several other planets have been discovered, making the number of those
belonging to our planetary system 'sixteen' instead of 'eleven'. Of these,
Astrea, Hebe, Flora, and Iris are members of the remarkable group of
asteroids between Mars and Jupiter. Astrea and Hebe were discovered by
Hencke at Driesen, the one in 1846 and the other in 1847; Flora and Iris
were both discovered in 1847 by Mr. Hind, at the South Villa Observatory,
Regent's Park. It would appear from the latest determinations of their
elements, that the small planets have the following order with respect to
mean distance from the Sun: Flora, Iris, Vesta, Hebe, Astrea, Juno, Ceres,
Pallas. Of these, Flora has the shortest period (about 3 1/4 years). The
planet Neptune, which, after having been predicted by several astronomers,
was actually observed on the 25th of September, 1846, is situated on the
confines of our planetary system beyond Uranus. The discovery of this
planet is not only highly interesting from the importance attached to it as
a question of science, but also from the evidence it affords of the care and
unremitting labor evinced by modern astronomers in the investigation and
comparison of the older calculations, and the ingenious application of the
results thus obtained to the observation of new facts. The merit of having
paved the way for the discovery of the planet Neptune is due to M. Bouvard,
who, in his persevering and assiduous efforts to deduce the entire orbit of
Uranus from observations made during the forty years that succeeded the
discovery of that planet in 1781, found the results yielded by theory to be
at variance with fact, in a degree that had no parallel in the history of
astronomy. This startling discrepancy, which seemed only to gain additional
weight from every attempt made by M. Bouvard to correct his calculations,
led Leverrier, after a careful modification of the tables of Bouvard, to
establish the proposition that there was "a formal incompatibility between
the observed motions of Uranus and the hypothesis that he was acted on
'only' by the Sun and known planets, according to the law of universal
gravitation." Pursuing this idea, Leverrier arrived at the conclusion that
the disturbing cause must be a 'planet', and finally, after an amount of
labor that seems perfectly overwhelming, he, on the 31st of August, 1846,
laid before the French Institute a paper, in which he indicated the exact
spot in the heavens where this new planetary body would be found, giving the
following data for its various elements: mean distance from the Sun, 36.154
times that of the Earth; period of revolution, 217.387 years; mean long.,
Jan. 1st, 1847, 318 degrees 47'; mass, 1/9300th; heliocentric long., Jan
1st1847, 326 degrees 32'. Essential difficulties still intervened, however,
and as the remoteness of the planet rendered it improbable that its disk
would be discernible by any telescopic instrument, no other means remained
for detecting the suspected body but its planetary motion, which could only
be ascertained by mapping, after every observation, the quarter of the
heavens scanned, and by a comparison of the various maps. Fortunately for
the verification of Leverrier's predictions, Dr. Bremiker had just completed
a map of the precise region in which it was expected the new planet would
apper, this being one of a series of maps made for the Academy of Berlin, of
the small stars along the entire zodiac. By means of this valuable
assistance, Dr. Galle, of the Berlin Observatory, was led, on the 25th of
September, 1846, by the discovery of a star of the eighth magnitude, not
recorded in Dr. Bremiker's map, to make the first observation of the planet
predicted by Leverrier. By a singular coincidence, Mr. Adams, of Cambridge,
had predicted the appearance of the planet simultaneously with M. Leverrier;
but by the concurrence of several circumstances much to be regretted, the
world at large were not made acquainted with Mr. Adams's valuable discovery
until subsequently to the period at which Leverrier published his
observations. As the data of Leverrier and Adams stand at present, there is
a discrepancy between the predicted and the true distance, and in some other
elements of the planet; it remains therefore, for these or future
astronomers to reconcile theory with fact, or perhaps, as in the case of
Uranus, to make the new planet the means of leading to yet greater
discoveries. It would appear from the most recent observations, that the
mass of Neptune, instead of being, as at first stated, 1/9300th, is only
about 1/23000th that of the Sun, while its periodic time is now given with a
greater probability at 166 years, and its mean distance from the Sun nearly
30. The planet appears to have a ring, but as yet no accurate observations
have been made regarding its system of satellites. See 'Trans. Astron.
