Longitude

To locate positions in east and west directions, cartographers use lines of longitude, also known as meridians.  longitude is the distance in degrees east or west of the prime meridian, which is the reference point for longitude. The prime meridian represents 0° longitude. In 1884, astronomers decided that the prime meridian should go through Greenwich, England, home of the Royal Naval Observatory. Points west of the prime meridian are numbered from 0° to 180° west longitude (W); points east of the prime meridian are numbered from 0° to 180° east longitude (E).

Semicircles Unlike lines of latitude, lines of longitude are not parallel. Instead, they are large semicircles that extend vertically from pole to pole. For instance, the prime meridian runs from the north pole through Greenwich, England, to the south pole. The line of longitude on the opposite side of Earth from the prime meridian is the 180° meridian. There, east lines of longitude meet west lines of longitude. This meridian is also known as the International Date Line, and will be discussed later in this section.

Degrees of longitude Degrees of latitude cover relatively consistent distances. The distances covered by degrees of longitude, however, vary with location. As shown in Figure 2.2, lines of longitude converge at the poles into a point. Thus, one degree of longitude varies from about 111 km at the equator to 0 km at the poles.

Using coordinates. Both latitude and longitude are needed to locate positions on Earth precisely. For example, it is not sufficient to say that Charlotte, North Carolina, is located at 35°14´ N because that measurement includes any place on Earth located along the 35°14´ line of north latitude. The same is true of the longitude of Charlotte; 80°50´ W could be any point along that longitude from pole to pole. To locate Charlotte, use its complete coordinates — latitude and longitude. 

Time zones Earth is divided into 24 time zones. Why 24? Earth takes about 24 hours to rotate once on its axis. Thus, there are 24 times zones, each representing a different hour. Because Earth is constantly spinning, time is always changing. Each time zone is 15° wide, corresponding roughly to lines of longitude. To avoid confusion, however, time zone boundaries have been adjusted in local areas so that cities and towns are not split into different time zones.