anthelion

   The term anthelion comes from the Greek words anti (against, opposite to) and helios (Sun). It translates loosely as counter-Sun. The term is used in meteorology and astronomy to denote arare" physical illusion consisting of a diffuse, achromatic patch of light manifesting itself at the anthelion point, i.e. the spot on the horizon opposite the Sun, and at the same altitude as the Sun. It has been suggested that anthelia arise from the intersecting or overlapping of "anthelic arcs and/or related "halos, such as the parhelic circle, diffuse " anthelic arcs, and Tricker's and Wegener's anthelic arcs. But since they also occur in the absence of these halos, it has been suggested that the anthelion may deserve to be classified as a separate type of halo. As to the mediation of anthelia, various hypotheses exist. These revolve around the notion of ice crystals or ice prisms with a particular orientation present in cirrus clouds and the reflection of sunlight from either the inner or the outer surfaces of these crystals. In the past the term anthelion was used in a broader sense to denote a variety of physical illusions occurring at the side of the sky facing the Sun. Thus the French astronomer and author Nicolas Camille Flammarion (1842-1925) includes under the heading anthelia such phenomena as the "Brocken spectre and the "Ulloa circle.

   References

   Flammarion, C. (1873). The atmosphere.Trans-lated by Pitman, C.B. Edited by Glaisher, J. London: Sampson Low, Marston, Low, & Searle.

   Lynch, D.K., Schwartz, Pt. (1979). Origin of the anthelion. Journal of the Optical Society of America, 69, 383-386.

   Lynch, D.K., Livingston, W. (1995). Color and light in nature. Cambridge: Cambridge University Press.