A rare halo phenomenon appearing as a bright spot in the sky directly opposite the sun, caused by light reflecting through ice crystals in the atmosphere.
From Greek anti- (opposite) + helios (sun). Named in the 18th century by atmospheric opticians studying halo phenomena, it's the optical 'opposite' or counterpart to the sun.
Anthelions are so rare that many atmospheric scientists go their entire careers without seeing one — when they do occur, it's usually in polar regions or high mountains where ice crystals are perfectly arranged, making them meteorological unicorns.
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