Recent Progress on a New Distance to NGC 4258

We report on our ongoing, high-resolution study of H 2 O masers in the innermost parsec of NGC 4258. Over thirty epochs of VLBA and VLA data, taken over six years, are being used to monitor the velocities, accelerations, positions and proper motions of water masers rotating in a warped, Keplerian disk about a supermassive central object. Our extensive monitoring results in an improved accuracy in the distance determination to this galaxy (i) via a reduction in experimental random errors due to a longer time-baseline than in previous work; and (ii) via a better modeling of sources of systematic error, such as disk eccentricity. These data can therefore yield an extremely accurate geometric distance to NGC 4258.