People around the world see fewer and fewer stars in the night sky. The change of star visibility can be explained by the increase of 7% – 10% of sky brightness every year. A study published recently in the journal Science revealed the analysis results of more than 50000 times of naked eye observation of the night sky between 2011 and 2022.

In a large part of the earth’s land surface, the sky continues to emit artificial twilight for a long time after sunset. This “skylight” is a form of light pollution that has a serious impact on the environment. It will affect animals that move day and night, and will also destroy an important part of human cultural heritage. It will also have a negative impact on star observation and astronomy.

The change of sky brightness with time has not been measured globally before. Although it can be measured by satellite in principle, the only current sensor that monitors the whole earth does not have enough accuracy or sensitivity.

Therefore, a promising method is to use people’s observation power, use the human eye as a sensor, and do so within the framework of civil science experiments. The “Night Earth” project launched by the National Science Foundation of the United States NOIRLab has been running since 2006, and people from all over the world can participate in this project.

Participants observed their night sky, and then reported which of the eight star charts in a group best matched what they saw using online tables. Each chart showed the sky under different levels of light pollution.
Christopher Keba, the first author of the study, from the German Earth Science Research Center and the Ruhr University in Bochum, said: “Personal contributions work together like a global sensor network, making new science possible.”

The researchers analyzed the image data taken by 51351 participants in cloudless and moonless nights from 2011 to 2022. These data come from 19262 locations around the world, including 3699 locations in Europe and 9488 locations in North America.

Keba said that in the urban environment, the speed of stars disappearing from human sight is amazing. The study found that the change in the number of visible stars can be explained by the increase in the brightness of the night sky. In Europe, they found that the brightness of the night sky increased by 6.5% every year; In North America, this proportion is 10.4%.

In order to make these figures easier to understand, Keba explained the influence of stargazing in a place where the brightness of the night sky increased by 9.6% every year (the global average). “If we continue to develop at this speed, a child who can see 250 stars at birth can only see 100 stars at his 18th birthday.”

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