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The Barycentric celestial reference system (BCRS) is a coordinate system used in astrometry to specify the location and motions of astronomical objects. It was created in 2000 by the International Astronomical Union (IAU) to be the global standard reference system for objects located outside the gravitational vicinity of Earth: planets, moons, and other Solar System bodies, stars and other objects in the Milky Way galaxy, and extra-galactic objects. The Geocentric Celestial Reference System (GCRS), also created by the IAU in 2000, is a similar standard coordinate system used to specify the location and motions of near-earth objects, such as satellites.〔 These systems make it easier for scientists and engineers to compile, share, compare, and convert accurate measurements worldwide, by establishing standards both of measure and of methodology, and providing a consistent framework of operations. The focus of the BCRS is on astronomy: exploration of the Solar System and the universe. The BCRS is the system currently used for expressing positional data in astronomical references, such as the Hipparcos star catalog. The focus of the GCRS is somewhat more on the navigation of Earth satellites and the geophysical applications they support. The proper functioning of the Global Positioning System (GPS) is directly dependent upon the accuracy of satellite measurements as supported by the GCRS. ==Purpose and Implementation== The BCRS was designed to support the extremely-high-precision measurements of position and motion required in astrometry.〔 One critical factor in achieving that precision lies in how general relativistic effects are determined and measured. Both systems incorporate standards that enable the consistency and ready comparability of the resulting spacetime coordinates among astrometric measurements taken worldwide. They provide a metric tensor to establish a consistent frame of reference for observations. The tensor achieves consistency in part through its standardization of the reference point for gravity. The geocentric system is simpler, being smaller and involving few massive objects: that coordinate system defines its center as the center of mass of the Earth itself. The barycentric system can be loosely thought of as being centered on the Sun, but the Solar System is more complicated. Even the much smaller planets exert gravitational force upon the Sun, causing it to shift position slightly as they orbit. Those shifts are very large in comparison to the measurement precisions that are required for astrometry. Thus, the BCRS defines its center of coordinates as the center of mass of the entire Solar System, its barycenter. This stable point for gravity helps to minimize relativistic effects from any observational frames of reference within the Solar System. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Barycentric celestial reference system」の詳細全文を読む スポンサード リンク
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