Not only is the starry night sky a thing of great beauty and cultural significance worldwide, but also life on Earth evolved under conditions that saw sunlit days and dark nights, illuminated only by gentle light from the stars and moon. Today, the night skies are seldom dark, but instead are lit with the glow of scattered anthropogenic light that pollutes the atmosphere. And, according to the results of a new study, the stars are rapidly becoming invisible to the human eye due to the presence of this artificial light.
The brightening of the night sky in this way is known as skyglow, and it has increased exponentially over much of the past century. However, it is difficult to measure the extent of this anthropogenic light accurately because the satellites that record the radiance globally are insensitive to wavelengths of light given off by modern LED lighting fixtures. This led Christopher Kyba from the Deutsches GeoForschungsZentrum, Potsdam, and colleagues from the National Optical-Infrared Astronomy Research Laboratory in Tucson, Arizona, to take a different approach to quantifying the changes in night sky brightness over the past decade.
The researchers used the online platform of “Globe at Night” to ask citizen scientists from around the world to assess the appearance of the night sky in their location. The participants were provided with a set of star maps at different levels of light pollution, and asked to select the one that best matched the night sky they could see. This method relied on the naked-eye observations of stars between 2011 and 2022, and quantified stellar visibility to observers on 51,351 different occasions.
This approach used the human visual system as a direct sensor, and produced an estimate of the naked eye limiting magnitude (NELM), the visual apparent size of the faintest star that can be seen. This measure is related to skyglow because, as background light levels increase, faint stars become invisible to the naked eye.
The findings of the research, published in the journal Science, indicate that the night skies are rapidly getting brighter due to artificial light. According to the researchers, the change in the number of visible stars reported by Globe at Night participants is equivalent to a 9.6 percent per year annual increase in sky brightness, averaged over the locations of participants.
This means that the sky’s brightness would double in less than eight years in these locations, and that a child born in an area with 250 visible stars, would be able to see only 100 of those stars when he or she turned 18 years. This increase is much higher than estimates of the evolution of artificial light emissions (around 2 percent yearly) based on radiance measurements taken by satellites.
Unfortunately, the only satellite instruments that currently monitor sky brightness over the whole Earth have limited resolution and sensitivity, and cannot detect light with wavelengths below 500 nm – modern LED lights usually have an emission peak between 400 and 500 nm. The satellites also do not detect light emissions that are horizontal, such as those emanating from windows or signage, which also contributes to skyglow.
There are effective methods for reducing light pollution, and many of them also reduce the consumption of electricity. Although they have been implemented to some extent in some cities and local areas, they have not seen widespread acceptance or use to date. However, people are becoming increasingly aware of the potential negative effects of light pollution and may influence policy makers to implement control measures in future.
Increased artificial lighting has been shown to affect several biological and physiological processes in animals and plants. For example, a 2021 study in Science Advances demonstrated the detrimental impacts of street lighting on local insect populations, and another in 2022 highlighted the harmful effects of nighttime lighting on ecosystems across Europe. In addition to its environmental consequences, skyglow limits human observation of starry skies and the Milky Way, and has changed the appearance of the night skies in inhabited regions all over the world.
The researchers acknowledge that their dataset derived from citizen scientists is biased towards observers in Europe and North America, and most of the data comes from inhabited urban areas. They fear that the problem of increasing skyglow will be even greater in developing countries, where the observers are few and far between.
In a related Perspective commentary on the study by Kyba and colleagues, Fabio Falchi and Salvador Bará say, “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. 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.”
“The visibility of stars is deteriorating rapidly, despite (or perhaps because of) the introduction of LEDs in outdoor lighting applications,” wrote the researchers. “Existing lighting policies are not preventing increases in skyglow, at least on continental and global scales. The loss of the starry night is an unprecedented loss for all cultures.”
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