While the moon appears to shine brightly in our night sky, it does not produce its own light.
The Source of Moonlight
Moonlight is primarily sunlight reflected off the lunar surface. The sun’s rays travel through space, striking the moon, and a portion of that light then bounces back towards Earth, making the moon visible to us. At any given moment, one half of the moon is illuminated by the sun, experiencing its “day” side.
The moon’s surface is composed of dark gray volcanic rock and dust, much like asphalt, making it less reflective than one might assume. Its average reflectivity, known as albedo, is relatively low, typically ranging from 7% to 12% of the sunlight that hits it. Despite this low albedo, the sheer intensity of the sun’s light and the moon’s proximity to Earth allow even this small reflected amount to appear bright in our night sky.
Why the Moon Appears to Change Shape
The moon’s apparent “shine” changes shape throughout the month, creating lunar phases. These phases are a direct result of the moon’s orbit around Earth and the varying angles at which we view the sunlit portion of its surface. As the moon completes an orbit, taking about a month to cycle through all its phases, the amount of its illuminated side visible from Earth changes.
For instance, during a “new moon,” the moon is positioned between the Earth and the sun, and the side facing Earth receives no direct sunlight, making it appear dark. As the moon continues its orbit, we begin to see a sliver of the sunlit side, progressing through phases like waxing crescent, first quarter, waxing gibbous, and eventually the full moon. A full moon occurs when the entire side facing Earth is fully illuminated by the sun. The illuminated portion then appears to shrink, entering the waning gibbous, third quarter, and waning crescent phases before returning to a new moon.
Factors Affecting Its Apparent Brightness
Beyond the basic reflection and phase cycle, several factors influence how bright the moon appears from Earth. One such factor is the moon’s albedo; its surface is quite dark and reflects only a small fraction of the sunlight it receives. The texture of the lunar surface, with its craters and uneven terrain, also plays a role, causing light to scatter in many directions rather than reflecting like a smooth mirror.
Earth’s atmosphere also significantly impacts the moon’s perceived brightness and color. Atmospheric particles can scatter or absorb moonlight, making it appear dimmer or even reddish, especially when the moon is low on the horizon. During a total lunar eclipse, when Earth passes directly between the sun and moon, Earth’s shadow blocks direct sunlight from reaching the moon. The reddish hue often seen during these eclipses occurs because some sunlight is refracted and scattered by Earth’s atmosphere, allowing only red and orange wavelengths to reach the moon’s surface and then reflect back to our eyes.
The moon’s slightly elliptical orbit around Earth also causes variations in its apparent size and brightness. When the moon is at its closest point to Earth, known as perigee, it can appear slightly larger and brighter than when it is farthest away, at apogee. This orbital variation means that even a full moon can vary in its luminosity depending on its distance from Earth.