Model of the contraction velocities, V_cc, of the OH spots, i.e. of their velocities towards a common centre of contraction (CC). V_ftd (represented with dotted lines) corresponds to the velocity from the straight line fitted to the observed data for the given distance to (α, δ)_m, as may be seen in Fig. 3. This velocity is assumed to be the radial component of the contraction velocity. Each spot has its own velocity (the two V_ftd, represented here, are different to each other in magnitude). The small ellipse corresponds to the small circle plotted in Fig. 5. For an assumed spot at a point in this circle, |V_ftd| is the magnitude of the transversal velocity, which is directed to the CONUS axis (and orientated perpendicular to it). The |V_ftd| of the spot at the large ellipse is smaller than that at the small one. The resulting contraction velocity, of a magnitude, V_cc, lies on the surface of the CONUS and it is directed to the CC point (the apex of the CONUS).