From MapHist.com - Glossary of Terms for historical maps

Glossary of Terms
Longitude
By Pierre Joppen
Jul 7, 2004, 08:44

Part of a spherical reference system used to locate positions on the surface of the Earth. The angular distance in degrees, minutes, and seconds measured from the centre of the Earth to a point east and west of the Greenwich Meridian (prime or international meridian).

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