first full-fledged optical realisation of the ICRS, that is to say, an optical reference frame built only on extragalactic sources." The axes of Gaia-CRF2 were aligned to a prototype version of the forthcoming ICRF3 using 2820 objects common to Gaia-CRF2 and to the ICRF3 prototype. Third Gaia celestial reference frame (Gaia–CRF3) The third Gaia celestial reference frame (Gaia–CRF3) is based on 33 months of observations of 1,614,173 extragalactic sources. As with The earlier Hipparcos and Gaia reference frames, the axes of Gaia-CRF3 were aligned to 3142 optical counterparts of ICRF-3 in the S/X frequency bands. In August 2021 the International Astronomical Union noted that the Gaia-CRF3 had "largely superseded the Hipparcos Catalogue" and was "de facto the optical realization of the Celestial Reference Frame within the astronomical community." Consequently, the IAU decided that Gaia-CRF3 shall be "the fundamental realization of the International Celestial Reference System (ICRS) ... for the optical domain." See also Astrometry Astronomy Barycentric and geocentric celestial reference systems International Terrestrial Reference System and Frame References Further reading Kovalevsky, Jean; Mueller, Ivan Istvan; Kołaczek, Barbara (1989) Reference Frames in Astronomy and Geophysics, Astrophysics and Space Science Library, Volume 154 Kluwer Academic Publishers External links International Celestial Reference System (ICRS) from USNO Overview of ICRS and ICRF IERS Conventions 2003 (defines ICRS and other related standards) ICRF page from the International Earth Rotation Service General information on the ICRS from IERS ICRS Product Center Astronomical coordinate systems Astrometry Frames of reference