Longitude is defined in terms of meridians, which are half-circles running from pole to pole. A reference meridian, called the prime meridian , is selected, and this forms the reference by which longitudes are defined. On the earth, the prime meridian passes through Greenwich, England; for this reason it is also called the Greenwich meridian. The longitude of a point P on the surface is defined as the angle that the plane containing the meridian passing through P subtends with respect to the plane containing the prime meridian. If P is to the east of the prime meridian, the longitude is positive; if P is to the west of the prime meridian, the longitude is negative. Longitude angles can range up to +180 degrees (180 degrees east), and down to -180 degrees (180 degrees west). The +180 and -180 degree longitude meridians coincide directly opposite the prime meridian.

Latitude and longitude coordinates on the earth are sometimes extended into space to form a set of celestial coordinates.

Latitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface.

Longitude (shown as a vertical line) is the angular distance of a point east-west of the Prime(Greenwich) Meridian. Lines of longitude are termed as meridians.