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this period of 223 lunations, or eighteen years and ten days, is a very important one in the calculation of eclipses. It is supposed to have been known to the Chaldeans, the earliest astronomers, the regular return of eclipses having been known as a physical fact for ages before their exact theory was understood. In this period there occur ordinarily 70 eclipses, 29 of the moon and 41 of the sun, visible in some part of the earth. Seven eclipses of either sun or moon at most, and two at least (both of the sun), may occur in a year.
(427.) The commencement, duration, and magnitude of a lunar elipse are much more easily calculated than those of a solar, being independent of the position of the spectator on the earth's surface, and the same as if viewed from its centre. The common centre of the umbra and penumbra lies always in the ecliptic, at a point opposite to the sun, and the path described by the moon in passing through it is its true orbit as it stands at the moment of the full moon. In this orbit, its position, at every instant, is known from the lunar tables and ephemeris; and all we have, therefore, to ascertain, is, the moment when the distance between the moon's centre and the centre of the shadow is exactly equal to the sum of the semidiameters of the moon and penumbra, or of the moon and umbra, to know when it enters upon and leaves them respectively. No lunar eclipse can take place, if, at the moment of the full moon, the sun be at a greater angular distance from the node of the moon's orbit than 11° 21', meaning by an eclipse the immersion of any part of the moon in the umbra, as its contact with the penumbra cannot be observed (see note to art. 421.).
(428.) The dimensions of the shadow, at the place where it crosses the moon's path, require us to know the distances of the sun and moon at the time. These are variable; but are calculated and set down, as well as their semidiameters, for every day, in the ephemeris, so that none of the data are wanting. The sun's distance is easily calculated from its elliptic orbit; but the moon's is a matter of more difficulty, by reason of the progressive motion of the axis of the lunar orbit. (Art 409.)
(429.) The physical constitution of the moon is better known to us than that of any other heavenly body. By the aid of telescopes, we discern inequalities in its surface which can be no other than mountains and valleys, — for this plain reason, that we see the shadows cast by the former in the exact proportion as to length which they ought to have, when we take into account the inclination of the sun's rays to that part of the moon's surface on which they stand. The convex outline of the limb turned towards the sun is always circular, and very nearly smooth; but the opposite border of the enlightened part, which (were the moon a perfect sphere) ought to be an exact and sharply defined ellipse, is always observed to be extremely ragged, and indented with deep recesses and prominent points. The mountains near this edge cast long black shadows, as they should evidently do, when we consider that the sun is in the act of rising or setting to the parts of the moon so circumstanced. But as the enlightened edge advances beyond them, i.e. as the sun to them gains altitude, their shadows shorten; and at the full moon, when all the light falls in our line of sight, no shadows are seen on any part of her surface. From micrometrical measures of the lengths of the shadows of the more conspicuous mountains, taken under the most favourable circumstances, the heights of many of them have been calculated. Messrs. Beer and Maedler in their elaborate work, entitled “ Der Mond,” have given a list of heights resulting from such measurements, for no less than 1095 lunar mountains, among which occur all degrees of elevation up to 3569 toises, (22823 British feet), or about 1400 feet higher than Chimborazo in the Andes. The existence of such mountains is further corroborated by their appearance, as small points or islands of light beyond the extreme edge of the enlightened part, which are their tops catching the sun-beams before the intermediate plain, and which, as the light advances, at length connect themselves with it, and appear as prominences from the general edge.
(430.) The generality of the lunar mountains present a striking uniformity and singularity of aspect. They are wonderfully numerous, especially towards the Southern portion of the disc, occupying by far the larger portion of the surface, and almost universally of an exactly circular or cup-shaped form, foreshortened, however, into ellipses towards the limb; but the larger have for the most part flat bottoms within, from which rises centrally a small, steep, conical hill. They offer, in short, in its highest perfection, the true volcanic character, as it may be seen in the crater of Vesuvius, and in a map of the volcanic districts of the Campi Phlegræi* or the Puy de Dôme, but with this remarkable peculiarity, viz. : that the bottoms of many of the craters are very deeply depressed below the general surface of the moon, the internal depth being often twice or three times the external height. In some of the principal ones, decisive marks of volcanic stratification, arising from successive deposits of ejected matter, and evident indications of lava currents streaming outwards in all directions, may be clearly traced with powerful telescopes. (See Pl. V. fig. 2.) | In Lord Rosse's magnificent reflector, the flat bottom of the crater called Albategnius is seen to be strewed with blocks not visible in inferior telescopes, while the exterior of another (Aristillus) is all hatched over with deep gullies radiating towards its center. What is, moreover, extremely singular in the geology of the moon is, that, although nothing having the character of seas can be traced, (for the dusky spots, which are commonly called seas, when closely examined, present appearances incompatible with the supposition of deep water,) yet there are large regions perfectly level, and apparently of a decided alluvial character.
