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What Is Earthshine?

Earthshine is a soft, faint glow on the shadowed part of the moon caused by the reflection of sunlight from the Earth.

Specifically, Earthshine happens when the light from the sun is reflected from the Earth's surface, to the moon, and then back to our eyes. Because of this double reflection of light, Earthshine is many, many times dimmer than the direct light of the sun on the moon. Earthshine is even more faint because the moon's "albedo" (a specific kind of reflectivity) is less than Earth's. Even though this dim light is only a reflection it can still illuminate some features of the moon.

Earthshine can be best seen during the crescent phases (the 1-5 day period before or after a New Moon). During this time the sun is mostly behind the moon from our perspective and bathing the Earth in a lot of direct light that is reflected onto the shadowed parts of the moon.

Moons orbiting other planets can also experience this phenomenon, generally called "planetshine".