The geometric albedo of an astronomical body is the ratio of its total brightness at zero phase angle to that of an idealised fully reflecting, diffusively scattering (Lambertian) disk with the same cross-section. The phase angle of a point on a periodic wave is the distance between the point and a specified reference point, expressed using an angular measure. ... If a surface exhibits Lambertian reflectance, light falling on it is scattered such that the amount of light an observer sees, the surface luminance, is the same regardless of their angle of view. ...

Zero phase angle corresponds to looking along the direction of illumination. For Earth-bound observers this occurs when the body in question is at opposition. Opposition is a term used in positional astronomy and astrology to indicate when one celestial body is on the opposite side of the sky when viewed from a particular place (usually the Earth). ...

The visual geometric albedo refers to this quantity when calculated taking into account only electromagnetic radiation in the visual range.

For very bright solid airless objects such as Saturn's moons Enceladus and Tethys whose Bond albedo is close to one, a strong opposition effect gives them a geometric albedo above unity (1.4 in the case of Enceladus) ^[1]. This is possible because regolith surfaces have a strong tendency to reflect light straight back to their source even at a low angle of incidence, whereas a Lambertian surface the radiation radiation very broadly. Note: This article contains special characters. ... In Greek mythology Enceladus was one of the Gigantes. ... Tethys can refer to: Tethys the titaness of Greek mythology Tethys the natural satellite of Saturn The Tethys Ocean existed between the continents of Gondwana and Laurasia before the opening of the Atlantic Ocean on Earth. ... The opposition effect is the brightening of a rough surface when illuminated from directly behind the observer. ... Regolith (Greek: blanket rock) is a layer of loose, heterogeneous material covering solid rock. ... Aviation Fig. ...

For the hypothetical case of a plane surface, the geometric albedo is the albedo of the surface when the illumination is provided by a beam of radiation that comes in perpendicular to the surface. Albedo is the measure of reflectivity of a surface or body. ... Perpendicular is a geometric term that may be used as a noun or adjective. ...

See also

• Anisotropy
• Lambertian surface

Look up anisotropy in Wiktionary, the free dictionary. ...

References

• NASA JPL glossary]
• K.P. Seidelmann, Ed. (1992) Explanatory Supplement to the Astronomical Almanac, University Science Books, Mill Valley, California.

1. ^ See the discussion here for explanation of this unusual value above one.