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Astrometry

Astrometry can be considered the oldest sub-field of science and has been important in history for sailing (maritime navigation), since navigators used to calculate their position on Earth upon the observation of stars.

Today, astrometry is still important for keeping time. The international time standard is the Coordinated Universal Time (UTC), that is basically the atomic time synchronized to Earth's rotation by means of exact observations.


Short history of astrometry

Positional astrometry was only qualitative in its earliest days. Quantitative astrometry dates back at least to the Greek astronomer Hipparcos (or Hipparchus). In the 2nd century B.C. he compiled the first catalogue of stars and in doing so invented the brightness scale (magnitude) 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.

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 and trigonometry, that are important fields of mathematics central so physics and other sciences.
  • 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 and positions.
A very important work that used both astrometry and astrophysics was made by Edwin Hubble. Cepheid variable stars served to measure the distance to supposed nebulae, which led to the discovery of galaxies. 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 doppler shift they cause in their parent star's spectrum. 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 gravitaional models in specific objects such as pulsars or supernovae. Also, astrometric results are used to determin the distribution of dark matter in the galaxy.


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