Camilla Rathsach walked along the lichen-covered sand, heading out from the lone village on Denmark’s remote Anholt island—a spot of land just a few kilometers wide in the middle of the Kattegat Strait, which separates the Danish mainland from Sweden. As Anholt Town’s 45 streetlights receded into the distance, moonlit shadows reached out to embrace the dunes. Rathsach looked up, admiring the Milky Way stretching across the sky. Thousands of stars shone down. "It’s just amazing," she says. "Your senses heighten and you hear the water and feel the fresh air."

This dark-sky moment was one of many Rathsach experienced while visiting the island in 2020 for work on her master’s thesis on balancing the need for outdoor lighting and darkness. Having grown up in urban areas, Rathsach wasn’t used to how bright moonlit nights could be. And after speaking with the island’s residents, who value the dark sky deeply and navigate with little outdoor light, she realized that artificial lighting could be turned down at night depending on the moon’s phase.

At Aalborg University in Denmark, she worked with her graduate supervisor, Mette Hvass, to present a new outdoor lighting design for Anholt’s church. Rathsach and Hvass picked the church for their project because it is a central meeting place for the community yet it currently has no outdoor lights. They thought lighting would make it easier for people to navigate but wanted to preserve the inviting ambiance of moonlight.

One of the guiding principles of designing sustainable lighting is to start with darkness, and add only the minimum amount of light required. Darkness and natural light sources are important to many species, and artificial light can be downright dangerous.

"Lights can attract and disorient seabirds during their flights between colony and foraging sites at sea," says Elena Maggi, an ecologist at the University of Pisa in Italy who is not involved in the project. Anholt’s beaches host a variety of breeding seabirds, including gulls and terns, and the island is a stopover for many migrating birds. The waters around the island are also home to seals, cod, herring, and seagrass. Though scientists have made progress in understanding the effects of artificial light at night on a range of species, such as turtles, birds, and even corals, there is still more to learn.

"We still don’t know exactly how artificial light might interact with other disturbances like noise and chemical pollution, or with rising ocean temperatures and acidification due to climate change," says Maggi.

The scientists’ final design for the church includes path lighting and small spotlights under the window arches, along with facade lighting under the eaves shining downward. To preserve the dark sky, path lighting would turn off on bright moonlit nights, and facade lighting would shut off on semi-bright or bright nights. The window lighting would stay on regardless of the moon’s phase.

The adaptive lighting cooked up by Camilla Rathsach and Mette Hvass would automatically adjust to the availability of moonlight, tweaking this church’s lighting automatically to balance visibility and darkness. Mock-ups show how the church would be lit under no moonlight (first) and a full moon (second). Illustrations courtesy of Camilla Rathsach

"The contrast between the moon’s cold white light reflecting off the church’s walls and the warm orange lights in the windows would create a cozy, inviting experience," says Rathsach.

The moonlight adaptive lighting design project is part of a growing effort to balance the need for functional lighting in the town and to protect the darkness. Recently, the town’s public streetlights were swapped for dark-sky friendly lamps, says Anne Dixgaard, chairman of Dark Sky Anholt.

Dixgaard also organizes a yearly walk out to Anholt’s beach, where skywatchers can learn about the night sky. "People really value Anholt’s dark sky and want to preserve it," she says.

Rathsach and Hvass are working on the moonlight adaptive design project in hopes that it will be implemented one day, but they still have some challenges to overcome. Moonlight is a relatively faint light source, so detecting it using sensors is challenging, and lights would need to adjust automatically on nights with intermittent cloud cover. Yet big initiatives often begin with small steps.

"This work is something new and unexpected," says Maggi. "It’s a very interesting approach to mitigating the negative effects of artificial light at night."