An anthelion (plural anthelia, from late

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

ανθηλιος, "opposite the sun") is a rare

optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...

of the

halo HALO, halo, halos or haloes may refer to: Most common meanings * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head * ''Halo'' (franchise), a sci-fi video game series (2001–2021) Arts and en ...

family. It appears on the

parhelic circle A parhelic circle is a type of halo, an optical phenomenon appearing as a horizontal white line on the same altitude as the Sun, or occasionally the Moon. If complete, it stretches all around the sky, but more commonly it only appears in secti ...

opposite to the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

as a faint white spot, not unlike a sundog, and may be crossed by an X-shaped pair of diffuse arcs. How anthelia are formed is disputed. Walter Tape, among others, has argued they are not separate haloes, but simply where various haloes caused by horizontally oriented column-shaped

ice crystal Ice crystals are solid water (known as ice) in symmetrical shapes including hexagonal columns, hexagonal plates, and dendritic crystals. Ice crystals are responsible for various atmospheric optical displays and cloud formations. Formation ...

s coincide on the parhelic circle to create a bright spot. If this theory is correct, anthelia should only appear together with these other haloes. However, anthelia occur unaccompanied by other plate crystal haloes, thus scientists have produced alternative explanations. The Dutch professor S.W. Visser proposed they form by two exterior light reflections in quadrangular prisms, while Robert Greenler has suggested two interior reflections in column-shaped crystals produces the phenomenon. While the anthelion area is usually sparse on haloes, in a complex display it features various rare optic phenomena: Flanking the anthelion on the parhelic circle are two 120° parhelia (and two Liljequist parhelia) caused by plate crystals. The Tricker and

diffuse arc Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

s are produced in singly oriented column crystals and form an

ankh The ankh or key of life is an ancient Egyptian hieroglyphic symbol used to represent the word for "life" and, by extension, as a symbol of life itself. The ankh has a T-shape topped by a droplet-shaped loop. It was used in writing as a tri ...

-like shape passing through the anthelion.

Wegener arc Wegener may refer to: Astronomy * 29227 Wegener, a main-belt asteroid * Wegener (lunar crater) * Wegener (Martian crater) Places * Wegener Range, an Antarctic mountain range * Mount Wegener, an Antarctic mountain in the Read Mountains in the ...

s occasionally cross the sky to converge in the anthelion. (including fish eye photo a.o.)

References

External links

Earth Science Picture of the Day, April 26, 2006
- Photo of an anthelion and anthelic arcs display in Germany February 2006. Atmospheric optical phenomena {{climate-stub

See also

References

External links