Forty years ago this month, the Beatles began their second major tour of America with a performance at Shea Stadium in Queens. It's an event worth noting: More than 55,000 people attended that night, Aug. 15, 1965. It set a world record at that time for a pop concert, and it was the biggest public moment of the Beatles' remarkable career.

It's also worth noting that these days we seem to be reconstructing a shadow history of the band and its achievements. That is, almost every year now we observe some milestone of the Beatles.

Last year it was the anniversary of the group's astonishing 1964 appearance on "The Ed Sullivan Show." Two years from now, June 2007, the occasion will be a commemoration of 1967's "Sgt. Pepper's Lonely Hearts Club Band" — an epochal work that still stands as popular music's most famous and form-breaking album. Commentators from all over the world will weigh in on that one.

Which raises a number of questions: Why do we continue to pore over the Beatles' high points? Why is it that those lifetime-ago moments still fascinate us?

In part, of course, it's simply because there's such an undeniable epic arc in both the Beatles' story and in their music. Certainly, they possessed an extraordinarily intuitive skill for filling the needs of their times, and for realizing the potential of their own talents.

But there's another reason, just as important, that accounts for the lasting appeal of their history: The Beatles demonstrated that musical and social change could emanate from the shared spirit of the same body politic.

Rock 'n' roll, of course, had already shown it could stir cultural tumult. In the 1950s, Elvis Presley and numerous rhythm-and-blues and rockabilly artists had brought new audiences and sensibilities into the mainstream. Rough, rude and provocatively rhythmic music — from both black and white upstarts — had broken through the barriers, meeting fierce opposition, until the new spirit was almost tamed.

Whether they meant to or not, the Beatles raised the stakes on all this, and they did it right from the start.

Their American debut, on "The Ed Sullivan Show" on Feb. 9, 1964, coincided with my 13th birthday. I certainly didn't understand everything I was seeing — the girls in the audience sticking their tongues out leeringly at the group, the whole shock-of-the-new effect of these four men who looked so foreign and who commanded their melodies with such assurance and their instruments with such synchronous force — but I knew, as millions of others did, that I was witnessing something seismic.

The next day, the Beatles' performance was the only thing we talked about at school. The girls loved the band members' long hair, the boys seemed unnerved by it, but everyone agreed that the Beatles and their music was an awakening.

In the days following, the arguments and reactions around the country only grew. While Elvis Presley had already shown us something about using rebellious style as a means of change, the Beatles helped incite something stronger in American youth that night — something that started as a consensus, as a shared joy, but that in time would seem like the prospect of power — a new kind of youth mandate.

I wasn't at Shea, but I saw the Beatles a week later at Memorial Coliseum in my hometown, Portland, Ore. I was 14 and had won tickets to the concert in a local drawing, which I count among my life's luckiest moments.

I could see them on stage — small, suited figures moving and playing, looking holy in the blinding luminescence of flashbulbs and house lights. The collective yowling scream of the audience — to this day, the loudest thing I've heard — seemed to emerge from a mass fever dream. All these years later it still moves me to realize how jolting and transcendent it was to be in a room — no matter how large — when the Beatles played.

And from film clips of the Aug. 15 concert, Shea was the same. Everybody there that night — the thousands upon thousands of screaming teenagers ("supersonic seagulls" as Paul McCartney recently described them), the legion of exhausted policemen, even the Beatles themselves — seemed overwhelmed by the intensity of the event and its implications.

The poet Allen Ginsberg attended the same performance I did at Memorial and rendered the experience in his poem “Portland Coliseum”:

The million children

the thousand worlds

bounce in their seats, bash

each other’s sides, press

legs together nervous

Scream again & claphand

become one Animal

in the New World Auditorium

— hands waving myriad

snakes of thought

screetch beyond hearing

while a line of police with

folded arms stands

Sentry to contain the red

sweatered ecstasy

that rises upward to the

wired roof.

Ginsberg understood what he was witnessing: mass fervor that great — especially from the young — has always felt threatening. That’s because it can seem unruly, powerful enough to upset traditions and values or to incite dangerous action.

