Stars are becoming rare. Not because they are decreasing, but because we don’t see them anymore. Because over much of the Earth’s surface, the sky glows in an artificial twilight long after sunset. The so-called light pollution not only affects stargazing, but also affects many behaviors and physiological processes of living beings. This is the conclusion of an analysis published in the journal “Science”, for which researchers evaluated more than 50,000 observations as part of a citizen science project from 2011 to 2022.
“The speed with which stars become invisible to people in urban environments is dramatic,” says Christopher Kyba of the German GFZ Research Center for Geosciences in Potsdam and Ruhr University Bochum in the study. In Europe, they found an increase in brightness of 6.5% per year. In North America, it reaches 10.4%.
“If development continues at this rate, a child born in a place where 250 stars are visible will only be able to see 100 stars by age 18,” says Kyba, describing the surprising development.
unsuitable satellites
The change in sky brightness over time has yet to be measured globally. While satellites could, in principle, do this, the only current sensors that monitor the entire Earth lack sufficient accuracy and sensitivity, the study says. Researchers therefore used the human eye as a sensor and relied on the power of dough as part of citizen science experiments. The “Globe at Night” project, initiated by NOIRLab of the US National Science Foundation, has been running since 2006. People from all over the world can participate.
Participants look up at the night sky and then use an online form to indicate which of eight star charts best matches what they see. Each map shows the sky under different levels of light pollution.
“Each person’s contributions work together like a global network of sensors and enable new scientific discoveries,” explains Kyba. Observations are from 19,262 locations worldwide, including 3,699 locations in Europe and 9,488 locations in North America.
The researchers were able to verify that the observations and satellite data differ enormously. At the observers’ locations, the artificial brightness measured by satellite even dimmed slightly.
Blue light from LEDs
Kyba believes the difference between the observations and measurements is likely due to changes in lighting practices. “Satellites are most sensitive to light directed upwards in the sky, but most of the skylight is horizontally emitted light,” he explains. “So if advertisements and facade lights become more frequent, larger or brighter, they can have a big impact on sky brightness without making much of a difference in satellite imagery.”
Another factor cited by scientists is the switch from orange sodium vapor lamps to white LEDs, which emit much more blue light. “Our eyes are more sensitive to blue light at night, and blue light scatters more in the atmosphere and contributes more to the brightness in the sky,” says Kyba. “But the only satellites that can image the entire Earth at night are not sensitive to the wavelength range of blue light.”
However, the Citizen Science approach also has its limits because not enough observations are available in all regions of the world. “If we had broader participation, we could identify trends for other continents and possibly for individual states and cities,” explains US co-author Constance Walker. “The project isn’t finished yet, so check it out tonight and let us know what you see,” the astronomer asks people.