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[[File:Iapetus by Saturnlight.jpg|thumb|right|220px|[[Saturn]]'s moon [[Iapetus (moon)|Iapetus]] lit by Saturnshine. This is an enhanced picture; the planetshine is too dim by contrast to be visible to the unaided eye.]]

[[File:Plane of Ecliptic.jpg|thumb|right|220px|The [[Moon]] lit by [[earthlight (astronomy)|earthshine]], captured by the lunar-prospecting [[Clementine mission|''Clementine'' spacecraft]] in 1994. ''Clementine'''s camera reveals (from right to left) the [[Moon]] lit by earthshine, the [[Sun]]'s glare rising over the Moon's dark [[wiktionary:limb|limb]], and the planets [[Saturn]], [[Mars]], and [[Mercury (planet)|Mercury]] (the three dots at lower left).]]

'''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]] can be measured.

The most observed and familiar example of planetshine is [[earthlight (astronomy)|earthshine]] on the [[Moon]], which is most visible from the night side of [[Earth]] when the [[lunar phase]] is crescent or nearly [[new moon|new]],<ref name="NASA earthshine">{{cite web |title=Earthshine |url=http://earthobservatory.nasa.gov/IOTD/view.php?id=83782 |publisher=[[NASA]] |access-date=2014-10-06}}</ref> without the atmospheric [[diffuse sky radiation|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–Huygens|''Cassini'' space probe]] used [[Saturn]]'s shine to image portions of the [[moons of Saturn|planet's moons]], even when they do not reflect direct sunlight.

==Earthshine==<!-- This section is linked from [[Lunar phase]] -->

{{main|Earthlight (astronomy)}}

[[File:Earthshine diagram.svg|thumb|left|300px|Diagram of planetshine]]

Earthshine is visible [[earthlight (astronomy)|earthlight]] reflected from the [[Moon]]'s night side. It is also known as the Moon's ''ashen glow'' or as "the new Moon with the old Moon in her arm".<ref>e.g. in the Scots ballad of [[Sir Patrick Spens]]</ref>

[[File:Leonardo-Earthshine.png|left|thumb|150px|[[Leonardo da Vinci]]'s sketch of crescent Moon with earthshine as part of his ''[[Codex Leicester]]'', written between 1506 and 1510]]

Earthshine is most readily visible from a few nights before until a few nights after a [[new moon]], during the (waxing or waning) crescent phase. When the [[lunar phase]] is new as viewed from [[Earth]], Earth would appear nearly fully sunlit from the Moon. [[Sunlight]] is reflected from Earth to the night side of the Moon. The night side appears to glow faintly, and the entire disk of the Moon is dimly illuminated.

[[File:Earth illuminates.jpg|right|thumb|Earthshine reflected from the Moon, as seen through a telescope. The bright region is directly illuminated by the Sun, while the rest of the Moon is illuminated by sunlight reflected from Earth.]]

[[Leonardo da Vinci]] explained the phenomenon in the early 16th century when he realized that both Earth and the Moon reflect sunlight at the same time. Light is reflected from Earth to the Moon and back to Earth as earthshine.

Earthshine is used to help determine the current [[albedo]] of Earth. The data are used to analyze global [[cloud cover]], a climate factor. Oceans reflect the least amount of light, roughly 10%. Land reflects 10–25% of sunlight, and clouds reflect around 50%. Thus, the part of Earth where it is [[daytime]] and from where the Moon is visible determines how bright the earthshine on the Moon appears at any given time.

[[File:Earthshine.jpg|left|300px|thumb|Earthshine reflected from the Moon during [[conjunction (astronomy)|conjunction]] with [[Venus]] (left)]]

Studies of earthshine can be used to show how the Earth's cloud cover varies over time. Preliminary results show a 6.5% dip in cloud cover between 1985 and 1997 and a corresponding increase between 1997 and 2003. This has implications for climate research, especially with regards to [[global warming]]. All clouds contribute to an increased albedo, however some clouds have a net warming effect because they trap more heat than they reflect, while others have a net cooling effect because their increased [[albedo]] reflects more radiation than they trap heat. So while the Earth's albedo is measurably increasing, the uncertainty over the amount of heat trapped means the overall effect on global temperature remains unclear.<ref>{{cite web |url=http://skyandtelescope.com/news/article_1287_1.asp |last=Shiga |first=David |title=Moon Study Tracks Changes in Earth’s Cloud Cover |work=[[Sky & Telescope]] |date=25 June 2004}}</ref>

