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<html> <head><script type="text/javascript" src="/_static/js/bundle-playback.js?v=HxkREWBo" charset="utf-8"></script> <script type="text/javascript" src="/_static/js/wombat.js?v=txqj7nKC" charset="utf-8"></script> <script>window.RufflePlayer=window.RufflePlayer||{};window.RufflePlayer.config={"autoplay":"on","unmuteOverlay":"hidden"};</script> <script type="text/javascript" src="/_static/js/ruffle/ruffle.js"></script> <script type="text/javascript"> __wm.init("https://web.archive.org/web"); __wm.wombat("http://aa.usno.navy.mil:80/faq/docs/ICRS_doc.html","20060805073908","https://web.archive.org/","web","/_static/", "1154763548"); </script> <link rel="stylesheet" type="text/css" href="/_static/css/banner-styles.css?v=S1zqJCYt" /> <link rel="stylesheet" type="text/css" href="/_static/css/iconochive.css?v=3PDvdIFv" />  <title>ICRS Narrative</title> <meta name="description" content="The International Celestial Reference System (ICRS) is the fundamental celestial reference system adopted by the International Astronomical Union (IAU) for high-precision positional astronomy. The ICRS, with its origin at the solar system barycenter and space fixed axis directions, is meant to represent the most appropriate coordinate system for expressing reference data on the positions and motions of celestial objects."> <meta name="keywords" content="International Celestial Reference System, International Celestial Reference Frame, ICRS, ICRF, astronomical reference system, astronomical reference frame, star catalog, VLBI, Hipparcos, astrometry, right ascension, declination, J2000, J2000.0"> <meta name="owner" content=" George Kaplan Astronomical Applications Dept. U.S. Naval Observatory 3450 Massachusetts Ave, NW Washington, DC 20392-5420 Revised: 18 Nov 1998 GHK 2 Jan 2003 GHK 24 Jan 2003 GHK 18 May 2004 GHK "> <base href="https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/"> <link rel="stylesheet" href="https://web.archive.org/web/20060805073908cs_/http://aa.usno.navy.mil/css/default.css" type="text/css"> <script language="JavaScript">  </script> </head> <body bgcolor="#eeeeee" text="black" link="navy" vlink="maroon" alink="red">    <table width="100%"><tr> <td align="left" class="large">U.S. Naval Observatory</td> <td align="right" class="large">Astronomical Applications Department</td> </tr></table> <table width="100%"><tr> <td width="10%"> <table border="1" cellpadding="1" cellspacing="0" align="center" bgcolor="black"> <tr><td><a href="https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/" target="_top"><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/AALogoSmall.jpg" width="201" height="108" alt="AA Home" border="0"></a></td></tr> </table> </td> <td width="10"></td> <td align="center"><h1>The International Celestial Reference System <hr width="40%" size="3" noshade> ICRS</h1></td> </tr></table> <br> <center><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bar.gif" alt="bar" width="640" height="5" border="0"></center> <br>    <div class="med"> <p>The <i>International Celestial Reference System (ICRS)</i> is the fundamental celestial reference system adopted by the International Astronomical Union (IAU) for high-precision positional astronomy. The ICRS, with its origin at the solar system barycenter and "space fixed" axis directions, is meant to represent the most appropriate coordinate system for expressing reference data on the positions and motions of celestial objects.</p> </div> <div class="med"> <center> <table bgcolor="C0E0FF" border="0" cellspacing="0" cellpadding="5"> <tr><td width="40"> </td> <td><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bullet.gif">  <a href="faq/docs/ICRS_doc.html#BKGD">Background</a></td> <td width="40"> </td> </tr> <tr><td width="40"> </td> <td><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bullet.gif">  <a href="faq/docs/ICRS_doc.html#RDEV">Recent Developments</a></td> <td width="40"> </td> </tr> <tr><td width="40"> </td> <td><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bullet.gif">  <a href="faq/docs/ICRS_doc.html#IMPL">ICRS Implementation</a></td> <td width="40"> </td> </tr> <tr><td width="40"> </td> <td><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bullet.gif">  <a href="faq/docs/ICRS_doc.html#DATA">Data in the ICRS</a></td> <td width="40"> </td> </tr> <tr><td width="40"> </td> <td><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bullet.gif">  <a href="faq/docs/ICRS_doc.html#AUTH">Authorizing Resolution</a></td> <td width="40"> </td> </tr> <tr><td width="40"> </td> <td><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bullet.gif">  <a href="faq/docs/ICRS_doc.html#REFS">References</a></td> <td width="40"> </td> </tr> </tr> <tr><td width="40"> </td> <td><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bullet.gif">  <a href="data/docs/ICRS_links.html#LINKS">Important Links</a></td> <td width="40"> </td> </tr> </table> </center> </div> <div class="med"> <br> <a name="BKGD"></a> <h3>Background</h3> <img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/refsys.gif" align="right" width="262" height="175" hspace="10" vspace="5"> <p>The ICRS is a set of specifications defining a high precision coordinate system with its origin at the solar system barycenter and "space fixed" (kinematically non-rotating) axes. The specifications include a metric tensor, a prescription for establishing and maintaining the axis directions, a list of benchmark objects with precise coordinates for each one, and standard models and algorithms that allow these coordinates to be transformed into observable quantities for any location and time.