The fundamental function of astrometry is to provide astronomers with a reference frame in which to report their observations. 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. 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. The Hipparcos Star Catalog, produced from data obtained from the satellite Hipparcos, gives an optical catalog associated with the ICRF. 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. Additional catalogs were compiled for the 23,882 double/multiple stars and 11,597 variable stars also analyzed during the Hipparcos mission. A new catalog, “Tycho,” drew together a database of 1,058,332 to within 20-30 mas. During its four-year run, the positions, parallaxes, and proper motions of 118,218 stars were determined with an extremely high degree of accuracy. Operated from 1989 to 1993, Hipparcos measured large and small angles on the sky with much greater precision than any previous optical telescopes. 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. This technology made astrometry less expensive, opening the field to amateurs who wished to look into it. In the 1980s, charge-coupled devices (CCDs) replaced photographic plates and reduced optical uncertainties to one milliarcsecond. Automated plate-measuring machines and more sophisticated computer technology of the 1960s allowed for larger compilations of star catalogs to be achieved more efficiently. Given that stellar parallaxes are very difficult to measure, only about 60 of them had been obtained by the end of the nineteenth century. He made the first measurement of stellar parallax: 0.3 arcsec for the binary star 61 Cygni. ![]() His cataloging of 3222 stars was refined in 1807 by Friedrich Bessel, the father of modern astrometry. These measurements proved too insignificant for his telescope, but he discovered the aberration of light and the nutation of Earth’s axis. James Bradley first tried to measure stellar parallaxes in 1729. In doing so, he also invented the brightness scale still in use today. 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. ![]() 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. 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. It has helped with the discovery of extrasolar planets and many previously unobserved Solar System objects. In addition, it is useful for precise time-keeping and tracking near-Earth objects. ![]() For example, it is valuable for studies in celestial mechanics, stellar dynamics, and galactic astronomy. 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.
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