The lights of Dunedin spill across the harbour and reflect off the clouds above. Photo: Paul le Compte
If you're finding it hard to see the stars in the sky where you live - you're not alone.
Data collected by citizen scientists around the world over the past 12 years shows that the night sky is disappearing due to rapid increases in light pollution.
The data, reported today in the journal Science, indicates the change in visibility reported is equivalent to an average increase in sky brightness of 9.6 percent per year.
"The rate at which people are reporting that they see fewer stars was shockingly fast," said Dr Christopher Kyba, who led the study.
"[At that rate], if a child was born in a place where you could see 250 stars, by the time they're 18, in that place, you would only see 100."
Connie Walker, an astronomer and founding member of the Globe At Night program at NOIRLab in the US, said the finding indicated we were in danger of losing our night sky.
"If we lose the night sky, we lose part of our cultural heritage and we can't get it back if we're not good stewards," Walker said.
Light pollution also affects our environment and health.
"But the really good thing is that light pollution is one of the very easiest problems to redress," she said.
"Every single person on Earth can do something about it."
The power of people
Urban centres are surrounded by a halo produced by artificial light known as sky glow.
Satellites that whiz around our planet pick up specks of light coming from our growing towns and cities, but they can't measure the total amount of light pollution.
"Satellite data is lacking in very, very many ways," Walker said.
For a start they cannot pick up light in the blue wavelength that comes from modern LED lights.
They also only pick up light coming directly upwards, not sideways from billboards or buildings in general.
But humans can.
The Globe At Night program asks citizen scientists to count the number of stars they can see in constellations such as Orion and the Southern Cross throughout the year.
Kyba, from the Deutsches GeoForschungsZentrum Potsdam in Germany, worked with colleagues to analyse more than 51,000 observations taken on cloudless nights between 2011 and 2022.
Around 68 percent of the observations were from North America and Europe, with a one-off campaign in Australia, and a smattering of observations from Asia, Africa and South America.
Light pollution is a local phenomenon, and varies from the global average depending upon your specific location.
At a continental level, North America experienced an average increase in light pollution of 10 percent, while Europe experienced an average increase of 6.5 percent.
While there's not enough data for the other continents, Kyba said the study provided a big picture snapshot that represented changes across the globe.
Professor Fred Watson, Australia's astronomer-at-large, said the result was striking.
"It's surprising that that is so much more than what we see from the satellite images," said Watson, who is a dark-sky advocate but was not involved with this study.
Satellite images suggest light pollution is increasing on average by about 2 percent each year.
What's happening in Australia?
The bulk of the Australian data in the study comes from a Guinness World Record attempt to record the night sky in 2020.
More than 11,000 people - 9000 of them in Australia - recorded their observations of the Southern Cross in a single night.
"We broke the Globe at Night website," said Marnie Ogg, founder of the Australasian Dark Sky Alliance.
Although there is only data for 2020, Ogg said the light pollution in Australia was likely to be increasing in line with the figures in the Science study.
"We are so unaware of what's happening generally in Australia, because we are a dark, inky country," she said.
"We've had access to the night sky and we haven't seen the impacts greatly, we haven't had to do anything.
"But some of the statistics on urban light growth, in Victoria specifically, are massive," she said, adding that "around a third of the state is lit up now".
The problem with LED lights
Research into what is driving the increase in light pollution detected by citizen scientists is ongoing, but Kyba said there were two likely culprits.
One is the light being generated by sources such as advertising billboards; the other is the move towards using LED lights.
"A lot of cities are switching from the old orange, high pressure sodium lights to white LEDs," he said.
"When that happens, the satellite says the city got darker, but in fact, for humans, it may have even gotten brighter."
While cheaper and more energy efficient, Ogg said LED lights had made light pollution worse on the ground.
"Now we're putting in downward facing lights - yep, that's great - but at much higher intensity," she said.
"So the impacts are not so great in the sky, but they're very, very much happening on the ground now."
Professor Watson said blue light added to sky glow because it scattered more than other colours, even when the LED light was pointed downwards.
LED light also affected human night vision.
"It washes out the visibility of the stars because your eyes see more of the scattered light sensitive to the blue part of the spectrum," he said.
Spotting stars for science
The Globe At Night continues to collect data from citizen scientists around the world each month to monitor light pollution.
"We need measurements at the same location over many years," Walker said.
It was also important that more people outside of North America and Europe contributed to fill in the gaps.
"If we were to get ten times or 100 times as many people taking part, then instead of saying something about Europe and North America, we would start to be able to say something about individual countries," Kyba said.
Given there was already a chunk of 2020 data from Australia, it would not take long to build up a picture with more observations, he added.
This month, through Globe at Night, you can count the stars in the constellation of Orion.
- ABC
