satellite, and although his telescope was of less than four inches
aperture, he satisfied himself that none with a diameter of as much
as twenty-three miles could be in existence. As it happened, he
was right. The pair of moons detected by Professor Asaph Hall with
the Washington twenty-six refractor, August 11 and 17, 1877, are
unquestionably below that limit of size. Neither of them can well
be more than ten miles across. Their names, “Deimos” and “Phobos,”
are taken from the _Iliad_, where Fear and Panic are introduced as
attendants upon the God of War. Deimos revolves in 30 hours and 18
minutes at a distance of 14,600 miles from the centre of Mars. And
since the planet rotates in 24 hours, 37 minutes, the diurnal motion
of the sphere from east to west is so nearly neutralized by the
orbital circulation of the satellite from west to east that nearly
132 hours elapse between its rising and its setting. During the
interval, it changes four times from new to full, and _vice versâ_.
Phobos is more effective in illumination, both because it is
larger and because it is less distant. At the Martian equator, its
brightness is equal to 1/60th that of our moon, but beyond 69° of
latitude it is permanently shut out from view by the curvature of the
globe.
THE PLANETOIDS.—CAMILLE FLAMMARION
On the first day of the last century (January 1, 1801), Piazzi, an
astronomer devoted to the sky, was observing at Palermo the small
stars of the constellation Taurus, and noting their exact positions,
when he remarked one which he had never seen before. The following
evening (January 2) he directed his telescope again toward the
same region of the sky, and remarked that the star was no longer
at the point where he had seen it the day before, and that it had
retrograded by 4′. It continued to retrograde up to the 12th,
stopped, and then moved in the direct way—that is to say, from west
to east. What was this moving star? The idea that it might be a
planet did not immediately occur to the mind of the observer, and he
took it for a comet, as William Herschel had done in 1781, when he
discovered Uranus.
However, the skilful Sicilian observer was a member of an association
which had for its special object the search for an unknown planet
between Mars and Jupiter. From the earliest times of modern astronomy
Kepler had described the disproportion, the void which exists between
the orbit of Mars and that of Jupiter. If we omit, in fact, the
orbit of the small planets or asteroids, we notice that the four
planets, Mercury, Venus, the earth, and Mars, are in some measure
crowded quite close to the sun, while Jupiter, Saturn, Uranus, and
Neptune extend far into immensity. The law of Titius indicates a
number, the number 28, as not being represented by any planet. It
was in 1772 that this _savant_ published this relation in a German
translation which he had made of the _Contemplation de la Nature_
of Charles Bonnet. Bode, Director of the Berlin Observatory, was so
astonished at the coincidence that he announced this arithmetical
relation as being a real law of nature, and spoke of it in such a
way that it is generally known only by his name. He even organized
an association of twenty-four astronomers to explore each hour of
the Zodiac and search for the unknown. This systematic exploration
had not yet produced any result when, by the merest chance, Piazzi
saw his moving star, and at first believed it to be a comet. But on
receipt of the news, Bode was convinced that this was the looked-for
planet.
The new planet was found to be at the distance 2.77, and to revolve
within a few days of the predicted period. Piazzi gave to the new
body the name of _Ceres_, the protecting divinity of Sicily in the
“good old times” of mythology.
The gap being thus filled up at the distance 28 by the discovery of
Ceres, no one thought that other planets might exist there; and if
Piazzi had supposed so, he might have at once discovered a dozen of
the small bodies which revolve in this region. An astronomer of
Bremen, Olbers, observed this planet on the evening of March 28,
1802, when he perceived in the constellation of the Virgin a star of
the seventh magnitude which was not marked on Bode’s chart, which
he used. The following day he found it had changed its place, and
recognized by this fact that it was a second planet. But it was much
more difficult to give citizenship to it than to its elder sister,
because, the gap being filled up, it was not required, and it was
more inconvenient than agreeable. They looked upon it, then, as a
comet until its motion proved that it revolved in the same region as
Ceres at the distance 2.77, and in 1,685 days (the period of Ceres is
1,681 days). They gave it the name of _Pallas_.
