Observed for the first time on April 22, 1970, Earth Day was created by environmental activists as an unofficial holiday on which people could reflect on the planet's ecology and engage in pro-environmental activities. The cause was taken up by U.S. senator Gaylord Nelson (1916–) of Wisconsin, whose support helped make Earth Day a reality. It is estimated that some twenty million people across the United States participated in the first Earth Day through demonstrations, exhibits, and teach-ins in more than ten thousand communities.

Images of the Earth as a fragile, blue-green ball hanging in dark space were just beginning to filter into the public's consciousness, especially after the first moon landing in 1969. That year also saw two environmental disasters that captured public attention: the Santa Barbara, California, oil spill and a fire in the Cuyahoga River in Cleveland, Ohio. These graphic portraits helped stimulate support for Earth...