Red Spot, observed on the southern hemisphere of this planet for the
last three years. This interesting object, seen first in July, 1878,
disappeared for a time, reappeared on September 25 of the same year, and
has remained visible until now. When seen by me in September, it was
much elongated, and sharply pointed on one side, like a spear-head, but
it subsequently acquired an irregular form, with short appendages
protruding from its northern border. At first, the changes were great
and frequent, but at length it acquired the regular oval form, which,
with but slight modifications, it has retained until now. During the
month of November, 1880, I noticed two small black specks upon this Red
Spot, and they were seen again in January of the succeeding year, by Mr.
Alvan Clark, Jr. When the spot had attained its oval shape, it appeared
part of the time surrounded with a white luminous ring of cloudy forms
which, however, was changing more or less all the time, being sometimes
invisible. The color of this curious spot is a brilliant rosy red,
tinged with vermilion, and altogether different in shade from the
pinkish color of the equatorial belt. The size of the spot varies, but
of late its changes have been slight. Its longer diameter may be
estimated at 8,000 miles, and the shorter at 2,200 miles. The Red Spot
is represented on Plate IX. with its natural color, and as it appears at
the moments most favorable for observation. In ordinary cases its color
does not appear so brilliant, but paler.
It is difficult to account for the color of the equatorial belt and that
of the Red Spot; but it is known, at least, that the material to which
they are due cannot be situated at the level of the general surface
visible to us, and especially that of the cloudy forms of the equatorial
zone. Undoubtedly the red layer lies deeper than the superficial
envelope of the planet, although it does not seem to be very deeply
depressed.
Jupiter is attended by four satellites, which revolve around the planet
at various distances, and shine like stars of the 6th and 7th magnitude.
It is said that under very favorable circumstances, and in a very clear
sky, the satellites can be seen with the naked eye, but this requires
exceptionally keen eyes, since the glare of the planet is so strong as
to overpower the comparatively faint light of the satellites. However, I
myself have sometimes seen, without the aid of the telescope, two or
three of the satellites as a single object, when they were closely
grouped on the same side of Jupiter.
The four moons of Jupiter are all larger than our Moon, except the
first, which has about the same diameter. They range in size from 2,300
to 3,400 miles in diameter, the third being the largest; the
determination of their diameter is by no means accurate, however, as it
is difficult to measure such small objects with precision. Their mean
distance from the centre of Jupiter varies from 267,000 to 1,192,000
miles, the first satellite, the nearest to the planet, being a little
farther from Jupiter then our satellite is from us. The four satellites
revolve around the planet in orbits whose planes have a slight
inclination to the equator of Jupiter, and consequently to the ecliptic.
The diameter of the largest satellite is nearly half that of the Earth,
or 3,436 miles; while its volume is five times that of our Moon. The
period of revolution of these satellites varies from 1d. 18h. for the
first, to 16d. 16h. for the last.
Owing to the slight inclination of the plane of their orbits to that of
the planet, the three first satellites, and generally the fourth, pass
in front of the disk and also through the shadow of the planet at every
revolution, and are accordingly eclipsed. Their passages behind
Jupiter's disk are called occultations; those in front of it, transits.
The eclipses, the occultations and the transits of the moons of Jupiter
are interesting and important phenomena; the eclipses being sometimes
observed for the rough determination of longitudes at sea.
The satellites in transit present some curious phenomena. When they
enter the disk, they appear intensely luminous upon its grayish border;
but as they advance, they seem by degrees to lose their brightness,
until they finally become undistinguishable from the luminous surface of
Jupiter. It sometimes happens, however, that the first, the third and
the fourth satellites, after ceasing to appear as bright spots, continue
to be visible as dark spots upon the bright central portions of the
planet's disk; but in these cases their disks appear smaller than the
shadows they cast. Undoubtedly these satellites have extensive
atmospheres, since they sometimes pass unperceived across the central
parts of Jupiter, this being probably when their atmospheres are
condensed into clouds, strongly reflecting light; while when these
clouds are absent, we can see their actual surface, with traces of the
dark spots upon them similar to those on Mars.
