In all of time, only 24 human beings have flown to or around the moon, looked back, and seen Earth as a small blue sphere in the blackness of space.
Their numbers are dwindling. Of the 12 who walked on the moon's surface, only nine are alive today, and the youngest is 71.
So British director David Sington and his crew set out to seize the moment -- now, before the opportunity passes, to bring the Apollo astronauts together in one film, and ask them how the experience affected them.
"I think these guys are very sane individuals. They are very down to earth, because, in some sense, they really know what Earth is," says Sington. "There are 7 billion of us on the planet, and nine of those 7 billion have stood on another heavenly body."
From Buzz Aldrin, who made the first landing with Neil Armstrong, to Gene Cernan, who made the last landing in 1972, Sington and his colleagues got 10 Apollo astronauts to share some of their deepest thoughts.
"I called the moon my home for three days of my life, and I'm here to tell you about it," said a pensive Cernan as the camera rolled. Then he shrugged as if he didn't believe himself. "That's -- that's science fiction."
The documentary is called "In the Shadow of the Moon." Having already been voted best documentary at the Sundance Film Festival, it is opening this weekend in New York and Los Angeles. It will spread to theaters around the country by early October.
The film consists entirely of interviews with the astronauts. The filmmakers added pictures from the Apollo flights, much of the visuals recently restored and copied in high definition.
Looking back now, several astronauts say they learned far more about themselves than about the moon.
"My father was born shortly after the Wright brothers. He could barely believe that I went to the moon," says Charles Duke, who flew on Apollo 16 in 1972.
He pauses and wrinkles his nose. "But my son Tom was 5 -- and he didn't think it was any big deal."
"I think if you do something as drastically different like flying to the moon and coming back again," says Michael Collins, who orbited the moon in Apollo 11's command ship while Armstrong and Aldrin landed, "then, by comparison, a lot of other things that used to seem important don't seem quite as much so."
In the years after their travels, a few of the men struggled with depression. Some became environmentalists. Some turned deeply spiritual.
"I felt that the world," says Gene Cernan at one point, "was just too beautiful to have happened by accident. There has to be something bigger than you and bigger than me." He continues, "And I mean this in a spiritual sense, not a religious sense. There has to be a creator of the universe who stands above the religions that we ourselves create to govern our lives."
The men remark how desolate the moon seemed, how delicate Earth seemed -- and how very, very good it was to come home to it.
"Since that time," says Alan Bean, who flew the second moon landing on Apollo 12, "I have not complained about the weather one single time -- I'm glad there's weather. I've not complained about traffic -- I'm glad there are people around me."
Bean became a painter after he left the space program, and he closes the film with a broad smile.
"I feel blessed every single day. Not a day goes by that I don't think this is great. This was wonderful. Somebody had to go, and they happened to pick me."