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diff --git a/src/slalib/sun67.htx/node212.html b/src/slalib/sun67.htx/node212.html new file mode 100644 index 0000000..35a5044 --- /dev/null +++ b/src/slalib/sun67.htx/node212.html @@ -0,0 +1,171 @@ +<!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>Mean Place to Apparent Place</TITLE> +<META NAME="description" CONTENT="Mean Place to Apparent Place"> +<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="next" HREF="node213.html"> +<LINK REL="previous" HREF="node211.html"> +<LINK REL="up" HREF="node197.html"> +<LINK REL="next" HREF="node213.html"> +</HEAD> +<BODY > +<BR> <HR> +<A NAME="tex2html2571" HREF="node213.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2569" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2563" HREF="node211.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="tex2html2572" HREF="node213.html">Apparent Place to Observed Place</A> +<BR> +<B>Up:</B> <A NAME="tex2html2570" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2564" HREF="node211.html">Mean Place Transformations</A> +<BR> <HR> <P> +<P><!--End of Navigation Panel--> +<H2><A NAME="SECTION000512000000000000000"> +Mean Place to Apparent Place</A> +</H2> +The <I>geocentric apparent place</I> of a source, or <I>apparent place</I> +for short, +is the <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" + SRC="img3.gif" + ALT="$[\,\alpha,\delta\,]$"> if viewed from the centre of the Earth, +with respect to the true equator and equinox of date. +Transformation of an FK5 mean <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" + SRC="img3.gif" + ALT="$[\,\alpha,\delta\,]$">, equinox J2000, +current epoch, to apparent place involves the following effects: +<P><UL> +<LI> Light deflection - the gravitational lens effect of + the sun. +<LI> Annual aberration. +<LI> Precession/nutation. +</UL> +The <I>light deflection</I> is seldom significant. Its value +at the limb of the Sun is about +<P> <IMG WIDTH="31" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" + SRC="img294.gif" + ALT="$1\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.74$"> ; it falls off rapidly with distance from the +Sun and has shrunk to about +<P> <IMG WIDTH="31" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" + SRC="img295.gif" + ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.02$"> at an elongation of <IMG WIDTH="26" HEIGHT="14" ALIGN="BOTTOM" BORDER="0" + SRC="img296.gif" + ALT="$20^\circ$">.<P> +As already described, the <I>annual aberration</I> +is a function of the Earth's velocity +relative to the solar system barycentre (available through the +SLALIB routine +sla_EVP) +and produces shifts of up to about + <IMG WIDTH="31" HEIGHT="18" ALIGN="BOTTOM" BORDER="0" + SRC="img265.gif" + ALT="$20\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.5$"> . +<P> +The <I>precession/nutation</I>, from J2000 to the current epoch, is +expressed by a rotation matrix which is available through the +SLALIB routine +sla_PRENUT. +<P> +The whole mean-to-apparent transformation can be done using the SLALIB +routine +sla_MAP. As a demonstration, here is a program which lists the +<I>North Polar Distance</I> (<IMG WIDTH="54" HEIGHT="27" ALIGN="MIDDLE" BORDER="0" + SRC="img297.gif" + ALT="$90^\circ-\delta$">) of Polaris for +the decade of closest approach to the Pole: +<P><PRE> + IMPLICIT NONE + DOUBLE PRECISION PI,PIBY2,D2R,S2R,AS2R + PARAMETER (PI=3.141592653589793238462643D0) + PARAMETER (D2R=PI/180D0, + : PIBY2=PI/2D0, + : S2R=PI/(12D0*3600D0), + : AS2R=PI/(180D0*3600D0)) + DOUBLE PRECISION RM,DM,PR,PD,DATE,RA,DA + INTEGER J,IDS,IDE,ID,IYMDF(4),I + DOUBLE PRECISION sla_EPJ2D + + CALL sla_DTF2R(02,31,49.8131D0,RM,J) + CALL sla_DAF2R(89,15,50.661D0,DM,J) + PR=+21.7272D0*S2R/100D0 + PD=-1.571D0*AS2R/100D0 + WRITE (*,'(1X,'// + : '''Polaris north polar distance (deg) 2096-2105''/)') + WRITE (*,'(4X,''Date'',7X''NPD''/)') + CALL sla_CLDJ(2096,1,1,DATE,J) + IDS=NINT(DATE) + CALL sla_CLDJ(2105,12,31,DATE,J) + IDE=NINT(DATE) + DO ID=IDS,IDE,10 + DATE=DBLE(ID) + CALL sla_DJCAL(0,DATE,IYMDF,J) + CALL sla_MAP(RM,DM,PR,PD,0D0,0D0,2000D0,DATE,RA,DA) + WRITE (*,'(1X,I4,2I3.2,F9.5)') (IYMDF(I),I=1,3),(PIBY2-DA)/D2R + END DO + + END +</PRE> +<P> +For cases where the transformation has to be repeated for different +times or for more than one star, the straightforward +sla_MAP +approach is apt to be +wasteful as both the Earth velocity and the +precession/nutation matrix can be re-calculated relatively +infrequently without ill effect. A more efficient method is to +perform the target-independent calculations only when necessary, +by calling +sla_MAPPA, +and then to use either +sla_MAPQKZ, +when only the <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" + SRC="img3.gif" + ALT="$[\,\alpha,\delta\,]$"> is known, or +sla_MAPQK, +when full catalogue positions, including proper motion, parallax and +radial velocity, are available. How frequently to call +sla_MAPPA +depends on the accuracy objectives; once per +night will deliver sub-arcsecond accuracy for example. +<P> +The routines +sla_AMP +and +sla_AMPQK +allow the reverse transformation, from apparent to mean place. +<P> +<BR> <HR> +<A NAME="tex2html2571" HREF="node213.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2569" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2563" HREF="node211.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="tex2html2572" HREF="node213.html">Apparent Place to Observed Place</A> +<BR> +<B>Up:</B> <A NAME="tex2html2570" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2564" HREF="node211.html">Mean Place Transformations</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> |