Soc.', and 'The Planet Neptune', 1848, by J. P. Nicholl.) -- Tr.
We may, with no incondsiderable degree of probability, include within the
domain of our Sun, in the immediate sphere of its central force, a rotating
ring of vaporous matter, lying probably between the orbits of Venus and
Mars, but certainly beyond that of the Earth,* which appears to us in
p 92
a pyramidal form, and is known as the 'Zodiacal Light'; and a host of very
small asteroids, whose orbits either intersect, or very nearly approach,
that of our earth, and which present us with the phenomena of aerolites and
falling or shooting stars.
[footnote] * "If there should be molecules in the zones diffused by the
atmosphere of the Sun of too volatile a nature either to combine with one
another or with the planets, we must suppose that they would, in circling
round that luminary, present all the appearances of zodiacal light, without
opposing any appreciable resistance to the different bodies composing the
planetary system, either owing to their extreme rarity, or to the similarity
existing between their motion and that of the planets with which they come
in contact." -- Laplace, 'Expos. du Syst. du Monde' (ed. 5), p. 415.
When we consider the complication of variously-formed bodies which revolve
round the Sun in orbits of such dissimilar eccentricity--although we may not
be disposed, with the immortal author of the 'Mecanique Celeste', to regard
the largr number of comets as nebulous stars, passing from one central
system to another,* we yet can not fail to acknowledge that the planetary
system, especially so called (that is, the group of heavenly bodies which,
together with their satellites, revolve with but slightly eccentric orbits
round the Sun), constitutes but a small portion of the whole system with
respect to individual numbers, if not to mass.
[footnote] *Laplace, 'Exp. du Syst. du Monde', p. 396, 414.
It has been proposed to consider the telescopic planets, Vesta, Juno, Ceres,
and Pallas, with their more closely intersecting, inclined, and eccentric
orbits, as a zone of separation, or as a middle group in space; and if this
view be adopted, we shall discover that the interior planetary group
(consisting of Mercury, Venus, the Earth, and Mars) presents several very
striking contrasts* when compared with the exterior group, comprising
Jupiter, Saturn, and Uranus.
[footnote] *Littrow, 'Astronomie', 1825, bd.xi., 107. Mädler, 'Astron.',
1841, § 212. Laplace, 'Exp. du Syst. du Monde', p. 210.
The planets nearest the Sun, and consequently included in the inner group,
are of more moderate size, denser, rotate more slowly and with nearly equal
velocity (their periods of revolution being almost all about 24 hours), are
less compressed at the poles, and with the exception of one, are without
satellites. The exterior planets, which are further removed from the Sun,
are very considerably larger, have a density five times less, more than
twice as great a velocity in the period of their rotation round their axes,
are more compressed at the poles, and if six satellites may be ascribed to
Uranus, have a quantitative preponderance in the number of their attendant
moons, which is as seventeen to one.
p 93
Such general considerations regarding certain characteristic properties
appertaining to whole groups, can not, however, be applied with equal
justice to the individual planets of every group, nor to the relations
between the distances of the revolving planets from the central body, and
their absolute size, density, period or rotation, eccentricity, and the
inclination of their orbits and the axes. We know as yet of no inherent
necessity, no mechanical natural law, similar to the one which teaches us
that the squares of the periodic times are proportional to the cubes of the
major axes, by which the above-named six elements of the planetary bodies
and the form of their orbit are made dependent either on one another, or on
their mean distance from the Sun. Mars is smaller than the Earth and Venus,
although further removed from the Sun than these last-named planets,
approaching most nearly in size to Mercury, the nearest planet to the Sun.
Saturn is smaller than Jupiter, and yet much larger than Uranus. The zone
of the telescopic planets, which have so inconsiderable a volume,
immediately procede Jupiter (the greatest in size of any of the planetary
bodies), if we consider them with regard to distance from the Sun; and yet
the disks of these small asteroids, which scarcely admit of measurement,
have an areal surface not much more than half that of France, Madagascar, or
Borneo. However striking may be the extremely small density of all the
colossal planets, which are furthest removed from the Sun, we are yet unable
in this respect to recognize any regular succession.*
[footnote] *See Kepler, on the increasing density and volume of the planets
in proportion with their increase of distance from the Sun, which is
described as the densest of all the heavenly bodies; in the 'Epitome Astran.
