The astrometry (Latin astrum = star and geometry for measuring ) is the geometric part of the field of astronomy and as such the opposite of astrophysics. It is also called positional astronomy or classical astronomy and includes the measurement and calculation of star positions (known star positions ) and their movements in well-defined reference systems. This makes it the basis for many astronomical research and in particular of celestial mechanics. Until the establishment of astrophysics, which began in 1860 after the invention of spectroscopy, astrometry made ​​and Spherical Astronomy from the bulk of the entire astronomy.

After de Vegt ¹ Astrometry is the science of geometrical structure of the universe (location, movement and distance of the stars ) or the measurement of the sky. At the same time it gives a coordinate basis for geodesy - ie the shape of the earth.

Tasks of astrometry

Considered concretely, astrometry means today:

  • Creation of catalogs with accurate positions and proper motions of stars
  • Structure of the fundamental reference coordinate system of Astronomy and Earth Sciences
  • Spatial structure of astronomical databases
  • Development of measurement methods and instruments one hand, terrestrial (optical telescopes and sensors, infrared, radio telescopes, etc.)
  • Other hand, with astrometry (see Hipparcos and the subsequent spacecraft Project Gaia) and interplanetary space probes

The most important institution for these aspects is the rake Astronomical Institute ( ARI) in Heidelberg. It operates astrometry, stellar and Astronomical Services in the form of ephemerides and almanacs, calendars bases and bibliographies.

Historical and cross-connections

Until the advent of astrophysics after 1850 - mainly due to the spectral analysis and astrophotography - was (according to Karl Schütte ) astrometry synonymous with astronomy at all. Only in the 20th century, people began to speak of astrometry or positional astronomy - in contrast to astrophysics, which from 1950 dominated astronomy.

Between about 1960 and 1990, the astrometry led almost a niche because their ( increasingly, however, the geodesics ) devoted almost 10% of astronomers. But when began the era of the astrometry and the CCD sensors, this changed and today bring the high-precision measurement methods of astrometry also essential impulses among others for celestial mechanics, space, cosmology, and the Milky Way research.

Among the pioneers of the "classical " astrometry primarily include

  • Hipparchus, on the back of the first star catalog with over 1000 stars and discovered the slow coordinate shifts caused by the precession
  • Ptolemy, who summarized the astronomical theories of his time in the Almagest
  • Tycho Brahe, who yet .. without a telescope - achieved measurement accuracy up to 0.01 °
  • The people participating in the Himmelspolizey astronomers of Europe, which in 1800 the first accurate star catalogs created (eg Giuseppe Piazzi )
  • Friedrich Argelander and his 325,000 stars comprehensive Bonner Durchmusterung, which further developed the German Astronomical Society of the system of AGK catalogs
  • Simon Newcomb, the definition of the fundamental system had almost 100 years stock
  • The Astronomical Computing Center Heidelberg and the U.S. Naval Observatory,

While for the modern astrometry, in particular project groups the astrometry satellite Hipparcos and GAIA are mentioned.

Since the development of optoelectronic sensors and the Very Long Baseline Interferometry astrometry is experiencing a renaissance. Their cross-connections for Geodesy become stronger, the importance of high-precision coordinate systems increases. International tasks such as monitoring the Earth's rotation with radio astronomy and GPS, space and satellite projects such as Galileo or GAIA are interdisciplinary and giving young astronomers new career opportunities. In the definition of time astronomers systems must cooperate with physics and a further three to four disciplines.

Two-to four-dimensional astrometry

The 2-D part of the astrometry belongs to spherical astronomy and deals only with the direction of incidence of light sources from space - in theory, measurement technology, the coordinate systems and for various corrections to the apparent direction of celestial objects ( planets, stars, galaxies) as to their true direction.

Three-dimensional star positions are determined by measuring the parallax - that apparent annual shifts that are detectable from opposite points of the earth's orbit. This can star distances up to 100 light years are derived, with Hipparcos and other methods far beyond.

4-D could finally call the stellar dynamics, based on proper motions. They are obtained from exact Sternörtern of widely separated periods. Your addition to the spatial velocity vector is the radial velocity, a result of the spectral analysis and thus the transition to astrophysics. It is similar to distance determinations by means of photometry.

The dynamics of distant objects is more explored more astrophysically, the further they are away. This limit, however, continue to be extended through space and astrometry.

Benefits for Astronomical Research

Precise stellar coordinates, distance and speed data fertilize many aspects of astronomy. Some of these are:

  • Better spatial image of the stellar distribution and motion conditions
  • Dynamics of the Milky Way in our environment
  • More precise determination of the distribution of stars with respect to the combination of luminosity and spectral type in the Hertzsprung -Russell diagram
  • More accurate basis for measuring the earth and the solar system
  • More accurate prediction of occultations of stars by planets, and minor planets ( asteroids ).
  • Basis for high-precision astrometry to the most distant galaxies
  • Connection of the optical coordinate frame to that of the radio - interferometry quasars; see VLBI, geodesy.