A halo occurs when the light from the sun or the moon is refracted and reflected by ice crystals in the atmosphere, usually in a thin layer of high cirrostratus clouds. Under certain circumstances a second, or outer, halo appears, which is fainter than the inner one. At times white or colored luminous arcs are also seen lying somewhat parallel to the horizon and passing through the source of light, called mock suns, parhelia, or sun dogs for the sun, and paraselenae for the moon. A single mock sun, the anthelion, directly opposite the sun, may be added. In general a white halo results from the reflection of light by ice crystals, while one which appears as colored rings results from the refraction of light by ice crystals.
Halos are more brilliant and complex near the poles than in other parts of the world.
The theory attributing their formation to the presence of ice crystals was first suggested by the 17th cent. French philosopher Descartes. Similar to a halo and sometimes confused with it is the sun's corona . In X-ray electron diffraction, the term halos refers to the broad rings that appear on a photographic film as a result of the diffraction of a monoenergetic beam of X rays or electrons from a crystalline powder located at the center of the camera.