Already for well over a half century, the OMEGA Speedmaster has been a part of those great moments of humanity that tested the limits of physical endurance and courage. Among these were the first manned moon landing in July 1969 and every manned NASA mission since March of 1965. The OMEGA Speedmaster Professional Chronograph, as the only piece of equipment to have been along for all manned NASA space flight programs – from Gemini in the mid-1960s through today’s International Space Station – occupies a very special place in the history of space exploration. Even before they received NASA’s stamp of approval, the astronauts on two one-man Mercury missions had chosen it as their timepiece.
When Buzz Aldrin set foot on the lunar surface in July 1969, the OMEGA Speedmaster Professional earned its characteristic nickname “Moon Watch.” The Speedmaster was designed from the start to stand out from other chronographs. A black dial, luminous hands, a tachymeter, a waterproof case, a screw-in bottom, a curved crystal and an inner case to protect the movement – all of it went into giving this instrument watch its strong character. A small second counter complements the chronograph counters for minutes and hours to complete the distinctive Speedmaster dial. The central chronograph second hand can be started and stopped with the upper lateral pusher and set to zero with its lower counterpart. The caliber 321 was replaced in 1968 by the 861. This improved version used state-of-the-art technology and is still manufactured today under the changed designation 1861. The name change occurred in 1996, when Omega introduced mechanical movements with a high-quality rhodinized finish.
Some more information about the history of the moonwatch you can find here (google translated).