What imaginary line divides the Earth into Eastern and Western Hemispheres?

The Prime Meridian is the imaginary line at 0° longitude that, together with the 180th meridian, divides Earth into the Eastern and Western Hemispheres.

Longitude measures how far east or west a location lies from this reference line. The modern standard is the IERS Reference Meridian, which passes very close to the historic Greenwich Meridian in London. Because Earth is constantly moving and because older measurements relied on local astronomical observations, the modern geodetic line is about 102 metres from the famous line marked at Greenwich.

The choice of a prime meridian is a human convention, unlike the Equator, which is defined by Earth’s rotation and poles. In 1884, the International Meridian Conference selected Greenwich as the international reference. The opposite line, at 180° longitude, forms the other half of the great circle and broadly aligns with the International Date Line.

A common mix-up is the Equator: it divides the Northern and Southern Hemispheres, not the Eastern and Western ones.