Earthlight (astronomy)

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When the Moon is at crescent phase, the darker side is illuminated by indirect sunlight reflected from Earth, while the other side by direct sunlight.

Earthlight is the dim illumination of the otherwise dark side of the Moon by sunlight reflected from Earth's surface and clouds. It is also known as earthshine (see also planetshine) or the Moon's ashen glow. Earthlight on a waxing crescent is called "the old Moon in the new Moon's arms", while that on a waning crescent is called "the new Moon in the old Moon's arms".[citation needed]

This phenomenon is most visible from Earth at night (or astronomical twilight) a few days before or after the day of new moon,[1] when the lunar phase is a thin crescent. On these nights, the entire lunar disk is both directly and indirectly sunlit, and is thus unevenly bright enough to see. Earthlight on a waxing crescent Moon is most clearly seen in the western sky after dusk, while that on a waning crescent Moon in the eastern sky before dawn.

The term earthlight would also be suitable for an observer on the Moon seeing Earth during the lunar night, or for an astronaut inside a spacecraft looking out the window. Arthur C. Clarke uses it in this sense in his novel Earthlight. The Oxford English Dictionary recognizes the word.

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