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thus seen is so near to the edge of the disc, that it is muchforeshortened, and very unfavourably situated for observation.
As a summary of these remarks, we may therefore state thatit is not strictly true to say that the Moon always turns thesame face to the Earth . But it is true that it revolves onceupon its axis (with almost absolute uniformity), during eachone of its revolutions round the Earth ; and that, if its orbitround the Earth were exactly perpendicular to that axis, andits motion in it exactly uniform, an observer at the centre ofthe Earth would always see the very same half of its surface.If we are only speaking in general terms, we may be permittedto use the phrase, that it always turns the same face to theEarth . In a more accurate statement we must allow for thelibratory effects produced by the two causes we have named,and for the small additional effect produced by the movementof an observer in space, owing to the Earth ’s daily rotation,or to his own journeying to different parts of its surface. Itis by allowing for these effects that we find that an addi-tional -p-tli of the Moon ’s surface may be seen, although ingeneral very obliquely; which added to J makes up
altogether nearly fths of the whole.