It began with an oil spill, a plane ride, and a 53-year-old politician from Wisconsin.

In September 1969, U.S. Senator Gaylord Nelson had just visited the site of a major spill off the coast of Santa Barbara. As he boarded a flight out of the area, the longtime environmental champion began to wonder if the type of grassroots protests inspired by the Vietnam War could send a message about the dire straits of the country's land, air, water, and public health. A few days later, he publicly floated the notion of a national campus teach-in on the environment, and met with an immediate and positive response.

DID YOU KNOW?
Earth Day founder Gaylord Nelson, a senator from Wisconsin, also authored legislation that preserved the Appalachian Trail corridor and created the National Trails System.

Over the next seven months, an event began to take shape that would inspire 20 million participants across the country, from suburban mothers to inner-city activists, from college students to union members. Nelson hired Denis Hayes, a 25-year-old grad student, to oversee the core planning and publicity with the help of a small staff, but much of the hyper-local event "organized itself," as the senator put it. This so-called "Earth Day" was a national uprising that sent a serious message to the federal government, and led to landmark legislation during the 1970s including the Clean Air Act, Clean Water Act, and Endangered Species Act.

Earth Day came to symbolize the birth of the modern environmental movement. It was an enormous success. And its organizers, fearing a massive anti-climax, hoped it would never happen again.

"We tried to get people not to do it [the next year]," says Hayes. "We couldn't stomp the life out of it."

The event has only continued to grow. Today, a billion people celebrate Earth Day each April in more than 180 countries, making it the world's largest secular holiday. "The success of Earth Day is proved by the fact that it's hard to imagine a time when our society didn't...celebrate Earth Day," says writer and activist Bill McKibben. "But it was only 40 years ago that it seemed a completely novel idea to think about the planet in that way."

This "novel idea" has taken on a certain comforting familiarity. Each year, the Washington, D.C.-based Earth Day Network (EDN) develops a theme, makes educational materials available to schools, and organizes national events. At the same time, and true to the day's origins, locally organized events sprout across the country. Hayes, who remains honorary chair of EDN, describes it as "a combination of a holiday and a recruiting device. It focuses the attention of tens of millions of students on the environment. It's an opportunity to get people around the world thinking of the same issues and thinking of one another." This year, to mark the fortieth anniversary, EDN is focusing on climate change, hoping to inspire a "billion acts of green" - and throwing a whopper of a party, of course.

It's become a defining environmental event, but Earth Day is not without its critics. Some say the holiday's feel-good aspect and focus on individual actions has made it irrelevant, a fiesta of drum circles and face painting that belies its gritty, political origins. Others level a graver accusation: that Earth Day has sold out, with corporations like Coca-Cola and Wal-Mart increasingly using the occasion and the brand to tout their own greenness and encourage consumers to buy their way to sustainability. (Attention shoppers: It's now possible to buy an "Every Day Is Earth Day" page-a-day calendar and a Mickey Mouse Earth Day tote bag, among other items.)

LEARN MORE
This year's Earth Day will be held on April 22, 2010. A list of events can be found at www.earthday.net. Celebrate Earth Day during AMC's Earth Day Welcome Weekend at the Highland Center. Enjoy outdoor programs while learning about environmental stewardship and conservation For details, visit www.outdoors.org.

Critics cry greenwashing, but Hayes says it's not that simple. "The movement started as an effort to enlist the government to constrain corporate behavior," he says. "Today, the government is not a hugely reliably ally anymore. Now we're just as likely to be working with thoughtful corporations to move the government in a more productive direction." His point is underscored by this description on EDN's website: "Earth Day 2010 is a pivotal opportunity for individuals, corporations, and governments to join together and create a global green economy."

Those new alliances are not the only sign of changing times; new forms of media and communication are also putting a fresh spin on Earth Day. Organizers in New York City, for example, have designed a social-networking campaign called "b the E" that asks people to submit photos or videos of themselves flashing a three-fingered E and an explanation of what they care about. "It's an opportunity to get people of all ages and backgrounds to say, 'Hey, this is what I'm doing to be involved,'" says Elizabeth Broad, deputy director of Earth Day New York. "It celebrates the holistic nature of the movement."

That would likely please Nelson, who participated in Earth Day until his death in 2005. In one reflection on the day's evolution, he issued the following admonition: "We are not free to decide about whether or not our environment 'matters.' It does matter, apart from any political exigencies. We disregard the needs of our ecosystem at our mortal peril. That was the great lesson of Earth Day. It must never be forgotten."