![]() ![]() The definition of the ICRS thus is independent of the earth's motion, rotation, and its equator and is no longer a dynamical system, rather a quasi-inertial reference system with the assumption that those quasars do not move noticeably along the sky due to their enormous distances and thus are used as fixed, fiducial points to define the ICRS. The catalog of about 600 compact, extragalactic sources (mainly quasars) form the International Celestial Reference Frame (ICRF), the practical realization of the ICRS. In 1997 the International Astronomical Union (IAU) adopted the International Celestial Reference System (ICRS) based on more precise Very Long Baseline Interferometry (VLBI) observations with radio telescopes to define the axes of the celestial coordinate system. rotation free) based on celestial mechanics and law of gravitation. These observations were tied into the complex motion and rotation of the earth as well as other solar system bodies (sun, major and minor planets) to be able to establish a dynamical reference frame, which is made inertial (i.e. (H.G.Walter, O.J.Sovers, Astrometry of Fundamental Catalogues, Springer 2000). In the past these fundamental observations were provided by transit circle (meridian circle) telescopes at optical wavelengths for about 1500 bright stars. These types of observations allow us to define a coordinate system (directions of 3 orthogonal axes) from first principles, without prior knowledge of the coordinates of stars. The celestial coordinate system is established by large-angle, fundamental observations. ![]() A more detailed narrative with figures about celestial coordinate systems is given by G.Kaplan. An object on the celestial equator has Dec = 0. the angular distance from the equatorial plane with +90 degree for the celestial north pole and -90 degree for the celestial south pole. Declination (Dec) is the angle orthogonal to RA, i.e. Right ascension (RA) is the angle counted in this plane from 0 to 24 hour, similar to the geographic longitude. The most commonly used system is the equatorial celestial coordinate system which has the plane of earth's equator projected onto the celestial sphere as fundamental plane. The diameter of a large coin as seen from a distance of about 6000 km (New York to London) corresponds to an angle of 1 mas.Īstronomers use celestial coordinates on the sky to define a position (direction unit vector) of a celestial object as seen from a specific location (which can be on earth or in space) in a way similar to geographic latitude and longitude. A milliarcsecond (mas) is 1/1000 arcsec and a microarcsecond is 1/1000 mas. Progress in astrometry in the recent decade called for smaller angular units. The smallest angular separations or resolutions seen through an ordinary telescope on the ground is about 1 arcsec, limited by the turbulence of earth's atmosphere. The full moon in the sky substends an angle of about 1/2 degree or 30 arcmin as seen from earth. A degree is subdivided into 60 arcminutes (arcmin) and 1 arcmin equals 60 arcseconds (arcsec). A full circle can be divided into 360 degrees. Measurements of distances to celestial objects by triangulation for example is at the core of astrometry and it forms the basis of all astrophysics without knowing the distances to planets, satellites, stars, and galaxies, no correct understanding of the cosmos in which we live can be achieved.Īstrometry is about measuring angles, dealing with errors in angular measures and changes of angles with time (angular velocity), and derivation of astrophysical quantities from those measurements. astrometry) is misleading, because astrometry also is certainly part of physics or astrophysics. The term astrophysics, often used to distinguish most of current astronomical research from the classical astronomy (i.e. ![]() angles between celestial objects as seen on the celestial sphere were measured but also the "quality of light", specifically the light intensity (photometry) and color (spectroscopy, light intensity as function of color or wavelength). Toward the end of the 19th century not only the directions, i.e. Historically, astrometry was all that astronomy was about until about the 19th century. Astrometry is now one of many fields of research within astronomy. Norbert Zacharias, IAU vice-president commission 8 (Astrometry)Īstrometry is the science which deals with the positions and motions of celestial objects. ![]()
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