and Mairan ('Aurore Bor.', p. 16), have even maintained that the phenomenon
observed in Persia in 1668 was the zodiacal light. Delambre ('Hist. de
l'Astron. Moderne', t. ii., p. 742), in very decided trms ascribes the
discovery of this light to the celebrated traveler Chardin; but in the
'Couronnement de Soliman', and in several passages of the narrative of his
travels (éd. de Langlès. t. iv., p. 326; t. x., p. 97), he only applies
the term niazouk (nyzek), or "petite lance," to "the great and famous comet
which appeared over nearly the whole world in 1668, and whose head was so
hidden in the wewst that it could not be perceived in the horizon of
Ispahan" ('Atlas du Voyage de Chardin', Tab. iv.; from the observations at
Schiraz). The head or nucleus of the comet was, however, visible in the
Brazils and in India (Pingré, 'Cométogr.', t. ii., p. 22). Regarding the
conjectured identity of the last great comet of March, 1843, with this,
which Cassini mistook for the zodiacal light, see Schum., 'Astr. Nachr.',
1843, No. 476 and 480. In Persian, the term "nizehi âteschîn"(fiery
spears or lances) is also applied to the rays of the rising or setting sun,
in the same way as "nayâzik," according to Freytag's Arabic Lexicon,
signifies "stellæ cadentes." The comparison of comets to lances and swords
was, however, in the Middle Ages, very common in all languages. The great
comet of 1500, which was visible from April to June, was always termed by
the Italian writers of that time 'il Signor Astone' (see my 'Examen Critique
de l'Hist. de la Géographie', t. v., p. 80). All the hypotheses that have
been advanced to show that Descartes (Cassini, p. 230; Mairan, p. 16), and
even Kepler (Delambre, t. i., p. 601), were acquainted with the zodiacal
light, appear to me altogether untenable. Descartes ('Principes', iii.,
art. 136, 137) is very obscure in his remarks on comets, observing that
their tails are formed "by oblique rays, which, falling on different parts
of the planetary orbs, strike the eye laterally by extraordinary
refraction," and that they might be seen morning and evening, "like a long
beam," when the Sun is between the comet and the Earth. This passage no
more refers to the zodiacal light than those in which Kepler ('Epit. Astron.
Copernicanæ', t. i., p. 57, and t. ii., p. 893) speaks of the existence of
a solar atmosphere (limbus circa solem, coma lucida), which, in eclipses of
the Sun, prevents it "from being quite night:" and even more uncertain, or
indeed erroneous, is the assumption that the "trabes quas [Greek word]
vocant" (Plin., ii., 26 and 27) had reference to the tongue-shaped rising
zodiacal light, as Cassini (p. 231, art. xxxi.) and Mairan (p. 15) have
maintained. Every where among the ancients the trabes are associated with
the bolides (ardores et faces) and other fiery meteors, and even with
long-barbed comets. (Regarding [Greek words] . see Schäfer, 'Schol. Par.
ad Apoll. Rhod.', 1813, t. ii., p. 206; Pseudo-Aristot., 'de Mundo, 2, 9;
'Comment. Alex. Joh. Philop. et Olymp. in Aristot. Meteor.', lib. i., cap.
vii., 3, p. 195, Ideler; Seneca, 'Nat. Quæst.', i., 1.)
We may conjecture, with much probability, that the remarkable light on the
elevated plains of Mexico, seen for forty nights consecutively i8n 1509, and
observed in the eastern horizon rising pyramidally from the earth, was the
zodiacal light. I found a notice of this phenomenon in an ancient Aztec
MS., the 'CodexTelleriano-Remensis',* preserved in the Royal Library at
Paris.
