Antisun (EE17)

On the horizontal circle, in rare cases, a brightening can be seen opposite the sun. This whitish brightening is called the anthelion. It lies at the height of the sun, 180° from the sun in azimuth. The best chances are during displays with a bright horizontal circle and bright column halos such as the upper tangent arc or the Parry arc. An anthelion can also occur in moonlight. Bright white patches of cloud opposite the sun can be mistaken for it. It can only be identified with certainty together with the horizontal circle or the anthelic arcs.

According to current understanding, the anthelion is not an independent type of halo. It forms where the horizontal circle and several anthelic arcs intersect. This explanation comes from W. Tape. The halos involved are the horizontal circle, the diffuse anthelic arcs and the anthelic arcs of Tricker, Wegener and Hastings. Except for the horizontal circle, these halos require almost ideally horizontally oriented column crystals. Therefore, according to this theory, the anthelion should appear together with other column halos if the ice crystals are dispersed across the sky. However, the anthelion has also been observed without other column halos. Earlier explanations regarded the anthelion as an independent type of halo. According to the historical theory of Visser, it forms in quad prisms with two vertical side faces. In this process, the sunlight is reflected twice externally on the prism surfaces and then reflected back at an angle of 180°. Greenler made a complementary contribution. According to him, light that is reflected twice inside column prisms also contributes to the anthelion.