Transit tangential velocity of the planet-moon barycenter during ingress (v_t,_in), midtransit (v_t), and egress (v_t,_eg) as a function of the orientation of the periastron with respect to the line of sight (c_b). (a) The planet-moon barycenter has an orbital period of 30 days around a Sunlike star. The upper panel shows v_t,in (red solid line), v_t (black dashed line), and v_t,eg (blue dashed line) for orbital eccentricities of 0.2, 0.1, and 0.01. The lower panel shows the maximum variation of the transit velocity, which we illustrate as Δv_t = o_t,_in/o_t,_in - 1 and in units of permille. (b) Same as (a), but with a circumstellar orbital period of 365.25 days. The resulting variation of the in-transit tangential velocity is reduced by almost one order of magnitude, as shown in the lower panel.