The moon’s appearance changes nightly, shifting from a slender sliver to a brilliant orb and sometimes vanishing entirely. This predictable cycle involves the moon, Earth, and the Sun. The varying shapes are not actual changes to the moon itself, but rather how much of its illuminated surface is visible from our planet.
How Sunlight and Orbit Create Phases
The moon does not produce its own light; instead, it shines by reflecting sunlight. The Sun consistently illuminates one half of the moon, creating a day side and a night side, much like Earth. As the moon travels in its orbit around Earth, our perspective changes, allowing us to see different portions of this sunlit half.
The moon completes one orbit around Earth approximately every 27 Earth days. However, because Earth is also moving in its orbit around the Sun, it takes the moon about 29.5 days to complete a full cycle of phases from one new moon to the next. This period is known as a synodic month. The moon’s rotation is synchronized with its orbit, meaning the same hemisphere always faces Earth.
Decoding the Lunar Phases
The lunar cycle begins with the New Moon, a phase where the moon is positioned between the Earth and the Sun, making it largely invisible from our planet. Following the New Moon, the visible illuminated portion of the moon gradually increases, a process called waxing. The Waxing Crescent appears as a thin sliver of light, growing larger each day.
The First Quarter moon shows half of its disk illuminated, resembling a half-circle. This is followed by the Waxing Gibbous phase, where more than half of the moon is lit, continuing to expand. The cycle culminates in the Full Moon, when the entire side facing Earth is fully illuminated by the Sun.
After the Full Moon, the illuminated portion begins to decrease, a process known as waning. The Waning Gibbous phase appears as a decreasing, bright shape, transitioning to the Last Quarter moon, which again shows half illumination, but on the opposite side from the First Quarter. The cycle concludes with the Waning Crescent, a diminishing sliver, before returning to the New Moon.
Other Ways the Moon’s Look Transforms
Beyond the regular phases, other phenomena can alter the moon’s appearance. A total lunar eclipse occurs when Earth passes directly between the Sun and moon, casting a shadow on the lunar surface. During such an event, the moon can take on a reddish or coppery hue, often called a “blood moon.” This coloring happens because sunlight, filtered through Earth’s atmosphere, scatters blue light and bends longer red and orange wavelengths toward the moon.
The moon’s elliptical orbit also affects its apparent size. When the moon is closest to Earth in its orbit and coincides with a Full Moon, it can appear slightly larger and brighter, a phenomenon popularly termed a “supermoon.” Conversely, when a Full Moon occurs when farthest from Earth, it appears smaller, sometimes called a “micromoon.” Atmospheric conditions on Earth can also influence the moon’s perceived color, making it appear yellow, orange, or even blue, depending on the amount of dust or particles in the air that scatter light.