Untersonnenbogen (EE62)

The subsun arc, known in the scientific literature as the subhelic arc, resembles the heliac arc in appearance. It is colorless and crosses the subsun. Its extensions reach far across the sky. At a solar altitude of about 27°, it passes exactly through the 120° parhelia. At 0° solar altitude, the subsun arc coincides exactly with the heliac arc. From a solar altitude of about 55°, the subsun arc disappears below the horizon. Between the anthelion and the zenith, the subsun arc touches the apex of the loop of Tricker’s anthelic arc.

The sub-solar arc is formed by column crystals with a horizontal longitudinal axis or by column crystals in Parry orientation. The sunlight enters through one of the end faces of the crystal and exits after two internal reflections through the other. This light path is comparable to the light path in a kaleidoscope.