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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.

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.

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 scatters the radiation very broadly.

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.


  • 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.
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