The Moon’s glow is not intrinsic; it is a dark, rocky body that generates no light of its own. Its appearance as a bright orb in the night sky is entirely a result of redirecting light from an external source. The visible glow is simply sunlight that has traveled 93 million miles to reach the Moon’s surface and then bounced back toward Earth.
The Sun’s Illumination
The Sun is responsible for the Moon’s radiance. Solar photons travel unimpeded through space until they strike the Moon’s surface, where they are either absorbed or scattered. The level of illumination on the Moon is constant.
The different shapes of the Moon we observe, known as phases, are a consequence of the changing geometry between the Sun, Earth, and Moon. As the Moon orbits Earth, the angle at which we view the sunlit portion shifts, making it appear as a crescent, half, or full circle. The proportion of that illuminated side visible to an observer on Earth varies throughout the lunar cycle.
How the Lunar Surface Reflects Light
The Moon redirects sunlight through diffuse reflection, a scattering process distinct from mirror-like, or specular, reflection. When sunlight strikes the uneven surface, it is scattered in many directions, which is why the Moon appears equally bright regardless of the viewing angle. The lunar surface is covered in regolith, a layer of fine, powdery material composed of pulverized rock and dust from meteorite impacts.
The overall reflectivity of the Moon is measured by its albedo, a ratio indicating the fraction of incident light that is reflected. The Moon has a low average albedo of approximately 0.12, reflecting only about 12% of the sunlight that hits it. The lunar regolith is quite dark, similar in reflectivity to an asphalt road, despite the perception of brightness against the black backdrop of space.
The appearance of brightness is enhanced by the opposition effect, which occurs when the Moon is nearly full and viewed from Earth. At this time, the Sun, Earth, and Moon are almost perfectly aligned, causing the light to strike the surface straight on. This alignment minimizes the shadows cast by the tiny, irregularly shaped particles of regolith, leading to a noticeable increase in overall brightness.
Secondary Glows and Atmospheric Effects
Beyond the primary reflection of direct sunlight, other phenomena modify how the Moon’s glow is perceived. One notable effect is Earthshine, a faint illumination of the unlit portion of a crescent Moon. This occurs when sunlight reflects off Earth’s surface and clouds, bounces onto the Moon’s dark side, and then reflects back to our eyes. Earth is significantly more reflective than the Moon, and when the Moon appears as a thin crescent, the Earth as viewed from the Moon is nearly full, maximizing the effect.
The color of the Moon’s glow can also be altered by the Earth’s atmosphere, particularly when the Moon is near the horizon. When moonlight passes through a greater thickness of the atmosphere, shorter wavelengths of light, such as blue, are scattered away by air molecules, a process known as Rayleigh scattering. This leaves the longer wavelengths, like red and yellow, to dominate the light that reaches the observer, causing the Moon to appear a warm color.