Meridians and parallels—the basis for measuring time, distance,
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 "§." Lines of latitude south
of the equator are described as degrees south (S). The equator is at 0§.
The North Pole is at 90§ N, and the South Pole is at 90§ 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 0§ longitude at Greenwich
to 180§ 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 42§ 30' N, 83§ 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 15§ westward from Greenwich and advances by one hour
as pilots fly every 15§eastward until reaching the International Dateline
halfway around the globe (or at 180§ longitude). Pilots use GMT to refer
to the time of day rather than using local time zones.