The Apollo 11 mission was, in most respects, a feat of extraordinary precision.
Traveling at a maximum velocity of about seven miles a second, the Saturn V rocket would have launched the crew far off course in the event of even a slight navigational error. From nearly 240,000 miles away, Houston’s Mission Control could track the spacecraft’s position to within 30 feet. The command module’s guidance computer kept time to the millisecond.
And yet for all that precision, no one can say with absolute certainty when, exactly, Neil Armstrong first set foot on the moon.
Most of the details of the moment are canonical: Armstrong took his one small step on July 20, 1969—50 years ago this past Saturday. The step took place just after 10:56 eastern time that night. And Armstrong bookended the step with the lines “Okay, I’m going to step off the [lunar module] now” and “That’s one small step for man, one giant leap for mankind.” (Or was it “one small step for a man,” as Armstrong insisted?) At some point during the roughly eight-second interval between those two lines, he became the first human being to walk on the moon. But when exactly he did so is unclear.
The night of the moon landing, NASA told the press that Armstrong had stepped onto the lunar surface at 10:56:20 p.m., and The New York Times reported that same time stamp on its front page the next morning. The real-time transcription of the mission’s air-to-ground voice transmission suggests that Armstrong took the step sometime between 10:56:43 and 10:56:48. And when NASA’s official Apollo 11 mission report went public in November 1969, it pinpointed first contact five seconds earlier, at 10:56:15.