2003-05-01 - 11:01 a.m.
Bishops Ring was first described after the eruption of the Krakatoa volcano on August 27, 1883. This gigantic explosion, the sound of which could even be heard at a distance of 4,800 km in Africa, threw a vast quantity of dust into the atmosphere which caused unbelievable colourful sunrises and sunsets for several years The first observation of this ring was published in the Japan Gazette on August 30, 1883. There the appearance of the ring is described as a �faint halo� around the sun. The first exact description was made by Sereno Bishop who observed the phenomenon on September 5, 1883, in Honolulu:
�Let me draw your special attention to the very strange corona or halo that extends about 20 to 30 degrees away from the sun..It could be seen here every day, and the whole day long. A whitish veil with a shade of pink and violet or purple shadow in front of the blue background. I don�t know any other report on such a corona. It is a hardly remarkable object.�
The ring then became named after Bishop.
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 colours with the red on the outside one can conclude that the phenomenon is caused by diffraction because halos always have their red part on their inside. In average, the radius of the ring is about 28�. This 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.
Very homogeneous and small aerosols, such as those responsible for the appearing of Bishop�s Ring, are mainly caused by volcanic eruptions. So Bishop�s Ring was observed for a longer period of time in Japan after the eruption of Mt. Pinatubo. To see a bright and clear Bishop�s Ring again, we will have to wait for the next great volcanic eruption.