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Home > Astrometry

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 timeFor alternate uses of "time", see Time (disambiguation). Time quantifies or measures the interval between events, or the duration of events. Time has long been perceived as a dimension in which each event has a definite (but not necessarily unique) positi, in that UTC is basically the atomic timeTemps Atomique International TAI or International Atomic Time is a very accurate and stable time scale. It is an average of the time kept by many caesium clocks ( atomic clocks) all over the world, and has been available since 1955. True high-precision TA synchronized to EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla's rotation by means of exact observations.

There have been several important advances in astrometry.

• SundialA sundial measures apparent solar time, usually by measuring the position of the shadow of the sun as it changes through the day. A set of markings are made on the surface on which the shadow is cast, allowing the user to read the time. There are a few cos were effective at measuring time.
• AstrolabeFrom the 16th century to the 18th century (when the sextant was invented) the astrolabe was the chief navigational instrument. An astrolabe consists of a circle marked in degrees (similar to a protractor) with a rotating arm attached at its center. When ts were invented for measuring celestial angles.
• Astrometric applications led to the development of spherical geometrySpherical geometry is the geometry of the two- dimensional surface of a sphere. It is an example of a non-Euclidean geometry. In plane geometry the basic concepts are points and lines. On the sphere, points are defined in the usual sense. The equivalents
• Careful measurement of planetary motion s by Tycho BraheTycho Brahe ( December 14, 1546 October 24, 1601) was a Danish nobleman, well known as an astronomer/ astrologer (the two were not yet distinct) and alchemist. He had Uraniborg built, which became an early " research institute". For purposes of publicatio 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 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 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 determin the distribution of dark matter in the galaxy.

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Topics: Astrometry Astrometric Stars Celestial Astronomy Measuring...

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AstrometryAstrometry is a part of Astronomy and deals with the positions of stars and other celestial bodies, their distances and movements. Part of astrometry involves creating the cosmic distance ladder. It is one of the oldest subfields of the science, the succe