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After this disquisition upon lunar uninhabitability it may appear somewhat inconsistent for us to attempt a description of the scenery of the moon and some other effects that would be visible to a spectator, and of which he would be otherwise sensible, during a day and a night upon her surface. But we can offer the sufficient apology that an imaginary sojourn of one complete lunar day and night upon the moon affords an opportunity of marshalling before our readers some phenomena that are proper to be noticed in a work of this character, and that have necessarily been passed over in the series of chapters on consecutive and special points that have gone before. It may be urged that, in depicting the moon from such a standpoint as that now to be taken, we are describing scenes that never have been such in the literal sense of the word, since no eye has ever beheld them. Still we have this justification—that we are invoking the conception of things that actually exist; and that we are not, like some imaginary voyagers to the moon, indulging in mere flights of fancy. Although it is impossible for a habitant of this earth fully to realise existence upon the moon, it is yet possible, indeed almost inevitable, for a thoughtful telescopist—watching the moon night after night, observing the sun rise upon a lunar scene, and noting the course of effects that follow till it sets—it is almost inevitable, we say, for such an observer to identify himself so far with the object of his scrutiny, as sometimes to become in thought a lunar being. Seated in silence and in solitude at a powerful telescope, abstracted from terrestrial influences, and gazing upon the revealed details of some strikingly characteristic region of the moon, it requires but a small effort of the imagination to suppose one’s self actually upon the lunar globe, viewing some distant landscape thereupon; and under these circumstances there is an irresistible tendency in the mind to pass beyond the actually visible, and to fill in with what it knows must exist those accessory features and phenomena that are only hidden from us by distance and by our peculiar point of view. Where the material eye is baffled, the clairvoyance of reason and analogy comes to its aid.
Let us then endeavour to realize the strange consequences which the position and conditions of the moon produce upon the aspect of a lunar landscape in the course of a lunar day and night.
The moon’s day is a long one. From the time that the sun rises upon a scene[17] till it sets, a period of 304 hours elapses, and of course double this interval passes between one sunrise and the next. The consequences of this slow march of the sun begin to show themselves from the instant that he rises above the lunar horizon. Dawn, as we have it on earth, can have no counterpart upon the moon. No atmosphere is there to reflect the solar beams while the luminary is yet out of actual sight, and only the glimmer of the zodiacal light heralds the approach of day. From the black horizon the sun suddenly darts his bright untempered beams upon the mountain tops, crowning them with dazzling brilliance while their flanks and valleys are yet in utter darkness. There is no blending of the night into day. And yet there is a growth of illumination that in its early stages may be called a twilight, and which is caused by the slow rise of the sun. Upon the earth, in central latitudes, the average time occupied by the sun in rising, from the first glint of his upper edge till the whole disc is in sight, is but two minutes and a quarter. Upon the moon, however, this time is extended to a few minutes short of an hour, and, therefore, during the first few minutes a dim light will be shed by the small visible chord of the solar disc, and this will give a proportionately modified degree of illumination upon the prominent portion of the landscape, and impart to it something of the weird aspect which so strikes an observer of a total solar eclipse on earth when the scene is lit by the thin crescent of the re-appearing sun. This impaired illumination constitutes the only dawn that a lunar spectator could behold. And it must be of short duration; for when, in the course of half an hour, the solar disc has risen half into view the lighting would no doubt appear nearly as bright to the eye as when the entire disc of the sun is above the horizon. In this lunar sunrise, however, there is none of that gilding and glowing which makes the phenomenon on earth so gorgeous. Those crimson sky-tints with which we are familiar are due to the absorption of certain of the polychromous rays of light by our atmosphere. The blue and violet components of the solar beams are intercepted by our envelope of vapour, and only the red portions are free to pass; while on the moon, as there is no atmosphere, this selective absorption does not occur. If it did, an observer gazing from the earth upon the regions of the moon upon which the sun is just rising would see the surface tinted with rosy light. This, however, is not the case; the faintest lunar features just catching the sun are seen simply under white light diluted to a low degree of brightness. Only upon rare occasions is the lunar scenery suffused with coloured illumination, and these are when, as we shall presently have to describe, the solar rays reach the moon after traversing the earth’s atmosphere during an eclipse of the sun.
