= Longitude
{title2=$\phi=\operatorname{atan2}(y,x)$}
<Longitude> is an angular coordinate around a chosen polar axis, measured from a reference half-plane. On Earth the conventional reference is the Greenwich meridian. In a Cartesian coordinate convention it is $\phi=\operatorname{atan2}(y,x)$, with a specified range such as $(-\pi,\pi]$.
Together with <latitude> and a radius it describes a point using <spherical coordinates>. Longitude is undefined at the poles because every meridian meets there. Its numerical jump at the chosen meridian cut is a coordinate discontinuity, not a discontinuity of the sphere itself. On an oblate reference ellipsoid, geographic latitude needs an additional geodetic convention.
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