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.
See also
Great Circle, Latitude, Meridian, North Pole, Oblate Spheroidal Coordinates, Prolate Spheroidal Coordinates, South Pole, Sphere, Spherical Coordinates
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Cite this as:
Weisstein, Eric W. "Longitude." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Longitude.html