Dark mass impact on astrometry. (a) ΔRA_I, IIIa(f) = RA_IIIa(f)−RA_I(f) together with . (b) Same for Dec together with . (c) together with the analogous quantity for model (IIIb); note the different scales on the left and right vertical axes. Subscripts indicate the models to which the respective quantities correspond (Eqs. (I)–(IIIb)). The vertical line within the shaded blue region marks the true anomaly of model (I) in 2021.96, which is approximately the time of publication.