VIII. The moon's "waxing and waning" is due to the fact that the moon is not self-luminous but receives its light from the sun and to the additional fact that it makes a complete revolution around the earth with reference to the sun in 29-1/2 days. When the earth is on the side of the moon that faces the sun we see the full moon, that is, the whole illuminated hemisphere. But when we are on the side of the moon that is turned away from the sun we face its unilluminated hemisphere and we say that we have a 'new moon.' Once in every 29-1/2 days the earth is in each of these positions with reference to the moon and, of course, in the interval of time between these two phases we are so placed as to see larger or smaller parts of the illuminating hemisphere of the moon, giving rise to the other visible phases. When the moon is between the earth and the sun she is said to be in _conjunction_, and is invisible to us for a few nights. This is the phase called _new moon_. As she emerges from conjunction we see the moon as a delicate crescent in the west just after sunset and she soon sets below the horizon. Half of the moon's surface is illuminated, but we can see only a slender edge with the horns turned away from the sun. The crescent appears a little wider each night, and, as the moon recedes 13° further from the sun each night, she sets correspondingly later, until in her first quarter half of the illuminated hemisphere is turned toward us. As the moon continues her progress around the earth she gradually becomes gibbous and finally reaches a point in the heavens directly opposite the sun when she is said to be in _opposition_, her whole illumined hemisphere faces us and we have _full moon_. She then rises in the east as the sun sets in the west and is on the meridian at midnight. As the moon passes from opposition, the portion of her illuminated hemisphere visible to us gradually decreases, she rises nearly an hour later each evening and in the morning is seen high in the western sky after sunrise. At her _third quarter_ she again presents half of her illuminated surface to us and continues to decrease until we see her in crescent form again. But now her position with reference to the sun is exactly the reverse of her position as a waxing crescent, so that her horns are now turned toward the west away from the sun, and she appears in the eastern sky just before sunrise. The moon again comes into conjunction and is lost in the sun's rays and from this point the whole process is repeated.