The moon's halo or lunar halo is an optical illusion that causes a large bright ring to surround the moon. This striking and often beautiful halo around the moon is caused by the refraction of moonlight from ice crystals in the upper atmosphere. A lunar halo is created when light is refracted, reflected, and dispersed through ice crystals suspended in cirrus or cirrostratus clouds located at an altitude of 6,000 meters and higher, up to 12,000 meters. Source: https://www.space.com/moon-halo