A useful starting line became a global standard
Longitude tells you how far east or west a place lies around Earth. But unlike the equator, which follows the planet’s shape, zero longitude has no unique natural position: any meridian could serve as the starting line. For centuries, mapmakers and seafaring nations used different ones, making charts harder to compare.
Greenwich was already a practical choice. The Royal Observatory there produced astronomical observations that helped navigators determine longitude, and British ships commonly relied on charts built around Greenwich. Since British charts were already widely used, choosing their zero line meant less disruption for mariners and mapmakers.
Agreement made the line useful everywhere
In 1884, an international conference in Washington adopted the Greenwich meridian as the prime meridian—the agreed line from which longitude is measured. The decision did not make Greenwich geographically special; it made one existing system the common reference point.
That shared reference also helped coordinate time. Once a place’s longitude could be stated relative to the same zero line, maps, navigation calculations, and time standards could be aligned instead of translated between competing systems. The line’s lasting importance comes from that practical agreement: Greenwich became zero because a widely used chart system gave the world a workable place to begin.
