Star visibility eroding rapidly as night sky gets brighter: study
Issued on: 19/01/2023 -
Washington (AFP) – Light pollution is growing rapidly and in some places the number of stars visible to the naked eye in the night sky is being reduced by more than half in less than 20 years, according to a study released Thursday.
The researchers, whose findings were published in the journal Science, said the increase in light pollution -- skyglow -- that they found was much larger than that measured by satellite observations of Earth at night.
For the study of the change in global sky brightness from artificial light, the researchers used stellar observations from 2011 to 2022 submitted by more than 51,000 "citizen scientists" around the world.
Participants in the "Globe at Night" project run by the US National Optical-Infrared Astronomy Research Laboratory were given star maps and asked to compare them to the night sky at their location.
The change in the number of visible stars reported was equivalent to a 9.6 percent per year annual increase in sky brightness, averaged over the locations of the participants, the researchers said.
Over an 18-year period, given such star brightness change, a location with 250 visible stars would see that number reduced to 100.
Most of the naked-eye star observations came from Europe and the United States said Christopher Kyba, one of the authors of the study, but there was also good participation in Uruguay, South Africa and Japan.
"The global trend in skyglow that we measure likely underestimates the trend in countries with the most rapid increases in economic development, because the rate of change in light emission is highest there," the researchers said.
The study coincided with the replacement of many outdoor lights with light-emitting diodes (LEDs), but the researchers said the impact on skyglow from the transition to LEDs is unclear.
"Some researchers have predicted that it will be beneficial; others, that it could be harmful because of spectral changes or a rebound effect, in which the high luminous efficacy of LEDs leads to more or brighter lights being installed or longer hours of operation," they said.
According to the study, the global LED market share for new general lighting grew from under one percent in 2011 to 47 percent in 2019.
"The visibility of stars is deteriorating rapidly, despite (or perhaps because of) the introduction of LEDs in outdoor lighting applications," the researchers said.
"Existing lighting policies are not preventing increases in skyglow, at least on continental and global scales."
'Confronted with the cosmos'
Kyba, a physicist at the German Research Center for Geoscience, told AFP that while the team was able to evaluate erosion of star visibility due to skyglow, not a lot of research has been done on its ecological impact.
"There's tons of research on light shining directly on animals and plants," he said. "But it's really hard to do experiments on the impact of skyglow.
"You're not going to do something like just turn off New York City and see what happens in the East River."
Science aside, light pollution has changed the character of the night sky.
"For all of human history, when people went outside at nighttime, they were sort of confronted with the cosmos, at least on clear nights with no moon," Kyba said.
"You'd walk outside and there's the stars, there's the Milky Way. It's there and it's shining down on you," he said.
"And now that's like a really unusual experience," he said. "It surely makes a difference to us as people that we don't have this experience that used to be a very universal experience."
The Globe at Night campaign hosts an interactive data map at globeatnight.org and is seeking volunteers to collect more observations in 2023.
© 2023 AFP
Stars disappear before our eyes, citizen scientists report
NOIRLab’s Globe at Night educational program reveals how increasing light pollution is robbing us of the night sky
Peer-Reviewed PublicationIMAGE: A STARTLING ANALYSIS FROM GLOBE AT NIGHT — A CITIZEN SCIENCE PROGRAM RUN BY NSF’S NOIRLAB — CONCLUDES THAT STARS ARE DISAPPEARING FROM HUMAN SIGHT AT AN ASTONISHING RATE. THE STUDY FINDS THAT, TO HUMAN EYES, ARTIFICIAL LIGHTING HAS DULLED THE NIGHT SKY MORE RAPIDLY THAN INDICATED BY SATELLITE MEASUREMENTS. THE STUDY SHOWCASES THE UNIQUE CONTRIBUTIONS THAT CITIZEN SCIENTISTS CAN MAKE IN ESSENTIAL FIELDS OF RESEARCH. THIS GRAPHIC ILLUSTRATES HOW THE GREATER THE AMOUNT OF LIGHT POLLUTION, AND THEREFORE SKYGLOW, THE FEWER THE STARS THAT ARE VISIBLE. view more
CREDIT: NOIRLAB/NSF/AURA, P. MARENFELD
A startling analysis from Globe at Night — a citizen science program run by NSF’s NOIRLab — concludes that stars are disappearing from human sight at an astonishing rate. The study finds that, to human eyes, artificial lighting has dulled the night sky more rapidly than indicated by satellite measurements. The study published in the journal Science showcases the unique contributions that citizen scientists can make in essential fields of research.
