Fundamental astronomy

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The fundamental astronomy is the branch of astronomy that deals with the definition of reference systems on Earth and the sky , and its links with the orbits of Earth , Moon, planets and satellites in these reference systems. It thus provides some important fundamentals for astronomy and geodesy . In the underlying measurement methods, it is linked to position astronomy .

The name is based on fundamental subjects in other areas of knowledge ( fundamental theology , fundamental modeling , analysis and theorems of mathematics, fundamental systems , etc.).

Coordinates, reference systems and reductions

The position of stars and other celestial bodies was usually given in the coordinates right ascension and declination , which refer to the celestial equator and the earth's orbit ( ecliptic ). These coordinate systems are, however, subject to slow changes (especially precession ), which were discovered in ancient times .

Other effects such as nutation , the polar movements of the earth's axis and small irregularities in the earth's rotation were discovered between 1800 and 1950. They make fundamental astronomy a complex matter and require close cooperation with geodesy and geophysics . As a result, global coordinates can now be determined with centimeter accuracy and, since 1980, their annual changes due to plate tectonics can be determined.

The mentioned reference systems fall into two groups: earth- fixed and space-fixed coordinate systems. Since the earth rotates in the space-fixed system, we have to know the exact position of the earth's axis on the earth's surface and in space at any point in time for the transformation between the earth- and the spatially-fixed system , as well as the angular speed of the earth's rotation (the astronomical day length LOD, Length of day). This task is carried out in international cooperation by the Earth Rotation Service IERS .

In concrete terms, the transformation of the rotating earth and the earth's orbit into the space-fixed system means that formulas for the relationships between the two systems are required in order to

  • to be able to convert terrestrial points for any point in time into the "star system" (in everyday language: which star is at the zenith right above me ?),
  • and vice versa, to be able to specify the current position of each star relative to one's own location - e.g. B. as azimuth and zenith distance .

You have to do both steps

  • convert the stars themselves from the fundamental system (see e.g. FK6 ) to their current position and vice versa, and
  • take into account the polar movement of the earth's axis within the earth's body.
  • Each measurement must also be " reduced " to normal conditions . These include a .: Astronomical refraction , small instrument errors and tilt effects due to the influence of temperature .

Classical fundamental astronomy

The classic fundamental astronomy (up to about 1970) was based on precise astro-geodetic angle measurements of terrestrial observatories and their analysis by the methods of Astrometrie and the celestial mechanics . The following are / were measured primarily:

  1. exact Lotrichtungen , resulting from the two components latitude and longitude composed
  2. astronomical azimuths
  3. absolute directions to fundamental stars
  4. relative celestial coordinates (direction measurements of fixed stars and planets ) on specially mounted telescopes
  5. photographic directions to stars with long focal length images on photo plates or CCD sensors.

The instruments used to make these measurements are above all

Revolution through satellite methods and VLBI

The space revolutionized the fundamental astronomy by new measurements to artificial satellites , and to or from space probes . Furthermore, from around 1980 radio astronomy was able to provide high-precision direction measurements according to quasars (see VLBI and interferometry ). As a result, the classic reference systems have been improved in several ways:

See also

Web links