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<H3><A NAME="SECTION00054100000000000000">
SLALIB support for precession and nutation</A>
</H3>
SLALIB offers a choice of three precession models:
<UL>
<LI> The old Bessel-Newcomb, pre IAU 1976, ``FK4'' model, used for B1950
star positions and other pre-1984.0 purposes
(sla_PREBN).
<LI> The new Fricke, IAU 1976, ``FK5'' model, used for J2000 star
positions and other post-1984.0 purposes
(sla_PREC).
<LI> A model published by Simon <I>et al.</I> which is more accurate than
the IAU 1976 model and which is suitable for long
periods of time
(sla_PRECL).
</UL>
In each case, the named SLALIB routine generates the <IMG WIDTH="51" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img259.gif"
ALT="$(3\times3)$"><I>precession
matrix</I> for a given start and finish time. For example,
here is the Fortran code for generating the rotation
matrix which describes the precession between the epochs
J2000 and J1985.372 (IAU 1976 model):
<P><PRE>
DOUBLE PRECISION PMAT(3,3)
:
CALL sla_PREC(2000D0,1985.372D0,PMAT)
</PRE>
<P>
It is instructive to examine the resulting matrix:
<P><PRE>
+0.9999936402 +0.0032709208 +0.0014214694
-0.0032709208 +0.9999946505 -0.0000023247
-0.0014214694 -0.0000023248 +0.9999989897
</PRE>
<P>
Note that the diagonal elements are close to unity, and the
other elements are small. This shows that over an interval as
short as 15 years the precession isn't going to move a
position vector very far (in this case about <IMG WIDTH="25" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img260.gif"
ALT="$0^{\circ}
\hspace{-0.37em}.\hspace{0.02em}2$">).
<P>
For convenience, a direct <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> to <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> precession routine is
also provided
(sla_PRECES),
suitable for either the old or the new system (but not a
mixture of the two).
<P>
SLALIB provides only one nutation model, the new, IAU 1980 model,
implemented in the routine
sla_NUTC.
This returns the components of nutation
in longitude and latitude (and also provides the obliquity) from
which a nutation matrix can be generated by calling
sla_DEULER
(and from which the <I>equation of the equinoxes</I>, described
later, can be found). Alternatively,
the nutation matrix can be generated in a single call by using
sla_NUT.
<P>
A rotation matrix for applying the entire precession/nutation
transformation in one go can be generated by calling
sla_PRENUT.
<P>
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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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