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Prime Meridian

Published by Pacific Islands Ocean Observing System (PacIOOS) | National Oceanic and Atmospheric Administration, Department of Commerce | Metadata Last Checked: April 21, 2026 | Last Modified: 2016-01-25T00:00:00.000+00:00
The Prime Meridian is the meridian (line of longitude) at which the longitude is defined to be 0 degrees. The Prime Meridian and its opposite, the Antimeridian (at +/-180 degrees longitude), form a "great circle" that divides the Earth into the Eastern and Western Hemispheres. By international convention, the Prime Meridian passes through the Royal Observatory, Greenwich, in southeast London, known as the International Meridian or Greenwich Meridian.

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