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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 also instrumental for keeping time, in that UTC is basically the atomic time synchronized to Earth's rotation by means of exact observations.
There have been several important 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 proved the Copernican principle, that Earth revolves about the Sun.
- The sextant dramatically improved measurement of celestial angles.
- The development of cheap charge coupled devices, software, and telescopes allowed for large-scale amateur astrometric observation of minor planets.
Another was the use of Cepheid variable stars to measure the distance to nebulae, which led to the discovery of other galaxies by Edwin Hubble. Hubble used triangulation on nearby Cepheids, and correlated the Cepheid's period to their absolute brightness. Then by measuring the period and brightness of Cepheids in nebulae, he established their distance by their brightness.
Hubble used Cepheids to discover and calibrate distance with the red shift shown by distant galaxies.
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 will utilize astrometric techniques to detect gas giants around other stars, and perhaps even terrestrial planets nearby.
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.
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.
- Hipparcos Space Astrometry Mission (ESA -- 1989-93)
- Gaia Probe (ESA -- Planned for 2009-14)
- Astrometric binary
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