Circular orbital speed ($v_c=\sqrt{GM/r}$)

For a test body on a circular Kepler orbit about a point mass $M$, the centripetal acceleration $v_c^2/r$ equals $GM/r^2$, so $v_c=\sqrt{GM/r}$. For a spherical extended source use the enclosed mass $M(<r)$ instead. The formula is not valid for an arbitrary nonspherical gravitational potential.