Q:  Roughly how far away is the Andromeda Galaxy, in kpc?


           The distance to M31 is about 760 kpc.



   Q:  Compute half that distance.  Where would that "halfway" point
           lie in the diagram above?


           Half of the distance is about 380 kpc ... which lies
           just at the right-hand edge of the diagram.  If we tried
           to use the motions of satellites with radial distances
           from the Milky Way of more than 300 or 350 kpc, those
           motions would be perturbed by the gravitational influence
           of the Andromeda Galaxy.