Wasting light by sending it upwards is not just a problem for stargazers. Poorly designed public and private lighting uses several times more fossil-fuel-generated power and costs more than angled lights that focus on the ground.

Ahead of Earth Hour on March 28, when people are encouraged to turn off lights and appliances for an hour to support action against climate change, Australian astronomers are backing an international campaign to use lights more intelligently.

"There's an extraordinary amount of poor-quality lighting in Sydney - they're designed to look good in the day but they spill upward and waste a huge amount of power at night," said Nick Lomb, the curator of astronomy at Sydney Observatory. "A lot of the building floodlights spill in all directions, the billboards are lit up from below so the light just slides off, and the security lights radiate all over the place but still leave these dark shadows."

Of course, less light means the stars are more visible, too.

Dr Lomb said he was looking forward to Earth Hour, when a rare view of the planet Saturn, tilted so its rings are invisible, can be seen from the observatory in The Rocks.

Under normal conditions, the view of the planet would have been obscured by the beam of light that rises vertically from the arch of the Harbour Bridge.

"With the amount of upward spill [of light] that's around now, I'm worried that soon the Southern Cross will become a triangle as far as Australia's cities are concerned," said Professor Fred Watson, the head of the Anglo-Australian Observatory at Siding Spring, near Coonabarabran in NSW.

This year had been designated an International Year of Astronomy, and a campaign against "light pollution" had made some inroads into building planning approvals in Australia and overseas, Professor Watson said.

Sydney was still a relatively dark city by world standards, but it had growing islands of brightness around the central business district and suburban hubs such as North Sydney and Parramatta, he said. The sky was noticeably darker in Pittwater than in Chatswood.

WWF Australia, which is organising Earth Hour with the support of the Herald's publisher, Fairfax Media, wants "light pollution" addressed as a way of reducing power consumption and therefore greenhouse gas emissions.

"For the hour of Earth Hour and the 365 days that follow, we need people to be aware and take action to reduce their emissions," said WWF Australia's chief executive, Greg Bourne.