Approximately 7,000 people gather on Independence Mall in Philadelphia on the first Earth Day--April 22, 1970.

Designer Harvey Hirsch, now based in New Jersey, was a student at New York's School of the Visual Arts when an ad agency approached his class for creative ideas to increase the impact of the first Earth Day.

"One of my ideas was to shut Fifth Avenue down to cars and trucks for the day," he recalls via email. "I was not sure they had the power to do that, but eventually they were able to close Fifth Avenue from Central Park to 23rd Street. ... That seemed to be the catalyst to making the Earth Day concept gain traction."