_PLANETS AND SATELLITES_--(_continued_)
"The analogy between Mars and the earth is perhaps by far the greatest
in the whole solar system." So Herschel wrote in 1783,[965] and so we
may safely say to-day, after six score further years of scrutiny. The
circumstance lends a particular interest to inquiries into the physical
habitudes of our exterior planetary neighbour.
Fontana first caught glimpses, at Naples in 1636 and 1638,[966] of dusky
stains on the ruddy disc of Mars. They were next seen by Hooke and
Cassini in 1666, and this time with sufficient distinctness to serve as
indexes to the planet's rotation, determined by the latter as taking
place in a period of twenty-four hours forty minutes.[967] Increased
confidence was given to this result through Maraldi's precise
verification of it in 1719.[968] Among the spots observed by him, he
distinguished two as stable in position, though variable in size. They
were of a peculiar character, showing as bright patches round the poles,
and had already been noticed during sixty years back. A current
conjecture of their snowy nature obtained validity when Herschel
connected their fluctuations in extent with the progress of the Martian
seasons. The inference of frozen precipitations could scarcely be
resisted when once it was clearly perceived that the shining polar zones
did actually by turns diminish and grow with the alternations of summer
and winter in the corresponding hemisphere.
This, it may be said, was the opening of our acquaintance with the state
of things prevailing on the surface of Mars. It was accompanied by a
steady assertion, on Herschel's part, of permanence in the dark
markings, notwithstanding partial obscurations by clouds and vapours
floating in a "considerable but moderate atmosphere." Hence the presumed
inhabitants of the planet were inferred to "probably enjoy a situation
in many respects similar to ours."[969]
Schröter, on the other hand, went altogether wide of the truth as
regards Mars. He held that the surface visible to us is a mere shell of
drifting cloud, deriving a certain amount of apparent stability from the
influence on evaporation and condensation of subjacent but unseen
areographical features;[970] and his opinion prevailed with his
contemporaries. It was, however, rejected by Kunowsky in 1822, and
finally overthrown by Beer and Mädler's careful studies during five
consecutive oppositions, 1830-39. They identified at each the same dark
spots, frequently blurred with mists, especially when the local winter
prevailed, but fundamentally unchanged.[971] In 1862 Lockyer established
a "marvellous agreement" with Beer and Mädler's results of 1830, leaving
no doubt as to the complete fixity of the main features, amid "daily,
nay, hourly," variations of detail through transits of clouds.[972] On
seventeen nights of the same opposition, F. Kaiser of Leyden obtained
drawings in which nearly all the markings noted in 1830 at Berlin
reappeared, besides spots frequently seen respectively by Arago in 1813,
by Herschel in 1783, and one sketched by Huygens in 1672 with a
writing-pen in his diary.[973] From these data the Leyden observer
arrived at a period of rotation of 24h. 37m. 22·62s., being just one
second shorter than that deduced, exclusively from their own
observations, by Beer and Mädler. The exactness of this result was
practically confirmed by the inquiries of Professor Bakhuyzen of
Leyden.[974] Using for a middle term of comparison the disinterred
observations of Schröter, with those of Huygens at one, and of
Schiaparelli at the other end of an interval of 220 years, he was
enabled to show, with something like certainty, that the time of
rotation (24h. 37m. 22·735s.) ascribed to Mars by Mr. Proctor[975] in
reliance on a drawing executed by Hooke in 1666, was too long by _nearly
one-tenth of a second_. The minuteness of the correction indicates the
nicety of care employed. Nor employed vainly; for, owing to the
comparative antiquity of the records available in this case, an almost
infinitesimal error becomes so multiplied by frequent repetition as to
produce palpable discrepancies in the positions of the markings at
distant dates. Hence Bakhuyzen's period of 24h. 37m. 22·66s. is
undoubtedly of a precision unapproached as regards any other heavenly
body save the earth itself.
Two facts bearing on the state of things at the surface of Mars were,
then, fully acquired to science in or before the year 1862. The first
was that of the seasonal fluctuations of the polar spots; the second,
that of the general permanence of certain dark gray or greenish patches,
perceived with the telescope as standing out from the deep yellow ground
of the disc. That these varieties of tint correspond to the real
diversities of a terraqueous globe, the "ripe cornfield"[976] sections
representing land, the dusky spots and streaks, oceans and straits, has
long been the prevalent opinion. Sir J. Herschel in 1830 led the way in
ascribing the redness of the planet's light to an inherent peculiarity
of soil.[977] Previously it had been assimilated to our sunset glows
rather than to our red sandstone formations--set down, that is, to an
atmospheric stoppage of blue rays. But the extensive Martian atmosphere,
implicitly believed in on the strength of some erroneous observations by
Cassini and Römer in the seventeenth century, vanished before the sharp
occultation of a small star in Leo, witnessed by Sir James South in
1822;[978] and Dawes's observation in 1865,[979] that the ruddy tinge is
deepest near the central parts of the disc, certified its
non-atmospheric origin. The absolute whiteness of the polar snow-caps
was alleged in support of the same inference by Sir William Huggins in
1867.[980]
All recent operations tend to show that the atmosphere of Mars is much
thinner than our own. This was to have been expected _à priori_, since
the same proportionate mass of air would on his smaller globe form a
relatively sparse covering.[981] Besides, gravity there possesses less
than four-tenths its force here, so that this sparser covering would
weigh less, and be less condensed, than if it enveloped the earth.
Atmospheric pressure would accordingly be of about two and a quarter,
instead of fifteen terrestrial pounds per square inch. This corresponds
with what the telescope shows us. It is extremely doubtful whether any
features of the earth's actual surface could be distinguished by a
planetary spectator, however well provided with optical assistance.
Professor Langley's inquiries[982] led him to conclude that fully twice
as much light is absorbed by our air as had previously been
supposed--say 40 per cent. of vertical rays in a clear sky. Of the sixty
reaching the earth, less than a quarter would be reflected even from
white sandstone; and this quarter would again pay heavy toll in escaping
back to space. Thus not more than perhaps ten or twelve out of the
original hundred sent by the sun would, under the most favourable
circumstances, and from the very centre of the earth's disc, reach the
eye of a Martian or lunar observer. The light by which he views our
world is, there is little doubt, light reflected from the various strata
of our atmosphere, cloud or mist-laden or serene, as the case may be,
with an occasional snow-mountain figuring as a permanent white spot.
This consideration at once shows us how much more tenuous the Martian
air must be, since it admits of topographical delineations of the
Martian globe. The clouds, too, that form in it seem in general to be
rather of the nature of ground-mists than of heavy cumulus.[983]
Occasionally, indeed, durable and extensive strata become visible.
During the latter half of October, 1894, for instance, a region as large
as Europe remained apparently cloud-covered. Yet most recent observers
are unable to detect the traces of aqueous absorption in the Martian
spectrum noted by Huggins in 1867[984] and by Vogel in 1873.[985]
Campbell vainly looked for them,[986] visually in 1894,
spectrographically in 1896; Keeler was equally unsuccessful;[987]
Jewell[988] holds that they could, with present appliances, only be
perceived if the atmosphere of Mars were much richer in water-vapour
than that of the earth. There can be little doubt, however, that its
supply is about the minimum adequate to the needs of a _living_, and
perhaps a life-nuturing planet.
The climate of Mars seems to be unexpectedly mild. Its _theoretical_
mean temperature, taking into account both distance from the sun and
albedo, is 34° C. below freezing.[989] Yet its polar snows are both less
extensive and less permanent than those on the earth. The southern white
hood, noticed by Schiaparelli in 1877 to have survived the summer only
as a small lateral patch, melted completely in 1894. Moreover, Mr. W. H.
Pickering observed with astonishment the disappearance, in the course of
thirty-three days of June and July, 1892, of 1,600,000 square miles of
southern snow.[990] Curiously enough, the initial stage of shrinkage in
the white calotte was marked by its division into two unequal parts, as
if in obedience to the mysterious principle of duplication governing so
many Martian phenomena.[991] Changes of the hues associated respectively
with land and water accompanied in lower latitudes, and were thought to
be occasioned by floods ensuing upon this rapid antarctic thaw. It is
true that scarcity of moisture would account for the scantiness and
transitoriness of snowy deposits easily liquefied because thinly spread.
But we might expect to see the whole wintry hemisphere, at any rate,
frost-bound, since the sun radiates less than half as much heat on Mars
as on the earth. Water seems, nevertheless, to remain, as a rule,
uncongealed everywhere outside the polar regions. We are at a loss to
imagine by what beneficent arrangement the rigorous conditions naturally
to be looked for can be modified into a climate which might be found
tolerable by creatures constituted like ourselves.
Martian topography may be said to form nowadays a separate
sub-department of descriptive astronomy. The amount of detail become
legible by close scrutiny on a little disc which, once in fifteen years,
attains a maximum of about 1/5000 the area of the full moon, must excite
surprise and might provoke incredulity. Spurious discoveries, however,
have little chance of holding their own where there are so many
competitors quite as ready to dispute as to confirm.
The first really good map of Mars was constructed in 1869 by Proctor
from drawings by Dawes. Kaiser of Leyden followed in 1872 with a
representation founded upon data of his own providing in 1862-64; and
Terby, in his valuable _Aréographie_, presented to the Brussels Academy
in 1873[992] a careful discussion of all important observations from the
time of Fontana downwards, thus virtually adding to knowledge by
summarising and digesting it. The memorable opposition of September 5,
1877, marked a fresh epoch in the study of Mars. While executing a
trigonometrical survey (the first attempted) of the disc, then of the
unusual size of 25" across, G. V. Schiaparelli, director of the Milan
Observatory, detected a novel and curious feature. What had been taken
for Martian continents were found to be, in point of fact,
agglomerations of islands, separated from each other by a network of
so-called "canals" (more properly _channels_).[993] These are obviously
extensions of the "seas," originating and terminating in them, and
sharing their gray-green hue, but running sometimes to a length of three
or four thousand miles in a straight line, and preserving throughout a
nearly uniform breadth of about sixty miles. Further inquiries have
fully substantiated the discovery made at the Brera Observatory. The
"canals" of Mars are an actually existent and permanent phenomenon. An
examination of the drawings in his possession showed M. Terby that they
had been seen, though not distinctively recognised, by Dawes, Secchi,
and Holden; several were independently traced out by Burton at the
opposition of 1879; all were recovered by Schiaparelli himself in 1879
and 1881-82; and their indefinite multiplication resulted from Lovell's
observations in 1894 and 1896.
When the planet culminated at midnight, and was therefore in opposition,
December 26, 1881, its distance was greater, and its apparent diameter
less than in 1877, in the proportion of sixteen to twenty-five. Its
atmosphere was, however, more transparent, and ours of less impediment
to northern observers, the object of scrutiny standing considerably
higher in northern skies. Never before, at any rate, had the true aspect
of Mars come out so clearly as at Milan, with the 8-3/4-inch Merz
refractor of the observatory, between December, 1881, and February,
1882. The canals were all again there, but this time they were--in as
many as twenty cases--_seen in duplicate_. That is to say, a twin-canal
ran parallel to the original one at an interval of 200 to 400
miles.[994]
We are here brought face to face with an apparently insoluble enigma.
