A Bishop's Ring is a diffuse brown or bluish

halo HALO, halo, halos or haloes may refer to: Most common meanings * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head * ''Halo'' (franchise), a sci-fi video game series (2001–2021) Arts and en ...

observed around the sun. It is typically observed after large

volcanic eruptions A volcanic eruption occurs when material is expelled from a volcanic vent or fissure. Several types of volcanic eruptions have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior h ...

. The first recorded observation of a Bishop's Ring was by Rev.

Sereno Edwards Bishop Sereno Edwards Bishop (February 7, 1827 – March 23, 1909) was a scientist, Presbyterian minister and publisher. He was an avid proponent of the United States annexation of the Hawaiian Islands, and aligned himself with the political faction who ...

Honolulu Honolulu ( ; ) is the List of capitals in the United States, capital and most populous city of the U.S. state of Hawaii, located in the Pacific Ocean. It is the county seat of the Consolidated city-county, consolidated City and County of Honol ...

, after the

Krakatoa Krakatoa (), also transcribed (), is a caldera in the Sunda Strait between the islands of Java and Sumatra in the Indonesian province of Lampung. The caldera is part of a volcanic island group (Krakatoa archipelago) comprising four islands. Tw ...

eruption of August 27, 1883. This gigantic explosion threw a vast quantity of dust and volatile gases into the atmosphere. Sulfate aerosols remained in the stratosphere, causing colorful sunrises and sunsets for several years. The first observation of this ring was published in 1883, being described as a “faint halo” around the sun. Bishop observed the phenomenon on September 5, 1883; the phenomenon was subsequently named after him, and was the subject of an 1886 professorial dissertation (''Habilitationsschrift'') by

Albert Riggenbach Albert Riggenbach (22 August 1854 – 28 February 1921), also known as Albert Riggenbach-Burckhardt, was a Swiss meteorologist and co-author, with Hugo Hildebrandsson and Léon Teisserenc de Bort, of one of the first cloud atlases, the ''Internati ...

. Most observations agree that the inner rim of the ring is whitish or bluish white and that its outside is reddish, brownish or purple. The area enclosed by the ring is significantly brighter than its surroundings. From the sequence of colors with the red on the outside one can conclude that the phenomenon is caused by

diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...

because halos always have their red part on their inside. On average, the radius of the ring is about 28°, but it can vary between 10° and 30°, depending on the dust size.Asano, S. (1993) "Estimation of the size distribution of Pinatubo volcanic dust from Bishop's Ring simulations." ''Geophysical Research Letters'' 20(6): 447–450. The maximum of 30° is a rather big radius which can only be caused by very small dust particles (0.002 mm) which all have to be of about the same size. Sulfur compound aerosols derived from volcanic eruptions have been found to be the source for the Bishop's Ring effect.Kenneth Sassen, Thomas Peter, Beiping P. Luo, and Paul J. Crutzen (1994) "Volcanic Bishop’s ring: evidence for a sulfuric acid tetrahydrate particle aureole," ''Applied Optics'' 33: 4602–4606. A Bishop's Ring was observed for a long period of time in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

after the eruption of Mt. Pinatubo.

References

{{reflist, 2

External links

Photograph
of a Bishop's Ring, with commentary.
Meteorology glossary entry
for Bishop's Ring. Atmospheric optical phenomena