What does earthshine mean?
Earthshine is the faint, silvery illumination visible on the dark portion of the Moon when only a thin crescent is directly lit by the Sun. It is caused by sunlight striking the Earth, being reflected off its oceans, clouds, and landmasses, and then bouncing onward to the Moon's night side, which reflects a little of it back toward our eyes. The result is the familiar and beautiful image of what poets once called 'the old Moon in the new Moon's arms.' The effect is strongest near the new Moon, when the crescent is slim and the Earth, viewed from the Moon, appears nearly full and at its brightest. Beyond its visual charm, earthshine has scientific value: astronomers monitor its brightness to estimate how much solar energy Earth reflects back into space, a key figure known as albedo that matters for climate research. The word itself is transparent and descriptive, combining 'earth' and 'shine,' and it belongs chiefly to astronomical writing and sky-watching conversation.
nounThe faint light reflected from the Earth that illuminates the dark portion of the Moon, making it faintly visible especially when the Moon is a thin crescent.
- Sunlight reflected from the Earth's surface, clouds, and oceans onto the night side of the Moon, making the otherwise dark portion of the lunar disc faintly visible.
"With the crescent Moon low in the western sky, the rest of its disc glowed softly with earthshine."
"Through binoculars, the whole lunar disc was visible, the dark portion lit by gentle earthshine."
"Astronomers measure earthshine to study how clouds and ice reflect the Sun's energy back into space."
Rarely pluralized; the noun is normally treated as uncountable since it names a diffuse glow rather than countable objects.
"Photographs comparing successive crescents captured subtle differences in their earthshines."
That ghostly glow on the dark part of a crescent Moon is literally Earth bouncing sunlight back at you — Leonardo da Vinci figured out why it happens over 500 years ago.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of earthshine
Earthshine is a straightforward English compound of 'earth' and 'shine,' formed from native Old English roots rather than borrowed from Latin or Greek; 'earth' descends from Old English 'eorthe,' of Germanic origin. The phenomenon had long been observed and described before the modern term settled into use, with earlier writers speaking of the Moon being lit by light from the Earth. Cognate formations using the same element include 'earthquake' and 'earthbound.'
How earthshine is actually used
Earthshine is an astronomical term used mainly in scientific and observational contexts; it is uncountable and typically appears without an article in general statements ('the Moon was lit by earthshine'), though 'the earthshine' occurs when referring to a particular observation.
Easily confused with earthshine
Moonshine most commonly refers to illegally distilled whisky or to nonsense talk, whereas earthshine is sunlight reflected from Earth onto the Moon's dark side.
Earthlight is a near-synonym for earthshine as seen on the Moon, but it can also mean the Earth's own reflected light as seen from space, so the terms are not always interchangeable.