Schiaparelli regards the "germination" of his canals as a periodical
phenomenon depending on the Martian seasons. It is, assuredly, not an
illusory one, since it was plainly apparent, during the opposition of
1886, to MM. Perrotin and Thollon at Nice,[995] and to the former, using
the new 30-inch refractor of that observatory, in 1888; Mr. A. Stanley
Williams, with the help of only a 6-1/2-inch reflector, distinctly
perceived in 1890 seven of the duplicate objects noted at Milan,[996]
and the Lick observations, both of 1890 and of 1892, together with the
drawings made at Flagstaff and Mexico during the last favourable
oppositions of the nineteenth century, brought unequivocal confirmation
to the accuracy of Schiaparelli's impressions.[997] Various conjectures
have been hazarded in explanation of this bizarre appearance. The
difficulty of conceiving a physical reality corresponding to it has
suggested recourse to an optical rationale. Proctor regarded it as an
effect of diffraction;[998] Stanislas Meunier, of oblique reflection
from overlying mist-banks;[999] Flammarion considers it possible that
companion-canals might, under special circumstances, be evoked by
refraction as a kind of mirage.[1000] But none of these speculations are
really admissible, when all the facts are taken into account. The view
that the canals of Mars are vast rifts due to the cooling of the globe,
is recommended by the circumstance that they tend to follow great
circles; nevertheless, it would break down if, as Schiaparelli holds,
the fluctuations in their visibility depend upon actual obliterations
and re-emergencies. Fantastic though the theory of their artificial
origin appear, it is held by serious astronomers. Its vogue is largely
due to Mr. Lowell's ingenious advocacy. He considers the Martian globe
to be everywhere intersected by an elaborate system of irrigation-works,
rendered necessary by a perennial water-famine, relieved periodically by
the melting of the polar snows. Nor does he admit the existence of
oceans, or lakes. What have been taken for such are really tracts
covered with vegetation, the bright areas intermixed with them
representing sandy deserts. And it is noteworthy in this connection that
Professor Barnard obtained in 1894,[1001] with the great Lick refractor,
"suggestive and impressive views" disclosing details of light and shade
on the gray-green patches so intricate and minute as almost to preclude
the supposition of their aqueous nature.
The closeness of the terrestrial analogy has thus of late been much
impaired. Even if the surface of Mars be composed of land and water,
their distribution must be of a completely original type. The
interlacing everywhere of continents with arms of the sea (if that be
the correct interpretation of the visual effects) implies that their
levels scarcely differ;[1002] and Schiaparelli carries most observers
with him in holding that their outlines are not absolutely constant,
encroachments of dusky upon bright tints suggesting extensive
inundations.[1003] The late N. E. Green's observations at Madeira in
1877 indicated, on the other hand, a rugged south polar region. The
contour of the snow-cap not only appeared indented, as if by valleys and
promontories, but brilliant points were discerned outside the white
area, attributed to isolated snow-peaks.[1004] Still more elevated, if
similarly explained, must be the "ice island" first seen in a
comparatively low latitude by Dawes in January, 1865.
On August 4, 1892, Mars stood opposite to the sun at a distance of only
34,865,000 miles from the earth. In point of vicinity, then, its
situation was scarcely less favourable than in 1877. The low altitude of
the planet, however, practically neutralised this advantage for northern
observers, and public expectation, which had been raised to the highest
pitch by the announcements of sensation-mongers, was somewhat
disappointed at the "meagreness" of the news authentically received from
Mars. Valuable series of observations were, nevertheless, made at Lick
and Arequipa; and they unite in testifying to the genuine prevalence of
surface-variability, especially in certain regions of intermediate tint,
and perhaps of the "crude consistence" of "boggy Syrtes, neither sea,
nor good dry land." Professor Holden insisted on the "enormous
difficulties in the way of completely explaining the recorded phenomena
by terrestrial analogies";[1005] Mr. W. H. Pickering spoke of
"conspicuous and startling changes." They, however, merely overlaid, and
partially disguised, a general stability. Among the novelties detected
by Mr. Pickering were a number of "lakes," or "oases" (in Lowell's
phraseology), under the aspect of black dots at the junctions of two or
more canals;[1006] and he, no less than the Lick astronomers and M.
Perrotin at Nice,[1007] observed brilliant clouds projecting beyond the
terminator, or above the limb, while carried round by the planet's
rotation. They seemed to float at an altitude of at least twenty miles,
or about four times the height of terrestrial cirrus; but this was not
wonderful, considering the low power of gravity acting upon them. Great
capital was made in the journalistic interest out of these imaginary
signals from intelligent Martians, desirous of opening communications
with (to them) problematical terrestrial beings. Similar effects had,
however, been seen before by Mr. Knobel in 1873, by M. Terby in 1888,
and at the Lick Observatory in 1890; and they were discerned again with
particular distinctness by Professor Hussey at Lick, August 27,
1896.[1008]
The first photograph of Mars was taken by Gould at Cordoba in 1879.
Little real service in planetary delineation has, it is true, been so
far rendered by the art, yet one achievement must be recorded to its
credit. A set of photographs obtained by Mr. W. H. Pickering on Wilson's
Peak, California, April 9, 1890, showed the southern polar cap of Mars
as of moderate dimensions, but with a large dim adjacent area.
Twenty-four hours later, on a corresponding set, the dim area was
brilliantly white. The polar cap had become enlarged in the interim,
apparently through a wide-spreading snow-fall, by the annexation of a
territory equal to that of the United States. The season was towards the
close of winter in Mars. Never until then had the process of glacial
extension been actually (it might be said) superintended in that distant
globe.
Mars was gratuitously supplied with a pair of satellites long before he
was found actually to possess them. Kepler interpreted Galileo's anagram
of the "triple" Saturn in this sense; they were perceived by Micromégas
on his long voyage through space; and the Laputan astronomers had even
arrived at a knowledge, curiously accurate under the circumstances, of
their distances and periods. But terrestrial observers could see nothing
of them until the night of August 11, 1877. The planet was then within
one month of its second nearest approach to the earth during the last
century; and in 1845 the Washington 26-inch refractor was not in
existence.[1009] Professor Asaph Hall, accordingly, determined to turn
the conjecture to account for an exhaustive inquiry into the
surroundings of Mars. Keeping his glaring disc just outside the field of
view, a minute attendant speck of light was "glimpsed" August 11. Bad
weather, however, intervened, and it was not until the 16th that it was
ascertained to be what it appeared--a satellite. On the following
evening a second, still nearer to the primary, was discovered, which, by
the bewildering rapidity of its passages hither and thither, produced at
first the effect of quite a crowd of little moons.[1010]
Both these delicate objects have since been repeatedly observed, both in
Europe and America, even with comparatively small instruments. At the
opposition of 1884, indeed, the distance of the planet was too great to
permit of the detection of both elsewhere than at Washington. But the
Lick equatoreal showed them, July 18, 1888, when their brightness was
only 0·12 its amount at the time of their discovery; so that they can
now be followed for a considerable time before and after the least
favourable oppositions.
The names chosen for them were taken from the Iliad, where "Deimos" and
"Phobos" (Fear and Panic) are represented as the companions in battle of
Ares. In several respects, they are interesting and remarkable bodies.
As to size, they may be said to stand midway between meteorites and
satellites. From careful photometric measures executed at Harvard in
1877 and 1879, Professor Pickering concluded their diameters to be
respectively six and seven miles.[1011] This is on the assumption that
they reflect the same proportion of the light incident upon them that
their primary does. But it may very well be that they are less
reflective, in which case they would be more extensive. The albedo of
Mars is put by Müller at 0·27; his surface, in other words, returns 27
per cent. of the rays striking it. If we put the albedo of his
satellites equal to that of our moon, 0·17, their diameters will be
increased from 6 and 7 to 7-1/2 and 9 miles, Phobos, the inner one,
being the larger. Mr. Lowell, however, formed a considerably larger
estimate of their dimensions.[1012] It is interesting to note that
Deimos, according to Professor Pickering's very distinct perception,
does not share the reddish tint of Mars.
Deimos completes its nearly circular revolutions in thirty hours
eighteen minutes, at a distance from the surface of its ruling body of
12,500 miles; Phobos traverses an elliptical orbit[1013] in seven hours
thirty-nine minutes twenty-two seconds, at a distance of only 3,760
miles. This is the only known instance of a satellite circulating faster
than its primary rotates, and is a circumstance of some importance as
regards theories of planetary development. To a Martian spectator the
curious effect would ensue of a celestial object, seemingly exempt from
the general motion of the sphere, rising in the west, setting in the
east, and culminating twice, or even thrice a day; which, moreover, in
latitudes above 69° north or south, would be permanently and altogether
hidden by the intervening curvature of the globe.
* * * * *
The detection of new members of the solar system has come to be one of
the most ordinary of astronomical events. Since 1846 no single year has
passed without bringing its tribute of asteroidal discovery. In the last
of the seventies alone, a full score of miniature planets were
distinguished from the thronging stars amid which they seem to move;
1875 brought seventeen such recognitions; their number touched a minimum
of one in 1881; it rose in 1882, and again in 1886, to eleven; dropped
to six in 1889, and sprang up with the aid of photography to
twenty-seven in 1892. That high level has since, on an average, been
maintained; and on January 1, 1902, nearly 500 asteroids were recognised
as revolving between the orbits of Mars and Jupiter. Of these,
considerably more than one hundred are claimed by one investigator
alone--Dr. Max Wolf of Heidelburg; M. Charlois of Nice comes second with
102; while among the earlier observers Palisa of Vienna contributed 86,
and C. H. F. Peters of Clinton (N. Y.), whose varied and useful career
terminated July 19, 1890, 52 to the grand total. The construction by
Chacornac and his successors at Paris, and more recently by Peters at
Clinton, of ecliptical charts showing all stars down to the thirteenth
and fourteenth magnitudes respectively, rendered the picking out of moving
objects above that brightness a mere question of time and diligence. Both,
however, are vastly economised by the photographic method. Tedious
comparisons of the sky with charts are no longer needed for the
identification of unrecorded, because simulated stars. Planetary bodies
declare themselves by appearing upon the plate, not in circular, but in
linear form. Their motion converts their images into trails, long or short
according to the time of exposure. The first asteroid (No. 323) thus
detected was by Max Wolf, December 22, 1891.[1014] Eighteen others were
similarly discovered in 1892, by the same skilful operator; and ten more
through Charlois's adoption at Nice of the novel plan now in exclusive use
for picking up errant light-specks. Far more onerous than the task of their
discovery is that of keeping them in view once discovered--of tracking out
their paths, ixing their places, and calculating the disturbing effects
upon them of the mighty Jovian mass. These complex operations have come to
be centralised at Berlin under the superintendence of Professor Tietjen,
and their results are given to the public through the medium of the
_Berliner Astronomisches Jahrbuch_.
