This generation will stop seeing the stars: light pollution grows 10% a year

Sirius, Vega, Arthur, Canopus and perhaps two of the three that make up the Alpha Centauri constellation.

Those will be the only stars most northern hemisphere city dwellers will see before the end of this century.

At least, those are the calculations made by the scientists who, with the help of thousands of people who look at the sky each night, have measured the night brightness generated by artificial lights: it has not stopped increasing in the last decade until darkening the night. celestial dome.

The problem of light pollution has not stopped growing since astronomers had to leave the cities to see the stars, already in the last century.

But its magnitude has grown exponentially so far this year.

A work published in 2016 estimated that 83% of the world's population has polluted night skies.

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 generated by artificial lights was growing at a rate of 2.2 % year.

But things are much worse.

"A child born in an area where 250 stars are visible will only be able to see 100 when they turn 18"

Christopher Kyba, researcher at the German Research Center for Geosciences in Potsdam

The artificial night brightness of the Earth would be increasing by 9.6% each year for at least a decade, according to research published today in

.

That means almost five times the figures obtained so far.

To put it in perspective, Kyba, also lead author of this new work, states that "at this rate of change, a child born in an area where 250 stars are visible will only be able to see 100 by the time they are 18 years old."

And if it reached 80, "perhaps only five of the brightest stars would still be visible," adds the German researcher.

There is a lot of difference between the 2.2% increase in the study of 2017 and the 9.6% of the current job.

It's not that one of the numbers is wrong, both are right, but they were obtained by very different paths.

Until now, almost all research on light pollution on a global scale has relied on data from satellites.

There are some, such as the Suomi NPP of the NASA space agency and NOAA meteorological agency (both from the US), dedicated to measuring the artificial light emitted from Earth.

It might seem that these artifacts, from above, capture the night glow very well, but they don't.

On the one hand, radiometers or satellite cameras do not register well horizontal light pollution, the light emitted by facades, shop windows or advertising signs that also obscures the view of the sky.

More important is what Alejandro Sánchez de Miguel, a researcher at the Faculty of Physical Sciences at the Complutense University, 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 is precisely the blue LEDs that are leading the great transition in outdoor lighting, with LED technology replacing amber or yellow sodium lamps.

Blue is also, as Sánchez de Miguel, who has not participated in the present work by Kyba, adds,

On the left, an image of eastern Calgary, Canada, taken by astronauts on the International Space Station in 2010. On the right, the same area in 2021. The change from amber to blue is due to the deployment of outdoor lighting with technology LED.NASA Johnson Space Center and GFZ Potsdam

The human factor is the second major contribution (after the number of night brightness) of the new study published in

.

This time, the calculations have not been based on what the satellites saw from above, but on what humans saw from below.

In fact, the results 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, promoted by the National Science Foundation (NSF), an agency United States government.

Participants had to look at the sky and choose from a series of seven star maps, the one that best fit what they saw.

Thus, thousands of records were collected.

"Individual observations are not very precise, but the power of the method comes from the combination of thousands of them, since the mean of all those imprecise observations is actually very stable," Kyba says.

The other advantage of this method is that it is based on human perception.

“Imagine that with an instrumental observation I find that the red component of the sky brightness has decreased by 70%, but the blue component has increased by 30% [which is happening with the LED transition].

Has the situation improved or worsened?

It is very difficult to be sure with the instrument.

With 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 come 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.

“Looking at images from the International Space Station of Earth's night hemisphere, people are left in awe of the beauty of the city lights.

They do not realize that 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).

About this study, he recalls that "the human eye is not better, it simply considers the exact pass band of its sensitivity to communicate the perceived brightness to the brain."

The passband or 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 hiding the visibility of the stars, the eye is a good sensor and satellites do not have this pass band," he adds.

Falchi, who published a paper a month ago on the impact of night glare on the world's telescopes, has published, together with his colleague from the Department of Applied Physics at the University of Santiago de Compostela, Salvador Bará, a commentary in Science on the work of Kyba.

In one of his paragraphs he says: “Looking at the images and videos of the International Space Station from the night hemisphere of the Earth, people generally marvel at the

of the city lights, as if they were the lights of a Christmas tree. Christmas.

They do not realize that they are images of contamination.”

And they add: “It is like admiring the beauty of the rainbow colors that gasoline creates in the water and not realizing that it is chemical contamination.”

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