Forty years ago on Monday, Neil Armstrong became the most famous man on the planet by talking a short walk off of it. Since then he's tried to live with that fact, and also live it down. Only rarely -- on major anniversary dates, like Monday's -- does he show up on television, and then only fleetingly. He hasn't leveraged his fame for higher office or some grand cause, nor has he sold it willy-nilly. If the subject is Neil Armstrong, the first man to set foot on the moon tends to turn churlish. He will defer, deflect or refuse to answer. When his little home town of Wapakoneta, Ohio, sought to honor him with a parade on the 25th anniversary of his moonwalk, Armstrong sent his regrets. He once pleaded to a newspaper reporter, 10 years after his feat: 'How long must it take before I can cease to be known as a spaceman?' As if such a thing were possible. Or even desirable. It's not fair to call Armstrong a 'recluse,' as many accounts of his life after Apollo 11 invariably have. He's no cosmic J.D. Salinger or Howard Hughes, shunning the world out of spite or madness. Armstrong makes the occasional public appearance and speech, as he did Sunday at the Smithsonian and as he will do again Monday at NASA's official commemoration of the moon landing. He's also appeared in two NASA video productions over the past five years. What's more, after resisting would-be biographers for years, he finally caved to his family's prodding and sat for more than 50 hours of interviews with Auburn University historian James Hansen for a 2005 biography, 'First Man: The Life of Neil A. Armstrong.' Yet for the 40th anniversary, Armstrong has once again carefully rationed himself. He told planners at the Smithsonian and NASA that he would speak at their events, but not as the keynoter, not at length and only in conjunction with other Apollo alumni. A book-signing at the Air and Space Museum featuring his Apollo 11 crew mates, Buzz Aldrin and Michael Collins, was out of the question (Armstrong stopped signing anything some years ago when brokers began peddling bogus signatures on the Internet). Media interviews? Not a chance. 'He's always been this way,' says one person involved in planning the events. Carol Armstrong says her husband averages about 10 interview requests per month. He turns them all down, usually without reply (he did not respond to a request for this article). 'I think he thinks it's all been said before,' Carol says from their home near Cincinnati. A decade ago, when The Post sought an interview, Armstrong e-mailed his regrets, adding with Garbo-like brevity: 'I am comfortable with my level of public discourse.' Those who know Armstrong say his behavior has been consistent over the arc of his 78 years. Even before the world insisted on lionizing him, he was his own man, faithful to his standards: Reject personal glory. Avoid focusing on the self. Keep what's private private. Until Hansen revealed it, some of Armstrong's closest working associates never knew that the Armstrong and his first wife, Janet, had a 2-year-old daughter who died of a brain tumor a few years before Armstrong went into space. 'Neil has a very strict sense of what's appropriate to be involved in, and has since he was a boy,' says Hansen, a former NASA historian who spent nearly three years corresponding with Armstrong before winning his cooperation on the book. Armstrong has such a hard time speaking about himself in the first person, Hansen says, that 'he felt he couldn't write an autobiography or a memoir.' Adds Hansen: 'Neil was very much the same person after Apollo 11 as he was before it. The pragmatism, the modesty, the shyness were there from an early age. I don't see any radical changes in (him) throughout his life.' In his limited public utterances, Armstrong has always turned the subject away from himself. He usually deflects credit to the 400,000 people who built and maintained the vehicles and managed the bureaucracy that enabled him and Aldrin to reach the moon. In his own book, 'Men From Earth,' Aldrin wrote that he thought the man who preceded him onto the lunar surface had worked his way through his career 'carefully watching everything he did and said.' Talkative and opinionated, Aldrin may be the anti-Armstrong. In his post-Apollo career, Aldrin has done what Armstrong would find inconceivable. He once did a guest voice on 'The Simpsons,' sat for a hilarious interview on 'Da Ali G Show,' made a rap video with Snoop Dogg and Quincy Jones, and loaned his name to a computer game, Buzz Aldrin's Race Into Space. Just in time for the 40th anniversary of the moon landing, there's Aldrin in an ad for -- what?! -- Louis Vuitton luggage. Aldrin once punched a guy who accused him of 'lying' about the moon landing. Someone once described Aldrin and Armstrong as 'amiable strangers,' but Hansen says that's inaccurate. 