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.