2003ApJ...587..208R

In 1979, Menten and coworkers accurately determined the position of Sgr A* on an infrared image by aligning the infrared image with positions measured for SiO masers, associated with infrared-bright evolved stars, at radio wavelengths. We now report greatly improved radio positions and, for the first time, proper motions of many stellar SiO masers at the Galactic center. These positions and motions, coupled with better infrared imaging, allow a much improved location of Sgr A* on infrared images. With current data, infrared stars can be placed in a reference frame tied to Sgr A*, to an accuracy of ~10 mas in position and ~1 mas/yr in motion. The position of Sgr A* is, within uncertainties, consistent with stellar accelerations and the measured orbital focus of the star S2. The star S2 was observed within 16 mas (~130 AU in projection) of Sgr A* on 2002 May 2. Finally, we find that the central stellar cluster moves with Sgr A* to within ~70 km/s.