  We consider null geodesics in the exterior region of a subextremal Kerr
spacetime. We show that most well-known fundamental properties of null
geodesics can be represented on one plot. In particular, one can see
immediately that the ergoregion and trapping are separated in phase space.
Furthermore, we show that from the point of view of any timelike observer
outside of the black hole, trapping can be understood as a smooth set of
spacelike directions on the celestial sphere of the observer. Finally, we
discuss some applications of these insights.
