The imaginary line drawn from the North Pole extending to the South Pole and running through Greenwich is the Prime Meridian.
Just as the Equator divides the north and south hemispheres, the Prime Meridian divides the globe into east and west hemispheres.
All lines on the Earth’s surface which run north to south as per the Prime Meridian are called Meridians of Longitude. Depending on which side of the Prime Meridian they lie, they are named east or west.
Longitude is the distance east or west of the Prime Meridian measured in degrees.
000° is the prime meridian (through Greenwich).
The longitude of every point between must either be:
- Measured in degrees East from 000° to 180°, or
- Measured in degrees West from 000° to 180°
The Meridian at 180° (180th meridian or antemeridian) east or west of the Prime Meridian divides the Earth into eastern and western hemispheres. (This is also used for the basis for the international date line, the line does not run straight though, it zigzags to avoid political and country borders. It also moves to avoid cutting some countries in half).
That’s a lot of words – a diagram is easier.
TIP: If you come across a longitude written that is more than 180°, you will instantly recognise that there is an error somewhere.