Illustration of the use of optical wavelength interferometry to determine precise positions of stars. Courtesy NASA/JPL-Caltech.

Astrometry is the branch of astronomy that relates to precise measurements and explanations of the positions and movements of stars and other celestial bodies. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth Although once thought of as an esoteric field with little useful application for the future, 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. Kinematics ( Greek κινειν, kinein, to move is a branch of Classical mechanics which describes the motion of objects without The Solar System consists of the Sun and those celestial objects bound to it by Gravity. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply

History

The 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. Hipparchus ( Greek; ca 190 BC &ndash ca 120 BC was a Greek Astronomer, Geographer, and Mathematician of the Hellenistic Timocharis of Alexandria (ca 320 BC - 260 BC was a Greek Astronomer and Philosopher. For the crater see Aristillus (crater. Aristillus (fl ca 280 BC was a Greek astronomer who created the first Precession refers to a change in the direction of the axis of a rotating object In doing so, he also invented the brightness scale still in use today. ^[1]

James Bradley first tried to measure stellar parallaxes in 1729. This article describes the English astronomer for other people sharing the name see James Bradley (disambiguation James Bradley (March 1693 Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between 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. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. The aberration of light (also referred to as astronomical aberration or stellar aberration) is an astronomical phenomenon which produces an Apparent motion Nutation is a slight irregular motion (etymologically a "nodding" in the Axis of rotation of a largely axially symmetric object such as a Gyroscope His cataloguing of 3222 stars was refined in 1807 by Friedrich Bessel, the father of modern astrometry. Friedrich Wilhelm Bessel (22 July 1784 &ndash 17 March 1846 was a German Mathematician, Astronomer, and systematizer of the Bessel functions He made the first measurement of stellar parallax: 0. 3 arcsec for the binary star 61 Cygni. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. Not be confused with 16 Cygni, a more distant system containing two G-type stars harboring the Gas giant planet 16 Cygni Bb.

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. A charge-coupled device ( CCD) is an analog Shift register, that enables the transportation of analog signals (electric charges through successive stages (capacitors 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. The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member Hipparcos (an Acronym for Hi gh P recision Par allax Co llecting S atellite) was an Astrometry mission 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. The proper motion of a Star is the measurement of its change in position in the sky over time after Improper motions are accounted for 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. ^[2]

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. ^[3]

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. Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena See also Inertial frame A frame of reference in Physics, may refer to a Coordinate system or set of axes within which to Celestial mechanics is the branch of Astrophysics that deals with the motions of Celestial objects The field applies principles of Physics, historically Stellar dynamics is the branch of Astrophysics which describes in a statistical way the collective motions of Stars subject to their mutual Gravity. Galactic astronomy is the study of our own Milky Way galaxy and all its contents In observational astronomy, astrometric techniques help identify stellar objects by their unique motions. Observational astronomy is a division of the astronomical Science that is concerned with getting data in contrast with Theoretical astrophysics which is It is instrumental for keeping time, in that UTC is basically the atomic time synchronized to Earth's rotation by means of exact observations. For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of International Atomic Time ( TAI, from the French name Temps Atomique International) is a high-precision atomic Time standard that tracks EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 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. The cosmic distance ladder (also known as the Extragalactic Distance Scale) is the succession of methods by which astronomers determine the Distances to celestial Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply

Astronomers use astrometric techniques for the tracking of near-Earth objects. Near-Earth objects ( NEOs) are Asteroids Comets and large Meteoroids whose orbits bring them close ( Perihelion distance AU 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. An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 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. The Space Interferometry Mission, also known as SIM PlanetQuest, is a planned space telescope being developed by the U The Space Interferometry Mission, also known as SIM PlanetQuest, is a planned space telescope being developed by the U A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our Solar-Type, Solar Analog, and Solar Twin stars are those stars that are particularly similar to the Sun, with solar twin being more similar than solar analog

Astrometric measurements are used by astrophysicists to constrain certain models in celestial mechanics. Astrophysics is the branch of Astronomy that deals with the Physics of the Universe, including the physical properties ( Luminosity, Celestial mechanics is the branch of Astrophysics that deals with the motions of Celestial objects The field applies principles of Physics, historically By measuring the velocities of pulsars, it is possible to put a limit on the asymmetry of supernova explosions. Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves Asymmetry is the absence of or a violation of a Symmetry. In organisms Due to how cells divide in Organisms asymmetry in organisms is A supernova (plural supernovae or supernovas) is a stellar Explosion. Also, astrometric results are used to determine the distribution of dark matter in the galaxy. In Physics and cosmology, dark matter is hypothetical Matter that does not interact with the electromagnetic force but whose presence can be inferred from

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. ^[4]

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. TemplateInfobox Planet.--> 90377 Sedna (ˈsɛdnə) is a Trans-Neptunian Education Brown is a Huntsville Alabama native and graduated from Virgil Grissom High School in 1983 Palomar Observatory is a privately owned Observatory located in San Diego County California, 90 miles (145 Km) Southeast of Mount Wilson The Samuel Oschin telescope is a 48-inch (122-m aperture Schmidt camera at the Palomar Observatory in northern San Diego County, California 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. ^[5][6]

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. Statistics is a mathematical science pertaining to the collection analysis interpretation or explanation and presentation of Data.

In fiction

• In the fictional Star Trek: Voyager, the Astrometrics lab is the set for various scenes. Fiction is the telling of stories which are not real More specifically fiction is an imaginative form of Narrative, one of the four basic Rhetorical modes. Set construction is a process by which a set designer works in collaboration with the director of the production to create the set for a theatrical film or television
• In the reimagined TV Show Battlestar Galactica an Astrometrics lab is stated in dialogue multiple times. Battlestar Galactica is a franchise of Science fiction Films and television series, the first of which was produced in 1978

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

• Kovalevsky, Jean (2004). Fundamentals of Astrometry. Cambridge University Press. ISBN 0-521-64216-7.  
• Walter, Hans G. (2000). Astrometry of fundamental catalogues: the evolution from optical to radio reference frames. New York: Springer. ISBN 3540674365.  
• Kovalevsky, Jean (1995). Modern Astrometry. Berlin; New York: Springer. 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
• Hall of Precision Astrometry. University of Virginia Department of Astronomy. Retrieved on 2006-08-10. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 612 BC - Killing of Sinsharishkun, King of Assyrian Empire
• 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