Astrometry

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Illustration of the use of optical wavelength interferometry to determine precise positions of stars. Courtesy NASA/JPL-Caltech.

Astrometry is a branch of astronomy that deals with the positions of stars and other celestial bodies, their distances and movements.

It is one of the oldest subfields of the science, the successor to the more qualitative study of positional astronomy. Astrometry dates back at least to Hipparchus, who compiled the first catalogue of stars visible to him and in doing so invented the brightness scale basically still in use today. Modern astrometry was founded by Friedrich Bessel with his Fundamenta astronomiae, which gave the mean position of 3222 stars observed between 1750 and 1762 by James Bradley.

Apart from the fundamental function of providing astronomers with a reference frame to report their observations in, astrometry is also fundamental for fields like celestial mechanics, stellar dynamics and galactic astronomy. In observational astronomy, astrometric techniques help identify stellar objects by their unique motions. It is instrumental for keeping time, in that UTC is basically the atomic time synchronized to Earth's rotation by means of exact observations. Astrometry is also involved in creating the cosmic distance ladder because it is used to establish parallax distance estimates for stars in the Milky Way.

1 Advances in astrometry
2 Astrometrics
3 References
4 In fiction
5 See also

[edit] Advances in astrometry

Sundials were effective at measuring time.
Astrolabes were invented for measuring celestial angles.
Astrometric applications led to the development of spherical geometry
Careful measurement of planetary motions by Tycho Brahe, followed by analysis by Johannes Kepler proved the Copernican principle, that Earth revolves about the Sun.
The sextant dramatically improved measurement of celestial angles.
James Bradley measured stellar aberration with a precise transit telescope.
The advent of photography and the Astrograph in the late 19th century allows the imaging of a much larger number of fainter stars for astrometric measurement and position archiving in large photographic catalogs.
The development of charge coupled devices (CCDs), and their adoption by astronomers in the 1980s, improved the precision of professional astrometric work.
The development of inexpensive CCDs, software, and telescopes allowed for large-scale amateur astrometric observation of minor planets.
From 1989 to 1993, the European Space Agency's Hipparcos satellite performed astrometric measurements resulting in a catalogue of positions accurate to 20-30 milliarcsec for over a million stars.

Astronomers use astrometric techniques for the tracking of near-Earth objects. It has been also been used to detect extrasolar planets by measuring the displacement they cause in their parent star's apparent position on the sky, due to their mutual orbit around the center of mass of the system. NASA's planned Space Interferometry Mission (SIM PlanetQuest) will utilize astrometric techniques to detect terrestrial planets orbiting 200 or so of the nearest solar-type stars.

Astrometric measurements are used by astrophysicists to constrain certain models in celestial mechanics. By measuring the velocities of pulsars, it is possible to put a limit on the asymmetry of supernova explosions. Also, astrometric results are used to determine the distribution of dark matter in the galaxy.

[edit] Astrometrics

Astrometrics is the science of stellar measurements and motion. Astrometrics was used, during the 1990s, to detect extrasolar gas giants orbiting various solar systems. This was done by observing the "stellar wobble" of a star and calculating what kinds of gravitational forces would cause such motion; it was then determined that planetary forces must be affecting the stars in question.

[edit] References

Jean Kovalevsky and P. Kenneth Seidelman, Fundamentals of Astrometry, Cambridge University Press, 2004, ISBN 0-521-64216-7.
Hall of Precision Astrometry (English). University of Virginia Department of Astronomy. Retrieved on 2006-08-10.