An Etymological Dictionary of Astronomy and Astrophysics
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The → prime meridian that separates east from west in the same way that the Equator separates north from south. It is defined by the position of the → Airy transit circle.

A borough in southeast London, England, on the Thames River. It is the site of the original Royal Observatory, through which passes the prime meridian, or longitude 0°; → meridian.