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.