Zero degrees longitude is known as the Prime Meridian and separates the Eastern and Western Hemispheres. It extends from the North Pole to the South Pole and passes through Greenwich, England. In the Pacific Ocean, at 180 degrees, is the International Date Line, which also separates the hemispheres.
The Prime Meridian is important as a reference point in navigation. It is similar to the Equator, which is at zero degrees latitude and divides the Northern and Southern Hemispheres. Coordinated Universal Time is also determined by the Prime Meridian. This was previously known as Greenwich Mean Time, because the Prime Meridian is located in that city. All time zones that exist around the world use the time zone at the Prime Meridian as their starting point.