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in its speed in its orbit round the Earth , that it does notalways show us exactly the same face. In fact, according asthe Moon travels, during any given time, rather more quicklyor more slowly than would accord with its mean rate ofmotion, we see a little more, which we should not otherwisesee, upon one side, or the other, of the disc usually visible to us.This effect is technically termed a Libration.
But in addition to this it is also found that the Moon ’s axisof rotation is nof quite perpendicular to the plane in which shetravels round the Earth . The result of which is, that, in differentparts of her path, we see somewhat beyond each pole alter-nately, just as the Sun shines, or an observer upon it wouldsee (although to a much greater extent), beyond the two polesof the Earth , alternately, in the course of a year. In this waywe once more obtain a view, at different times, of somewhatmore than exactly one-half of the Lunar Disc.
There is also a third kind of Libration, which deservesmention, called the Diurnal Libration. As any place of obser-vation is carried round by the rotation of the Earth uponits axis, it is thereby changed in position to a considerableextent in the course of twelve hours ; in fact, the change for aplace upon the Equator (as we have explained in discussingthe determination of the Sun’s distance by observations ofMars ; Lect. I., page 16) is nearly equal to a distance of 8,000miles, or to the length of the Earth ’s diameter. Aud fromthese two different positions there may be seen two slightlydifferent views of the Moon , which if put together would includerather more than one-half of its surface.
A similar effect may be obtained by two observers situated ata considerable distance apart upon the Earth . They will see,at the same instant, the one a little beyond one part of theedge of the disc which would bound the view of an observerat the Earth ’s centre, and the other a little beyond anotherpart of it. By the combined effect of all these causes,opportunities occur, which enable us in a long succession ofobservations, made upon suitable occasions, to see nearlythree-fifths of the whole surface of the Moon . It must,however, be allowed that a good deal of the outlying portions