On that day — the Sept. 11 that requires no year — the sun set on crushed buildings in a reimagined world. It set on a recontoured skyline and a haunted city. The equations of life no longer worked. That’s the way it seemed.

A decade now since the tall towers fell in New York and the Pentagon was gashed open and a diverted plane dropped into a field near Shanksville, Pa., people know where they were when they heard the unheard-of.

They were a school bus driver grabbing a coffee and a doughnut at a Dunkin’ Donuts in Biddeford, Me. They were driving to work in Pasadena, Md., sitting at a red light on Crain Highway, the radio humming. They were setting up for another busy lunch at a restaurant in Panama City, Fla.

They were on the way to teach music in Spring Park, Minn. They were getting a car loan in Butte, Mont., chewing over the rate. They were about to fill a lower-molar cavity in Kew Gardens, Queens.

People repeated the same thing: My life will be changed forever.

It was called the saddest day in American history. It was called the worst day in American history.

The memories remain fresh and overwhelming. The trembling ground, the wall of smoke that shut off the sun, the choking dust, the ghastliness of the jumping people — the grievous loss of life and the epic acts of heroism. Exhausted phone lines that wouldn’t connect to those who might have answers. People listening to car radios, reports of more planes in the sky, fears of more killers to come.

Also, the aching days and weeks and months after.

In Lower Manhattan, cordoned off with sawhorses for blocks around the smoldering World Trade Center, the odious scent that persisted for months and wafted through the city. Was it burning tires? Unsettled souls?

Residents moving about in dust masks. The rats dislodged from their homes. The flatbed trucks and garbage trucks panting back and forth, loading the seemingly limitless detritus.

People buying parachutes and canoes, a way to get out the next time. Buying bulletproof vests and ammunition. The prolonged hunt for remains. Funeral after funeral.

And Gary Condit and Chandra Levy and the past tumble of news excised from the nation’s front pages, because the news — all the news — was 9/11, everything twisting and turning out of that day.

The attacks unhinged the lives of families — the fathers, mothers, husbands, wives, children of the nearly 3,000 people who did not return home. There was also more nuanced, distanced loss: A man lost two former Navy shipmates from back in the day. A man in England lost two online Scrabble partners.

Paul Simon said he didn’t know if he could ever complete another album. A woman wrote on a remembrance site that she regretted that she had had children, that she had brought their innocence into a world no longer fathomable to her.

But there has been a chasm between expectations and reality. The prophecy of more attacks on the United States has not been the case, not yet at least. Bumbling attempts got close — involving underwear and a shoe and a 1993 Nissan Pathfinder — but the actuality has been that terrorist acts on American soil in the succeeding years have been, as always, largely homegrown.

So many things were expected to be different that have not been. Time passes, and passes some more. Exigencies of living hammer away impatiently. People — most of them, at least — began to become themselves. New York, which by its nature accommodates so much, was willing to absorb 9/11 and keep moving.

Already we have fifth graders not yet born on that day. The people known as “Wall Street,” celebrated as martyrs and heroes in the days after the attacks, have been vilified for boundless greed. We are back as a nation of ideological divides and uncivilized political intransigence. Bridges fall, roads crack.

What has stuck? Shedding shoes and getting patted down at the airport. Navigating barriers to enter big buildings — smile for the camera. Every so often, the police rummaging through selected bags at the subway station. All this information being collected on who we are and what we do, snooping that is more accepted than objected to. A nagging suspicion of Muslims. A pair of distant wars that refuse easy endings, with a price tag of $1.3 trillion and climbing. The certainty that any full reckoning must include the cost of shortchanging the country’s future.

An underlying sense of the sinister out there somewhere. See something, say something.

The killing of Osama bin Laden has not closed the book. Nor has 10 years.

Yet a lot crowds into 3,600 days in a speeded-up, interwoven world. For most people, the influence of 9/11 on day-to-day life is felt much less intensely than the arrival of Facebook and Twitter. Or the eruption of nagging, pontificating voices on cable TV. Or the suffocating recession.

Ultimately, each person attaches an individual meaning to 9/11, if possible. Outside of the families of the victims, most people’s lives may not present themselves as remarkably different. But there is residue, lingering wisps of Sept. 11.

A Birthday to Hate

Angela Landon, sitting in her house in Bangor, Me., that day, feeding a bottle to her 10-month-old. Pregnant with the third of what would be four children, all girls, and her mother calling. The terrible news. On her birthday.

“My mother used to call me every year and say, ‘What a beautiful day to be born,’ ” she said. “After 9/11, what was I supposed to say? ‘It’s a beautiful day to die’?”

She hated her birthday for a while. A tiny price in the scheme of a wicked day, but a price paid.

The year after, Ms. Landon had no appetite for celebration, but her family insisted, even dressed up as the Wiggles, the Australian children’s entertainers. “They tried to make me feel good,” she said. “But I didn’t feel good.”

