Light passing through the hexagonal ice prisms is deflected twice, which produces deviation angles ranging from 22° to 50°. Lesser deviation results in a brighter halo along the inner edge of the circle, while greater deviation contribute to the weaker outer part of the halo. As no light is refracted at smaller angles than 22° the sky is darker inside the halo.
(Source: kenobi-wan-obi, via thequestioningmark)
