Prime Meridian

The meridian defined as 0° longitude from which east and west longitude are measured.

The internationally adopted reference passes through the Airy Transit Circle at Greenwich for historical mapping purposes, while modern geodetic reference systems realise zero longitude through Earth-centred coordinate frameworks that differ slightly from the historic line on the ground.

Longitude is measured up to 180° east or west. The prime meridian is a longitude origin, not a magnetic reference and not by itself a time-zone boundary in every jurisdiction.