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.
[2] If the halo occurs due to light from the Moon rather than the Sun, it is known as a paraselenic circle.
[3] Even fractions of parhelic circles are less common than sun dogs and 22° halos.
While parhelic circles are generally white in colour because they are produced by reflection, they can however show a bluish or greenish tone near the 120° parhelia and be reddish or deep violet along the fringes.
The reflection can be either external (e.g. without the light passing through the crystal) which contributes to the parhelic circle near the Sun, or internal (one or more reflections inside the crystal) which creates much of the circle away from the Sun.
Because an increasing number of reflections makes refraction asymmetric some colour separation occurs away from the Sun.
[citation needed] The intensity distribution of the parhelic circle is largely dominated by 1-3-2 and 1-3-8-2 rays (cf.
Artificial parhelic circles can be realized by experimental means using, for instance, spinning crystals.
Photo: John Bortniak, NOAA , January 1979.
Photo: Cindy McFee, NOAA , December 1980. [ 1 ]
