In geography and geodesy, a meridian is the locus connecting points of equal longitude, which is the angle east or west of a given prime meridian. In other words, it is a line of longitude. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. On a Mercator projection or on a Gall-Peters projection, each meridian is perpendicular to all circles of latitude. A meridian is half of a great circle on Earth's surface. The length of a meridian on a modern ellipsoid model of Earth has been estimated as 20,003.93 km (12,429.87 mi).
Meridians run between the North and South poles.
The astronomic prime meridian at Greenwich, England. The geodetic prime meridian is actually 102.478 meters east of this point since the adoption of WGS84.
Geodesy is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D. It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems.
A modern instrument for geodetic measurements using satellites
Geoid, an approximation for the shape of the Earth; shown here with vertical exaggeration (10000 vertical scaling factor).
Height measurement using satellite altimetry
GPS Block IIA satellite orbits over the Earth.
