Earthlight is the

diffuse reflection Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ''ideal'' dif ...

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

reflected from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

's surface and

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

s. Earthshine (an example of

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

), also known as the Moon's ashen glow, is the dim illumination of the otherwise unilluminated portion of the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

by this indirect sunlight. Earthlight on the Moon during the waxing

crescent A crescent shape (, ) is a symbol or emblem used to represent the lunar phase (as it appears in the northern hemisphere) in the first quarter (the "sickle moon"), or by extension a symbol representing the Moon itself. In Hindu iconography, Hind ...

is called "the old Moon in the new Moon's arms", while that during the waning crescent is called "the new Moon in the old Moon's arms".

Visibility

At Earth

Earthlight has a calculated maximum

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

of −17.7 as viewed from the Moon. When the Earth is at maximum phase, the total radiance at the lunar surface is approximately from Earthlight. This is only 0.01% of the radiance from direct Sunlight. Earthshine has a calculated maximum apparent magnitude of −3.69 as viewed from Earth. This phenomenon is most visible from Earth at

night Night, or nighttime, is the period of darkness when the Sun is below the horizon. Sunlight illuminates one side of the Earth, leaving the other in darkness. The opposite of nighttime is daytime. Earth's rotation causes the appearance of ...

(or astronomical

twilight Twilight is daylight illumination produced by diffuse sky radiation when the Sun is below the horizon as sunlight from the upper atmosphere is scattered in a way that illuminates both the Earth's lower atmosphere and also the Earth's surf ...

) a few days before or after the day of new moon, when the

lunar phase A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. Because the Moon is tidally locked with the Earth, the same hemisphere is always facing the Earth. In common usage, the four maj ...

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 Dawn is the time that marks the beginning of twilight before sunrise. It is recognized by the diffuse sky radiation, appearance of indirect sunlight being Rayleigh scattering, scattered in Earth's atmosphere, when the centre of the Sun's disc ha ...

during the waning crescent (in the eastern sky). High contrast photography is also able to reveal the night side of the moon illuminated by Earthlight during a solar eclipse. Radio frequency transmissions are also reflected by the moon; for example, see Earth–Moon–Earth communication.

At the Moon

The term ''earthlight'' would also be suitable for an observer on the Moon seeing Earth during the lunar night, or for an

astronaut An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a List of human spaceflight programs, human spaceflight program to serve as a commander or crew member of a spa ...

inside a

spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...

looking out the window. Arthur C. Clarke uses it in this sense in his 1955 novel '' Earthlight''. No person has been on the Moon during its night and experienced earthshine. Earthshine is in the lunar night about 43 times brighter, and sometimes even 55 times brighter than a night on Earth illuminated by the light of the full moon. Only on the far side and during lunar eclipses on the near side does the night become dark (and darker than a moonless night on Earth). Operations on the Moon and remote studying of the Moon and Earth is enabled by Earthlight, and has been used for research, for example to study Earth's environment by studying the amount of Earthlight it produces. Earthlight allows also some places on the surface of the lunar near side to be illuminated that are never illuminated by the Sun.

History

The phenomenon was sketched and remarked upon in the 16th century by

Leonardo da Vinci Leonardo di ser Piero da Vinci (15 April 1452 - 2 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially rested o ...

, who thought that the illumination came from reflections from the Earth's oceans (we now know that clouds account for much more reflected intensity than the oceans). This is why this phenomenon is sometimes called the Da Vinci Glow. It is referenced in " The Ballad of Sir Patrick Spens" (

Child Ballad The Child Ballads are List of the Child Ballads, 305 traditional ballads from England and Scotland, and their American variants, anthologized by Francis James Child during the second half of the 19th century. Their lyrics and Child's studies ...

No. 58), in the phrase "‘A saw the new muin late yestreen/ Wi the auld muin in her airm." Astronaut Dr Sian Proctor was moved by seeing and experiencing earthlight from orbit as mission pilot of Inspiration4 space mission and wrote the poem, "Earthlight". In 2024, Proctor authored EarthLight: The Power of EarthLight and the Human Perspective on the concept and nature of earthlight.

References

External links