The unexpected discoveries of Ceres and Pallas led astronomers to
revise the catalogues of stars and celestial charts. Harding was of
the number of the zealous revisers. He was soon rewarded for his
trouble. On September 1, 1804, at ten o’clock in the evening, he saw
in the constellation of Pisces a star of the eighth magnitude which
was not noted in the _Histoire Céleste_ of Lalande. On September
4, he found it had perceptibly changed its place: it was a new
planet. It received the name of _Juno_. Its distance from the sun
is expressed by the number 2.67, and its revolution is performed in
1,592 days.
After these three discoveries, Olbers, noticing that the orbits of
these planets crossed each other in the constellation of the Virgin,
advanced the hypothesis that they might be nothing else but fragments
of a large shattered planet. Mechanics show that, in this case, the
fragments would again pass every year—that is to say, at each of
their revolutions—through the spot where the catastrophe took place.
Olbers then set himself to explore the constellation Virgo carefully,
and found on March 29, 1807, a fourth small planet, to which he gave
the name of _Vesta_. Its distance is but 2.36, and its revolution
only 1.326 days. This is the brightest of the small planets, and it
is sometimes seen with the naked eye (when we know where it is), like
a star of the sixth magnitude.
It seems surprising that after these brilliant beginnings
thirty-eight years should then have passed without the discovery of a
single planet, for it was only in 1845 that the fifth, _Astræa_, was
discovered by Hencke (who should not be confused with the astronomer
Encke), a simple amateur astronomer, postmaster at Berlin, who amused
himself by constructing charts of the stars. The principal reason
for this must be attributed to the want of good star-charts, for to
find these little moving points the first thing necessary is to have
a very precise chart of the region of the Zodiac which we observe,
in order to see whether one of the stars observed is in motion. The
earliest good Zodiacal charts are those which the Academy of Berlin
commenced to publish in 1830, taking as a basis the zones of Bessel
continued by Argelander. Those of the Paris Observatory, which are
more perfect, were only begun in 1854.
These small planets are all telescopic, invisible to the naked eye,
with the exception of Vesta, and sometimes Ceres, which good sight
can occasionally succeed in distinguishing; they are of the seventh,
eighth, ninth, tenth, and eleventh magnitudes, and even still
smaller, and it was for this reason also that so long an interval
of time elapsed between the fourth and fifth discoveries. It is
probable that all the small planets of any importance are now known,
but that a great number—several hundreds, perhaps—still remain to be
discovered of which the average brightness does not exceed that of
stars of the twelfth magnitude, and of which the diameter is but a
few miles. The diameter of the largest, Vesta, may be estimated at
400 kilometres (248 miles).
Hencke found successively the 5th and the 6th in 1845 and 1847;
Hind, the English astronomer, the 7th and 8th in 1847; Graham, an
English observer, the 9th in 1848; Gasparis, an Italian astronomer,
the 10th and 11th in 1849 and 1850, and afterward seven others. Hind
has further discovered eight others; Goldschmidt, a German painter
(a naturalized Frenchman), discovered fourteen between 1852 and
1861.[27] They are now discovered by swarms; Paliser alone has found
sixty-eight since 1874.
The names given to these small bodies commenced with the mythological
army of divinities of the earth and ancient heaven; but even before
the list had been exhausted certain scientific, or even national
or political, circumstances caused the preference to be given to
more modern names. It was thus that the 11th, discovered at Naples,
received the name of Parthenope; the 12th, discovered in England,
that of Victoria; the 20th, that of Massilia; the 21st, that of
Lutetia; the 25th, that of Phocæa, before even Urania had been
restored to the skies; the 45th was named in honor of the Empress
of the French; the 54th, in honor of the illustrious Alexander von
Humboldt; etc. The 87th, 107th, 141st, 154th, and 169th have been
named in honor of a young astronomer who has devoted his best years
to the culture of astronomy.
A rather curious fact is that they have put Wisdom (_Sapientia_)
in the sky only at the 275th, discovered in 1888; Bellona has been
placed there since the 28th (1854).
Of all this number of planets, the nearest to the sun is No. 149,
Medusa, of which the distance is 2.17—that is to say, about twice
as far from the sun as the earth; and the most distant is No. 279,
Thule, of which the distance is 4.26, about 4¼ times our distance.
