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<P><DL>
<DT><A NAME="412">...SLALIB</A>
<DD>The name isn't an acronym;
it just stands for ``Subprogram Library A''.
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<DT><A NAME="27724">...papers;</A>
<DD>One frame not included in
Figure 1 is that of the Hipparcos catalogue. This is currently the
best available implementation in the optical of the <I>International
Celestial Reference System</I> (ICRS), which is based on extragalactic
radio sources observed by VLBI. The distinction between FK5 J2000
and Hipparcos coordinates only becomes important when accuracies of
50 mas or better are required. More details are given in
Section 4.14.
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<DT><A NAME="27833">...<I>equinox</I>.</A>
<DD>With
the introduction of the International Celestial Reference System (ICRS), the
connection between (i) star coordinates and (ii) the Earth's orientation
and orbit has been broken. However, the orientation of the
International Celestial Reference Frame (ICRF) axes was, for convenience,
chosen to match J2000 FK5, and for most practical purposes ICRF coordinates
(for example entries in the Hipparcos catalogue) can be regarded as
synonymous with J2000 FK5. See Section 4.14 for further details.
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<DT><A NAME="27903">...used.</A>
<DD>An equinox is, however, not required for coordinates
in the International Celestial Reference System. Such coordinates must
be labelled simply ``ICRS'', or the specific catalogue can be mentioned,
such as ``Hipparcos''; constructions such as ``Hipparcos, J2000'' are
redundant and misleading.
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<DT><A NAME="28038">...catalogues.</A>
<DD>This was
true until the inception of the International Celestial Reference
System, which is based on the idea of axes locked into the
distant background. The coordinates
of the extragalactic sources which realize these
axes have no individual significance; there is a ``no net rotation''
condition which has to be satisfied each time any revisions take
place.
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</DL><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>
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