... Piotrovich Main Astronomical Observatory, Pulkovo, St. Petersburg, Russia Received May 24, 2006; in final form, July 7, 2006 Abstract--We have used two astrometric methods developed at the Main Astronomical Observatory of the Russian Academy of Sciences--the method of apparent-motion parameters (AMP) and a direct geometrical method (DGM)--to derive the orbit of the star S2 around the Galactic center, and thereby the mass of the supermassive black hole at the Galactic center. ... Relative positions. ...