From the variation in the brightness of these satellites, which is said
to be always observed in the same part of their orbit, William Herschel
was led to suppose that these bodies, like our Moon, rotate upon their
axes in the same period in which they move round the planet, so that
they always present the same face to Jupiter; but these conclusions have
been denied. From my observations it is apparent, however, that the
light reflected by them varies in intensity as well as in color. But
this is rather to be attributed to the presence of an atmosphere
surrounding these bodies, which when cloudy reflects more light than
when clear, with corresponding changes in the color of the light.
The satellites in transit are sometimes preceded or followed, according
to the position of the Sun, by a round black spot having about the same
size as the satellite itself. This black spot is the shadow of the
satellite cast on the vapory envelope of Jupiter, similar to the shadow
cast by the Moon on the Earth, during eclipses of the Sun; in fact, all
the Jovian regions traversed by these shadows have the Sun totally
eclipsed. Sometimes it happens that the shadow appears elliptical. This
occurs either when it is observed very near the limb, or when entering
upon a round, cloud-like spot. This effect is attributable to the
perspective under which the shadow is seen on the spherical globe or
spot.
The proper motion of the satellites in the Jovian sky is much more rapid
than that of the Moon in our sky. During one Jovian day of ten hours,
the first satellite advances 84°; the second, 42°; the third, 20° and
the fourth, 9°. The first satellite passes from New Moon to its first
quarter in a little more than a Jovian day, while the fourth occupies
ten such days in attaining the same phase.
In density, as well as in physical constitution, Jupiter differs widely
from the interior planets, and especially from the Earth; and, as has
been shown, it is surrounded by a dense, opaque, cloudy layer, which is
almost always impenetrable to the sight, and hides from view the
nucleus, which we may conceive to exist under this vaporous envelope. In
1876, the year of the great Jovian disturbances, I observed frequently
in the northern hemisphere of the planet a very curious phenomenon,
which seems to prove that its cloudy envelope is at times partially
absent in some places, its vapors being apparently either condensed, or
transported to other parts of its surface, and that, therefore, a
considerable part of the real globe of the planet was visible at these
places. The phenomenon consisted in the deformation of the northern
limb, which had a much shorter radius on all of this hemisphere situated
northward of the white belt which adjoins the equatorial zone. The
deformation of the limb on both sides, where it passed from a longer to
a shorter radius, was abrupt, and at right angles to the limb, forming
there a steplike indentation which was very prominent. The polar segment
having a smaller radius, appeared unusually dark, and was not striped,
as usual, but uniform in tint throughout. On September 27th, the third
satellite passed over this dark segment, and emerged from the western
border, a little below the place where the limb was abruptly deformed,
as above described. When the satellite had fully emerged from this limb,
it was apparent that if the portion of the limb having a longer radius
had been prolonged a little below, and as far as the satellite, it would
have enclosed it within its border, and thus retarded the time of
emersion. The depth of deformation of the limb was accordingly greater
than the diameter of the third satellite, and certainly more than 4,000
miles. That the phenomenon was real, is proved by the fact that the
egress of this satellite occurred at least four minutes sooner than the
time predicted for it in the American Ephemeris. Other observations seem
to point in the same direction, since some of the satellites which were
occulted have been seen through the limb of Jupiter by different
astronomers, as if this limb was sometimes semi-transparent. Another
observation of mine seems to confirm these conclusions. On April 24th,
1877, at 15h. 25m. the shadow of the first satellite was projected on
the dark band forming the northern border of the equatorial belt, the
shadow being then not far from the east limb. Close to this shadow, and
on its western side, it was preceded by a secondary shadow, which was
fainter, but had the same apparent size. This round dark spot was not
the satellite itself, as I had supposed at first, since this object was
yet outside of the planet, on the east, and entered upon it only at 16h.
