Two coordinates are needed to define a point on its surface. The second coordinate used on the Earth is called Longitude . It is made up of a series of great circles running through both poles and set at right angles to the Equator. These circles are not parallel to each other. They are shown on the right sphere above.
All lines of Longitude are identical.
The zero line of Longitude is called the Zero Meridian or Prime Meridian . Its position must be decided by international agreement. The Earth's Prime Meridian was fixed by astronomers and geographers to run through Greenwich Observatory in South East London, England. The zero meridian is now usually called the Greenwich Meridian . It passes mainly through the Pacific Ocean.