§ 17. During the time that the Earth is performing her journey round the Sun, the Moon, our constant attendant, is continually moving round the Earth, _and completes one of these revolutions in little more than Twentyseven days_.—Very important and interesting to us are the changes in appearance which she exhibits during each of these revolutions.—You probably will know, that the Moon does not appear to us, at all times, the same. Sometimes she is hardly at all visible, (at least not with the naked eye); at other times only a small rim of her is seen, which by degrees becomes larger and larger, until finally she appears in her full round form. After this she begins again to diminish, changes again into a small luminous rim, and finally disappears entirely from our sight. These successive changes in the Moon’s appearance are called _the Moon’s Phases_, or the _waxing and waning of the Moon_. The time during which the Moon is not seen is called _New Moon_; the time during which she exhibits her full shape is called the _Full Moon_; and the different periods of her waxing and waning (when she appears to us in the form of a crescent) are called _Quarters_. Thus we speak of the _First_ and of the _Last_ Quarter of the Moon. The First Quarter takes place after New Moon; the last Quarter after Full Moon. The following diagram, Plate No. VII, may serve to represent to you the Moon’s phases as seen from our Earth. When the Moon is in _a_, then the light of the Sun falls just on that side of it which is turned from the Earth. It is then, we have New Moon. When in _b_, a small brim of the Moon is seen, because a small portion of its lighted surface is then turned towards the Earth.—When in _c_ half of her lighted surface is turned towards us, and we have the First Quarter. In _d_ a still greater portion of the Moon’s lighted surface is visible, and in _e_, we have Full Moon, because her whole lighted surface is then turned towards the Earth. In _f_ the moon commences to wane (to grow smaller,) and in _g_ the last quarter commences; finally, when passed through the point _h_, we have in _a_, again New Moon. For familiar illustration you may also take a white ivory ball, holding it before a lighted candle, which may take the place of the Sun. When the ball is in a straight line between your eye and the candle it will appear to you all dark; because the lighted part is then entirely turned toward the candle (away from you), and you have the same case which is represented to you in the diagram, when the Moon is in _a_. But if you move the ball a little to the right, you will perceive a streak of light, similar to the First Quarter represented in the Diagram, when the Moon is in _c_. If moved still farther to the right, so that the whole lighted part of the ball is seen, it will resemble the Full Moon; represented in the Diagram, when the Moon is in _e_. [Illustration: _N^o. VII._ ] [Illustration: _N^o. VIII._ ]