4m. I watched closely this strange phenomenon, and at 16h. 45m., when
the shadow had already crossed about ¾ of the disk, it was still
preceded by the secondary, or mock shadow, as it may be called; the same
relative distance having been kept all the while between the two
objects, which had therefore traveled at the same rate. It is obvious
that this dark spot could not be one of the planet's markings, since the
shadow of the first satellite moves more quickly on the surface of
Jupiter than a spot on the same surface travels by the effect of
rotation, so that in this case the shadow would soon have passed over
this marking, and left it behind, during the time occupied by the
observation. From these observations it seems very probable that Jupiter
has a nucleus, either solid or liquid, which lies several thousand
mites below the surface of its cloudy envelope. It is also probable that
the uniformly shaded dark segment seen in 1876, was a portion of the
surface of this nucleus itself. When the cloudy envelope is
semi-transparent at the place situated on a line with an occulted
satellite and the eye of an observer, this satellite may accordingly
remain visible for a time through the limb, as shown by observation. The
phenomenon of the mock shadow may also be attributed to a similar cause,
where semi-transparent vapors receive the shadow of a satellite at their
surface, while at the same time part of this shadow, passing through the
semi-transparent vapors, may be seen at the surface of the nucleus, or
of a layer of opaque clouds situated at some distance below the surface.
Some astronomers are inclined to think that Jupiter is at a high
temperature, and self-luminous to a certain extent. If this planet is
self-luminous to any degree, we might expect that some light would be
thrown upon the satellites when they are crossing the shadow cast into
space by the planet; but when they cross this shadow they are totally
invisible in the best telescopes, a proof that they do not receive much
light from the non-illuminated side of Jupiter. It would, indeed, seem
probable that some of the intensely white spots occasionally seen on the
equatorial belt of the planet are self-luminous in a degree, yet not
enough to render the satellites visible while they are immersed in
Jupiter's shadow. It does not seem impossible that the planet should
have the high temperature attributed to it, when we remember the
terrific storms observed in its atmosphere, which, owing to the great
distance of Jupiter from the Sun, do not seem to be attributable to this
body, but rather to some local cause within the envelope of the planet.
Astronomy, which is a science of observation, is naturally silent with
regard to the inhabitants of Jupiter. If there are any such inhabitants,
they are confined to the domain of conjecture, under the dense cloudy
envelope of the planet. The conditions of habitability on Jupiter must
differ very widely from those of our globe. Comparatively little direct
light from the Sun reaches the surface of the globe of Jupiter, except
that which passes through the narrow openings forming the dark clouds.
All the rest of the planet's surface, being covered perpetually by
opaque clouds, receives only diffused light. On Jupiter there are
practically no seasons, since its axis is nearly perpendicular to its
orbit. The force of gravity on the surface of Jupiter being more than
double what it is on the Earth, living bodies would there have more than
double the weight of similar bodies on the Earth. Furthermore, Jupiter
only receives 0.011 of the light and heat which we receive from the Sun;
and its year is nearly equal to 12 of our years. If there are living
beings on Jupiter, they must, then, be entirely different from any known
to us, and they may have forms never dreamed of in our most fantastic
conceptions.
The two round black spots represented towards the central parts of Plate
IX. are the shadows of the first and second satellite; while the two
round white spots seen on the left of the disk, are the satellites
themselves, as they appeared at the moment of the observation. The first
satellite and its shadow are the nearest to the equator; while the
second satellite and its shadow are higher, the last being projected on
the Great Red Spot.[2] The row of dark circular spots represented on the
northern, or lower hemisphere, when they first appeared, had some
resemblance to Sun-spots without a penumbra, with bright markings around
them, resembling faculæ. These round spots subsequently enlarged
considerably, until they united along the entire line, encircling the
planet, and finally forming a narrow pink belt, which is still visible.
[Footnote 2: By an accidental error in enlarging the original drawing,
the satellites and shadows appear in Plate IX. of double their actual
size. The error is one easy of mental correction.]
