Passing through the luminary and parallel to the horizon, there is a white luminous circle, the parhelic circle (P), on which a number of images of the luminary appear.
The most brilliant are situated at the intersections of the inner halo and the parhelic circle; these are known as parhelia (denoted by the letter p in the figures) (from the Gr.
The other images on the parhelic circle are the paranthelia (q) and the anthelion (a) (from the Greek av-ri, opposite, and iXcos, the sun).
Thomas Young explained the parhelic circle (P) as due to reflection from the vertical faces of the long prisms and the bases of the short ones.
When the sun is near the horizon the rays fall upon the principal section of the prisms; the minimum deviation for such rays is 22 °, and consequently the parhelia are not only on the inner halo, but also on the parhelic circle.
As the sun rises, the rays enter the prisms more and more obliquely, and the angle of minimum deviation increases; but since the emergent ray makes the same angle with the refracting edge as the incident ray, it follows that the parhelia will remain on the parhelic circle, while receding from the inner halo.
Double internal reflection by a triangular prism would form a single coloured image on the parhelic circle at about 98° from the sun.
The emerging rays are parallel to their original direction and form a colourless image on the parhelic circle opposite the sun.