Fundamental system (astronomy)

The fundamental system of astronomy is a precise coordinate frame for the positions of stars ( star locations ) and extra-galactic objects. In the last few decades the respective catalogs of fundamental stars represented the best approximation to the inertial space - today the fundamental system is realized even more precisely via quasars .

In the past, the underlying measurements were only carried out visually or photographically (see classic astrometry ). Today, radio astronomy in the form of a precise network of quasars and its measurement using VLBI methods also plays a decisive role.

Star catalogs as a database for fixed points

The top of the earth rotates in this coordinate frame and its movements can therefore be modeled . The frame is defined in the form of a star directory of hundreds of so-called fundamental stars . Since the end of the 19th century these have been in particular:

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 °
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.	118,000	Hipparcos catalog	1998	1989-1993	1989-1993
FK6	4,150	Sixth Catalog of Fundamental Stars	1999, 2000	Ø 1992

Quasars as a database for fixed points

Newer coordinate systems are mostly based on measurements of the positions of quasars . Examples of such catalogs are ICRF and GCRF2 .

Heaven, earth and earth rotation

These coordinates are not only part of the “daily bread” for astronomers, but also for many of those scientists who deal with planet Earth or some of its phenomena. After all, for example, all geographic coordinates and even the exact time systems in physics are based on astronomical measurements of direction and time.

The fact that earth and sky coordinates are connected can be easily understood with a picture: One can imagine the earth as a spherical stable top in space, the axis of rotation of which points to a pole on an imaginary celestial sphere.

We are used to always picture north “above” because we naturally place the “ world axis ” in the earth axis . Actually this is nothing more than the traditional geocentric system of antiquity , with which Copernicus only partially broke.

When the earth rotates as a “top” in this “cage” made up of louder surrounding stars , it makes sense to contrast the circles of latitude with similar circles on a celestial sphere that envelops the earth.

While mankind first had to agree on the prime meridian of Greenwich for the longitudes , for its astronomical counterpart, right ascension, one would literally be offered: the spring equinox , when the sun crosses the celestial equator every year from the southern hemisphere.

Mathematically, the spring equinox is the line of intersection of the equator with the so-called ecliptic (plane of the earth's orbit or the annual apparent sun path through the constellations ).

Variable earth axis

However, these practical levels for astronomers are slowly changing. Just as every toy spinning top wobbles a bit, so does the earth - only much slower and more regularly. This effect is called precession and its duration of approx. 25,800 years is called a Platonic year . During this time, the earth's axis describes a clearly definable cone with an opening angle of currently 23.43 °, which can now be calculated with an accuracy of 0.01 "(0.000005%). This also includes a second effect called nutation - one from the moon caused "tremors" at a rhythm of 18.613 years (the nutation period), which is just as precisely modeled.

These effects are measured by special instruments and methods of astrometry and geodesy ; the most important are the space procedures VLBI (Very Long Baseline Interferometry, direction measurement according to quasars ), space laser and GPS , as well as the earth-bound meridian circle and astrolabe or PZT (photographic zenith telescope); the latter two have lost their importance over the past decade. In addition, a kind of space scanner was added a few years ago , the Hipparcos satellite .

The astronomical-geodetic model of the earth and its orbit described here is our fundamental system of astronomy - and currently represents the best implementation of an inertial system.

Terrestrial fundamental system

Its terrestrial equivalent - "brought down" to the earth rotating in it - is called the International Terrestrial Reference System (ITRS). It is permanently marketed through fundamental stations (such as Wettzell in Bavaria) and numerous GPS measuring stations.

The relationship to the surveying points of the respective national survey is established through coordinate transformations and in the third dimension through height measurements and the geoid .

Web links

Remarks

↑ The first FK (Auwers 1879) only covered 60% of the starry sky (up to declination −10 °).
↑ Hipparcos is not a FK in the strict sense of the word, but has only been precisely adapted to the FK5 system and has 'stiffened' it. The new system (FK6) has gained significantly in accuracy thanks to the measurements of the astrometric satellite ( Hipparcos | 1989–1993).

[1] The first FK (Auwers 1879) only covered 60% of the starry sky (up to declination −10 °).

[2] Hipparcos is not a FK in the strict sense of the word, but has only been precisely adapted to the FK5 system and has 'stiffened' it. The new system (FK6) has gained significantly in accuracy thanks to the measurements of the astrometric satellite ( Hipparcos | 1989–1993).