Just as Sept. 11 was unthinkable, Sunday was inevitable: the 10th anniversary of a day that stands alone. In history. In memory.
Three-thousand six-hundred fifty-two days have now passed. At 8:46 a.m. - the time when the first plane slammed into 1 World Trade Center - 87,648 hours will have gone by. Another 5,258,880 minutes. Another 315,532,800 seconds.
Once more, the families gathered at ground zero, where 2,749 died, and in Washington and in Pennsylvania to pay tribute to the 224 who died there.
Once more, there was an outpouring of grief. Once more, there was the sound of bells tolling sadly. Once more, there were speeches. Once more, the names were recited.
Mayor Michael R. Bloomberg said that the attacks had turned "a perfect blue-sky morning" into "the blackest of nights."
He added, "We can never unsee what happened here."
President Obama read Psalm 46, which talks about God as "our refuge and strength, a very present help in trouble," and Gov. Andrew M. Cuomo of New York read from President Franklin D. Roosevelt's 1941 State of the Union address, the famous "four freedoms" speech - freedom of speech, freedom of religion, freedom from want and freedom from fear "anywhere in the world."
The 10th anniversary dawned on a city and a nation that has changed immutably, with continuing wars in Afghanistan and Iraq and persistent security worries at home. And no longer is ground zero a scarred reminder of what was, but a symbol of resurgence, with the National September 11 Memorial about to open and a not-yet-finished skyscraper. It now stands 961 feet above the street where thousands fell.
This Sept. 11 began with the towers that will take their place of the ones that were destroyed a decade ago illuminated in red, white and blue stripes.
What was then the site of the World Trade Center is surrounded by construction fences, and evidence of what happened is everywhere: There are flags on the new Tower One, the "Freedom Tower." The subway station nearby has exit sign that identify it as the "Rector Street 9/11 Memorial," with the "11" written to look like the twin towers.
Ten years ago, it was just another morning - a Tuesday, a day when ordinary people did the most ordinary of things: Scrambling to work, hurriedly kissing their families goodbye, running for the train. And then there was the dark gash and the ball of fire high up in one of the buildings, and a few minutes later, a second gash, a second ball of fire and a plume of smoke visible for miles.
On Sunday, President and Mrs. Obama arrived and shook hands with former President George W. Bush and Mrs. Bush, with state and city officials and with relatives of those who died. Then the President and the former president and their wives walked to the 30-foot waterfalls that are part of the new memorial. In the moments they stood there, the 16 big pumps sent 52,000 gallons of water flowing over the edge.
One measure of how Sept. 11 changed everything was how little grumbling there was last week as motorists waited to crawl through police checkpoints. Sept. 11 redefined the bridges and tunnels beyond those checkpoints as something that generations of commuters had never imagined: potential targets.
Sept. 11 redefined so much more.
Sept. 11 put New York, a city that had not faced combat in more than 200 years, on the front lines in a global war on terrorism. Sept. 11 made slogans created by Madison Avenue like "If you see something, say something" as widespread as "Loose lips sink ships" once was.
Sept. 11 brought color-coded threat levels (though the Department of Homeland Security, itself a post-Sept. 11 creation, phased them out several months ago).
Still travellers worry: Is it safe to fly? Since Sept. 11, airline passengers have had to pull off their shoes and empty their pockets, and they felt embarrassed when they forgot they had a too-big bottle of shampoo or mouthwash in their carry-on.
And still there were episodes when terrorists on international flights tried to set off plastic explosives hidden in their shoes or sewn into their underwear.
Is it safe to open the mail? A week after the Sept. 11 attacks, letters containing anthrax killed 5 people and infected 17 others. It took the F.B.I. five years to conclude that an Army microbiologist had been responsible. In the confusion at first, people hoarded antibiotics, and officials briefly grounded crop-dusting airplanes.
But this anniversary played out against a different backdrop than the first anniversary, in 2002, or the fifth, in 2006. For the first time, Osama bin Laden was dead. "We've taken the fight to Al Qaeda like never before," Mr. Obama declared Saturday in his weekly radio address.