From the glowing arc of the Milky Way to dozens of intricate constellations, the unaided human eye should be able to perceive several thousand stars on a clear, dark night. Unfortunately, growing light pollution has robbed about 30% of people around the globe and approximately 80% of people in the United States of the nightly view of their home galaxy. A new paper published in the journal Science concludes that the problem is getting rapidly worse.
New citizen-science-based research sheds alarming light on the problem of ‘skyglow’ — the diffuse illumination of the night sky that is a form of light pollution. The data for this study came from crowd-sourced observations collected from around the world as part of Globe at Night, a program run by NSF’s NOIRLab and developed by NRAO astronomer Connie Walker. The research reveals that skyglow is increasing more rapidly than shown in satellite measurements of Earth's surface brightness at night.
“At this rate of change, a child born in a location where 250 stars were visible would be able to see only abound100 by the time they turned 18,” said Christopher Kyba, a researcher at the German Research Centre for Geosciences and lead author of the paper detailing these results.
Light pollution is a familiar problem that has many detrimental effects, not only on the practice of astronomy. It also has an impact on human health and wildlife, since it disrupts the cyclical transition from sunlight to starlight that biological systems have evolved alongside. Furthermore, the loss of visible stars is a poignant loss of human cultural heritage. Until relatively recently, humans throughout history had an impressive view of the starry night sky, and the effect of this nightly spectacle is evident in ancient cultures, from the myths it inspired to the structures that were built in alignment with celestial bodies.
Despite being a well-recognized issue, however, the changes in sky brightness over time are not well documented, particularly on a global scale.
Globe at Night has been gathering data on stellar visibility every year since 2006 [1]. Anyone can submit observations through the Globe at Night web application on a desktop or smartphone. After entering the relevant date, time and location, participants are shown a number of star maps. They then record which one best matches what they can see in the sky without any telescopes or other instruments.
This gives an estimate of what is called the naked eye limiting magnitude, which is a measure of how bright an object must be in order to be seen. This can be used to estimate the brightness of skyglow, because as the sky brightens, the fainter objects disappear from sight.
The authors of the paper analyzed more than 50,000 observations submitted to Globe at Night between 2011 and 2022, ensuring consistency by omitting entries that were affected by factors including cloud cover and moonlight. They focused on data from Europe and North America, since these regions had a sufficient distribution of observations across the land area as well as throughout the decade studied. The paper notes that the sky is likely brightening more quickly in developing countries, where satellite observations indicate the prevalence of artificial lighting is growing at a higher rate.
After devising a new method to convert these observations into estimates of the change in skyglow, the authors found that the loss of visible stars reported by Globe at Night indicates an increase in sky brightness of 9.6% per year over the past decade. This is much greater than the roughly 2% per year global increase in surface brightness measured by satellites.
“This shows that existing satellites aren't sufficient to study how Earth's night is changing,” said Kyba. “We've developed a way to ‘translate’ Globe at Night observations of star visibility made at different locations from year to year into continent-wide trends of sky brightness change. That shows that Globe at Night isn't just an interesting outreach activity, it's an essential measurement of one of Earth's environmental variables.”
Existing satellites are not well suited to measuring skyglow as it appears to humans, because there are no current instruments monitoring the whole Earth that can detect wavelengths shorter than 500 nanometers, which corresponds to the color cyan, or greenish blue. Shorter wavelengths, however, contribute disproportionately to skyglow, because they scatter more effectively in the atmosphere. White LEDs, now increasingly commonly used in high-efficiency outdoor lighting, have a peak in emission between 400 and 500 nanometers.
