Sirius, Vega, Arthur, Canopus and maybe two of the three that make up the Alpha Centauri constellation. These will be the only stars most Northern Hemisphere city dwellers will see before the end of this century. At least, that’s the calculations of the scientists who, with the help of thousands of people gazing up at the sky each night, have measured the nighttime brightness produced by artificial lights: it has steadily increased over the past decade up to the blackout night sky dome.
The problem of light pollution has not stopped growing since astronomers had to leave cities to see the stars as early as the last century. But its scale has grown exponentially so far this year. A paper published in 2016 estimates that 83% of the world’s population has polluted the night sky. The following year, another study led by Christopher Kyba, a researcher at the German Research Center for Geosciences in Potsdam, confirmed that Earth’s nights were getting brighter: the glow produced by artificial light was increasing by 2.2% annually. But things are much worse.
“A child born in an area where 250 stars are visible can only see 100 at 18.”
Christopher Kyba, research associate at the German Research Center for Geosciences in Potsdam
According to a study published today in Science, Earth’s artificial nighttime brightness would increase by 9.6% every year for at least a decade. This means almost five times the previously determined numbers. To put it in perspective, Kyba, also lead author of this new paper, explains that “at that rate of change, a child born in an area where 250 stars are visible can only see 100 by the time they are 18 years old.” And at 80 “perhaps only five of the brightest stars can still be seen,” adds the German researcher.
There’s a big difference between the 2.2% increase in the 2017 study and the 9.6% increase in the current job. It’s not that either number is wrong, both are correct, but they were obtained in very different ways. To date, almost all light pollution research on a global scale has relied on data from satellites. There are some, like the NASA space agency’s Suomi NPP and the NOAA meteorological agency (both from the US) that are dedicated to measuring the artificial light emitted by the earth. It might appear that these artifacts do a very good job of catching the night light from above, but they don’t.
On the one hand, radiometers or satellite cameras do not register horizontal light pollution, the light from facades, shop windows or advertising signs, which also obstructs the view of the sky. More importantly, Alejandro Sánchez de Miguel, a researcher at Complutense University’s Faculty of Physical Sciences, recalls: “These satellites are focused on the infrared and the red and green parts of the spectrum. In the blue they are simply blind. And it’s the blue LEDs that are spearheading the big shift in outdoor lighting, with LED technology replacing amber or amber sodium lamps. Blue is also, adds Sánchez de Miguel, who was not involved in Kyba’s current work, “the one that best spreads across the sky and to which the human eye is most sensitive at night”.
Left, an image of eastern Calgary, Canada, taken by astronauts on the International Space Station in 2010. Right, the same area in 2021. The change from amber to blue is due to the use of outdoor LED technology lighting .NASA Johnson Space Center and GFZ Potsdam
The human factor is the second major contributor (after the nighttime brightness number) of the new study, published in Science. This time the calculations are not based on what the satellites saw from above, but on what humans saw from below. In fact, the findings of Kyba’s work are supported by the observations of more than 51,000 observations made by people around the world who installed an app from the Globe at Night project funded by the National Science Foundation (NSF), an agency of the US government. Participants had to look up at the sky and choose from a series of seven star charts the one that best matched what they were seeing. Thousands of records were thus collected.
“Individual observations are not very accurate, but the strength of the method comes from combining thousands of them, because the mean of all those imprecise observations is actually very stable,” says Kyba. The other advantage of this method is that it is based on human perception. “Imagine that during an instrumental observation I find that the red part of the sky brightness has decreased by 70%, but the blue part has increased by 30%. [lo que está sucediendo con la transición LED]. Has the situation improved or worsened? It is very difficult to be sure with the instrument. Through human observations, we know immediately what it means for humans,” adds the German scientist.
The work has a weak point, which is also the same human factor. The vast majority of observations are from North America, Europe and East Asia (Japan and South Korea). That is, the conclusions of the work would only be valid for these areas. In the rest of the planet, we should continue to trust what the satellites are recording.
“When viewing images of the International Space Station from Earth’s night hemisphere, people are struck by the beauty of the city lights. They don’t realize they are images of contamination.”
Fabio Falchi, researcher at the Istituto di Scienza e Tecnologia dell’Inquinamento Luminoso (Italy)
Fabio Falchi is a researcher at the Istituto di Scienza e Tecnologia dell’Inquinamento Luminoso (Italy). Referring to this study, he recalls that “the human eye is no better, it simply takes into account the precise passband of its sensitivity to transmit perceived brightness to the brain.” The passband or passband (passband) refers to the range of frequencies that can pass through a filter, in this case the human eye. “If we want to study the effect of light pollution on obscuring star visibility, the eye is a good sensor, and satellites don’t have that passband,” he adds.
Falchi, who a month ago published a paper on the effects of night glare on the world’s telescopes, together with his colleague from the Department of Applied Physics at the University of Santiago de Compostela, Salvador Bará, published a commentary in Science on the work by Kyba. In one of his paragraphs he says: “When you look at the images and videos of the International Space Station from Earth’s nighttime hemisphere, people generally marvel at the beauty of the city lights as if they were the lights of a Christmas tree. Christmas. They don’t realize they’re images of contamination.” And they add, “It’s like admiring the beauty of the rainbow colors that gasoline creates in water and not realizing it’s a chemical contaminant.”
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