She doesn’t hate her birthday anymore. Last year, on her 40th, it was everyone out to Chuck E. Cheese’s, and a merry time. But she gets emotional. “Ten years later, it’s really hard,” she said. “My little ones don’t understand yet. They know my birthday is 9/11, and they know something happened on 9/11 and I’ve explained it, but they don’t get it.”

For the first couple of years, whenever she had to display her ID to cash a check or give her birth date over the phone to the bank, people would suck in their breath. That doesn’t happen now. “It’s kind of out of sight, out of mind,” she said. “And that bothers me.”

Her oldest daughter, Erika, 17, got on the phone, and she said it was a hard story to tell and a hard story to hear. What did Sept. 11 mean to her?

“I grew up in a proud-to-be-American household,” she said. “So I love my country. It was scary. I remember getting off the bus and my mother running and hugging me. My friends don’t talk about it now. It’s not a big ordeal here. But it’s always my mother’s birthday, so it’s always, always there.”

Civic Life Even Nastier

People shake their heads when they think back.

Jonathan Zimmerman, a professor of education and history at New York University, said: “I remember people saying, ‘We’re all going to be New Yorkers.’ People said we’re all going to be serious. That’s hilarious to talk about. Reality TV was in its infancy. There was no ‘Jersey Shore.’ Imagine if it did spawn a new seriousness.”

He said what we all see: “Civic life is even more frayed, even more polarized, even nastier.”

Dalton Conley, dean for the social sciences at N.Y.U., said, “I think the ironic thing is, the area less affected in terms of daily life and fundamental change is actually New York City, the epicenter of the event. Our own university expected we would fall off the map.”

Did he know anyone who had taken the narrative of that day and done something really bold, gone the distance? He said he did. The father of a friend of his daughter, a Wall Street man who went to war.

Crossing the Line

Out of the shock and the ruin, Gerard Decatrel tried to imagine New York’s tomorrow, the twists it might take, because there had to be something transformative. Imagination, in those days, could take you a lot of places.

He worked at Morgan Stanley in Times Square, a trader of foreign exchange options. He was 30. He lived in Manhattan, a family man.

As he constructed outcomes, he decided there were some he could accept and those he couldn’t.

“I drew a line,” he said. “I could bear it if New York became like Jerusalem and there were conventional attacks going on all the time. But if there were any biological or chemical attacks, I said I would join the military.”

He couldn’t entirely explain the impulse. He didn’t know anyone who had perished in the towers. Taking up arms would mean entering an alternate space, leaving behind a wife, a 4-year-old daughter, a 1-year-old son.

“I don’t know, but I took it personally,” he said. “I’d been a New Yorker all my life.”

That fall, the mysterious anthrax attacks visited the wrung-out and quaking city. There it was. His line had been crossed. He joined the Marines. Morgan Stanley said it understood; go, and his job would be waiting for his safe return. His wife said all right. He didn’t know then that she was humoring him. She thought they would reject him because he was too old.

He had to commit to training and six years in the service, eight years of his life altogether. He moved to Virginia, Florida, California.

And then, indeed, Iraq for three seven-month deployments as a pilot of a Cobra attack helicopter. He flew more than 500 missions. He shot at the enemy and the enemy shot back, but “they weren’t very good shots,” he said. “And they didn’t have the best weapons.”

The weather, he felt, was the biggest danger, the blinding sandstorms that could reduce visibility to zero and yet you flew, flew on hope. He felt old — almost everyone else was so young. Two pilots in his squad were killed.

But he made the sort of permanent friends you made in no other context.

He was discharged from the Marines last September. He is back in his city. He works again for Morgan Stanley, trading options once more, battling the cranky markets.

He had done something. He had served. Everything adds up to something. He would say he was different.

“I feel I have more confidence and a different perspective,” he said. “Something goes wrong in the market and everyone’s freaking out. Well, I’m not. No one’s dying. The market can’t freak me out.”

Reshaping History

George W. Bush understood 9/11 as a declaration of war. To others, it was an immense hate crime. Either way, it catapulted the country into what seems a permanent state of war.

David Blight, a history professor at Yale University, observed that an event’s meaning is always made by the subsequent history. “That’s how memory works,” he said. “Memory is always about the present.”

He added: “That innocence that we live above history, that we’re not vulnerable, that we control our own fate, got a big, big hit. I think this still lingers. But I think we are pleasantly recycling that.”

Which is not to say the day didn’t leave obligations, impose debts some people felt they had to pay.

Opening Tiny Doors

The sorrow needed to go somewhere. There were places to receive it, online receptacles, and the flutter of contributions arrived from all over. On Sept. 19, 2001, one came from Colleen Casey of Bolingbrook, Ill. She expressed the accepted condition that many people felt: “I do not expect my life to ever be as it was before.”

She offered up a poem, “I Needed the Quiet,” that she had discovered when she was 14 and her father died of a heart attack. It helped her; maybe it could help others.

And she wrote, “I will try to live my life better.”

Americans had died by going to work. She felt she had to earn their sacrifice.

Now Ms. Casey lives in Addison, Ill., a materials license reviewer for the Nuclear Regulatory Commission. Same job, new home. She is 54 and single.