Copern. in' vii. 'libros digesta', 1618-1622, p. 420. Leibnitz also
inclined to the opinions of Kepler and Otto von Guericke, that the planets
increase in volume in proportion to their increase of distance from the Sun.
See his letter to the Magdeburg Burgomaster (Mayence, 1671), in Leibnitz,
'Deutschen Schriften, herausg. von Guhrauer', th. i., 264.
Uranus appears to be denser than Saturn, even if we adopt the smaller mass,
1/24605, assumed by Lamont; and, notwithstanding the inconsiderable
difference of density observed in the innermost planetary group,* we find
both Venus and Mars less dense than the Earth, which lies between them.
[footnote] *On the arrangement of masses, see Encke, in Schum., 'Astr.
Nachr', 1843 Nr. 488, 114.
The time of rotation certainly diminishes with increasing solar distance,
but yet it is greater in Mars than in the Earth, and in Saturn than in
Jupiter. The elliptic
p 94
orbits of Juno, Pallas, and Mercury have the greatest degree of
eccentricity, and Mars and Venus, which immediately follow each other, have
the least. Mercury and Venus exhibit the same contrasts that may be
observed in the four smaller planets, or asteroids, whose paths are so
closely interwoven.
The eccentriciities of Juno and Pallas are very nearly identical, and reach
three times as great as those of Ceres and Vesta. The same may be said of
the inclination of the orbits of the planets toward the plane of projection
of the ecliptic, or in the position of their axes of rotation with relation
to their orbits, a position on which the relations of climate, seasons of
the year, and length of the days depend more than on eccentricity. Those
planets that have the most elongated elliptic orbits, as Juno, Pallas, and
Mercury, have also, although not to the same degree their orbits most
strongly inclined toward the ecliptic. Pallas has a comet-like inclination
nearly twenty-six times greater than that of Jupiter, while in the little
planet Vesta, which is so near Pallas, the angle of inclination scarcely by
six times exceeds that of Jupiter. An equally irregular succession is
observed in the position of the axes of the few planets (four or five) whose
planes of rotation we know with any degree of certainty. It would appear
from the position of the satellites of Uranus, two of which, the second and
fourth, have been recently observed with certainty, that the axis of this,
the outermost of all the planets is scarcely inclined as much as 11 degrees
toward the plane of its orbit, while Saturn is placed between this planet,
whose axis almost coincides with the plane of its orbit, and Jupiter, whose
axis of rotation is nearly perpendicular to it.
In this enumeration of the forms which compose the world in space, we have
delineated them as possessing an actual existence, and not as objects of
intellectual contemplation, or as mere links of a mental and causal chain of
connection. The planetary system, in its relations of absolute size and
relative position of the axes, density, time of rotation, and different
degrees of eccentricity of the orbits, does not appear to offer to our
apprehension any stronger evidence of a natural necessity than the
proportion observed in the distribution of land and water on the Earth, the
configuration of continents, or the height of mountain chains. In these
respects we can discover no common law in the regions of space or in the
inequalities of the earth's crust. They are 'facts' in nature that have
arisen from the conflict of manifold forces acting under unknown
p 95
conditions, although man considers as 'accidental' whatever he is unable to
explain in the planetary formation on purely genetic principles. If the
planets have been formed out of separate rings of vaporous matter revolving
round the Sun, we may conjecture that the different thickness, unequal
density, temperature, and electro-magnetic tension of these rings may have
given occasion to the most various agglomerations of matter, in the same
manner as the amount of tangential velocity and small variations in its
direction have produced so great a differencein the forms and inclinations
of the elliptic orbits. Attractions of mass and laws of gravitation have no
doubt exercised an influence here, no less than in the geognostic relations
of the elevations of continents; but we are unable from the present forms to
draw any conclusions regarding the series of conditions through which they
have passed. Even the so-called law of the distances of the planets from
the Sun, the law of progression (which led Kepler to conjecture the
existence of a planet supplying the link that was wanting in the chain of
connection between Mars and Jupiter), has been found numerically inexact for
the distances between Mercury, Venus, and the Earth, and a variance with the
conception of a series, owing to the necessity for a supposition in the case
of the first member.
