The first Earth Day proclamation is issued by Mayor of San Francisco Joseph Alioto (3/21/70)

The Earth Day most often celebrated in the United States—and in many other countries around the world—first took place on April 22, 1970. It was a nationwide teach-in about the environment, dreamed up by U.S. Senator Gaylord Nelson. A Democrat from Wisconsin, Senator Nelson had been instrumental earlier in introducing conservation in John F. Kennedy’s presidency. Gaylord Nelson’s Earth Day was modeled on the anti-war teach-in demonstrations that Vietnam war protesters had used successfully to educate people about their issues.

On the first Earth Day, more than 20 million people turned out at thousands of colleges, universities and communities all across America for an environmental teach-in day, which sparked a global environmental reawakening. More than half a billion people in 175 countries now celebrate Earth Day on April 22.