On April 22 each year, people around the world get together to celebrate nature, get educated on environmental issues and learn how to live greener lives. Earth Day began with a bang in 1970 as a nationwide protest and has since evolved into a global phenomenon, with thousands of events and initiatives engaging millions of people and spurring additional action.

Why was this date chosen, and what kind of impact has Earth Day had on environmental legislation and grassroots initiatives? These 18 Earth Day facts shed some light on the annual tradition and how it has changed over the past four decades.

1. Earth Day first began on April 22, 1970, when 20 million people participated in rallies across the United States, celebrating nature and decrying activities that put it at risk.

Gaylord Nelson2. Sen. Gaylord Nelson (at right), a Democrat from Wisconsin, came up with the idea for Earth Day in 1969. Inspired by the anti-Vietnam War "teach-ins" that took place at college campuses all over the nation, Nelson envisioned a large-scale environmental demonstration that would catch the attention of the federal government.

3. Nelson was later awarded the Presidential Medal of Freedom for his role as founder of Earth Day. Along with Harvard student Denis Hayes, Nelson went on to found the Earth Day Network.

4. In New York, then Mayor John Lindsay shut down part of Fifth Avenue to speak at a rally, and in Washington, D.C., Congress went into recess so its members could talk to their constituents about the environment at Earth Day events.

5. Earth Day had an immediate impact. By the end of the year, the United States saw some of its first major political efforts to protect the environment, including the founding of the U.S. Environmental Protection Agency (EPA).

6. Within five years, the EPA had banned the insecticide DDT  and Congress passed the Safe Drinking Water Act and set emissions and efficiency standards for vehicles.

7. The first Earth Day also changed public attitudes. According to the EPA, "Public opinion polls indicate that a permanent change in national priorities followed Earth Day 1970. When polled in May 1971, 25 percent of the U.S. public declared protecting the environment to be an important goal, a 2,500 percent increase over 1969."

8. By 1990, Earth Day was celebrated across the globe by 10 times as many people — 200 million.

9. There are actually two Earth Days. The second is the Spring Equinox Earth Day, which originated in San Francisco. Conservationist John McConnell chose March 21 because he felt it represented equilibrium and balance. McConnell founded the Earth Society Foundation, which organizes this event.

10. The Spring Equinox Earth Day Event is still held annually. Ever since the United Nations signed the Earth Day Proclamation written by McConnell in 1970, the Earth Society Foundation has rung the U.N. Peace Bell at U.N. Headquarters in New York to mark the occasion.

11. San Francisco's role in Earth Day is particularly fitting, given the origins of its name. The city is named after Saint Francis, who was the patron saint of ecology.

12. In 2009, the United Nations designated April 22 to be International Mother Earth Day.

13. Among people who oppose environmental action, a rumor has spread that April 22 was chosen because it's the birthday of Vladimir Lenin, the founder of the Soviet Union. In 2004, Capitalism Magazine posited that environmentalists share Lenin's goal of destroying private property.

14. In reality, the date was chosen in 1970 simply because it fell on a Wednesday, when organizers believed that many people would be able to get out of work to participate.

15. Many cities have turned Earth Day into a week-long celebration with events that educate children about the environment and encourage greater participation by the community in local green causes.

16. The Earth Day Network works with hundreds of thousands of schools around the globe, helping to integrate environmental themes into the curriculum to ensure that Earth Day has a year-round, lasting impact.

17. More than 94 million people have pledged "Acts of Green" through the Earth Day Network, sharing how they plan to make a difference for the environment.

18. By 2010, Earth Day's 40th anniversary, more than 1 billion people in more than 180 countries around the world were estimated to have celebrated, whether by attending events or simply spreading the word on Facebook.

Know of other Earth Day facts? Leave us a note in the comments below.

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