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<H2><A NAME="SECTION0004141000000000000000">SLA_PRECL - Precession Matrix (latest)</A>
<A NAME="xref_SLA_PRECL"> </A><A NAME="SLA_PRECL"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Form the matrix of precession between two epochs, using the
model of Simon <I>et al</I>. (1994), which is suitable for long
periods of time.
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_PRECL (EP0, EP1, RMATP)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>EP0</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>beginning epoch</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>EP1</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>ending epoch</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>RMATP</EM></TD>
<TH ALIGN="LEFT"><B>D(3,3)</B></TH>
<TD ALIGN="LEFT" NOWRAP>precession matrix</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The epochs are TDB Julian epochs.
<DT>2.
<DD>The matrix is in the sense:
<BLOCKQUOTE><B>v</B><SUB>1</SUB> = <B>M</B><IMG WIDTH="7" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img72.gif"
ALT="$\cdot$"><B>v</B><SUB>0</SUB></BLOCKQUOTE>
where <B>v</B><SUB>1</SUB> is the star vector relative to the
mean equator and equinox of epoch EP1, <B>M</B> is the
<IMG WIDTH="39" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img18.gif"
ALT="$3\times3$"> matrix RMATP and
<B>v</B><SUB>0</SUB> is the star vector relative to the
mean equator and equinox of epoch EP0.
<DT>3.
<DD>The absolute accuracy of the model is limited by the
uncertainty in the general precession, about
<IMG WIDTH="23" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img32.gif"
ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.3$"> per
1000 years. The remainder of the formulation provides a
precision of 1 milliarcsecond over the interval from 1000AD
to 3000AD,
<IMG WIDTH="23" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img25.gif"
ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.1$"> from 1000BC to 5000AD and
<IMG WIDTH="17" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img140.gif"
ALT="$1\hspace{-0.05em}^{'\hspace{-0.1em}'}$"> from 4000BC to 8000AD.
</DL></DL>
<P> <DL>
<DT><STRONG>REFERENCE:</STRONG>
<DD>Simon, J.L. <I>et al</I>., 1994. <I>Astr.Astrophys.</I> <B>282</B>,
663.
</DL>
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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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