The Moon does not generate its own light, unlike the Sun, which produces energy through nuclear fusion. Instead, the Moon acts as a mirror in space, capturing and redirecting the Sun’s rays toward Earth. The light we see illuminating the night sky is entirely sunlight that has traveled over 93 million miles to reach the Moon’s surface. This light is then reflected back across a quarter of a million miles to our eyes.
The Physics of Reflected Light
The fundamental difference between the Sun and the Moon lies in light emission versus reflection. The Sun is a luminous body that produces and emits its own light. Conversely, the Moon is a non-luminous body that shines only by reflecting light from an external source when solar photons strike the lunar surface and bounce off toward Earth.
The Moon’s surface is covered in a pulverized layer of rock and dust called regolith, which is surprisingly dark. This darkness is quantified by the Moon’s albedo, a measure of how much light a surface reflects, which averages only about 0.12. This means the Moon reflects only about 12% of the sunlight that hits it, comparable in reflectivity to worn asphalt. The bright glow we observe is a result of the Sun’s power, which makes even a small fraction of reflected light appear brilliant against the black backdrop of space.
Why We See Changing Moon Shapes
The varying appearance of the Moon throughout the month, known as the lunar phases, is direct proof of the Sun’s role in its illumination. The phases are not caused by the Earth casting a shadow on the Moon, but by the changing geometry of the Sun, Earth, and Moon system. As the Moon completes its 29.5-day orbit around Earth, the proportion of its sunlit side visible from our perspective constantly shifts.
The Sun always illuminates exactly half of the Moon, creating a distinct day side and night side, just as it does for Earth. When the Moon is positioned between the Earth and the Sun, the illuminated half faces away from us, resulting in the New Moon phase where the Moon is essentially invisible. As the Moon moves in its orbit, we begin to see a small sliver of the illuminated side, leading to the crescent phases.
The Full Moon occurs when the Earth is positioned between the Sun and the Moon, allowing us to see the entire sunlit hemisphere. At the quarter phases, the Sun, Earth, and Moon form a right angle, which means we see precisely half of the Moon illuminated. These predictable changes demonstrate that the Moon’s appearance is solely dictated by the angle at which solar light strikes its surface and the viewing angle from Earth.
The Phenomenon of Earthshine
The observation known as Earthshine further reinforces the principle of celestial reflection. This phenomenon is a faint glow that illuminates the dark portion of the Moon, particularly visible during the thin crescent phases. The light responsible for this glow originates with the Sun, but it takes a two-step journey to reach our eyes. The initial solar light travels to Earth, where it reflects off our planet’s bright features, such as clouds and oceans. This reflected light, sometimes called Earthlight, then travels to the Moon, gently illuminating the lunar surface that is otherwise in shadow before bouncing back to Earth.