The Prime Meridian also known as the Greenwich Meridian, is the imaginary vertical line passing through the Royal Greenwich Observatory in London. It is the meridian at which the longitude is 0 degrees. The Prime Meridian along with the 180 degree longitude, known as the International Date Line divide the Earth into Eastern and Western Hemispheres. These two meridians encircle the earth like a longitudinal equator.
Here’s a Google Maps Mashup depicting the countries through which the Prime Meridian passes.
