The Moon’s appearance from Earth transforms throughout the month. This changing view, from a sliver to a full circle and back, occurs because our perspective of its illuminated surface shifts as it journeys through space, not because the Moon changes its physical shape.
The Moon’s Journey Around Earth
The Moon orbits Earth, completing one revolution in about 27.3 days relative to the stars. However, the phases we observe follow a 29.5-day cycle, known as a synodic month. This longer period accounts for Earth’s simultaneous movement around the Sun, requiring the Moon to travel slightly further to realign. The Moon’s elliptical path also means its distance from Earth varies slightly during its orbit.
How Sunlight Illuminates the Moon
The Moon shines by reflecting sunlight. The Sun always illuminates half of the Moon’s spherical surface, creating a day and night side. This reflection occurs diffusely due to the Moon’s rough surface, scattering light in many directions. Only a small amount of incident sunlight, typically 3% to 12%, is reflected.
The Mechanics of Moon Phases
The Moon’s phases result from the changing angles at which we view its sunlit half as it orbits Earth. When the Moon is between the Sun and Earth, its illuminated side faces away, creating the “New Moon” phase, which appears dark. As the Moon progresses, a growing sliver of its illuminated surface becomes visible, leading to the “Waxing Crescent.”
The “First Quarter” phase occurs when half of the Moon’s illuminated side is visible, appearing as a half-circle. More than half becomes illuminated next, entering the “Waxing Gibbous” phase, which expands until the “Full Moon.” During a Full Moon, Earth is positioned between the Sun and Moon, and we see the entire sunlit face.
After the Full Moon, the illuminated portion visible from Earth begins to decrease. This period is called “Waning Gibbous,” followed by the “Third Quarter” (or Last Quarter), where half of the Moon is visible on the opposite side from the First Quarter. The cycle concludes with the “Waning Crescent” as the visible portion shrinks to a thin sliver before returning to the New Moon. This continuous cycle creates the predictable progression of lunar phases approximately every 29.5 days.
Beyond Phases Other Visual Changes
Beyond its regular phases, the Moon displays other visual variations unrelated to its illumination cycle. Its apparent size changes slightly because its elliptical orbit means it is sometimes closer to Earth (perigee) and sometimes farther away (apogee), making it appear marginally larger or smaller.
The Moon’s color can also vary, often appearing yellow, orange, or red, especially near the horizon. This color change occurs because Earth’s atmosphere scatters shorter wavelengths of light, like blue, allowing longer wavelengths, such as red and orange, to pass through. Atmospheric conditions like dust, smoke, or a lunar eclipse can enhance these color effects.