The hitherto disscovered principal planets that revolve round our Sun are
attended certainly by fourteen, and probably by eighteen secondary planets
(moons or satellites). The principal planets are, therefore, themselves the
central bodies of subordinate systems. We seem to recognize in the fabric
of the universe the same process of arrangement so frequently exhibited in
the development of organic life, where we find in the manifold combinations
of groups of plants or animals the same typical form repeated in the
'subordinate classes'. The secondary planets or satellites are more
frequent in the external region of the planetary system, lying beyond the
intersecting orbits of the smaller planets or asteroids; in the inner region
none of the planets are attended by satellites, with the exception of the
Earth, whose moon is relatively of great magnitude, since its diameter is
equal to a fourth of that of the Earth, while the diameter of the largest of
all known secondary planets -- the sixth satellite of Saturn -- is probably
about one seventeenth, and the largest of Jupiter's moons, the third, only
about one twenty-sixth part that of the primary planet or central body. The
planets which are attended by the largest number of satellites are most
remote from the Sun,
p 96
and are at the same time the largest, most compressed at the poles, and the
least dense. According to the most recent measurements of Mädler, Uranus
has a greater planetary compression than any other of the planets, viz.,
1/9.92d. In our Earth and her moon, whose mean distance from one another
amounts to 207,200 miles, we find that the differences of mass* and diameter
between the two are much less considerable than are usually observed to
exist between the principal planets and their attendant satellites, or
between bodies of different orders in the solar system.
[footnote] *If, according to Burckhardt's determination, the Moon's radius
be 0.2725 and its volume 1/49.00th, its density will be 0.5596, or nearly
five ninths. Compare, also, Wilh. Beer and H. Madler, 'der Mond', 2, 10,
and Madler, 'Ast.', 157. The material contents of the Moon are, according
to Hansen, nearly 1/34th (and ädler 1/40.6th) that of the Earth, and its
mass equal to 1/87.73d that of the Earth. In the largest of Jupiter's
moons, the third, the relations of volume to the central body are 1/15370th,
and of mass 1/11300th. On the polar flattening of Uranus, see Schum,
'Astron. Nachr.', 1844, No. 493.
While the density of the Moon is five ninths less than that of the Earth, it
would appear, if we may sufficiently depend upon the determinations of their
magnitudes and masses, that the second of Jupiter's moons is actually denser
than that great planet itself. Among the fourteen satellites that have been
investigated with any degree of certainty, the system of the seven
satellites of Saturn presents an instance of the greatest possible contrast,
both in absolute magnitude and in distance from the central body. The sixth
of these satellites is probably not much smaller than Mars, while our moon
has a diameter which does not amount to more than half that of the latter
planet. With respect to volume, the two outer, the sixth and seventh of
Saturn's satellites, approach the nearest to the third and brightest of
Jupiter's moons. The two innermost of these satellites belong perhaps,
together with the remote moons of Uranus to the smallest cosmical bodies of
our solar system, being only made visible under favorable circumstances by
the most powerful instruments. They were first discovered by the forty-foot
telescope of William Herschel in 1789, and were seen again by John Herschel
at the Cape of Good Hope, by Vico at Rome, and by Lamont at Munich.