[footnote] *Humboldt, 'Monumens des Peuples Indigènes de l'Amérique', t.
ii., p. 301. The rare manuscript which belonged to the Archbishop of
Rheims, Le Tellier, contains various kinds of extracts from an Aztec ritual,
an astrological calendar, and historical annals, extending from 1197 to
1549, and embracing a notice of different natural phenomena, epochs of
earthquakes and comets (as, for instance, those of 1490 and 1529), and of
(which are important in relation to Mexican chronology) solar eclipses. In
Camargo's manuscript 'Historia de Tlascala', the light rising in the east
almost to the zenith is, singularly enough, described as "sparkling, and as
if sown with stars." The description of this phenomenon, which lasted forty
days, can not in any way apply to volcanic eruptions of Popcatepetl, which
lies very near, in the southeastery direction. (Prescott, 'History of the
Conquest of Mesico', vol. i., p. 284.) Later commentators have confounded
this phenomenon, which Montezuma regarded as a warning of his misfortunes,
with the "estrella que humeava" (literally, 'which spring forth'; Mexican
'choloa, to leap or spring forth'). With respect to the connection of this
vapor with the star Citlal Choloha (Venus) and with "the mountain of the
star" (Citialtepetl, the volcano of Orizaba), see my 'Monumens', t. ii., p.
303.
This phenomenon, whose primordial antiquity can scarcely be doubted, and
which was first noticed in Europe by Childrey and Dominicus Cassini, is not
the luminous solar atmosphere itself, since this can not, in accordance with
mechanical laws, be more compressed than in the relation of 2 to 3, and
consequently can not be diffused beyond 9/20ths of Mercury's heliocentric
distance. These same laws teach us that the altitude of the extreme
boundaries of the atmosphere of a cosmical
p 141
body above its equator, that is to say, the point at which gravity and
centrifugal force are in equilibrium, must be the same as the altitude at
which a satellite would rotate round the central body simultaneously with
the diurnal revolution of the latter.*
[footnote] *Laplace, 'Expos. du Syst. du Monde', p. 270; 'Mécanique
Céleste', t. ii., p. 169 and 171; Schubert, 'Astr.', bd. iii., § 206.
This limitation of the solar atmosphere in its present concentrated
condition is especially remarkable when we compare the central body of our
system with the nucleus of other nebulous stars. Herschel has discovered
several, in which the radius of the nebulous matter surrounding the star
appeared at an angle of 150". On the assumption that the parallax is not
fully equal to 1", we find that the outermost nebulous layer of such a star
must be 150 times further from the central body than our Earth is from the
Sun. If, therefore, the nebulous star were to occupy the place of our Sun,
its atmosphere would not only include the orbit of Uranus, but even extend
eight times beyond it.•
[footnote] *Arago, in the 'Annuaire', 1842, p. 408. Compare Sir John
Herschel's considerations on the volume and faintness of light of planetary
nebulæ, in Mary Somerville's 'Connection of the Physical Sciences', 1835,
p. 108. The opinion that the Sun is a nebulous star, whose atmosphere
presents the phenomenon of zodiacal light, did not originate with Dominicus
Cassini, but was first promulgated by Mairan in 1730 ('Traité de l'Aurore
Bor.', p. 47 and 263; Arago, in the 'Annuaire', 1842, p. 412). It is a
renewal of Kepler's views.
Considering the narrow limitation of the Sun's atmosphere, which we have
just described, we may with much probability regard the existence of a very
compressed annulus of nebulous matter,* revolving freely in space between
the orbits of Venus and Mars, as the material cause of the zodiacal light.
[footnote] *Cominicus Cassini was the first to assume, as did subsequently
Laplace, Schubert, and Poisson, the hypothesis of a separate ring to explain
the form of the zodiacal light. He says distinctly, "If the orbits of
Mercury and Venus were visible (throughout their whole extent), we should
invariably observe them with the same figure and in the same position with
regard to the Sun, and at the same time of the year with the zodiacal
light." ('Mém. de l'Acad.', t. viii., 1730, p. 218, and Biot, in the
'Comptes Rendus', 1836, t. iii., p. 666.) Cassini believed that the
nebulous ring of zodiacal light consisted of innumerable small planetary
bodies revolving round the Sun. He even went so far as to believe that the
fall of fire-balls might be connected with the passage of the Earth through
the zodiacal nebulous ring. Olmsted, and especially Biot (op. cit., p.
673), have attempted to establish its connection with the November
phenomenon -- a connection which Olbers doubts. (Schum., 'Jahrb.', 1837, s.
281.) Regarding the question whether the place of the zodiacal light
perfectly coincides with that of the Sun's equator, see Houzeau, in Schum.,
'Astr. Nachr.', 1843, No. 492, s. 190.
