Earthlight (astronomy)

Earthlight is the diffuse reflection of sunlight reflected from Earth's surface and clouds. Earthshine (an example of planetshine), also known as the Moon's ashen glow, is the dim illumination of the otherwise unilluminated portion of the Moon by this indirect sunlight. Earthlight on the Moon during the waxing crescent is called "the old Moon in the new Moon's arms",[1] while that during the waning crescent is called "the new Moon in the old Moon's arms".[2]

During the crescent phase, the darker side of the Moon reflects indirect sunlight, reflected from Earth, while the other side reflects direct sunlight.

An 80 mm shot of a crescent Moon with a 10-second exposure, revealing earthshine. The green ghost image was caused by a UV filter on the lens.

This phenomenon is most visible from Earth at night (or astronomical twilight) a few days before or after the day of new moon,[3] 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. Earthshine is most clearly seen after dusk during the waxing crescent (in the western sky) and before dawn during the waning crescent (in the eastern sky).

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.[4]

Arthur C. Clarke uses it in this sense in his novel Earthlight. The Oxford English Dictionary recognizes the word.