Model fitting results for knot C6. Knot C6 moves along the precessing common trajectory during the whole observation period. In the upper left panel the solid black line (modeling the inner trajectory) and the dashed red line (modeling the entire trajectory) coincide. In the upper right panel three modeled trajectoryies are shown for t₀ (black dashed line) and t₀ ± 0.5 yr (or ϕ ±   0.61 rad; green and magneta lines), demonstrating that most of the observation data points are within the position angle range defined by the two additional modeled trajectories and the precession period is determined within an uncertainty of ~ ± 0.5 yr.