Astrometry is a branch of astronomy that involves precise measurements and explanations of the positions and movements of stars and other celestial bodies. As such, it provides astronomers with a frame of reference within which to report their observations.
Although it was once regarded as an esoteric field with little practical significance, astrometry has proved extremely useful in a range of areas of contemporary astronomical research. For example, it is valuable for studies in celestial mechanics, stellar dynamics, and galactic astronomy. In addition, it is useful for precise time-keeping and tracking near-Earth objects. It has helped with the discovery of extrasolar planets and many previously unobserved Solar System objects. Also, it is useful for the study of dark matter in the galaxy and in developing models for the physical origin of the Solar System.
The history of astrometry is linked to the history of star catalogs, which gave astronomers reference points by which they could track the movements of objects in the sky. This type of work can be dated back to about 190 B.C.E., when Hipparchus used the catalog of his predecessors Timocharis and Aristillus to discover Earth’s precession. In doing so, he also invented the brightness scale still in use today.
James Bradley first tried to measure stellar parallaxes in 1729. These measurements proved too insignificant for his telescope, but he discovered the aberration of light and the nutation of Earth’s axis. His cataloging 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.
Given that stellar parallaxes are very difficult to measure, only about 60 of them had been obtained by the end of the nineteenth century. Automated plate-measuring machines and more sophisticated computer technology of the 1960s allowed for larger compilations of star catalogs 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 amateurs who wished to look into it.
In 1989, the European Space Agency's Hipparcos satellite took astrometry into orbit, where it could be less affected by Earth's mechanical forces and optical distortions from the 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 four-year run, the positions, parallaxes, and proper motions of 118,218 stars were determined with an extremely high degree of accuracy. A new catalog, “Tycho,” drew together a database of 1,058,332 to within 20-30 mas. Additional catalogs were compiled for the 23,882 double/multiple stars and 11,597 variable stars also analyzed during the Hipparcos mission.
In 1994, using data from about 400 radio sources beyond the Milky Way galaxy, the International Astronomical Union (IAU) established the International Celestial Reference Frame (ICRF) as the fundamental frame of reference, replacing earlier catalogs. The Hipparcos Star Catalog, produced from data obtained from the satellite Hipparcos, gives an optical catalog associated with the ICRF.
Today, the catalog most often used is USNO-B1.0, an all-sky catalog that tracks the proper motions, positions, magnitudes, and other characteristics of over one billion stellar objects. Over 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 arcseconds.
The fundamental function of astrometry is to provide astronomers with a reference frame in which to report their observations. In addition, it is vitally important for fields such as celestial mechanics, stellar dynamics, and galactic astronomy.
The accurate positions and movements of stars allow scientists to generate a two-dimensional map of the sky at a particular moment in time. To obtain a three-dimensional picture, researchers take into account the parallaxes (which provide distances to the stars) and radial velocities of the celestial objects. With that information, one can calculate the three-dimensional position and velocity of each celestial object.
In observational astronomy, astrometric techniques help identify stellar objects by their unique motions. It is instrumental for keeping time—Coordinated Universal Time (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 also been used to detect extrasolar planets by measuring the displacement they cause in the parent star's apparent position in the sky, because of 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 highly significant 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, researchers 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 Earth’s motion during this time, and then calculate the heliocentric distance to this object. 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.
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 gaining an understanding of the Solar System and how its past, present, and future are interrelated with other objects in the universe.
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.
All links retrieved April 22, 2016.
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