Cosmos: A Sketch of the Physical Description of the Universe, Vol. 1

by Alexander von Humboldt

The first observation of the phenomenon may have been made two or three years prior to this period; but, notwithstanding, the merit of having (in the spring of 1683) been the first to investigate the phenomenon in all its relations in space is incontestably due to Dominicus Cassini. The light which he saw at Bologna in 1668, and which was observed at the same time in Persia by the celebrated traveler Chardin (the court astrologers of Ispahan called this light, which had never before been observed, 'nyzek', a small lance), was not the zodiacal light, as has often been asserted,* but the p 140 enormous tail of a comet, whose head was concealed in the vapory mist of the horizon, and which, from its length and appearance, presented much similarity to the great comet of 1843.

[footnote] *Dominicus Cassini ('Mém. de l'Acad.', t. viii., 1730, p. 188), 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.