The whole world's longitude is measured from one line in London
In 1884, 22 nations agreed that zero degrees would run through a telescope in Greenwich.
Every point on Earth has a longitude measured east or west from a single reference: the Prime Meridian, longitude 0 degrees, running through the Royal Observatory in Greenwich, London.
That choice was a decision, not an accident of geography. In October 1884, delegates from 25 countries met in Washington, D.C. at the International Meridian Conference. 22 of them voted to fix the prime meridian on the meridian passing through the Greenwich transit instrument. Greenwich won partly because most of the world’s shipping already used charts based on it.
The meridian does more than split the eastern and western hemispheres. From 1884 it also became the anchor of the global time-zone system — the reason clocks worldwide are still set as offsets from Greenwich Mean Time.
The line is precise to a telescope’s crosshairs: the Airy Transit Circle’s eyepiece literally defined zero degrees longitude for the planet.
Sources & references
2 referencesWell-established. Corroborated by 2 independent sources.
