The 0˚ meridian (also called the prime meridian) is drawn through Greenwich, England. Meridians are numbered east and west from the prime meridian.
Meridians of longitude are imaginary half-circles running from the North Pole to the South Pole. They are sometimes called lines of longitude. Unlike parallels of latitude that are different sizes, all lines of longitude are the same length. Since every meridian must cross the equator, and since the equator is a circle, the equatorial circle can be divided into 360°. These divisions of the equatorial circle are used to label the meridians.
Longitude is the distance east or west of the prime meridian, and longitude is measured in degrees from 0˚ to 180˚. Places to the east of the prime meridian have east longitude.
East and west longitude meet at the 180˚ meridian, which runs through the Pacific ocean basin. Therefore, most of the United States lies in the western hemisphere. Only a small portion of Alaska (part of the Aleutian Islands) crosses the 180˚ meridian into the eastern hemisphere. The complete circle around the earth made by the prime meridian (0˚) and the 180˚ meridian divide the earth into eastern and western hemispheres.
To learn more, see Exploring Our Fluid Earth.