Why does the phases of the moon depend on the sun earth and moon?

The phases of the moon depend on the positions of the Sun, Earth, and Moon relative to each other. As the Moon orbits the Earth, different portions of its illuminated half are visible from Earth, creating the various phases. For example, during a new moon, the Moon is between the Earth and the Sun, making it invisible, while a full moon occurs when the Earth is between the Sun and the Moon, allowing the entire illuminated side to be seen. Thus, the alignment of these three celestial bodies determines the lunar phases.