FK5

FK5 stands for the Fifth Fundamental Catalog, a historical star catalog that provided high-precision astrometric data for a set of fundamental stars used to define the optical reference frame of the sky. The catalog gives mean positions and proper motions for its stars at epoch J2000.0, along with cross-identifications and, in some cases, magnitudes. The FK5 was designed to improve upon its predecessor, the FK4, and to transfer the reference frame from the B1950 to the J2000 system, serving as a cornerstone for accurate celestial coordinates in the late 20th century.

Contents and data in the FK5 include the right ascension and declination of each fundamental star at epoch J2000.0, together with proper motions in right ascension and declination. The catalog also provides standard errors for these quantities and references to the underlying measurements. The FK5 was compiled under the auspices of the International Astronomical Union and relied on ground-based astrometry and the then-current precession-nutation models to link coordinates to the J2000 frame.

Historical context and legacy: The FK5 was published in the late 1980s as part of an ongoing effort to establish a stable, long-term celestial reference frame for astrometry. It played a key role in bridging FK4 and the modern, inertial reference frames. In the long term, the FK5 was superseded by more accurate catalogs and the adoption of the International Celestial Reference System (ICRS), realized by radio and optical reference sources such as Hipparcos and Gaia. Today, the FK5 is mainly of historical and methodological interest, illustrating the evolution of celestial reference frames.