Adorning one wall is a plaque with badges, one for each Sentinel who has ever served. Today, there’s a blank space where the 627th badge will go, inscribed with the name of Specialist Andrew Selga. It’s Oct. 16, the day before the 20-year-old will receive that badge. He’s one of 12 active Sentinels, all members of the Army’s 3rd U.S. Infantry Regiment, more commonly known as the Old Guard. While they have a very visible role at the Tomb, most of the Sentinels’ work is hidden from visitors — visitors who, on this drab October day, will peer at them through a veil of chilly mist.

8 a.m.

The gates of Arlington Cemetery open at 8 a.m., exactly the time of the first Changing of the Guard at the Tomb of the Unknowns. There are no school groups yet, and the trams carrying visitors have yet to begin their rounds. Still, the ceremony progresses, as it will nine more times today.

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Selga’s day started three hours earlier, when he and the seven other men on the schedule arrived at Tomb Quarters, where they stay for their 24-hour shifts. And “24-hour” means 24 hours of work — they don’t sleep, they don’t relax. They prepare to guard the Tomb, they guard the Tomb and they work on getting better at guarding the Tomb.

9 a.m.

There’s a small crowd now, a few dozen teenagers, some in hoodies and jeans, some shivering in skirts. After the changing ceremony, four students from Independence Middle School in Independence, Ohio — winners of an eighth-grade essay contest — perform the first wreath-laying of the day. They stand a little straighter than 13-year-olds usually do as a Sentinel escorts them to the Tomb. Staff Sgt. Christopher Carney instructs military personnel in uniform to salute and says “it is appropriate” for civilians to place their hands on their hearts. As taps echoes toward the District, most people have one hand on their heart and the other on their iPhone, trying to get a good photo.

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Before laying the wreath, the students received a briefing from one of the Sentinels.

“It was kind of scary when he first walked up, because he was so tall,” Christina Meyers says. They were soon put at ease: “He was really gentle,” Andrea Klima says. “Not like the way they are out there,” gesturing at a Sentinel sternly walking back and forth.

Near the entrance to the Tomb Quarters, the next group of wreath-laying kids is getting instructions from a Sentinel in a polo shirt and khakis. “Look down at the stairs,” he says. “People try to look forward and they trip and fall. Don’t do that.”

10 a.m.

The crowds are starting to come in; the lines on the steps are now four people deep. Carney again instructs the crowd to maintain an atmosphere of silence and respect. There are three wreath-laying ceremonies this time: three groups of teenagers with serious faces, three playings of taps.

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The rhythm of the day is becoming clear. The crowd starts to thicken at 15 minutes before the hour. After the ceremonies it disperses, leaving behind a few dozen people who watch the guard and take the occasional picture in front of the Tomb, unsure if they should smile.

11 a.m.

One of the wreath-layings this time is by Mayer Middle University, another middle school. Two boys from Gresham, Ore., honor other boys from we don’t know where. After taps, three volleys sound from elsewhere in the cemetery as someone else’s boy or girl is laid to rest.

In Tomb Quarters, the two Sentinel trainees might be practicing in front of a three-way mirror. Each day, they’re required to do three hours of “mirror time,” during which they spin their rifles, check their posture and stumble in private so they’re perfect in public.

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During their time as trainees (usually six to nine months), future Sentinels are not allowed to laugh or smile within the Quarters, except in very limited “safe zones.” They are also, in military parlance, not allowed to “acknowledge furniture.” That means they “can’t sit down, can’t touch it, and if it’s in the way, they can’t move it,” Selga says.

Noon

It’s still cloudy, but the Sentinels have donned sunglasses; the glare off the marble magnifies the sunshine struggling to break through. During the rifle inspection, the incoming Sentinel knocks a medal off of his chest while spinning his weapon. It hits the marble with a tiny clang that startles everyone but the Sentinels, who carry on. (After the ceremony a trainee brings the medal to the Sentinel, who steps into the guardhouse to reattach it to his uniform.)

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At 12:03, a volley of gunshots from a full honor funeral — much louder than the rifles that have been firing all day — makes everyone jump but the Sentinels. Who carry on.

Those who visit the Tomb between the Changing of the Guard ceremonies usually stay for less than 10 minutes: Watching a guy walk can keep one’s attention only for so long. Unless you’re the guy walking.