A large number of these small bodies are remarkable for their great
eccentricity and for their high inclination to the ecliptic, an
inclination so great that some of them leave the Zodiac; thus, Pallas
(2) goes 34 degrees from the ecliptic; Euphrosyne (31) and Anna
(265) and Istria (183), to 26 degrees. They are sometimes northern
circumpolar stars, always above the horizon, sometimes southern
stars, not arising above the horizon of Paris. All these orbits are
so interlaced with each other that, if they were material hoops, we
could by means of one or two taken by chance raise all the others.
Are they globes? Yes, doubtless, for the most part. But several among
the smaller ones may be polyhedral, and may have proceeded from
subsequent explosions; the variations of brightness which have been
sometimes observed seem to imply surfaces irregularly broken.
Are they _worlds_? Why not? Is not a drop of water shown in the
microscope peopled with a multitude of various beings? Does not a
stone in a meadow hide a world of swarming insects? Is not the leaf
of a plant a world for the species which inhabit and prey upon it?
Doubtless among the multitude of small planets there are those which
must remain desert and sterile because the conditions of life (of
any kind) are not found united. But we can not doubt that on the
majority the ever-active forces of nature have produced, as in our
world, creations appropriate to these minute planets. Let us repeat,
moreover, that for nature there is neither great nor little. And
there is no necessity to flatter ourselves with a supreme disdain for
these little worlds, for in reality the inhabitants of Jupiter would
have more right to despise us than we have to despise Vesta, Ceres,
Pallas, or Juno: the disparity is greater between Jupiter and the
earth than between the earth and these planets.
FOOTNOTES:
[27] Goldschmidt passionately loved astronomy, and I have found
among his papers, which his family left me, numerous observations
and remarks which show how he loved the study of the sky. His
greatest ambition had been, at first, to possess a small telescope,
in order to make some observations, and the best day of his life
was that on which he found one in the possession of a dealer in old
stores. He hastened to direct it to the sky from his modest studio,
situated in one of the most frequented streets of Paris (Rue de
l’Ancienne-Comédie), above the Café Procope, formerly used as a
rendezvous by the stars of literature. There, _from his window_, he
discovered, in 1852, the 21st small planet, which received from Arago
the name of Lutetia; then, in 1854, the 32d (Pomona); then, in 1855,
the 36th (Atlanta); and afterward eleven others, all from his window.
Having often removed in search of a pure atmosphere, he finally
retired to Fontainebleau, where the forest offered him on all sides
admirable subjects for painting; and here he died in 1866.
JUPITER.—AGNES M. CLERKE
Jupiter is by far the most important member of the solar family.
The aggregate mass of all the other planets is only two-fifths of
his, which 316 earths would be needed to counterbalance. His size is
on a still more colossal scale than his weight, since in volume he
exceeds our globe 1,380 times. His polar and equatorial diameters
measure respectively 84,570 and 90,190 miles, giving a mean diameter
of 88,250 miles, and a polar compression of 1/16th. The corresponding
equatorial protuberance rises to 2,000 miles, so that the elliptical
figure of the planet strikes an observer at the first glance. This
at once indicates rapid axial movement; and Jupiter’s rotation is
accordingly performed in nine hours and fifty-five minutes, with an
uncertainty of a couple of minutes.
The numbers just given imply that this great planet is of somewhat
slight consistence, and its mean density is, in fact, a little less
than that of the sun. The sun is heavier than an equal bulk of water
in the proportion of 1.4 to 1, Jupiter in the proportion of 1.33 to
1. The earth is thus more than four times specifically heavier than
the latter globe. Three Jupiters would keep in equipoise four equal
globes of water, while the earth would turn the scale against five
and a half aqueous models of itself. This low density, an unfailing
characteristic of all the giant planets, is charged with meaning.
It at once gives us to understand that, in crossing the zone of
asteroids, we enter upon a different planetary region from that left
behind. The bodies revolving there are on an immensely larger scale
of magnitude than those on the hither side; they are of solar, rather
than terrestrial, density; they rotate much more rapidly, and are
in consequence of a more elliptical shape; they display, and most
likely possess, no solid surface; they are attended by retinues of
satellites.
