What is "shiranui"

Shiranui (不知火; lit. "unknown fire") is a Japanese term given to the optical or supernatural phenomenon similar to will-o'-the-wisp. See Shiranui (Optical phenomenon) It is also a name associated with:

is a kaika told about in Kyushu. They are said to appear on days of the noon moon such the kaijitsu (29th or 30th day) of the seventh month of the lunisolar calendary when the wind is weak, in the Yatsushiro Sea and the Ariake Sea. Furthermore, they can be seen in modern times, but they have been determined to be an atmospheric optical phenomena.