The location was in the shade cast by the Survivor Tree, an oak that was recently planted at the World Trade Centre for a second time. The first time was in the 1970s, but the tree was later engulfed in rubble on 11 September 2001.
Remarkably, it was found alive though badly damaged, then nursed back to health and finally replanted at its old home last December. It now stands 9 metres (30ft) tall.
Close to the oak stood Payton Wall. She was four years old when her father, Glen Wall, died in the Twin Towers. Now 14, she wrote a letter to the president describing how she coped with that loss. By happenstance, Obama read the letter on Monday, the morning after he had orchestrated the killing of the architect of 9/11, Osama bin Laden.
It happened under the same cloudless New York sky that had famously been a feature of 9/11 itself. On that day, almost 10 years ago, the beauty of the crystal clear blue sky seemed to mock the terrible events that were to unfold beneath it.
But on this occasion, with the knowledge that 9/11's architect had been confined to a watery grave, the beauty of the day seemed more in tune with events. Before laying the wreath, Obama walked through the memorial plaza that is now taking shape at the heart of Ground Zero. He saw the two giant footprints of the Twin Towers that form the physical and aesthetic heart of the site, which will become reflective pools and the largest manmade waterfalls in America. In the past week the first of the 2,976 names of those who died in New York, Washington and Pennsylvania have appeared, etched in bronze plates that have just been set out along the pools' edges.
In time, when the memorial area is complete, the sound of falling water and rustling leaves of the Survivor Tree and some 400 other oaks will suffuse the space that once was filled with 1.8m tonnes of rubble and molten steel beneath a mushroom cloud of dust.
Before arriving at Ground Zero, Obama paid homage to the 343 New York firefighters who lost their lives when the two towers collapsed. He visited the Pride of Midtown, a firehouse in Manhattan which lost an entire shift – 15 men – on that day, leaving 28 children fatherless.
He told the surviving rescue workers that he hoped the killing of Bin Laden had brought them some comfort. "Obviously you can't bring back the friends you lost," he said. "But what happened on Sunday sent a message around the world that when we say we will never forget, we mean what we say.
"Our commitment to make sure that justice was done was something that transcends politics. It doesn't matter which administration it was, who was in charge, we were going to make sure that the perpetrators received justice. That's some comfort, I hope."
Later, he visited a police precinct in lower Manhattan, along with New York's 9/11 mayor, Rudy Giuliani. Obama told the officers gathered to greet him: "We did what we said we were going to do."
This being Ground Zero, wherever Obama went he was followed like a faithful dog by the record of history. In particular, he was shadowed by the memory of a previous president's visit to the same place.
Not far from the spot where Obama laid his wreath, that president stood in 2001 on top of a crushed fire truck, his arms around a firefighter, speaking through a megaphone. "Can you hear me?" one of the rescue workers shouted out to him, just four days after the towers came down.
"I can hear you, the rest of the world hears you, and the people who brought those buildings down will hear all of us soon."
He was right. Twenty three days after he made those off-the-cuff comments, George Bush began the war in Afghanistan. He didn't know then, he couldn't know then, that it would take America another nine years and seven months to be heard by the main culprit.
After Bush spoke, rescue workers at the site broke out into a chorus of "USA! USA!" It was the same chorus that was heard on Sunday night all round the perimeter of Ground zero when news came through of Bin Laden's death.
Thomas Von Essen was the commissioner of the New York fire department in 2001 when the tragedy happened, and recalled spending that day with Bush. "You could see from his face when he saw the mountain of rubble that it had a profound impact on him," he said. "I'm not a fan of politicians, they are so insincere, but I saw his face I knew he really understood what people had been through."
Von Essen said that he was glad that Obama had repeated the presidential visit nearly a decade later. But there was an aspect to the proceedings he was not so happy about.
"It was courageous of the president to authorise the raid on Bin Laden and I'm glad it worked out; it could easily have gone wrong. But this is now the political side – getting the pictures and taking the credit. I'm a little tired of all that."
TJ, a cement mason from Brooklyn who was one of the first people to start clearing the site after the Twin Towers collapsed, remembers helping to pull the fire truck on which Bush stood out of the rubble. He had no qualms about the commemoration.
"Obama's paying his respects just like he should. I don't think it's political. He's coming here to pay respects to the dead – it's what it is, no more no less."
Just outside the security fence around Ground Zero, bouquets of roses had been placed on the side of the road alongside a makeshift altar consisting of an empty bottle of Johnnie Walker blue label, some well-worn military boots and the badge of the 4th Infantry Division from the Iraq war. "Today let us pray for peace," someone had written.
Amid the crowd, a hawker was selling badges. They were stamped with the date 1 May 2011, and the words "Mission accomplished".
