Buzz Aldrin salutes the flag while standing on the moon.

Here in the northern hemisphere, summer officially arrived at 11:54 a.m. eastern time on Friday, June 21 — the longest day of the calendar year. But the non-astronomers among us know that summer doesn’t truly begin until July 1, when lazy, hazy days return and vacation season starts in earnest.

This year, however, July also ushers in a month-long reflection on the 50th anniversary of mankind’s first landing on the moon, a singular moment unequaled in human history that even today remains the crowning achievement of post-war American exceptionalism.

Most readers know that Neil Armstrong and Buzz Aldrin became the first humans to walk on the moon on July 20, 1969 — culminating a so-called “space race” unleashed more than a decade earlier.

But for those less than 50 years of age — and especially millennials — it’s virtually impossible to comprehend the level of popular interest in the space program during the 1960s.

At a time when the country’s social fabric was increasingly frayed by the civil rights movement, a spiraling counter-culture and a hugely unpopular war, NASA provided Americans — not only with a vision of the future — but a tangible pathway to the stars.

Even before President John Kennedy’s 1961 challenge to land a man on the moon and return him safely, astronauts already were rock stars. Later immortalized by what author Tom Wolfe termed “the right stuff,” the Mercury Seven (all of whom had been military pilots — four seeing extensive combat in either World War II or Korea) firmly established themselves as daredevil brainiacs with advanced degrees who worked hard, partied hard and exhibited a swagger that bordered on reckless disregard for personal safety.

Virtuous and American as apple pie, NASA’s best and brightest were the vanguard of a generation that married giant leaps in technology and engineering to an indomitable spirit of exploration and adventure.

Mission launches (and splashdowns) were appointment viewing, with network programming suspended for hours while families crowded black and white TV sets listening to radio transmissions between Mission Control and the space capsule, and watching network anchors narrate animation sequences once the spacecraft outdistanced visual range.

That’s hard to explain to younger Americans accustomed to hundreds of cable channels and streaming options.

There were setbacks, some better known than others. Nearly two decades before the Challenger shuttle disaster, Apollo 1 crew members Virgil “Gus” Grissom, Ed White and Roger Chafee were killed by a flash fire during a launch rehearsal test — prompting a 20-month suspension of manned flights.

And four years before the dramatic Apollo 13 mission that ended with all three crewmen returning safely to earth, Gemini 8 — piloted by none other than Neil Armstrong — likewise survived a primary in-space system failure.

Through it all the space program prevailed, gaining momentum and fusing national unity, pride and purpose with childlike wonder — a quality that arguably reached its apex six months before the moon landing with the crew of Apollo 8 broadcasting a Christmas Eve message from mankind’s first lunar orbit.

Watching on live television or listening to transistor radios, hushed listeners overheard crew members take turns reading from the Book of Genesis, followed by the emotional impact of commander Frank Borman offering Christmas wishes and God’s blessings “to all of you on the good earth.”

No doubt these, and other, pivotal moments will be revisited during the numerous documentaries and print retrospectives published in upcoming weeks.

They, too, should make for appointment viewing — if only to remind us what can be accomplished with unity, pride and purpose.