powerful telescope, surrounded by its rings, and having its body striped
with dark belts, somewhat similar, but broader and less strongly marked
than those of Jupiter. In telescopes of inferior power, but still
sufficient to see the ring distinctly, we should scarcely discern the
belts at all. We might, however, observe the shadow cast upon the ring
by the planet, (as seen in the figure on the right, on the upper side;)
and, in favorable situations of the planet, we might discern glimpses of
the shadow of the ring on the body of the planet, on the lower side
beneath the ring. To see the division of the ring and the satellites
requires a better telescope than is in possession of most observers.
With smaller telescopes, we may discover an oval figure of peculiar
appearance, which it would be difficult to interpret. Galileo, who first
saw it in the year 1610, recognised this peculiarity, but did not know
what it meant. Seeing something in the centre with two projecting arms,
one on each side, he concluded that the planet was triple-shaped. This
was, at the time, all he could learn respecting it, as the telescopes he
possessed were very humble, compared with those now used by astronomers.
The first constructed by him magnified but three times; his second,
eight times; and his best, only thirty times, which is no better than a
common ship-glass.
It was the practice of the astronomers of those days to give the first
intimation of a new discovery in a Latin verse, the letters of which
were transposed. This would enable them to claim priority, in case any
other person should contest the honor of the discovery, and at the same
time would afford opportunity to complete their observations, before
they published a full account of them. Accordingly, Galileo announced
the discovery of the singular appearance of Saturn under this disguise,
in a line which, when the transposed letters were restored to their
proper places, signified that he had observed, "that the most distant
planet is triple-formed."[13] He shortly afterwards, at the request of
his patron, the Emperor Rodolph, gave the solution, and added, "I have,
with great admiration, observed that Saturn is not a single star, but
three together, which, as it were, touch each other; they have no
relative motion, and are constituted of this form, oOo, the middle one
being somewhat larger than the two lateral ones. If we examine them with
an eyeglass which magnifies the surface less than one thousand times,
the three stars do not appear very distinctly, but Saturn has an oblong
appearance, like that of an olive, thus, {oblong symbol}. Now, I have
discovered a court for Jupiter, (alluding to his satellites,) and two
servants for this old man, (Saturn,) who aid his steps, and never quit
his side."
It was by this mystic light that Galileo groped his way through an
organization which, under the more powerful glasses of his successors,
was to expand into a mighty orb, encompassed by splendid rings of vast
dimensions, the whole attended by seven bright satellites. This system
was first fully developed by Huyghens, a Dutch astronomer, about forty
years afterwards.[14] It requires a superior telescope to see it to
advantage; but, when seen through such a telescope, it is one of the
most charming spectacles afforded to that instrument. To give some idea
of the properties of a telescope suited to such observations, I annex an
extract from an account, that was published a few years since, of a
telescope constructed by Mr. Tully, a distinguished English artist. "The
length of the instrument was twelve feet, but was easily adjusted, and
was perfectly steady. The magnifying powers ranged from two hundred to
seven hundred and eighty times; but the great excellence of the
telescope consisted more in the superior distinctness and brilliancy
with which objects were seen through it, than in its magnifying powers.
With a power of two hundred and forty, the light of Jupiter was almost
more than the eye could bear, and his satellites appeared as bright as
Sirius, but with a clear and steady light; and the belts and spots on
the face of the planet were most distinctly defined. With a power of
nearly four hundred, Saturn appeared large and well defined, and was one
of the most beautiful objects that can well be conceived."
That the ring is a solid opaque substance, is shown by its throwing its
shadow on the body of the planet on the side nearest the sun, and on the
other side receiving that of the body. The ring encompasses the
equatorial regions of the planet, and the planet revolves on an axis
which is perpendicular to the plane of the ring in about ten and a half
hours. This is known by observing the rotation of certain dusky spots,
which sometimes appear on its surface. This motion is nearly the same
with the diurnal motion of Jupiter, subjecting places on the equator of
the planet to a very swift revolution, and occasioning a high degree of
compression at the poles, the equatorial being to the polar diameter in
the high ratio of eleven to ten.
