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We are using the NRAO Very Long Baseline Array (VLBA) and the Japanese VERA project to measure trigonometric parallaxes and proper motions of masers found in high-mass star-forming regions across the Milky Way. Early results from 18 sources locate several spiral arms. The Perseus spiral arm has a pitch angle of 16 ◦ ±3 ◦ , which favors four rather than two spiral arms for the Galaxy. Combining distances, proper motions, and radial velocities yields complete 3-dimensional kinematic information. We find that star forming regions on average are orbiting the Galaxy ≈ 15 km s −1 slower than expected for circular orbits. By fitting the measurements to a model of the Galaxy, we estimate the distance to the Galactic center R0 = 8.4 ± 0.6 kpc and a circular rotation speed Θ0 = 254 ± 16 km s −1. The ratio Θ0/R0 can be determined to higher accuracy than either parameter individually, and we find it to be 30.3±0.9 km s −1 kpc −1, in good agreement with 1
Reid et al. (Tue,) studied this question.
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