The stars are still there, we just can't see them anymore as artificial light is now changing that view at an alarming pace, creating a brightening night that future generations may barely recognize

A child looking up at the night sky today could see hundreds of stars from home. But if the current rate of artificial skyglow continues, that same view could look dramatically different by the time the child turns 18. The stars themselves will not disappear. Instead, increasing artificial light scattered through Earth’s atmosphere could make many of them impossible to see.

A 2023 study published in Science examined 51,351 naked-eye observations collected between 2011 and 2022 and found that sky brightness at participating locations was increasing by an average of 9.6 per cent each year. If that rate continued, a child who could see 250 stars today could see fewer than 100 from the same place at 18, as quoted in a report by ScienceDaily.

The figure is a projection based on the measured average, not a prediction for every town.

How did people measure the changing night sky?

The observations came from Globe at Night, a citizen-science programme run by NSF’s NOIRLab. Volunteers looked at a familiar constellation and chose the chart that most closely matched the faintest stars visible to them.

The approach effectively turned human observers into rough sensors for measuring changes in sky brightness.

As artificial light increases, it scatters through the atmosphere and brightens the dark background of the sky. Faint stars lose contrast against that glow and gradually disappear from view, even though their light is still reaching Earth.

Researchers filtered out observations affected by factors such as twilight, moonlight and snow before comparing the remaining reports with a global sky-brightness model.

The data came from 19,262 locations, although observations were not evenly spread around the world. Most came from North America and Europe. The study estimated annual brightening of 6.5 per cent in Europe and 10.4 per cent in North America, compared with 9.6 per cent across all included locations.

That means the 9.6 per cent figure represents an average for participating locations rather than a measurement covering every inhabited part of the planet.

Why did satellites see a slower change?

The ground-based observations showed a much faster deterioration in the number of stars people could see than an earlier satellite study suggested.

A 2017 Science Advances study using the VIIRS Day/Night Band instrument found that the area of Earth covered by artificial outdoor lighting expanded by about 2.2 per cent each year between 2012 and 2016. Total measured radiance increased by about 1.8 per cent annually.

The difference partly comes down to what the two methods actually measure.

The satellite instrument is relatively insensitive to wavelengths below 500 nanometres, including much of the blue light produced by white LEDs. It is also less capable of detecting light travelling sideways, which can spread through the atmosphere and contribute to skyglow far from the original source.

Blue light also scatters particularly efficiently in the atmosphere. This means changes in the colour and direction of outdoor lighting can have a major effect on what people see overhead without producing the same change in satellite measurements.

Satellites measure light leaving Earth in particular wavelengths and directions. Human observers, meanwhile, measure something much more direct: which stars remain visible through the glow.

How widespread is light pollution?

The problem was already extensive before the latest measurements showed the night sky brightening further.

The 2016 World Atlas of Artificial Night Sky Brightness combined satellite observations, ground measurements and modelling of how light travels through the atmosphere. Researchers estimated that more than 80 per cent of humanity lived beneath light-polluted skies.

The Milky Way was already hidden from more than one-third of the world’s population, including around 60 per cent of Europeans and nearly 80 per cent of North Americans.

Artificial light does not remain confined to cities. Light can scatter through the atmosphere and produce bright skyglow hundreds of kilometres from its source. Even places that appear dark at ground level can therefore have noticeably brighter skies overhead.

The loss of visible stars is also affected by the way human vision and astronomical brightness work. Star brightness follows a logarithmic magnitude scale, and many more stars exist toward the faint end of naked-eye visibility. Losing a relatively small amount of visibility can therefore remove a surprisingly large number of stars.

Could the rate of brightening change?

The 9.6 per cent annual increase describes the trend that best matched the observations gathered from 2011 to 2022. It does not mean every location becomes brighter by exactly that amount each year, nor does it guarantee that the same rate will continue after 2022.

Individual observations can be affected by weather, aerosols, eyesight and dark adaptation, as well as local lighting conditions. The large dataset and statistical modelling help identify a wider trend despite those differences, but they cannot eliminate every uncertainty.

Artificial skyglow is also not simply an unavoidable result of having lights at night. Its strength depends on how much light is used, where the light is directed, its colour and how long it remains switched on.

Shielding a light so it illuminates the ground rather than the sky addresses a different issue from simply replacing a bulb with a more efficient one.

The example of a child seeing 250 stars today and fewer than 100 at 18 is therefore an illustration of the measured trajectory, not a fixed prediction of what every child will experience. The stars will remain overhead. The question is whether future generations will still be able to see them.

The stars are not going anywhere, but the darkness that allows people to see them is changing. The 51,351 observations show how quickly artificial skyglow can alter the view from the ground, while the difference between human observations and satellite measurements highlights how much of the problem can be missed from orbit. For the next generation, the night sky may depend as much on decisions made on Earth as on what remains above it.

FAQs

Why are fewer stars visible?
Artificial light scatters through the atmosphere, making faint stars harder to see.

Will the stars actually disappear?
No. They remain overhead, but artificial skyglow can hide them from view.