The familiar glow of the Moon in our night sky often prompts the question: how does it produce its own light? The simple answer is, it doesn’t. The Moon, a rocky body, acts like a mirror, reflecting light from the Sun.
The Moon as a Reflector of Sunlight
The light we perceive from the Moon is entirely sunlight that has traveled through space, struck the lunar surface, and then bounced back towards Earth. The Sun’s rays illuminate one half of the Moon at any given time, creating a “day side” and a “night side,” just as on Earth.
Despite its apparent brightness, the Moon’s surface is not highly reflective. Its average reflectivity, known as albedo, is quite low, ranging from about 7% to 14%, comparable to asphalt. However, the intensity of the Sun’s light, coupled with the Moon’s proximity to Earth, allows even this small percentage of reflected light to appear luminous. This reflected light, called moonlight, takes approximately 1.26 seconds to travel from the Moon to Earth.
Why We See Different Lunar Phases
The changing appearance of the Moon, known as its phases, results from our shifting view of its sunlit portion as it orbits Earth. One half of the Moon is always illuminated by the Sun. However, the amount of that illuminated half visible from Earth varies depending on the Moon’s position relative to our planet and the Sun. This causes the Moon to appear to change its shape over approximately 29.5 days, completing a full cycle of phases.
For instance, when the Moon is between Earth and the Sun, its sunlit side faces away from us, appearing as a largely invisible “new moon.” As it moves in orbit, more of the illuminated surface becomes visible, culminating in a “full moon” when the entire sunlit side faces Earth. The lunar phases are not due to the Moon generating more or less light, but rather a changing perspective on which part of its illuminated face we can see.
Common Misconceptions About Moonlight
A common misunderstanding is that moonlight is fundamentally different from sunlight. In reality, it is simply sunlight reflected from the lunar surface. While significantly dimmer than direct sunlight due to the Moon’s low reflectivity and the distance it travels, its origin is the same.
Another common misconception revolves around the “dark side of the Moon.” This phrase often implies a perpetually unlit hemisphere, which is inaccurate. Every part of the Moon experiences both day and night as it rotates, just like Earth. The term “dark side” actually refers to the “farside” of the Moon, the hemisphere that never faces Earth due to tidal locking. The farside is generally brighter than the nearside because it has fewer dark volcanic plains.