One-fifth of us have lost sight of Milky Way

by Heather Catchpole

SYDNEY: Light pollution has caused one-fifth of the world's population – mostly in mainland Europe, Britain and the U.S. – to lose their ability to see the Milky Way in the night sky.

"The arc of the Milky Way seen from a truly dark location is part of our planet's natural heritage," said Connie Walker, and astronomer from the U.S. National Optical Astronomy Observatory in Tucson, Arizona.

Yet "more than one fifth of the world population, two thirds of the U.S. population and one half of the European Union population have already lost naked eye visibility of the Milky Way."

Star-free night

The phenomenon, caused by the reflection of manmade light by the Earth's atmosphere, impacts astronomical research and can even affect human health, warned Walker, who will present her research on Wednesday at a meeting of the American Astronomical Society in Pasadena, California.

The effects of light pollution on human health can be as mild as the disruption of the circadian rhythm leading to problems sleeping, but it can also be serious, she said.

One study of 147 Israeli communities, published in 2008 in the journal Chronobiology International, found some evidence for an increased risk of breast cancer for women living in areas with the most light pollution. This is thought to be due to unnatural light at night affecting levels of hormones such as melatonin and estrogen.

Light pollution comes in a variety of forms such as 'over illumination', 'light trespass' and 'sky glow' – the orange glow that hangs over cities and is produced by upwards directed light.

Walker's research has found that cities using light fixtures that direct just 3% of their light upwards can almost double the sky glow experienced by astronomical observatories 100 km away. "Allowing 10% direct uplight increases this figure to 570%," said Walker, who is chair of the U.S. Dark Skies Working Group, part of the Dark Skies Awareness program, a global citizen science effort to raise awareness of light pollution.

GLOBE at night

"The point of raising awareness of light pollution is that it touches many areas of people's lives, from simply not being able to see the natural heritage of a starry night sky to affecting... the habits of animals, energy consumption, economic resources, and astronomical research," she said.

One project called GLOBE at Night, teaches members of the public "to record the brightness of the night sky by matching its appearance toward the constellation Orion with star maps of progressively fainter stars," said Walker.

These measurements are then submitted online and are used to create global maps of levels of light pollution. Over the last four years, the annual, two-week long GLOBE at Night events have resulted in 35,000 measurements contributed from over 100 countries.

Data from this project and others allowed Walker to estimate how much of the world's population is still able to see the Milky Way on a clear night.

John Norris, an astronomer from the Australian National University's (ANU) Mount Stromlo and Siding Spring Observatories said light pollution was less of an issue in Australia, where cities are widely distributed.

But he said emission lines from the mercury and sodium in fluorescent or sodium streetlights can still create background light interference for astronomers observing at optical wavelengths.

"To get the same quality of data [as from a non light-affected area] I would have to observe longer to be able to subtract the background light pollution from the light of the star," said Norris, who added that astronomers "jealously seek to guard the darkness" of observatories.

"At the Anglo-Australian Observatory and ANU we seek to have agreements with government and local councils. If people want to build something that is going to produce light pollution they have to [first] seek approval and meet certain requirements," he said. "It's a win/win situation because it is more energy efficient to have a downwards-facing lamp rather than lighting up the sky."