There had been small riots at rock ’n’ roll concerts in the 1950s — chairs thrown, fisticuffs — but the threat implicit in 1960s music was something else: it was about setting things loose, about changing or upending the world.

The barricade of policemen I saw that day at the Beatles' show — the same line Ginsberg had seen — certainly acted as if they were seeing something more than mania. The scream the Beatles brought forth in America was just too unforeseen and too big. It could help shake the order of things, and in time it would.

That August in 1965, we didn't fathom where the power in this sort of communion might lead. We didn't know where we were going with the Beatles, and they didn't know where they were headed.

The music that followed their 1966 retirement from live performances turned often hopeful and generous (not to mention unbelievably creative), and more important, compassionate. "Sgt. Pepper" is often viewed as whimsical or naive, and yet songs like "She's Leaving Home," "Getting Better" and "A Day in the Life" gave voice to the combined senses of hope, strangeness and anxiety that marked the lives of many in that period.

By the end of the 1960s, though, the Beatles' songs had grown more mournful, frightened and angry. John Lennon grew suspicious of his audience's politics in "Revolution" and of the whole world in "The Ballad of John and Yoko," whereas Paul McCartney's "Let It Be" and "The Long and Winding Road" played like doleful prayers of solitude. By 1969, the two men — who had once exemplified collaboration — could barely sing to each other across a gulf of mutual recrimination.

All this, sadly, reflected the tenor of the time. The spirit of Western youth — especially American — descended from bliss to disillusionment, as political assassinations, the madness of Vietnam, the strife over civil rights and political protests, the effects of unmonitored drug use and the violence of the Manson family and Altamont all bore down, taking a steady toll.

The Beatles came to their bitter, nasty end in April 1970 — the one event we tend not to commemorate. It's more pleasurable remembering the big bang of the "Ed Sullivan" appearance and the Shea Stadium concert. But the sort of promises born in those moments may no longer be possible.

It's true, of course, that subsequent mass popular music events like 1985's Live Aid and this year's Live 8 concerts have followed through on some of what the Beatles made possible, albeit in cautious, inoffensive ways.

It's also true, though, that the sort of youth power that the Beatles helped awaken is simply no longer even considered. The cultural perspective that defines youth has changed drastically.

We've infantilized adolescents. We view them as children whose judgments are immature, who have to be protected from influences that may steer them in wrong directions — directions that may threaten decency or disrupt social authority.

True, the same things were said about teenagers in the 1950s and 1960s, but part of our ambition WAS to dispute mores and intimidate hegemony. Today, the pressures against such instincts for adolescents come from both within their peer group and the culture at large. Teenagers now are themselves often the harshest critics of young nonconformists.

Meanwhile, watchdogs across the spectrum — from Bill O'Reilly to the Rev. Al Sharpton to Sen. Hillary Rodham Clinton — worry over the effects that rude rap or scandalous video games may be having on the young. And today's conservative mind-set stigmatizes the sort of insurrectionary voice that the Beatles, Bob Dylan, the Rolling Stones and others exercised in much of their 1960s music.

Last year, when R.E.M., Bruce Springsteen, Pearl Jam, the Dixie Chicks and others played concerts to promote a defeat of President Bush, their efforts were seen as a risky anomaly.

It was as if songs like the Beatles' "Revolution" or Buffalo Springfield's "For What It's Worth," James Brown's "Say It Loud (I'm Black and I'm Proud)" and Marvin Gaye's "What's Going On" had never filled the air in those years conveying a sense of political and generational transformation that, for a time, seemed imminent and irrefutable.

Maybe this sort of reflection seems too far a stretch from the joys felt on those warm nights in the summer of 1965. There's no denying that above all else the Beatles were fun; had they not been, they would not have enjoyed so much effect or such staying power.

But fun on the level that the Beatles managed to achieve — at least in those days — implied more than a collective, thrilling scream. We remember the Beatles for their music and spectacle, but we celebrate them because, when they stood before their American audiences in 1964 and 1965, we witnessed the social and cultural power that a pop group and its audience could create and share.

From there, I guess, you measure how much we've learned, or how much we've lost.