The Royal Observatory, Greenwich is famous around the world as the home of Greenwich Mean Time. Widely used across the country from the 1850s onwards, Greenwich Mean Time became the legal Time of Great Britain in 1880. It was adopted in principle as the basis of universal time in 1884, thereby securing its future place at the heart of our global system of Time Zones.

From the early 1820s until the 1960s, time related work occupied by far the greatest portion of staff time at the Observatory. It involved both the determination of time by astronomical means, together with its dissemination (the Greenwich Time Service), and also the testing and repair of chronometers for the Admiralty.

Accurate measurement of the local time differences between the Observatory and other locations around the world to determine their relative longitudes also formed part of the Observatory’s work – achieving the required level of precision however, was often easier said than done.


The astronomical basis of timekeeping

The Greenwich Time Service (the dissemination of time)

The Time Department Control Rooms and Time Desk


Astronomical Regulators

Quartz and atomic clocks

Journeyman clocks


Alarum Clocks

Other clocks and chronometers owned by the Observatory


Pendulum experiments

Longitude determinations


Articles by members of the Royal Observatory staff

The steady march of atomic time. Humpry Smith (former Head of Time Department), New Scientist 11 February 1982