Page:Astronomy for Everybody.djvu/146

While the earth is making this motion the moon will have moved around the orbit in the direction of the arrows, so as to have reached the point N. At the moment when the lines EM and FN are parallel to each other, the moon will have completed her actual revolution, and will seem to be in the same place among the stars as before. But the sun is now in the direction FS. The moon therefore has to continue its motion before it catches up to the sun. This requires a little more than two days, and makes the whole time between two new moons twenty-nine and a half days.

The varying phases of the moon depend upon its position with respect to the sun. Being an opaque globe, without light of its own, we see it only as the light of the sun illuminates it. When it is between us and the sun its dark hemisphere is turned toward us, and it is entirely invisible. The time of this position in the almanacs is called "new moon," but we cannot commonly see the moon for nearly two days after this time, because it is lost in the bright twilight of evening. On the second and third day, however, we see a small portion of the illuminated globe, having the familiar form of a thin crescent. This crescent we commonly call the new moon, although the time given in the almanac is several days earlier.

In this position, and for several days longer, we may, if the sky is clear, see the entire face of the moon, the dark parts shining with a faint gray light. This light is that which is reflected from the earth to the moon. An inhabitant of the moon, if there were such, would