The _cui bono?_ however, began to be agitated. Was it worth while to
maintain a staff of astronomers for the sole purpose of keeping hold
over the identity of the innumerable component particles of a cosmical
ring? The prospect, indeed, of all but a select few of the asteroids
being thrown back by their contemptuous captors into the sea of space
seemed so imminent that Professor Watson provided by will against the
dereliction of the twenty-two discovered by himself. But the fortunes of
the whole family improved through the distinction obtained by one of
them. On August 14, 1898, the trail of a rapidly-moving, star-like
object of the eleventh magnitude imprinted itself on a plate exposed by
Herr Witt at the Urania Observatory, Berlin. Its originator proved to be
unique among asteroids. "Eros" is, in sober fact,
'one of those mysterious stars
Which hide themselves between the Earth and Mars,'
divined or imagined by Shelley.[1015] True, several of its congeners
invade the Martian sphere at intervals; but the proper habitat of Eros
is within that limit, although its excursions transcend it. In other
words, its mean distance from the sun is about 135, as compared with the
Martian distance of 141 million miles. Further, its orbit being so
fortunately circumstanced as to bring it once in sixty-seven years
within some 15 millions of miles of the earth, it is of extraordinary
value to celestial surveyors. The calculation of its movements was much
facilitated by detections, through a retrospective search,[1016] of many
of its linear images among the star-dots on the Harvard plates.[1017]
The little body--which can scarcely be more than twenty miles in
diameter--shows peculiarities of behaviour as well as of position. Dr.
von Oppolzer, in February, 1901,[1018] announced it to be extensively
and rapidly variable. Once in 2 hours 38 minutes it lost about
three-fourths of its light,[1019] but these fluctuations quickly
diminished in range, and in the beginning of May ceased
altogether.[1020] Evidently, then, they depend upon the situation of the
asteroid relatively to ourselves; and, so far, events lent countenance
to M. André's eclipse hypothesis, since mutual occultations of the
supposed planetary twins could only take place when the plane of their
revolutions passed through the earth, and this condition would be
transitory. Yet the recognition in Eros of an "Algol asteroid" seems on
other grounds inadmissible;[1021] nor until the phenomenon is
conspicuously renewed--as it probably will be at the opposition of
1903--can there be much hope of finding its appropriate rationale.
The crowd of orbits disclosed by asteroidal detections invites attentive
study. D'Arrest remarked in 1851,[1022] when only thirteen minor planets
were known, that supposing their paths to be represented by solid hoops,
not one of the thirteen could be lifted from its place without bringing
the others with it. The complexity of interwoven tracks thus illustrated
has grown almost in the numerical proportion of discovery. Yet no two
actually intersect, because no two lie exactly in the same plane, so
that the chances of collision are at present _nil_. There is only one
case, indeed, in which it seems to be eventually possible. M. Lespiault
has pointed out that the curves traversed by "Fidés" and "Maïa" approach
so closely that a time may arrive when the bodies in question will
either coalesce or unite to form a binary system.[1023]
The maze threaded by the 500 asteroids contrasts singularly with the
harmoniously ordered and rhythmically separated orbits of the larger
planets. Yet the seeming confusion is not without a plan.
The established rules of our system are far from being totally
disregarded by its minor members. The orbit of Pallas, with its
inclination of 34° 42', touches the limit of departure from the ecliptic
level; the average obliquity of the asteroidal paths is somewhat less
than that of the sun's equator;[1024] their mean eccentricity is below
that of the curve traced out by Mercury, and all without exception are
pursued in the planetary direction--from west to east.
The zone in which these small bodies travel is about three times as wide
as the interval separating the earth from the sun. It extends perilously
near to Jupiter, and dovetails into the sphere of Mars.
Their distribution is very unequal. They are most densely congregated
about the place where a single planet ought, by Bode's Law, to revolve;
it may indeed be said that only stragglers from the main body are found
more than fifty million miles within or without a mean distance from the
sun 2·8 times that of the earth. Significant gaps, too, occur where some
force prohibitive of their presence would seem to be at work. The
probable nature of that force was suggested by the late Professor
Kirkwood, first in 1866, when the number of known asteroids was only
eighty-eight, and again with more confidence in 1876, from the study of
a list then run up to 172.[1025] It appears that these bare spaces are
found just where a revolving body would have a period connected by a
simple relation with that of Jupiter. It would perform two or three
circuits to his one, five to his two, nine to his five, and so on.
Kirkwood's inference was that the gaps in question were cleared of
asteroids by the attractive influence of Jupiter. For disturbances
recurring time after time--owing to commensurability of periods--nearly
at the same part of the orbit, would have accumulated until the shape of
that orbit was notably changed. The body thus displaced would have come
in contact with other cosmical particles of the same family with
itself--then, it may be assumed, more evenly scattered than now--would
have coalesced with them, and permanently left its original track. In
this way the regions of maximum perturbation would gradually have become
denuded of their occupants.
We can scarcely doubt that this law of commensurability has largely
influenced the present distribution of the asteroids. But its effects
must have been produced while they were still in an unformed, perhaps a
nebular condition. In a system giving room for considerable modification
through disturbance, the recurrence of conjunctions with a dominating
mass at the same orbital point need not involve instability.[1026] On
the whole, the correspondence of facts with Kirkwood's hypothesis has
not been impaired by their more copious collection.[1027] Some chasms of
secondary importance have indeed been bridged; but the principal stand
out more conspicuously through the denser scattering of orbits near
their margins. Nor is it doubtful that the influence of Jupiter in some
way produced them. M. de Freycinet's study of the problem they
present[1028] has, however, led him to the conclusion that they existed
_ab origine_, thus testifying rather to the preventive than to the
perturbing power of the giant planet.
The existence, too, of numerous asteroidal pairs travelling in
approximately coincident tracks, must date from a remote antiquity. They
result, Professor Kirkwood[1029] believed, from the divellent action of
Jupiter upon embryo pigmy planets, just as comets moving in pursuit of
one another are a consequence of the sundering influence of the sun.
Leverrier fixed, in 1853,[1030] one-fourth of the earth's mass as the
outside limit for the combined masses of all the bodies circulating
between Mars and Jupiter; but it is far from probable that this maximum
is at all nearly approached. M. Berberich[1031] held that the moon would
more than outweigh the whole of them, a million of the lesser bodies
shining like stars of the twelfth magnitude being needed, according to
his judgment, to constitute her mass. And M. Niesten estimated that the
whole of the 216 asteroids discovered up to August, 1880, amounted in
_volume_ to only 1/4000th of our globe,[1032] and we may safely
add--since they are tolerably certain to be lighter, bulk for bulk, than
the earth--that their proportionate _mass_ is smaller still. A fairly
concordant result was published in 1895 by Mr. B. M. Roszel.[1033] He
found that the lunar globe probably contains forty times, the
terrestrial globe 3,240 times the quantity of matter parcelled out among
the first 311 minor planets. The actual size of a few of them may now be
said to be known. Professor Pickering, from determinations of
light-intensity, assigned to Vesta a diameter of 319 miles, to Pallas
167, to Juno 94, down to twelve and fourteen for the smaller members of
the group.[1034] An albedo equal to that of Mars was assumed as the
basis of the calculation. Moreover, Professor G. Müller[1035] of Potsdam
examined photometrically the phases of seven among them, of which
four--namely, Vesta, Iris, Massalia, and Amphitrite--were found to
conform precisely to the behaviour of Mars as regards light-change from
position, while Ceres, Pallas, and Irene varied after the manner of the
moon and Mercury. The first group were hence inferred to resemble Mars
in physical constitution, nature of atmosphere, and reflective capacity;
the second to be moon-like bodies.
Finally, Professor Barnard, directly measuring with the Yerkes refractor
the minute discs presented by the original quartette, obtained the
following authentic data concerning them:[1036] Diameter of Ceres, 477
miles, albedo = 0·18; diameter of Pallas, 304 miles, albedo = 0·23;
diameter of Vesta, 239 miles, albedo = 0·74; diameter of Juno, 120
miles, albedo = 0·45. Thus, the rank of premier asteroid proves to
belong to Ceres, and to have been erroneously assigned to Vesta in
consequence of its deceptive brilliancy. What kind of surface this
indicates, it is hard to say. The dazzling whiteness of snow can hardly
be attributed to bare rock; yet the dynamical theory of gases--as Dr.
Johnstone Stoney pointed out in 1867[1037]--prohibits the supposition
that bodies of insignificant gravitative power can possess aerial
envelopes. Even our moon, it is calculated, could not permanently hold
back the particles of oxygen, nitrogen, or water-gas from escaping into
infinite space; still less, a globe one thousand times smaller. Vogel's
suspicion of an air-line in the spectrum of Vesta[1038] has,
accordingly, not been confirmed.
* * * * *
Crossing the zone of asteroids on our journey outward from the sun, we
meet with a group of bodies widely different from the "inferior" or
terrestrial planets. Their gigantic size, low specific gravity, and
rapid rotation, obviously from the first threw the "superior" planets
into a class apart; and modern research has added qualities still more
significant of a dissimilar physical constitution. Jupiter, a huge globe
86,000 miles in diameter, stands pre-eminent among them. He is, however,
only _primus inter pares_; all the wider inferences regarding his
condition may be extended, with little risk of error, to his fellows;
and inferences in his case rest on surer grounds than in the case of the
others, from the advantages offered for telescopic scrutiny by his
comparative nearness.
Now the characteristic modern discovery concerning Jupiter is that he is
a body midway between the solar and terrestrial stages of cosmical
existence--a decaying sun or a developing earth, as we choose to put
it--whose vast unexpended stores of internal heat are mainly, if not
solely, efficient in producing the interior agitations betrayed by the
changing features of his visible disc. This view, impressed upon modern
readers by Mr. Proctor's popular works, was anticipated in the last
century. Buffon wrote in his _Époques de la Nature_ (1778):[1039]--"La
surface de Jupiter est, comme l'on sait, sujette à des changemens
sensibles, qui semblent indiquer que cette grosse planète est encore
dans un état d'inconstance et de bouillonnement."
Primitive incandescence, attendant, in his fantastic view, on planetary
origin by cometary impacts with the sun, combined, he concluded, with
vast bulk to bring about this result. Jupiter has not yet had time to
cool. Kant thought similarly in 1785;[1040] but the idea did not commend
itself to the astronomers of the time, and dropped out of sight until
Mr. Nasmyth arrived at it afresh in 1853.[1041] Even still, however,
terrestrial analogies held their ground. The dark belts running parallel
to the equator, first seen at Naples in 1630, continued to be
associated--as Herschel had associated them in 1781--with Jovian
trade-winds, in raising which the deficient power of the sun was
supposed to be compensated by added swiftness of rotation. But opinion
was not permitted to halt here.
In 1860 G. P. Bond of Cambridge (U.S.) derived some remarkable
indications from experiments on the light of Jupiter.[1042] They showed
that fourteen times more of the photographic rays striking it are
reflected by the planet than by our moon, and that, unlike the moon,
which sends its densest rays from the margin, Jupiter is brightest near
the centre. But the most perplexing part of his results was that Jupiter
actually seemed to give out more light than he received. Bond, however,
rightly considered his data too uncertain for the support of so bold an
assumption as that of original luminosity, and, even if the presence of
native light were proved, thought that it might emanate from auroral
clouds of the terrestrial kind. The conception of a sun-like planet was
still a remote, and seemed an extravagant one.
