  We report measurement of the trigonometric parallax of W51 Main/South using
the Very Long Baseline Array (VLBA). We measure a value of 0.185 +/- 0.010 mas,
corresponding to a distance of 5.41 (+0.31/-0.28) kpc. W51 Main/South is a
well-known massive star-forming region near the tangent point of the
Sagittarius spiral arm of the Milky Way. Our distance to W51 yields an estimate
of the distance to the Galactic center of Ro = 8.3 +/- 0.46 (statistical) +/-
1.0 (systematic) kpc by simple geometry. Combining the parallax and proper
motion measurements for W51, we obtained the full-space motion of this massive
star forming region. We find W51 is in a nearly circular orbit about the
Galactic center. The H2O masers used for our parallax measurements trace four
powerful bipolar outflows within a 0.4 pc size region, some of which are
associated with dusty molecular hot cores and/or hyper- or ultra-compact HII
regions.