</p> <p>In this context is useful to distinguish between <i>reference system</i> and a <i>reference frame</i>. A <i>reference system</i> is the complete specification of how a celestial coordinate system is to be formed. It defines the origin and fundamental planes (or axes) of the coordinate system. It also specifies all of the constants, models, and algorithms used to transform between observable quantities and reference data that conform to the system. A <i>reference frame</i> consists of a set of identifiable fiducial points on the sky along with their coordinates, which serves as the practical realization of a reference system.</p> <p>For example, the fundamental plane of an astronomical reference system has conventionally been the extension of the Earth's equatorial plane, at some date, to infinity. The <i>declination</i> of a star or other object is its angular distance north or south of this plane. The <i>right ascension</i> of an object is its angular distance measured eastward along the equator from some defined reference point. This reference point, the right ascension origin, has traditionally been the <i>equinox</i>: the point at which the Sun, in its yearly circuit of the celestial sphere, crosses the equatorial plane moving from south to north. The Sun's apparent yearly motion lies in the <i>ecliptic</i>, the plane of the Earth's orbit. The equinox, therefore, is a direction in space along the nodal line defined by the intersection of the ecliptic and equatorial planes; equivalently, on the celestial sphere, the equinox is at one of the two intersections of the great circles representing these planes. Because both of these planes are moving, the coordinate systems that they define must have a date associated with them; such a reference system must be therefore specified as "the equator and equinox of [some date]".</p> <p>Of course, such a reference system is an idealization, because the theories of motion of the Earth that define how the two planes move are imperfect. In fact, the very definitions of these planes are problematic for high precision work. Even if the fundamental planes are defined without any reference to the motions of the Earth, there is no way to magically paint them on the celestial sphere at any particular time. Therefore, in practice, we use a specific reference frame - a set of fiducial objects with assigned coordinates - as the practical representation of an astronomical reference system. The scheme is completely analogous to how terrestrial reference systems are established using benchmarks on the Earth's surface.</p> <p><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/RA_Dec.gif" align="left" width="305" height="180" hspace="10" vspace="5">Most commonly, a reference frame consists of a catalog of precise positions (and motions, if measurable) of stars or extragalactic objects as seen from the solar system barycenter at a specific epoch (now usually "J2000.0", which is 12h <a href="https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/faq/docs/TT.html" target="_top" onclick="window.open('https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/faq/docs/TT.html', 'TT', 'toolbar=no location=no directories=no menubar=no width=500 height=500 scrollbars=yes'); return false">TT</a> on 1 January 2000). Each object's instantaneous position, expressed as right ascension and declination, indicates the object's angular distance from the catalog's equator and origin of right ascension. Any two such objects in the catalog therefore uniquely orient a spherical coordinate system on the sky - a reference frame.</p> <p>A modern astrometric catalog contains data on a large number of objects (N), so the coordinate system is vastly overdetermined. The quality of the reference frame defined by a catalog depends on the extent to which the coordinates of all possible pairs of objects (~N<sup>2</sup>/2) serve to define the identical equator and right ascension origin, within the expected random errors. Typically, every catalog contains <i>systematic errors</i>, that is, errors in position that are similar in direction and magnitude for objects that are in the same area of the sky, or are of the same magnitude (flux) or color (spectral index). Systematic errors mean that the reference frame is warped, or is effectively different for different classes of objects. Obviously, minimizing systematic errors when a catalog is constructed is as important (if not more so) than minimizing the random errors.</p> <p>To be useful, a reference frame must be implemented at the time of actual observations, and this requires the computation of the geocentric coordinates of the catalog objects at arbitrary dates and times. The accuracy with which we know the motions of the objects (unless assumed zero) are an essential factor in this computation. Astrometric star catalogs list <i>proper motions</i>, which are the projection of each star's space motion onto the celestial sphere, expressed as an angular rate in right ascension and declination per unit time. Because the tabulated proper motions are never perfect (even if assumed zero), any celestial reference frame deteriorates with time. Moreover, systematic errors in the proper motions can produce time-dependent warpings and spurious rotations in the frame. Therefore, the accuracy and consistency of the proper motions are critical to the overall quality, utility, and longevity of reference frames defined by stars.</p> <p>The positions of solar system objects can also be used to define a reference frame. For each solar system body involved, an <i>ephemeris</i> (pl. <i>ephemerides</i>) is used, which is simply a table of the celestial coordinates of the body as a function of time (or an algorithm that yields such a table). A reference frame defined by the ephemerides of one or more solar system bodies is called a <i>dynamical reference frame</i>. Because the ephemerides used incorporate the theories of motion of the Earth as well as that of the other solar system bodies, dynamical reference frames embody in a very fundamental way the moving equator and ecliptic, hence the equinox. They have therefore been used to properly align star catalog reference frames (the star positions were systematically adjusted) on the basis of simultaneous observations of stars and planets. However, dynamical reference frames are not very practical for establishing a coordinate system for day-to-day astronomical observations. The ICRS does not involve a dynamical reference frame.