How Dark Is Chile's Night Sky? Measuring Light Pollution in the Coquimbo Region

Northern Chile has some of the clearest, darkest skies on Earth, and over the last half century they have made it one of the world's great centres of astronomy. The Coquimbo Region, a semi-desert strip between the Andes and the Pacific a few hundred kilometres north of Santiago, calls itself the Región Estrella, the Region of the Stars. Its mountaintops host the AURA-NOIRLab programmes, the Carnegie Institution's Las Campanas Observatory, ESO's La Silla Observatory and the future Giant Magellan Telescope, and stargazing tourism has become part of its economy.

But light pollution is growing almost everywhere, and fast. About 99% of people in the United States and Europe live under light-polluted nights, and more than a third of the world's population can no longer see the Milky Way. The spread of LED street lighting adds a new twist: where older sodium lamps glowed in a few narrow yellow-orange lines, LEDs give off a broad spectrum rich in blue and green light.

A research group on artificial light at night, working in the region since 2019 at about twenty sites, set out to measure how much of that has reached Chile's sky. Their first survey compared four places, from nearly untouched to fully urban.

How they measured

The group uses a Sky Quality Camera: a Canon digital SLR with a fish-eye lens that photographs the whole sky at once, with software that calibrates every pixel. Each picture gives two maps:

• Sky brightness, in astronomers' units of magnitudes per square arcsecond. The scale runs backwards: a higher number means a darker sky.
• Colour temperature, in kelvin. It tells natural light from artificial light, since the lamps on the horizon show up as warm, low-temperature spots.

They went out only on cloudless nights around new Moon, with the Moon well below the horizon, and then traced each bright patch on the horizon to the town or road producing it.

Four sites, from dark to bright

Fray Jorge National Park. A UNESCO Biosphere Reserve on the coast, famous for patches of temperate rainforest kept alive by sea fog in the middle of dry scrub, and since 2013 the first certified Starlight Reserve in South America. Its sky is still very close to pristine: the darkest point measured 22.25 and the zenith 22.05, and models suggest artificial light adds only about 4% to its overall brightness. That puts it among the darkest measured places on Earth. Yet three glows are already visible low on the horizon: La Serena–Coquimbo, the city of Ovalle, and Greater Valparaíso and Santiago, at least 300 km away. The authors note that nothing is yet being done to protect the park's night sky or monitor it.

Fray Jorge National Park: the photograph (left), with the Milky Way arching overhead, the sky-brightness map (centre) and the colour-temperature map (right). Image: Angeloni and others, CC BY 4.0.

Las Campanas Observatory. At about 2,300 m, founded in 1969 and home to the twin 6.5 m Magellan telescopes. Two glows stand out: La Serena–Coquimbo to the south-west and the town of Vallenar to the north. A row of fainter, warm sources on the western horizon traces the newly relit Ruta 5, the Pan-American Highway, which passes about 20 km away. Another study had estimated that the highway's lights were over half of the artificial light at the observatory; the camera, though, recorded about the same brightness toward the highway as in the opposite direction.

Las Campanas Observatory. The telescope dome in the lower left is the only obstruction. Image: Angeloni and others, CC BY 4.0.

Collowara Astrotourism Observatory. A visitor observatory near Andacollo, a mining town of about 11,000 people, about 2 km from both the town and an open-pit mine. Here the sky is visibly split in two. The western half, facing La Serena–Coquimbo and Andacollo, is about 70% brighter than the eastern, and about 130% brighter within 30 degrees of the horizon. Artificial light has made the zenith 54% brighter than natural and the sky as a whole 130% brighter, and it has wiped out most stars of the fifth magnitude and fainter, the dimmest the naked eye can see. The camera may even have caught the glow of San Juan and Mendoza, in Argentina, hundreds of kilometres away across the Andes, reflected off high clouds that night.

Collowara: the western half of the sky, toward the town, the mine and the coast, is much brighter than the eastern. Image: Angeloni and others, CC BY 4.0.

La Serena–Coquimbo. Chile's fourth-largest metropolitan area, with 448,784 people at the 2017 census, measured from the sports field of a school. The Milky Way is completely gone and even fairly bright stars are hard to see; the sky is so bright that the maps need a different colour scale. Between two visits half a year apart, in January and June, the sky's colour shifted: the cold, bluish light that had filled one part of the sky had spread over almost all of it, a colour uncomfortable to the naked eye in places. The authors blame the spread of white-blue LED lighting. Even the darkest point of this sky was brighter than the brightest point at any of the other sites.

La Serena–Coquimbo in June: bright everywhere, and bluish over most of the sky. Image: Angeloni and others, CC BY 4.0.

What the comparison shows

• A small town can light up a big sky. Collowara's sky is about as bright as that of Flagstaff, Arizona, a city of nearly 140,000 that in 2001 became the world's first Dark Sky Community. The authors take that both as praise for Flagstaff's long-standing lighting rules and as proof that protecting the sky need not hold back growth.
• La Serena–Coquimbo is the biggest offender everywhere. It is the strongest source of artificial light on the horizon at every site, from the Andes near the region's northern border to the coastal forest near its south.
• A Chilean city, a European sky. La Serena–Coquimbo's sky matches, within a few tenths of a magnitude, that of downtown Padova, a city of similar size in one of the most densely populated and light-polluted parts of Italy, and Padova's light is even warmer in colour. If Chile's skies are still dark overall, the authors conclude, that is thanks to a small, scattered population and its geography, not to any effective control of light pollution.

Watching the city every night

To follow the change, the group installed an automatic sky-brightness meter (a TESS-W photometer) at the AURA-NOIRLab headquarters in La Serena in March 2023. Over its first months it logged more than 115,000 readings. They fall into two clusters: clear nights (about 37% of the time) and overcast ones (40%), which are far brighter, because clouds reflect the city's light back down.

Zenith sky brightness over La Serena under all weather: one peak for clear nights, one for overcast ones. Image: Angeloni and others, CC BY 4.0.

With public funding, the group is building a network of about 40 such stations across the region, at observatories, nature reserves and rural schools. The goal, the authors write, is to protect a night sky that is natural and cultural heritage.

Sources

• Rodolfo Angeloni, Juan Pablo Uchima Tamayo, Marcelo Jaque Arancibia and others, "Towards a Spectro-Photometric Characterization of the Chilean Night Sky. A first quantitative assessment of ALAN across the Coquimbo Region," arXiv:2312.10542, licensed under CC BY 4.0. Changed: rewritten in hubnx's own words and shortened for a general reader, the methods and tables summarized; the figures are the authors', and the maps drawn over other publishers' base maps are left out. This page is shared under the same licence.