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diff --git a/src/slalib/sun67.htx/node204.html b/src/slalib/sun67.htx/node204.html new file mode 100644 index 0000000..cc880e2 --- /dev/null +++ b/src/slalib/sun67.htx/node204.html @@ -0,0 +1,127 @@ +<!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>SLALIB support for precession and nutation</TITLE> +<META NAME="description" CONTENT="SLALIB support for precession and nutation"> +<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="previous" HREF="node203.html"> +<LINK REL="up" HREF="node203.html"> +<LINK REL="next" HREF="node205.html"> +</HEAD> +<BODY > +<BR> <HR> +<A NAME="tex2html2491" HREF="node205.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2489" HREF="node203.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2485" HREF="node203.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="tex2html2492" HREF="node205.html">Mean Places</A> +<BR> +<B>Up:</B> <A NAME="tex2html2490" HREF="node203.html">Precession and Nutation</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2486" HREF="node203.html">Precession and Nutation</A> +<BR> <HR> <P> +<P><!--End of Navigation Panel--> +<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> +<BR> <HR> +<A NAME="tex2html2491" HREF="node205.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2489" HREF="node203.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2485" HREF="node203.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="tex2html2492" HREF="node205.html">Mean Places</A> +<BR> +<B>Up:</B> <A NAME="tex2html2490" HREF="node203.html">Precession and Nutation</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2486" HREF="node203.html">Precession and Nutation</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> |