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'''Astrometry''' is the branch of [[astronomy]] that relates to precise measurements and explanations of the positions and movements of [[star]]s and other celestial bodies. Although once thought of as an esoteric field with little useful application for the future,{{Citation needed|date=April 2008}} the information obtained by astrometric measurements is now very important in contemporary research into the [[kinematics]] and physical origin of our [[Solar System]] and our [[Galaxy]], the [[Milky Way]].

==History==
The history of astrometry is linked to the history of [[star catalogue]]s, which gave astronomers reference points for objects in the sky so they could track their movements. This can be dated back to [[Hipparchus]], who around 190 BC used the catalogue of his predecessors [[Timocharis]] and [[Aristillus]] to discover the earth’s [[precession]]. In doing so, he also developed the brightness scale still in use today.<ref>Walter, Hans G. (2000).</ref>  Hipparchus compiled a catalogue with at least 850 stars and their positions.<ref>p. 109, ''Star maps: history, artistry, and cartography'', Nick Kanas, Springer, 2007, ISBN 0387716688.</ref>  Hipparchus's successor, [[Ptolemy]], included a catalogue of 1,022 stars in his work the ''[[Almagest]]'', giving their location, coordinates, and brightness.<ref>p. 110, Kanas 2007.</ref>

In the 10th century, [[Abd al-Rahman al-Sufi]] carried out observations on the [[star]]s and described their positions, [[apparent magnitude|magnitude]]s, brightness, and [[colour]], and gave drawings for each constellation, in his ''[[Book of Fixed Stars]]''.   [[Ibn Yunus]] observed more than 10,000 entries for the sun's position for many years using a large [[astrolabe]] with a diameter of nearly 1.4 metres. His observations on [[eclipse]]s were still used centuries later in [[Simon Newcomb]]'s investigations on the motion of the moon, while his other observations inspired [[Laplace]]'s ''Obliquity of the Ecliptic'' and ''Inequalities of Jupiter and Saturn's''.{{Clarify|date=August 2009}}<ref name=Zaimeche>{{Citation |last=Zaimeche |first=Salah |year=2002 |url=http://www.muslimheritage.com/topics/default.cfm?ArticleID=234 |title=The Muslim Pioneers of Astronomy |publisher=Foundation for Science Technology and Civilisation |accessdate=2008-01-22 }}</ref>  In the 15th century, the [[Timurid]] astronomer [[Ulugh Beg]] compiled the ''[[Zij-i-Sultani]]'', in which he catalogued 1,019 stars.  Like the earlier catalogs of Hipparchus and Ptolemy, Ulugh Beg's catalogue is estimated to have been precise to within approximately 20 [[minutes of arc]].<ref>p. 49, "Astrometry", ''History of astronomy: an encyclopedia'', John Lankford, Taylor & Francis, 1997, ISBN 081530322X.</ref>

In the 16th century, [[Tycho Brahe]] used improved instruments to measure star positions more accurately than previously, with a precision of 15–35 [[arcseconds|second]]s.<ref>pp. 2–3, ''Fundamentals of astrometry'', Jean Kovalevsky and P. Kenneth Seidelmann, Cambridge University Press, 2004, ISBN 0521642167.</ref>  [[Taqi al-Din]] measured the [[right ascension]] of the stars at the [[Istanbul observatory of al-Din]] using the "observational clock" he invented.<ref name=Tekeli>{{cite encyclopedia | first = Sevim | last = Tekeli | title = Taqi al-Din | year = 1997 | encyclopedia = Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures | publisher = [[Kluwer Academic Publishers]] | isbn = 0792340663 | url = http://www.springer.com/philosophy/philosophy+of+sciences/book/978-1-4020-4425-0 }}</ref>

[[James Bradley]] first tried to measure stellar [[parallax]]es in 1729. The stellar movement proved too insignificant for his [[telescope]], but he instead discovered the [[aberration of light]] and the [[nutation]] of the Earth’s axis. His cataloguing of 3222 stars was refined in 1807 by [[Friedrich Bessel]], the father of modern astrometry. He made the first measurement of stellar parallax: 0.3 [[arcsec]] for the [[binary star]] [[61 Cygni]]. 

Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century.  Automated plate-measuring machines and more sophisticated computer technology of the 1960s allowed for larger compilations of star catalogues to be achieved more efficiently. In the 1980s, [[charge-coupled device]]s (CCDs) replaced photographic plates and reduced optical uncertainties to one milliarcsecond.  This technology made astrometry less expensive, opening the field to an amateur audience.

In 1989, the [[European Space Agency]]'s [[Hipparcos]] satellite took astrometry into orbit, where it could be less affected by mechanical forces of the Earth and optical distortions from its atmosphere. Operated from 1989 to 1993, Hipparcos measured large and small angles on the sky with much greater precision than any previous optical telescopes. During its 4-year run, the positions, parallaxes, and [[proper motions]] of 118,218 stars were determined with an incredible degree of accuracy. A new catalogue “Tycho” drew together a database of 1,058,332 to within 20-30 mas. Additional catalogues were compiled for the 23,882 double/multiple stars and 11,597 variable stars also analyzed during the Hipparcos mission.<ref>{{cite web
 | author=Staff | date=[[June 1]], [[2007]]
 | url=http://www.rssd.esa.int/index.php?project=HIPPARCOS
 | title=The Hipparcos Space Astrometry Mission
 | publisher=European Space Agency
 | accessdate=2007-12-06 }}</ref>

Today, the catalogue most often used is USNO-B1.0, an all-sky catalogue that tracks proper motions, positions, magnitudes and other characteristics for over one billion stellar objects. During the past 50 years, 7,435 Schmidt plates were used to complete several sky surveys that make the data in USNO-B1.0 accurate to within 0.2 arcsecond.<ref>Kovalevsky, Jean (1995).</ref>

==Applications==

Apart from the fundamental function of providing [[astronomer]]s with a [[Frame of reference|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 [[Coordinated Universal Time|UTC]] is basically the [[International Atomic Time|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]].

Astrometry has also been used to support claims of [[Methods_of_detecting_extrasolar_planets#Astrometry|extrasolar planet detection]] by measuring the displacement the proposed planets 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 analog|solar-type stars]].

Astrometric measurements are used by [[astrophysicist]]s to constrain certain models in [[celestial mechanics]].  By measuring the velocities of [[pulsar]]s, 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.

Astronomers use astrometric techniques for the tracking of [[near-Earth objects]]. Astrometry is responsible for the detection of many record-breaking solar system objects. To find such objects astrometrically, astronomers use telescopes to survey the sky and large-area cameras to take pictures at various determined intervals. By studying these images, we can notice solar system objects by their movements relative to the background stars, which remain fixed. Once a movement per unit time is observed, astronomers compensate for the amount of parallax caused by the earth’s motion during this time and the heliocentric distance to this object is calculated. Then, using this distance and other photographs, more information about the object, such as parallax, proper motion, and the semimajor axis of its orbit, can be obtained.<ref>{{cite web
 | first=Chadwick | last=Trujillo | coauthors=Rabinowitz, David
 | date=[[June 1]], [[2007]]
 | url=http://www.gps.caltech.edu/%7Embrown/papers/ps/sedna.pdf
 | format=PDF
 | title=Discovery of a candidate inner Oort cloud planetoid
 | publisher=European Space Agency
 | accessdate=2007-12-06 }}</ref>

