The
anthelion is an
atmospheric effect that appears as a white spot on the
parhelic circle opposite the
sun. It is distinguished from the
antisolar point, which is defined as a point directly opposite the
sun on the
celestial sphere. When standing on the
Earth's surface, if you can see the sun, the antisolar point is through the ground. The anthelion is visible with the sun, at the same elevation offset around the
zenith by 180°.
The anthelion anchors the three antehlic arcs: the diffuse arcs, the Tricker arc, and the Wegener arc.
Get this: no one knows quite for sure why it appears. There are three competing theories.
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Walter Tape, a math professor at the University of Alaska Fairbanks, believes that it's not a separate effect at all, just a brightening where weak anthelic arcs converge and intensify each other. If this was the case, and the anthelion was formed by the same needle-shaped crystals in a Parry orientation, one would always expect to see a bright anthelion coincident with the other athelic arcs. But bright anthelia have been observed alone, which casts doubt on Tape's theory.
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The Dutch professor S.W. Visser presented another theory, that they are formed by rare quadruple prisms with two of their edge faces oriented vertically to reflect light. Visser may be violating Occam's buzzkill in establishing this explanation.
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Still another theory is offered by Robert Greenler, a physicist at the University of Wisconsin-Milwaukee, who believes they are caused by two simple internal reflections in needle shaped crystals.
What is defined below is actually a glory, which appears at the antisolar point. (Hey cool. How often do you get to correct him?)
Sources
- http://amsglossary.allenpress.com/glossary/search?p=1&query=anthelion
- http://www.meteoros.de/arten/ee17e.htm