Only since it was adopted and enforced by Zöllner in 1865,[1043] can it
be regarded as permanently acquired to science. The rapid changes in the
cloud-belts both of Jupiter and Saturn, he remarked, attest a high
internal temperature. For we know that all atmospheric movements on the
earth are sun-heat transformed into motion. But sun-heat at the distance
of Jupiter possesses but 1/27, at that of Saturn 1/100 of its force
here. The large amount of energy, then, obviously exerted in those
remote firmaments must have some other source, to be found nowhere else
than in their own active and all-pervading fires, not yet banked in with
a thick solid crust.
The same acute investigator dwelt, in 1871,[1044] on the similarity
between the modes of rotation of the great planets and of the sun,
applying the same principles of explanation to each case. The fact of
this similarity is undoubted. Cassini[1045] and Schröter both noticed
that markings on Jupiter travelled quicker the nearer they were to his
equator; and Cassini even hinted at their possible assimilation to
sun-spots.[1046] It is now well ascertained that, as a rule (not without
exceptions), equatorial spots give a period some 5-1/2 minutes shorter
than those in latitudes of about 30°. But, as Mr. Denning has pointed
out,[1047] no single period will satisfy the observations either of
different markings at the same epoch, or of the same markings at
different epochs. Accelerations and retardations, depending upon
processes of growth or change, take place in very much the same kind of
way as in solar maculæ, inevitably suggesting similarity of origin.
The interesting query as to Jupiter's surface incandescence has been
studied since Bond's time with the aid of all the appliances furnished
to physical inquirers by modern inventiveness, yet without bringing to
it a categorical reply. Zöllner in 1865, Müller in 1893, estimated his
albedo at 0·62 and 0·75 respectively, that of fresh-fallen snow being
0·78, and of white paper 0·70.[1048] But the disc of Jupiter is by no
means purely white. The general ground is tinged with ochre; the polar
zones are leaden or fawn coloured; large spaces are at times stained or
suffused with chocolate-browns and rosy hues. It is occasionally seen
ruled from pole to pole with dusky bars, and is never wholly free from
obscure markings. The reflection, then, by it, as a whole, of about 70
per cent. of the rays impinging upon it, might well suggest some
original reinforcement.
Nevertheless, the spectroscope gives little countenance to the
supposition of any considerable permanent light-emission. The spectrum
of Jupiter, as examined by Huggins, 1862-64, and by Vogel, 1871-73,
shows the familiar Fraunhofer rays belonging to reflected sunlight. But
it also shows lines of native absorption. Some of these are identical
with those produced by the action of our own atmosphere, especially one
or more groups due to aqueous vapours; others are of unknown origin;
and it is remarkable that one among the latter--a strong band in the
red--agrees in position with a dark line in the spectra of some ruddy
stars.[1049] There is, besides, a general absorption of blue rays,
intensified--as Le Sueur observed at Melbourne in 1869[1050]--in the
dusky markings, evidently through an increase of depth in the atmospheric
strata traversed by the light proceeding from them.
All these observations, however (setting aside the stellar line as of
doubtful significance), point to a cool planetary atmosphere. One
spectrograph, it is true, taken by Dr. Henry Draper, September 27,
1879,[1051] seemed to attest the action of intrinsic light; but the
peculiarity was referred by Dr. Vogel, with convincing clearness, to a
flaw in the film.[1052] So far, then, native emissions from any part of
Jupiter's diversified surface have not been detected; and, indeed, the
blackness of the shadows cast by his satellites on his disc sufficiently
proves that he sends out virtually none but reflected light.[1053] This
conclusion, however, by no means invalidates that of his high internal
temperature.
The curious phenomena attending Jovian satellite-transits may be
explained, partly as effects of contrast, partly as due to temporary
obscurations of the small discs projected on the large disc of Jupiter.
At their first entry upon its marginal parts, which are several times
less luminous than those near the centre, they invariably show as bright
spots, then usually vanish as the background gains lustre, to reappear,
after crossing the disc, thrown into relief, as before, against the
dusky limb. But instances are not rare, more especially of the third and
fourth satellites standing out, during the entire middle part of their
course, in such inky darkness as to be mistaken for their own shadows.
The earliest witness of a "black transit" was Cassini, September 2,
1665; Römer in 1677, and Maraldi in 1707 and 1713, made similar
observations, which have been multiplied in recent years. In some cases
the process of darkening has been visibly attended by the formation, or
emergence into view, of spots on the transiting body, as noted by the
two Bonds at Harvard, March 18, 1848.[1054] The third satellite was seen
by Dawes, half dark, half bright, when crossing Jupiter's disc, August
21, 1867;[1055] one-third dark by Davidson of California, January 15,
1884, under the same circumstances;[1056] and unmistakably spotted, both
on and off the planet, by Schröter, Secchi, Dawes, and Lassell.
The first satellite sometimes looks dusky, but never absolutely black,
in travelling over the disc of Jupiter. The second appears uniformly
white--a circumstance attributed by Dr. Spitta[1057] to its high albedo.
The singularly different aspects, even during successive transits, of
the third and fourth satellites, are connected by Professor Holden[1058]
with the varied luminosity of the segments of the planetary surface they
are projected upon, and W. H. Pickering inclines to the same opinion;
but fluctuations in their own brightness[1059] may be a concurrent
cause. Herschel concluded in 1797 that, like our moon, they always turn
the same face towards their primary, and as regards the outer satellite,
Engelmann's researches in 1871, and C. E. Burton's in 1873, made this
almost certain; while both for the third and fourth Jovian moons it was
completely assured by W. H. Pickering's and A. E. Douglass's
observations at Arequipa in 1892,[1060] and at Flagstaff in
1894-95.[1061] Strangely enough, however, the interior members of the
system have preserved a relatively swift rotation, notwithstanding the
enormous checking influence upon it of Jove-raised tides.
All the satellites are stated, on good authority, to be more or less
egg-shaped. On September 8, 1890, Barnard saw the first elongated and
bisected by a bright equatorial belt, during one of its dark
transits;[1062] and his observation, repeated August 3, 1891, was
completely verified by Schaeberle and Campbell, who ascertained,
moreover, that the longer axis of the prolate body was directed towards
Jupiter's centre.[1063] The ellipticity of its companions was determined
by Pickering and Douglass; indeed, that of No. 3 had long previously
been noticed by Secchi.[1064] No. 3 also shows equatorial stripes,
perceived in 1891 by Schaeberle and Campbell,[1065] and evident later to
Pickering and Douglass;[1066] nor need we hesitate to admit as authentic
their records of similar, though less conspicuous markings on the other
satellites. A constitution analogous to that of Jupiter himself was thus
unexpectedly suggested; and Vogel's detection of lines--or traces of
lines--in their spectra, agreeing with absorption-rays derived from
their primary, lends support to the conjecture that they possess gaseous
envelopes similar to his.
The system of Jupiter, as it was discovered by Galileo, and investigated
by Laplace, appeared in its outward aspect so symmetrical, and displayed
in its inner mechanism such harmonious dynamical relations, that it
might well have been deemed complete. Nevertheless, a new member has
been added to it. Near midnight on September 9, 1892, Professor Barnard
discerned with the Lick 36-inch "a tiny speck of light," closely
following the planet.[1067] He instantly divined its nature, watched its
hurried disappearance in the adjacent glare, and made sure of the
reality of his discovery on the ensuing night. It was a delicate
business throughout, the Liliputian luminary subsiding into invisibility
before the slightest glint of Jovian light, and tarrying, only for brief
intervals, far enough from the disc to admit of its exclusion by means
of an occulting plate. The new satellite is estimated to be of the
thirteenth stellar magnitude, and, if equally reflective of light with
its next neighbour, Io (satellite No. 1), its diameter must be about one
hundred miles. It revolves at a distance of 112,500 miles from Jupiter's
centre, and of 68,000 from his bulging equatorial surface. Its period of
11h. 57m. 23s. is just two hours longer than Jupiter's period of
rotation, so that Phobos still remains a unique example of a secondary
body revolving faster than its primary rotates. Jupiter's innermost moon
conforms in its motions strictly, indeed inevitably, to the plane of his
equatorial protuberance, following, however, a sensibly elliptical path
the major axis of which is in rapid revolution.[1068] Its very
insignificance raises the suspicion that it may not prove solitary.
Possibly it belongs to a zone peopled by asteroidal satellites. More
than fifteen thousand such small bodies could be furnished out of the
materials of a single full-sized satellite spoiled in the making. But we
must be content for the present to register the fact without seeking to
penetrate the meaning of its existence. Very high and very fine
telescopic power is needed for its perception. Outside the United
States, it has been very little observed. The only instruments in this
country successfully employed for its detection are, we believe, Dr.
Common's 5-foot reflector and Mr. Newall's 25-inch refractor.
In the course of his observations on Jupiter at Brussels in 1878, M.
Niesten was struck with a rosy cloud attached to a whitish zone beneath
the dark southern equatorial band.[1069] Its size was enormous. At the
distance of Jupiter, its measured dimensions of 13" by 3" implied a real
extension in longitude of 30,000, in latitude of something short of
7,000 miles. The earliest record of its appearance seems to be by
Professor Pritchett, director of the Morrison Observatory (U.S.), who
figured and described it July 9, 1878.[1070] It was again delineated
August 9, by Tempel at Florence.[1071] In the following year it
attracted the wonder and attention of almost every possessor of a
telescope. Its colour had by that time deepened into a full brick-red,
and was set off by contrast with a white equatorial spot of unusual
brilliancy. During three ensuing years these remarkable objects
continued to offer a visible and striking illustration of the compound
nature of the planet's rotation. The red spot completed a circuit in
nine hours fifty-five minutes thirty-six seconds; the white spot in
about five and a half minutes less. Their _relative_ motion was thus no
less than 260 miles an hour, bringing them together in the same meridian
at intervals of forty-four days ten hours forty-two minutes. Neither,
however, preserved continuously the same uniform rate of travel. The
period of each had lengthened by some seconds in 1883, while sudden
displacements, associated with the recovery of lustre after recurrent
fadings, were observed in the position of the white spot,[1072]
recalling the leap forward of a reviving sun-spot. Just the opposite
effect attended the rekindling of the companion object. While
semi-extinct, in 1882-84, it lost little motion; but a fresh access of
retardation was observed by Professor Young[1073] in connection with its
brightening in 1886. This suggests very strongly that the red spot is
_fed from below_. A shining aureola of "faculæ," described by Bredichin
at Moscow, and by Lohse at Potsdam, as encircling it in September,
1879,[1074] was held to strengthen the solar analogy.
