On July 21st 1969, Neil Armstrong was the first human to set foot on the Moon, wearing a Speedmaster. In 1970 the watch proved invaluable when after a series of explosions on board of the Apollo XIII a number of important instruments out of order. There was no contact possible with Flight Control, and the astronauts had to calculate and time the return into the atmosphere. An error of a few seconds could have been fatal. Divers chrono Divers have their own time piece as well.
Basic requirement is that the case is waterproof.
That is why a divers chronograph has round pushers, a screw on crown, and a screw back case which is provided with rubber gasket-rings to keep water out. At a depth of 30 meters there is hardly any daylight penetrating, so luminescent numereals and hands on a dark dial are necessary. The most important feature of any divers watch and an essential part of every divers equipmentis the diving ring. This is a rotating bezel around the dial or the crystal. It is devided in minutes and often has a triangular fluorescent marking at 12.