Earth Day was founded after an accumulation of high-profile pollution incidents, such as the Santa Barbara oil spill, led to increased public concern about the environment in the late 1960s. On April 22, 1970, Sen. Gaylord Nelson led a national teach-in on the environment, known today as Earth Day.

Teach-ins were held on college and university campuses in the ’60s to address various subjects of interest and importance, said Keith Woodhouse, director of the Environmental Policy and Culture Program at Northwestern University, in a phone interview. Nelson’s national teach-in was a demonstration held in multiple cities across the nation against the degradation of the environment.

Nelson left it up to towns, community groups, schools and individuals to decide what their activities would focus on for the first Earth Day, said Woodhouse. An estimated 20 million Americans participated in some activity on April 22, 1970, and this accomplishment led Earth Day to become an annual event. 

Although the purpose of Earth Day has shifted over time, it has maintained its core purpose as a celebration of the Earth and a call for action against the pollution and degradation of the environment, first in the United States and later on an international scale, Woodhouse said.