Determinations of the 'true' diameter of satellites, made by the measurement
of the apparent size of their small disks, are subjected to many optical
difficulties; but numerical astronomy, whose task it is to predetermine by
calculation the motions of the heavenly bodies as they will appear when
viewed from the Earth, is directed almost
p 97
exclusively to motion and mass, and but little to volume. The absolute
distance of a satellite from its central body is greatest in the case of the
outermost or seventh satellite of Saturn, its distance from the body round
which it revolves amounting to more than two millions of miles, or ten times
as great a distance as that of our moon from the Earth. In the case of
Jupiter we find that the outermost or fourth attendant moon is only
1,040,000 miles from that planet, while the distance between Uranus and its
sixth satellite (if the latter really exist) amounts to as much as 1,360,000
miles. If we compare, in each of these subordinate systems, the volume of
the satellite, we discover the existence of entirely new numerical
relations. The distances of the outermost satellites of Uranus, Saturn, and
Jupiter are when expressed in semi-diameters of the main planets, as 91, 64,
and 27. The outermost satellite of Saturn appears, therefore, to be removed
only about one fifteenth further from the center of that planet than our
moon is from the Earth. The first or innermost of Saturn's satellites is
nearer to its central body than any other of the secondary planets, and
presents, moreover, the only instance of a period of revolution of less than
twenty-four hours. Its distance from the center of Saturn may, according to
Mädler and Wilhelm Beer, be expressed as 2.47 semi-diameters of that
planet, or as 80,088 miles. Its distance from the surface of the main
planet is therefore 47,480 miles, and from the outer-most edge of the ring
only 4916 miles. The traveler may form to himself an estimate of the
smallness of this amount by remembering the statement of an enterprising
navigator, Captain Beechey, that he had in three years passed over 72,800
miles. If, instead of absolute distances, we take the semi-diameters of the
principal planets, we shall find that even the first or nearest of the moons
of Jupiter (which is 26,000 miles further removed from the center of that
planet than our moon is from that of the Earth) is only six semi-diameters
of Jupiter from its center, while our moon is removed from us fully 60 1/3d
semi-diameters of the Earth.
In the subordinate systems of satellites, we find that the same laws of
gravitation which regulate the revolutions of the principal planets round
the Sun likewise govern the mutual relations existing between these planets
among one another and with reference to their attendant satellites. The
twelve moons of Saturn, Jupiter, and the Earth all most like the primary
planets from west to east, and in elliptic orbits, deviating
p 98
but little from circles. It is only in the case of one moon, and perhaps in
that of the first and innermost of the satellites of Saturn (0.068), that we
discover an eccentricity greater than that of Jupiter; according to the very
exact observations of Bessel, the eccentricity of the sixth of Saturn's
satellites (0.029) exceeds that of the Earth. On the extremest limits of
the planetary system, where, at a distance nineteen times greater than that
of our Earth, the centripetal force of the Sun is greatly diminished, the
satellites of Uranus (which most striking contrasts from the facts observed
with regard to other secondary planets. Instead, as in all other
satellites, of having their orbits but slightly inclined toward the ecliptic
and (not excepting even Saturn's ring, which may be regarded as a fusion of
agglomerated satellites) moving from west to east, the satellites of Uranus
are almost perpendicular to the ecliptic, and move retrogressively from east
to west, as Sir John Herschel has proved by observations continued during
many years. If the primary and secondary planets have been formed by the
condensation of rotating rings of solar and planetary atmospheric vapor,
there must have existed singular causes of retardation or impediment in the
vaporous rings revolving round Uranus, by which, under the relations with
which we are unacquainted, the revolution of the second and fourth of its
satellites was made to assume a direction opposite to that of the rotation
of the central planet.
It seems highly probable that the period of rotation of 'all' secondary
planets is equal to that of their revolution round the main planet, and
therefore that they always present to the latter the same side.
Inequalities, occasioned by sight variations in the revolution, give rise to
fluctuations of from 6 degrees to 8 degrees, or to an apparent libration in
longitude as well as in latitude. Thus, in the case of our moon, we
sometimes observe more than the half of its surface, the eastern and
northern edges being more visible at one time, and the western or southern
at another. By means of this libration* we are enabled to see the annular
mountain Malapert (which occasionally conceals the Moon's south pole), the
arctic landscape round the crater of Gioja, and the large gray plane near
Endymion which exceeds in superficial extent the 'Mare Vaporum'.
[footnote] *Beer and Madler, op. cit., 185, s.208, and § 347, s. 332; and
ix their 'Phys. Kenntniss der himml. Korper', s. 4 und 69, Tab. 1 (Physical
History of the Heavenly Bodies).