prime meridian

longitude line; noontime line, the line on which all other time zones are based (Greenwich, England)

The Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory in London — it is the meridian at which longitude is defined to be 0 degrees. The prime meridian, and the opposite 180th meridian (at 180° longitude), which the International Date Line generally follows, separate the Eastern and Western Hemispheres. The Prime Meridian and the 180th meridian form a great circle which encircles the Earth like a longitudinal equator.
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Noun
1. meridian at zero degree longitude from which east and west are reckoned (usually the Greenwich longitude in England)
(hypernym) meridian, longitude, line of longitude

Meridian of longitude 0 degrees, used as the origin for measurements of longitude. The meridian of Greenwich, England, is the internationally accepted prime meridian on most charts. However, local or national prime meridians are occasionally used.

prime meridian
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