THE MOON.
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If we next suppose the Moon , after having been reduced fromFull, half-way towards being New, to continue to approachnearer and nearer to the latter position, one boundary, pmq,of the enlightened part will remain unaltered, while the other,pnq, will always be a semi-circle on the Moon ’s surfacepassing through p and Q. But pnq will be gradually turnedmore and more away from our line of sight, and will drawnearer and nearer to pmq. As it does so it will necessarilyappear to form a curve, on the disc of which we have previouslyspoken, which will always pass through p to q, but willgradually change from the straight line poq, first having theform of a very much flattened ellipse, then of one more curved,and so on; until at last, just before New Moon , it will nearlycoincide with the circle pmq.
The appearances will, in succession, be such as are shown in
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a a • a cl.
the drawing in Fig . XVII., taken in order from left to right.The same drawings, taken from right to left, may illustrate:, theincrease of the phase of a New Moon , during the first quarterof a month. A similar explanation will also account for theappearances which the Moon presents when it is more thanhalf full, during which time it is technically said to be gibbous.
,p ... p p “ p
Fig. XVIII.—Phases of the Moon.
[mi
In this case the elliptic boundary is simply turned the otherway, as in the drawings in Fig . XVIII., which, according as
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