Where east and west begin
The prime meridian is the reference line for 0° longitude, used to measure positions east and west around Earth. The historic Greenwich meridian was chosen by an international conference in 1884.
Unlike the equator, it does not have a unique natural position. People had to agree where their zero would go.
Why Greenwich?
Many charts already used Greenwich as a reference. Choosing it kept an established navigational habit, as NOAA explains.
Royal Museums Greenwich describes earlier alternatives, including Paris. A coordinate system is much easier to share once everyone starts counting from the same place.
Your phone has a twist
A GPS receiver’s modern zero lies about 102 metres east of the historic line at Greenwich. Satellite-based measurements use a later reference meridian.
The old marker has not wandered off. The reference system changed. That is a small distance with a lovely lesson: even a line labelled zero comes with a history.
Explore ten geography terms to picture whenever you fancy another short read.