Meridian (geography)
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A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations with a given longitude. Each is half of a great circle on the Earth's surface.
The metre was originally defined as 1 part in 10,000,000 of the length of the meridian passing through Paris from the North Pole to the equator.
The meridian passing through the former Royal Observatory in Greenwich, England is known as the Prime Meridian, and is established by convention as the origin of longitude measurement.