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°.
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