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opposition surge
The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. The term is most widely used in astronomy, where generally it refers to the sudden noticeable increase in the brightness of a celestial body such as a planet, moon, or comet as its phase angle of observation approaches zero. It is so named because the reflected light from the Moon and Mars appear significantly brighter than predicted by simple Lambertian reflectance when at astronomical opposition. Two physical mechanisms have been proposed for this observational phenomenon: shadow hiding and coherent backscatter.
==Overview==
The phase angle is defined as the angle between the observer, the observed object and the source of light. In the case of the solar system, the light source is the Sun, and the observer is situated on Earth. At zero phase angle, the sun is directly behind the observer and the object is directly in front of him, fully illuminated.
As the phase angle of an object lit by the sun decreases, the object's brightness rapidly increases. This is mainly due to the increased area lit, but is also partly due to the intrinsic brightness of the part that is sunlit. This is affected by such factors as the angle at which light reflected from the object is observed. For this reason, a full moon is more than twice as bright as the moon at first or third quarter, even though the visible area illuminated appears to be exactly twice as large.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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