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Definition: Prime Meridian from Merriam-Webster's Geographical Dictionary

The meridian of 0° long. which runs through the original site of the Royal Greenwich Observatory at Greenwich, England, and from which other longitudes are reckoned.

Summary Article: Prime Meridian from Encyclopedia of Time: Science, Philosophy, Theology, & Culture

The prime meridian is an imaginary half-circle reference line extending north and south from one pole to the other and passing through Greenwich, England. It marks 0° longitude and divides the western and eastern hemispheres of the earth. The line that meets it at both poles and is exactly 180 degrees away from the prime meridian is the international date line. The prime meridian is the standard basis for determining time throughout the world. It is the starting point for all the world's time zones. This meridian is the location of UTC (universal time coordinated) as well as Greenwich mean time (GMT). Noon GMT is defined as the time at which the sun crosses the Greenwich (prime) meridian.

An international agreement in 1884 established the prime meridian in its current location. This line runs through the transit circle telescope, built in 1850 by Sir George Biddell Airy, the seventh astronomer royal, at the Royal Observatory Meridian Building in Greenwich, England. The crosshairs in the eyepiece of the transit circle telescope precisely define 0° longitude for the world. It is located at 51° 28' 38' north latitude. As the earth's crust is moving very slightly all the time, this exact position moves very slightly.

Longitude is a measure of both time and location on the earth. As the earth spins on its axis, a specific location and time can be determined relative to the prime meridian. This north-south line marks noon of the day that begins at the international date line and is the origin from which east or west longitude is measured. One degree of longitude equals 4 minutes of time the world over, but the distance of 1° longitude varies depending on the latitude of the location. Every 15° east measured from the prime meridian marks another hour later; every 15° to the west marks another hour earlier.


Lines of latitude and longitude appeared on maps at least 3 centuries before the Christian era. Hipparchus was the first astronomer to determine the difference in longitude and chose Rhodes as the location for his prime meridian, that is, his 0° east or west. By 150 CE, Ptolemy plotted grid lines on 27 maps of his first world adas. The equator was set from previous astronomical observations, and he chose a line running through the Canary Islands as die prime meridian for his maps.

Later mapmakers moved the prime meridian to die Azores, Cape Verde Islands, Rome, Copenhagen, Jerusalem, St. Petersburg, Pisa, Lisbon, Rio, Tokyo, Paris, and Philadelphia. This placement of the line diat marks 0° east or west is purely political, as it is not based on any natural phenomenon.

Beginning in 1667, and continuing for over 200 years, French cartographers used the longitude line running through the Paris Observatory to be the prime meridian. Other meridian lines were given names such as the Rose Line but were never used universally. The English king Charles II founded the Royal Observatory at Greenwich in 1675 for the purpose of improving navigation, and its longitude became another prime meridian.

The development of the accurate chronometer by Englishman John Harrison made it possible for English ships to determine their location based on comparison of their time and the time at a known longitude. Harrison was a clockmaker and not an astronomer, but he was able to solve the problem of determining longitude at sea. This was recognized in 1773.

Ocean charts drove the need for one internationally agreed-upon prime meridian. In 1871, the first International Geographic Congress (IGC) met in Antwerp, Belgium. The general view was that the navigation passage charts for all nations should use the Greenwich meridian as zero, so when ships exchanged longitude at sea, locations and times would match. This did not apply to land maps and coastal charts. In 1875, the second IGC met in Rome. France was willing to accept the Greenwich prime meridian if the rest of the world would accept the metric measurement system. In October 1884, at the invitation of U.S. President Chester Arthur, the International Meridian Conference was held in Washington, D.C., and 41 delegates from 25 nations voted 22 to 1 for Greenwich as the location of the prime meridian. Santo Domingo was against it, and France and Brazil abstained. The United States had already chosen Greenwich as the basis for its national time zones. Seventy-two percent of the world's commerce already used sea charts that used Greenwich as the prime meridian.

International commerce and communication required one standard. Since 1884, the world has had only one prime meridian.

See also

Harrison, John, Latitude, Longitude, Time, Measurements of, Time, Teaching, Time Zones

  • Barnett, J. E. ( (1998).). Time's pendulum: The quest. New York and London: Plenum Trade.
  • Raymo, C. ( (2006).). Walking zero: Discovering cosmic space and time along the prime meridian. New York: Walker.
  • Sobel, D. ( (1995).). Longitude: The true story of a lone genius who solved the greatest scientific problem of his time. New York: Penguin Books.
  • Chenhall, Ann L.
    Copyright © 2009 by SAGE Publications, Inc.

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