Why Does the Moon Have Phases?

The Moon produces no light of its own; it reflects sunlight. As it orbits Earth every 29.5 days (a synodic month), the portion of its sunlit side visible to us changes. At new moon, the Moon is between Earth and the Sun, so its illuminated face points away from us. At full moon, Earth is between the Sun and Moon, and we see the entire sunlit hemisphere. The waxing and waning crescents and quarters are intermediate viewing angles. The Moon always keeps the same face toward Earth due to tidal locking, but the lit portion we see shifts throughout the cycle.