  The water maser in the mildly active nucleus in the nearby galaxy NGC4258
traces a thin, nearly edge-on, subparsec-scale Keplerian disk. Using the
technique of very long baseline interferometry, we have detected the proper
motions of these masers as they sweep in front of the central black hole at an
orbital velocity of about 1100 km/s. The average maser proper motion of 31.5
microarcseconds per year is used in conjunction with the observed acceleration
of the masers to derive a purely geometric distance to the galaxy of 7.2 +- 0.3
Mpc. This is the most precise extragalactic distance measured to date, and,
being independent of all other distance indicators, is likely to play an
important role in calibrating the extragalactic distance scale.
