The Challenge: Measure Latitude and Longitude
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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 northsouth 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).