UTO, Kumamoto Prefecture--A high school science club here enlisted the help of local fishermen to reproduce a rare type of atmospheric optical phenomenon that can be glimpsed in the Yatsushiro Sea and nowhere else in Japan.

Members of the Earth Science Group at the Kumamoto prefectural Uto High School’s Science Club have been studying Shiranui, as the mirage is called, as it had not been sighted for several decades.

In the early hours of Sept. 3, the team members were convinced they had observed and photographed Shiranui.

The Earth Science Group members were able to reproduce Shiranui last year in an indoor experiment, which allowed them to determine what conditions are favorable for the phenomenon’s occurrence.

Shiranui, which roughly translates as “nobody-knows fire,” is seen around the day of Hassaku, or Aug. 1 of the lunar calendar, in late summer.

Fire or lamps on fishing boats are the light source for Shiranui, which characteristically takes the form of spots of light that appear stretched horizontally.

Mentions of Shiranui in literature go as far back as “Nihon Shoki” (The Chronicles of Japan) and “Manyoshu” (Collection of Ten Thousand Leaves), books that were compiled in the eighth century. Edo Period (1603-1867) scholars Arai Hakuseki (1657-1725), Kamo no Mabuchi (1697-1769) and Motoori Norinaga (1730-1801) also mentioned the phenomenon in their works.

The Yatsushiro Sea, an inland stretch of water in this southwestern prefecture, also goes by the name of the Shiranui Sea.

The image in a photo that was taken in the town of Shiranuhi, now part of the city of Uki, in September 1988, however, is believed to be the last officially available record of Shiranui’s emergence. The photo is currently administered by the Uki municipal board of education.

The Earth Science Group’s members began studying Shiranui out of curiosity about why the phenomenon hadn’t been seen for so long.

Successive members of the group waited for the apparition at Eino-o Tsurugijinja shrine, a long-famed Shiranui observation spot in Uki’s Shiranuhi-machi district, every year for the past six years.

They photographed how city lights on the other side of the sea, 12.5 kilometers away, appeared separated above and below in the early hours of Oct. 1 last year. There was no doubt it was a mirage, but of a variety called “inferior mirage,” which can also be seen at other locations across Japan.

A mirage is caused by refraction of light rays that travel through air layers with uneven temperatures.

The high school Earth Science Group members confirmed through their experiment that a mirage causes light to appear divided vertically in many cases but can also cause it to appear separated horizontally when there is a breeze.

They presented their findings at a “junior session” during a meeting of the Geological Society of Japan (GSJ) in September last year, for which they won top awards.

Simulations based on the conditions of their experiment led them to believe that Shiranui is most likely to emerge when the sea is the shallowest in the small hours on Sept. 3, which was when Hassaku day fell this year.

The students talked with officials of the Yatsushiro fisheries cooperative association, who agreed to send three of their fishing boats to three locations at mouths of rivers 4 to 10 km southwest of Eino-o Tsurugijinja shrine.

The team members conjectured that an inflow of river water, which is colder than seawater, would create unevenness in air temperatures there.

The fishing boats were fitted out with 500-watt LED lights. The students set up a 600-millimeter telephoto lens for three hours from midnight Sept. 2 to capture the lights.

Of the three light sources, the students found that the one aboard a boat anchored at the mouth of the Ozayagawa river, 8.5 km from the shrine, appeared stretched in a horizontal direction.

It marked the first time the students had seen light extended horizontally instead of being separated vertically. They concluded it that was a Shiranui apparition.

The students went on to report on the results of their observations at a junior session of a GSJ meeting in Yamagata, capital of the prefecture of the same name, on Sept. 8, where they hoped to hear expert comment.

“The Shiranui this time was so small that it could only be seen through a telephoto lens,” said Eiki Honda, an Uto High School teacher who is adviser to the club. “I hope to work with the students to do further research to find out under what conditions Shiranui would be visible with the naked eye.”

“Shiranui may emerge only at locations that are so remote that you can see it only through a telephoto lens,” said Naoto Komeda, a second-year student who leads the Earth Science Group, before the successful observation. “Even if our attempt fails, we will press on with our study to find out why Shiranui can no longer be seen.”