</p> <p>Descriptions of reference frames and reference systems often refer to three coordinate axes, which are simply the set of right-handed cartesian axes that correspond to the usual celestial spherical coordinate system. The xy-plane is the equator, the z-axis points toward the north celestial pole, and the x-axis points toward the origin of right ascension. Although in principle this allows us to specify the position of any celestial object in rectangular coordinates, the distance scale is not established to high precision beyond the solar system. What a reference system actually defines is the way in which the two conventional astronomical <i>angular</i> coordinates, right ascension and declination, overlay real observable points in the sky.</p> <br> <a name="RDEV"></a> <h3>Recent Developments</h3> <p>The establishment of celestial reference systems is coordinated by the <a href="https://web.archive.org/web/20060805073908/http://www.iau.org/" target="_top">International Astronomical Union (IAU)</a>. The previous astronomical reference system was based on the equator and equinox of J2000.0 determined from observations of planetary motions, together with the IAU (1976) System of Astronomical Constants and related algorithms <a href="faq/docs/ICRS_doc.html#1">[1]</a>. The reference frame that embodied this system for practical purposes was the <a href="https://web.archive.org/web/20060805073908/http://vizier.u-strasbg.fr/viz-bin/Cat?I/149A" target="_top">Fifth Fundamental Catalogue (FK5)</a> <a href="faq/docs/ICRS_doc.html#2">[2]</a>. The FK5 is a catalog of 1535 bright stars (to magnitude 7.5), supplemented by a fainter extension of 3117 additional stars (to magnitude 9.5). The FK5 was the successor to the FK3 and FK4 catalogs, all compiled from catalogs of meridian observations taken in the visual band (many such observations were, in fact, taken by eye). The formal uncertainties in the star positions of the FK5 at the time of its (somewhat delayed) publication in 1988 were about 30-40 milliarcseconds over most of the sky, but the errors are considerably worse when systematic trends are taken into account.</p> <p>In recent years, the most precise wide-angle (global) astrometry has been conducted not in the optical regime but at radio wavelengths, involving the techniques of <a href="https://web.archive.org/web/20060805073908/http://lupus.gsfc.nasa.gov/brochure/brochure.html" target="_top">Very Long Baseline Interferometry (VLBI)</a> and pulsar timing. Uncertainties of radio  <img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/VLBI_antenna.jpg" align="left" hspace="10" vspace="5"> source positions listed in all-sky VLBI catalogs are now typically less than one milliarcsecond, and often a factor of ten better. Furthermore, because these radio sources are very distant extragalactic objects (mostly quasars) that are not expected to show measurable intrinsic motion, a reference frame defined by VLBI positions should be "more inertial" (less subject to spurious rotation) than a reference frame defined by galactic objects such as stars or pulsars. The VLBI catalogs do have the disadvantage that their origin of right ascension is somewhat arbitrary; there is no real equinox in VLBI catalogs, since VLBI has little sensitivity to the ecliptic plane. However, this problem has turned out to be more conceptual than practical, since methods have been developed to relate the VLBI right ascension origin to the equinox as conventionally defined.</p> <p>Because of these considerations, since the mid 1980s, astronomical measurements of the Earth's rotation - from which astronomical time is determined - have depended heavily on VLBI, with classical methods based on star transits being phased out. Hence the situation evolved where the definition of the fundamental astronomical reference frame (the FK5) became irrelevant to some of the most precise and important astrometric measurements. VLBI revealed, in addition, that the models of the Earth's precession and nutation that were part of the old system were inadequate for modern astrometric precision. In particular, the "constant of precession" - a measurement of the long-term rate of change of the orientation of the Earth's axis in space - had been overestimated by about 0.3 arcseconds per century. Moreover, the success of the European Space Agency <a href="https://web.archive.org/web/20060805073908/http://astro.estec.esa.nl/Hipparcos/" target="_top">Hipparcos astrometric satellite</a>, launched in 1989, promised to provide a new, very accurate set of star coordinates in the optical regime.</p> <p>Thus, beginning in 1988, a number of IAU working groups began considering the requirements for a new fundamental astronomical reference system <a href="faq/docs/ICRS_doc.html#3">[3]</a> <a href="faq/docs/ICRS_doc.html#4">[4]</a>. The resulting series of IAU resolutions, passed in 1991, 1994, 1997, and 2000 <a href="faq/docs/ICRS_doc.html#5">[5]</a> <a href="faq/docs/ICRS_doc.html#6">[6]</a> <a href="faq/docs/ICRS_doc.html#7">[7]</a>, <a href="faq/docs/ICRS_doc.html#8">[8]</a>, effectively form the specifications for the ICRS. The axes of the ICRS are defined by the adopted positions of a specific set of extragalactic objects, which are assumed to have no measurable proper motions. The ICRS axes are consistent, to better than 0.1 arcsecond, with the equator and equinox of J2000.0 defined by the dynamics of the Earth. However, the ICRS axes are meant to be regarded as fixed directions in space that have an existence independent of the dynamics of the Earth or the particular set of objects used to define them at any given time.