Effect of the spacetime geometry on the emission direction K. A hot spot (red disk) is orbiting around a black hole (black disk). The observer is located face-on toward the negative Z axis. In a Newtonian spacetime, the direction of emission (i.e., the unit vector K along the projection of the null four-vector k normal to the emitter’s four-velocity) is exactly vertical toward the negative Z axis (dashed pale blue arrow). This is the case illustrated in the non-relativistic Fig. 3. Special relativistic light aberration leads to an additional azimuthal component (solid light blue arrow). General relativistic light bending leads to an additional radial component (solid dark blue arrow). We note that the direction of emission in Schwarzschild spacetime is along the sum of the two solid arrows, given that the special relativistic aberration is of course also included in the Schwarzschild geometry. The various vectors are approximately to scale for a Keplerian hot spot at a few gravitational radii. The aberration and light bending effects are not small corrections to an approximately vertical direction; they lead to strong distortions of the apparent emission direction (on the of order tens of percents).