The conspicuous visibility of this astonishing object lasted three
years. When the planet returned to opposition in 1882-83, it had faded
so considerably that Riccò's uncertain glimpse of it at Palermo, May 31,
1883, was expected to be the last. It had, nevertheless, begun to
recover in December, and presented to Mr. Denning in the beginning of
1886 much the same aspect as in October, 1882.[1075] Observed by him in
an intermediate stage, February 25, 1885, when "a mere skeleton of its
former self," it bore a striking likeness to an "elliptical ring"
descried in the same latitude by Mr. Gledhill in 1869-70. This, indeed,
might be called the preliminary sketch for the famous object brought to
perfection ten years later, but which Mr. H. C. Russell of Sydney saw
and drew still unfinished in June, 1876,[1076] before it had separated
from its matrix, the dusky south tropical belt. In earlier times, too, a
marking "at once fixed and transient" had been repeatedly perceived
attached to the southernmost of the central belts. It gave Cassini in
1665 a rotation-period of nine hours fifty-six minutes,[1077] reappeared
and vanished eight times during the next forty-three years, and was last
seen by Maraldi in 1713. It was, however, very much smaller than the
recent object, and showed no unusual colour.[1078]
The assiduous observations made on the "Great Red Spot" by Mr. Denning
at Bristol and by Professor Hough at Chicago afforded grounds only for
negative conclusions as to its nature. It certainly did _not_ represent
the outpourings of a Jovian volcano; it was in no sense attached to the
Jovian soil--if the phrase have any application to that planet; it was
_not_ a mere disclosure of a glowing mass elsewhere seethed over by
rolling vapours. It was, indeed, certainly not self-luminous, a
satellite projected upon it in transit having been seen to show as
bright as upon the dusky equatorial bands. A fundamental objection to
all three hypotheses is that the rotation of the spot was variable. It
did not then ride at anchor, but floated free. Some held that its
surface was depressed below the average cloud-level, and that the cavity
was filled with vapours. Professor Wilson, on the other hand, observing
with the 16-inch equatorial of the Goodsell Observatory in Minnesota,
received a persistent impression of the object "being at a higher level
than the other markings."[1079] A crucial experiment on this point was
proposed by Mr. Stanley Williams in 1890.[1080] A dark spot moving
faster along the same parallel was timed to overtake the red spot
towards the end of July. A unique opportunity hence appeared to be at
hand of determining the relative vertical depths of the two formations,
one of which must inevitably, it was thought, pass above the other. No
forecast included a third alternative, which was nevertheless adopted by
the dark spot. It evaded the obstacle in its path by skirting round its
southern edge.[1081] Nothing, then, was gained by the conjunction,
beyond an additional proof of the singular repellent influence exerted
by the red spot over the markings in its vicinity. It has, for example,
gradually carved out a deep bay for its accommodation in the gray belt
just north of it. The effect was not at first steadily present. A
premonitory excavation was drawn by Schwabe at Dessau, September 5,
1831, and again by Trouvelot, Barnard, and Elvins in 1879; yet there was
no sign of it in the following year. Its development can be traced in
Dr. Boeddicker's beautiful delineations of Jupiter, made with the
Parsonstown 3-foot reflector, from 1881 to 1886.[1082] They record the
belt as straight in 1881, but as strongly indented from January, 1883;
and the cavity now promises to outlast the spot. So long as it survives,
however, the forces at work in the spot can have lost little of their
activity. For it must be remembered that the belt has a shorter
rotation-period than the red spot, which, accordingly (as Mr. Elvins of
Toronto has pointed out), breasts and diverts, by its interior energy, a
current of flowing matter, ever ready to fill up its natural bed, and
override the barrier of obstruction.
The famous spot was described by Keeler in 1889, as "of a pale pink
colour, slightly lighter in the middle. Its outline was a fairly true
ellipse, framed in by bright white clouds."[1083] The fading
continuously in progress from 1887 was temporarily interrupted in 1891.
The revival, indeed, was brief. Professor Barnard wrote in August, 1892:
"The great red spot is still visible, but it has just passed through a
crisis that seemingly threatened its very existence. For the past month
it has been all but impossible to catch the feeblest trace of the spot,
though the ever-persistent bay in the equatorial belt close north of it,
and which has been so intimately connected with the history of the red
spot, has been as conspicuous as ever. It is now, however, possible to
detect traces of the entire spot. An obscuring medium seems to have been
passing over it, and has now drifted somewhat preceding the spot."[1084]
The object is now always inconspicuous, and often practically invisible,
and may be said to float passively in the environing medium.[1085] Yet
there are sparks beneath the ashes. A rosy tinge faintly suffused it in
April, 1900,[1086] and its absolute end may still be remote.
The extreme complexity of the planet's surface-movements has been
strikingly evinced by Mr. Stanley Williams's detailed investigations. He
enumerated in 1896[1087] nine principal currents, all flowing parallel
to the equator, but unsymmetrically placed north and south of it, and
showing scant signs of conformity to the solar rule of retardation with
increase of latitude. The linear rate of the planet's equatorial
rotation was spectroscopically determined by Bélopolsky and Deslandres
in 1895. Both found it to fall short of the calculated speed, whence an
enlargement, by self-refraction, of the apparent disc was
inferred.[1088]
Jupiter was systematically photographed with the Lick 36-inch telescope
during the oppositions of 1890, 1891, and 1892, the image thrown on the
plates (after eightfold direct enlargement) being one inch in diameter.
Mr. Stanley Williams's measurements and discussion of the set for 1891
showed the high value of the materials thus collected, although much
more minute details can be seen than can at present be photographed. The
red spot shows as "very distinctly annular" in several of these
pictures.[1089] Recently, the planet has been portrayed by Deslandres
with the 62-foot Meudon refractor.[1090] The extreme actinic feebleness
of the equatorial bands was strikingly apparent on his plates.
In 1870, Mr. Ranyard[1091]--whose death, December 14, 1894, was a
serious loss to astronomy--acting upon an earlier suggestion of Sir
William Huggins, collected records of unusual appearances on the disc of
Jupiter, with a view to investigate the question of their recurrence at
regular intervals. He concluded that the development of the deeper
tinges of colour, and of the equatorial "port-hole" markings girdling
the globe in regular alternations of bright and dusky, agreed, so far as
could be ascertained, with epochs of sun-spot maximum. The further
inquiries of Dr. Lohse at Bothkamp in 1873[1092] went to strengthen the
coincidence, which had been anticipated _à priori_ by Zöllner in
1871.[1093] Moreover, separate and distinct evidence was alleged by Mr.
Denning in 1899 of decennial outbreaks of disturbance in north temperate
regions.[1094] It may, indeed, be taken for granted that what Hahn terms
the universal pulse of the solar system[1095] affects the vicissitudes
of Jupiter; but the law of those vicissitudes is far from being so
obviously subordinate to the rhythmical flow of central disturbance as
are certain terrestrial phenomena. The great planet, being in fact
himself a "semi-sun," may be regarded as an originator, no less than a
recipient, of agitating influences, the combined effects of which may
well appear insubordinate to any obvious law.
It is likely that Saturn is in a still earlier stage of planetary
development than Jupiter. He is the lightest for his size of all the
planets. In fact, he would float in water. And since his density is
shown, by the amount of his equatorial bulging, to increase
centrally,[1096] it follows that his superficial materials must be of a
specific gravity so low as to be inconsistent, on any probable
supposition, with the solid or liquid states. Moreover, the chief
arguments in favour of the high temperature of Jupiter, apply, with
increased force, to Saturn; so that it may be concluded, without much
risk of error, that a large proportion of his bulky globe, 73,000 miles
in diameter, is composed of heated vapours, kept in active and agitated
circulation by the process of cooling.
His unique set of appendages has, since the middle of the last century,
formed the subject of searching and fruitful inquiries, both theoretical
and telescopic. The mechanical problem of the stability of Saturn's
rings was left by Laplace in a very unsatisfactory condition.
Considering them as rotating solid bodies, he pointed out that they
could not maintain their position unless their weight were in some way
unsymmetrically distributed; but made no attempt to determine the kind
or amount of irregularity needed to secure this end. Some observations
by Herschel gave astronomers an excuse for taking for granted the
fulfilment of the condition thus vaguely postulated; and the question
remained in abeyance until once more brought prominently forward by the
discovery of the dusky ring in 1850.
The younger Bond led the way, among modern observers, in denying the
solidity of the structure. The fluctuations in its aspect were, he
asserted in 1851,[1097] inconsistent with such a hypothesis. The fine
dark lines of division, frequently detected in both bright rings, and as
frequently relapsing into imperceptibility, were due, in his opinion, to
the real nobility of their particles, and indicated a fluid formation.
Professor Benjamin Peirce of Harvard University immediately followed
with a demonstration, on abstract grounds, of their non-solidity.[1098]
Streams of some fluid denser than water were, he maintained, the
physical reality giving rise to the anomalous appearance first disclosed
by Galileo's telescope.
The mechanism of Saturn's rings, proposed as the subject of the Adams
Prize, was dealt with by James Clerk Maxwell in 1857. His investigation
forms the groundwork of all that is at present known in the matter. Its
upshot was to show that neither solid nor fluid rings could continue to
exist, and that the only possible composition of the system was by an
aggregated multitude of unconnected particles, each revolving
independently in a period corresponding to its distance from the
planet.[1099] This idea of a satellite-formation had been, remarkably
enough, several times entertained and lost sight of. It was first put
forward by Roberval in the seventeenth century, again by Jacques Cassini
in 1715, and with perfect definiteness by Wright of Durham in
1750.[1100] Little heed, however, was taken of these casual
anticipations of a truth which reappeared, a virtual novelty, as the
legitimate outcome of the most refined modern methods.
The details of telescopic observation accord, on the whole, admirably
with this hypothesis. The displacements or disappearance of secondary
dividing-lines--the singular striated appearance, first remarked by
Short in the eighteenth century, last by Perrotin and Lockyer at Nice,
March 18, 1884[1101]--show the effects of waves of disturbance
traversing a moving mass of gravitating particles;[1102] the broken and
changing line of the planet's shadow on the ring gives evidence of
variety in the planes of the orbits described by those particles. The
whole ring-system, too, appears to be somewhat elliptical.[1103]
The satellite-theory has derived unlooked-for support from photometric
inquiries. Professor Seeliger pointed out in 1888[1104] that the
unvarying brilliancy of the outer rings under all angles of
illumination, from 0° to 30°, can be explained from no other point of
view. Nor does the constitution of the obscure inner ring offer any
difficulty. For it is doubtless formed of similar small bodies to those
aggregated in the lucid members of the system, only much more thinly
strewn, and reflecting, consequently, much less light. It is not,
indeed, at first easy to see why these sparser flights should show as a
dense dark shading on the body of Saturn. Yet this is invariably the
case. The objection has been urged by Professor Hastings of Baltimore.
The brightest parts of these appendages, he remarked,[1105] are more
lustrous than the globe they encircle; but if the inner ring consists of
identical materials, possessing presumably an equal reflective capacity,
the mere fact of their scanty distribution would not cause them to show
as dark against the same globe. Professor Seeliger, however,
replied[1106] that the darkening is due to the never-ending swarms of
their separate shadows transiting the planet's disc. Sunlight is not,
indeed, wholly excluded. Many rays come and go between the open ranks of
the meteorites. For the dusky ring is transparent. The planet it
encloses shows through it, as if veiled with a strip of crape. A
beautiful illustration of its quality in this respect was derived by
Professor Barnard from an eclipse of Japetus, November 1, 1889.[1107]
The eighth moon remained steadily visible during its passage through the
shadow of the inner ring, but with a progressive loss of lustre in
approaching its bright neighbour. There was no breach of continuity. The
satellite met no gap, corresponding to that between the dusky ring and
the body of Saturn, through which it could shine with undiminished
light, but was slowly lost sight of as it plunged into deeper and deeper
gloom. The important facts were thus established, that the brilliant and
obscure rings merge into each other, and that the latter thins out
towards the Saturnian globe.
