Six of seven longtime members leaving the National Symphony Orchestra are pictured: from left, musicians Perry Holley, Yvonne Caruthers, Jacqueline Anderson, Sylvia Alimena and Elizabeth Adkins, and music librarian Marcia Farabee. (Marvin Joseph/The Washington Post)

Two hundred fifty-one years. It’s longer than the United States has been a country. It’s longer than the time elapsed since Mozart’s death. And it represents the collected years of experience that the National Symphony Orchestra is losing at the end of this season.

Thursday night, the NSO is honoring seven departing members: six musicians and one librarian, Marcia Farabee, each of whom has served between 28 and 47 years.

Mass departures from orchestras are sometimes signals that a music director is cleaning house. This one, however, seems to be largely a matter of coincidence. The violinist Perry Holley, 67, has reached the age at which “Social Security kicks in.” The horn player Sylvia Alimena, 54, developed a focal dystonia of the jaw and tongue, a condition that causes involuntary muscle contractions and which she sadly calls “a career-ender.” Elisabeth Adkins, the orchestra’s associate concertmaster, is leaving to become a professor at Texas Christian University.

And the cellist Yvonne Caruthers, 61, who has been with the orchestra since 1978, says, “I’m still young enough and healthy enough and have enough energy; it seemed like the door was open to try something else.”

Their combined years is more than three times the age of the NSO. In that cumulative time, the players logged untold miles on airplanes and trains and buses through Europe and Asia and South America and every corner of the United States. They’ve played under dozens of conductors, five music directors and one principal conductor, Ivan Fischer, who started some rehearsals by having the winds and brass play a Bach chorale. The violinist Jacqueline Anderson, hired by Howard Mitchell in 1967, even remembers the pre-Kennedy Center days when the orchestra performed at DAR Constitution Hall.

“The orchestra didn’t have a very good feeling about itself in the Mitchell days,” Anderson says. That changed with the advent of the Hungarian conductor Antal Dorati. “He had such a wonderful dignity and nobility about his music-making,” Anderson says. “I remember him making a speech to the orchestra that we really should take pride in ourselves. That was not something anyone had ever talked about before.”

How has the orchestra changed? When Anderson arrived, there were five women in the NSO; today, there are 32, including a female concertmaster, Nurit Bar-Josef — not counting the five who are departing. (Charlotte Davis Paramore, the fifth woman present when Anderson arrived and one of the seven being honored, is prevented by indisposition from talking to the media or attending Thursday night’s ceremony.)

The demands of the job have also changed. “The vast array of genres that the musicians are asked not only to be knowledgable about, but to play at the top of their form, has increased,” says Farabee — from Strauss’s “Der Rosenkavalier” to a hip-hop concert.

Farabee knows better than anyone what the orchestras are playing: As librarian, she’s the one who gets the music into the musicians’ folders. She’s the one who remembers that Mstislav Rostropovich was meticulous about the string players’ bowings, although he “for the most part left the winds alone.” She’s the one who knows that some conductors own relatively little music, and some, like Hugh Wolff, a longtime assistant conductor at the NSO, “has a vast library of his own and has had all his music professionally prepared,” she says, with “meticulous” markings.

These are the things an audience seldom gets to know about.

Ask a 100-member orchestra for an opinion, the saying goes, and you’ll get 105 different answers. The departing players offer a range of perspectives — which add up to vivid images of an institutional past. For many, this began with Rostropovich, or “Slava,” who went on a hiring spree after he took over as music director in 1977. “He knew exactly what he was looking for,” says Caruthers, the first cellist he hired. “I was totally intimidated to come and play for him; it was like playing for God. But when you sat down face to face, he was so welcoming and encouraging that you played your heart out.”

“He certainly was not technically a wonderful conductor,” Adkins says. “But his musicianship was so compelling, it would come from his body, it would come from his eyes, it would come from the top of his head and go directly to your heart.”

Rostropovich’s successor, Leonard Slatkin, had an uneven tenure with the NSO. Yet the departing players remember him as “a brilliant man” and “technically advanced.” Not everyone was fond of his championship of American music. But there were positive sides. After a couple of seasons, Caruthers says, “we could sight-read anything they put in front of us.”

As for the current music director, Christoph Eschenbach: To one player, he’s “distant,” to another, “a very kind fellow.” Farabee says, “I’ve never seen anyone so focused on the performance.”

“I think we’re still discovering how to play with him,” Adkins says. “He has an interesting mix of very strong opinions, and he also wants very spontaneous music-making during a concert.”

You’re not going to find consensus about 251 years of history. But there are some things about which nearly everyone agrees. Most players cite the 1990 tour of Russia, when Rostropovich returned to his native country for the first time since 1974, as a highlight. Alimena remembers audiences climbing onto the roof of the Moscow Conservatory to look down through the skylights. “You can’t write fiction that good,” Caruthers says.

Other highlights are more individual. Anderson fondly remembers the concerts of Marvin Hamlisch, the orchestra’s principal pops conductor from 2000 to 2011. “Even though he wasn’t by any means a good conductor, he was wonderfully creative,” she says. “I don’t necessarily like doing pops, but I just loved his concerts.”

In general, the players are sorry to leave an ensemble that they describe as warm, friendly, and supportive. “I’m still convinced I’m going to fall to pieces,” Alimena says of Thursday’s ceremony.

They are also keenly aware that the orchestra landscape is changing. A colleague of Anderson’s recently commented, she says, “that we, he and I and other people of our vintage, kind of rode the crest of the wave of orchestral life. We saw the best years: started out kind of meager, built up very high, and now things are much more difficult. I am very grateful in hindsight to have been able to do that.”

“It was all just living the dream,” Caruthers says, remembering playing in Vienna and driving around Oman on the orchestra’s most recent tour. “You pinch yourself. Am I here? Yes? I am?”

And now, alas, she’s not.

The NSO and Christoph Eschenbach will play an all-Bruckner program, including the 8th Symphony, on Thursday, Friday and Saturday at the Kennedy Center Concert Hall.