“Since human eyes are more sensitive to these shorter wavelengths at nighttime, LED lights have a strong effect on our perception of sky brightness,” said Kyba. “This could be one of the reasons behind the discrepancy between satellite measurements and the sky conditions reported by Globe at Night participants.”
Beyond wavelength differences, space-based instruments do not measure light emitted horizontally very well, such as from illuminated signs or windows, but these sources are significant contributors to skyglow as seen from the ground. Crowd-sourced observations will therefore always be invaluable for investigating the direct human effects of sky brightness.
“The increase in skyglow over the past decade underscores the importance of redoubling our efforts and developing new strategies to protect dark skies,” said Walker. “The Globe at Night dataset is indispensable in our ongoing evaluation of changes in skyglow, and we encourage everyone who can to get involved to help protect the starry night sky.”
More information
[1] From 2006 to 2010, Globe at Night data were collected based on a paper rather than an online form, so they were incompatible and were not included in this analysis.
This research was presented in a paper accepted for the journal Science (DOI: 10.1126/science.abq7781).
NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O'odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.
CosmoView Episodio 59: Stars D [VIDEO] | EurekAlert! Science News ReleasesJOURNAL
Science
METHOD OF RESEARCH
Observational study
Visibility of stars in the night sky declines faster than previously thought
This is shown by a study in Science based on a worldwide Citizen Science project on light pollution that has collected data over the last eleven years.
Peer-Reviewed PublicationIMAGE: LIGHT POLLUTION IMPACT 1 – FROM EXCELLENT DARK SKY (LEFT) TO INNER CITY SKY (RIGHT) view more
CREDIT: NOIRLAB/NSF/AURA, P. MARENFELD
People around the world are seeing fewer and fewer stars in the night sky. The change in star visibility can be explained by an increase in the sky brightness of 7-10 percent per year. The rate of change is faster than satellite measurements of artificial light emissions on Earth would at first suggest. This is the finding of a study published in the journal Science, conducted by a research group led by Christopher Kyba of the GFZ German Research Centre for Geosciences and the Ruhr-Universität Bochum with colleagues from the GFZ and the US National Science Foundation’s NOIRLab. They 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.
Light pollution background
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 NSF's NOIRLab since its inception, emphasizes. 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.
Need for suitable measurement methods
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.
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.
With Citizen Science...
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 contributions of individual people work together as if they were a global sensor network, making new science possible,” says Christopher Kyba from the GFZ German Research Centre for Geosciences in Potsdam and the Ruhr University Bochum. Together with his GFZ colleague Yigit Öner Altıntas and Constance E. Walker and Mark Newhouse from NOIRLab, he 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.
...to surprising findings
“The rate at which stars are becoming invisible to people in urban environments is dramatic,” sums 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.”
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).
Causes for the difference between measurements from Earth and from space
Christopher Kyba believes that the difference between human observation and satellite measurements is probably due to changes in lighting practices: "Satellites are most sensitive to light that is directed upwards towards the sky. But it is horizontally emitted light that accounts for most of the skyglow," Kyba explains. “So, if advertisements and facade lighting become more frequent, bigger or brighter, they could have a big impact on skyglow without making much of a difference on satellite imagery.”
Another factor the authors cite is the widespread 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 is more likely to be scattered in the atmosphere, so contributes more to skyglow,” Kyba says. “But the only satellites that can image the whole Earth at night are not sensitive in the wavelength range of blue light.”
Limits of the study and further potential
However, the Citizen Science approach also has its limitations. For example, the number of participants from different regions of the world determines the significance of spatial and temporal trends. So far people from North America and Europe have had the largest participation in the experiment, and half of the Asian contributions come from a single country: Japan. “The most data comes from the regions of Earth where skyglow is currently most prevalent. That’s useful, but it means that we can’t say much about skyglow change in regions with few observations,” Kyba emphasizes. Especially in developing countries, rapid changes in artificial skyglow are suspected, but there have been few observations so far.
