#meridian__longitude__line_of_longitude  an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator; "all points on the same meridian have the same longitude"
  supertype:  #great_circle__greatcircle  a circular line on the surface of a sphere formed by intersecting it with a plane passing through the center
  subtype:  #observer's_meridian  a meridian that passes through the observer's zenith
  subtype:  #prime_meridian__primemeridian  meridian at zero degree longitude from which east and west are reckoned (usually the Greenwich (England) longitude)
  subtype:  #magnetic_meridian  an imaginary line passing through both magnetic poles of the Earth
  subtype:  #date_line__dateline__International_Date_Line  an imaginary line on the surface of the earth following (approximately) the 180th meridian

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