Patrick Cicero's sign at the Tucson march and rally for science Saturday conveyed a message in four partial differential equations.

Cicero, an antenna engineer at Raytheon Missile Systems, said Maxwell's equations are the foundation of our knowledge of electro-magnetism.

They "make Tweets possible," read his sign. "Respect Science!"

Cicero was one of about 200 scientists and science-advocates who marched from Armory Park to El Presidio Park Sunday morning, where a crowd of about 2,500 assembled for an Earth Day Rally for Science — one of about 400 "sister" events to the Washington, D.C. march.

Other groups assembled at nearby parking lots to walk to the rally in groups.

A longer march had been called off when organizers couldn't raise funds to blockade city streets. When the "Women's March for Science" group decided to march from Armory Park anyhow, Tucson Police provided an escort that created a rolling closure of a route along Sixth Avenue and West Alameda Street.

At El Presidio Park, the scientists, many clad in lab coats that proclaimed "Science, not Silence," listened to music and speeches and crowded around 40 science-outreach booths, where their kids handled snakes, insects and desert tortoises.

Shady spots were at a premium as the temperature rose into the 90s at midday.

Snapping cellphone photos of each others' signs seemed to be the most popular activity. Among the messages:

"You are the result of 3.8 billion years of evolution. Act like it."

"There is no Planet B."

"Be like a proton: Stay positive."

"Grab 'em by the data."

"It's not magic; it's science."

"We are part of the solution."

"Got polio? Me neither. Thanks science."

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