As
p 142
yet we certainly know nothing definite regarding its actual material
dimensions; its augmentation* by emanations from the tails of myriads of
comets that come within the Sun's vicinity; the singular changes affecting
its expansion, since it sometimes does not apper to extend beyond our
Earth's orbit; or, lastly, regarding its conjectural intimate connection
with the more condensed cosmical vapor in the vicinity of the Sun.
[footnote] *Sir John Herschel, 'Astron.', § 487.
The nebulous particles composing this ring, and revolving round the sun in
accordance with planetary laws, may either be self-luminous or receive light
from that luminary. Even in the case of a terrestrial mist (and this fact
is very remarkable), which occurred at the time of the new moon at midnight
in 1743, the phosphorescence was so intense that objects could be distinctly
recognized at a distance of more than 600 feet.
I have occasionally been astonished in the tropical climates of south
america, to observe the variable intensity of the zodiacal light. As i
passed the nights, during many months, in the open air, on the shores of
rivers and on ilanos, i enjoyed ample opportunities of carefully examining
this phenomenon. When the zodiacal light had been most intense, i have
observed that it would be perceptibly weakened for a few minutes, until it
again suddenly shone forth in full brilliancy. In some few instances i have
thought that i could perceive -- not exactly a reddish coloration, nor the
lower portion darkened in an arc-like form, nor even a scintillation, as
mairan affirms he has observed -- but a kind of flickering and wavering of
the light.*
[footnote] *Arago, in the 'Annuaire', 1832, p. 246. Several physical facts
appear to indicate that, in a mechanical separation of matter into its
smallest particles, if the mass be very small in relation to the surface,
the electrical tension may increase sufficiently for the production of light
and heat. Experiments with a large concave mirror have not hitherto given
any positive evidence of the presence of radiant heat in the zodiacal light.
(Lettre de M. Matthiessen à M. Arago, in the 'Comptes Rendus', t. xvi.,
1843, Avril, p. 687.)
Must we suppose that changes are actually in progress in the nebulous ring?
or is it not more probable that, although I could not, by my meteorological
instruments, detect any change of heat or moisture near the ground, and
small stars of the fifth and sixth magnitudes appeared to shine with equally
undiminished intensity of light, processes of condensation may be going on
in the uppermost strata of the air, by means of which the transparency, or
rather, the reflection of light, may be modified in some peculiar and
unknown manner?
p 143
An assumption of the existence of such meteorological causes on the confines
of our atmosphere is strengthened by the "sudden flash and pulsation of
light," which, according to the acute observations of Olbers, vibrated for
several seconds through the tail of a comet, which appeared during the
continuance of the pulsations of light to be lengthened by several degrees,
and then again contracted.*
[footnote] *"What you tell me of the changes of light in the zodiacal
light, and of the causes to which you ascribe such changes within the
tropics, is of the greatr interest to me, since I have been for a long time
past particularly attentive, every spring, to this phenomenon in our
northern latitudes. I, too, have always believed that the zodiacal light
rotated; but I assumed (contrary to Poisson's opinion, which you have
communicated to me) that it completely extended to the Sun, with
considerably augmenting brightness. The light circle which, in total solar
eclipses, is seen surrounding the darkened Sun, I have regarded as the
brightest portion of the zodiacal light. I have convinced my self that this
light is very different in different years, often for several successive
years being very bright and diffused, while in othr years it is scarcely
perceptible. I tyhink that I find the first trace of an allusion to the
zodiacal light in a letter from Rothmann to Tycho, in which he mentions that
in the spring he has observed the twilight did not close until the sun was
24ºdegrees below the horizon. Rothmann must certainly have confounded the
disappearance of the setting zodiacal light in the vapors of the western
horizon with the actual cessation of twilight. I have failed to observe the
pulsations of the light, probably on account of the faintness with which it
appears in these countries. You are, however, certainly right in ascribing
those rapid variations in the light of the heavenly bodies, which you have
perceived in tropical climates, to our own atmosphere, and especially to its
higher regions. This is especially in the clearest weather, that these
tails exhibit pulsations, commencing from the head, as being the lowest
part, and vibrating in one or two seconds through the entire tail, which
thus appears rapidly to become some degrees longer, but again as rapidly
contracts. That these undulations, which were formerly noticed with
attention by Robert Hooke, and in more recent times by Schröter and
Chladni, 'do not actually occur in the tails of the comets', but are
produced by our atmosphere, is obvious when we recollect that the individual
parts of those tails (which are many millions of miles in length) lie 'at
very different distances' from us, and that the light from their extreme
points can only reach us at intervals of time which differ several minutes
from one another. Whether what you saw on the Orinoco, not at intervals of
seconds, but of minutes, were actual coruscations of the zodiacal light, or
whether they belonged exclusively to the upper strata of our atmosphere, I
will not attempt to decide; neither can I explain the remarkable 'lightness
of whole nights', nor the anomalous augmentation and prolongation of the
twilight in the year 1831, particularly if, as has been remarked, the
lightest part of these singular twilights did not coincide with the Sun's
place below the horizon." (From a lettr written by Dr. Olbers to myself,
and dated Bremen, Marth 26th, 1833.)