Saturn's ring, in its revolution around the sun, _always remains
parallel to itself_. If we hold opposite to the eye a circular ring or
disk, like a piece of coin, it will appear as a complete circle only
when it is at right angles to the axis of vision. When it is oblique to
that axis, it will be projected into an ellipse more and more flattened,
as its obliquity is increased, until, when its plane coincides with the
axis of vision, it is projected into a straight line. Please to take
some circle, as a flat plate, (whose rim may well represent the ring of
Saturn,) and hold it in these different positions before the eye. Now,
place on the table a lamp to represent the sun, and holding the ring at
a certain distance, inclined a little towards the lamp, carry it round
the lamp, always keeping it parallel to itself. During its revolution,
it will twice present its edge to the lamp at opposite points; and
twice, at places ninety degrees distant from those points, it will
present its broadest face towards the lamp. At intermediate points, it
will exhibit an ellipse more or less open, according as it is nearer one
or the other of the preceding positions. It will be seen, also, that in
one half of the revolution, the lamp shines on one side of the ring, and
in the other half of the revolution, on the other side.
Such would be the successive appearances of Saturn's ring to a spectator
on the sun; and since the earth is, in respect to so distant a body as
Saturn, very near the sun, these appearances are presented to us nearly
in the same manner as though we viewed them from the sun. Accordingly,
we sometimes see Saturn's ring under the form of a broad ellipse, which
grows continually more and more acute, until it passes into a line, and
we either lose sight of it, altogether, or, by the aid of the most
powerful telescopes, we see it as a fine thread of light drawn across
the disk, and projecting out from it on each side. As the whole
revolution occupies thirty years, and the edge is presented to the sun
twice in the revolution, this last phenomenon, namely, the disappearance
of the ring, takes place every fifteen years.
[Illustration Fig. 61.]
You may perhaps gain a still clearer idea of the foregoing appearances
from the following diagram, Fig. 61. Let A, B, C, &c., represent
successive positions of Saturn and his ring, in different parts of his
orbit, while _a b_ represents the orbit of the earth. Please to remark,
that these orbits are drawn so elliptical, not to represent the
eccentricity of either the earth's or Saturn's orbit, but merely as the
projection of circles seen very obliquely. Also, imagine one half of the
body of the planet and of the ring to be above the plane of the paper,
and the other half below it. Were the ring, when at C and G,
perpendicular to C G, it would be seen by a spectator situated at _a_ or
_b_ as a perfect circle; but being inclined to the line of vision
twenty-eight degrees four minutes, it is projected into an ellipse. This
ellipse contracts in breadth as the ring passes towards its nodes at A
and E, where it dwindles into a straight line. From E to G the ring
opens again, becomes broadest at G, and again contracts, till it
becomes a straight line at A, and from this point expands, till it
recovers its original breadth at C. These successive appearances are all
exhibited to a telescope of moderate powers.
The ring is extremely _thin_, since the smallest satellite, when
projected on it, more than covers it. The thickness is estimated at only
one hundred miles. Saturn's ring shines wholly by _reflected light_
derived from the sun. This is evident from the fact that that side only
which is turned towards the sun is enlightened; and it is remarkable,
that the illumination of the ring is greater than that of the planet
itself, but the outer ring is less bright than the inner. Although we
view Saturn's ring nearly as though we saw it from the sun, yet the
plane of the ring produced may pass between the earth and the sun, in
which case, also, the ring becomes invisible, the illuminated side being
wholly turned from us. Thus, when the ring is approaching its node at E,
a spectator at _a_ would have the dark side of the ring presented to
him. The ring was invisible in 1833, and will be invisible again in
1847. The northern side of the ring will be in sight until 1855, when
the southern side will come into view. It appears, therefore, that there
are three causes for the disappearance of Saturn's ring: first, when the
edge of the ring is presented to the sun; secondly, when the edge is
presented to the earth; and thirdly, when the unilluminated side is
towards the earth.