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==Retroreflection==

{{see also|Backscatter|Opposition surge}}

Features on [[Earth]], the [[Moon]], and some other bodies have, to some extent, [[retroreflector|retroreflective]] properties. Light striking them is [[backscatter]]ed, or [[diffuse reflection|diffusely reflected]] preferentially back in the direction from which it has come rather than in other directions. If the light comes from the Sun, it is reflected preferentially back toward the Sun and in nearby directions. For example, when [[lunar phase|its phase]] is full, the Moon reflects light preferentially toward the Sun and also Earth, which is in almost the same direction. As viewed from Earth, the [[full Moon]] therefore [[opposition surge|appears brighter]] than it would if it [[scattering|scattered]] light uniformly in all directions. Similarly, near [[new moon]], sunlight that has been backscattered from Earth toward the Sun and also the Moon, which is in almost the same direction, and then backscattered again from the Moon toward Earth appears much brighter, as viewed from Earth, than it would without the retroreflective effects.

The retroreflection is produced by spheres of transparent material on the reflecting surface. When it encounters a transparent sphere, light is preferentially [[heiligenschein|reflected and refracted]] in a path, within the sphere, which exits it in the direction from which it entered. On Earth, the spheres are droplets of water in clouds. On the Moon, large numbers of solid glassy spheres are found on the surface. They are thought to have been formed from drops of molten [[ejecta]], produced by [[impact event]]s, which cooled and solidified before falling back to the surface.

== Ringshine ==

[[File:Saturn eclipse crop.jpg|thumb|Ringshine on [[Saturn]] as it eclipses the Sun, seen from behind from the [[Cassini orbiter]].]]

[[File:PIA21055 - Pandora Up Close.jpg|thumb|Very faint ringshine can be seen on [[Pandora (moon)|Pandora]]'s dark side.]]

Ringshine is when sunlight is reflected by a planet's ring system onto the planet or onto the moons of the planet. This has been observed in many of the photos from the [[Cassini orbiter]].<ref>{{cite web|title=Cassini Solstice Mission: Saturn by Ringshine|url=http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=3236|publisher=NASA|accessdate=25 June 2011}}</ref>

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==Search for terrestrial planets==

[[File:The crescent Moon and earthshine over ESO's Paranal Observatory.jpg|thumb|Crescent Moon and earthshine over [[ESO]]'s [[Paranal Observatory]].]]

Scientists at [[NASA]]'s Navigator Program, which specializes in the detection of terrestrial planets, have backed the launch of a [[Terrestrial Planet Finder]] (TPF) mission.<ref>{{cite web |url=http://planetquest.jpl.nasa.gov/TPF/tpf_index.cfm |title=Archived copy |accessdate=2008-03-03 |deadurl=yes |archiveurl=https://web.archive.org/web/20080218101411/http://planetquest.jpl.nasa.gov/TPF/tpf_index.cfm |archivedate=18 February 2008 |df=dmy-all }}</ref> TPF would detect light reflected by planets orbiting stars to investigate whether they could harbor life. It would use advanced telescope technologies to look for life-marks in the light reflected from the planets, including water, oxygen and methane.

The [[European Space Agency]] has a similar mission, named [[Darwin (ESA)|Darwin]], under consideration. This will also study the light from planets to detect the signatures of life.<ref>[http://www.esa.int/esaSC/120382_index_0_m.html]</ref>

Unlike many traditional astronomical challenges, the most serious challenge for these missions is not gathering enough photons from the faint planet, but rather detecting a faint planet that is extremely close to a very bright star. For a terrestrial planet, the contrast ratio of planet to its host stars is approximately ~10⁻⁶-10⁻⁷ in the thermal infrared or ~10⁻⁹-10⁻¹⁰ in the optical/near infrared. For this reason, Darwin and Terrestrial Planet Finder-I will work in the thermal infrared. However, searching for terrestrial planets in the optical/near infrared has the advantage that the [[diffraction limit]] corresponds to a smaller angle for a given size telescope. Therefore, [[NASA]] is also pursuing a Terrestrial Planet Finder-C mission that will search for and study terrestrial planets using the optical (and near infrared) wavelengths. While Terrestrial Planet Finder-C aims to study the light of extrasolar planets, Darwin and Terrestrial Planet Finder-I will search for thermal infrared light that is reradiated (rather than scattered) by the planet.