This atmosphere of ours is the most influential element in beautifying our terrestrial scenery, and the absence of such an appendage from the moon is the great modifying cause that affects lunar scenery as compared with that of the earth. We are accustomed to the sun with its dazzling brightness—overpowering though it be—subdued and softened by our vaporous screen. Upon the moon there is no such modification. The sun’s intrinsic brilliancy is undiminished, its apparent distance is shortened, and it gleams out in fierce splendour only to be realised, and then imperfectly, by the conception of a gigantic electric light a few feet from the eye. And the brightness is rendered the more striking by the blackness of the surrounding sky. Since there is no atmosphere there can be no sky-light, for there is nothing above the lunar world to diffuse the solar beams; not a trace of that moisture which even in our tropical skies scatters some of the sun’s light and gives a certain degree of opacity or blueness, deep though it be, to the heavens by day. Upon the moon, with no light-diffusing vapour, the sky must be as dark or even darker than that with which we are familiar upon the finest of moonless nights. And this blackness prevails in the full blaze of the lunar noon-day sun. If the eye (upon the moon) could bear to gaze upon the solar orb (which would be less possible than upon earth) or could it be screened from the direct beams, as doubtless it could by intervening objects, it would perceive the nebulous and other appendages which we know as the corona, the zodiacal light, and the red solar protuberances: or if these appendages could not be viewed with the sun above the horizon they would certainly be seen in glorious perfection when the luminary was about to rise or immediately after it had set.
And, notwithstanding the sun’s presence, the planets and stars would be seen to shine more brilliantly than we see them on the clearest of nights; the constellations would have the same configurations, though they would be differently situated with respect to the celestial pole about which they would appear to turn, for the axis of rotation of the moon is directed towards a point in the constellation Draco. The stars would never twinkle or change colour as they appear to us to do, for scintillation or twinkling is a phenomenon of atmospheric origin, and they would retain their full brightness, down even to the horizon, since there would be no haze to diminish their light. The planets, and the brighter stars at least, would be seen even when they were situated very near to the sun. The planet Mercury, so seldom detected by terrestrial gazers, would be almost constantly in view during the lunar day, manifesting his close attendance on the central luminary by making only short excursions of about two (lunar) days’ length, first on one side and then on the other. Venus would be nearly as continuously visible, though her wanderings would be more extensive on either side. The zodiacal light also, which in our English latitude and climate is but rarely seen and in more favourable climes appears only when the sun itself is hidden beneath the horizon, would upon the moon be seen as a constant accompaniment to the luminary throughout his daily course across the lunar sky. The other planets would appear generally as they do to us on earth, but, never being lost in daylight, their courses among the stars could be traced with scarcely any interruption.
One planet, however, that adorns the sky of the lunar hemisphere which is turned towards us deserves special mention from the conspicuous and highly interesting appearance it must present. We allude to the earth. To nearly one-half of the moon (that which we never see) this imposing object can never be visible; but to the half that faces us the terrestrial planet must appear almost fixed in the sky. A lunar spectator in (what is to us) the centre of the disc, or about the region north of the lunar mountains Ptolemy and Hipparchus, would have the earth in his zenith. From regions upon the moon a little out of what is to us the centre, a spectator would see the earth a little declining from the zenith, and this declination would increase as the regions corresponding to the (to us) apparent edge of the moon were approached, till at the actual edge it would be seen only upon the horizon. From the phenomena of libration (explained in [Chap. VI.]) the earth would appear from nearly all parts of the lunar hemisphere to which it is visible at all to describe a small circle in the sky. To an observer, however, upon the (to us) marginal regions of the lunar globe, it would appear only during a portion of the lunar day—being visible in fact only in that part of its small circular path which happened to lie above the observer’s horizon: in some regions only a portion of the terrestrial disc would make its brief appearance. From the lunar hemisphere beyond this marginal line the earth can never be seen at all.
