Greenwich Mean Time or its modified form, British Summer Time, is the legal standard time of the United Kingdom. Greenwich Royal Observatory was set up in 1675, and one of its main objectives was to establish a standard of time in connection with the determination of longitude at sea. Its famous Octagon Room, designed by Sir Christopher Wren, was equipped in 1676 with two 'great' clocks by Thomas Tompion, both with 13 ft. Pendulums and a year's going time. The two Tompion year clocks, fitted with dead-beat escapements of a type first proposed by Richard Towneley of Burnley, were later somewhat modified by Tompion and eventually succeeded by clocks with the Graham type of dead-beat escapement and mercury-compensated pendulums. From 1872 to 1925 the standard timekeeper was a Dent clock with G. B. Airy's modification of the spring-detent escapement, zinc-and-steel compensation pendulum and barometric compensation. This was followed in 1925 by a Shortt free-pendulum clock with its pendulum swinging almost entirely freely in a case at constant air pressure, served by a slave pendulum which impulsed the free pendulum and was afterwards synchronized by a signal from it. The error of the Dent clock was about 0. 01 seconds per day, and of the Shortt clock a few thousandths of a second. The Shortt clock was superceded in 1942 by a group of quartz-crystal vibrating at 100 kilocycles per second, electronic gearing reducing this to 50 cycles per second, the final time indication being a continuously rotating hand. Before 1955 the Greenwich timekeeper was checked against the stars by a transit instrument and since then by a photo-zenith tube. A new precision method of measuring time, developed between 1945 and 1955, was the atomic clock, in which the timekeeping element is a molecular or atomic vibration. The caesium atomic clock, brought into practical use by Louis Essen and J. V. L. Parry at the National Physical Laboratory, Teddington, in 1955, was accurate to one part in 10 to the tenth power, equivalent to an error of one second in 300 years. This clock is not free-running, but is used to synchronize a system of quartz-crystal clocks. Greenwich Mean Time has been based on the atomic clock since 1967. A second National Physical Laboratory atomic clock of 1959 was seven times more accurate than the first, and was later improved still further to an accuracy of one part in 10 to the twelfth power, corresponding to an error of one second in 30,000 years. Accuracy of this order, when combined with astronomical observation, makes it possible to detect variations in the rotation of the Earth itself, revealing an annual fluctuation together with a slow drift. By 1967 the atomic clock had been shown to be a more accurate timekeeper than the Earth itself, which led to an international decision to base the unit of time, i. E. The second, not fundamentally on the rotation of the Earth but on an atomic vibration, and the General Conference on Weights and Measures thus defined the second as the period of 9,192,631,770 vibrations of an atom of caesium 133 in zero magnetic field. The work of Greenwich Observatory was gradually transferred to Herstmonceux, Sussex between 1946 and 1958, but Greenwich Mean Time remains the time of the meridian of Greenwich itself. From 1st January 1972 the world time signals broadcast from many countries have been based on atomic time and converted in steps to Universal Time to allow for the slowing down of the Earth's rotation.