QU loop illustration in a non-relativistic context. Top panel: geometry of the problem. The black hole is represented by the black disk. The hot spot (red disk) orbits in the equatorial XY plane around the black hole (black disk). The Z axis is normal to the equatorial plane. We consider an observer looking face-on at the black hole located toward the negative Z axis. The north direction of the observer’s screen is assumed to lie along the −Y axis. The θ and φ angles of the spherical coordinates are represented. The hot spot rotates in the positive φ direction. The green vector K represents the direction of emission of the photon (we discarded any relativistic effect here), and the blue vector B is the magnetic field, assumed to be toroidal. The polarization vector F = K × B is shown in pink. Successive positions of the hot spot are labeled from 1 to 5. Bottom-left panel: rotation of the polarization vector on the sky plane of the observer with the Stokes directions of Fig. 1 overlaid. Bottom-right panel: associated QU plane and QU loops. The polar coordinates in this plane are , where F_LP is the linearly polarized flux and χ_o is the observed EVPA.