  We have measured the distance to the massive star-forming region W3OH in the
Perseus spiral arm of the Milky Way to be 1.95 $\pm$ 0.04 kilo-parsecs
($5.86\times10^{16}$ km). This distance was determined by triangulation, with
the Earth's orbit as one segment of a triangle, using the Very Long Baseline
Array. This resolves a long-standing problem of a factor of two discrepancy
between different techniques to determine distances. The reason for the
discrepancy is that this portion of the Perseus arm has anomalous motions. The
orientation of the anomalous motion agrees with spiral density-wave theory, but
the magnitude is somewhat larger than most models predict.