As, however, the separate particles of a comet's tail, measuring millions of
miles,
p 144
are very unequally distant from earth, it is not possible, according to the
laws of the velocity and transmission of light, that we should be able, in
so short a period of time, to perceive any actual changes in a cosmical body
of such vast extent. There considerations in no way exclude the realith of
the changes that have been observed in the emanations from the more
condensed envelopes around the nucleus of a comet, nor that of the sudden
irradiation of the zodiacal light, from internal molecular motion, nor of
the increased or diminished reflection of light in the cosmical vapor of the
luminous ring, but should simply be the means of drawing our attention to
the differences existing between that which appertains to the air of heaven
(the realms of universal space) and that which belongs to the strata of our
terrestrial atmosphere. It is not possible, as well-attested facts prove,
perfectly to explain the operations at work in the much-contested upper
boundaries of our atmosphere. The extraordinary lightness of whole nights
in the year 1831, during which small print might be read at midnight in the
latitudes of Italy and the north of Germany is a fact directly at variance
with all that we know, according to the most recent and acute researches on
the crepuscular theory, and of the height of the atmosphere.*
[footnote] *Biot, 'Traité d'Astron. Physique', 3ème éd., 1841, t. i., p.
171, 238 and 312.
The phenomena of light depend upon conditions still less understood, and
their variability at twilight, as well as in the zodiacal light, excite our
astonishment.
We have hitherto considered that which belongs to our solare system -- that
world of material forms governed by the Sun -- which includes the primary
and secondary planets, comets of short and long periods of revolution,
meteoric asteroids, which move thronged together in streams, either
sporadically or in closed rings, and finally a luminous nebulous ring, that
revolves round the Sun in the vicinity of the Earth, and for which, owing to
its position, we may retain the name of zodiacal light. Every where the law
of periodicity governs the motions of these bodies, however different may be
the amount of tangential velocity, or the quantity of their agglomerated
material parts; the meteoric asteroids which enter our atmosphere from the
external regions of universal space are alone arrested in the course of
their planetary revolution, and retained within the sphere of a larger
planet. In the solar system, whose boundaries determine the attractive
force of the central body, comets are made to revolve in their elliptical
p 145
orbits at a distance 44 times greater than that of Uranus; may, in those
comets whose nucleus appears to us, from its inconsiderable mass, like a
mere passing cosmical cloud, the Sun exercises its attractive force on the
outermost parts of the emanations radiating from the tail over a space of
many millions of miles. Central forces, therefore, at once constitute and
maintain the system.
Our Sun may be considered as at rest when compared to all the large and
small, dense and almost vaporous cosmical bodies tht appertain to and
revolve around it; but it actually rotates around the common center of
gravity of the whole system, which occasionally falls within itself, that is
to say, remains within the material circumference of the Sun, whatever
changes may be assumed by the position of the planets. A very different
phenomenon is that presented by the translatory motion of the Sun, that is,
the progressive motion of the center of gravity of the whole solar system in
universal space. Its velocity is such* that, according to Bessel, the
relative motion of the Sun, and that of 61 Cygni, is not less in one day
than 3,336,000 geographical miles.
