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The objects covered by the catalog compared to the entire firmament. For color legend see description page

The FK5 , full title: Fifth Fundamental Catalog (fifth fundamental catalog), is the most important fundamental catalog of astronomy of the last 20 years and defined the celestial (celestial) coordinate system until about 2010 . The FK6 replaced the FK5 since 2000. FK6 contains fewer stars, but measured much more precisely than in FK5. FK5 has served astronomy and geodesy since 1988 as the most precise implementation of a reference system for the coordinates right ascension and declination . With its underlying constants, it also provided a precise model for tasks in the planetary system , as well as an extensive inertial system for extragalactic astronomy.

The FK5 contains 1535 fundamental stars (ie stars measured to at least 0.05 ″) and 3117 supplement stars (supplement, publ. 1991). These star locations were determined from dozens of previous catalogs and with new relative and absolute methods of astrometry . The measurements from different epochs were transformed to the two standard epochs B1950.0 and J2000.0 .

The 1535 stars go back to the previous catalog FK4 (Heidelberg 1963) and to another 3115, somewhat less precisely defined stars. Overall, they have a nearly homogeneous distribution and accuracy (better than 0.05 ″) over the entire sky.

The system also contains precise proper motions of each star. They are partly based on measurements up to 100 years ago, so that the positions can be converted a few decades into the future and into the past. They relate with great accuracy to a stationary ("inertial") system. It only had to be changed by about 0.01 ″ when transitioning from the FK4.

Characteristic values ​​of the FK5, its predecessors FK3 and FK4 , the successor FK6 and the satellite-supported (relative) Hipparcos catalog
Short name Number of stars title Published Measurement places Measurement of own movements Overlap
Auwers, A., 1879 539 Fundamental catalog for zone observations on the Nördl. sky 1879 Ø 1860 ≈1850-1870 up to dec. = −10 ° Note 1
Peters, J., 1907 925 New FK Berliner Astr. Yearbook based on the principles of Auwers 1907 Ø 1880 1745-1900 up to dec. = −89 °
FK3 873 Third fundamental catalog of the Berlin Astronomical Yearbook 1937 1912-1915 from here over the whole sky,
with epochs 1900, 1950, 2000
FK3sup 662 (Additional stars, Volume II) 1938 Ø 1913 1845-1930
FK4 1,535 Fourth Fundamental Catalog 1963 Ø 1950
FK4sup 1,111 Supplement Stars FK4 / 5 ≈1965
FK5 1,535 Fifth Fundamental Catalog 1988 Ø 1975
FK5sup 3.117 Supplement Stars of FK5 1991
Hipp. Note 2 118,000 Hipparcos catalog 1998 1989-1993 1989-1993
FK6 4,150 Sixth Catalog of Fundamental Stars 1999, 2000 Ø 1992
Note 1The first FK (Auwers 1879) covered only 60% of the starry sky (up to declination −10 °).
Note 2Hipparcos is not an FK in the strict sense of the word, but has only been precisely adapted to the FK5 system and has 'stiffened' it. However, the new system (FK6) has gained significantly in accuracy thanks to the measurements made by the astrometric satellite ( Hipparcos , 1989–1993).