Which imaginary line passes through Greenwich in London at 0° longitude?

The prime meridian is the imaginary line that passes through Greenwich at 0° longitude.

Longitude measures positions east or west of a reference meridian. In 1884, representatives at the International Meridian Conference in Washington, D.C., selected the meridian passing through the Royal Observatory at Greenwich as the international zero line. This decision helped standardize maps, navigation, and global timekeeping.

The modern reference used by satellite navigation is the International Reference Meridian, which lies roughly 102 meters east of the historic Greenwich line because Earth’s measurement systems were refined. The prime meridian is not the same as the Equator: the Equator divides Earth into northern and southern hemispheres, while the prime meridian divides east and west for longitude. Together, latitude and longitude identify locations on Earth.