The visibility of stars in the night sky is diminishing faster than previously thought

The visibility of stars in the night sky is diminishing faster than previously thought

Light Pollution Effect 1 – from excellent dark skies (left) to inner city skies (right). Credit: NOIRLab/NSF/AURA, P. Marenfeld

People all over the world are seeing fewer stars in the night sky. The change in the visibility of the stars can be explained by the increase in the brightness of the sky by 7-10% per year. The rate of change is faster than satellite measurements of artificial light emissions on Earth would initially suggest.

This is the result of a study published in the journal Scienceswhich was conducted by a research group led by Christopher Kyba of the German GFZ Research Center for Geosciences and Ruhr-Universität Bochum with colleagues from the GFZ and the US National Science Foundation’s NOIRLab.

They analyzed more than 50,000 naked eye observations made by citizen scientists around the world from 2011 to 2022 as part of the Citizen Science Project “Globe at Night”. The results show that citizen science data is an important complement to previous measurement methods.

light pollution background

Over much of the Earth’s surface, the sky continues to glow with artificial auroras long after sunset. This “heavenly glow” is a form of light pollution that has dangerous effects on the environment, and thus should be a focus of research, says Constance Walker, co-author of the study and head of the NSF NOIRLab’s Globe at Night project since its inception. Confirms.

After all, many of the behaviors and physiological processes of organisms are determined by daily and seasonal cycles – and thus affected by light. “The glow in the sky affects both diurnal and nocturnal animals and also destroys an important part of our cultural heritage,” says Walker. The appearance of the sky changes at night, with negative effects on stars and astronomy.

Credit: NOIRLab

The need for appropriate measurement methods

The change in celestial glow has never been measured globally before. 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.

Hence a promising approach is to use the observational power of people using the human eye as a sensor, and in doing so – in Citizen Science experiments – to rely on the power of the crowd. Initiated by NOIRLab of the US National Science Foundation, the “Globe at Night” project has been running since 2006. People from all over the world can participate in this project.

Citizen science

Participants look at their night sky, then report which combination of eight-star charts best matches what they see using an online form. Each graph shows the sky under different levels of light pollution.

The visibility of stars in the night sky is diminishing faster than previously thought

Light Pollution Effect 2 – From excellent dark skies (left) to inner city skies (right). Credit: NOIRLab/NSF/AURA, P. Marenfeld

“The contributions of individuals work together as if they were a global sensor network, making new science possible,” says Christopher Kyba of the German GFZ Research Center for Geosciences in Potsdam and the University of Bochum. Together with his GFZ colleague Yigit Oner Altinas, Constance E. Walker and Mark Neuhaus of NOIRLab, he analyzed data from 51,351 participants worldwide on cloud- and moon-free nights between 2011 and 2022. It was obtained from 19,262 locations around the world, including That’s 3,699 locations in Europe and 9,488 locations in North America.

In order to calculate the rate of change in sky brightness from this data and to take into account that observers have also been in different locations over the years, they used a global model of sky brightness based on satellite data From 2014.

Surprising results

The rate at which the stars become invisible to the people in them urban environments dramatic,” summarizes Christopher Kyba, lead author of the study. The researchers found that the change in the number of visible stars could be explained by an increase in night sky brightness. In Europe, they found a 6.5% increase in brightness annually that matches the data; in North America 10.4%.

To put these numbers into a more understandable context, Kyba explains the consequences of seeing stars in a place with an increase of 9.6% annually, which is average over all locations worldwide. “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 there by his 18th birthday.”

Based on the slower growth in upward emission seen in the satellite data, the researchers were surprised by the speed of this development in the celestial glow. In fact, for observer locations, artificial brightness measured by satellite decreased slightly (by 0.3% annually in Europe, by 0.8% in North America).

Reasons for the difference between measurements from Earth and from space

Christopher Kyba thinks the difference between human observations and satellite measurements is probably due to changes in lighting practices. “Satellites are most sensitive to light directed toward the sky,” Kiba explains. “But light emitted horizontally is responsible for most of the sky’s glow.” “So, if advertisements and facade lighting become more frequent, larger, or brighter, they can have a significant effect on sky glow without making a significant difference to satellite images.”

Another factor cited by the authors is the widespread shift from orange sodium vapor lamps to white LEDs, which emit more blue light. “Our eyes are more sensitive to blue light at night, and blue light is more likely to be scattered by the atmosphere, so it contributes more to the glow of the sky,” Kiba says. “But the only satellites that can image the entire Earth at night are not sensitive in the wavelength range blue light. ”

The visibility of stars in the night sky is diminishing faster than previously thought

Astronaut photos of parts of Calgary (Canada) show examples of how lighting has changed from 2010 to 2021: new lighting has been installed and many streetlights have been converted from orange high-pressure sodium to white LED. (Note: the images were not taken with the same settings, and have different spatial resolutions. Thus, the 2010 image appears somewhat brighter.

Study limits and other possibilities

However, Citizen Science’s approach also has its limitations. For example, the number of participants from different regions of the world determines the importance of spatial and temporal trends. So far people from North America and Europe had the largest participation in the experiment, and half of the Asian contributions come from one country: Japan.

“Most of the data comes from regions of the Earth where cyan glow is currently prevalent. This is useful, but it means we can’t say much about cyan glow change in regions that have few observations,” Kiba asserts. Rapid changes in artificial celestial glow are suspected, especially in developing countries, but there have been few observations so far.

Two conclusions: lighting policy and citizen science

The researchers drew two main conclusions from their findings: On the one hand, they show that current lighting policies, such as the use of LEDs, have not yet achieved any improvement, at least on a continental level, despite the growing awareness of light pollution. .

“On the other hand, we were able to demonstrate that Citizen Science’s data is an important complement to previous measurement methods,” says Kyba.

Constance Walker adds, “If we had a broader involvement, we could identify trends for other continents, and perhaps even individual states and cities. The project is ongoing, so feel free to take a look tonight and tell us what you see.”

more information:
Christopher CM Kyba, citizen scientists who reported rapid global declines in stargazing from 2011 to 2022, Sciences (2023). DOI: 10.1126/science.abq7781.

More information on “Globe at Night” can be found here. here.

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