The night sky is disappearing, about ten percent a year

In 2023, a team led by Christopher Kyba published an analysis of more than 50,000 observations submitted by members of the public to the Globe at Night project, which asks people to compare the stars they can see with a set of reference charts. Between 2011 and 2022, the visible sky lost stars quickly. The researchers estimated that the brightness of the night sky was increasing by nearly ten percent per year.

Ten percent sounds moderate until it compounds. At 9.6 percent a year, sky brightness doubles about every seven and a half years and rises more than fivefold in eighteen. By the study’s estimate, a child born where 250 stars are visible would be able to see fewer than 100 by their eighteenth birthday. The sky of your childhood is not, in any literal sense, the sky you can show your children.

The figure is striking partly because instruments had missed it. Satellite measurements had suggested a slower increase, in part because many satellite sensors cannot detect the blue-rich light of modern white LEDs, which scatter strongly in the atmosphere. It took tens of thousands of people looking up and comparing what they saw to a chart. In an age of very expensive observatories, one of the most important measurements of the changing sky was made with unaided eyes.

A third of humanity

The Milky Way is already hidden from more than a third of humanity, according to the most recent world atlas of artificial sky brightness, including roughly 60 percent of Europeans and nearly 80 percent of North Americans. For most people who have ever lived, a sky crowded with stars was an ordinary feature of night. For most people alive now, it is something you travel to see.

For most people alive now, a sky crowded with stars is something you travel to see.

The efficiency trap

One irony in this story concerns the technology meant to help. LEDs use far less energy per unit of light than the sodium lamps they replaced, and they were widely adopted as a climate measure. But cheaper light tends to mean more light, a pattern economists have long recognized in other fuels. Whiter light also scatters more in the air. A city can cut its electricity bill and brighten its sky at the same time, and both numbers would be reported as outcomes of the same program.

Streaks in the data

Professional observatories are built far from cities, but they face a newer problem. Starlink alone now has more than 10,000 satellites in orbit, and other constellations are following. Sunlight glinting off them leaves bright streaks across long-exposure images. For wide-field survey telescopes such as the Vera C. Rubin Observatory, simulations with about 40,000 low-orbit satellites suggest that around 10 percent of images could carry at least one trail, and that most exposures near twilight could be affected.

Satellite operators have worked with astronomers on darker coatings and changes to how satellites are oriented, and the International Astronomical Union has set up a center to coordinate those efforts. Software can mask many streaks. But masking is a loss of data, the faintest trails are the hardest to remove, and radio astronomers face interference that is harder still. There is also no equivalent of a dark-sky preserve in orbit. A remote mountain can escape a city. Nowhere on Earth is out of sight of a satellite.

Not only astronomy

Artificial light at night affects much more than telescopes. Migrating birds are drawn off course by illuminated buildings. Insects are trapped circling streetlights, a contributing factor in their decline. Sea turtle hatchlings crawl toward bright roads instead of the ocean. Human sleep and hormone cycles respond to light, too.

The encouraging part is that light pollution is unusually easy to reduce. Unlike carbon dioxide, it does not accumulate. When a light goes off, its pollution ends at the speed of light. Shielded fixtures that point light downward, warmer color temperatures, lower brightness and switching lights off when no one needs them all work immediately, and save energy. Dozens of communities have been certified as dark sky places for doing exactly that.

What kind of loss this is

Astronomy will survive; it will move further away and spend more money. What is harder to replace is the ordinary experience of stepping outside and seeing the structure of the galaxy we live in. Careful looking at points of light gave us the size of the universe, and looking back from space gave us a sense of the planet. The night sky was the first scientific instrument, the place where people first noticed patterns, kept calendars and asked what the lights were. It would be a strange inheritance to lose it to lighting nobody needs.

Sources

• Kyba et al., “Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022”, Science, 2023
• Falchi et al., “The new world atlas of artificial night sky brightness”, Science Advances, 2016
• Rubin Observatory: Low Earth orbit satellites FAQ
• Globe at Night