In the early 1950s, as track athletes came closer and closer to breaking the four-minute mile, some people wondered if it could be done, or did the mark represent a barrier human beings were physiologically unable to surmount? Such talk was misbegotten, though. The concepts of “four minutes” and “one mile” are arbitrary numbers on independent scales. If writers in 1954 had framed the question as “can a human being run 5,280 feet in 240 seconds,” it would have had no allure at all. The neat symmetry of “four minutes” and “one mile” made all the difference.

I have been thinking about the arbitrary nature of numbers recently as the avalanche of 50th anniversaries from the 1960s continues. So many significant events: from “I Have a Dream” to the Kennedy assassination to Beatlemania to the Tonkin Gulf Resolution to the election of LBJ to the march on Selma to the personal ones I blogged about in March and April to the many more still to come (the first teach-ins, Dylan goes electric, passage of Medicare and the Voting Rights Act, the Watts riots, and that takes us only through the summer).

Fifty is just a number—no more intrinsically significant than 49 or 51—but because we live in a base-10 world, we endow it with special characteristics. And so these 50th anniversaries resonate strongly with us. Fifty is important too because of what it represents in the human lifespan. Fifty years is enough to pass from young to old, from mature to elderly. Those of us who can claim to have lived through 50 years know that it brings changes unimaginable by one end from the other. But so do 49 and 51.

Fifty years is useful to us as an aesthetically pleasing mirror that shows where we’ve gone, its corners more nicely rounded than 49 or 51.

Fifty years ago this week, the Beatles were #1 at WOKY in Milwaukee, as they had been so often in 1964. “Ticket to Ride” is different from those moptop-shaking pop songs of ’64, however, pointing in a direction that seems logical and inevitable to us now, but certainly did not seem so then. The Rolling Stones have birthed their first monster riff on “The Last Time,” but in a few weeks will unleash another, more iconic one on “Satisfaction.” The Moody Blues are a pop group on “Go Now,” but they could not have imagined then that 50 years in the future they might still be trading on the name, playing a sort of pop music unimaginable in 1965. Herman’s Hermits, Petula Clark, Wayne Fontana and the Mindbenders, Dave Clark Five, Peter and Gordon, the Searchers, the Zombies—it’s no wonder people thought the Sir Douglas Quintet was British, because everybody else was.

“Wooly Bully,” “Help Me Rhonda,” the Supremes’ “Back in My Arms Again,” “It’s Not Unusual” by Tom Jones—we occasionally have to remind ourselves that these, too, were once current hits, new on some music director’s desk, jockeying for position on the radio and the record charts like Taylor Swift and Katy Perry today, because now they’re as elemental as the sun and the air. That’s because for us—children of the baby boom and slightly later—they always have been.