THE PLANET SATURN
PLATE X
Saturn, which is next to Jupiter in order of distance from the Sun,
while not the largest, is certainly the most beautiful and interesting
of all the planets, with his grand and unique system of rings, and his
eight satellites, which, like faithful servants, attend the planet's
interminable journey through space.
Seen with the naked eye, Saturn shines in the night like a star of the
first magnitude, whose dull, soft whiteness is, however, far from
attaining the brilliancy of Venus or Jupiter, although it sometimes
approaches Mars in brightness. Saturn hardly ever exhibits the
phenomenon of scintillation, or twinkling, a peculiarity which makes it
easily distinguishable among the stars and planets of the heavens.
The synodical period of Saturn occupies 1 year and 13 days, so that
every 378 days, on an average, this planet holds the same position in
the sky relatively to the Sun and the Earth.
The mean distance of Saturn from the Sun is a little over 9½ times that
of our globe, or 872,000,000 miles. Owing to the orbital eccentricity,
this distance may increase to 921,000,000 miles, when the planet is in
aphelion; or decrease to 823,000,000 miles, when it is in perihelion;
Saturn being therefore 98,000,000 miles nearer to the Sun when in
perihelion than in aphelion. If gravitation were free to exert its
influence alone, Saturn would fall into the Sun in 5 years and 2 months.
The distance of Saturn from the Earth varies, according to the position
of the two planets in their respective orbits. At the time of
opposition, when the Earth lies between the Sun and Saturn, this
distance is smallest; while, on the contrary, at the time of
conjunction, when the Sun lies between the Earth and Saturn, it is
greatest. Owing, however, to the eccentricity of the orbits of Saturn
and our globe, and the inclination of their planes to each other, and
owing also to the variable heliocentric longitude of the perihelion, the
distance of the two planets from each other at their successive
conjunctions and oppositions is rendered extremely variable. At present
it is when the oppositions of Saturn occur in December that this planet
comes nearest to us; while when the conjunctions take place in June, the
distance of Saturn from the Earth is the greatest possible. In the
former case the distance of the planet from our globe is only
730,000,000 miles; while in the last it is 1,014,000,000 miles, the
difference between the nearest and farthest points of Saturn's approach
to us being no less than 284,000,000 miles, or over three times the mean
distance of the Sun from the Earth.
From the great variations in the distance of Saturn from the Earth,
necessarily result corresponding changes in the brightness and apparent
diameter of this body. When it is farthest from us, its angular diameter
measures but 14"; while, when it is nearest, it measures 20".
The orbit of Saturn is inclined 2°30' to the ecliptic, and its
eccentricity, which equals 0,056, is over three times that of the
Earth's orbit.
This planet revolves around the Sun in a period of 29 years and 5½
months, or 10,759 terrestrial days, which constitutes its sidereal year.
The extension of the immense curve forming the orbit of this planet, is
no less than 5,505,000,000 miles, which is traversed by the planet with
a mean velocity of a little less than 6 miles per second, or three times
less than the motion of our globe in space.
The real dimensions of the globe of Saturn are not yet known with
accuracy, and the equatorial diameter has been variously estimated by
observers, at from 71,000 to 79,000 miles. If we adopt the mean of these
numbers, 75,000 miles, the circumference of the Saturnian equator would
measure 235,620 miles, or 9½ times the circumference of our globe; the
surface of Saturn would be 86 times, and its volume over 810 times that
of the Earth.
However great the volume of Saturn, its mass is proportionally small,
being only 90 times greater than that of our globe; the mean density of
the materials composing this planet being less than that of cork, and
only 0.68 the density of water. The force of gravitation at the surface
of Saturn is greater, by a little over ⅑, than it is at the surface of
the Earth; a body falling in a vacuum at its surface, would travel 17.59
feet during the first second.
From observations of markings seen on the surface of Saturn, and from
the study of their apparent displacements on the disk, William Herschel
found that the planet rotated upon its axis in 10h. 16m. 0.24s. Since
Herschel's determination, new researches have been made, and lately,
Professor Hall, noticing a bright spot, followed it for nearly a month,
observing its transits across the central meridian of the disk. From
these observations he has obtained for the rotation period 10h. 14m.
