Light Pollution Management: How a Small Japanese Town Maintains Its Dark Skies

Light pollution is a growing global challenge. It obstructs the view of the sky and makes it difficult for astronomers to observe celestial events.

Effects of Light Pollution

Light pollution affects more than just the night sky; it can also affect human health and safety. Although one of the reasons for increased lighting is safety, aiming lights often results in light trespass. It also creates an unintended effect, aiming bright lights directly at people's eyes, usually blinding them to dangers hidden in shadows. Moreover, light pollution also damages other forms of life, like migrating birds and ocean populations.

In recent decades, organizations like the International Astronomical Union, the American Union, and the DarkSky International have been joined by officials to work on solutions to light pollution. The increase in the use of LEDs for lighting poses the same problems brought by incandescent lighting. Experts continue to work on recommendations for the wise use of such forms of illumination to mitigate the problems of light pollution.

Defending the Dark Skies

In the southwestern part of Okayama Prefecture in Japan, a semi-rural area called Bisei Town is home to several astronomical observation posts, including the Bisei Astronomical Observatory. This facility displays a 40-inch (101 centimeters) telescope and other devices and is open for public viewing throughout the year.

Since the town is a designated dark-sky place, the locals are adamant about keeping their view of the stars. In 1989, the town enacted light pollution ordinances, making it the first local government in the country to pass such a law.

The town and observatory collaborated with Panasonic to create a dark sky-friendly region. As part of this program, all the public lighting in the city is replaced with LEDs with a temperature of 3,000 K or less. As a result of this effort, Bisei was certified as a "DarkSky Community" by DarkSky International.

Although the locals cooperate with the programs to maintain their dark skies, they also have to contend with light pollution from other cities in the area. Some very bright, higher-temperature LEDs are still in use in neighboring towns where the populations reach a half million or more. Their light continues to create problems for Bisei.

Japanese astronomers Ryosuke Itoh and Syota Maeno worked together to monitor how the view at the Bisei Observatory is affected by the light pollution ordinances in Bisei town. All of the fluorescent lights in the town have been replaced with LED lamps, hoping it would reduce sky brightness in the nearby region. However, light pollution from these and lights farther away is still scattered by the atmosphere, resulting in a perceptible sky glow.

Itoh and Maeno used a specially equipped sky meter to get a photometric value for sky brightness. They used sky spectra dating between 2006 and 2023 to see if the data was affected by changes in lighting.

It was found that Bisei town itself has an observed sky brightness on the Bortle Scale of class 4, corresponding to a rural/suburban transition zone. This means that shifting to lower color-temperature LEDs as a substitute for incandescent lamps reduces some light pollution in town. However, a very definite spectral line around 4,500 A was observed by Itoh and Maeno, which they call a "blue hump." This comes from the bright white, higher-temperature LEDs still used in nearby towns, although they cannot identify directly all the specific sources.