Astrometry

Astrometry is the branch of astronomy that relates to precise measurements and explanations of the positions and movements of stars and other celestial bodies. Although once thought of as an esoteric field with little useful application for the future,Fact|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

Missing information|pre photogragaphy description (setting circle, etc), photography, Astrograph, plate-measuring machine description - usage - link|date=March 2008The history of astrometry is linked to the history of star catalogues, 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. [Walter, Hans G. (2000).]

Astrometry was studied extensively in Islamic astronomy, which produced many star catalogues during the Islamic Golden Age. In 850, Alfraganus wrote "Kitab fi Jawani" ("A compendium of the science of stars"), which gave revised values for the obliquity of the ecliptic, the precessional movement of the apogees of the sun and the moon, and the circumference of the earth. [Harvard reference |last=Dallal |first=Ahmad |contribution=Science, Medicine and Technology |editor-last=Esposito |editor-first=John |title=The Oxford History of Islam |year=1999 |publisher=Oxford University Press, New York |p=164] Albatenius (853-929) gave times for the new moon and lengths for the solar year and sidereal year, and worked on the phenomenon of parallax. [Harvard reference |last=Wickens |first=G. M. |contribution=The Middle East as a world Centre of science and medicine |editor-last=Savory |editor-first=Roger M. |year=1976 |title=Introduction to Islamic Civilization |pages=111-118 |publisher=Cambridge University Press |isbn=052109948X (cf. Harvard reference |last=Zaimeche |first=Salah |year=2002 |url=http://www.muslimheritage.com/topics/default.cfm?ArticleID=235 |title=The Muslim Pioneers of Astronomy |publisher=Foundation for Science Technology and Civilisation |accessdate=2008-01-22 )]

In the 10th century, Azophi carried out observations on the stars and described their positions, magnitudes, 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 eclipses 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".Harvard reference |last=Zaimeche |first=Salah |year=2002 |url=http://www.muslimheritage.com/topics/default.cfm?ArticleID=235 |title=The Muslim Pioneers of Astronomy |publisher=Foundation for Science Technology and Civilisation |accessdate=2008-01-22 ] Abu-Mahmud al-Khujandi relatively accurately computed the axial tilt to be 23°32'19" (23.53°), [Citation|first=Richard P.|last=Aulie|year=1994|date=March 1994|title=Al-Ghazali Contra Aristotle: An Unforeseen Overture to Science In Eleventh-Century Baghdad|journal=Perpectives on Science and Christian Faith|volume=45|pages=26-46 (cf. cite web|url=http://www.1001inventions.com/index.cfm?fuseaction=main.viewSection&intSectionID=441|title=References
publisher=1001 Inventions|accessdate=2008-01-22)] which was a significant improvement over the Greek and Indian estimates of 23°51'20" (23.86°) and 24°, [Harvard reference |last=Saliba |first=George |authorlink=George Saliba |year=2007 |url=http://www.youtube.com/watch?v=IkL5Klh9WlY |title=Lecture at SOAS, London - Part 3/7 |publisher=Muslim Heritage & YouTube |accessdate=2008-01-22 ] and still very close to the modern measurement of 23°26' (23.44°).

In the 15th century, the Timurid prince and astronomer Ulugh Beg compiled the "Zij-i-Sultani", in which he catalogued 1,019 stars. In the 16th century, Taqi al-Din accurately measured the right ascension of the stars at the Istanbul observatory of al-Din using the "observational clock" he invented.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 ]

James Bradley first tried to measure stellar parallaxes 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 devices (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. [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 ]

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. [Kovalevsky, Jean (1995).]

Applications

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 instrumental for keeping time, in that UTC is basically the 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.

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.

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.

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. [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 ]

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 Samual Oschin 48 inch Schmidt telescope 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. [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] [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 ]

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 statistical methods to compute data estimates and error ranges.

Computer programs

* Astrometrica
* MPO (computer program)

In fiction

* In the fictional ', the Astrometrics"' lab is the set for various scenes.
* In the reimagined TV Show Battlestar Galactica an Astrometrics lab is stated in dialogue multiple times.

See also

* Astrometric binary
* Ephemeris
* Equatorium
* Gaia Probe (ESA -- Planned for 2011-17)
* Hipparcos Space Astrometry Mission (ESA -- 1989-93)
* Spherical astronomy
* Star cartography

References

Further reading

* cite book
first=Jean | last=Kovalevsky
coauthor=Seidelman, P. Kenneth | year=2004
title=Fundamentals of Astrometry
publisher=Cambridge University Press
id=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
id=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