</p> <p>The promotion, maintenance, extension, and use of the ICRS were the responsibilities of the IAU Working Group on the International Celestial Reference System from 1997 until 2003. These responsibilities are now diffused throughout <a href="https://web.archive.org/web/20060805073908/http://www.iau.org/Organization/divcom/div1.html" target="_top">IAU Division 1 (Fundamental Astronomy)</a>, with leading roles played by <a href="https://web.archive.org/web/20060805073908/http://www.pha.jhu.edu/iau_comm8/comm8.html" target="_top">Commission 8 (Astrometry)</a> and <a href="https://web.archive.org/web/20060805073908/http://danof.obspm.fr/iaucom19/" target="_top">Commission 19 (Rotation of the Earth)</a>.</p> <br> <a name="IMPL"></a> <h3>ICRS Implementation</h3> <p><i>The Defining Extragalactic Frame</i></p> <img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/HDF_Earth[tiny].jpg" align="right" hspace="10" vspace="5"> <p>The <a href="https://web.archive.org/web/20060805073908/http://hpiers.obspm.fr/webiers/results/icrf/Icrf.html" target="_top"><i>International Celestial Reference Frame (ICRF)</i></a> is a catalog of adopted positions of 608 extragalactic radio sources observed with VLBI, all strong (>0.1 Jy) at S and X bands (wavelengths 13 and 3.6 cm) <a href="faq/docs/ICRS_doc.html#9">[9]</a>. Most have faint <a href="https://web.archive.org/web/20060805073908/http://rorf.usno.navy.mil/rorf_optical.shtml" target="_top">optical counterparts</a> (typically m<sub>V</sub>>18) and the majority are quasars. Of these objects, 212 are <i>defining sources</i> that establish the orientation of the ICRS axes, with origin at the solar system barycenter. Typical position uncertainties for the defining sources are of order 0.5 milliarcsecond; the orientation of the axes is defined from the ensemble to an accuracy of about 0.02 milliarcseconds. As described below, these axes correspond closely to what would conventionally be described as "the equator and equinox of J2000.0".</p> <p>The <a href="https://web.archive.org/web/20060805073908/http://www.iers.org/" target="_top">International Earth Rotation Service (IERS)</a> monitors the radio sources involved in the ICRF. This monitoring is necessary because, at some level, most of the sources are variable in both flux and <a href="https://web.archive.org/web/20060805073908/http://rorf.usno.navy.mil/rrfid.shtml" target="_top">structure</a> and the centers of emission can display spurious motions. It is possible that, eventually, the defining list of sources will have to be amended to maintain the fixed orientation of the overall frame.</p> <p><i>The Frame at Optical Wavelengths</i></p> <p>The ICRS is realized at optical wavelengths by stars in the <a href="https://web.archive.org/web/20060805073908/http://astro.estec.esa.nl/Hipparcos/catalog.html" target="_top">Hipparcos Catalogue</a> of 118,218 stars, some as faint as visual magnitude 12 <a href="faq/docs/ICRS_doc.html#10">[10]</a>. Only stars with uncomplicated and well-determined proper motions (e.g., no known binaries) are used for the ICRS realization. This subset, referred to as the Hipparcos Celestial Reference Frame (HCRF), comprises 85% of the stars in the Hipparcos catalog. Hipparcos star coordinates and proper motions are given within the ICRS (J2000.0) coordinate system but are listed for epoch J1991.25. (That is, the catalog effectively represents a snapshot of the motion of the stars through space taken on 2 April 1991.) At the catalog epoch, Hipparcos uncertainties for stars brighter than 9th magnitude have median values somewhat better than 1 milliarcsecond in position and 1 milliarcsecond/year in proper motion <a href="faq/docs/ICRS_doc.html#10">[10]</a> <a href="faq/docs/ICRS_doc.html#11">[11]</a>. Thus, projected to epoch J2000.0, typical Hipparcos star position errors are in the range 5-10 milliarcseconds.</p> <p><i>Standard Algorithms</i></p> <p>At its General Assembly in 2000 <a href="faq/docs/ICRS_doc.html#8">[8]</a> the IAU defined a system of space-time coordinates for (1) the solar system, and (2) the Earth, within the framework of General Relativity, by specifying the form of the metric tensors for each and the 4-dimensional space-time transformation between them. The former is called the Barycentric Celestial Reference System (BCRS), and the latter, the Geocentric Celestial Reference System (GCRS). Since the IAU definitions of the BCRS and GCRS concern only relativity, they can be thought of as defining two <i>families</i> of reference systems. The ICRS can be considered a particular implementation of the BCRS, i.e., a member of the BCRS family. In practical terms, this means that models used in the analysis of observations contributing to the ICRS, or the prediction of observables from ICRS data, should be consistent with the specified metric.