Why Do Moon Phases Occur?

The different moon phases occur from its revolutions around the Earth. The moon goes through distinct phases with one complete rotation around the world, including a new phase, first quarter phase, third quarter phase and full moon. Variations in light and shape of the moon change within these phases too, as the moon goes through a waxing gibbous stage en route to the full moon stage, then transforms into a waning gibbous before reaching the third quarter phase.

After reaching the third quarter mark, the moon regresses in its circular shape, becoming darker and more narrow in a waning crescent stage. The moon appears completely dark in the new moon stage, then appears as a light, silver-colored sliver during the waxing crescent phase, which leads to the first quarter. The moon completes each phase in a week. Half the time, the sun illuminates one half of the moon's surface, dividing it equally into light and dark spheres. The new moon phase casts the moon in a shadow because it lies on the same side of the sky as the sun. During the full moon, when the moon appears largest and brightest, the moon exists in the same part of the sky as the sun, receiving its full light.