The Brightening Dark
Fifty-one thousand people counting stars from their own doorsteps found that the night sky is getting brighter almost five times as fast as satellites had measured. Most of the people on Earth can no longer see a truly dark sky.
The amount the night sky has brightened every year, going by how many fewer stars observers could see between 2011 and 2022. At that rate, sky brightness doubles about every 8 years. Kyba et al., Science, 2023 · 51,351 Globe at Night observations
Nine shades of night
Amateur astronomers grade the sky on the Bortle scale. A class 1 sky shows stars down to magnitude 8. In a city centre the faintest star you can see is about magnitude 4, so most of the sky's stars are gone.
Excellent dark site. Milky Way casts shadows
Truly dark site. Milky Way looks veined
Rural sky, with glow on the horizon
Rural to suburban. Light domes over towns
Suburban. Milky Way faint overhead
Bright suburban. Sky grey near zenith
Suburban to urban. Milky Way gone
City sky. Bright enough to read by
Inner city. Only the brightest stars and planets
Brighter than the satellites can see
Satellites put the rise in Earth's artificial light at about 2% a year. Observers on the ground found a 9.6% yearly rise. One likely reason for the gap is that orbiting sensors barely register the blue light from white LEDs.
Implied rise in sky brightness from 2011 to 2022 at 9.6% a year
Brighter by the time a child born under it turns 18, if the rate holds (calculated)
Who has lost the Milky Way
The 2016 World Atlas of artificial night sky brightness was the first to map skyglow over the whole planet. Losing the Milky Way is mostly a problem of the rich world.
People on Earth for whom the Milky Way is hidden. Among North Americans it is nearly 4 in 5.
People in the US and Europe who live under a sky with measurable light pollution
Singapore's population, which lives under skies too bright for eyes to fully adapt to night vision. The atlas rated it the most light-polluted country.
What the glow costs
Skyglow isn't only about stars. It wastes energy, and it pulls night-migrating birds into lit glass.
Birds killed at US buildings each year
Median estimate, with a range of 365 to 988 million. Building strikes are the second-largest direct human cause of US bird deaths, after cats. Loss et al., 2014
Fewer collisions with half the windows dark
That was the spring figure at Chicago's McCormick Place, and it was 6× in fall. Researchers estimate deaths there could drop 59%. Van Doren et al., 2021
Wasted on US outdoor light each year
At least 30% of US outdoor lighting is wasted, mostly by unshielded fixtures. That waste emits about 21 million tonnes of CO₂ a year. DarkSky International estimate
The fastest pollution to clean up
Light pollution stops the moment the light goes off. There is nothing to clean up afterwards.
Use fixtures that point all their light down. Light that goes sideways or up never reaches the ground you meant to light.
Pick bulbs of 3000K or lower. Blue-rich white light scatters more in the sky.
Use motion sensors and timers so lights are on only when someone is there.
Turn off unneeded building lights during the spring and fall migration peaks.
Submit an observation to Globe at Night. Data from volunteers like these produced the 9.6% figure.
Street lighting is a public budget line, and dark-sky ordinances can cut it.
For almost all of human history, half of every day came with a sky full of stars. Within one lifetime it has become something you travel to see.