Prime Meridian
The Prime Meridian is the reference line for 0 degrees longitude, used to measure east-west position and connect geography with global timekeeping.
What the Prime Meridian is
The Prime Meridian is the chosen zero line for longitude. It runs from pole to pole and gives mapmakers, navigators, surveyors, and software a shared starting point for measuring positions east or west around Earth.
Why Greenwich became the standard
Before the late nineteenth century, different countries and mapmakers used different zero meridians, often tied to national observatories or capitals. That made charts, timekeeping, and long-distance navigation harder to align. In 1884, delegates at the International Meridian Conference chose the meridian through the Royal Observatory at Greenwich as the international reference for longitude.
Longitude and hemispheres
Longitude values increase east and west from 0 degrees to 180 degrees. The Prime Meridian and the 180th meridian form a great circle that is commonly used to divide Earth into the Eastern and Western hemispheres. Unlike the Equator, this division is based on a chosen convention rather than a natural halfway line between poles.
Time and the meridian
The Greenwich meridian became closely tied to world time because local noon depends on when the Sun crosses a meridian. Greenwich Mean Time helped standardize railway, maritime, and international schedules, while modern Coordinated Universal Time is now the global time standard used with time zones and digital systems.
Greenwich and the modern zero line
The bronze strip and laser line at Greenwich mark the historic Airy meridian, defined by the Airy Transit Circle telescope. Modern satellite-based geodesy defines zero longitude by the International Reference Meridian, which is slightly east of the historic line because it is based on Earth's center and global reference frames rather than a local vertical line at Greenwich.
Why it matters
The Prime Meridian is a reminder that some map lines are conventions with real consequences. A single reference for longitude supports navigation, time zones, GPS, mapping databases, and international communication. It lets a place be described in a way that can be understood across languages, borders, and technologies.