In 1884, the International Meridian Conference recommended that the prime meridian "to be employed as a common zero of longitude and standard of time-reckoning throughout the globe" pass through the "centre of the transit instrument at the Observatory of Greenwich". Today, tourists visiting its meridian line must walk east approximately 102 m before their satellite-navigation receivers indicate zero longitude. This offset can be accounted for by the difference between astronomical and geodetic coordinates-deflection of the vertical-in the east-west direction at Greenwich, and the imposed condition of continuity in astronomical time. The coordinates of satellite-navigation receivers are provided in reference frames that are related to the geocentric reference frame introduced by the Bureau International de l'Heure (BIH) in 1984. This BIH Terrestrial System provided the basis for orientation of subsequent geocentric reference frames, including all realizations of the World Geodetic System 1984 and the International Terrestrial Reference Frame. Despite the lateral offset of the original and current zerolongitude lines at Greenwich, the orientation of the meridian plane used to measure Universal Time has remained essentially unchanged.
Numerous time scales exist to address specific user requirements. Accurate dynamical time scales (barycentric, geocentric and terrestrial) have been developed based on the theory of relativity. A family of time scales has been developed based on the rotation of the Earth that includes Universal Time (specifically UT1), which serves as the traditional astronomical basis of civil time. International Atomic Time (TAI) is also maintained as a fundamental time scale based on the output of atomic frequency standards. Coordinated Universal Time (UTC) is an atomic scale for worldwide civil timekeeping, referenced to TAI, but with epoch adjustments via so-called leap seconds to remain within one second of UT1. A review of the development of the time scales, the status of the leap-second issue, and user considerations and perspectives are discussed. A description of some more recent applications for time usage is included.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.