[[50000 Quaoar]] and [[90377 Sedna]] are two solar system objects discovered in this way by [[Michael E. Brown]] and others at Caltech using the [[Palomar Observatory]]’s [[Samuel Oschin telescope]] of {{convert|48|in|m}} and the Palomar-Quest large-area CCD camera. The ability of astronomers to track the positions and movements of such celestial bodies is crucial to the understanding of our Solar System and its interrelated past, present, and future with others in our Universe.<ref>{{cite web
 | first=Robert Roy | last=Britt
 | date=[[October 7]], [[2002]]
 | url=http://www.space.com/scienceastronomy/quaoar_discovery_021007.html
 | title=Discovery: Largest Solar System Object Since Pluto
 | publisher=SPACE.com | accessdate=2007-12-06
}}</ref><ref>{{cite web
 | first=Whitney | last=Clavin
 | date=[[May 15]], [[2004]]
 | url=http://www.nasa.gov/vision/universe/solarsystem/planet_like_body.html
 | title=Planet-Like Body Discovered at Fringes of Our Solar System
 | publisher=NASA | accessdate=2007-12-06 }}</ref>

== Statistics ==

A fundamental aspect of astrometry is error correction. Various factors introduce errors into the measurement of stellar positions, including atmospheric conditions, imperfections in the instruments and errors by the observer or the measuring instruments. Many of these errors can be reduced by various techniques, such as through instrument improvements and compensations to the data. The results are then analyzed using [[statistics|statistical methods]] to compute data estimates and error ranges.

== Computer programs ==
* Astrometrica
* MPO (computer program)

== In fiction ==

* In the [[fiction]]al ''[[Star Trek: Voyager]]'', the '''Astrometrics''' lab is the [[set (drama)|set]] for various [[scene (fiction)|scene]]s.
* In the reimagined TV Show [[Battlestar Galactica]] an Astrometrics lab is stated in dialogue multiple times.

== See also ==

* [[Astrometric binary]]
* [[Ephemeris]]
* [[Equatorium]]
* [[Gaia probe|Gaia Probe]] (ESA—Planned for 2011-17)
* [[Hipparcos|Hipparcos Space Astrometry Mission]] (ESA—1989-93)
* [[Spherical astronomy]]
* [[Star cartography]]
* [[Methods_of_detecting_extrasolar_planets#Astrometry|Methods of detecting extrasolar planets - Astrometry]]

==References==
{{Reflist}}

===Further reading===
* {{cite book 
 | first=Jean | last=Kovalevsky
 | coauthor=Seidelman, P. Kenneth | year=2004
 | title=Fundamentals of Astrometry
 | publisher=Cambridge University Press
 | isbn=0-521-64216-7 }}
* {{cite book 
 | first=Hans G. | last=Walter
 | year=2000
 | title=Astrometry of fundamental catalogues: the evolution from optical to radio reference frames
 | publisher=Springer
 | location=New York
 | isbn=3540674365 }}
* {{cite book 
 | first=Jean | last=Kovalevsky
 | year=1995
 | title=Modern Astrometry
 | publisher=Springer
 | location=Berlin; New York
 | id=ISBN 354042380X }}

==External links==
* http://www.astrometry.org/ A site for popularization of Astrometry and Celestial Mechanics, by the Scientia Astrophysical Organization.
* http://ad.usno.navy.mil/ Astrometry Department of the U.S. Naval Observatory
* {{cite web
 | url = http://www.astro.virginia.edu/~rjp0i/museum/engines.html
 | title = Hall of Precision Astrometry
 | publisher = University of Virginia Department of Astronomy
 | accessdate = 2006-08-10 }}
* http://www.nasa.gov/vision/universe/solarsystem/planet_like_body.html  
* http://www.space.com/scienceastronomy/quaoar_discovery_021007.html
* http://www.gps.caltech.edu/~mbrown  Mike Brown's Caltech Home Page
* http://www.gps.caltech.edu/%7Embrown/papers/ps/sedna.pdf  Scientific Paper describing Sedna's discovery
* http://www.rssd.esa.int/index.php?project=HIPPARCOS

[[Category:Astrometry| ]]
[[Category:Astronomical sub-disciplines]]
[[Category:Astrological aspects]]