The moon, a familiar sight in our sky, does not produce its own light. We see it because it reflects sunlight.
The Moon’s Illumination
The “moonlight” we observe is sunlight reflected from its surface. Like Earth, half of the moon is always illuminated by the Sun. Sunlight strikes the moon’s rocky terrain, reflecting a portion back into space.
The moon’s surface is relatively dark, reflecting only about 12% of the sunlight it receives. Despite this low reflectivity, the moon appears bright to us because of its relative proximity to Earth, averaging about 239,000 miles (385,000 kilometers) away.
How Moonlight Reaches Our Eyes
The light we see from the moon is sunlight that has traveled approximately 93 million miles to reach the lunar surface, reflected, and then traveled another 239,000 miles back to Earth. This reflected light travels as waves, and when these waves enter our eyes, our brains interpret them as the moon’s visible glow. It takes approximately 1.26 seconds for moonlight to reach Earth’s surface.
The moon’s surface is rough, leading to what is known as diffuse reflection. This means that sunlight hitting the moon scatters in many directions, rather than reflecting like a mirror. This scattered light allows us to perceive the moon clearly against the night sky. The moon’s apparent brightness also varies with how much of its sunlit portion faces Earth.
Variations in Lunar Appearance
The moon’s appearance changes throughout the month, presenting different phases such as crescent, half, and full. These phases occur because as the moon orbits Earth, the amount of its sunlit surface visible from our perspective continuously changes. The cycle, which repeats approximately every 29.5 days, progresses from a new moon, where the illuminated side faces away from Earth, to a full moon, where the entire sunlit side is visible.
The moon can often be seen during the daytime due to its brightness and its orbital path. It is above the horizon for about 12 hours each day, and its reflected sunlight is bright enough to overcome the scattered light from the Sun in Earth’s atmosphere. The moon is most easily observed during the day around its quarter phases, when it is high enough in the sky and sufficiently illuminated.
Sometimes, the moon appears in shades of red, orange, or yellow, particularly when it is low on the horizon or during a lunar eclipse. This color variation is caused by Earth’s atmosphere scattering shorter wavelengths of light, like blue and violet, through a process called Rayleigh scattering. Longer wavelengths, such as red and orange, pass through the atmosphere more directly, making the moon appear warmer in color. During a total lunar eclipse, Earth’s atmosphere filters sunlight, allowing only red and orange hues to reach the moon, which then reflect back to us, creating a “blood moon” effect.