The Moon, Earth’s natural satellite, regularly changes its appearance in our night sky. These varying appearances are known as the phases of the Moon, which describe the amount of the Moon’s sunlit surface visible from Earth. This predictable cycle offers a continuous spectacle.
The Dance of Light and Orbit
The phases of the Moon result from the Sun’s illumination, the Moon’s orbit around Earth, and our changing perspective from Earth. The Moon does not produce its own light; it reflects sunlight. The Sun always illuminates one half of the Moon, while the other half remains in darkness.
As the Moon orbits Earth, a cycle taking approximately 29.5 days, the portion of its illuminated side visible to us changes. Our view shifts because the Moon’s position relative to the Sun and Earth alters. Although half of the Moon is always lit by the Sun, we only observe a specific fraction of that illuminated half depending on its orbital path.
The Moon’s orbit around Earth is also slightly tilted, by about five degrees, relative to Earth’s orbit around the Sun. This tilt means the Moon is usually either above or below the plane of Earth’s shadow. This orbital geometry dictates how much of the sunlit lunar surface is presented to our view.
Observing the Phases
From Earth, we observe a continuous cycle of lunar appearances. The cycle begins with the New Moon, when the Moon is positioned between Earth and the Sun, making the side facing us completely dark. After the New Moon, a slender sliver of light marks the Waxing Crescent phase, where “waxing” indicates the illuminated portion is growing.
Next, the First Quarter Moon appears, showing exactly half of the Moon’s face illuminated from our perspective, resembling a half-circle. As more of the Moon becomes lit, it transitions into the Waxing Gibbous phase, where more than half but not all of the Moon’s surface is visible. This leads to the Full Moon, when the entire side of the Moon facing Earth is fully illuminated, appearing as a complete circle.
Following the Full Moon, the illuminated area begins to shrink, entering the Waning Gibbous phase, where “waning” signifies the visible light is decreasing. The Third Quarter (or Last Quarter) Moon then appears as another half-circle, but with the opposite side illuminated compared to the First Quarter. Finally, the Moon becomes a Waning Crescent, showing only a thin sliver of light that continues to diminish until it returns to the New Moon phase.
Dispelling Common Misunderstandings
A frequent misconception is that Earth’s shadow causes the phases of the Moon. If Earth’s shadow were responsible, it would appear curved differently than the gradual progression of light we observe, and it would occur far more frequently.
Lunar eclipses are separate phenomena, occurring only when Earth passes directly between the Sun and a Full Moon, casting a shadow on the Moon. These events are infrequent. Another misunderstanding suggests that clouds obscure parts of the Moon, creating the phases; however, the Moon does not have clouds, and Earth’s clouds have no impact on its appearance.