WASHINGTON - Earth Day has arrived, and so has the March for Science - a global event on six continents (and cheered on by scientists on a seventh, Antarctica). Thousands of people have gathered on grounds of the Washington Monument for a four-hour rally that will culminate in a march down Constitution Avenue to the foot of Capitol Hill.

"We are at a critical juncture. Science is under attack," said Cara Santa Maria, a science communicator who is one of several emcees of the rally. "The very idea of evidence and logic and reason is being threatened by individuals and interests with the power to do real harm.

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"We're gathered here today to fight for science," she went on as the crowd cheered." We're gathered to fight for education. To fight for knowledge. And to fight for Planet Earth."

She was followed by the musician Questlove, who said "many people" are refusing to follow scientific facts, and he gestured toward the White House. "That guy over there," he said in a whisper - organizers have said this is not a partisan event - and then he waved, said "Hi," and made a fast gesture with his middle finger that someone not paying close attention might well have missed.

YouTube star Tyler DeWitt soon took the stage with another pointed message: Experts need to learn how to explain things in a way regular folks can understand.

"Ditch the jargon!" he said. "Make it understandable. Make people care. Talk to them, not at them. We cannot complain about slashed funding if we can't tell taxpayers why science matters."

Out in the crowd, someone was carrying a sign certain to baffle anyone lacking an understanding of P Values and the null hypothesis. But most of the signs were more straightforward:

"In peer review we trust"

"The oceans are rising, and so are we"

"If you're not part of the solution, you're part of the precipitate"

As it happens, the National Math Festival is also in Washington - so there's an unusual number of people in town who can recite Pi to more than five digits.

Some signs took a shot at the current occupant of the White House:

"Hey Trump - Think You Can Stifle Science? Ask Galileo How That Worked Out!

"Empirical Data Trumps Imperial Alt-Facts"

Three federal scientists, approached by a reporter, refused to give their names for fear of repercussions at work.

Another person was dressed as the Muppet "Beaker" and when interviewed would say only "Meep."

Next to Beaker, however, was Erik Molvar, director of the Western Watersheds Project, who had travelled from Wyoming. Molvar is a sage grouse expert who studies the impact of livestock on grouse habitat; politicians supporting the livestock industry ignore his research into cheatgrass, which is highly flammable and leads to damaging wildfires, he said. "Livestock spread cheatgrass like mosquitos spread the zika virus," he said.

Emily Fink, 28, and Kayla Denson, 29, are biomedical researchers who drove seven hours from Buffalo, New York, to attend the march, and they said they fear the Trump administration's proposed budget cuts will imperil their careers.

"It feels like we're getting our foot in the door right as the door is closing on us," Fink said.

Fink brought several copies of her resume to the march and held up a neon sign that read, "are you looking for a highly motivated post doc? Ask for CV." She thought the march might be a good networking opportunity, though so far no one had asked for a resume.

"I'm a meteoriticist," said Conel Alexander, 56, as he wrote down his occupation to make sure he spelled it correctly. He studies meteorites, and was with a group from the Carnegie Institution for Science. "Most people just think I'm a meteorologist," he said.

Speaking of meteorology: Favorable weather forecasts took a turn by week's end and this turned out to be a soggy Saturday. Marchers gathered under gray skies and there was a forecast of periodic showers.

Maryam Zaringhalam and Kelly Fleming, both 28, came with poster board signs they'd made at an event Friday night with the group 500 Women Scientists. Zaringhalam, a molecular biologist, and Fleming, a chemical engineer, had been concerned about the way diversity issues were dealt with by the march organizing committee.

"But I thought, people are going to be taking pictures at the march and this is what I want them to see," Zaringhalam said. "I want them to see someone who looks like me."

Zaringhalam is an Iranian American who was in Iran when President Trump issued his executive order on immigration. Though she is an American citizen, Zaringhalam worried she would not be allowed back into the country.

At their sign-making event Friday night, a passerby had asked what the women were doing. When they told him, he responded, "You don't look like scientists."

"I think he thought he was flirting," Fleming said, making a face. Zaringhalam came up with the motto for her sign: "this is what a scientist looks like."

Carol Trosset, 57, an anthropologist at Carleton College, traveled to Washington from Northfield, Minnesota, having never been to a political rally before. She wore the lab coat - now quite the vintage item - that had belonged to her late mother, who had been a Ph.D chemist in the 1940s and 1950s.

"I thought, what should I wear? I'll wear mom's lab coat," Trosset said. Her mother was also a naturalist, collecting data at their home in Cincinnati recording when birds would arrive and flowers would bloom. Trosset has begun analyzing her mother's data and sees clear signs of a warming climate.

Brooke Hardesty, 16, waited nervously in front of the science tent. She was looking for the other "Science Teens," fellow high school students who are slated to speak at the rally. From far flung cities around the country -- Hardesty is from Buckeye, Arizona -- they'd previously communicated only through social media and Skype. On Saturday Hardesty met her fellow nerds for the first time in person.

"I've never been around so many other people who are excited about science," she said.

The teens, chatting awkwardly, discussed who they're most excited to see -- "Bill Nye!" they said in unison -- and talked about how surreal this experience has been.

"I'm kind of surprised they let a bunch of teenagers do this," said Sam Rosenberg, 16, of Poolesville, Maryland.

"We're even verified on Twitter!" Hardesty agreed.

The lineup for the rally includes some prominent names in science and environmentalism, including Nye, chief executive of the Planetary Society and an honorary co-chair of the march; climate scientist Michael Mann; and the coordinator of the first Earth Day, Denis Hayes.

But the organizers wanted to erase the stereotype of science as a stodgy enterprise dominated by older white men, and the lineup includes speakers from a broad range of ages, backgrounds and expertise. They include Taylor Richardson, a 13-year-old aspiring astronaut who raised $17,000 this year to send other girls to see the film "Hidden Figures"; chemist Mary Jo Ondrechen, a member of the Mohawk Nation and chair of the board of the American Indian Science and Engineering Society; and Gallaudet University biologist Caroline Solomon, who is deaf.

"We need more girls in computer science. We need more diversity in computer science," said speaker Kavya Kopparapu, 16, a student at Thomas Jefferson High School for Science and Technology in Virginia and founder of the Girls Computing League.

"In my future career I don't want to be known as a girl that happens to be a computer scientist. I want to be known as a computer scientist that happens to be a girl," she told the crowd.

No politicians were given speaking roles, though some reportedly planned to show up for the march. The march began as a notion batted around online on Reddit after the Women's March on Washington, which was held Jan. 21, the day after President Trump's inauguration. The idea snowballed, then gained additional mass after it was endorsed by virtually every mainstream science organization.

Some scientists in recent weeks have said they worried the march would politicize the broader scientific enterprise and signal an alignment with left-leaning ideologies. The march's website offered an answer to that concern: "In the face of an alarming trend toward discrediting scientific consensus and restricting scientific discovery, we might ask instead: can we afford not to speak out in its defense? There is no Planet B."

The Washington march may be the biggest gathering - organizers have a permit for up to 75,000 people - but there are similar events in more than 600 cities on six continents. Seven researchers in Antarctica went on Twitter to express their support for the march. Thousands of people gathered in Sydney, Australia, according to the Sydney Morning Herald. Science advocates went on social media to post bulletins from marches in Austria, England, Malawi, among other places.