= Polar angle
{title2=$\theta$}
In the usual physics convention for <spherical coordinates>, the polar angle is the <angle> between the positive $z$ axis and the <position vector>. For $r=\sqrt{x^2+y^2+z^2}>0$, it satisfies $\cos\theta=z/r$ with $0\leq\theta\leq\pi$. The north and south axes have $\theta=0$ and $\theta=\pi$, respectively; the <plane> $z=0$ has $\theta=\pi/2$. The angle is undefined at the origin. In <plane polar coordinates>, the same phrase usually denotes the oriented in-plane angle instead, so the reference axis and convention should always be stated. The <azimuth> supplies the other angular coordinate in three dimensions.
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