Representation of a H i particle in the orbital plane. θ is the angular position of the particle, and the horizontal black dotted lines correspond to the maximum values of θ for which the particle can be observed from the Earth. The radial velocity V_r ⋆ and total velocity V are displayed as black vectors, separated by the angle ϵ (positive in our representation). The velocity of the particle as seen from the Earth, V_r ⊕, is displayed as a red vector. It is the total velocity vector projected on the star/Earth line of sight. For the sake of clarity, the diagram is not to scale.