= Azimuthal angle in cylindrical coordinates
{title2=$\phi$}
= Azimuthal angle
{synonym}
The azimuthal <angle> $\phi$ in <cylindrical coordinates> is measured in the horizontal plane from a chosen reference axis. In the usual mathematical convention $x=s\cos\phi$, $y=s\sin\phi$, so positive $\phi$ runs counterclockwise from positive $x$ when viewed from positive $z$. This same angular coordinate occurs in <spherical coordinates>. It is defined modulo $2\pi$ away from the polar axis and is nonunique on the axis.
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