</p> <p>In 2000, the IAU also adopted new models for the computation of the Earth's instantaneous orientation within the ICRS. The new models include what is referred to as the <i>IAU 2000A precession-nutation model</i>, a new definition of the celestial pole, and a new reference point, called the <i>Celestial Ephemeris Origin</i>, for measuring the rotational angle of the Earth around its instantaneous axis. Some aspects of the models were not finalized until late 2002. A set of algorithms that incorporate these models, used to transform ICRS catalog data to observable quantities (and vice versa), is given in the <a href="https://web.archive.org/web/20060805073908/http://maia.usno.navy.mil/conv2000.html" target="_top">IERS Conventions (2003)</a>. Numerical values for fundamental astronomical constants, and computer code implementing the new algorithms, are also given. It should be noted that the newly adopted precessional rate of the Earth's axis has not yet been incorporated into a dynamically self-consistent theory. An IAU Working Group is studying the the issue and will make a recommendation for a new theory of precession. Therefore, the expressions for precession currently found in the IERS Conventions (2003) are likely to be superseded sometime in 2004. However, the effect of that change on computed observables should be negligible for data acquired in the decades around 2000.</p> <p>The new Earth orientation models are, of course, relevant only to fundamental observations made from the surface of the Earth. Astrometric observations taken from space platforms, or those that are differential in nature (based on reference objects that are all within a small field), are not affected by the new models. The IERS Standards are focused on observations of the first type, and the document is not meant to be a complete astrometric textbook. Some algorithms required for optical observations may be found in Volumes 1 and 3 of the Hipparcos Catalogue documentation <a href="faq/docs/ICRS_doc.html#10">[10]</a>. Kaplan has numerically compared some of the algorithms (those involving relativity) used in radio and optical data analysis and found good consistency despite very different paradigms <a href="faq/docs/ICRS_doc.html#12">[12]</a>.</p> <p>A collection of computer subroutines in Fortran that implement IAU-sanctioned algorithms is now available, called the <a href="https://web.archive.org/web/20060805073908/http://www.iau-sofa.rl.ac.uk/" target="_top">Standards of Fundamental Astronomy (SOFA)</a>. (C versions are contemplated for the future.) The collection is managed by an international panel, the SOFA Reviewing Board, which works under the auspices of IAU Division 1 (Fundamental Astronomy). The board solicits code from the astrometric and geodetic community that implements IAU-sanctioned models. Subroutines are adapted to established coding standards and validated for accuracy before being added to the SOFA collection.</p> <p>For ground-based applications requiring accuracies of no better than 50 milliarcseconds between about 1990 and 2010, the algorithms described in Chapter 3 of the <a href="https://web.archive.org/web/20060805073908/http://www.uscibooks.com/seid.htm" target="_top"><i>Explanatory Supplement to the Astronomical Almanac</i></a> <a href="faq/docs/ICRS_doc.html#13">[13]</a> can still be used with ICRS data. See also the <a href="software/novas/novas_info.html">NOVAS</a> software package, which implements these algorithms. A revision of the <i>Explanatory Supplement</i>, to incorporate descriptions of the new IAU models, is in preparation. A new version of NOVAS, implementing the new models, is now available in a <a href="software/novas/new_novas_f/">beta test release</a>.</p> <p><i>Relationship to Other Systems</i></p> <p>The orientation of the ICRS axes is consistent with the equator and equinox of J2000.0 represented by the FK5, within the errors of the latter. Since, at J2000.0, the errors of the FK5 are significantly worse than those of Hipparcos, the ICRS can be considered to be a refinement of the FK5 system <a href="faq/docs/ICRS_doc.html#10">[10]</a> at (or near) that epoch. <p>The ICRS can also be considered to be a good approximation (at least as good as the FK5) to the conventionally defined dynamical equator and equinox of J2000.0 <a href="faq/docs/ICRS_doc.html#14">[14]</a>. In fact, the equator is well determined fundamentally from the VLBI observations that are the basis for the entire ICRS, and the ICRS pole is within 20 milliarcseconds of the dynamical pole. The zero point of VLBI-derived right ascensions is arbitrary, but traditionally has been set by assigning to the right ascension of source 3C 273B a value derived from lunar occultation timings - the Moon's ephemeris thus providing an indirect link to the dynamical equinox. The ICRS origin of right ascension was made to be consistent with that in a group of VLBI catalogs previously used by the IERS, each of which had been individually aligned to the lunar occultation right ascension of 3C 273B. The difference between the ICRS origin of right ascension and the dynamical equinox has been independently measured by two groups that used different definitions of the equinox, but in both cases the difference found was less than 0.1 arcsecond.</p> <p>Because of its consistency with previous reference systems, implementation of the ICRS will be transparent to any applications with accuracy requirements of no better than 0.1 arcseconds near epoch J2000.0. That is, for applications of this accuracy, the distinctions between the ICRS, FK5, and dynamical equator and equinox of J2000.0 are not significant.</p> <p><i>ICRS Overview Papers</i></p> <p>Feissel and Mignard have written a good, concise review of the ICRS adoption and its implications <a href="faq/docs/ICRS_doc.html#14">[14]</a>. More recently, Seidelmann and Kovalevsky published a broader review of the ICRS and the new Earth orientation models <a href="faq/docs/ICRS_doc.html#15">[15]</a>. Both papers contain technical details and references not covered here.</p> <br> <a name="DATA"></a> <h3>Data in the ICRS</h3> <img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/0241+622.jpg" align="right" hspace="10" vspace="5"> <p>Although the ICRF and HCRF are its basic radio and optical realizations, the ICRS is gradually being extended to fainter magnitudes and other wavelengths. Thus, an increasing amount of fundamental astronomical data is being brought within the new system. Clearly, the <a href="https://web.archive.org/web/20060805073908/http://ad.usno.navy.mil/dens_wg/dens.html" target="_top">densification of the ICRS</a> at optical and infra-red wavelengths will facilitate its wider use, and a number of projects to this end are in progress.