The Prime Meridian

The Prime Meridian is the imaginary line running between the North and South Poles that's used to indicate 0° longitude. It passes through Greenwich in southeast London, and is therefore sometimes referred to as the Greenwich Meridian. An international conference held in Washington D.C. in 1884 designated "the meridian passing through the center of the transit instrument at the Observatory of Greenwich as the initial meridian for longitude." During the 1950s, the Greenwich Observatory moved to Herstmonceux Castle in East Sussex, where the skies were deemed to be clearer. In 1990, it moved again, to the Institute of Astronomy at the University of Cambridge, but the original site in Greenwich continues to serve as the location for 0° longitude.

As the prime meridian, the north–south line at Greenwich is used as the reference point for all other meridians of longitude, which are numbered east or west of it. The current system employs 24 standard meridians of longitude 15° apart, starting with the Prime Meridian. The lines all run from the North to the South Pole, at right angles to the Equator. These meridians are the centers of 24 standard time zones. In theory, standard time becomes successively one hour earlier at each 15° longitude west of the Prime Meridian, and one hour later at each 15° longitude east. Time is the same throughout each zone. In practice, many of the zones have been subdivided or their shape altered for convenience.

The mean solar time at Greenwich is now called Universal Time (formerly called Greenwich Mean Time or GMT).

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