Zero longitude was a choice, not a natural scar on Earth
A prime meridian is the agreed 0° line of longitude. Greeks from Eratosthenes to Ptolemy floated Atlantic island baselines and Mercator tried the Canaries; only in 1884 did 22 countries vote for Greenwich. Other worlds still have to pick their own zeros, often by choosing a crater.
On a spheroid, a prime meridian and its 180° anti-meridian form a great ellipse splitting the body into hemispheres. Earth and Moon longitudes run 0° to 180° east and west; other Solar System bodies usually count 0° to 360°, with west longitudes used when rotation is prograde like Earth's.
Greek longitude thinking runs through Eratosthenes in Alexandria, Hipparchus on Rhodes, and Strabo's city lists; Ptolemy first attached longitude numbers consistently, basing zero near the "Fortunate Isles"—often read as Canaries, though his maps fit Cape Verde better—west of Africa's tip so Europe stayed positive. India's fourth-century Surya Siddhanta placed a traditional zero through Avanti (Ujjain).
Printed Ptolemy maps after 1477 and early globes followed him, yet explorers hunted a "natural" zero—Columbus even reported a mid-Atlantic compass oddity. Mercator's 1541 globe ran precisely through Fuerteventura; later charts chased magnetic hypotheses via the Azores until Ortelius's 1570 atlas mixed options. Eighteenth-century longitude prizes and John Harrison's chronometer, plus Flamsteed's star charts, made a single naval reference practical.
By the eighteenth century most European states counted from their own capitals: Paris for France, Berlin for Prussia, Copenhagen for Denmark, Greenwich for Britain. Nevil Maskelyne tipped the balance with 49 issues of the Nautical Almanac, published between 1765 and 1811 and computed from the Royal Observatory, whose figures even French translations kept. At the International Meridian Conference in Washington, D.C., in October 1884, 22 countries voted for Greenwich; France pushed for a neutral line and stuck with Paris until 1911. Since 1984 the formal standard has been the IERS Reference Meridian, fixed by satellite laser ranging and radio observations of quasars instead of George Airy's 1851 transit circle. It runs very close to the old line, and plate motion has nudged the surface zero a few centimetres since.
Source: Prime meridian
