Star visibility falls as light pollution enhances night sky brightness: Study

People around the world are seeing fewer and fewer stars in the night sky, owing to an increase in the night sky brightness due to light pollution, according to a study.

The change in star visibility can be explained by an increase in the sky brightness of 7-10 per cent per year, the study said.

The rate of change is faster than satellite measurements of artificial light emissions on Earth would at first suggest, the study said.

The study analysed more than 50,000 naked-eye observations by citizen scientists around the world from 2011 to 2022 as part of the "Globe at Night" Citizen Science Project. The results show that citizen science data are an important supplement to previous measurement methods.

The findings of the study have been published in the journal Science.

Over a large part of the Earth's land surface, the sky continues to glow with an artificial twilight long after sunset.

This "skyglow" is a form of light pollution that has serious effects on the environment and should therefore be the focus of research, as Constance Walker, co-author of the study and head of the Globe at Night project of US National Science Foundation's NOIRLab since its inception, emphasised.

After all, many behaviours and physiological processes of living creatures are determined by daily and seasonal cycles - and thus influenced by light.

"Skyglow affects both diurnal and nocturnal animals and also destroys an important part of our cultural heritage," says Walker.

The appearance of the night sky is changing, with negative effects on stargazing and astronomy, the study said.

The change in skyglow over time has not previously been measured globally. While it could in principle be measured by satellites, the only current sensors that monitor the entire Earth do not have sufficient accuracy or sensitivity, the study said.

A promising approach is therefore to use the observational power of people using the human eye as a sensor, and in doing so - within the framework of Citizen Science experiments - to rely on the power of the crowd.

The “Globe at Night" project, initiated by the US National Science Foundation's NOIRLab, has been running since 2006. People all over the world can participate in this project.

Participants look at their night sky, and then report which of a set of eight star charts best matches what they see using an online form. Each chart shows the sky under different levels of light pollution, the study said.

"The contributions of individual people work together as if they were a global sensor network, making new science possible," said Christopher Kyba from the GFZ German Research Centre for Geosciences in Potsdam and the Ruhr University Bochum.

Kyba's team has analyzed data from 51,351 participants around the world taken on cloud- and moon-free nights between 2011 and 2022. They were obtained from 19,262 locations worldwide, including 3,699 locations in Europe and 9,488 locations in North America.

In order to calculate a rate of change in sky brightness from this data and to take into account that the observers were also at different locations over the years, they made use of a global model for sky brightness based on satellite data from 2014.

"The rate at which stars are becoming invisible to people in urban environments is dramatic," summed up Christopher Kyba, lead author of the study.

The researchers found that the change in the number of visible stars can be explained by increases in night sky brightness. In Europe, they found a 6.5 per cent increase in brightness per year matched the data; in North America, it's 10.4 per cent.

To put these numbers into a more understandable context, Kyba explains the consequences for seeing stars in a place with a 9.6 per cent per year increase, which was the average over all locations worldwide.

"If the development were to continue at that rate, a child born in a place where 250 stars are visible will only be able to see 100 stars there on his 18th birthday," said Kyba.

Based on the slower growth in upward emissions seen in satellite data, the researchers were surprised by the speed of this development in skyglow.

In fact, for the locations of the observers, the artificial brightness measured by satellite had slightly decreased - by 0.3 percent per year in Europe, by 0.8 percent in North America.