The global coordinate system is constructed to match the surface of the Earth. It consists of 360 divisions that intersect at the North Pole and the South Pole and parallel lines that circle the Earth and are parallel to the equator. The lines that intersect the poles are called lines of longitude or meridians. Those parallel to the equator are called lines of latitude or parallels. Medians are perpendicular to the equator.

The equator divides the Earth into the Northern and Southern hemispheres. Lines of latitude north of the equator are described as degrees north (N), represented by the symbol "º." Lines of latitude south of the equator are described as degrees south (S). The equator is at 0º. The North Pole is at 90º N, and the South Pole is at 90º S. Lines of latitude are equally spaced from each other. Each degree of latitude is approximately 60 nautical miles or 69 statute miles (111 kilometers) from the next.

he prime meridian divides the Earth into the Eastern and Western hemispheres. The prime meridian is the line of longitude that runs through Greenwich, England. Points on the globe are measured from Greenwich in an eastward or westward direction in units called degrees, from 0º longitude at Greenwich to 180º east or west. All the lines of longitude converge at the two poles and are equally spaced from each other only at the equator.

Each degree of latitude and longitude is divided into 60 minutes, indicated by an apostrophe, and each minute is divided into 60 seconds, indicated by a quotation mark.

Every location on the Earth's surface can be described in terms of its latitude and longitude, counting degrees north or south of the equator and east or west of Greenwich. When describing a position, the latitude is listed first. Minutes and seconds can be used to give a more precise location description. For instance, a part of Detroit, Michigan, is located at 42º 30' N, 83º W.

Time is also measured from Greenwich. The time at Greenwich is referred to as Greenwich Mean Time (GMT). Universal Coordinated Time (UTC), or ZULU. The globe is divided into 24 time zones—12 to the east of Greenwich and 12 to the west of Greenwich. Time retreats by one hour as pilots fly every 15º westward from Greenwich and advances by one hour as pilots fly every 15ºeastward until reaching the International Dateline halfway around the globe (or at 180º longitude). Pilots use GMT to refer to the time of day rather than using local time zones.