, he admitted to be incomplete. There
was, and is, outstanding variability, which seems to intimate the
presence of an atmosphere and the formation of clouds. But no
positive knowledge has yet been gained regarding the physical state
of Saturn’s moons. We may, nevertheless, conjecture that, since
tidal friction has destroyed the rotation (as regards Saturn) of the
remotest member of the family, it has not spared those more exposed
to its grinding-down action. All presumably rotate in the same time
that they revolve.
The five inner satellites move in approximately circular orbits; the
three outer in ellipses about twice as eccentric as the terrestrial
path. All, Japetus only excepted, keep strictly to the plane of the
rings. And since this makes an angle of 27° with the planet’s orbit,
eclipses are much less frequent here than in the Jovian system. They
can only occur when Saturn is within a certain distance (different
for each) from the node of the satellite-orbit. Even Mimas (No. I),
although it wheels round the ring at an interval of only 34,000
miles, often slips outside the obliquely projected shadow-cone. Its
distance from Saturn’s centre is 118,000 miles, and it completes
a circuit in 22½ hours. Perpetually wrapped in the glare of its
magnificent primary, it is a very shy object, only to be caught
sight of in its timid excursions by the very finest telescopes. Like
all the Saturnian moons, except Titan, and, by a rare conjunction,
Japetus, it is far too much contracted to be visible in transit
across the disk.