Millions of homes and businesses around the world will be in the dark tonight as part of Earth Hour, an annual event meant to raise awareness about climate change and the environment.

The popular event, now in its seventh year, encourages individuals and organizations around the world to turn off all of their non-essential lights for one hour. This year, it’s scheduled to start at 8:30 p.m. during the participants’ local time.

More than 7,000 cities and towns in 152 countries and territories took part in Earth Hour last year. The campaign broke records to become the largest voluntary action for the environment, and seems to continue to grow with each passing year.

Whether you’ve participated in Earth Hour before or are thinking about taking part for the first time, here are a few things you might not have known about the campaign.

Creating awareness about climate change

The idea for Earth Hour was first conceived by the World Wildlife Federation in Australia in 2005 in response to scientific data revealing the devastating impact of climate change. The environmental advocacy group teamed up with advertising agency Leo Burnett and came up with the idea of a large-scale 'lights-out' event. Back then, the project was called "The Big Flick."

It was born in Sydney, but soon went international

The inaugural Earth Hour was held in Sydney, Australia, in 2007 at 7:30 p.m. About 2.2. million people and 2,100 businesses across the city participated. It was deemed such a success that there were plans to make Earth Hour a national event in Australia. Those plans were quickly scrapped once organizers realized how much international interest the campaign had drummed up. Earth Hour went global the following year with 371 cities and towns in more than 35 countries taking part.

Held in March to coincide with the equinoxes

Earth Hour is held every year in late March, around the time of the Spring and Autumn equinoxes in the northern and southern hemispheres. This is when sunset times are almost the same in both hemispheres. Organizers say this ensures the greatest visual impact for a global 'lights out' event.

Major landmarks also ‘go dark’ for Earth Hour

Major landmarks around the world have taken part in Earth Hour, including the Sydney Opera House, the Great Pyramids, the Empire State Building and the Las Vegas Strip. Even the so-called City of Lights has gone dark, with the Eiffel Tower shutting off its power.  

Lights on Toronto's CN Tower were turned off during Earth Hour's first international campaign in 2008, as thousands of people across the city joined in.  

Not to be left behind, websites have also found ways to take part in Earth Hour. The Google homepage in many countries has 'gone dark' by switching its usually-white background to black.