The extent of the moon’s librations, taking them all and in combination into account, amounts to about seven degrees of arc of latitude or longitude upon the moon, both in the north-south and east-west directions. And taking into account the whole effect of them, we may conclude that our view of the moon’s surface, instead of being confined to one half, is extended really to about four-sevenths of the whole area of the lunar globe. The remaining three-sevenths must for ever remain a _terra incognita_ to the habitants of this earth, unless, indeed, from some catastrophe which it would be wild fancy to anticipate, a period of rotation should be given to the moon different from that which it at present possesses. Some highly fanciful theorists have speculated upon the possible condition of the invisible hemisphere, and have propounded the absurd notion that the opposite side of the moon is hollow, or that the moon is a mere shell; others again have urged that the hidden half is more or less covered with water, and others again that it is peopled with inhabitants. There is, however, no good reason for supposing that what we may call the back of the moon has a physical structure essentially different from the face presented towards us. So far as can be judged from the peeps that libration enables us to obtain, the same characteristic features (though of course with different details) prevail over the whole lunar surface.
The speculative ideas held by the philosophers of the pre-telescopic age, touching the causes which produced the inequalities of light and shade upon the moon, received their _coup de grâce_ from the revelations of Galileo’s glasses. Our satellite was one of the earliest objects, if not actually the first, upon which the Florentine turned his telescope; and he found that the inequalities upon her surface were due to differences in its configuration analogous to the continents and islands, and (as might then have been thought) the seas of our globe. He could trace, even with his moderate means, the semblance of mountain-tops upon which the sun shone while their lower parts were in shadow, of hills that were brightly illuminated upon their sides towards the sun, of brightly shining elevations, and deeply shadowed depressions, of smooth plains, and regions of mountainous ruggedness. He saw that the boundary of sunlight upon the moon was not a clearly defined line, as it would be if the lunar globe were a smooth sphere, as the Aristotelians had asserted, but that the terminator was uneven and broken into an irregular outline. From these observations the Florentine astronomer concluded that the lunar world was covered not only with mountains like our globe, but with mountains whose heights far surpassed those existing upon the earth, and whose forms were strangely limited to circularity.