Equatorial plane ($\theta =\pi /2$)

Given a polar axis, the equatorial plane passes through the sphere's centre and is perpendicular to that axis. In spherical coordinates with polar angle $\theta$, it is the plane $\theta =\pi /2$. For motion in a spherically symmetric gravitational field, conservation of angular momentum permits rotation of axes so that a nonradial orbit lies in such a plane. This is a choice of orientation, rather than an additional physical restriction on a spherically symmetric problem.