Saturn's ring _revolves_ in its own plane in about ten and a half hours.
La Place inferred this from the doctrine of universal gravitation. He
proved that such a rotation was necessary; otherwise, the matter of
which the ring is composed would be precipitated upon its primary. He
showed that, in order to sustain itself, its period of rotation must be
equal to the time of revolution of a satellite, circulating around
Saturn at a distance from it equal to that of the middle of the ring,
which period would be about ten and a half hours. By means of spots in
the ring, Dr. Herschel followed the ring in its rotation, and actually
found its period to be the same as assigned by La Place,--a coincidence
which beautifully exemplifies the harmony of truth.
Although the rings have very nearly the same centre with the planet
itself, yet, recent measurements of extreme delicacy have demonstrated,
that the coincidence is not mathematically exact, but that the centre of
gravity of the rings describes around that of the body a very minute
orbit. "This fact," says Sir J. Herschel, "unimportant as it may seem,
is of the utmost consequence to the stability of the system of rings.
Supposing them mathematically perfect in their circular form, and
exactly concentric with the planet, it is demonstrable that they would
form (in spite of their centrifugal force) a system in a state of
unstable equilibrium, which the slightest external power would subvert,
not by causing a rupture in the substance of the rings, but by
precipitating them unbroken upon the surface of the planet." The ring
may be supposed of an unequal breadth in its different parts, and as
consisting of irregular solids, whose common centre of gravity does not
coincide with the centre of the figure. Were it not for this
distribution of matter, its equilibrium would be destroyed by the
slightest force, such as the attraction of a satellite, and the ring
would finally precipitate itself upon the planet. Sir J. Herschel
further observes, that, "as the smallest difference of velocity between
the planet and its rings must infallibly precipitate the rings upon the
planet, never more to separate, it follows, either that their motions in
their common orbit round the sun must have been adjusted to each other
by an external power, with the minutest precision, or that the rings
must have been formed about the planet while subject to their common
orbitual motion, and under the full and free influence of all the acting
forces.
"The rings of Saturn must present a magnificent spectacle from those
regions of the planet which lie on their enlightened sides, appearing
as vast arches spanning the sky from horizon to horizon, and holding an
invariable situation among the stars. On the other hand, in the region
beneath the dark side, a solar eclipse of fifteen years in duration,
under their shadow, must afford (to our ideas) an inhospitable abode to
animated beings, but ill compensated by the full light of its
satellites. But we shall do wrong to judge of the fitness or unfitness
of their condition, from what we see around us, when, perhaps, the very
combinations which convey to our minds only images of horror, may be in
reality theatres of the most striking and glorious displays of
beneficent contrivance."
Saturn is attended by _seven satellites_. Although they are bodies of
considerable size, their great distance prevents their being visible to
any telescope but such as afford a strong light and high magnifying
powers. The outermost satellite is distant from the planet more than
thirty times the planet's diameter, and is by far the largest of the
whole. It exhibits, like the satellites of Jupiter, periodic variations
of light, which prove its revolution on its axis in the time of a
sidereal revolution about Saturn, as is the case with our moon, while
performing its circuit about the earth. The next satellite in order,
proceeding inwards, is tolerably conspicuous; the three next are very
minute, and require powerful telescopes to see them; while the two
interior satellites, which just skirt the edge of the ring, and move
exactly in its plane, have never been discovered but with the most
powerful telescopes which human art has yet constructed, and then only
under peculiar circumstances. At the time of the disappearance of the
rings, (to ordinary telescopes,) they were seen by Sir William Herschel,
with his great telescope, projected along the edge of the ring, and
threading, like beads, the thin fibre of light to which the ring is then
reduced. Owing to the obliquity of the ring, and of the orbits of the
satellites, to that of their primary, there are no eclipses of the
satellites, the two interior ones excepted, until near the time when
the ring is seen edgewise.
