How long did Apollo 11 spend on the Moon?

The lunar module Eagle landed on July 20, 1969, at 20:17 UTC in the Sea of Tranquility and lifted off on July 21 at 17:54 UTC for the rendezvous with Michael Collins in lunar orbit. Between those two moments, 21 hours and 36 minutes passed on the surface. Only a small part of that was work outside: Neil Armstrong and Edwin Aldrin's moonwalk lasted 2 hours and 31 minutes. The rest of the time went to checking systems, preparing for the walk, eating, and an uncomfortable rest period in the cramped cabin.

The Eagle was a minimal vehicle, built just for that tight schedule. Power came from batteries, cooling from evaporating water, and the suits had barely any reserves left after the moonwalk. Orbital mechanics added another constraint: liftoff had to fall within a window when Collins, in the command module, would pass over the landing site at the right moment. Longer stays required a bigger vehicle, more consumables, and experience that was still missing in 1969; these came with later Apollo flights and their lunar rovers.

The flight plan called for a rest period before the moonwalk; the crew skipped it and went out earlier. Outside, both astronauts set up measuring instruments and the flag, collected rocks and dust, took photographs, and spoke briefly with the White House. What followed was the return to the cabin, repressurization, discarding equipment no longer needed, and an attempt to sleep in a space that, because of the lunar dust tracked in, smelled of burnt gunpowder. Only then did they fire the ascent stage.