  We analyse the massless particles orbiting a spherically symmetric,
asymptotically flat black hole with a radius equal to the photon sphere and a
circular geodesic. Asymptotic observers record the orbital period of the null
circular geodesic as the lowest among all possible paths around the compact
object. We proceed with the analytical study of massless particle motion to
demonstrate the instability of an orbit with null circularity. Furthermore, our
results apply to any asymptotically flat and spherically symmetric spacetime.