"The firmament of Saturn will unquestionably present to view a more
magnificent and diversified scene of celestial phenomena than that of
any other planet in our system. It is placed nearly in the middle of
that space which intervenes between the sun and the orbit of the
remotest planet. Including its rings and satellites, it may be
considered as the largest body or system of bodies within the limits of
the solar system; and it excels them all in the sublime and diversified
apparatus with which it is accompanied. In these respects, Saturn may
justly be considered as the sovereign among the planetary hosts. The
prominent parts of its celestial scenery may be considered as belonging
to its own system of rings and satellites, and the views which will
occasionally be opened of the firmament of the fixed stars; for few of
the other planets will make their appearance in its sky. Jupiter will
appear alternately as a morning and an evening star, with about the same
degree of brilliancy it exhibits to us; but it will seldom be
conspicuous, except near the period of its greatest elongation; and it
will never appear to remove from the sun further than thirty-seven
degrees, and consequently will not appear so conspicuous, nor for such a
length of time, as Venus does to us. Uranus is the only other planet
which will be seen from Saturn, and it will there be distinctly
perceptible, like a star of the third magnitude, when near the time of
its opposition to the sun. But near the time of its conjunction it will
be completely invisible, being then eighteen hundred millions of miles
more distant than at the opposition, and eight hundred millions of miles
more distant from Saturn than it ever is from the earth at any
period."[15]
URANUS.--Uranus is the remotest planet belonging to our system, and is
rarely visible, except to the telescope. Although his diameter is more
than four times that of the earth, being thirty-five thousand one
hundred and twelve miles, yet his distance from the sun is likewise
nineteen times as great as the earth's distance, or about eighteen
hundred millions of miles. His revolution around the sun occupies nearly
eighty-four years, so that his position in the heavens, for several
years in succession, is nearly stationary. His path lies very nearly in
the ecliptic, being inclined to it less than one degree. The sun
himself, when seen from Uranus dwindles almost to a star, subtending, as
it does, an angle of only one minute and forty seconds; so that the
surface of the sun would appear there four hundred times less than it
does to us. This planet was discovered by Sir William Herschel on the
thirteenth of March, 1781. His attention was attracted to it by the
largeness of its disk in the telescope; and finding that it shifted its
place among the stars, he at first took it for a comet, but soon
perceived that its orbit was not eccentric, like the orbits of comets,
but nearly circular, like those of the planets. It was then recognised
as a new member of the planetary system, a conclusion which has been
justified by all succeeding observations. It was named by the discoverer
the _George Star_, (Georgium Sidus,) after his munificent patron, George
the Third; in the United States, and in some other countries, it was
called _Herschel_; but the name _Uranus_, from a Greek word, (= Ouranos=,
_Ouranos_,) signifying the oldest of the gods, has finally prevailed. So
distant is Uranus from the sun, that light itself, which moves nearly
twelve millions of miles every minute, would require more than two hours
and a half to pass to it from the sun.
And now, having contemplated all the planets separately, just cast your
eyes on the diagram facing page 236, Fig. 53, and you will see a
comparative view of the various magnitudes of the sun, as seen from each
of the planets.
Uranus is attended by _six satellites_. So minute objects are they, that
they can be seen only by powerful telescopes. Indeed, the existence of
more than two is still considered as somewhat doubtful. These two,
however, offer remarkable and indeed quite unexpected and unexampled
peculiarities. Contrary to the unbroken analogy of the whole planetary
system, _the planes of their orbits are nearly perpendicular to the
ecliptic_, and in these orbits their motions are retrograde; that is,
instead of advancing from west to east around their primary, as is the
case with all the other planets and satellites, they move in the
opposite direction. With this exception, all the motions of the planets,
whether around their own axes, or around the sun, are from west to east.
