Since 1969 Omega has been able to promote its Speedmaster chronograph as the "first watch on the moon." NASA had chosen the Swiss-made timekeeper for all its astronauts four years earlier, so it was natural that Buzz Aldrin should have been wearing it when he took his momentous step onto the lunar surface.

Before NASA made its selection, the Speedmaster and its rivals were subjected to rigourous testing: simmered for hours at a temperature of 93 degrees, then immediately frozen at –18, left in pure oxygen for two days, struck, compressed, decompressed, vibrated... The Speedmaster was the only one to emerge well within the allowed deviation of 5 seconds per day.

The Speedmaster is still NASA's choice, and Soviet astronauts also came to appreciate its qualities when they discovered it during the Apollo-Soyuz link-up in 1975.

A timepiece that goes into space faces challenges unknown on earth. The atmospheric pressure inside the watch might make the glass explode in conditions of near-zero gravity. The glass of the Speedmaster is fixed by a clamp ring able to withstand five times the pressure that it experiences in space. It is vital that the internal pressure should remain constant, otherwise the lubricating oil which adheres to the moving parts would leak, preventing the mechanism from working properly, and obscuring the glass.

A vacuum inside the watch would also make the piece run much too fast: it would prevent the balance wheel (which regulates the movement of the gear train) from working properly.

The glass itself is both thick enough and elastic enough to withstand the huge differences in temperature that it encounters during a space flight.

Extremely accurate timekeeping is a matter of life and death for an astronaut on the moon, whose oxygen reserves are measured in minutes and seconds rather than in volume. The same applies to the fuel in his manoeuvre reactors, and to the batteries supplying energy.

Rival manufacturers have attempted to capture the NASA contract from Omega, but in two subsequent trials Omega still came out on top. The 1998 Speedmaster Professional X-33 has already been nicknamed the "Mars Watch."

But what happened to Buzz Aldrin's original moon watch? And why wasn't Neil Armstrong wearing one?

Unfortunately Aldrin's watch disappeared in the early 1970s when it was being transported to the Smithsonian Institution, and its fate remains a mystery. There was much excitement in 2001 – and flurry of claims of ownership – when a retired Californian sales clerk announced that he had bought the missing watch from a man who had picked it up on a Santa Barbara beach. But its authenticity could not be proved, and both NASA and Aldrin eventually decided it was not the Moon Watch.

As for Armstrong's watch, it did not leave the lunar module. The module's electronic counter had broken down, and Armstrong left his watch on board as a back-up.