The annual event is celebrated at the end of March close to the spring and autumn equinoxes in the northern and southern hemispheres respectively. During this time, both hemispheres experience sunset at about the same time, ensuring the best visual impact when the normally glimmering city skylines go dark.
The first Earth Hour was observed in 2007 in Sydney, at the urging of the Australian chapter of the World Wildlife Fund (WWF). That year over two million households and 2,100 businesses went dark for an hour. This small sacrifice helped save 10% of the electricity consumed by the city (the equivalent of carbon dioxide emitted by 48,000 cars) in a normal hour.
The impact made by this simple act spurred the world into action. In 2008, over 50 million people from over 5,000 cities joined in, and the numbers have only been growing since. Earth Hour is now the biggest voluntary environmental celebration in the world with entire city skylines going dark. Famous landmarks and buildings like New York's Empire State building,Toronto's CN Tower and Paris' Eiffel Tower also observe the event.