I spent the day at UCLA, reviewing the data-analysis work of the Galactic Center Group there, for reporting to the Keck Foundation. It was a great day on a great project. They have collected large amounts of data (for more than 20 years!), both imaging and spectroscopy, to tie down the orbits of the stars near the Galactic Center black hole, and also to tie down the Newtonian reference frame. The approach is to process imaging and spectroscopy into astrometric and kinematic measurements, and then fit those measurements with a physical model. Among the highlights of the day were arguments about priors on orbital parameters, and descriptions of post-Newtonian terms that matter if you want to test General Relativity. Or test for the presence of dark matter concentrated at the center of the Galaxy.