Sometimes referred to as Lat/Lon, the Geographic coordinate system is a spherical coordinate system composed of parallels of latitude (Lat) and meridians of longitude (Lon). Both divide the circumference of the Earth into 360 degrees. Degrees are further subdivided into minutes and seconds (60 sec = 1 minute, 60 min = 1 degree).
Because the Earth spins on an axis between the North and South Poles, this allows construction of concentric, parallel circles, with a reference line exactly at the north-south center, termed the Equator. The series of circles north of the Equator is termed north latitudes and runs from 0° latitude (the Equator) to 90° North latitude (the North Pole), and similarly southward. Position in an east-west direction is determined from lines of longitude. These lines are not parallel, and they converge at the poles. However, they intersect lines of latitude perpendicularly.
Unlike the Equator in the latitude system, there is no natural zero meridian. In 1884, it was finally agreed that the meridian of the Royal Observatory in Greenwich, England, would be the prime meridian. Thus, the origin of the geographic coordinate system is the intersection of the Equator and the prime meridian. Note that the 180° meridian is the international date line.
The figure above shows the graticule of meridians and parallels on the global surface.