Volume 4

Cosmos, a sketch of a physical description of the universe / by Alexander von Humboldt ; translated from the German by E.C. Otté.

  • Humboldt, Alexander von, 1769-1859.
Date:
1849-1858
    without gradual diminution of the star’s brilliancy; just so the egress or reappearance of the star. In the case of the few exceptions which have been described, the cause may have consisted in accidental changes of our atmosphere. If, however, the Earth’s Moon is destitute of a gaseous envelope, the stars must appear then, in the absence of all diffuse light, to rise upon a black sky j32 no air-wave can there convey sound, music, or language. To our imagination, so apt presumptuously to stray into the unfathomable, the Moon is a voiceless wilderness. The phenomenon of apparent adherence on and within the Moon’s edge,33 sometimes observed in the occultation of stars, can scarcely be considered as a consequence of irradiation, which, in the narrow crescent of the Moon, on account of the very different intensity of the light in the ash-coloured part of the Moon, and in that which is immediately illuminated by the Sun, certainly makes the latter appear as if surrounding the former. Arago saw during a total eclipse of the Moon, a star distinctly adhere to the slightly luminous disc of the Moon during the conjunction. It still continues to be a subject of discussion between Arago and Plateau whether the phenomenon here mentioned depends upon deceptive percep- tion and physiological causes,34 or upon the aberration of 32 Mädler, in Schumacher’s Jahrbuch fur 1840, p. 188. 33 Sir John Herschel (Outlines, p. 247) directs attention to the ingress of such double stars as cannot be seen separately by the telescope, on account of the too great proximity of the individual stars of which they consist. 34 Plateau, Sur V Irradiation, in the Mem. de V Acad. Roy ale des Sciences et Belles-Lettres de Bruxelles, tom. xi. p. 142, and the supplementary volume of* Poggendorffs Annalen, 1842, pp. 79-128, 193-232, and 405 and 443. “ The probable cause of the irradiation is an irritation produced by the light upon the retina, and spreads a little beyond the outline of the image.’*
    sphericity and refrangibility of the eye.35 Those eases in ■which it has been asserted that a disappearance and reap- pearance, and then a repeated disappearance, have been observed during an occultation, may probably indicate the ingress to have taken place at a part of the Moon’s edge which happened to be deformed by mountain declivities and deep chasms. The great differences in the reflected light from particular regions of the illuminated disc of the Moon, and especially the absence of any sharp boundary between the inner edge of the illuminated and ash-coloured parts in the Moon’s phases, led to the formation of several very rational theories as to the inequalities of the surface of our satellite, even at a very remote period. Plutarch says distinctly, in the small but very remarkable work On the Face in the Moon, that we may suppose the spots to be partly deep chasms and valleys, partly mountain peaks, “ which cast long shadows, like 85 Arago, in the Comptes Itendus, tom. viii. 1839, pp. 713 and 883. “ Les phenomenes d'irradiation signales par M. Plateau sont regardes par M. Arago comme les effets des aberrations de refrangibilite et de sphericite de Teeii, combines avec l’indistinction de la vision, consequence des circonstances dans lesquelles les observateurs se sont places. Des mesures exactes prises sur des disques noirs a fond blanc et des disques blancs a fond noir, qui etoient places au Palais du Luxembourg, visibles a l’observatoire, n’ont pas indique les effets de 1’irradiation.” “The phenomena of irradiation pointed out by M. Plateau are regarded by M. Arago as the effects of the aberration of sphericity and refrangibility of the eye, combined with the indistinctness of vision consequent upon the circumstances in which the observers are placed. The exact measurement taken of the black discs upon a white ground, and the white discs upon a black ground which were placed upon the palace of Luxembourg, and visible at the Observatory, did not present any phenomena of irradiation.”
