The glowing orb of the Moon is a familiar sight, but the source of its light is often misunderstood. It is a common misconception that the Moon generates its own light like a star. The light we see is not produced internally but is instead a result of external illumination. The Moon functions as a reflector, taking light from the Sun and redirecting it toward Earth.
The Sun: The Moon’s Only Light Source
The illumination of the Moon is derived from the Sun, making sunlight the source of what we perceive as “moonlight.” The Moon, like Earth and the other planets, is a rocky, non-luminous body that does not undergo the nuclear fusion necessary to emit its own light. This separates satellites, which merely reflect light, from stars, which actively emit radiation.
When sunlight strikes the Moon’s surface, the light is scattered in all directions, with a small fraction traveling the approximately 239,000 miles to reach our eyes. Because the Sun is intensely bright, the light the Moon reflects is sufficient to make it the second brightest object visible in Earth’s night sky. At any given moment, one-half of the Moon is illuminated by the Sun, while the other half is in darkness.
How the Lunar Surface Reflects Light
The physical characteristics of the Moon’s surface play a direct role in how much light is reflected. The measure of a celestial body’s reflectivity is called albedo, a ratio that compares the amount of light reflected to the amount of light received. Despite its brilliant appearance against the blackness of space, the Moon is actually a poor reflector, with an average albedo of only about 0.07 to 0.12. This means the lunar surface reflects only about 7% to 12% of the sunlight that strikes it.
The Moon’s surface is comparable to common dark materials like worn asphalt or coal. It is covered in regolith, a fine, powdery, gray material resulting from billions of years of meteorite impacts. The texture of this regolith causes light to be scattered diffusely, rather than specularly like a mirror. This is one reason the Moon’s brightness appears relatively uniform across its face at full phase. This low albedo is partly a result of space weathering, a process where the lunar soil is darkened by constant exposure to the solar wind and micrometeorite bombardment.
Why We See Changing Shapes of Illumination
The changing shapes of the Moon we observe, known as lunar phases, are not due to the Moon losing or gaining light, but rather to the changing geometry of the Sun-Earth-Moon system. The Moon is a sphere, and exactly half of it is always illuminated by the Sun, creating a day side and a night side. The phases we see are merely the varying amounts of that illuminated half visible from our perspective on Earth as the Moon completes its orbit.
When the Moon is positioned between the Earth and the Sun, the illuminated side faces entirely away from us, resulting in the New Moon phase. As the Moon moves along its orbit, our viewing angle gradually changes, and we begin to see a sliver of the sunlit portion. The cycle progresses from a crescent to the First Quarter, where we see exactly half of the illuminated face, and then to a gibbous shape as more of the day side comes into view. The Full Moon occurs when the Earth is positioned between the Sun and the Moon, allowing us to see the entire sunlit hemisphere.
The Role of Earthshine
While the Sun is the primary light source, a secondary phenomenon called Earthshine subtly illuminates the Moon’s night side. This effect is most noticeable during the crescent phases, when a thin sliver of the Moon is brightly lit by the Sun, but the rest of the dark disk is faintly visible. Earthshine is caused by sunlight reflecting off Earth’s clouds, oceans, and landmasses, with that light then traveling to the Moon.
When the Moon appears as a thin crescent to us, the Earth appears nearly full and intensely bright from the Moon’s perspective. The Earth’s albedo is much higher than the Moon’s, reflecting about 30% of the sunlight it receives. This reflected Earthlight gently illuminates the portion of the Moon that is experiencing its lunar night, a ghostly glow that has historically been referred to as “the old Moon in the new Moon’s arms.”