The second US Naval Observatory (USNO) CCD Astrograph Catalog, UCAC2 was released in 2003 July. Positions and proper motions for 48,330,571 sources mostly stars) are available on 3 CDs, supplemented with Two Micron All Sky Survey photometry for 99.5% of the sources. The catalog covers the sky area from -90° to +40° declination, going up to +52° in some areas; this completely supersedes the UCAC1 released in 2001. Current epoch positions are obtained from observations with the USNO 8 inch (0.2 m) Twin Astrograph equipped with a 4K CCD camera. The precision of the positions are 15–70 mas, depending on magnitude, with estimated systematic errors of 10 mas or below. Proper motions are derived by using over 140 ground- and spacebased catalogs, including Hipparcos/Tycho and the AC2000.2, as well as yet unpublished remeasures of the AGK2 plates and scans from the NPM and SPM plates. Proper-motion errors are about 1–3 mas/yr for stars to 12th magnitude, and about 4–7 mas yr
1 for fainter stars to 16th magnitude.