An estimated twenty million Americans took part in the first Earth Day on April 22, 1970. Virtually every community from Maine to California hosted activities. Congress adjourned for the day. All the television networks gave it significant coverage. In New York, hundreds of thousands of people jammed Fifth Avenue from Fourteenth Street all the way to Central Park to listen to politicians, scientists, and celebrities. In San Jose, California, college students held a funeral for the internal combustion engine, and buried a new car.

An Earth flag is being held by a member of the crowd at the Capitol Building in Washington D.C., for Earth Day 1990. (© Todd A. Gipstein/Corbis. Reproduced by permission.)

Earth Day arrived at the close of the 1960s—a time of cultural and political turmoil. At its core was a growing recognition that unconstrained growth could produce a legacy of poisoned streams, filthy...