'I'm not even sure `amiable' is the right word. Neil did not appreciate how (Aldrin) went off in such strong, aggressive ways with his ideas. They worked well together, but I'm not sure there was much personal rapport. Buzz never figured Neil out.' From time to time, Hansen says, Aldrin would contact him and ask for help to persuade Armstrong to attend some event -- a reflection, Hansen says, of the astronauts' uneasy relationship. Hansen says Armstrong's reticence may have been reinforced by the example of Charles Lindbergh, another 20th-century pioneer who knew much about the soul-twisting powers of fame. The two men met in 1968, and Lindbergh and his wife, Anne Morrow Lindbergh, were Armstrong's guests for the Apollo 11 launch. They corresponded until Charles Lindbergh's death in 1974. A solitary country boy who dreamed of flying, Armstrong grew into a skilled Navy combat pilot (he flew 78 missions in the Korean War), an engineer and a test pilot who flew the experimental X-15 rocket plane to the edge of outer space. As a pilot and astronaut, his unflappable calm was more than a personality trait; it was a survival skill. On his first space mission, in 1966, Armstrong docked Gemini 8 with a second vehicle, but the craft immediately fell into a continuous, stomach-churning roll. Armstrong fired the vehicle's reentry controls, aborting the mission but saving the spacecraft, himself and pilot David Scott. As Tom Wolfe described him in 'The Right Stuff,' Armstrong's facial expression 'hardly ever changed. You'd ask him a question, and he would just stare at you with those pale-blue eyes of his, and you'd start to ask the question again, figuring he hadn't understood, and -- click -- out of his mouth would come forth a sequence of long, quiet, perfectly formed, precisely thought-out sentences, full of anisotropic functions and multiple-encounter trajectories. ... It was as if his hesitations were just data punch-in intervals for his computer.' Armstrong left NASA two years after the mission and retreated to Ohio. He became an aerospace engineering professor at the University of Cincinnati, where he taught throughout the 1970s. This period was quieter, but not without its pressures, too. Campus security had to keep a close eye on outsiders who came to the school hoping to catch a glimpse of him. He had a hard time simply eating in public; one colleague recalled in 1999 that Armstrong would interrupt his meals and retreat to his car if he spotted autograph-seekers. Perhaps Armstrong's most uncharacteristic act came in 1979. As his university career wound down, he decided to appear in a series of ads for Chrysler, a company that was then, as now, struggling for its survival. The ads are dimly remembered today, but for Armstrong they marked the beginning of his most financially rewarding period. He eventually did endorsements for General Time and the Bankers Association of America, and slowly prospered by accepting appointments to the boards of corporations that, Hansen says, Armstrong could justify on the basis of offering engineering advice. The list included companies such as Learjet, Taft Broadcasting, United Airlines, Marathon Oil and Morton-Thiokol. The latter company manufactured the rocket boosters that exploded in the 1986 Challenger space shuttle accident. (Armstrong joined Thiokol two years after serving on the federal commission that investigated the disaster.) Armstrong is retired now, and he and his wife -- he divorced and remarried in 1994 -- spend most of their time at leisure, playing golf, traveling, skiing in Colorado during the winter. In reflection, Armstrong has confided in colleagues that he never wanted to be defined by Apollo 11, and by the few hours he spent walking in moondust. Though he is wary of offending NASA, he said as much publicly, in a rare TV appearance in 2005, to promote Hansen's book on '60 Minutes': 'We'd all like to be recognized not for one piece of fireworks, but for the ledger of our daily work.' This seems understandable as a personal matter, but odd -- and impossible -- as a matter of history. The first man on the moon doesn't feel his life is defined by being the first man on the moon? As if the world would remember much more than Armstrong's 'one piece of fireworks'? Would his name belong beside Magellan's or Marco Polo's if not for Armstrong's singular achievement? Hansen can't help agreeing with those who believe Armstrong could have been a more forceful advocate for space exploration, that his reserve was damning. By being reticent he created mystery, and hence a vacuum that was filled with 'craziness' -- such as a long-standing rumor in the Muslim world that Armstrong converted to Islam after hearing the call to prayer on the moon. Not true. But 'because of Neil's personality, he's his own worst enemy,' Hansen says. 'People want to project all kinds of things on him.' Enigmas can be like that. The other view is that Armstrong was as heroic after his return to Earth as he was on his journey beyond it. In a culture that crushes and disfigures the famous, Armstrong was Olympian in his discipline and humility, never tarnishing the grand moment that fate handed him. The ultimate professional, he did what was asked of him, and then went home, spurning the laurels. Indeed, maybe Armstrong knows more than anyone alive about fame and its limitations, that it is cheap, wearying and filled with constant, jangling pressures. What kind of ego requires that? And who needs it -- especially if, like Neil Armstrong, your legacy will never be challenged and your name will live on long after you have departed this Earth? 