</p> <p>As described above, the <a href="https://web.archive.org/web/20060805073908/http://hpiers.obspm.fr/webiers/results/icrf/Icrf.html" target="_top">ICRF</a> consists of the adopted positions of about 600 extragalactic radio sources, a third of which are defining sources. In its original presentation, the ICRF contained 608 extragalactic radio sources, including 212 defining sources. All observational data were part of a common catalog reduction <a href="faq/docs/ICRS_doc.html#9">[9]</a> and thus the adopted coordinates of all the sources are in the ICRS. Of the 396 non-defining sources, 294 are <i>candidate sources</i> that do not meet all of the accuracy and observing history requirements of the defining sources but which may at some later time be added to the defining list. The remaining 102 <i>other sources</i> show excess apparent position variation and are of lower astrometric quality. In 1999, in the <a href="https://web.archive.org/web/20060805073908/http://www.iers.org/iers/products/icrf/icrf-ext1.html" target="_top">ICRF-Extension 1</a>, the positions of the candidate and other sources were refined and 59 new sources were added (the positions of the defining sources were not changed). ICRF-Extension 2 has been finalized and will soon be adopted by the IERS.</p> <p>A list of radio sources to be used as calibrators for the Very Long Baseline Array and the Very Large Array (VLBA Calibrator Survey - VCS1) <a href="faq/docs/ICRS_doc.html#16">[16]</a> has been put in the ICRS.</p> <p>The <a href="https://web.archive.org/web/20060805073908/http://astro.estec.esa.nl/Hipparcos/catalog.html" target="_top">Tycho Catalogue</a> contains about one million stars observed with the Hipparcos satellite's star mapper <a href="faq/docs/ICRS_doc.html#10">[10]</a>. The Tycho Catalogue can be considered an extension of the Hipparcos Catalogue that is more complete at magnitudes 8-11 but less accurate. The astrometric accuracy varies strongly with magnitude, but the median uncertainties of Tycho positions and proper motions are a factor of 30 worse than those in the Hipparcos Catalogue. The large proper motion uncertainties make Tycho unsuitable for precise astrometric work; projected to epoch J2000.0, most Tycho star positions will be in error by several tenths of an arcsecond. The Tycho catalog has now been entirely superceded by the Tycho-2 catalog.</p> <p>The <a href="https://web.archive.org/web/20060805073908/http://ad.usno.navy.mil/proj/ACT/" target="_top">ACT Reference Catalog</a> has also been superceded by the Tycho-2 catalog.</p> <p>The <a href="https://web.archive.org/web/20060805073908/http://www.astro.ku.dk/~erik/Tycho-2/" target="_top">Tycho-2 Catalogue</a> <a href="faq/docs/ICRS_doc.html#17">[17]</a> combines a re-analysis of the Hipparcos star mapper observations with data from 144 earlier ground-based star catalogs. The ground-based catalogs include the <a href="https://web.archive.org/web/20060805073908/http://ad.usno.navy.mil/ac/" target="_top">Astrographic Catalogue (AC)</a>, a large photographic project carried out near the beginning of the 20th century involving 20 observatories worldwide. Tycho-2 contains 2,539,913 stars, and combines the accuracy of the recent Tycho position measurements with proper motions derived from a time baseline of almost a century. Proper motion uncertainties are 1-3 milliarcsecond/year. At epoch J2000.0, the Tycho-2 positions of stars brighter than 9th magnitude will typically be in error by 20 milliarcseconds. However, the positional accuracy degrades quite rapidly for magnitudes fainter than 9, so that 12th magnitude stars may be expected to have a median J2000.0 position error exceeding 100 milliarcseconds.</p> <p>For descriptions of other star catalogs on the ICRS, see the U.S. Naval Observatory Astrometry Department's <a href="https://web.archive.org/web/20060805073908/http://ad.usno.navy.mil/star/star_cats_rec.shtml" target="_top">catalog information and recommendations</a>.</p> <p>The Jet Propulsion Laboratory DE405/LE405 planetary and lunar ephemerides (usually just referred to as DE405) <a href="faq/docs/ICRS_doc.html#18">[18]</a> have been aligned to the ICRS. These ephemerides provide the positions and velocities of the nine major planets and the Moon with respect to the solar system barycenter, in rectangular coordinates. The data is represented in Chebyshev series form and Fortran subroutines are provided to read and evaluate the series for any date and time. DE405 spans the years 1600 to 2200; a long version, DE406, spans the years -3000 to +3000 with lower precision.</p> <p>The data listed in the <a href="publications/docs/almanacs.html#astalm"><i>Astronomical Almanac</i></a> is in the ICRS beginning with the 2003 edition. Planetary and lunar ephemerides are derived from DE405/LE405. New models for Earth rotation, based on the IAU resolutions of 2000, are now being used in the preparation of the book and will be first reflected in the 2006 almanac. The precession algorithm listed in the IERS Conventions (2003) will be used for 2006, but it is anticipated that this algorithm will be replaced for later editions by a dynamically consistent precession model recommended by the IAU Working Group on Precession and the Ecliptic.