The azimuthal coordinate on the surface of a sphere (
in spherical
coordinates) or on a spheroid (in prolate
or oblate spheroidal coordinates).
Longitude is defined such that 
. Lines of constant longitude are generally
called meridians. The other angular coordinate on the
surface of a sphere is called the latitude.
The shortest distance between any two points on a sphere is the so-called great circle distance, which can be directly computed from the latitude and longitudes of two points.
