Planetshine

Planetshine is the dim illumination, by sunlight reflected from a planet, of all or part of the otherwise dark side of any moon orbiting the body. Planetlight is the diffuse reflection of sunlight from a planet, whose albedo can be measured.

The most observed and familiar example of planetshine is earthshine on the Moon, which is most visible from the night side of Earth when the lunar phase is crescent or nearly new,^[1] without the atmospheric brightness of the daytime sky. Typically, this results in the dark side of the Moon being bathed in a faint light.

Planetshine has also been observed elsewhere in the Solar System. In particular, the Cassini space probe used Saturn's shine to image portions of the planet's moons, even when they do not reflect direct sunlight. The New Horizons space probe similarly used Charon's shine to discover albedo variations on Pluto's dark side.^[2]

Although using a geocentric model in 510 AD, Indian mathematician and astronomer Aryabhata was the first to correctly explain how planets and moons have no light of their own, but rather shine due to the reflection of sunlight in his Aryabhatiya.^[3]