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ICRS Links

The International Celestial Reference System (ICRS) is now the basis for fundamental measurements of the positions and motions of celestial bodies. The ICRS defines standard constants, data, and algorithms for the processing and interpretation of precise astrometric observations, effectively establishing a space-fixed coordinate system on the sky.

This page summarizes important links relating to the implementation of the ICRS. A separate narrative description of the ICRS is also available.

The Defining Extragalactic Frame

The International Celestial Reference Frame (ICRF) is a catalog of adopted positions of 608 extragalactic radio sources, 212 of them defining the ICRS axes.

The Frame at Optical Wavelengths

The ICRS is realized at optical wavelengths by a subset of the stars in the Hipparcos Catalogue; specifically, the ~100,000 stars without C, G, O, V or X flags in field H59 (85% of the total catalog).

Standard Algorithms

Algorithms, constants, and computer subroutines that can be used to transform ICRS catalog data to observable quantities (and vice versa) are given in the IERS Conventions (2003). A separate collection of computer subroutines that implement IAU-sanctioned algorithms, the Standards of Fundamental Astronomy (SOFA), is under construction under the auspices of IAU Division 1 (Fundamental Astronomy).

For applications requiring accuracies of no better than 0.1 arcsecond between 1990 and 2010, previously published algorithms can still be used.

The latest version of NOVAS implements the new IAU models. A revision of the Explanatory Supplement to reflect the new models is in progress.

Data in the ICRS

Three important star catalogs ranging in visual magnitude from the brightest to 16th magnitude are Hipparcos (see links above), Tycho-2, and UCAC. The Tycho-2 Catalogue combines a re-analysis of the Hipparcos star mapper observations with data from 144 earlier ground-based star catalogs, including the century-old Astrographic Catalogue (AC). Tycho-2 contains 2.5 million stars. The long time baseline provides very good proper motions. The UCAC project used ground-based observations taken between 1998 and 2004 with an accuracy of 20 mas for well exposed stars. Proper motions are provided using various earlier epoch data. The UCAC second data release contains 46 million stars down to 16th magnitude.

A subgroup of the IAU Working Group on the ICRS is tasked with promoting the densification of the ICRS at optical and infra-red wavelengths, and a number of projects to this end are in progress.

The Jet Propulsion Laboratory DE405/LE405 planetary and lunar ephemerides provide the positions and velocities of the nine major planets and the Moon with respect to the solar system barycenter, in rectangular coordinates, with respect to the ICRS axes.

The Millennium Star Atlas is based on the Hipparcos and Tycho catalogs and is the product of a collaboration between the European Space Agency and Sky Publishing Corporation. It shows stars to visual magnitude 11 in 1548 charts in 3 printed volumes.

The data published in the Astronomical Almanac is in the ICRS beginning with the 2003 edition; complete implementation was done by the 2006 edition.

IAU Resolutions

A number of resolutions have been passed by the International Astronomical Union (IAU) since 1991 that relate to celestial reference systems, time scales, and the rotation of the Earth. The ICRS was established by Resolution B2 of 1997, which was amended by Resolution B1.2 of 2000. An authoritative explanation and methods of implementation are found in USNO Circular 179.