The heliac arc
is an ice-based atmospheric effect
. It makes a giant Jesus fish
in the sky of varying distortion.
This extremely rare, faint white arc forms a giant fish shape in the sky with its caudal fin resting on the horizon, its intersection at the sun, and its nose high in the sky. It changes shape with the sun's elevation. When the sun is near the horizon, the tail rests about 22° apart and the loop is so large and high it passes the zenith. As the sun rises, the loop slowly shrinks and the tail slowly widens. With the sun at 30° the nose is near the upper Parry arc. With the sun at 65°, the loop is nearly impossible to see against the sun and the tail wraps around the sky and meets on the opposite horizon.
The heliac arc forms when singly-oriented needle-shaped ice crystals float with their long axis horizontal, and any of their edge faces at a 60° angle from the horizon. Additionally, these crystals must be so oriented in an extremely thin cirrostratus layer, or the effect is lost in the general diffusion of a cloud. It is these precise angle and distribution requirements that makes this arc so rare. The ray path is actually simple reflection: Sunlight bounces off internal and external faces. Any internal refraction cancels itself out, denying this arc color.
"Heliac" (synomymous with "Heliacal" in that ugly twist of English that lets "graphic" and "graphical" mean the same thing) means "passing away from or into the light of the sun." This arc is the only loop arc whose intersection meets within the sun. The other loop arcs (diffuse arc, Tricker arc, and Wegener anthelic arc) all rest their intersections at the anthelion.
(Where there is an astonishing south pole display that includes a heliac arc)