The sun himself turns on his axis from west to east; all the primary
planets revolve around the sun from west to east; their revolutions on
their own axes are also in the same direction; all the secondaries, with
the single exception above mentioned, move about their primaries from
west to east; and, finally, such of the secondaries as have been
discovered to have a diurnal revolution, follow the same course. Such
uniformity among so many motions could have resulted only from forces
impressed upon them by the same Omnipotent hand; and few things in the
creation more distinctly proclaim that God made the world.
Retiring now to this furthest verge of the solar system, let us for a
moment glance at the aspect of the firmament by night. Notwithstanding
we have taken a flight of eighteen hundred millions of miles, the same
starry canopy bends over our heads; Sirius still shines with exactly the
same splendor as here; Orion, the Scorpion, the Great and the Little
Bear, all occupy the same stations; and the Galaxy spans the sky with
the same soft and mysterious light. The planets, however, with the
exception of Saturn, are all lost to the view, being too near the sun
ever to be seen; and Saturn himself is visible only at distant
intervals, at periods of fifteen years, when at its greatest elongations
from the sun, and is then too near the sun to permit a clear view of his
rings, much less of the satellites that unite with the rings to compose
his gorgeous retinue. Comets, moving slowly as they do when so distant
from the sun, will linger much longer in the firmament of Uranus than in
ours.
Although the sun sheds by day a dim and cheerless light, yet the six
satellites that enlighten and diversify the nocturnal sky present
interesting aspects. "Let us suppose one satellite presenting a surface
in the sky eight or ten times larger than our moon; a second, five or
six times larger; a third, three times larger; a fourth, twice as large;
a fifth, about the same size as the moon; a sixth, somewhat smaller;
and, perhaps, three or four others of different apparent dimensions: let
us suppose two or three of those, of different phases, moving along the
concave of the sky, at one period four or five of them dispersed through
the heavens, one rising above the horizon, one setting, one on the
meridian, one towards the north, and another towards the south; at
another period, five or six of them displaying their lustre in the form
of a half moon, or a crescent, in one quarter of the heavens; and, at
another time, the whole of these moons shining, with full enlightened
hemispheres, in one glorious assemblage, and we shall have a faint idea
of the beauty, variety, and sublimity of the firmament of Uranus."[16]
_The New Planets,--Ceres, Pallas, Juno, and Vesta._--The commencement of
the present century was rendered memorable in the annals of astronomy,
by the discovery of four new planets, occupying the long vacant tract
between Mars and Jupiter. Kepler, from some analogy which he found to
subsist among the distances of the planets from the sun, had long before
suspected the existence of one at this distance; and his conjecture was
rendered more probable by the discovery of Uranus, which follows the
analogy of the other planets. So strongly, indeed, were astronomers
impressed with the idea that a planet would be found between Mars and
Jupiter, that, in the hope of discovering it, an association was formed
on the continent of Europe, of twenty-four observers, who divided the
sky into as many zones, one of which was allotted to each member of the
association. The discovery of the first of these bodies was, however,
made accidentally by Piazzi, an astronomer of Palermo, on the first of
January, 1801. It was shortly afterwards lost sight of on account of its
proximity to the sun, and was not seen again until the close of the
year, when it was re-discovered in Germany. Piazzi called it _Ceres_, in
honor of the tutelary goddess of Sicily, and her emblem, the sickle,
([Planet: Ceres]) has been adopted as its appropriate symbol.
The difficulty of finding Ceres induced Dr. Olbers, of Bremen, to
examine with particular care all the small stars that lie near her path,
as seen from the earth; and, while prosecuting these observations, in
March, 1802, he discovered another similar body, very nearly at the same
distance from the sun, and resembling the former in many other
particulars. The discoverer gave to this second planet the name of
_Pallas_, choosing for its symbol the lance, ([Planet: Pallas]) the
characteristic of Minerva.