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)

A brief history of the Time Department from 1923 to 1948

Extracts from the Reports of the Astronomer Royal relating to the Time Service (1937–1960)


Clocks and Chronometers

Astronomical Regulators

Quartz and atomic clocks

Journeyman clocks


Alarum Clocks

Other clocks and chronometers owned by the Observatory


Longitude determinations and Pendulum experiments

Pendulum experiments

Longitude determinations


In pictures

The Time Department Control Rooms and Time Desk


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

Time related activities at the Royal Observatory Greenwich. John Pilkington (1986). When Humphry Smith retired as Head of the Time Department in June 1977, John Pilkngton took over his role. In this paper, published as part of the Proceedings of the Eighteenth Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting, Washington, DC, 2-4 Dec 1986, he presents a bleak outlook for the future of the Department describing how a once World renowned pioneering department at the forefront of technology had evolved into a bit-part player in global timekeeping. Trying to remain diplomatic, he appears to have omitted the fact that the Observatory’s six atomic clocks were being left to run down and stop over the next couple of years.