Abstract
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 zero-longitude lines at Greenwich, the orientation of the meridian plane used to measure Universal Time has remained essentially unchanged.
Highlights
The Airy Transit Circle is a nineteenth-century telescopic instrument at the Royal Observatory, Greenwich, famous as the origin for global longitude
The 1884 International Meridian Conference recommended that the prime meridian, Λ0 = 0, pass through the Airy Transit Circle at Greenwich (Explanatory Supplement 1961), establishing the mean solar time determined at the Airy Transit Circle—Greenwich Mean Time (GMT)—as an internationally recognized basis for global timekeeping and navigation
The 102-m offset between the Airy Transit Circle and zero longitude indicated by a GNSS receiver is attributable to the fact that continuity in the UT1 time series was maintained in the Bureau International de l’Heure (BIH) reference frames, as geodetic longitudes replaced astronomical longitudes when space-geodetic methods were introduced
Summary
The Airy Transit Circle is a nineteenth-century telescopic instrument at the Royal Observatory, Greenwich, famous as the origin for global longitude. Its longitude is 00◦00 05.3 W (Howse 1997) in the International Terrestrial Reference Frame (ITRF), and in the World Geodetic System (WGS 84) used by the U.S Global Positioning System (GPS). 2 and 3, the authors show that the deflection of the vertical (DoV) can account for the entire longitude shift at Greenwich. 4, the authors compare astrogeodetic and gravimetric DoVs and their uncertainties, using a recent global gravitational model, for a number of astronomical observatories that contributed to past determinations of UT1. This provides information about the overall orientation between the former (astronomical) and modern (geodetic)
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