International Celestial Reference System

The International Celestial Reference System (ICRS) is a relativistic celestial coordinate system whose axes are defined by distant extragalactic sources, chiefly observed in radio wavelengths.

Celestial coordinate system: A system for specifying directions or positions on the celestial sphere. The ICRS is a precise, standardized instance of this broader kind of coordinate system.
Very-long-baseline interferometry: A radio astronomy technique that combines widely separated antennas to achieve extremely high angular resolution. It measures radio-source directions with the precision needed to realize the ICRS axes.
International Astronomical Union: The international organization that coordinates astronomical standards, definitions, and nomenclature. The IAU adopted the ICRS as the standard celestial reference system in 1997.
Gaia (spacecraft): A European Space Agency space observatory that measures stellar positions, distances, and motions. Gaia's optical astrometry realizes the celestial system in a dense catalog of stars.
Reference frame: A coordinate system together with conventions and physical standards that make measurements and positions operationally meaningful. The ICRS is realized as a frame through measured source positions and adopted conventions.
Quasar: An extremely luminous active galactic nucleus powered by accretion onto a supermassive black hole. Compact quasars provide many of the distant radio sources used to anchor the celestial frame.
FK5: A fundamental star catalog and celestial reference frame based on the fifth fundamental catalog. The ICRS superseded FK5, whose orientation depended on an equinox and stellar observations.
Astrometry: The branch of astronomy that measures the positions and motions of celestial objects. The ICRS supplies the common celestial orientation for modern high-precision astrometry.
International Celestial Reference Frame: The observational realization of the ICRS through measured positions of extragalactic radio sources. It supplies the physical source-based realization of the ICRS axes.
International Celestial Reference Frame 3: The third realization of the International Celestial Reference Frame, based on very-long-baseline interferometry observations. It is a specific radio-source realization of the ICRS.

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