For the first time, too, there was tangible progress toward fulfilling the promise to rebuild - a promise made in the aftermath of the attacks but delayed by squabbling over architects, plans and finances. Buildings are rising between Church and West Streets in Lower Manhattan, and the National September 11 Memorial will open to the public on Monday. Relatives of those who died at the World Trade Center will get a first look on Sunday.
If they were to measure it, they would see that the memorial covers about half of the 16-acre World Trade Center site. They will see that the names of the dead have been inscribed on the walls of two reflecting pools that now fill the footprints of the old towers - pools that hold 550,000 gallons of water and are lined with 3,968 panels of granite, each weighing 420 pounds. A museum is to open nearby next year. For the memorial and the museum together, the plans called for some 8,151 tons of steel and 49,900 cubic yards of concrete.
This time, there will be other reminders. The U.S.S. New York, commissioned in 2009 and made with seven-and-a-half tons of steel from the twin towers, spent the weekend at anchor in the Hudson River. On Sunday morning it was to cruise to Lower Manhattan, stopping within sight of the new tower at the trade center site.
Other ceremonies and services were planned. The New York City Fire Museum will honour the 343 fire-fighters who died with the dedication of the bunker coat and helmet that a Fire Department chaplain, Mychal Judge, was wearing on Sept. 11 when he died. Marble Collegiate Church in Manhattan will have a "trialogue," a three-way discussion with Shamsi Ali, the imam of the Islamic Cultural Center of New York; Rabbi Michael S. Friedman, the associate rabbi of Central Synagogue in Manhattan; and Michael B. Brown, the church's senior minister.
At night, an interfaith ceremony on the south side of Pier 40, a park at the west end of Houston Street, will be led by the Rev. Alfonso Wyatt, the vice president of the Fund for the City of New York.
The ceremony at ground zero brought together the officials who were in office 10 years ago - Mr. Bush, Gov. George E. Pataki of New York, Gov. Donald T. DiFrancesco of New Jersey and Mayor Rudolph W. Giuliani - with their successors: Mr. Obama, Gov. Andrew M. Cuomo of New York, Gov. Chris Christie of New Jersey and Mr. Bloomberg.
As at past observances, there will be music. The cellist Yo-Yo Ma, who performed at the one-year anniversary ceremony, played the slow Sarabande movement from Bach's Suite for Cello No. 1. James Taylor sang "You Can Close Your Eyes," and Paul Simon sang "The Sound of Silence."
The ceremony is to pause six times: twice to remember the planes that hit the towers, twice to remember when the towers collapsed, once for the attack on the Pentagon and once the plane that went down in a field in Pennsylvania. The first moment of silence was at 8:46 a.m., when American Airlines Flight 11 sliced into 1 World Trade Center - the north tower - 17 minutes before United Airlines Flight 175 hit the south tower.
And still what happened on that morning seems as impossible as it did in those first few minutes, when one friend called another and said something like, "Go turn on the television. A plane has crashed into the World Trade Center."
Or when, in the seconds before the picture came on, an anchor was heard saying something like, "Wait. These are live pictures, not the tape? So that was a different plane, and it hit the other one?"
Like the day when John F. Kennedy was assassinated in 1963, or the day when the space shuttle Challenger exploded in 1986 or the day when Pearl Harbour was attacked in 1941, Sept. 11 was one of those days that divided things into "before" and "after."
New Yorkers still talk about what a bright morning that was, after a thunder-and-lightning show in the sky the night before. They talk about how late-summer days are forever different. They talk about how the foreboding that has replaced the promise in the pink of the sunrise and so much joy in the deep blue of the midmorning sky.
And they talk about what the World Trade Center was, a city-within-the-city that dominated the skyline. Below 14th Street, it was a direction-finder as sure as the "N" on any compass. It had been bombed in 1993. The damage had been repaired, but the two buildings remained a target for Al Qaeda.