Astrometry

The Astrometrie ( gr. Ἄστρον = star and μέτρον, métron = dimension, measuring) is the geometric portion of astronomy and as such, the counterpart of astrophysics . It is also called positional astronomy or classical astronomy and includes the measurement and calculation of star positions (so-called star locations ) and their movements in precisely defined reference systems . This makes it the basis of much astronomical research and especially celestial mechanics . Until the establishment of astrophysics, which began around 1860 after the invention of spectroscopy, astrometry and spherical astronomy made up the majority of all astronomy.

According to de Vegt , astrometry is the science of the geometric structure of the universe (location, movement and distance of the stars) or the measurement of the sky . At the same time, it provides a coordinate basis for geodesy - i.e. the measurement of the earth .

Tasks of astrometry

More specifically, astrometry means today:

Creation of catalogs with precise positions and movements of stars
Construction of the fundamental reference coordinate system of astronomy and geosciences
Development of spatial astronomical databases
Development of measurement methods and instruments
- on the one hand terrestrial (optical telescopes and sensors , infrared, radio telescopes , etc.),
- on the other hand with astrometric satellites (see Hipparcos as well as the subsequent space probe project Gaia ) and interplanetary space probes
Implementation of the relevant measurements and international measurement campaigns
Reduction of measurements and standardization of the corresponding procedures.

The most important institution for these aspects is the Astronomical Computing Institute (ARI) in Heidelberg . It operates astrometry, stellar dynamics and astronomical services in the form of ephemeris and yearbooks , calendar bases and bibliographies .

Historical and cross connections

Until the advent of astrophysics after 1850 - mainly through spectral analysis and astrophotography - (according to Karl Schütte ) astrometry was synonymous with astronomy in general. It was not until the 20th century that people began to speak of astrometry or positional astronomy - in contrast to astrophysics , which dominated astronomy from 1950 onwards.

Between 1960 and 1990 astrometry almost led a niche existence, as barely 10% of astronomers (but increasingly geodesics ) devoted themselves to it . But when the era of astrometry satellites and CCD sensors began, this changed and today the high-precision measurement methods of astrometry also bring significant impulses and the like. a. for celestial mechanics , space travel, cosmology and stellar dynamics or Milky Way research .

The pioneers of "classic" astrometry are above all

Hipparchus , on the first star catalog with over 1000 stars back and the slow coordinates offset by the precession discovered
Ptolemy , who summarized the astronomical theories of his time in the Almagest
Tycho Brahe , who - still without a telescope - achieved measuring accuracies of up to 0.01 °
the astronomers of Europe participating in the Sky Police, who created the first precise star catalogs around 1800 (e.g. Giuseppe Piazzi )
Friedrich Wilhelm Bessel , who succeeded in the first measurement of a fixed star distance
Friedrich Argelander and his 325,000 star Bonn survey , which the German Astronomical Society further developed into the system of AGK catalogs
Simon Newcomb , whose definition of the fundamental system lasted almost 100 years
the Heidelberg Astronomical Computing Institute and the US Naval Observatory
the project groups of the astrometric satellites Hipparcos and Gaia

Astrometry has experienced a renaissance since the development of optoelectronic sensors and Very Long Baseline Interferometry . Their links to geodesy are becoming stronger, and the importance of high-precision coordinate systems is increasing. International tasks such as monitoring the earth's rotation with radio astronomy and GPS , space travel and satellite projects such as Galileo or GAIA are becoming interdisciplinary and give young astronomers new career opportunities. In defining the time systems , astronomers have to cooperate with physics and another three to four disciplines.

Two to four dimensional astrometry

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

The star locations become three-dimensional by measuring parallaxes - those apparent annual shifts that can be determined from opposite points on the earth's orbit . From this, star distances of up to 100 light years can be derived, and with Hipparcos and other methods, far more.

Finally, 4-D could be called stellar dynamics , which is based on proper movements . They are obtained from precise asterisk words from widely spaced epochs . Its addition to the spatial velocity vector gives the radial velocity , a result of the spectral analysis and thus the transition to astrophysics . The situation is similar when determining distances using photometry .

The dynamics of distant objects are explored more physically the further away they are. However, space travel and astrometric satellites are constantly expanding this limit .

Use for astronomical research

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

Better spatial picture of the star distribution and the movement conditions
Dynamics of the Milky Way in our environment
More precise determination of the star distribution in terms of the combination of luminosity and spectral type in the Hertzsprung-Russell diagram
More precise basis for measuring the earth and the solar system
More precise prediction of star coverages by planets and minor planets ( asteroids ).
Basis for high-precision astrometry up to the most distant galaxies
Connection of the optical coordinate frame to that of radio interferometry with quasars ; see VLBI , Geodesy .

literature

Julius Redlich: A look into the most general network of terms in astrometry . Beyer publishing house, Langensalza 1907.
Rudolf Sigl : Geodetic Astronomy . 3. Edition. Verlag Wichmann, Heidelberg 1991, ISBN 3-87907-190-X .
Albert Schödlbauer : Geodetic Astronomy - Basics and Concepts . De Gruyter, Berlin / New York 2000, ISBN 3-11-015148-0 .
P. Brosche, Harald Schuh : New developments in astrometry and their significance for geodesy . In: Journal of Surveying (ZfV) . 1999, ISSN 0044-3689 , p. 343-350 (vol. 124).
Jean Kovalevsky, (et al.): Fundamentals of astrometry . Cambridge Univ. Press, Cambridge 2004, ISBN 0-521-64216-7 .
Jean Kovalevsky: Modern astrometry . Springer, Berlin 2002, ISBN 3-540-42380-X .
Stephen Webb: Measuring the universe - the cosmological distance ladder . Springer, London 2001, ISBN 1-85233-106-2 .
Michael Perryman: Astronomical Applications of Astrometry: Ten Years of Exploitation of the Hipparcos Satellite Data . Cambridge Univ. Press, Cambridge 2008, ISBN 978-0-521-51489-7 .

Web links

Wikibooks: Astronomical Calculations for Amateurs / Positional Astronomy - Learning and Teaching Materials

Individual evidence

↑ Until the Hipparcos mission, these (weak) stars were not measured precisely enough, so that the coverage lines on earth were often too unsafe for mobile measuring teams. Now the Tycho catalog solves the problem with an accuracy of about ± 100 m.

[1] Until the Hipparcos mission, these (weak) stars were not measured precisely enough, so that the coverage lines on earth were often too unsafe for mobile measuring teams. Now the Tycho catalog solves the problem with an accuracy of about ± 100 m.