[footnote] *Bessel, in Schum., 'Jahrb. für' 1839, s. 51; probably four
millions of miles daily, in a relative velocity of at the least 3,336,000
miles, or more than couble the velocity of revolution of the Earth in her
orbit round the Sun.
This change of the entire solar system would remain unknown to us, if the
admirable exactness of our astronomical instruments of measurement, and the
advancement recently made in the art of observing, did not cause our advance
toward remote stars to be perceptible, like an approximation to the objects
of a distant shore in apparent motion. The proper motion of the star 61
Cygni, for instance, is so considerable, that it has amounted to a whole
degree in the course of 700 years.
The amount or quantity of these alterations in the fixed stars (that is to
say, the changes in the relative position of self-luminous stars toward each
other), can be determined with a greater degree of certainty than we are
able to attach to the genetic explanation of the phenomenon. After taking
into consideration what is due to the precession of the equinoxes, and the
nutation of the earth's axis produced by the action of the Sun and Moon on
the spheroidal figure of our globe, and what may be ascribed to the
transmission of light, that is to say, to its aberration, and to the
parallax formed by the diametrically opposite position of the Earth in its
course round the Sun, we still find that there is a residual portion
p 146
of the annual motion of the fixed stars due to the translation of the whole
solar system in universal space, and to the true proper motion of the stars.
The difficult problem of numerically separating these two elements, the
true and the apparent motion, has been effected by the careful study of the
direction of the motion of certain individual stars, and by the
consideration of the fact that, if all the stars were in a state of absolute
rest, they would appear perspectively to recede from the point in space
toward which the Sun was directing its course. But the ultimate result of
this investigation, confirmed by the calculus of probabilities, is, that our
solar system and the stars both change their places in space. According to
the admirable researches of d'Argelander at Abo, who has extended and more
perfectly developed the work begun by William Herschel and Prevost, the Sun
moves in the direction of the constellation Hercules, and probably, from the
combination of the observations made of 537 stars, toward a point lying (at
the equinox of 1792.5) at 257ºdegrees 49.'7 R.A., and 28ºdegrees 49.'7
N.D. It is extremely difficult, in investigations of this nature, to
separate the absolute from the relative motion, and to determine what is
aloone owing to the solar system.*
[footnote] *Regarding the motion of the solar system, according to Bradley,
Tobias Mayer, Lambert, Lalande, and William Herschel, see Arago in the
'Annuaire', 1842, p. 388-399' Argelander, in Schum., 'Astron. Nachr
., No. 363, 364, 398, and in the treatise 'Von der eigenen Bewegung des
Sonnensystems' (On the proper Motion of the Solar System), 1837, s. 43,
respecting Perseus as the central body of the whole stellar stratum,
likewise Otho Struve, in the 'Bull. de l'Acad. de St. Pétersb.', 1842, t.
x., No. 9, p. 137-139. The last-named astronomer has found, by a mo4re
recent combination, 261ºdegrees 23' R.A.+37ºdegrees 36' Decl. for the
direction of the Sun's motion; and, taking the mean of his own results with
that of Argelander, we have, by a combination of 797 stars, the formula
259ºdegrees 9' R.A.+34ºdegrees 36' Decl.
If we consider the proper, and not the perspective motions of the stars, we
shall find many that appear to be distributed in groups, having an opposite
direction; and facts hitherto observed do not, at any rate, render it a
necessary assumption that all parts of our starry stratum, or the whole of
the stellar islands filling space, should move round one large unknown
luminous or non-luminous central body. The tendency of the human mind to
investigate ultimate and highest causes certainly inclines the intellectual
activity, no less than the imagination of mankind, to adopt such an
hypothesis. Even the Stagirite proclaimed that "every thing which is moved
must be referable to a motor, and that there would be no end to
p 147
the concatenation of causes if there were not one primordial immovable
morot."*
[footnote] *Aristot., 'de Cælo', iii., 2, p. 301, Bekker: 'Phys.', viii.,
t, p. 256.
This material taken from pages 147-203
COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1
by Alexander von Humboldt
Translated by E C Otte
from the 1858 Harper & Brothers edition of Cosmos, volume 1
--------------------------------------------------