To refer to a specific place on the planet a coordinate system is needed. We’re using longitude (west/east) and latitude (north/south) to tell where we are. The latitude is 0 on the equator but for longitude a position for 0 – the prime meridian – had to be defined. On a conference in Washington, United States different alternatives were analysed and it was defined that the prime meridian should be the one going through the Royal Observatory at Greenwich, London (as it was used most often by that time).
Therefore if you go to Greenwich you can jump from west to east or divide yourself. The observatory was founded by Charles II in 1675 and became the central workplace for the royal astronomers. The well-known Greenwich Mean Time (GMT) was the time measured here – but it was replaced with the Coordinated Universal Time (UTC) in 1972. The Royal Observatory today contains an important museum.
England, United Kingdom