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International Celestial Reference System and its realizations
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<p>The International Celestial Reference System (ICRS) is the current standard <a href="/facts/Celestial_reference_system/23HhqvHj">celestial reference system</a> adopted by the <a href="/facts/International_Astronomical_Union/GMFPImXo">International Astronomical Union</a> (IAU). Its origin is at the <a href="/facts/Barycenter/Fw9renao">barycenter</a> of the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>, with axes that are intended to "show no global rotation with respect to a set of distant extragalactic objects". This fixed reference system differs from previous reference systems, which had been based on <a href="/facts/Catalogues_of_Fundamental_Stars/xhaTlLx4">Catalogues of Fundamental Stars</a> that had published the positions of stars based on direct "observations of [their] <a href="/facts/Equatorial_coordinate_system/dn5W4Eoo">equatorial coordinates</a>, right ascension and declination" and had adopted as "privileged axes ... the mean equator and the dynamical equinox" at a particular <a href="/facts/Epoch_(astronomy)/RDdj6SmA">date and time</a>. </p><p>The International Celestial Reference Frame (ICRF) is a realization of the International Celestial Reference System using reference celestial sources observed at radio wavelengths. In the context of the ICRS, a <a href="/facts/Frame_of_reference/15yYfB9M">reference <i>frame</i></a> (RF) is the physical realization of a reference <i>system,</i> i.e., the reference frame is the set of numerical coordinates of the reference sources, derived using the procedures spelled out by the ICRS. </p><p>More specifically, the ICRF is an <a href="/facts/Inertial_frame_of_reference/1U6TrdG3">inertial</a> <a href="/facts/Barycentric_coordinates_(astronomy)/Fw9renao">barycentric</a> reference frame whose axes are defined by the measured positions of <a href="/facts/Extragalactic_astronomy/WtKKEWru">extragalactic sources</a> (mainly <a href="/facts/Quasar/HS96328l">quasars</a>) observed using <a href="/facts/Very-long-baseline_interferometry/2XDaPPMw">very-long-baseline interferometry</a> while the <i>Gaia</i>-CRF is an inertial barycentric reference frame defined by optically measured positions of extragalactic sources by the <a href="/facts/Gaia_(spacecraft)/dlvcdkoN"><i>Gaia</i> satellite</a> and whose axes are rotated to conform to the ICRF. Although <a href="/facts/General_relativity/jVsoIg8X">general relativity</a> implies that there are no true inertial frames around <a href="/facts/Gravitation/bqAN7DdQ">gravitating</a> bodies, these reference frames are important because they do not exhibit any measurable angular <a href="/facts/Rotation/yChkz49x">rotation</a> since the extragalactic sources used to define the ICRF and the <i>Gaia</i>-CRF are extremely far away. The ICRF and the <i>Gaia</i>-CRF are now the standard reference frames used to define the positions of <a href="/facts/Astronomical_object/BCHLa0Q2">astronomical objects</a>. </p>