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previously appeared like a flat circular disc, we shall in likemanner now see the half of it, as though it were bounded by astraight line passing through the centre of that disc. In otherwords, the bounding edge of the bright portion when thePhase is lialf-fnll, which is really a semi-circle passing half-way round the Moon , will look like a straight line, because we
Fijr. XV.—Moon full. Moon half-full.
are unable to realize the protuberance of the central portionsof its globe towards us. What we see consequently appears, asin Fig. XV., to be one-half of a circle, although it is really one-half of a hemisphere.
Perhaps the following perspective drawing may help toillustrate this still more clearly. Let pmnq in Fig . XVI.
Fig . XVI.—One-fourth of the Moon ’s spherica surface is seen fromthe Earth as a flat semi-circle.
represent in perspective that half of the Moon ’s surface which issupposed to be seen illuminated. An eye at e, looking along e ntowards o, the Moon ’s centre, is unable to judge of the smalldifference between the distances ep, en, eq, and, the semi-circle pnq consequently appearing as if it were projected intothe straight line poq, the illuminated half-hemisphere pmnqis seen as the semicircle poqm.