= Orbital speed
{title2=$v$}
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Orbital <speed> is the magnitude of the <velocity> of an orbiting body in a specified reference frame. For a Newtonian <circular orbit> around a point source of <mass> $M$, equating $v^2/r$ to $GM/r^2$ gives $v=\sqrt{GM/r}$. In a curved <spacetime>, a speed inferred from coordinate radius and coordinate time generally differs from a speed measured by a local observer. For a circular orbit in the <Schwarzschild metric>, a static local observer measures $v_{\rm local}=r|d\phi/dt|/\sqrt{1-2GM/(c^2r)}$; $r|d\phi/dt|$ uses the clock normalization at infinity instead.
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