The Distance to the Perseus Spiral Arm in the Milky Way

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.