“Most of your thoughts are on what you’re doing,” Selga says. That’s walking 21 steps, turning, pausing for 21 seconds (which requires a perfectly functioning internal clock), turning, pausing for another 21 seconds and walking 21 steps back. And never slouching.

“Sometimes I’ll be thinking about something I’m doing and I’ll think about it so much the time will fly by,” Selga says.

1 p.m.

As the next change approaches, a crowd of men arrives. Some are in wheelchairs and most wear caps showing when they did their military service. Their trip was paid for by the Kansas chapter of the Honor Flight Network, which sends World War II, Korean War and Vietnam War veterans on visits to D.C.

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When it’s time for four Kansans to lay their wreath, the crowd is a little quieter and stands a little straighter. Even though civilians aren’t supposed to salute, the foursome does anyway. No one seems to mind.

2 p.m.

Another Honor Flight group, this one from York, Pa., arrives. With them is Lawrence Bocksel, a combat engineer who landed on Omaha Beach on D-Day.

“They’re the heroes of the world,” Bocksel says, looking at the tombstones around him. “They gave everything so we could have our tomorrows.” Bocksel’s tomorrows include his two daughters, who are with him, as are his grandchildren, some of whom buzz about wearing too-big York Honor Flight T-shirts.

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One daughter says she’s glad her dad got to travel here with other veterans, since he never talks about his experiences with anyone else.

“It was terrible,” he says. “Terrible. But it had to be done.” He doesn’t want to say any more.

3 p.m.

It’s warming up. Two more middle school groups lay wreaths. One of the wreath-layers forgets to put his hand on his heart and is quietly reminded by Staff Sgt. Brian Blackmore. Their classmates are starting to sweat beneath their D.C. and FBI sweatshirts.

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Though part of the ceremony is the inspection of the incoming guard’s uniform, what the public sees is largely ceremonial. The Sentinels are first checked in Quarters before proceeding to the Tomb. That inspection is just as rigorous as the one up top looks. The Sergeant of the Guard — the one who supervises the ceremony — notices what most people wouldn’t.

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“He can see loose strings on buttons, something as much as a fingerprint on a not-wiped-down piece of brass,” Selga says.

Today, the only adjustment a Sergeant of the Guard makes during the inspection at the Tomb is to pluck something invisible to the crowd off a Sentinel’s jacket.

4 p.m.

It’s the final push of tour buses and school groups. Some visitors carry shopping bags from the Air and Space Museum and White House Gifts. Among the later groups are people with reddened eyes dressed in black, who have come to Arlington for more personal reasons. They don’t talk much, even to one another.

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Though the sun has finally come out, the Sentinels seem impervious to the weather while guarding.

“If it’s a hot day, you might come down [to Tomb Quarters] and go, ‘Oof, it’s hot out there,’ ” Selga says. “Or take a deep breath or something that you’re not allowed to do out there.”

Though there are some allowances for the weather — in the summer the Sentinels switch every half hour instead of every hour, and they have cold-weather gear for the winter — there aren’t many. All the Sentinels wear the same uniform on the same day, and the Sergeant of the Guard makes that call. If a Sentinel feels it’s too cold for the summer uniform, “he can just suck it up,” Selga says.

5 p.m.

At the final change, there are no school groups, just tourists who hurried to make it on time. Every other time, the Sentinels who changed duties gave and acknowledged orders in that peculiar military bark; this time it’s a silent change. Retreat, played at nearby Fort Myer, floats through the air.

There’s no wreath-laying, no taps this time. Blackmore informs the small crowd that Arlington Cemetery is now a restricted military post.

Everyone has to leave, except the eight men who have 12 hours left in their workday. Someone will walk the Tomb until 6 p.m.; then, the Sentinels use the space in front for training. Even when that’s done, someone will stand guard, honoring three strangers through the night.

What is the Tomb of the Unknowns?

It’s four crypts, one for each war from World War I to Vietnam. Three of them each contain the remains of an unidentified service member. The sarcophagus (the big boxy thing) sits atop the World War I crypt. The other three lie in front of it. The Vietnam crypt is vacant: After exhumation in 1998, the body of Air Force 1st Lt. Michael Blassie was identified using DNA testing. There are no plans to inter another unknown, and the crypt cover now reads “Honoring and Keeping Faith with America’s Missing Servicemen, 1958-1975.”