The visibility of stars in the night sky quickly decreases

These astronaut photos of parts of Calgary (Canada) show examples of how lighting changed between 2010 and 2021. Here is a picture from 2010.
 Image Credit: Courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center, georeferencing by GFZ Potsdam

This is shown by a science study based on a worldwide Citizen Science project on light pollution, which has collected data in the past eleven years.

People see fewer and fewer stars in the night sky worldwide. The cause is probably light pollution in the evening and night hours, which increases by seven to ten percent per year. This rate of change is greater than satellite measurements of artificial light missions on Earth suggested. This is the finding of a study in the science magazine, carried out by a research group led by Dr. Christopher Kyba from the German GeoForschungsZentrum GFZ and the Ruhr University Bochum with researchers from the GFZ and the NOIRLab of the US National Science Foundation. As part of the Citizen Science project "Globe at Night", they evaluated more than 50,000 observations with the naked eye of civil scientists around the world from 2011 to 2022. The study also shows that the Citizen Science data are an important addition to previous measurement methods.

Here is a picture from 2021: New lighting was installed and many street lamps were converted from orange high sodium vapor to white LED lights. (The photos were not taken with the same settings and have a different spatial resolution. Therefore, the 2010 photo appears a little brighter.)
Image Credit: Courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center, georeferencing by GFZ Potsdam

Background light pollution

Long after sunset, the sky still shines in an - artificial - twilight above much of the earth's land surface. This artificial glow of the night sky is a form of light pollution that has a serious impact on the environment and should therefore be in the eye of research, as Constance Walker, co-author of the study and since its foundation head of the Globe at Night project of the NOIRLab of the NSF emphasizes. Because many behaviors and physiological processes of living beings are determined by daily and seasonal rhythms - and thus influenced by light. "The sky lights affect both day and night active animals and also destroy an important part of our cultural heritage," says Walker. The appearance of the night sky changes, with negative effects on star observation and astronomy.

Need for suitable measuring methods

The change in light pollution is global and has not been well known over time. In principle, artificial sky light can be measured by satellites. But the only instruments in space that can monitor the entire earth do not have sufficient measuring accuracy and sensitivity.

It is therefore a promising approach to use the observation power of humans and thus the human eye as a sensor, and - as part of Citizen Science experiments - to rely on the power of the many. The "Globe at Night" project has been running since 2006, initiated by the NOIRLab of the US National Science Foundation (NSF). People all over the world can participate in this.

With Citizen Science ...

The participants look at their night sky and then state in an online form which of eight star maps best suits what they see. Each map shows the sky under different degrees of light pollution.

"The contributions of the individual people work together like a global sensor network that enables us to take a completely new research approach," says Christopher Kyba. Together with Yigit Öner Altıntas as well as Constance E. Walker and Mark Newhouse, he evaluated data from 51,351 observations around the world, which were made between 2011 and 2022 on cloudless and moon-free nights. They represent 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 celestial brightness from this data and to take into account that the observers were also in other locations over the years, they also used a global model for celestial brightness based on satellite data from 2014.

... to surprising insights

"The speed at which stars become invisible to people in urban environments is dramatic," said Christopher Kyba, first author of the study. The researchers estimated the changes in celestial brightness based on the number of visible stars. This resulted in 6.5 percent more brightness per year for Europe and an increase of 10.4 percent for North America.

To illustrate these numbers, Kyba explains the consequences that would result for the observability of stars in a place with an increase in brightness of 9.6 percent per year. This corresponds to the currently determined global average. “If the development progresses like this, a child who is born in a place where 250 stars are visible will be there on his 18th. Birthday can only see 100 stars."

According to the analysis of the satellite data, the researchers did not expect the speed of this development. On the contrary, for the observers' locations, these even indicated that the artificial brightness decreased slightly (by 0.3 percent per year in Europe, by 0.8 percent in North America).

The brighter the lighting on the floor, the less you can see from the sky. In the country you can see significantly more stars than in the city center. 
Image Credit: NOIRlab Marenfeld

Causes of the difference between measurements from Earth and from space

Christopher Kyba believes that the difference between human observation and satellite measurements is likely due to changes in lighting practice: “Satellites are most sensitive to light directed up to the sky. But it is horizontally emitted light that makes up most of the sky,” explains Kyba. “So, if advertising and facade lighting become more frequent, larger or brighter, they could have a big impact without being reflected accordingly on the satellite images."

The authors cite the widespread switch from orange sodium vapor lamps to white LEDs, which also emit blue light. "Our eyes are more sensitive to blue light at night, and blue light is more scattered in the atmosphere, so it contributes more to the sky," says Kyba. “But the only satellite that can map the whole earth at night is not sensitive in the wavelength range of the blue light."

Limits of the study and other potential

However, the Citizen Science approach also has its limitations. The number of participants from different regions of the world determines the informative value about spatial and temporal trends. So far, people from North America and Europe in particular have participated in the experiment, and half of the Asian contributions come from a single country: Japan. “Most of the data comes from the regions of the world where the sky is currently most pronounced. This is useful, but it means that we cannot say much about the changes in the sky in regions with few observations,” emphasizes Kyba. Rapid changes in artificial sky lighting are suspected, especially in developing countries, but so far there have been few observations.

Two conclusions: lighting policy and citizen science

The researchers draw two main conclusions from their results: First, it can be seen that current lighting practice and policy, for example, has not yet improved, at least at the continental level, despite the growing awareness of light pollution, due to the increasing use of LEDs. "And on the other hand, we were able to demonstrate that the Citizen Science data is an important addition to the previous measurement methods," emphasizes Kyba. Constance Walker adds: “If we had broader participation, we could identify trends for other continents and possibly even for individual states and cities. The project is not yet complete, so have a look tonight and tell us what you see!"

Published in journal: Science

Research Material: Globe at Night project

Source/Credit: Ruhr University Bochum

Reference Number: en012023_01

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