In preparation for these missions, astronomers have performed detailed earthshine observations, since earthshine has the spectroscopic characteristics of light reflected by the Earth. Astronomers have paid particular attention to whether earthshine measurement can detect the [[red edge]], a spectral feature that is due to plants. The detection of a similar spectral feature in light from an extrasolar planet would be particularly interesting, since it might be due to a light-harvesting organism. While the [[red edge]] is almost certainly the easiest way to directly detect life on Earth via earthshine observations, it could be extremely difficult to interpret a similar feature due to life on another planet, since the wavelength of the spectral feature is not known in advance (unlike most atomic or molecular spectral features).

==Moon in Earthshine==

[[File:New Moon.jpg|thumb|500px|left|Image of the Moon approximately in its phase New Moon. At this time it is mostly lit only by [[Earthshine]] and [[Starlight]]]]

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==See also==

* [[André-Louis Danjon]]

* [[Ashen light]]

* [[Danjon scale]]

* [[List of reflected light sources]]

==References==

{{Reflist|30em}}

* Ford, E. B., Turner, E.L. & Seager, S. (2001) ``Characterization of extrasolar terrestrial planets from diurnal photometric variability'' Nature, Volume 412, Issue 6850, pp. 885–887. [http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001Natur.412..885F&link_type=PREPRINT&db_key=AST link] and [https://dx.doi.org/10.1038/35091009 preprint]

* Seager, S., Turner, E. L., Schafer, J., & Ford, E. B. (2005) ``Vegetation's Red Edge: A Possible Spectroscopic Biosignature of Extraterrestrial Plants'' Astrobiology, Volume 5, Issue 3, pp.&nbsp;372–390. ([https://dx.doi.org/10.1089/ast.2005.5.372 link] and [http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005AsBio...5..372S&link_type=PREPRINT&db_key=AST preprint])

*{{cite journal

| title = Earthshine and the Earth's albedo: 1. Earthshine observations and measurements of the lunar phase function for accurate measurements of the Earth's Bond albedo

| display-authors=4

| author=Qiu J

| author2=Goode PR

| author3=Palle E

| author4=Yurchyshyn V

| author5=Hickey J

| author6=Rodriguez PM

| author7=Chu MC

| author8=Kolbe E

| author9=Brown CT

| author10=Koonin SE

| journal=Journal of Geophysical Research: Atmospheres

| volume = 108

| issue = D22

| pages = 4709

| date = 2001

| doi = 10.1029/2003jd003610

| bibcode=2003JGRD..108.4709Q}}

Rush - Earthshine from album Vapor Trails (Remastered 2013). Music Lee, Lifeson. Lyrics Peart

== External links ==

* [https://web.archive.org/web/20030216133708/http://science.msfc.nasa.gov/headlines/y2002/12apr_earthshine.htm Science@NASA: Earthshine]

* [http://antwrp.gsfc.nasa.gov/apod/ap020419.html NASA Astronomy Picture of the Day], 19 April 2002

* [https://web.archive.org/web/20030214221353/http://www.space.com/scienceastronomy/planetearth/earth_shine_010417.html '''Earthshine' Linked to Solar Cycle, Climate Change''], Space.com

* [http://www.agu.org/sci_soc/prrl/prrl0113.html ''Scientists Watch Dark Side of the Moon to Monitor Earth's Climate''], American Geophysical Union

* [http://www.skytrip.de/mond5.htm Earthshine picture gallery on SkyTrip.de]

{{The Moon}}

[[Category:Lunar observation]]

[[Category:Light sources]]

[[Category:Observational astronomy]]

[[Category:Phenomena]]