The moon and its motion and shape.

The moon rises in the eastern part of the sky and sets in the western, and, like the sun, its exact position also depends on the time of the year. But its time of rising varies in a different way from that of the sun. If the moon rises at say, 8:00 P.M. one night, the next night it will rise at about 8:50 P.M., the next about 9:40 P.M., and so on; after about one month (50 min × 28 days = 1400 min = 24 hours) it will rise at 8:00 P.M. again. The moon rises, on the average, 50 minutes later each night. Thus moonrise can be at any time of day or night.

Unlike the sun, the moon appears to change shape. Sometimes it looks like a disc, other times like a half-disc, or like a very narrow crescent. These various shapes are called the "phases" of the moon. The phase of the moon is determined by the position of the sun at the time of the moon rise. If the moon rises at sunset, the moon is full: . If the moon rises at noon, it is at first quarter : . If it rises at sunrise, it is a new moon: (no moon) . If it rises at midnight, it is at last quarter: . Intermediate rising times give intermediate phases.

The path the moon takes across the sky depends on both the time of year (like the sun's path) and its phase. The new (or nearly new) moon is always on (nearly) the same path as the sun, so it is low in the sky in winter, high in the summer. The full (or nearly full) moon is opposite the sun in the sky (rises at sunset, sets at sunrise), so it is high in winter, low in summer. Full moonlight appears brighter in the winter than in the summer because the full moon is higher in the sky.

The length of time the moon is up depends on where it rises, just like the length of time the sun is up (the daylight hours) depends on where it rises. (And as we shall see, the length of time a star is up depends on the length of time the star is above the horizon but doesn't depend on the time of year or time of month.) The apparent position of the moon in the sky depends on where you are on the earth's surface; as you move around the earth, you observe the moon in exactly the same way you observe the sun: move north, the moon seems lower; move toward the equator, and the moon seems higher, etc.

As you probably have noticed, the moon's motion seems more complex than the sun's; this is because we see the moon as it goes around us, as we both go around the sun, and as we spin on our axis. When we look at the sun, we only see two effects (spinning on our axis, or "rotating", and traveling around the sun, or "revolving".) When we look at the stars, we (primarily) see only one of these motions, the diurnal (daily) rotation of the earth. So that their apparent motions are simplest of all.