Definition: Bishop's ring: A whitish ring, centred on the Sun or Moon, with a slightly bluish tinge on the inside and reddish brown on the outside.
A Bishop's ring is caused by diffraction of light passing through a cloud of very fine volcanic dust in the high atmosphere. The radius of the inner edge of the ring is typically between 22° and 28°, and the ring width is another 10° or more. Exact dimensions are determined by the size of the dust particles. The colours of a Bishop's ring are not very distinct. They are particularly faint in rings observed around the Moon, which usually show only a pale red fringe. The Bishop's ring is named after the Reverend S. Bishop, who first described the phenomenon after the eruption of Krakatoa volcano in 1883.