The Moon is not a light source; it does not generate its own light but acts as a secondary light source through reflection. The brilliant light we observe illuminating the night sky is entirely sunlight that has traveled to the Moon’s surface and then been scattered back toward Earth. Understanding the mechanism behind the Moon’s glow requires distinguishing between the two fundamental ways objects interact with light.
Emitted vs. Reflected Light
Light can be categorized into two types: emission and reflection. An object that emits light, such as the Sun or a simple lightbulb, is a true light source because it generates photons through the conversion of energy. The Sun produces light and heat through nuclear fusion. Similarly, a lightbulb filament gets hot enough to emit light as thermal radiation or uses electricity to excite atoms to release photons.
Reflection, in contrast, occurs when light strikes a surface and bounces off without the object itself generating the light. The energy’s wavelength generally remains unchanged, and the light’s origin can be traced back to an external source, like the Sun. Most everyday objects, including the trees and cars we see, are visible only because they reflect ambient light.
The Moon’s Reflective Surface
The light that reaches Earth from the Moon is sunlight scattered by its dusty, rocky surface. This process is a form of diffuse reflection, where the light bounces off the rough surface in many different directions. However, the Moon is surprisingly dark, quantified by its albedo, a measure of how much light a surface reflects. The Moon’s albedo is quite low, reflecting only about 12% of the sunlight that strikes it, making it comparable to old asphalt pavement.
The Moon appears bright in the night sky despite its low reflectivity because of the immense intensity of the incoming sunlight and the high contrast with the dark vacuum of space. When viewed from Earth, the Moon is the dominant light source in the darkness. The amount of light we see also changes depending on the geometry between the Sun, Earth, and Moon, which determines the lunar phases. During a full Moon, the Sun’s light hits the surface and bounces back most directly toward us, maximizing the reflected glow.