Earthshine

The Sun illuminates Earth, and Earth reflects some of that sunlight toward the Moon. Around new Moon, the Moon’s near side is turned mostly away from direct sunlight as seen from Earth, while Earth presents a largely sunlit hemisphere toward the Moon. A small fraction of Earth-reflected light illuminates the lunar near side and is reflected onward to an observer on Earth. The result is a dim glow on the portion of a crescent Moon that would otherwise appear dark. The same light path is sometimes described from the Moon as earthlight.

Earthshine is easiest to notice when the Moon is a thin crescent, near the time of new Moon, because the unlit-looking part of the lunar disk is then contrasted with the bright crescent. It can be seen with the unaided eye under suitable sky conditions, though the faint glow is easier to appreciate in photographs. Its visibility depends on lunar geometry, the Moon’s brightness, atmospheric conditions and the observer’s surroundings.

The amount of earthshine depends in part on how much sunlight Earth reflects, a property called albedo. Clouds, snow and ice are reflective, while oceans, vegetation and other surfaces have different reflectances. Earth’s cloud and surface patterns change over time, and the visible distribution of land and ocean also changes with Earth’s rotation. Consequently, the light returned from Earth to the Moon is not constant. Observational methods compare light from the earthshine-lit lunar region with light from the bright crescent and account for atmospheric effects and lunar reflectance.

Ground-based observatories have used measurements of earthshine alongside observations of the sunlit lunar crescent to estimate Earth’s broad-scale reflectance. Researchers at Big Bear Solar Observatory and collaborators have developed long-running measurements for studying albedo variations. Such measurements complement satellite observations: the methods sample reflected light in different viewing geometries and have different limitations. Interpreting earthshine records therefore requires calibration and averaging, rather than treating one night’s brightness as a direct measure of global climate.