Jupiter circulates round the sun in 11.86 years, in an orbit
deviating by less than one and a half degrees from the plane of the
ecliptic, but of thrice the eccentricity of the ellipse traced out
by the earth. With a mean distance from the sun of 483 millions
of miles, it accordingly approaches within 462 at perihelion, and
withdraws to 504 millions of miles at aphelion. Seasons it has none
worth mentioning; nor could they be of much effect even if they were
better marked.
Under propitious circumstances Jupiter comes within 369 million miles
of the earth. These occur when he is in opposition nearly at the
epoch of his perihelion passage. His maximum opposition distance, on
the other hand, is 411 million miles. He is then at aphelion. Thus,
at the most favorable opposition, he is 42 million miles nearer to us
than at the least favorable. The effect on his brightness is evident
to the eye. When his midnight culmination takes place in October, he
in fact sends us one and a half times more light than when the event
comes round to April. We need only recall the unusual splendor of his
appearance in September and October, 1892, when his lustre was double
that of Sirius. His opposition period, as we may call it, is 399 days.
The intrinsic brilliancy of his surfaces is surprising, especially
when we consider that it is somewhat deeply tinged with color.
The minimum diameter of the visible disk considerably exceeds the
maximum of that of Mars. Even with a low power it thus makes a
beautiful and interesting telescopic object. Its distinctive aspect
is that of a belted planet, the belts varying greatly in number and
arrangement. As many as thirty have, on occasions, been counted,
delicately ruling the disk from pole to pole. They are always
parallel to the equator, but are otherwise highly changeable, and
can not be too closely studied as an index to the planet’s physical
constitution. Two in particular are remarkable. They are called the
north and south equatorial belts, and inclose a lustrous equatorial
zone. The poles are shaded by dusky hoods.
This general scheme of markings, however, when viewed with one of the
great telescopes of the world, is so overlaid with minor particulars
as sometimes to be scarcely recognizable. One can not see the wood
for the trees. Lovely color-effects, too, come out under the best
circumstances of definition and aerial transparency. The tropical
belts may be summarily described as red; but they are of complex
structure, and their subordinate features and formations are marked
out, under the sway of alternating and tumultuous activities, by
strips and patches of vermilion, pink, purple, drab, and brown. The
intermediate space is divided into two bands by a line, or narrow
ribbon, pretty nearly coinciding with the equator, and rosy or
vivid scarlet in hue. The polar caps are sometimes of a delicate
wine-color, sometimes pale gray.
Professor Keeler made an elaborate study of the planet with the
Lick 36-inch in 1889, and executed a series of valuable drawings.
With a power of 320, the disk, he tells us, “was a most beautiful
object, covered with a wealth of detail which could not possibly be
accurately represented in a drawing.” Most of the surface was then
“mottled with flocculent and irregular cloud-masses. The edges of the
equatorial zone were brilliantly white, and were formed of rounded,
cloud-like masses, which, at certain places, extended into the red
belt as long streamers. These formed the most remarkable and curious
feature of the equatorial regions. They are the cause of the double
or triple aspect which the red belts present in small telescopes.”
Near their starting-points the streamers were white and sharply
defined, but became gradually diffused over the ruddy surface of
the belts. When at all elongated, they invariably flowed backward
_against_ the rotational drift, and were inferred to be cloud-like
masses expelled from the equatorial region, and progressively left
behind by its advance. This hypothesis was confirmed by the motion
of some bright points, or knots, on the streamers. “The portions of
the equatorial zone surrounding the roots of well-marked streamers
were somewhat brighter,” Professor Keeler continues, “than at other
places, and it is a curious circumstance that they were almost
invariably suffused with a pale olive-green color, which seemed to be
associated with great disturbance, and was rarely seen elsewhere.”
Now, if the material of the streamers had been simply a superficial
overflow, it should have carried with it into higher latitudes an
excess of linear rotational speed, and should hence have pushed its
way onward as it proceeded north and south. But, instead, it fell
behind; its velocity was less, not greater, than that of the belts
with which it eventually became incorporated. What are we to gather
from this fact? Evidently that the currents issuing north and south
were of eruptive origin. Their motion, in miles per second, was slow,
because they belonged to profound strata of the planet’s interior.
Their backward drift measured the depth from which they had been
flung upward.
