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<H2><A NAME="SECTION00055000000000000000">
Mean Places</A>
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The main effect of the precession/nutation is a steady increase of about
<IMG WIDTH="25" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img261.gif"
ALT="$50\hspace{-0.05em}^{'\hspace{-0.1em}'}$">/year in the ecliptic longitudes of the stars. It is therefore
essential, when reporting the position of an astronomical target, to
qualify the coordinates with a date, or <I>epoch</I>.
Specifying the epoch ties down the equator and
equinox which define the <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> coordinate system that is
being used.
<A NAME="tex2html5" HREF="footnode.html#27903"><SUP><IMG ALIGN="BOTTOM" BORDER="1" ALT="[*]" SRC="foot_motif.gif"></SUP></A> For simplicity, only
the smooth and steady ``general
precession'' part of the complete precession/nutation effect is
included, thereby defining what is called the <I>mean</I>
equator and equinox for the epoch concerned. We say a star
has a mean place of (for example)
<IMG WIDTH="104" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
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ALT="$12^{h}\,07^{m}\,58^{s}.09$"> <IMG WIDTH="102" HEIGHT="35" ALIGN="MIDDLE" BORDER="0"
SRC="img263.gif"
ALT="$-19^{\circ}\,44^{'}\,37^{''}.1$"> ``with respect to the mean equator
and equinox of epoch J2000''. The short way of saying
this is ``<IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> equinox J2000'' (<B>not</B> ``<IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> epoch J2000'',
which means something different to do with
proper motion).
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<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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