# Catalogues of Fundamental Stars

The fundamental system of astronomy is an accurate coordinate frame for the positions of stars and extragalactic objects. In recent decades, presented their respective catalogs of fundamental stars for the best approximation to inertial space is - today the fundamental system is realized in more detail about quasars.

The underlying measurements were previously only visually or photographically performed (see classical astrometry ). Today, the Radio Astronomy plays a crucial role in the form of a precision network 500 and its measurement by quasar VLBI method.

## Star catalogs as a database for fixed points

In this coordinate frame, the centrifugal earth rotates and can be modeled in its movements. The frame is defined in terms of a Sternverzeichnissess hundreds of so-called fundamental stars. Since the end of the 19th century, these were particular:

## Heaven, Earth and Earth's rotation

However, these coordinates are not just for astronomers to " daily bread," but for many of those scientists who deal with the planet earth or some of its phenomena. Because ultimately go to sample all geographic coordinates and even the exact time systems of physics to astronomical direction and time measurements back.

The fact that terrestrial and celestial coordinates can be related, easy to understand with a picture: One can imagine in space, has its axis of rotation to a pole at an imaginary celestial sphere, the Earth as a spherical stable gyroscope.

We are accustomed to our north is always "up " to imagine, because we put the " world axis " naturally in the earth's axis. Actually, this is nothing more than the traditional geocentric system of antiquity, with the only Copernicus has partially broken.

Now, when the Earth rotates as a " roundabout " in this "cage" out of sheer surrounding stars, lends itself to face the parallels similar circles on a celestial sphere surrounding the earth.

But whereas had some of the meridians long-winded to the zero meridian of Greenwich humanity, right ascension is suitable for their astronomical equivalent of such a formally: The vernal equinox, where the sun coming crosses the celestial equator every year from the southern hemisphere.

Mathematically, the vernal equinox is the intersection of the equator with the so-called ecliptic (plane of Earth's orbit and the apparent annual path of the sun through the constellations ).

### Mutable earth's axis

These handy for astronomers levels are slow variable. Just as every toy top wobbles a little, it is with the earth - only much slower and more regular. This effect is called precession and its period of 26,000 years, a Platonic year. The Earth's axis describes in this time a clearly definable cone with an opening angle of 23.44 ° is currently what you can expect ahead to exactly 0.01 " ( 0.000005 %) now This also includes a second effect called nutation -. A from the Moon caused " trembling" to the rhythm of 18,613 years ( the nutation ), which is also accurately modeled.

These effects are measured by special instruments and methods of astrometry and geodesy; the most important are the Space VLBI method ( direction measurement for quasars ), Space Laser and GPS, as well as earthbound the meridian circle and astrolabe or PZT; the latter two have become less important over the last decade. In addition came a few years ago, a sort of space scanner to the satellite Hipparcos.

Described here astronomic- geodetic model of the earth and its orbit is our fundamental system of astronomy - and is currently the best realization of an inertial system dar.

### Terrestrial fundamental system

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

The relationship with the measurement points of the respective State Survey is prepared by coordinate transformations and in the 3rd dimension by altimetry and the geoid.

However, these models have other effects and subtleties, their description would fill a book. Therefore, we refer to these details on some websites.