Precessing jet and relative angles. Left: sketch of the precessing cone. The angle ζ is the angle between the jet precession axis and the line of sight (l.o.s.) direction, ψ is the opening angle of the precession cone and η is the continuosly changing angle between the jet and the line of sight because of precession. Right: the angles ζ, η, ψ in spherical geometry. The angle Ω, describing in the precession period P₂ an angle 2π, defines the temporal dependence.