Where are all the stars?

As Earth Hour approaches, urban light pollution interrupts the visibility of stars.

HOPEFULLY, AS A CONSCIENTIOUS AG reader, you intend to celebrate Earth Hour. On 27 March at 8.30 p.m. local time, as many as 1 billion people in a thousand cities around the world will turn off their lights for an hour, continuing an annual event that had its origins in 2007 in Sydney.

Astronomers welcome the initiative of Earth Hour, because it draws attention to something that most of them feel passionate about. While they, too, worry about the carbon footprint of wasted light, and agonise over the plight of nocturnal animals confused by city lights, they also have a serious professional concern.

Our view of the Universe through optical (visible-light) telescopes depends on being able to detect faint images against a natural background luminosity of the sky itself. Even in the absence of artificial lighting, this sky-glow threatens to swamp the feeble celestial objects that astronomers are trying to investigate. Any kind of light pollution adds to this background, which is why the world’s largest telescopes are placed in remote locations.

You might be surprised to know, however, that city lights are effectively scattered by the Earth’s atmosphere across hundreds of kilometres. For example, astronomers using Australia’s largest optical telescope, the 3.9 m Anglo-Australian Telescope near Coonabarabran, NSW, are used to seeing the glow of Sydney, 350 km away, from the walkway around the dome.

This effect has been mapped on a worldwide scale by Pierantonio Cinzano, based at the University of Padua, Italy. Pier-antonio’s studies show that two-thirds of the world’s population live in light-polluted conditions, and about one-fifth can no longer see the Milky Way.

It’s also clear that light pollution is encroaching on many of the world’s major observatory sites. Remoteness is no longer enough. What’s ultimately needed is a reduction in urban waste-light, which can be achieved through better lighting design. As well as satisfying astronomers and reducing urban energy bills, this will have the far more important effect of reducing greenhouse-gas emissions on a scale vastly greater than Earth Hour.

Fred Watson is Astronomer-in-Charge of the Anglo-Australian Observatory near Coonabarabran, north-western NSW. Visit his website at www.fredwatson.com.au.