The meteoric constitution of these appendages was beautifully
demonstrated in 1895 by Professor Keeler,[1108] then director of the
Alleghany Observatory, Pittsburgh. From spectrographs taken with the
slit adjusted to coincidence with the equatorial plane of the system, he
determined the comparative radial velocities of its different parts. And
these supply a crucial test of Clerk Maxwell's theory. For if the rings
were solid, the swiftest rates of rotation should be at their outer
edges, corresponding to wider circles described in the same period;
while, if they are pulverulent, the inverse relation must hold good.
This proved to be actually the case. The motion slowed off outward, in
agreement with the diminishing speed of particles travelling freely,
each in its own orbit. Keeler's result was promptly confirmed by
Campbell,[1109] as well as by Deslandres and Bélopolsky.
A question of singular interest, and one which we cannot refrain from
putting to ourselves, is--whether we see in the rings of Saturn a
finished structure, destined to play a permanent part in the economy of
the system; or whether they represent merely a stage in the process of
development out of the chaotic state in which it is impossible to doubt
that the materials of all planets were originally merged. M. Otto Struve
attempted to give a definite answer to this important query.
A study of early and later records of observations disclosed to him, in
1851, an apparent progressive approach of the inner edge of the bright
ring to the planet. The rate of approach he estimated at about
fifty-seven English miles a year, or 11,000 miles during the 194 years
elapsed since the time of Huygens.[1110] Were it to continue, a collapse
of the system must be far advanced within three centuries. But was the
change real or illusory--a plausible, but deceptive inference from
insecure data? M. Struve resolved to put it to the test. A set of
elaborately careful micrometrical measures of the dimensions of Saturn's
rings, executed by himself at Pulkowa in the autumn of 1851, was
provided as a standard of future comparison; and he was enabled to renew
them, under closely similar circumstances, in 1882.[1111] But the
expected diminution of the space between Saturn's globe and his rings
had not taken place. A slight extension in the width of the system, both
outward and inward, was indeed, hinted at; and it is worth notice that
just such a separation of the rings was indicated by Clerk Maxwell's
theory, so that there is an _à priori_ likelihood of its being in
progress. Yet Hall's measures in 1884-87[1112] failed to supply evidence
of alteration with time; and Barnard's, executed at Lick in
1894-95,[1113] showed no sensible divergence from them. Hence, much
weight cannot be laid upon Huygens's drawings and descriptions, which
had been held to prove conclusively a partial filling up, since 1657, of
the interval between the ring and the planet.[1114]
The rings of Saturn replace, in Professor G. H. Darwin's view,[1115] an
abortive satellite, scattered by tidal action into annular form. For
they lie closer to the planet than is consistent with the integrity of a
revolving body of reasonable bulk. The limit of possible existence for
such a mass was fixed by Roche of Montpellier, in 1848,[1116] at 2·44
mean radii of its primary; while the outer edge of the ring-system is
distant 2·38 radii of Saturn from his centre. The virtual discovery of
its pulverulent condition dates, then, according to Professor Darwin,
from 1848. He conjectures that the appendage will eventually disappear,
partly through the dispersal of its constituent particles inward, and
their subsidence upon the planet's surface, partly by their dispersal
outward, to a region beyond "Roche's limit," where coalescence might
proceed unhindered by the strain of unequal attractions. One modest
satellite, revolving inside Mimas, would then be all that was left of
the singular appurtenances we now contemplate with admiration.
There seems reason to admit that Kirkwood's law of commensurability has
had some effect in bringing about the present distribution of the matter
composing them. Here the influential bodies are Saturn's moons, while
the divisions and boundaries of the rings represent the spaces where
their disturbing action conspires to eliminate revolving particles.
Kirkwood, in fact, showed, in 1867,[1117] that a body circulating in the
chasm between the bright rings known as "Cassini's division," would have
a period nearly commensurable with those of _four_ out of the eight
moons; and Meyer of Geneva subsequently calculated all such
combinations, with the result of bringing out coincidences between
regions of maximum perturbation and the limiting and dividing lines of
the system.[1118] This is in itself a strong confirmation of the view
that the rings are made up of independently revolving small bodies.
On December 7, 1876, Professor Asaph Hall discovered at Washington a
bright equatorial spot on Saturn, which he followed and measured through
above sixty rotations, each performed in ten hours fourteen minutes
twenty-four seconds.[1119] This, he was careful to add, represented the
period, not necessarily of the _planet_, but only of the individual
spot. The only previous determination of Saturn's axial movement
(setting aside some insecure estimates by Schröter) was Herschel's in
1794, giving a period of ten hours sixteen minutes. The substantial
accuracy of Hall's result was verified by Mr. Denning in 1891.[1120] In
May and June of that year, ten vague bright markings near the equator
were watched by Mr. Stanley Williams, who derived from them a rotation
period only two seconds shorter than that determined at Washington.
Nevertheless, similarly placed spots gave in 1892 and 1893 notably
quicker rates;[1121] so that the task of timing the general drift of the
Saturnian surface by the displacements of such objects is hampered, to
an indefinite extent, by their individual proper motions.
Saturn's outermost satellite, Japetus, is markedly variable--so variable
that it sends us, when brightest, just 4-1/2 times as much light as when
faintest. Moreover, its fluctuations depend upon its orbital position in
such a way as to make it a conspicuous telescopic object when west, a
scarcely discernible one when east of the planet. Herschel's
inference[1122] of a partially obscured globe turning always the same
face towards its primary seems the only admissible one, and is confirmed
by Pickering's measurements of the varying intensity of its light. He
remarked further that the dusky and brilliant hemispheres must be so
posited as to divide the disc, viewed from Saturn, into nearly equal
parts; so that this Saturnian moon, even when "full," appears very
imperfectly illuminated over one-half of its surface.[1123]
Zöllner estimated the albedo of Saturn at 0·51, Müller at 0·88, a value
impossibly high, considering that the spectrum includes no vestige of
original emissions. Closely similar to that of Jupiter, it shows the
distinctive dark line in the red (wave-length 618), which we may call
the "red-star line"; and Janssen, from the summit of Etna in 1867[1124]
found traces in it of aqueous absorption. The light from the ring
appears to be pure reflected sunshine unmodified by original atmospheric
action.[1125]
Uranus, when favourably situated, can easily be seen with the naked eye
as a star between the fifth and sixth magnitudes. There is indeed, some
reason to suppose that he had been detected as a wandering orb by savage
"watchers of the skies" in the Pacific long before he swam into
Herschel's ken. Nevertheless, inquiries into his physical habitudes are
still in an early stage. They are exceedingly difficult of execution,
even with the best and largest modern telescopes; and their results
remain clouded with uncertainty.
It will be remembered that Uranus presents the unusual spectacle of a
system of satellites travelling nearly at right angles to the plane of
the ecliptic. The existence of this anomaly gives a special interest to
investigations of his axial movement, which might be presumed, from the
analogy of the other planets, to be executed in the same tilted plane.
Yet this is far from being certainly the case.
Mr. Buffham in 1870-72 caught traces of bright markings on the Uranian
disc, doubtfully suggesting a rotation in about twelve hours in a plane
_not_ coincident with that in which his satellites circulate.[1126]
Dusky bands resembling those of Jupiter, but very faint, were barely
perceptible to Professor Young at Princeton in 1883. Yet, though almost
necessarily inferred to be equatorial, they made a considerable angle
with the trend of the satellites' orbits.[1127] More distinctly by the
brothers Henry, with the aid of their fine refractor, two gray parallel
rulings, separated by a brilliant zone, were discerned every clear night
at Paris from January to June, 1884.[1128] What were taken to be the
polar regions appeared comparatively dusky. The direction of the
equatorial rulings (for so we may safely call them) made an angle of 40°
with the satellites' line of travel. Similar observations were made at
Nice by MM. Perrotin and Thollon, March to June, 1884, a lucid spot near
the equator, in addition, indicating rotation in a period of about ten
hours.[1129] The discrepancy was, however, considerably reduced by
Perrotin's study of the planet in 1889 with the new 30-inch
equatoreal.[1130] The dark bands, thus viewed to better advantage than
in 1884, appeared to deviate no more than 10° from the satellites'
orbit-plane. No definitive results, on the other hand, were derived by
Professors Holden, Schaeberle, and Keeler from their observations of
Uranus in 1889-90 with the potent instrument on Mount Hamilton.
Shadings, it is true, were almost always, though faintly, seen; but they
appeared under an anomalous, possibly an illusory aspect. They
consisted, not of parallel, but of forked bands.[1131]
Measurements of the little sea-green disc which represents to us the
massive bulk of Uranus, by Young, Schiaparelli,[1132] Safarik, H. C.
Wilson[1133] and Perrotin, prove it to be quite distinctly _bulged_. The
compression at once caught Barnard's trained eye in 1894,[1134] when he
undertook at Lick a micrometrical investigation of the system; and he
was surprised to perceive that the major axis of the elliptical surface
made an angle of about 28° with the line of travel pursued by the
satellites. Nothing more can be learned on this curious subject for some
years, since the pole of the planet is just now turned nearly towards
the earth; but Barnard's conclusion is unlikely to be seriously
modified. He fixed the mean diameter of Uranus at 34,900 miles. But this
estimate was materially reduced through Dr. See's elimination of
irradiative effects by means of daylight measures, executed at
Washington in 1901.[1135]
The visual spectrum of this planet was first examined by Father Secchi
in 1869, and later, with more advantages for accuracy, by Huggins,
Vogel,[1136] and Keeler.[1137] It is a very remarkable one. In lieu of
the reflected Fraunhofer lines, imperceptible perhaps through feebleness
of light, six broad bands of original absorption appear, one
corresponding to the blue-green ray of hydrogen (F), another to the
"red-star line" of Jupiter and Saturn, the rest as yet unidentified. The
hydrogen band seems much too strong and diffuse to be the mere echo of a
solar line, and might accordingly be held to imply the presence of free
hydrogen in the Uranian atmosphere. This, however, would be difficult of
reconcilement with Keeler's identification of an absorption-group in the
yellow with a telluric waterband.
Notwithstanding its high albedo--0·62, according to Zöllner--proof is
wanting that any of the light of Uranus is inherent. Mr. Albert Taylor
announced, indeed, in 1889, his detection, with Common's giant
reflector, of bright flutings in its spectrum;[1138] but Professor
Keeler's examination proved them to be merely contrast effects.[1139]
Sir William and Lady Huggins, moreover, obtained about the same time a
photograph purely solar in character. The spectrum it represented was
crossed by numerous Fraunhofer lines, and by no others. It was, then,
presumably composed entirely of reflected light.
* * * * *
Judging from the indications of an almost evanescent spectrum, Neptune,
as regards physical condition, is the twin of Uranus, as Saturn of
Jupiter. Of the circumstances of his rotation we are as good as
completely ignorant. Mr. Maxwell Hall, indeed, noticed at Jamaica, in
November and December, 1883, certain rhythmical fluctuations of
brightness, suggesting revolution on an axis in slightly less than eight
hours;[1140] but Professor Pickering reduces the supposed variability to
an amount altogether too small for certain perception, and Dr. G. Müller
denies its existence _in toto_. It is true their observations were not
precisely contemporaneous with those of Mr. Hall[1141] who believes the
partial obscurations recorded by himself to have been of a passing kind,
and to have suddenly ceased after a fortnight of prevalence. Their less
conspicuous renewal was visible to him in November, 1884, confirming a
rotation period of 7·92 hours.
