The determination of time and frequency

In the measurement of frequency to a precision of 1 part in 108, or better, it is necessary to consider small variations that may exist in the unit of time. The determination of time is an astronomical process, and the paper reviews briefly the principles involved. Various factors affecting the uniformity of the observed time are described. Reference is made to the astronomical instruments employed or planned for use for the determination of time at the Royal Greenwich Observatory. The clocks and other equipment are next described, together with the methods currently employed in the operation of the time service. The standard of accuracy now attained is discussed in relation to both time and frequency, and it is shown that, for frequency, agreement between Greenwich and the U.S. Naval Observatory is normally within 1 part in 108. In practice, two time systems are now employed: Greenwich Mean Time, based directly on the astronomical observations and applicable to surveying and astronavigation, and a more uniform time system which is not yet precisely defined, suitable for accurate work in frequency measurement and in related scientific investigations.