The phases of the Moon change each month because the Moon orbits the Earth, and as it moves, the portion of the Moon that is illuminated by the Sun and visible from Earth changes. The Moon does not produce its own light; we see the sunlight reflected off its surface. As the Moon travels around the Earth in about 29.5 days, different amounts of its sunlit side are visible from Earth, causing the phases to progress from New Moon, to Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, Waning Crescent, and back to New Moon. This cycle is called a lunar month. The underlying cause of the changing phases is the relative positions of the Earth, Moon, and Sun. The Moon’s orbit around the Earth causes varying angles between these bodies, and this angle change alters how much of the illuminated half of the Moon faces Earth at any given time. The Earth's own shadow does not cause the phases (except during lunar eclipses), but the changing view of the sunlit Moon surface does. Additionally, the Moon's orbit is slightly tilted and elliptical, which leads to small variations in how we see the illuminated parts, but the main factor remains the orbital motion around the Earth combined with sunlight reflection. The full cycle takes about 29.5 days, longer than the Moon's orbital period of 27.3 days due to Earth moving around the Sun simultaneously, which changes the necessary orbit path for the Moon to return to the same phase position.
In summary, the Moon’s changing phases each month result from the Moon orbiting Earth while reflecting sunlight, and our changing viewpoint of which portion of that sunlit half is visible from Earth.
