The question of whether the Moon is a reflection of the Earth has a complex answer. Fundamentally, the Moon does not produce its own light; it shines exclusively by reflecting the light it receives from the Sun. The brilliant glow we see is sunlight that has traveled 93 million miles, struck the lunar surface, and then scattered back toward our eyes. Therefore, the Moon is primarily a reflection of the Sun, not the Earth.
How the Moon Shines
The visibility of the Moon is governed entirely by reflection, measured by a property called albedo. Albedo describes the fraction of incoming sunlight that a surface reflects back into space. The Moon’s surface, composed of dark, volcanic rock and dust, has a low albedo, reflecting only about 12% of the sunlight that strikes it.
This low reflectivity means the Moon is actually a relatively dark object, comparable in brightness to old asphalt. The remaining 88% of solar energy is absorbed, causing the Moon’s surface temperature to vary drastically. Despite its dim surface material, the Moon appears luminous to us because of its close proximity to Earth. This closeness ensures that even a small percentage of reflected sunlight appears bright enough to dominate the night sky.
The Cycles That Change the Moon’s Appearance
The changing shapes of the Moon, known as lunar phases, are caused by the shifting geometry of the Sun, Earth, and Moon system. As the Moon orbits our planet, the amount of the sunlit lunar surface visible from Earth changes over a roughly 29.5-day cycle. The Moon is always half-illuminated by the Sun, but our perspective on that illuminated half constantly shifts.
When the Moon is positioned between the Earth and the Sun, the illuminated side faces away from us, resulting in the new moon phase. As the orbit progresses, we begin to see a sliver of the sunlit side, known as a crescent. The full moon occurs when the Earth is positioned between the Sun and the Moon, allowing us to see the entire sunlit face.
Earth’s Light Reflected Back to the Moon
The underlying confusion points to a real, secondary phenomenon called earthshine. Earthshine is the faint illumination of the otherwise dark portion of the Moon, caused by reflected sunlight from Earth. For this to happen, sunlight must strike Earth, bounce off its atmosphere and surface, travel to the Moon, and then reflect back to our eyes.
The Earth is significantly more reflective than the Moon, possessing an average albedo of about 30% due to its bright clouds, oceans, and polar ice caps. When the Moon is a thin crescent, a nearly full Earth would be visible from the lunar surface, appearing much brighter than a full moon appears to us. This brilliant, reflected light illuminates the dark side of the Moon, making it faintly visible.
This dim glow is most easily observed on the unlit part of a thin crescent moon, sometimes poetically described as “the old moon in the new moon’s arms”. Earthshine is a secondary light source for the Moon, a subtle glow next to the brilliance of direct sunlight. While it confirms that Earth reflects light onto the Moon, this faint illumination is not the primary mechanism behind the Moon’s bright appearance.