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The concept of "Greenwich Mean Time," and its modern equivalent, Universal Time, is based on the changing angle between a "prime meridian" and some point on the celestial sphere. In 1884, at the International Meridian Conference, it was 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 Airy "transit instrument at the Observatory of Greenwich." Today, observatory visitors must walk approximately 102 m east before their satellite-navigation receivers indicate zero longitude. The need to maintain continuity in Universal Time by the Bureau International de l"Heure, in conjunction with a transition from astronomical to geodetic coordinates by 1984, when optical astronomical methods were replaced by modern techniques, is responsible for this offset. The difference between astronomical and geodetic longitudes is the deflection of the vertical in the east-west direction. Modern techniques enabled the establishment of a global reference frame centered at the center of mass of the Earth, through which the plane of the geodetic prime meridian passes. While the geodetic prime meridian does not intersect the Airy transit instrument at the surface of the Earth, its orientation with respect to the celestial sphere has remained intact.

DOI: 10.1007/978-3-319-59909-0_28

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