It was ascertained at first by indirect means that the orbit of
Neptune's satellite is inclined about 20° to his equator. Mr.
Marth[1142] having drawn attention to the rapid shifting of its plane of
motion, M. Tisserand and Professor Newcomb[1143] independently published
the conclusion that such shifting necessarily results from Neptune's
ellipsoidal shape. The movement is of the kind exemplified--although
with inverted relations--in the precession of the equinoxes. The pole of
the satellite, owing to the pull of Neptune's equatorial protuberance,
describes a circle round the pole of his equator in a retrograde
direction, and in a period of over five hundred years. The amount of
compression indicated for the primary body is, at the outside, 1/85;
whence it can be inferred that Neptune possesses a lower rotatory
velocity than the other giant planets. Direct verification of the trend
theoretically inferred for the satellite's movement was obtained by Dr.
See in 1899. The Washington 26-inch refractor disclosed to him, under
exceptionally favourable conditions, a set of equatorial belts on the
disc of Neptune, and they took just the direction prescribed by theory.
Their objective reality cannot be doubted, although Barnard was unable,
either with the Lick or the Yerkes telescope,[1144] to detect any
definite markings on this planet. Its diameter was found by him to be
32,900 miles.
The possibility that Neptune may not be the most remote body circling
round the sun has been contemplated ever since he has been known to
exist. Within the last few years the position at a given epoch of a
planet far beyond his orbital verge has been approximately fixed by two
separate investigators.
Professor George Forbes of Edinburgh adopted in 1880 a novel plan of
search for unknown members of the solar system, the first idea of which
was thrown out by M. Flammarion in November, 1879.[1145] It depends upon
the movements of comets. It is well known that those of moderately short
periods are, for a reason already explained, connected with the larger
planets in such a way that the cometary aphelia fall near some planetary
orbit. Jupiter claims a large retinue of such partial dependents,
Neptune owns six, and there are two considerable groups, the farthest
distances of which from the sun lie respectively near 100 and 300 times
that of the earth. At each of these vast intervals, one involving a
period of 1,000, the other of 5,000 years, Professor Forbes maintains
that an unseen planet circulates. He even computed elements for the
nearer of the two, and fixed its place on the celestial sphere;[1146]
but the photographic searches made for it by Dr. Roberts at Crowborough
and by Mr. Wilson at Daramona proved unavailing. Undeterred by
Deichmüller's discouraging opinion that cometary orbits extending beyond
the recognised bounds of the solar system are too imperfectly known to
serve as the basis of trustworthy conclusions,[1147] the Edinburgh
Professor returned to the attack in 1901.[1148] He now sought to prove
that the lost comet of 1556 actually returned in 1844, but with elements
so transformed by ultra-Neptunian perturbations as to have escaped
immediate identification. If so, the "wanted" planet has just entered
the sign Libra, and, being larger than Jupiter, should be possible to
find.
Almost simultaneously with Forbes, Professor Todd set about groping for
the same object by the help of a totally different set of indications.
Adams's approved method commended itself to him; but the hypothetical
divagations of Neptune having scarcely yet had time to develop, he was
thrown back upon the "residual errors" of Uranus. They gave him a
virtually identical situation for the new planet with that derived from
the clustering of cometary aphelia.[1149] Yet its assigned distance was
little more than half that of the nearer of Professor Forbes's remote
pair, and it completed a revolution in 375 instead of 1,000 years. The
agreement in them between the positions determined, on separate grounds,
for the ultra-Neptunian traveller was merely an odd coincidence; nor can
we be certain, until it is seen, that we have really got into touch with
it.
FOOTNOTES:
[Footnote 965: _Phil. Trans._, vol. lxxiv., p. 260.]
[Footnote 966: _Novæ Observationes_, p. 105.]
[Footnote 967: _Phil. Trans._, vol. i., p. 243.]
[Footnote 968: _Mém. de l'Ac._, 1720, p. 146.]
[Footnote 969: _Phil. Trans._, vol. lxxiv., p. 273.]
[Footnote 970: A large work, entitled _Areographische Fragmente_, in
which Schröter embodied the results of his labours on Mars, 1785-1803,
narrowly escaped the conflagration of 1813, and was published at Leyden
in 1881.]
[Footnote 971: _Beiträge_, p. 124.]
[Footnote 972: _Mem. R. A. Soc._, vol. xxxii., p. 183.]
[Footnote 973: _Astr. Nach._, No. 1,468.]
[Footnote 974: _Observatory_, vol. viii., p. 437.]
[Footnote 975: _Month. Not._, vols. xxviii., p. 37; xxix., p. 232;
xxxiii., p. 552.]
[Footnote 976: Flammarion, _L'Astronomie_, t. i., p. 266.]
[Footnote 977: Smyth, _Cel. Cycle_, vol. i., p. 148 (1st ed.).]
[Footnote 978: _Phil. Trans._, vol. cxxi., p. 417.]
[Footnote 979: _Month. Not._, vol. xxv., p. 227.]
[Footnote 980: _Phil. Mag._, vol. xxxiv., p. 75.]
[Footnote 981: Proctor, _Quart. Jour. of Science_, vol. x., p. 185;
Maunder, _Sunday Mag._, January, February, March, 1882; Campbell, _Publ.
Astr. Pac. Soc._, vol. vi., p. 273.]
[Footnote 982: _Am. Jour. of Sc._, vol. xxviii., p. 163.]
[Footnote 983: Burton, _Trans. Roy. Dublin Soc._, vol. i., 1880, p.
169.]
[Footnote 984: _Month. Not._, vol. xxvii., p. 179; _Astroph. Journ._,
vol. i., p. 193.]
[Footnote 985: _Untersuchungen über die Spectra der Planeten_, p. 20;
_Astroph. Journ._, vol. i., p. 203.]
[Footnote 986: _Publ. Astr. Pac. Soc._, vols. vi., p. 228; ix., p. 109;
_Astr. and Astroph._, vol. xiii., p. 752; _Astroph. Jour._, vol. ii., p.
28.]
[Footnote 987: _Ibid._, vol. v., p. 328.]
[Footnote 988: _Ibid._, vols. i., p. 311; iii., p. 254.]
[Footnote 989: C. Christiansen, _Beiblätter_, 1886, p. 532.]
[Footnote 990: _Astr. and Astrophysics_, vol. xi., p. 671.]
[Footnote 991: Flammarion, _La Planète Mars_, p. 574.]
[Footnote 992: _Mémoires Couronnés_, t. xxxix.]
[Footnote 993: Lockyer, _Nature_, vol. xlvi., p. 447.]
[Footnote 994: _Mem. Spettr. Italiani_, t. xi., p. 28.]
[Footnote 995: _Bull. Astr._, t. iii., p. 324.]
[Footnote 996: _Journ. Brit. Astr. Ass._, vol. i., p. 88.]
[Footnote 997: _Publ. Pac. Astr. Soc._, vol. ii., p. 299; Percival
Lowell, _Mars_, 1896; _Annals of the Lowell Observatory_, vol. ii.,
1900.]
[Footnote 998: _Old and New Astr._, p. 545.]
[Footnote 999: _L'Astronomie_, t. xi., p. 445.]
[Footnote 1000: _La Planète Mars_, p. 588.]
[Footnote 1001: _Month. Notices_, vol. lvi., p. 166.]
[Footnote 1002: _L'Astronomie_, t. viii.]
[Footnote 1003: _Astr. Nach._, No. 3,271; _Astr. and Astrophysics_, vol.
xiii., p. 716.]
[Footnote 1004: _Month. Not._, vol. xxxviii., p. 41; _Mem. Roy. Astr.
Soc._, vol. xliv., p. 123.]
[Footnote 1005: _Astr. and Astrophysics_, vol. xi., p. 668.]
[Footnote 1006: _Ibid._, p. 850.]
[Footnote 1007: _Comptes Rendus_, t. cxv., p. 379.]
[Footnote 1008: _Astr. Jour._, No. 384; _Publ. Astr. Pac. Soc._, vol.
vi., p. 109. _Cf. Observatory_ vol. xvii., pp. 295-336.]
[Footnote 1009: See Mr. Wentworth Erck's remarks in _Trans. Roy. Dublin
Soc._, vol. i., p. 29.]
[Footnote 1010: _Month. Not._, vol. xxxviii., p. 206.]
[Footnote 1011: _Annals Harvard Coll. Obs._, vol. xi., pt. ii., p. 217.]
[Footnote 1012: Young, _Gen. Astr._, p. 366.]
[Footnote 1013: Campbell, _Publ. Pac. Astr. Soc._, vol. vi., p. 270.]
[Footnote 1014: _Astr. Nach._, No. 3,319.]
[Footnote 1015: _Witch of Atlas_, stanza iii. I am indebted to Dr.
Garnett for the reference.]
[Footnote 1016: Recommended by Chandler, _Astr. Jour._, No. 452.]
[Footnote 1017: _Harvard Circulars_, Nos. 36, 37, 51.]
[Footnote 1018: _Astr. Nach._, No. 3,687.]
[Footnote 1019: Montangerand, _Comptes Rendus_, March 11, 1901.]
[Footnote 1020: Pickering, _Astroph. Jour._, vol. xiii., p. 277.]
[Footnote 1021: _Harvard Circular_, No. 58.]
[Footnote 1022: _Astr. Nach._, No. 752.]
[Footnote 1023: L. Niesten, _Annuaire_, Bruxelles, 1881, p. 269.]
[Footnote 1024: According to Svedstrup (_Astr. Nach._, Nos. 2,240-41),
the inclination to the ecliptic of the "mean asteroid's" orbit is = 6°.]
[Footnote 1025: _Smiths._ Report, 1876, p. 358; _The Asteroids_
(Kirkwood), p. 42, 1888.]
[Footnote 1026: Tisserand, _Annuaire_, Paris, 1891, p. B. 15; Newcomb,
_Astr. Jour._, No. 477; Backlund, _Bull. Astr._, t. xvii., p. 81;
Parmentier, _Bull. Soc. Astr. de France_, March, 1896; Observatory, vol.
xviii., p. 207.]
[Footnote 1027: Berberich, _Astr. Nach._, No. 3,088.]
[Footnote 1028: _Bull. Astr._, t. xviii., p. 39.]
[Footnote 1029: _The Asteroids_, p. 48; _Publ. Astr. Pac. Soc._, vols.
ii., p. 48; iii., p. 95.]
[Footnote 1030: _Comptes Rendus_, t. xxxvii., p. 797.]
[Footnote 1031: _Bull. Astr._, t. v., p. 180.]
[Footnote 1032: _Annuaire_, Bruxelles, 1881, p. 243.]
[Footnote 1033: _Johns Hopkins Un. Circular_, January, 1895;
_Observatory_, vol. xviii., p. 127.]
[Footnote 1034: _Harvard Annals_, vol. xi., part ii., p. 294.]
[Footnote 1035: _Astr. Nach._, Nos. 2,724-5.]
