Starting in New Zealand's remote Chatham Islands, thousands of cities, towns, and landmarks around the world will start to go dark for Earth Hour on Saturday evening.

Up to a billion people worldwide are expected to participate in this global voluntary blackout by switching off their lights from 8:30 p.m. to 9:30 p.m. local time.

The movement, sponsored by the conservation nonprofit WWF, is designed as a symbolic gesture in support of action against global warming.

Now in its third year, Earth Hour has been attracting some high-profile advocates.

United Nations Secretary-General Ban Ki-moon recently pledged his support for Earth Hour, saying it has the potential to be "the largest demonstration of public concern about climate change ever attempted."

Secretary-General Ban urged people to participate as a way of letting politicians know that they expect progress at the Global Climate Change Conference in Copenhagen, Denmark, in December, when world leaders will meet to draft a replacement for the Kyoto Protocol.

Other big names endorsing Earth Hour 2009 include actors Edward Norton and Cate Blanchett, musicians Alanis Morissette and Big Kenny, and the band Coldplay.

Landmarks at Night

Earth Hour began in Sydney, Australia, in 2007 with about two million participants.

By 2008 the event had spread to nearly 400 participating cities in 35 countries and 50 million participants. (See before-and-after pictures of Earth Hour 2008.)

As of press time, more than 2,800 cities, towns, and villages in 84 countries worldwide are expected to take part in Earth Hour 2009.

World landmarks such as the Empire State Building, the Las Vegas strip, the Eiffel Tower, Rio de Janiero's statue of "Christ the Redeemer," Athens's Acropolis, Egypt's Great Pyramids, and Rome's Colosseum will also slip temporarily into darkness.

"Sometimes it takes a while for a good idea to get out there, and this year we're really hitting our stride," said WWF spokesperson Leslie Aun.

Earth Hour: Energy Saver?

While Earth Hour is important as a symbolic gesture, it would be even more valuable if the energy savings of the event were known, said Mary-Elena Carr, associate director of the Columbia Climate Center in New York City.

"The issue is whether it goes beyond a 'really cool' event and leads to anything tangible," Carr said.

"If there was an idea of how much energy was being saved, people could take measures to lower their energy use in a systematic and practical way."

Unlike in previous years, WWF is not releasing energy-savings estimates for this year's Earth Hour.

"We think the value of Earth Hour is the lights going off," WWF's Aun said, "not the energy savings."