What are moon phases?
The moon circles the Earth every 29 days. The Earth's gravity pulls on it so that the same side of the moon is always facing us. Nonetheless, we see a different view, or phase, of the moon each night of the month. Just like the Earth, half of the moon is always in sunlight, and half in shadow. When the moon is between us and the sun, the lighted part is pointed away from us, so we don't see it -- this is the new moon. As the moon orbits the Earth, each night we see a greater part of the lighted side, until the whole sunlit side faces us (full moon). As the orbit continues, we then see less of the lighted side until the whole cycle is completed and starts again.