The spots, red, white, and black, constantly visible on the Jovian
surface, excite the highest curiosity. They are of all kinds and
qualities, and their histories and adventures are as diverse as they
are in themselves. Some are quite evanescent; others last for years.
At times they come in undistinguished crowds, like flocks of sheep,
then a solitary spot will acquire notoriety on its own account. White
spots appear in both ways; black spots more often in communities; and
it is remarkable that the former frequent distinctively, though not
exclusively, the Southern, the latter the Northern Hemisphere. Red
spots, too, develop pretty freely; but the attention due to them has
been mainly observed by one striking specimen.
The Great Red Spot has been present with us for at least nineteen
years; and it is a moot point whether its beginnings were not watched
by Cassini more than two centuries ago. Its modern conspicuousness,
however, dates from 1878. Then of a full brick-red hue, and strongly
marked contour, it measured 30,000 by nearly 7,000 miles, and might
easily have inclosed three such bodies as the earth. It has since
faded several times to the verge of extinction, and partially
recovered; but there has never been a time when it ceased to dominate
the planet’s surface-configuration. More than once it has been
replaced by a bare elliptical outline, as if through an effusion
of white matter into a mold previously filled with red matter; and
just such a sketch was observed by Gledhill in 1870. The red spot
is attached, on the polar side, to the southern equatorial belt.
It might almost be described as jammed down upon it; for a huge
gulf, bounded at one end by a jutting promontory, appears as if
scooped out of the chocolate-colored material of the belt to make
room for it. Absolute contact, nevertheless, seems impossible. The
spot is surrounded by a shining aureola, which seemingly defends it
against encroachments, and acts as a _chevaux-de-frise_ to preserve
its integrity. The formation thus constituted behaves like an
irremovable obstacle in a strong current. The belt-stuff encounters
its resistance, and rears itself up into a promontory or “shoulder,”
testifying to the solid presence of the spot, even though it be
temporarily submerged. The great red spot, the white aureola, and the
brownish shoulder are indissolubly connected.
The spot is then no mere cloudy condensation. Yet it has no real
fixity. Its period of rotation is inconstant. In 1870-80, it was
of 9 hours, 55 minutes, 34 seconds; in 1885-86, it was longer by 7
seconds. The object had retrograded at a rate corresponding to one
complete circuit of Jupiter in six years, or of the earth in seven
months. It is not then fast moored, but floats at the mercy of the
currents and breezes predominant in the strange region it navigates.
A quiescent condition is implied by the approximate constancy of its
rotation-period during the last ten years. With the paling of its
color, its “proper motion” slackens or ceases. This must mean that,
at its maxima of agitation, it is the scene of uprushes from great
depths, which, bringing with them a slower linear velocity, occasion
the observed laggings. It is not self-luminous, and shows no symptom
of being depressed below the general level of the Jovian surface.
Jupiter has no certain and single period of rotation. Nearly all
the spots that from time to time come into view on its disk are in
relative motion, and thus give only individual results. The great
red spot has the slowest drift of all (with the rarest exceptions),
while the black cohorts of the Northern Hemisphere outmarch all
competitors. Mr. Stanley Williams, as the upshot of long study, has
delimitated nine atmospheric surfaces with definite periods. They
are well marked, and evidently have some degree of permanence, yet
the velocities severally belonging to them are distributed with
extreme irregularity. Thus, two narrow, adjacent zones differ in
movement by 400 miles an hour. This state of things must obviously
be maintained by some constantly acting force, since friction, if
unchecked, would very quickly abolish such enormous discrepancies.
The rotational zones are unsymmetrically placed; there is no
correspondence between those north and south of the Jovian equator;
and, although the equatorial drift is quicker than that of either
tropic, it is outdone in 20° to 24° north latitude.
Jupiter’s equatorial rotation, as indicated by observations of
spots, is accomplished in 9 hours 50 minutes; but Bélopolsky and
Deslandres’s spectrographic determinations gave rates of approach and
recession falling somewhat short of the corresponding velocity.
[Illustration: Three Views of Saturn
Showing Varying Aspects of the Ring taken at Different Intervals: 1,
Feb. 2, 1862; 2, Nov. 3, 1858; 3, March 23, 1856]