Two conclusions: Lighting policy and Citizen Science
The researchers draw 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 brought about any improvement, at least on a continental level, despite growing awareness of light pollution.
“And on the other hand, we were able to demonstrate that the Citizen Science data represent an important supplement to the previous measurement methods,” Kyba emphasizes.
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 ongoing, so feel free to take a look tonight and let us know what you see!”
Further information on “Globe at Night” can be found here.
Light Pollution Impact 2 – From excellent dark sky (left) to inner city sky (right)
CREDIT
NOIRLab/NSF/AURA, P. Marenfeld
Astronaut photographs of parts of Calgary (Canada) show examples of how lighting changed from 2010-2021: New lighting has been installed and many streetlights have been converted from orange high pressure sodium to white LED. (Note: The photos are not taken with the same settings, and have different spatial resolution. Thus, the 2010 photo appears somewhat brighter.
CREDIT
Images courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center, georeferencing by GFZ Potsdam
JOURNAL
Science
METHOD OF RESEARCH
Data/statistical analysis
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022
ARTICLE PUBLICATION DATE
19-Jan-2023
The night sky is rapidly getting brighter, eroding star visibility worldwide
Observations from citizen scientists worldwide over the past 12 years reveal a worrisome trend – stars in the night sky are becoming more difficult to see due to rapidly growing light pollution. The change in their visibility as reported by citizen scientists is equivalent to a 9.6% per year annual increase in sky brightness, an increase faster than satellites have indicated, and which occurred despite policies to prevent increases in light pollution. To put this in perspective, the authors note that under such sky brightness change, a child born in an area where 250 stars were visible would likely see fewer than 100 stars in the same location 18 years later.
In many inhabited places on Earth, the night sky never fully grows dark. It instead glows with an artificial twilight caused by the scatter of anthropogenic light in the atmosphere. This type of light pollution, called skyglow, is responsible for the visible brightening of the night sky and the erosion of our ability to see stars. Although the ubiquity and luminance of skyglow have increased exponentially over much of the last century, its global change over time is not well understood. Satellites that can measure global skyglow are limited in resolution and sensitivity and are often blind to the wavelengths of light produced by modern LED lights that have come to dominate lighting over the past decade. To better understand how growing light pollution is affecting our view of the stars, Christopher Kyba and colleagues evaluated 51,351 citizen scientist observations of naked-eye stellar visibility from 2011-2022. To determine the background brightness of the night sky, Kyba et al. asked participants worldwide to compare star maps of the night sky at different levels of light pollution to what they could see with their own eyes through the online “Globe at Night” platform. According to the findings, the night sky has increased in brightness from artificial light by roughly 7 to 10% per year, which is equivalent to a doubling of the night sky’s brightness in less than eight years. This increase is much higher than estimates of the evolution of artificial light emissions (~2% yearly) based on radiance measurements taken by satellites. “Perhaps the most important message that the scientific community should glean from the Kyba et al. study is that light pollution is increasing, notwithstanding the countermeasures purportedly put into operation to limit it,” write Fabio Falchi and Salvador Bará in a related Perspective. “Awareness must greatly increase for artificial light at night to be perceived not as an always-positive thing, but as the pollutant it really is.”
For reporters interested in trends, several studies published in Science and Science Advances have reported the rapidly growing issue of light pollution and its environmental impact, including a Science study from March 1973 that discussed increases in outdoor illumination and the resultant impact on astronomy. A June 2016 Science Advances study by Fabio Falchi et al. presented a world atlas of artificial sky luminance that quantified the magnitude of artificial skyglow globally. In a November 2017 study also published in Science Advances, Kyba et al. used satellite data to show the expansion of Earth’s artificially lit outdoor surface area. Other recent studies have demonstrated the environmental risks associated with artificial lighting, including an August 2021 Science Advances study that demonstrated the detrimental impacts of street lighting on local insect populations and a September 2022 Science Advances study that highlighted the harmful effects of nighttime lighting to ecosystems across Europe.
JOURNAL
Science
ARTICLE TITLE
Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022
ARTICLE PUBLICATION DATE
20-Jan-2023
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