The lunar spectator whose situation enabled him to view the earth would see it as a moon; and a glorious moon indeed it must be. Its diameter would be four times as great as that of the moon itself as seen by us, and the area of its full disc 13 times as great. It would be seen to pass through its phases, just as does our satellite, once in a lunar day or a terrestrial month, and during that cycle of phases, since 29 of our days would be occupied by it, the axial rotation would bring all the features of its surface configuration into view so many times in succession. But the greatest beauty of this noble moon would be seen during the lunar night, in considering which we shall again allude to it; for when it is full-moon to the earth it is new-earth to the moon. At lunar midnight this globe of ours is fully illuminated; as morning nears, the earth-moon wanes, its disc slowly passing through the gibbous phases until at sunrise it would be just half-illuminated. During the long forenoon it assumes a crescent which narrows and narrows till at midday the sun is in line with the earth and the latter is invisible, save perhaps by a thin line of light marking its upper or lower edge, accordingly as the sun is apparently above or below it. In the lunar afternoon an illuminated crescent appears upon the opposite side of the terrestrial globe, and this widens and widens till it becomes a half disc by lunar sunset and a full disc by lunar midnight.
The sun in his daily course passes at various distances, sometimes above and sometimes below, the nearly stationary earth. Obviously it will at times pass actually behind it, and then the lunar spectator would behold the sublime spectacle of a total solar eclipse, and that under circumstances which render the phenomenon far more imposing than its counterpart can appear from the earth; for whereas, when we see the moon eclipse the sun, the nearly similar (apparent) diameters of the two bodies renders the duration of totality extremely short—at most 7 minutes—a lunar spectator, the earth appearing to him four times the diameter of the sun, and he and the earth being relatively stationary, would enjoy a view of the totality extending over several hours. During the passage of the solar disc behind that of the earth, a beautiful succession of luminous phenomena would be observed to follow from the refractions and dispersions which the sunbeams would suffer in passing tangentially through those parts of our atmospheric envelope which lie in their course; those, for instance, on the margin of the earth, as seen from the moon. As the sun passed behind the earth, the latter would be encircled upon the in-going side with a beautiful line of golden light, deepening in places to glowing crimson, due to the absorption, already spoken of, of all but the red and orange rays of the sun’s light by the vapours of our atmosphere. As the eclipse proceeded and totality came on, this ruddy glow would extend itself nearly, if not all, around the black earth, and so bright would it be, that the whole lunar landscape covered by the earth’s shadow would be illuminated with faint crimson light,[18] save, perhaps, in some parts of the far distance, upon which the earth had not yet cast its shadow, or off which the shadow had passed. Although the crimson light would preponderate, it would not appear bright and red alike all around the earth’s periphery. The circle of light would be, in fact, the ring of twilight round our globe, and it would only appear red in those places where the atmosphere chanced to be in that condition favourable for producing what on earth we know as red sunset and sunrise. We know that the sun, even in clear sky, does not always set and rise with the beautiful red glow, which may be determined by merely local causes, and will therefore vary in different parts of the earth. Now a lunar spectator watching the sun eclipsed by the earth, would see, during totality and at a coup d’œil, every point around our world upon which the sun is setting on one side and rising upon the other. To every part of the earth around what is then the margin, as seen from the moon, the sun is upon the horizon, shining through a great thickness of atmosphere, reddening it, and being reddened by it wherever the vaporous conditions conduce to that colouration. And at all parts where these conditions obtain, the lunar eclipse-observer would see the ring of light around the black earth-globe brilliantly crimsoned; at other parts it would have other shades of red and yellow, and the whole effect would be to make the grand earth-ball, hanging in the lunar sky, like a dark sphere in a circle of glittering gold and rubies.
During the early stages of the eclipse, this chaplet of brilliant-coloured lights would be brightest upon the side of the disappearing sun; at the time of central eclipse the radiance (supposing the sun to pass centrally behind the earth) would be equally distributed, and during the later stages it would preponderate upon the side of the reappearing sun. We have endeavoured to give a pictorial realization of this phenomenon and of the effect of the eclipse upon the lunar landscape, but such a picture cannot but fall very, very far short of the reality. (See [Plate XXII].)