    Mount Athos, whose shadow reaches Lemnos.”36 The spots cover about two-fifths of the whole disc. In a clear atmo- sphere, and under favourable circumstances in the position of the Moon, some of the spots are visible to the naked eye ; the ridge of the Apennines, the dark walled plain Grimaldus, the detached Mare Crisium, and Tycho 37 crowded round with numerous mountain ridges and craters. It has been affirmed, not without probability, that it was especially the aspect of the Apennine chain, which induced the Greeks to consider the spots on the Moon to be mountains, and at the same time to associate with them the shadow of Mount Athos, which in the solstices reached the Brazen Cow upon Lemnos. Another very fantastic opinion was that of Agesinax, dis- puted by Plutarch, according to which the Moon’s disc was supposed, like a mirror, to present to us again, catoptrically, the configuration and outline of our continent, and the outer sea (the Atlantic). A very similar opinion appears to have been preserved to this time as a popular belief among the people in Asia Minor.38 36 Plutarch, Moral, ed. Wytten. tom. iv. pp. 786-789. The shadow of Athos, which was seen by the traveller Pierre Belon (Observations de singularity tromies en Grece, Asie, etc. 1554, liv. i. chap. 25), reached the brazen cow in the mar- ket-town Myrine in Lemnos. 37 Proofs of the visibility of these four objects, see in Beer and Mädler, der Mond, pp. 241, 338, 191, and 290. It is scarcely necessary to mention that all which refers to the topography of the Moon’s surface is derived from the excel- lent work of my two friends, of whom the second, William Beer, was taken from us but too early. The beautiful Ueb er sichtsblatt, which Mädler published in 1837, three years after the large map of the Moon, consisting of three sheets, is to be recommended for the purpose of more easily becoming acquainted with the bearings. 38 Plut. De facie in orbe Lunce, pp. 726-729,Wytten. This
    By the careful application of large telescopes, it has gra- dually become possible to construct a topographical chart of the Moon, based upon actual observations; and since, in the opposition, the entire half-side of the Earth’s satellite pre- sents itself at the same moment to our investigation, we know more of the general and merely formal connection of the mountain groups in the Moon, than of the orography of a whole terrestrial hemisphere containing the interiors of Africa and Asia. Generally the darker parts of the disc are passage is, at the same time, not without interest for ancient geography.—See Humboldt, Examen critique de VHist, de la Geogr. tom. i. p. 145. With regard to other views of the ancients, see Anaxagoras and Democritus, in Plut. de plac. Philos. ii. 25; Parmenides, in Stob. pp. 419, 453, 516, and 563, ed. Heeren; Schneider, Eclogce physicce, vol. i. pp. 433- 443. According to a very remarkable passage in Plutarch's Life of Nicias, cap. 42, Anaxagoras himself, who calls “the mountainous Moon another Earth,” had made a drawing of the Moon’s disc. (Compare also Origines, Philosophumena, cap. 8, ed. Miilleri, 1851, p. 14.) I was once very much astonished to hear a very well-educated Persian, from Ispahan, who certainly had never read a Greek book, mention, when I showed him the Moon’s spots in a large telescope in Paris, the hypothesis of Agesinax, (alluded to in the text,) as to the reflection, as a widely-diffused popular belief in his country. “ What we see there in the Moon,” said the Persian, “ is ourselves; it is the map of our Earth.” One of the interlo- cutors in Plutarch’s Moon-dialogue, would not have expressed himself otherwise. If it can be supposed that men are inha- bitants of the Moon, destitute *of water and air, the Earth, with its spots, would also present to them such a map upon a nearly black sky by day, with a surface fourteen times greater than that which the full Moon presents to us, and always in the same position. But the constantly varying clouds and obscurities of our atmosphere, would confuse the outlines of the continents.—Compare Mädler’s Astron. p. 169, and Sir John Iierschel, Outlines, § 436.
    the lower and more level; the brighter parts, reflecting much Sun-light, are the more elevated and mountainous Kepler’s old description of the two as sea and land, has long been given up; and the accuracy of the explanation, and the oppo- sition, was already doubted by Hevel, notwithstanding the similar nomenclature introduced by him. The circumstance principally brought forward as disproving the presence of surfaces of water on the Moon, was that in the so-called seas of the Moon, the smallest parts showed themselves, upon closer examination aud very different illumination, to be completely uneven, polyhedric, and consequently giving much polarized light. Arago has pointed out, in opposition to the arguments which have been derived from the irregularities, that some of these surfaces may, notwithstanding the irregu- larities, be covered with water, and belong to the bottoms of seas of no great depth, since the uneven craggy bottom of the ocean of our planet is distinctly seen when viewed from a great height, on account of the preponderance of the light issuing from below its surface, over the intensity of that which is reflected from it. (Annuaire du Bureau des Lon- gitudes pour- 1836, pp. 339-343.) In the work of my friend, which will shortly appear, on astronomy and photo- metry, the probable aosence of -water upon our satellite will be deduced from other optical grounds, which cannot be developed in this place. Among the low plains, the largest surfaces are situated in the northern and eastern parts. The indistinctly bounded Oceanus Procellarum, has the greatest extension of all these, being 360,000 geographical miles. Connected with the Mare Imbrium (64,000 square miles), the Mare Nubium, and to some extent with the Mare Humo- rum, and surrounding insular mountain districts (the Riphcei, Kepler, Copernicus, and the Carpathians), this eastern part of the Moon’s disc presents the most decided contrast to the luminous south-western district, in which mountain is crowded