Contrary to urban myth, NASA did use the metric system for the Apollo Moon landings. SI units were used for arguably the most critical part of the missions – the calculations that were carried out by the Lunar Module’s onboard Apollo Guidance Computer (AGC) during the computer-controlled phases of the spacecraft’s descent to the surface of the Moon, and for the journey of the Ascent stage of the craft during its return to lunar orbit, where it would rendezvous with the Command and Service Module (CSM).As is the case in the UK with road signage, the use of metric units in the USA is often hidden from public view. The Apollo Guidance Computer is a good example of this. The computer display readouts were in units of feet, feet per second, and nautical miles – units that the Apollo astronauts, who had mostly trained as jet pilots, would have been accustomed to using. Internally, however, the computer’s software used SI units for all powered-flight navigation and guidance calculations, and values such as altitude and altitude rate were only converted to imperial units when they needed to be shown on the computer’s display.