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diff --git a/src/slalib/sun67.htx/node223.html b/src/slalib/sun67.htx/node223.html new file mode 100644 index 0000000..497a385 --- /dev/null +++ b/src/slalib/sun67.htx/node223.html @@ -0,0 +1,139 @@ +<!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>Geocentric Coordinates</TITLE> +<META NAME="description" CONTENT="Geocentric Coordinates"> +<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="node224.html"> +<LINK REL="previous" HREF="node222.html"> +<LINK REL="up" HREF="node197.html"> +<LINK REL="next" HREF="node224.html"> +</HEAD> +<BODY > +<BR> <HR> +<A NAME="tex2html2683" HREF="node224.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2681" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2675" HREF="node222.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="tex2html2684" HREF="node224.html">Ephemerides</A> +<BR> +<B>Up:</B> <A NAME="tex2html2682" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2676" HREF="node222.html">Calendars</A> +<BR> <HR> <P> +<P><!--End of Navigation Panel--> +<H2><A NAME="SECTION000517000000000000000"> +Geocentric Coordinates</A> +</H2> +The location of the observer on the Earth is significant in a +number of ways. The most obvious, of course, is the effect of latitude +on the observed <IMG WIDTH="66" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" + SRC="img28.gif" + ALT="$[\,Az,El~]$"> of a star. Less obvious is the need to +allow for geocentric parallax when finding the Moon with a +telescope (and when doing high-precision work involving the +Sun or planets), and the need to correct observed radial +velocities and apparent pulsar periods for the effects +of the Earth's rotation. +<P> +The SLALIB routine +sla_OBS +supplies details of groundbased observatories from an internal +list. This is useful when writing applications that apply to +more than one observatory; the user can enter a brief name, +or browse through a list, and be spared the trouble of typing +in the full latitude, longitude <I>etc</I>. The following +Fortran code returns the full name, longitude and latitude +of a specified observatory: +<P><PRE> + CHARACTER IDENT*10,NAME*40 + DOUBLE PRECISION W,P,H + : + CALL sla_OBS(0,IDENT,NAME,W,P,H) + IF (NAME.EQ.'?') ... (not recognized) +</PRE> +<P>(Beware of the longitude sign convention, which is west +ve +for historical reasons.) The following lists all +the supported observatories: +<P><PRE> + : + INTEGER N + : + N=1 + NAME=' ' + DO WHILE (NAME.NE.'?') + CALL sla_OBS(N,IDENT,NAME,W,P,H) + IF (NAME.NE.'?') THEN + WRITE (*,'(1X,I3,4X,A,4X,A)') N,IDENT,NAME + N=N+1 + END IF + END DO +</PRE> +<P> +The routine +sla_GEOC +converts a <I>geodetic latitude</I> +(one referred to the local horizon) to a geocentric position, +taking into account the Earth's oblateness and also the height +above sea level of the observer. The results are expressed in +vector form, namely as the distance of the observer from +the spin axis and equator respectively. The <I>geocentric +latitude</I> can be found be evaluating ATAN2 of the +two numbers. A full 3-D vector description of the position +and velocity of the observer is available through the routine +sla_PVOBS. +For a specified geodetic latitude, height above +sea level, and local sidereal time, +sla_PVOBS +generates a 6-element vector containing the position and +velocity with respect to the true equator and equinox of +date (<I>i.e.</I> compatible with apparent <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" + SRC="img3.gif" + ALT="$[\,\alpha,\delta\,]$">). For +some applications it will be necessary to convert to a +mean <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" + SRC="img3.gif" + ALT="$[\,\alpha,\delta\,]$"> frame (notably FK5, J2000) by multiplying +elements 1-3 and 4-6 respectively with the appropriate +precession matrix. (In theory an additional correction to the +velocity vector is needed to allow for differential precession, +but this correction is always negligible.) +<P> +See also the discussion of the routine +sla_RVEROT, +later. +<P> +<BR> <HR> +<A NAME="tex2html2683" HREF="node224.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2681" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2675" HREF="node222.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="tex2html2684" HREF="node224.html">Ephemerides</A> +<BR> +<B>Up:</B> <A NAME="tex2html2682" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2676" HREF="node222.html">Calendars</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> |