Capital Weather Gang's Angela Fritz explains how math is used to determine the timing and location of an eclipse, such as the one that crossed the U.S. on Aug. 21. (Claritza Jimenez,Daron Taylor,Angela Fritz/The Washington Post)

CARBONDALE, Ill. — The warning signs hang above hundreds of miles of highway, flashing the same message from Illinois to Tennessee: “SOLAR ECLIPSE. AUGUST 21. PLAN AHEAD.”

On Monday, the moon will pass in front of the sun and cast a shadow over a 70-mile-wide cross-section of the continent known as the “path of totality.” It will be the country’s first total solar eclipse in nearly 40 years, and an estimated 12 million people are expected to witness it. That estimate may well be low.

A good chunk of those people will watch from somewhere along Interstate 24. It’s a smooth, straight highway that cuts across the American heartland, passing cornfields, churches, Chik-fil-As and dozens of billboards bearing stern instructions not to leave your car to look at the sun.

This is the road to totality. And already, eclipse chasers are congregating here, ready for the moon’s shadow to fall on them.

Rose Gilbert arrived days ago. It took the Columbia, Md., resident 11 hours to drive herself, her husband, three of their daughters and Gilbert’s octogenarian parents to Nashville, where they’ve rented a house with a view of a lake and a wide-open stretch of sky.

“Suppose it’s cloudy?” asks her father, Carl Landi. He’s been skeptical about this whole endeavor since Gilbert first proposed it more than a year ago. (“Had it been up to me, I probably wouldn’t be here,” he confides privately.)

“Then we’ll get in the car and drive,” Gilbert replies, not missing a beat.

She and her husband, John, wouldn’t call themselves astronomy buffs. She’s a nurse; he’s a physician assistant. They don’t own telescopes or plan their vacations around celestial events.

But an eclipse is different, Rose says. “It’s two whole minutes of the sun being blocked.”

“That’s a once-in-a-lifetime experience for most people,” says John.

“It’s a no-brainer,” she responds.

Capital Weather Gang's Angela Fritz breaks down what will happen when a total solar eclipse crosses the U.S. on Aug. 21. (Claritza Jimenez,Daron Taylor,Angela Fritz/The Washington Post)

So here they are, the whole family. Their cameras are outfitted with solar filters. Their eclipse glasses are NASA-certified — Gilbert double-checked. Even Landi is grudgingly looking forward to the event. T-minus three days and ready to go.

Signs of the coming spectacle are evident to those who look. There’s an unusual abundance of out-of-state plates in Midwestern towns that rarely get tourists. Restaurants have announced economically awkward Monday afternoon closings between noon and 3 p.m. The billboard outside the Park Avenue Baptist Church in Paducah, Ky., asks: “We live on a planet that circles the sun and you don’t believe in miracles?”

Locals compare the eclipse mania to a fever. It started almost imperceptibly — a date on the calendar, a one-minute preview on the nightly news. Then came the special sections of the newspaper, the cartons full of cardboard solar glasses in every storefront and posters of the sun in every window. The obsession grew and grew. Now the whole region is half-delirious.

“I’ve heard some pretty apocalyptic-sounding things,” said Melanie Cochran, of Nashville. “Cellphones dying. Power lines overloaded. They say you should get all your grocery shopping done now, in case the stores run out of food.”

“Pfft,” Demeka Fritts, also of Nashville, lets out an exasperated breath. “Every newscast is eclipse and politics.”

Don’t be fooled by her tone: Fritts long ago made plans to watch the event from her sister’s rooftop. It’s been a while since she spent time gazing at the sky. The 38-year-old used to love looking at the stars, but now her job keeps her busy, and the lights in Nashville are too bright to see much. On Monday, she’ll stop and look up again. The whole country will. “It’s kind of cool,” she says.

Businesses are closing for the big event. Schools are sending their students home early — or asking them not to come in at all.

Shelley and John Henry Wells, of San Francisco, were supposed to be at an artist’s conference in the Smoky Mountains next week. A few months ago, they found out that the organizers had canceled all of Monday’s events; instead, attendees will be given a bagged lunch and a seat on a bus to a viewing location near Hendersonville, N.C.

Meanwhile, anyone who can turn the eclipse into a marketing opportunity has done so. The Warby Parker hipster eyewear chain is handing out branded solar glasses. A billboard for Harrah’s Casino in Metropolis, Ill., promises a $100,000 eclipse giveaway.

At the National Quilt Museum in Paducah, curators have pulled out of storage an award-winning 1989 work called “Corona II” — a fabric and thread depiction of the sun’s outer atmosphere as seen during an eclipse. On Friday, at least two dozen visitors came into the museum specifically to see it.

“It’s just magical,” said Laura Hendrickson, the museum’s registrar, her gaze tracing the quilt’s stunning, swirling design. “That’s the only way to describe it.”

That this quilt — the only one like it in the country — happens to hang within the path of totality seems a stunning cosmic coincidence. (Then again, the path of totality also encompasses “Carhenge.”)

Hendrickson confessed that she harbors a secret hope that something special will happen during Monday’s event. She’s a “megafan” of the TV show “Heroes,” in which characters gain superpowers from watching a total solar eclipse.

“The nerd in me is like, what if the eclipse happens and someone can fly?” Hendrickson laughed.

“I’ll be right here,” she said of her plans for the eclipse. Watching through solar glasses decorated with an image of “Corona II.” Waiting for something magical to occur.

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