Astrometry: Wikis


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Illustration of the use of interferometry in the optical wavelength range 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. 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.



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. In doing so, he also developed the brightness scale still in use today.[1] Hipparchus compiled a catalogue with at least 850 stars and their positions.[2] Hipparchus's successor, Ptolemy, included a catalogue of 1,022 stars in his work the Almagest, giving their location, coordinates, and brightness.[3]

In the 10th century, Abd al-Rahman al-Sufi 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.[4] 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.[5]

In the 16th century, Tycho Brahe used improved instruments to measure star positions more accurately than previously, with a precision of 15–35 seconds.[6] Taqi al-Din measured the right ascension of the stars at the Istanbul observatory of Taqi al-Din using the "observational clock" he invented.[7]

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.[8]

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.[9]


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.

Motion of barycenter of solar system relative to the Sun.

Astrometry has also been used to support claims of 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. Although, as of 2009, none of the extrasolar planets detected by ground-based astrometry has been verified in subsequent studies, astrometry is expected to be more accurate in space missions that are not affected by the distorting effects of the Earth's atmosphere. [10]. 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, and the European Space Agency's GAIA (due to launch in 2012), which will be applying astrometric techniques in its stellar census[11].

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.

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.[12]

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 48 inches (1.2 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.[13][14]


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 Star Trek: Voyager, 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


  1. ^ Walter, Hans G. (2000).
  2. ^ p. 109, Star maps: history, artistry, and cartography, Nick Kanas, Springer, 2007, ISBN 0387716688.
  3. ^ p. 110, Kanas 2007.
  4. ^ Zaimeche, Salah (2002). The Muslim Pioneers of Astronomy. Foundation for Science Technology and Civilisation. Retrieved 2008-01-22.  
  5. ^ p. 49, "Astrometry", History of astronomy: an encyclopedia, John Lankford, Taylor & Francis, 1997, ISBN 081530322X.
  6. ^ pp. 2–3, Fundamentals of astrometry, Jean Kovalevsky and P. Kenneth Seidelmann, Cambridge University Press, 2004, ISBN 0521642167.
  7. ^ Tekeli, Sevim (1997). "Taqi al-Din". Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures. Kluwer Academic Publishers. ISBN 0792340663.  
  8. ^ Staff (June 1, 2007). "The Hipparcos Space Astrometry Mission". European Space Agency. Retrieved 2007-12-06.  
  9. ^ Kovalevsky, Jean (1995).
  10. ^ Nature 462, 705 (2009) 8 December 2009 doi:10.1038/462705a
  11. ^ [1]ESA - Space Science - Gaia overview
  12. ^ Trujillo, Chadwick; Rabinowitz, David (June 1, 2007). "Discovery of a candidate inner Oort cloud planetoid" (PDF). European Space Agency. Retrieved 2007-12-06.  
  13. ^ Britt, Robert Roy (October 7, 2002). "Discovery: Largest Solar System Object Since Pluto". Retrieved 2007-12-06.  
  14. ^ Clavin, Whitney (May 15, 2004). "Planet-Like Body Discovered at Fringes of Our Solar System". NASA. Retrieved 2007-12-06.  

Further reading

  • Kovalevsky, Jean; Seidelman, P. Kenneth (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

Simple English

Astrometry is a part of astronomy that has to do with measuring the motion of objects in space such as stars. Two main ways to measure these objects are proper motion and parallax.

Proper motion is an object’s change in position over time when seen from the Earth. Just like anything else that moves, a star's proper motion is measured by looking towards the same place at different times and seeing how far the star has moved. The difference with stars is that, from Earth, they seem to move very slowly so we have to measure in arcseconds per year. When a star has a large proper motion, it usually means that it is nearby. Barnard’s Star, the star with the largest proper motion, only moves 1 degree across the sky (3600 arcseconds) in 348 years.

Parallax is the apparent difference in position of an object when seen from two or more different places. When an object is seen from a new place, there is a new line of sight going from the viewer to the object showing a different background in the distance. Parallax is measured using the angle between two lines of sight.

Parallax can also be used to measure distances to the nearest few thousand stars. These distances are measured in parsecs which are equal to about 3.26 light years.



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