FK4

The FK4 , full title Fourth Fundamental Catalog (fourth Fundamental Catalog ), is the most important fundamental catalog of the last 40 years and has represented the heavenly (Zälestischen) coordinate system for decades .

It served astronomy and geodesy from 1963 as the most precise realization 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 FK4 contains 1535 fundamental stars (i.e. stars measured to an accuracy of at least 0.1 ″) whose star locations were determined from relative and absolute methods of astrometry . The measurements from different epochs were transformed to the standard epoch B1950.0 .

The 1535 stars go back to 873 stars in the previous catalog FK3 (Heidelberg 1937 ) and its 661 supplement stars , which were a little less precisely defined at the time. Overall, they have a nearly homogeneous distribution and accuracy (average ± 0.1 ″) over the entire sky.

The system also contains precise proper movements of each star, so that their positions can be converted a few decades into the future and into the past. They relate with great accuracy to a stationary ("inertial") system.

Characteristic values of the FK4, its predecessor FK3 , its successors FK5 and FK6 as well as the satellite-based (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 1}The first FK (Auwers 1879) covered only 60% of the starry sky (up to declination −10 °).

^{Note 2}Hipparcos 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).

FK4

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