What causes the phases of the Moon?

The phases of the Moon are primarily caused by the relative positions of the Earth, Moon, and Sun. As the Moon orbits the Earth, the angle at which sunlight hits the Moon changes, creating the various appearances of the Moon we observe from Earth.

During the New Moon phase, the Moon is positioned between the Earth and the Sun, which means the side of the Moon that is illuminated by the Sun is facing away from us, making it virtually invisible. As the Moon continues its orbit, we see increasingly more of the illuminated side, leading to the Waxing Crescent, First Quarter, Waxing Gibbous, and finally the Full Moon phases. After the Full Moon, the process reverses as the Moon moves towards the New Moon phase again, showing the Waning Gibbous, Last Quarter, and Waning Crescent phases.

This cyclical pattern is purely a result of the interplay of the positions of the Earth, Moon, and Sun, not influenced significantly by how quickly the Earth rotates, how far the Moon is from the Earth, or the Sun’s brightness.