[Footnote 1036: _Month. Not._, vol. lxi., p. 69.]
[Footnote 1037: _Astroph. Jour._, vol. vii., p. 25.]
[Footnote 1038: _Spectra der Planeten_, p. 24.]
[Footnote 1039: Tome i., p. 93.]
[Footnote 1040: _Berlinische Monatsschrift_, 1785, p. 211.]
[Footnote 1041: _Month. Not._, vol. xiii., p. 40.]
[Footnote 1042: _Mem. Am. Ac._, vol. viii., p. 221.]
[Footnote 1043: _Photom. Unters._, p. 303.]
[Footnote 1044: _Astr. Nach._, No. 1,851.]
[Footnote 1045: _Mém. de l'Ac._, t. x., p. 514.]
[Footnote 1046: _Ibid._, 1692, p. 7.]
[Footnote 1047: _Month. Not._, vol. xliv., p. 63.]
[Footnote 1048: _Photom. Unters._, pp. 165, 273; _Potsdam Publ._, No.
30.]
[Footnote 1049: Vogel, _Sp. der Planeten_, p. 33, _note_.]
[Footnote 1050: _Proc. Roy. Soc._, vol. xviii., p. 250.]
[Footnote 1051: _Month. Not._, vol. xl., p. 433.]
[Footnote 1052: _Sitzungsberichte_, Berlin, 1895, ii., p. 15.]
[Footnote 1053: The anomalous shadow-effects recorded by Webb (_Cel.
Objects_, p. 170, 4th ed.) are obviously of atmospheric and optical
origin.]
[Footnote 1054: Engelmann, _Ueber die Helligkeitsverhältnisse der
Jupiterstrabanten_, p. 59.]
[Footnote 1055: _Month. Not._, vol. xxviii., p. 11.]
[Footnote 1056: _Observatory_, vol. vii., p. 175.]
[Footnote 1057: _Month. Not._, vol. xlviii., p. 43.]
[Footnote 1058: _Publ. Astr. Pac. Soc._, vol. ii., p. 296.]
[Footnote 1059: Pickering failed to obtain any photometric evidence of
their variability. _Harvard Annals_, vol. xi., p. 245.]
[Footnote 1060: _Astr. and Astroph._, vol. xii., pp. 194, 481.]
[Footnote 1061: _Annals Lowell Obs._, vol. ii., pt. i.]
[Footnote 1062: _Astr. Nach._, Nos. 2,995, 3,206; _Month. Not._, vols.
li., p. 556; liv., p. 134. Barnard remains convinced that the oval forms
attributed to Jupiter's satellites are illusory effects of their
markings. _Astr. Nach._, Nos. 3,206, 3,453; _Astr. and Astroph._, vol.
xiii., p. 272.]
[Footnote 1063: _Publ. Astr. Pac. Soc._, vol. iii., p. 355.]
[Footnote 1064: _Astr. Nach._, No. 1,017.]
[Footnote 1065: _Publ. Astr. Pac. Soc._, vol. iii., p. 359.]
[Footnote 1066: _Astr. Nach._, No. 3,432.]
[Footnote 1067: _Astr. Jour._, Nos. 275, 325, 367, 472; _Observatory_,
vol. xv., p. 425.]
[Footnote 1068: Tisserand, _Comptes Rendus_, October 8, 1894; Cohn,
_Astr. Nach._, No. 3,404.]
[Footnote 1069: _Bull. Ac. R. Bruxelles_, t. xlviii., p. 607.]
[Footnote 1070: _Astr. Nach._, No. 2,294.]
[Footnote 1071: _Ibid._, No. 2,284.]
[Footnote 1072: Denning, _Month. Not._, vol. xliv., pp. 64, 66;
_Nature_, vol. xxv., p. 226.]
[Footnote 1073: _Sidereal Mess._, December, 1886, p. 289.]
[Footnote 1074: _Astr. Nach._, Nos. 2,280, 2,282.]
[Footnote 1075: _Month. Not._, vol. xlvi., p. 117.]
[Footnote 1076: _Proc. Roy. Soc. N. S. Wales_, vol. xiv., p. 68.]
[Footnote 1077: _Phil. Trans._, vol. i., p. 143.]
[Footnote 1078: For indications relative to the early history of the red
spot, see Holden, _Publ. Astr. Pac. Soc._, vol. ii., p. 77; Noble,
_Month. Not._, vol. xlvii., p. 515; A. S. Williams, _Observatory_, vol.
xiii., p. 338.]
[Footnote 1079: _Astr. and Astrophysics_, vol. xi., p. 192.]
[Footnote 1080: _Month. Not._, vol. l., p. 520.]
[Footnote 1081: _Observatory_, vol. xiii., pp. 297, 326.]
[Footnote 1082: _Trans. R. Dublin Soc._, vol. iv., p. 271, 1889.]
[Footnote 1083: _Publ. Astr. Pac. Soc._, vol. ii., p. 289.]
[Footnote 1084: _Astr. and Astrophysics_, vol. xi., p. 686.]
[Footnote 1085: Denning, _Knowledge_, vol. xxiii., p. 200;
_Observatory_, vol. xxiv., p. 312; _Pop. Astr._, vol. ix., p. 448;
_Nature_, vol. lv., p. 89.]
[Footnote 1086: Williams, _Observatory_, vol. xxiii., p. 282.]
[Footnote 1087: _Month. Not._, vol. lvi., p. 143.]
[Footnote 1088: Bélopolsky, _Astr. Nach._, No. 3,326.]
[Footnote 1089: _Publ. Astr. Pac. Soc._, vol. iv., p. 176.]
[Footnote 1090: _Bull. Astr._, 1900, p. 70.]
[Footnote 1091: _Month. Not._, vol. xxxi., p. 34.]
[Footnote 1092: _Beobachtungen_, Heft ii., p. 99.]
[Footnote 1093: _Ber. Sächs. Ges. der Wiss._, 1871, p. 553.]
[Footnote 1094: _Month. Not._, vol. lix., p. 76.]
[Footnote 1095: _Beziehungen der Sonnenfleckenperiode_, p. 175.]
[Footnote 1096: A. Hall, _Astr. Nach._, No. 2,269.]
[Footnote 1097: _Astr. Jour._ (Gould's), vol. ii., p. 17.]
[Footnote 1098: _Ibid._, p. 5.]
[Footnote 1099: _On the Stability of the Motion of Saturn's Rings_, p.
67.]
[Footnote 1100: _Mém. de l'Ac._, 1715, p. 47; Montucla, _Hist. des
Math._, t. iv., p. 19; _An Original Theory of the Universe_, p. 115.]
[Footnote 1101: _Comptes Rendus_, t. xcviii., p. 718.]
[Footnote 1102: Proctor, _Saturn and its System_ (1865), p. 125.]
[Footnote 1103: Perrotin, _Comptes Rendus_, t. cvi., p. 1716.]
[Footnote 1104: _Abhandl. Akad. der Wiss._, Munich, Bd. xvi., p. 407.]
[Footnote 1105: _Smiths. Report_, 1880 (Holden).]
[Footnote 1106: Quoted by Dr. E. Anding, _Astr. Nach._, No. 2,881.]
[Footnote 1107: _Astr. and Astrophysics_, vol. xi., p. 119; _Month.
Not._, vol. l., p. 108.]
[Footnote 1108: _Astroph. Jour._, vol. i., p. 416.]
[Footnote 1109: _Ibid._, vol. ii., p. 127.]
[Footnote 1110: _Mém. de l'Ac. Imp._ (St. Petersb.), t. vii., 1853, p.
464.]
[Footnote 1111: _Astr. Nach._, No. 2,498.]
[Footnote 1112: _Washington Observations_, App. ii., p. 22]
[Footnote 1113: _Month. Not._, vol. lvi., p. 163.]
[Footnote 1114: T. Lewis, _Observatory_, vol. xviii., p. 379.]
[Footnote 1115: _Harper's Magazine_, June, 1889.]
[Footnote 1116: _Mém. de l'Acad. de Montpellier_, t. viii., p. 296,
1873.]
[Footnote 1117: _Meteoric Astronomy_, chap. xii. He carried the subject
somewhat farther in 1871. See _Observatory_, vol. vi., p. 335.]
[Footnote 1118: _Astr. Nach._, No. 2,527.]
[Footnote 1119: _Amer. Jour. of Sc._, vol. xiv., p. 325.]
[Footnote 1120: _Observatory_, vol. xiv., p. 369.]
[Footnote 1121: _Month. Not._, vol. liv., p. 297.]
[Footnote 1122: _Phil. Trans._, vol. lxxxii., p. 14.]
[Footnote 1123: _Smiths. Report_, 1880.]
[Footnote 1124: _Comptes Rendus_, t. lxiv., p. 1304.]
[Footnote 1125: Huggins, _Proc. R. Soc._, vol. xlvi., p. 231; Keeler,
_Astr. Nach._, No. 2,927; Vogel, _Astroph. Jour._, vol. i., p. 278.]
[Footnote 1126: _Month. Not._, vol. xxxiii., p. 164.]
[Footnote 1127: _Astr. Nach._, No 2,545.]
[Footnote 1128: _Comptes Rendus_, t. xcviii., p. 1419.]
[Footnote 1129: _Comptes Rendus_, t. xcviii., pp. 718, 967.]
[Footnote 1130: _V. J. S. Astr. Ges._, Jahrg. xxiv., p. 267.]
[Footnote 1131: _Publ. Astr. Pac. Soc._, vol. iii., p. 287.]
[Footnote 1132: _Astr. Nach._, No. 2,526.]
[Footnote 1133: _Ibid._, No. 2,730.]
[Footnote 1134: _Astr. Jour._, Nos. 370, 374.]
[Footnote 1135: _Astr. Nach._, No. 3,768.]
[Footnote 1136: _Ann. der Phys._, Bd. clviii., p. 470; _Astroph. Jour._,
vol. i., p. 280.]
[Footnote 1137: _Astr. Nach._, No. 2,927.]
[Footnote 1138: _Month. Not._, vol. xlix., p. 405.]
[Footnote 1139: Astr. Nach., No. 2,927; Scheiner's _Spectralanalyse_, p.
221.]
[Footnote 1140: _Month. Not._, vol. xliv., p. 257.]
[Footnote 1141: _Observatory_, vol. vii., pp. 134, 221, 264.]
[Footnote 1142: _Month. Not._, vol. xlvi., p. 507.]
[Footnote 1143: _Comptes Rendus_, t. cvii., p. 804; _Astr. and
Astroph._, vol. xiii., p. 291; _Astr. Jour._, No. 186.]
[Footnote 1144: _Astr. Jour._, Nos. 342, 436, 508.]
[Footnote 1145: _Astr. Pop._, p. 661; _La Nature_, January 3, 1880.]
[Footnote 1146: _Proc. Roy. Soc. Edinb._, vols. x., p. 429; xi., p. 89.]
[Footnote 1147: _Vierteljahrsschrift. Astr. Ges._, Jahrg. xxi., p. 206.]
[Footnote 1148: _Proc. Roy. Soc. Edinb._, vol. xxiii., p. 370; _Nature_,
vol. lxiv., p. 524.]
[Footnote 1149: _Amer. Jour. of Science_, vol. xx., p. 225.]