SSPL / Getty Images Schmitt is seen collecting Moon samples by a large lunar boulder, with part of the Lunar Rover in the foreground. Apollo 17, the sixth and last manned mission to the Moon, with astronauts Eugene Cernan, Ronald Evans and Schmitt on board, was launched on 7th December 1972.

Ron Evans fell sound asleep in the command module of Apollo 17 when he and his crewmates were waiting to take off. It wasn’t easy to doze off in an Apollo spacecraft, least of all when you’re flat on the back with your feet in the air, shoulder to shoulder with two other men, all of you inflated to cartoonish size inside heavy, pressurized suits, perched atop 363 ft. of explosive Saturn V rocket. But Apollo 17 was launching late on a December night — 40 Decembers ago this month, in fact — it was awfully quiet on the pad and a technical glitch caused a two-hour delay. So Evans fell asleep and soon started to snore, and his crewmates, Gene Cernan and Jack Schmitt amused themselves by griping to Mission Control about the infernal noise.

Before long, however, the whole crew was alert again, and then the engines lit with a sky-brightening light that could be seen as far away as North Carolina. Just before ignition, Cernan, the commander, who had flown to the moon before, had some words for his rookie crew. “I know you’re going to do your jobs,” he told them. “But make sure you do one other thing: enjoy this, have fun. You’re not going to get to do it again.”

(More: Time Talks with Gene Cernan, Commander of Apollo 17)

He had no idea how right he’d be. Cernan was one of only 24 men to fly out to the moon and one of only three — including Jim Lovell and John Young — to make the trip twice. Some of them walked, some of them orbited, some — the snakebit crew of Apollo 13 — simply swung around the far side and came home. But they were men who had the brass to make such a trip, backed by a country that had the brass to send them there in the first place. And then, on Dec. 13, 1972 at 11:34 PM Houston time, it all came to an end. After nine moon missions spanning four years, Cernan, standing in the moon’s Taurus-Littrow region, climbed back up the ladder of his LEM, intoned a few bittersweet words — we’d come in peace, we were leaving in peace, the Apollo program was ending but we’d surely be back — and that was that.

And yet we haven’t gone back. Forty years and 135 shuttle missions later, humans have gone round and round and round their home planet—just as John Glenn and Yuri Gagarin went round and round it when the idea was new — leaving any deeper space travel to the robots. Yet, if the Apollo program had to end — well, never mind that. It didn’t have to end. We chose to end it, to bring down our sails, just as we’d chosen to hoist them in the first place. But no matter how it ended, Apollo 17 was a worthy capstone to the entire lunar enterprise.

While Apollo 11’s Neil Armstrong and Buzz Aldrin spent only 21 hours on the lunar surface and just 2 hours and 31 minutes outside their spacecraft, Apollo 17 was on the ground for more than three days, and Cernan and Schmitt spent more than 22 hours outdoors, spread over three separate excursions. If the Apollo 11 landing site had been a baseball diamond and the lunar module landed on the pitcher’s mound, Armstrong and Aldrin never got out of the infield — except for a brief detour Armstrong made into shallow right field to investigate a crater. Cernan and Schmitt ventured up to five miles from the safety of their LEM, aided in no small part by the lunar rover — effectively a collapsible moon car — that they’d brought along.

(Photos: Apollo 17: Forty Years Later)

Living on the moon for so long wasn’t easy. There were the exquisitely aching hands that came from doing manual labor in pressurized gloves. There was the gunpowder-scented lunar soil that was so abrasive it would sand away the rubber grips of hammers, but so fine it would not just get under the astronauts’ nails, but deep into the very skin beneath, so that weeks or months would have to go by before the nails — and the dirt — could simply grow out. There was the terrible sense of wasted time, as the astronauts would sack out on tiny hammocks inside the tiny lunar excursion module (LEM), wondering what in the world they were doing indoors when the moon lay just outside the foil-thick walls.

“You’d get that stiff spacesuit off and instead of two of you in the cabin it might as well have been four,” Cernan told Time today in a wide-ranging coversation. “I’m trying to sleep with the little window shade closed and I’m thinking, I got on top of that Saturn V to get here and now I’m lying around inside this LEM. Why am I sleeping at all?”

The science all of the Apollo crews conducted was solid and the engineering they tested was brilliant, but it was something much more that drove the astronauts — or at least drove some of them — and it was that same thing that drove the country as a whole. During Apollo 10, Cernan’s first lunar trip, he looked out the window at the moon that was just a few dozen miles below and turned to Young, one of his crewmates. The two then had the following exchange, which was captured by the then-classified cabin recorders:

Cernan: Hey, let me ask you a question. Where do you suppose a planet like this comes from? Do you suppose it broke away from the — away from the Earth, like a lot of people say?

Young: Don’t ask me, babe.

Cernan: It sure looks different.

Young: I ain’t no cosmologist. I don’t care about nothing like that.

Cernan: Sure looks different.

Young has never been anyone’s idea of a poet, but he was surely playing up the rough-hewn space-jock act at least a little. In that same conversation, looking out at the surreally complete blackness around them, he mused, “First time I ever felt warm about the dark.”

(More: All Cracked Up: A Surprising Look Inside the Moon)

Cernan always felt warm about the dark — and the light and the gray and the million brilliant colors that were part of his space experiences. Forty years later he still does. “Picture my environment,” he says of the last look he took around at the moonscape before he ascended the LEM ladder. “I’m a quarter million miles from home on the surface of the moon. I’m looking down at my footprints and then over my shoulder at the crescent Earth, with its blues and whites, hovering over the southwest rim of the mountains. And I’m seeing that the Earth doesn’t just tumble through space. It moves with purpose and logic. It was too beautiful to have happened by accident. I felt like I was standing in a place where science had met its match. Science could no longer explain to me where I was.”

It’s partly the odd elusiveness of the moon that makes it so evocative a place. A 2,200-mi. wide sphere that hangs in space at barely arm’s reach — by cosmic standards at least — it nonetheless fades slowly from view each month, vanishing entirely even as it remains fixedly in place. Its lack of atmosphere means that to people standing on it or orbiting above it, there’s no gradual sunrise or sunset, no wash of reflected light that spills over from the daytime side to the nighttime side. It’s either brilliantly lit or invisibly black.

“You get into the shadow of the moon as you’re preparing to orbit and you can’t see it at all,” Cernan says. “It’s completely dark below but you just feel that something’s there, you sense some mass, some presence. Then all of a sudden you come around to an instantaneous sunrise and bam, there it is, there’s the moon. It’s been there all along.”

(From the Archives: Poised For the Leap)

Throughout human history, it’s been there for all of us, but it’s only in recent eras that we’ve begun to study it in a real and serious way. It’s the loss of that good science that frustrates Cernan when he contemplates America’s 40 years of space drift. But it’s the loss of that sense of surrendering to the pull of the moon, of Mars, of deeper space, that galls him more. Armstrong died last August, but in the last two years of his life, he, along with Cernan and Lovell, became vocal advocates for a space program that refocused itself — that pointed its prow toward a deep-space target and then bloody well went there. The three of them provoked the current NASA hierarchy in ways they dared not when they were astronauts angling for flight assignments — and they enjoyed that freedom. “I can say what I want these days,” Cernan laughs. “I’ve got nothing to lose.”

Cernan had the fun he wanted to have when he went to the moon, he did the work he went to do and he made the history he knew he’d make. He’s been happy to talk about it all for the last four decades, partly because he feels a responsibility to do so. “When the last of us is gone, it will all be told in the third person,” he says, “it will all be hearsay.” Whether there will ever be first-person explorers again will be for the generations left behind to decide.