What causes the different phases of the Moon?

The different phases of the Moon are caused by its orbit around Earth and the relative positions of the Moon, Earth, and the Sun. As the Moon orbits Earth, we see varying amounts of its sunlit side. The phases start with the new moon, when the Moon is between the Earth and the Sun and we can’t see it. As it moves, we see more of the illuminated side, progressing through the waxing crescent, first quarter, waxing gibbous, full moon, and then the phases reverse as it wanes. The entire cycle takes about 29.5 days.