What causes the phases of the Moon?

The phases of the Moon are caused by its orbit around Earth. As the Moon orbits Earth, the amount of sunlight reflected from its surface changes, giving rise to the different phases. When the Moon is between Earth and the Sun, we see the new moon phase. As it moves, we see the waxing crescent, first quarter, and waxing gibbous phases, with the full moon occurring when the entire side of the Moon facing Earth is fully illuminated. After the full moon, the Moon wanes through the gibbous, third quarter, and crescent phases before returning to the new moon.