</p> <br> <a name="AUTH"></a> <h3>Authorizing Resolution</h3> <p>The construction and implementation of the ICRS was authorized and supported by the <a href="https://web.archive.org/web/20060805073908/http://www.iau.org/" target="_top">International Astronomical Union (IAU)</a>. Resolution B2, passed by the 23rd General Assembly of the IAU in August 1997 <a href="faq/docs/ICRS_doc.html#7">[7]</a>, states that <ul> <li>from 1 January 1998, the IAU celestial reference system shall be the International Celestial Reference System (ICRS) as specified in the 1991 IAU Resolution on reference frames and as defined by the International Earth Rotation Service (IERS); <li>the corresponding fundamental reference frame shall be the International Celestial Reference Frame (ICRF) constructed by the IAU Working Group on Reference Frames; <li>the Hipparcos Catalogue shall be the primary realization of the ICRS at optical wavelengths; <li>the IERS should take appropriate measures, in conjunction with the IAU Working Group on Reference Frames, to maintain the ICRF and its ties to the reference frames at other wavelengths. </ul> <p>The "1991 IAU Resolution on reference frames" referred to above (passed by the 21st IAU General Assembly) <a href="faq/docs/ICRS_doc.html#5">[5]</a> recommended that an IAU working group establish a list of extragalactic radio sources that would be "candidates for primary sources defining the new conventional reference frame." </p> <p>At the subsequent IAU General Assembly in 2000, Resolution B1.2 <a href="faq/docs/ICRS_doc.html#8">[8]</a> restricted the number of Hipparcos stars that would be considered part of the optical realization of the ICRS. The relevant part of this resolution states that <ul> <li>Resolution B2 of the XXIIIrd IAU General Assembly (1997) be amended by excluding from the optical realization of the ICRS all stars flagged C, G, O, V and X in the Hipparcos Catalogue; <li>this modified Hipparcos frame be labelled the Hipparcos Celestial Reference Frame (HCRF). </ul> <p>Effectively, this change eliminated about 15% of the stars in the Hipparcos catalog, leaving those with well determined linear proper motions. The flags referred to are given in Hipparcos field H59.</p> <p>Resolutions B1.6, B1.7, and B1.8 of the 2000 General Assembly defined the IAU 2000A precession-nutation model, the Celestial Intermediate Pole, the Celestial Ephemeris Origin, and other quantities that, together, specify how the orientation of the Earth is to be computed. Other related resolutions dealt with time scales and relativity.</p> <p>The <a href="https://web.archive.org/web/20060805073908/http://maia.usno.navy.mil/iauc19/iaures.html" target="_top">full text of the relevant 1997 resolutions</a> has been posted by IAU Commission 19 (Rotation of the Earth); the <a href="https://web.archive.org/web/20060805073908/http://danof.obspm.fr/IAU_resolutions/Resol-UAI.html" target="_top">full text of the relevant 2000 resolutions</a> has been posted by the Observatory of Paris. The 1997 and 2000 resolutions are also printed in their entirety in IAU Information Bulletins <a href="https://web.archive.org/web/20060805073908/http://www.iau.org/IAU/Activities/publications/bulletin/IB81.html" target="_top">81</a> and <a href="https://web.archive.org/web/20060805073908/http://www.iau.org/IAU/Activities/publications/bulletin/pdf/ib88.pdf" target="_top">88</a>, respectively, and in the Proceedings of the two General Assemblies <a href="faq/docs/ICRS_doc.html#7">[7]</a> <a href="faq/docs/ICRS_doc.html#8">[8]</a>.</p> <br> <a name="REFS"></a> <h3>References</h3> <a name="1"></a> <p>[1]   Kaplan, G. H., ed. (1982): <cite>The IAU Resolutions on Astronomical Constants, Time Scales, and the Fundamental Reference Frame</cite>, U.S. Naval Observatory Circular No. 163.   [<a href="https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/publications/docs/usnopubs.html">ordering info</a>]</p> <a name="2"></a> <p>[2]   Fricke, W., Schwan, H., Lederle, T. (1988): <cite>Fifth Fundamental Catalogue (FK5)</cite>, Part I, The Basic Fundamental Stars. Veroffentlichungen Astronomisches Rechen-Institut, Heidelberg, No. 32.   [<a href="https://web.archive.org/web/20060805073908/http://vizier.u-strasbg.fr/viz-bin/Cat?I/149A" target="_top">catalog data</a>]</p> <a name="3"></a> <p>[3]   Lieske, J. H., Abalakin, V. K., eds. (1990): <cite>Inertial Coordinate System on the Sky</cite>, Proceedings of IAU Symposium 141.</p> <a name="4"></a> <p>[4]   Hughes, J. A., Smith, C. A., Kaplan, G. H. (1991): <cite>Reference Systems</cite>, Proceedings of IAU Colloquium 127.   [<a href="https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/publications/docs/usnopubs.html">ordering info</a>]</p> <a name="5"></a> <p>[5]   IAU (1992): <cite>Proceedings of the Twenty-First General Assembly, Buenos Aires 1991</cite>, Transactions of the IAU, Vol. XXI-B, pp. 41-63.</p> <a name="6"></a> <p>[6]   IAU (1996): <cite>Proceedings of the Twenty-Second General Assembly, The Hague 1994</cite>, Transactions of the IAU, Vol. XXII-B, pp. 24-60.</p> <a name="7"></a> <p>[7]   IAU (1998): <cite>Proceedings of the Twenty-Third General Assembly, Kyoto 1997</cite>, Transactions of the IAU, Vol. XXIII-B, pp. 36-51.   [<a href="https://web.archive.org/web/20060805073908/http://maia.usno.navy.mil/iauc19/iaures.html" target="_top">resolutions</a>]</p> <a name="8"></a> <p>[8]   IAU (2002): <cite>Proceedings of the Twenty-Fourth General Assembly, Manchester 2000</cite>, Transactions of the IAU, Vol. XXIV-B, pp. 33-57.   [<a href="https://web.archive.org/web/20060805073908/http://danof.obspm.fr/IAU_resolutions/Resol-UAI.htm" target="_top">resolutions</a>]</p> <a name="9"></a> <p>[9]   Ma, C., Feissel, M. (1997): <cite>Definition and Realization of the International Celestial Reference System by VLBI Astrometry of Extragalactic Objects</cite>, IERS Technical Note 23, Central Bureau of IERS - Observatoire de Paris.   [<a href="https://web.archive.org/web/20060805073908/http://hpiers.obspm.fr/webiers/results/icrf/Icrf.html" target="_top">catalog info</a>]   [<a href="https://web.archive.org/web/20060805073908/http://vizier.u-strasbg.fr/viz-bin/Cat?I/251" target="_top">catalog data</a>]</p> <a name="10"></a> <p>[10]   ESA (1997): <cite>The Hipparcos and Tycho Catalogues</cite>, European Space Agency pub. SP-1200 (17 volumes).   [<a href="https://web.archive.org/web/20060805073908/http://astro.estec.esa.nl/Hipparcos/catalog.html" target="_top">catalog info</a>]   [<a href="https://web.archive.org/web/20060805073908/http://vizier.u-strasbg.fr/viz-bin/Cat?I/239" target="_top">catalog data</a>]</p> <a name="11"></a> <p>[11]   Mignard, F. (1997): "Astrometric Properties of the Hipparcos Catalogue", <cite>Proceedings of the ESA Symposium 'Hipparcos - Venice '97'</cite>, European Space Agency pub. SP-402, pp. 5-10.   [<a href="https://web.archive.org/web/20060805073908/http://astro.estec.esa.nl/Hipparcos/venice-proc/oral01_02.pdf" target="_top">reprint</a>]</p> <a name="12"></a> <p>[12]   Kaplan, G. H. (1998): "High-Precision Algorithms for Astrometry: A Comparison of Two Approaches", <cite>Astronomical Journal</cite>, Vol. 115, pp. 361-372.   [<a href="https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/publications/reports/ghk98a.pdf">reprint</a>]</p> <a name="13"></a> <p>[13]   Seidelmann, P. K., ed. (1992): <cite>Explanatory Supplement to the Astronomical Alamanac</cite>, University Science Books.   [<a href="https://web.archive.org/web/20060805073908/http://www.uscibooks.com/seid.htm" target="_top">ordering info</a>]</p> <a name="14"></a> <p>[14]   Feissel, M., Mignard, F. (1998): "The Adoption of ICRS on 1 January 1998: Meaning and Consequences", <cite>Astronomy & Astrophysics</cite>, Vol. 331, pp. L33-L36.   [<a href="https://web.archive.org/web/20060805073908/http://link.springer-ny.com/link/service/journals/00230/bibs/8331003/2300l33/small.htm" target="_top">reprint</a>]</p> <a name="15"></a> <p>[15]   Seidelmann, P. K., Kovalevsky, J. (2002): "Application of the New Concepts and Definitions (ICRS, CIP and CEO) in Fundamental Astronomy", <cite>Astronomy & Astrophysics</cite>, Vol. 392, pp. 341-351.   [<a href="https://web.archive.org/web/20060805073908/http://www.edpsciences-usa.org/articles/aa/full/2002/34/aa2452/aa2452.html" target="_top">reprint</a>]</p> <a name="16"></a> <p>[16]   Beasley, A. J., Gordon, D., Peck, A. B., Petrov, L., MacMillan, D. S., Fomalont, E. B., Ma, C. (2002): "The VLBA Calibrator Survey - VCS1", Astrophysical Journal Supplement Series, Vol. 141, pp. 13-21.   [<a href="https://web.archive.org/web/20060805073908/http://www.journals.uchicago.edu/ApJ/journal/issues/ApJS/v141n1/55343/55343.html" target="_top">reprint</a>]   [<a href="https://web.archive.org/web/20060805073908/http://www.aoc.nrao.edu/vlba/VCS1" target="_top">catalog data</a>] </p> <a name="17"></a> <p>[17]   H鴊, E., Fabricius, C., Makarov, V. V., Urban, S., Corbin, T., Wycoff, G., Bastian, U., Schwekendiek, P., Wicenec, A. (2000): "The Tycho-2 Catalog of the 2.5 Million Brightest Stars", <cite>Astronomy & Astrophysics</cite>, Vol. 355, pp. L27-L30.   [<a href="https://web.archive.org/web/20060805073908/http://www.astro.ku.dk/~erik/Tycho-2/letter.ps" target="_top">reprint</a>]   [<a href="https://web.archive.org/web/20060805073908/http://www.astro.ku.dk/~erik/Tycho-2/" target="_top">catalog info</a>]   [<a href="https://web.archive.org/web/20060805073908/http://vizier.u-strasbg.fr/viz-bin/Cat?I/259" target="_top">catalog data</a>] </p> <a name="18"></a> <p>[18]   Standish, E. M. (1998): "JPL Planetary and Lunar Ephemerides, DE405/LE405", Jet Propulsion Laboratory Interoffice Memorandum IOM 312.F-98-048.   [<a href="https://web.archive.org/web/20060805073908/ftp://navigator.jpl.nasa.gov/ephem/export/de405.iom/">reprint</a>]   [<a href="https://web.archive.org/web/20060805073908/ftp://navigator.jpl.nasa.gov/ephem/export/">files</a>]   [<a href="https://web.archive.org/web/20060805073908/http://www.willbell.com/software/jpl.htm" target="_top">CD-ROM ordering info</a>]   [<a href="https://web.archive.org/web/20060805073908/http://heasarc.gsfc.nasa.gov/listserv/heafits/msg00050.html" target="_top">FITS version</a>] </p> <br> <a name="LINK"></a> <h3>Important Links</h3> <p>A summary of the <a href="data/docs/ICRS_links.html#LINKS">important links</a> to the data and algorithms of the ICRS System are given on a separate page.</p> <br> <table width="90%" border="0" cellspacing="5" cellpadding="0"> <tr><td align="right"><i>- George Kaplan<br> 24 Jan 2003</i></td></tr> </table> </div> <br> <div class="small"> <center> <p><a href="https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/about/staff/docs/ExternalLinksDisclaimer.html">Disclaimer for External Links</a></p> </center> </div> <br>    <p> <center><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bar.gif" alt="bar" width="640" height="5" border="0"></center> <br>  <font size="-1" face="Trebuchet MS, Verdana, Arial, sans-serif"><center> [<a href="publications/">Publications</a> | <a href="data/">Data Services</a> | <a href="software/">Software</a> | <a href="faq/">FAQ</a> | <a href="research/">Research</a> | <a href="about/">About AA</a>] <br> [<a href="news.html">News</a> | <a href="AAmap.html">Site Map</a> | <a href="finder/finder.html">Index</a> | <a href="https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/" target="_top">Home</a>] </center></font> <br> <center><img src="/web/20060805073908im_/http://aa.usno.navy.mil/graphics/bar.gif" alt="bar" width="640" height="5" border="0"></center> <div style="margin-top:10px;"> <table width="100%"> <tr> <td><div class="xsmall" style="font-family:verdana,arial,sans-serif;"> <a href="WebMaster.html">Need help?</a> </div> </td> <td align="right"> <div class="xsmall" style="font-family:verdana,arial,sans-serif;"> <script language="JavaScript">  </script> </div> </td> </tr> </table> </div> </body> </html>