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diff --git a/src/slalib/sun67.htx/node216.html b/src/slalib/sun67.htx/node216.html new file mode 100644 index 0000000..ab75104 --- /dev/null +++ b/src/slalib/sun67.htx/node216.html @@ -0,0 +1,110 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> +<!--Converted with LaTeX2HTML 97.1 (release) (July 13th, 1997) + by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds +* revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan +* with significant contributions from: + Jens Lippman, Marek Rouchal, Martin Wilck and others --> +<HTML> +<HEAD> +<TITLE>The Hipparcos Catalogue and the ICRS</TITLE> +<META NAME="description" CONTENT="The Hipparcos Catalogue and the ICRS"> +<META NAME="keywords" CONTENT="sun67"> +<META NAME="resource-type" CONTENT="document"> +<META NAME="distribution" CONTENT="global"> +<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso_8859_1"> +<LINK REL="STYLESHEET" HREF="sun67.css"> +<LINK REL="next" HREF="node217.html"> +<LINK REL="previous" HREF="node213.html"> +<LINK REL="up" HREF="node197.html"> +<LINK REL="next" HREF="node217.html"> +</HEAD> +<BODY > +<BR> <HR> +<A NAME="tex2html2611" HREF="node217.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2609" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2603" HREF="node215.html"> +<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="previous_motif.gif"></A> <A HREF="sun67.html#stardoccontents"><IMG ALIGN="BOTTOM" BORDER="0" + SRC="contents_motif.gif"></A> +<BR> +<B> Next:</B> <A NAME="tex2html2612" HREF="node217.html">Timescales</A> +<BR> +<B>Up:</B> <A NAME="tex2html2610" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2604" HREF="node215.html">Efficiency considerations</A> +<BR> <HR> <P> +<P><!--End of Navigation Panel--> +<H2><A NAME="SECTION000514000000000000000"> +The Hipparcos Catalogue and the ICRS</A> +</H2> +With effect from the beginning of 1998, the IAU adopted a new +reference system to replace FK5 J2000. The new system, called the +International Celestial Reference System (ICRS), differs profoundly +from all predecessors in that the link with solar-system dynamics +was broken; the ICRS axes are defined in terms of the directions +of a set of extragalactic sources, not in terms of the mean equator and +equinox at a given reference epoch. Although the ICRS and FK5 coordinates +of any given object are almost the same, the orientation of the new frame +was essentially arbitrary, and the close match to FK5 J2000 was contrived +purely for reasons of continuity and convenience. +<P> +A distinction is made between the reference <I>system</I> (the ICRS) +and <I>frame</I> (ICRF). The ICRS is the set of prescriptions and +conventions together with the modelling required to define, at any +time, a triad of axes. The ICRF is a practical realization, and +currently consists of a catalogue of equatorial coordinates for 608 +extragalactic radio sources observed by VLBI. +<P> +The best optical realization of the ICRF currently available is the +Hipparcos catalogue. The extragalactic sources were not directly +observable by the Hipparcos satellite and so the link from Hipparcos +to ICRF was established through a variety of indirect techniques: VLBI and +conventional interferometry of radio stars, photographic astrometry +and so on. The Hipparcos frame is aligned to the ICRF to within about +0.5 mas and 0.5 mas/year (at epoch 1991.25). +<P> +The Hipparcos catalogue includes all of the FK5 stars, which has enabled +the orientation and spin of the latter to be studied. At epoch J2000, +the misalignment of the FK5 frame with respect to Hipparcos +(and hence ICRS) are about 32 mas and 1 mas/year respectively. +Consequently, for many practical purposes, including pointing +telescopes, the IAU 1976-1982 conventions on reference frames and +Earth orientation remain adequate and there is no need to change to +Hipparcos coordinates, new precession/nutation models and so on. +However, for the most exacting astrometric applications, SLALIB +provides some support for Hipparcos coordinates in the form of +four new routines: +sla_FK52H and +sla_H2FK5, +which transform FK5 positions and proper motions to the Hipparcos frame +and <I>vice versa,</I> and +sla_FK5HZ and +sla_HFK5Z, +where the transformations are for stars whose Hipparcos proper motion is +zero. +<P> +Further information on the ICRS can be found in the paper by M.Feissel +and F.Mignard, Astron.Astrophys. 331, L33-L36 (1988). +<P> +<BR> <HR> +<A NAME="tex2html2611" HREF="node217.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2609" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2603" HREF="node215.html"> +<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="previous_motif.gif"></A> <A HREF="sun67.html#stardoccontents"><IMG ALIGN="BOTTOM" BORDER="0" + SRC="contents_motif.gif"></A> +<BR> +<B> Next:</B> <A NAME="tex2html2612" HREF="node217.html">Timescales</A> +<BR> +<B>Up:</B> <A NAME="tex2html2610" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2604" HREF="node215.html">Efficiency considerations</A> +<BR> <HR> <P> +<P><!--End of Navigation Panel--> +<ADDRESS> +<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I> +</ADDRESS> +</BODY> +</HTML> |