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J. M. Moran (Harvard-Smithsonian Center for Astrophysics)
The water masers in the nucleus of the nearby galaxy NGC\,4258 serve as test particles that allow the structure and motion of the accretion disk surrounding the nuclear black hole to be studied with exquisite detail. The full range of maser emission within 1000~km\,s-1 of the systemic velocity has been observed with VLBI at frequent intervals since 1994. Measurements of the proper motions and accelerations of the systemic maser features indicate that the distance to the galaxy is 7.2 ±0.3~Mpc, so that linear dimensions can be calculated with considerable precision. The accuracies of proper motions and accelerations can be expected to improve with time, to the extent allowed by the long-term stability of individual masers. The magnetic field strength in the maser medium is less than 300 mG. Further measurements in progress can be expected to reduce this limit considerably or provide a detection. Considerable effort has been expended to define the warp of the maser disk, a difficult task because of the limited distribution of observable masers. There is no clear evidence that the disk is resolved in the vertical direction, although a few masers may reside outside it. The precise azimuth positions of the masers in the disk can be estimated from deprojection techniques but require physical assumptions. Progress in this work will be discussed.