PHASES OF THE MOON.

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At the time of conjunction, or when the moon is between the earth and sun(as at a in the scene), she is invisible to the earth, because her enlightenedside is then turned towards the sun, and her dark side towards the earth. In ashort time after the conjunction (as in the next figure below) at x, she appearsto the earth like a slender crescent, to the eastward of the sun, a little afterhe sets; as represented by the figure m, in the outer circle. This crescentseems, as it were, to fill up, and the bright portion increases as the moonadvances in her orbit; and when she has performed a fourth part of a revolu-tion, and has arrived at b, she appears to be half illuminated; and is then saidto be in her first quarter. After describing the second quadrant of her orbit,she arrives at c, and is then opposite to the sun, as regards the earth, andshines with a round illuminated disc, which we term the full moon. Herappearance at that time is represented in our scene of the moon at the full.(Scene XIII.) ;

After the full, she begins to decrease gradually as she moves through theother half of her orbit, and when she arrives at d (her eastern half only beingilluminated), she is said to be in her third quarter, and has to us the appearanceof the corresponding figure in the outer circle: thence she continues todecrease, until she again disappears at the conjunction, in the position at a, asbefore.

Between the third quarter and the change, or in the position from d to a,the moon is frequently visible to us during the day, even when the sun shines;she then affords the opportunity of making a very beautiful experiment. Holda small white globe, at the length of the arm, between the eye and the moon,and if the sun Bhines on the globe, and it is held so that its upper part may justseem to touch the lower edge of the moon’s figure, we shall see the illu-minated portion of the globe corresponding exactly with that of the moon, andexhibiting a complete picture of its phase. The reason is plain—the sunenlightens the globe in the same manner as the moon; and, the figures beingsimilar, when we place ourselves in the described situation, the moon and theglobe have the same position as regards the observer, and, therefore, we mustsee the same illuminated portion of the one as of the other.

These different phases plainly demonstrate that the moon does not shineby any light of her own ; for if she did, being globular, she would constantlypresent a fully illuminated disc like the sun ; she is, therefore, demonstrably anopaque body.

If the moon may be supposed to be inhabited by beings like ourselves, it isevident, from this explanation of her appearances to the earth, that the earthmust also present phases, precisely of the same character, to the inhabitants ofthe moon, as she herself does to the earth, differing only in the order in whichthey take place. When the moon is in conjunction, or in the position a of thescene, the inhabitants of that satellite will see the earth at full. When themoon is in opposition, or in the position c, the earth appears to the inhabitantof the moon 10 be in conjunction ; he sees nothing of its disc. When the moon