Turning Off the Lights: What Earth Hour Teaches Us About Light Pollution in Cities

In this study, Chu Wing So and colleagues investigate how much a city’s night sky brightens because of decorative and advertising lights. They use Hong Kong’s annual Earth Hour--when many lights are voluntarily turned off for one hour--as a kind of natural experiment. The authors analyzed how the night sky responded to these temporary changes over more than a decade of events. Using cameras, light sensors, and spectrometers, they discovered that just a few highly visible buildings and billboards contribute a surprisingly large share of the city’s light pollution.

Introduction: Why Does Light Pollution Matter?

Light pollution happens when artificial light at night (ALAN) is used excessively or improperly. This disrupts ecosystems, confuses animals, and even affects human health and sleep patterns. It also obscures the stars, making astronomy and stargazing harder. Many sources of ALAN include necessary lights like streetlights for safety, but also decorative and advertising lights, which are less essential. Figuring out which lights matter most is challenging because cities don’t always track details like brightness or schedules. Blackouts and pandemics have revealed that turning off lights does make skies darker, but those situations are unpredictable. Earth Hour, an annual coordinated event when cities voluntarily switch off decorative lights for an hour, gives researchers a rare, repeatable way to study the impact of lighting.

Methods: Watching the Sky Go Dark

The team focused on the Tsim Sha Tsui (TST) district of Hong Kong, a dense commercial and tourist area full of brightly lit skyscrapers and billboards. They collected data during Earth Hours between 2011 and 2024. Observations included all-sky cameras, wide-field DSLR photos, videos from trams, and even government webcams. They measured how the sky’s brightness changed using light meters (NSB), and they analyzed the color of the skyglow using a portable spectrometer.

Results: What Happens When the Lights Go Out?

The most striking result came from comparing the sky before, during, and after Earth Hour. Turning off about 120 decorative and advertising lights in the central business districts (CBDs) reduced the zenith sky brightness by 23–53%, depending on weather and year. The darkest skies occurred when the weather was clear and participation was high. Notably, these reductions were much larger than what has been seen in similar studies in cities like Berlin. All-sky and wide-field images revealed that the lights switched off were mainly on hotel facades, skyscraper roofs, and giant LED billboards facing the harbor. Survey teams identified that most buildings near the harbor participated, while those on side streets and outside the CBD did not.

What Colors Disappear?

By analyzing the spectrum of the sky, the authors pinpointed which kinds of lights contributed most to skyglow. The strongest reductions occurred in the blue (445–500 nm), green (500–540 nm), and red (615–650 nm) parts of the spectrum, which are typical of LED billboards and video walls. They also observed a narrower feature at 585–595 nm, which comes from metal halide floodlights used on facades. In contrast, streetlights--which stayed on--did not contribute noticeably to the changes during Earth Hour.

Discussion: Lessons and Future Directions

Earth Hour provided a unique, repeatable experiment showing that just a small set of decorative and advertising lights--less than the total number of city lights--are responsible for much of the urban skyglow. Even when these lights were switched off, however, the sky remained much brighter than a natural dark sky, and the Milky Way remained invisible to the naked eye. The findings suggest that sustainable policies targeting decorative and billboard lighting, like setting earlier switch-off times or using less intense, warmer-colored lights, could make a big difference. For example, replacing blue-rich LEDs with amber-colored ones would reduce the skyglow significantly.

Limitations and Importance of Ground Measurements

The authors note that their instruments measured light differently from human eyes and that some horizontal light (like from video walls) is not fully captured by zenith-pointing sensors. They also observed that the shift to LED lighting--which is richer in blue light--is changing how cities glow at night. While satellite data can monitor light over large areas, ground-based observations like theirs remain essential to track individual buildings and billboards, which are often invisible in space-based images.

Conclusion: A Brighter (or Darker) Future?

This work demonstrates that even in a bright city like Hong Kong, modest changes--targeting the most glaring decorative and advertising lights--could significantly improve the night sky. Events like Earth Hour not only raise awareness but also help researchers understand and propose solutions to the problem of light pollution, with benefits for ecosystems, human health, and our view of the stars.

Source: So