Earth Day is celebrated each year on April 22 as a way to inspire appreciation and awareness for the Earth’s environment. Started by Sen. Gaylord Nelson of Wisconsin as an environmental teach-in, what began as a national movement went international in 1990 and is now celebrated in 175 nations. The date corresponds with the first day of Spring in the Northern Hemisphere and the first day of Autumn in the Southern Hemisphere. More than 20 million people participated in the first Earth Day and now an estimated 500 million take part annually.