23.8s., a result which agrees very closely with that obtained 82 years
earlier by Herschel, considering the fact that the markings from which
the period of rotation is ascertained are not fixed on the planet, but
are always more or less endowed with proper motion. The velocity of
rotation at the equator is 21,538 miles per hour, or nearly 6 miles per
second.
The axis of rotation of Saturn is inclined 64° 18' to the plane of the
orbit, so that its equator makes an angle of 25° 42' with the same
plane. The seasons of this planet therefore present greater extremes of
temperature than those of the Earth, but not quite so great variations
as the seasons of Mars.
The globe of Saturn is not a perfect sphere, but its figure is that of
an oblong spheroid, flattened at the poles. The polar compression of
Saturn is greater than that of any other planet, surpassing even that of
Jupiter. Though not yet determined with a great degree of accuracy, the
compression is known to be between ²⁄₁₈ and ⅒ of the equatorial
diameter; that is, a flattening of about 3,894 miles, at each pole, the
polar diameter being 7,788 miles shorter than the equatorial.
The internal condition of the planet Saturn, whether solid, liquid or
gaseous, cannot be discovered from the examination of its surface, as
its globe is enwrapped in a dense opaque layer of vapors and cloud-like
forms, through which the sight fails to penetrate. The appearance of
this vapory envelope is like that of _cumulus_ clouds, and one of its
characteristics is to arrange itself into alternate bright and dark
parallel belts, broader than those seen on Jupiter, and also more
regular and dark. These belts, which are parallel to the equator of the
planet, vary in curvature with the inclination of its axis of rotation
to the line of sight.
The belts of Saturn, like those of Jupiter, are not permanent, but keep
changing more or less rapidly. Sometimes they have been observed to be
quite numerous; while at other times they are few. Occasionally
conspicuous white or dark spots are seen on the surface, although the
phenomenon is quite rare. It is from the observation of such spots that
Saturn's period of rotation has been determined, as stated above. The
equatorial zone of Saturn always appears more white and brilliant than
the other parts, as it also appears more mottled and cloud-like. In late
years the globe has been characterized, and much adorned, by a pale
pinkish tint on its equatorial belt, resembling that of Jupiter, but
somewhat fainter. On either side of the equatorial belt there is a
narrower band, upon which the mottled appearance is visible. Below
these, one or two dark belts, separated by narrow white bands, are
usually seen; but, of late, the bands have been less numerous, being
replaced in high latitudes by a dark segment, which forms a polar cap to
Saturn. The globe of Saturn does not anywhere appear perfectly white,
and when compared with its ring, it looks of a smoky yellowish tint,
which becomes an ashy gray on its shaded parts. It usually appears
darker near the limb than in its central portions; although on some
occasions I have seen portions of the limb appear brighter, as if some
white spots were traversing it.
Some observers have seen the limb deformed and flattened at different
places, and W. Herschel even thought such a deformation to be a
permanent feature of this globe, which he termed diamond-shaped, or
"square shouldered." But this was evidently an illusion, since the
planet's limb usually appears perfectly elliptical, although it
occasionally appears as if flattened at some points, especially where it
comes in apparent contact with the shadow cast by the globe on the ring,
as observed by myself many times. But with some attention, it is
generally found that this deformation is apparent rather than real, and
is caused by the passage of some large dark spots over the limb, which
is thus rendered indistinguishable from the dark background upon which
it is projected.
What distinguishes Saturn from all known planets, or heavenly bodies,
and makes it unique in our universe, is the marvelous broad flat ring
which encircles its equator at a considerable distance from it. With a
low magnifying power this flat ring appears single, but when carefully
examined with higher powers, it is found to consist of several distinct
concentric rings and zones, all lying nearly in the same plane with the
planet's equator.
[PLATE X.--THE PLANET SATURN.
Observed on November 30, 1874, at 5h. 30m. P.M.]