Had she lived her life “better?”

You don’t remold yourself easily. She knew that. But there were tiny doors that she could open. She was shy. But she began to do more, not be moony about her own troubles, go to places she hadn’t gone, feed the fires.

She mentioned participating in a diabetes walk, another for a homeless shelter, one for suicide prevention.

She started doing water aerobics, wanting to improve her health.

“I’ve tried to spend more time listening, really listening to people I come across in daily life,” she said. “People need to be validated and heard.”

She has had her scrapes with adversity — two bad car crashes, her Subarus totaled. She bought a third Subaru. One had been white, one blue and now she was on red, completing the colors of her country’s flag.

She tries to be a little kinder. Now she gives money to those professing need, like men she spots at roadway intersections. The ones squatting there with the hand-scribbled signs: “Homeless” or “Help.” It was just something she got in her head to do. She always has a spare $20 and bottled water in the car to hand over.

“Although some of my friends think I’m nuts for doing this,” she said, “I’ve never, ever had any kind of adverse outcome. Just gratitude.”

She said: “We’re all trying to slog through life together. I’m trying to do a little more. That’s all I can do.”

‘We’re Wired to Cope’

The day burrowed into the mind, and who knew how deep and how long it would stay. But deep and long. That’s what so many accepted. People slack on couches, struggling to push the ache out of them.

But the lasting psychological toll, studies suggested, was nowhere as bad as many experts predicted. The preponderance of people, they got on.

“I think we are innately resilient,” said George A. Bonanno, professor of clinical psychology at Columbia University, who studies grief and trauma. “The norm is to be resilient.”

In part, this is because we get so much practice, from less singular but still powerful traumas like divorce or disease. “We’re wired to cope with traumatic events,” he said.

So 33 Chilean miners buried deep in the ground can come out of it with their sanity. People can watch a mudslide scoop up their home. They can see tall towers fall and keep going.

“Human history is full of tragedy, and within these tragedies there is room for growth,” said Grady Bray, a disaster psychologist based in Texas. “There is no growth in human beings without struggle. I’m convinced of that.”

A 5-Year-Old’s Premonition

Sasha Vaccaro finished cooking camp — shish kebab today, lots of fun — and was free for the afternoon. He slid into a seat at the Starbucks across the street from his Upper East Side home, sipped Passion Tea Lemonade. He was dressed cool, in a T-shirt and shorts. He is 15.

Sasha has a complicated life. He suffers from depression that can be crippling. He has been given diagnoses of aspects of Asperger syndrome, attention deficit hyperactivity disorder and obsessive-compulsive disorder. His younger brother is autistic. His parents are divorced.

He explained some things he copes with. “If I touch my body on one side, I have to touch it on the other,” he said. “If I have an itch, I have to scratch on the other side too. But I’ve gotten better at it. I try to ride out the wave.”

He overreacts to criticism. When he hears sad things, he gets very sad.

His Sept. 11 was this. He was in kindergarten four blocks from the trade center, playing the tambourine in music class. His father clutched him in his arms, carrying him away as the second plane of the suicidal fanatics sliced into the flank of the tower. He watched both buildings aflame. His father cried; so did he.

“Before, I thought the world was perfect and everyone was nice,” he said. “It’s when I stopped believing in God.”

The World Trade towers had been of outsize importance to the family. They used to go down there and lie flat on their backs, their feet grazing the base of a tower, and look up at the majestic presence.

What does 9/11 do to someone a decade later? Everyone has complications, and how do you filter them out and assign one cause to one effect? How would you ever do it in a boy with so much going on?

Year after year, Sasha didn’t talk about 9/11.

Then in March, he wrote a graphic novel to satisfy a school assignment to relate a pivotal moment. It was his 9/11 day, from morning pancakes to music class to calamity and tears. And also his sixth-sense moment: At school he had a premonition that something awful was imminent in the towers. He looked toward them and said to his father, “Daddy, twin tower alert! Twin tower alert!”

In getting it out, he thought, maybe he was finally confronting what no child should have to see. The graphic novel got a good grade. The class was absorbed.

On all fronts, he has been doing better of late. Therapy has helped his mass of issues. His medications are being cut back. His last school year was his best. He wants to be a neurosurgeon or a veterinarian. He had a gecko once.

He has not been back to ground zero. Maybe when it’s done, he’ll go see how it came out.

Down at the site, things were happening, 3,000 workers a day — the equal of the dead — belatedly putting together the replacements for the vanished buildings. A sandwich-board peddler promoted $22-per-ounce cash for unwanted sterling silver. The tourists jostled past, peering through the fence, watching steel sprout on a land of ghosts.

That day was 10 years ago, and one day it will be 20 years ago and 50 and 100, sinking further into history.

What does 9/11 mean?

Sasha wanted to think on that for a moment. His face tightened into deep thought. “I honestly don’t know,” he said. “I can’t understand why people would do that. I don’t know what to say. It’s just sadness. That’s all it will ever be. Lots and lots and lots of sadness.”