Distance of the perseus spiral arm in the Milky Way

The distance to the massive star-forming region W3OH in the Perseus spiral arm of the Milky Way has been measured to be 1.95±0.04 kiloparsecs (5.86×1016 km), determined by triangulation using the Very Long Baseline Array with the Earth's orbit as one segment of a triangle. This resolves a long-standing discrepancy of a factor of two. The reason for the discrepancy is that this portion of the Perseus arm has anomalous motions.