(431.) The moon has no clouds, nor any other decisive indications of an atmosphere. Were there any, it could not fail to be perceived in the occultations of stars and the phænomena of solar eclipses, as well as in a great variety of other phænomena. The moon's diameter, for example, as measured micrometrically, and as estimated by the interval between the disappearance and reappearance of a star in an occultation, ought to differ by twice the horizontal refraction at the moon's surface. No appretiable difference being perceived, we are entitled to conclude the non-existence of any atmosphere dense enough to cause a refraction of 1"i.e. having one 1980th part of the density of the earth's atmosphere. In a solar eclipse, the existence of any sensible refracting atmosphere in the moon, would enable us to trace the limb of the latter beyond the cusps, externally to the sun's disc, by a narrow, but brilliant line of light, extending to some distance along its edge. No such phænomenon is seen. Very faint stars ought to be extinguished before occultation, were any appretiable amount of vapour suspended near the surface of the moon. But such is not the case; when occulted at the bright edge, indeed, the light of the moon extinguishes small stars, and even at the dark limb, the glare in the sky caused by the near presence of the moon, renders the occultation of very minute stars unobservable. But during the continuance of a total lunar eclipse, stars of the tenth and eleventh magnitude are seen to come up to the limb, and undergo sudden extinction as well as those of greater brightness. Hence, the climate of the moon must be very extraordinary; the alternation being that of unmitigated and burning sunshine fiercer than an equatorial noon, continued for a whole fortnight, and the keenest severity of frost, far exceeding that of our polar winters, for an equal time. Such a disposition of things must produce a constant transfer of whatever moisture may exist on its surface, from the point beneath the sun to that opposite, by distillation in vacuo after the manner of the little instrument called a cryophorus. The consequence must be absolute aridity below the vertical sun, constant accretion of hoar frost in the opposite region, and, perhaps, a narrow zone of running water at the borders of the enlightened hemisphere.f It is possible, then, that evapo
* See Breislak's map of the environs of Naples, and Desmarest's of Auvergne. † From a drawing taken with a reflector of twenty feet focal length (h).
ration on the one hand, and condensation on the other, may to a certain extent preserve an equilibrium of temperature, and mitigate the extreme severity of both climates; but this process, which would imply the continual generation and destruction of an atmosphere of aqueous vapour, must, in conformity with what has been said above of a lunar atmosphere, be confined within very narrow limits.
(432.) Though the surface of the full moon exposed to us, must necessarily be very much heated, — possibly to a degree much exceeding that of boiling water, — yet we feel no heat from it, and even in the focus of large reflectors, it fails to affect the thermometer. No doubt, therefore, its heat (conformably to what is observed of that of bodies heated below the point of luminosity) is much more readily absorbed in traversing transparent media than direct solar heat, and is extinguished in the upper regions of our atmosphere, never reaching the surface of the earth at all. Some probability is given to this by the tendency to disappearance of clouds under the full moon, a meteorological fact, (for as such we think it fully entitled to rank*) for which it is necessary to seek a cause, and for which no other rational explanation seems to offer. As for any other influence of the moon on the weather, we have no decisive evidence in its favour.
(433.) A circle of one second in diameter, as seen from the earth, on the surface of the moon, contains about a square mile: Telescopes, therefore, must yet be greatly improved, before we could expect to see signs of inhabitants, as manifested by edifices or by changes on the surface of the soil. It should, however, be observed, that, owing to the small density of the materials of the moon, and the comparatively feeble gravitation of bodies on her surface, muscular force would there go six times as far in overcoming the weight of materials as on the earth. Owing to the want of air, however, it seems impossible that any form of life, analagous to those on earth, can
* From my own observation, made quite independently of any knowledge of such a tendency having been observed by others. Humboldt, however in his personal narrative, speaks of it as well known to the pilots and seamen of Spanish America : see note at the end of the chapter (h).