Yes, I am a Glutton for Punishment. Please Spank Me.
Proper motion is the measure of how far an object moves across the sky.
Most objects outside of our own solar system have tiny proper motions. You then measure how far it has moved with respect to background galaxies. These galaxies are extremely distant and their proper motion is almost zero. I am also measuring the proper motions of other distant objects, including the distant globular cluster Pal 13. Measuring the velocity and distance of these objects can reveal the mass of the Milky Way.