Astrometry

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

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 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.

Related Topics:
Cepheid variable - Nebula - Galaxies - Edwin Hubble - Triangulation - Absolute brightness

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Hubble used Cepheids to discover and calibrate distance with the red shift shown by distant galaxies.

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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.

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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.

Related Topics:
Near-Earth objects - Extrasolar planets - Space Interferometry Mission - SIM PlanetQuest - Terrestrial planets - Solar-type stars

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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.

Related Topics:
Astrophysicist - Celestial mechanics - Pulsar - Asymmetry - Supernova - Dark matter

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