Latitude lines run east to west from equator, labeled with N (north) or S (south). Longitude lines run north to south from prime meridian, labeled with E (east) or W (west). Coordinates are written as latitude followed by longitude, separated by commas
A frame of reference is an abstract coordinate system with specified origin and scale. Reference points are identified mathematically and physically. In n dimensions, n+1 reference points are sufficient to define a frame
Greenwich meridian was the international standard from 1884 to 1974. Sir George Airy established the meridian in 1851. By 1884, over two-thirds of ships used it as reference. France abstained from the vote, continuing to use Paris meridian
Meridian is a line connecting points of equal longitude. Length of meridian is 20,003.93 km on modern ellipsoid. Meridian is perpendicular to all circles of latitude
Two geometric objects are perpendicular if their intersection forms right angles at a foot. Perpendicularity extends to lines, planes, and segments. Perpendicularity is symmetric, meaning perpendicular lines are always parallel
Prime meridian is an arbitrary line of longitude at 0° longitude. Prime meridian and anti-meridian form great circle dividing Earth into hemispheres. Longitudes for Earth and Moon measured from prime meridian to 180°