​ANTHELION (late Gr. ἀνθήλιος, opposite the sun), the luminous ring or halo sometimes seen in Alpine or polar regions surrounding the shadow of the head of an observer cast upon a bank of cloud or mist. The halo diminishes in brightness from the centre outwards, and is probably due to the diffraction of light. Under favourable conditions four concentric rings may be seen round the shadow of the observer’s head, the outermost, which seldom appears, having an angular radius of 40°.