At the Royal Observatory in Greenwich, England, a brass strip marks zero degrees longitude. The Western Hemisphere is on one side, the Eastern Hemisphere is on the other.

Photograph by Bruce Dale
  • On October 13, 1884, the Greenwich meridian was adopted as the prime meridian, the imaginary north-south line where the longitude is 0 degrees. The Greenwich meridian divides the Earth into the Eastern and Western Hemispheres.

    Unlike parallels of latitude, which are defined by their distance from the Equator and the poles (mathematically defined regions), the prime meridian is arbitrary and had to be agreed upon by the scientific community. U.S. President Chester A. Arthur asked delegates from 25 nations to meet in Washington, D.C., in order to select a site. Participants voted on the meridian that passes through the Royal Observatory, Greenwich, in London, England.

    Today, the Greenwich meridian is visible in London at night in the form of a green laser beam pointing north from the Royal Observatory.

  • Term Part of Speech Definition Encyclopedic Entry
    adopt Verb

    to formally raise and care for a child of other biological parents.

    arbitrary Adjective

    determined by choice, not by standards or rules.

    arbitrary Adjective

    determined by choice, not by standards or rules.

    delegate Noun

    representative in government or a person who represents someone else.

    Equator Noun

    imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

    Encyclopedic Entry: equator
    hemisphere Noun

    half of a sphere, or ball-shaped object.

    Encyclopedic Entry: hemisphere
    laser Noun

    (acronym for light amplification by stimulated emission of radiation) an instrument that emits a thin beam of light that does not fade over long distances.

    latitude Noun

    distance north or south of the Equator, measured in degrees.

    Encyclopedic Entry: latitude
    longitude Noun

    distance east or west of the prime meridian, measured in degrees.

    Encyclopedic Entry: longitude
    meridian Noun

    line of longitude, dividing the Earth by north-south.

    prime meridian Noun

    imaginary line around the Earth running north-south, 0 degrees longitude.

    Encyclopedic Entry: prime meridian