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diff --git a/src/slalib/sun67.htx/node220.html b/src/slalib/sun67.htx/node220.html new file mode 100644 index 0000000..84db99f --- /dev/null +++ b/src/slalib/sun67.htx/node220.html @@ -0,0 +1,96 @@ +<!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>Sidereal Time: GMST, LAST</TITLE> +<META NAME="description" CONTENT="Sidereal Time: GMST, LAST"> +<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="node221.html"> +<LINK REL="previous" HREF="node219.html"> +<LINK REL="up" HREF="node217.html"> +<LINK REL="next" HREF="node221.html"> +</HEAD> +<BODY > +<BR> <HR> +<A NAME="tex2html2655" HREF="node221.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2653" HREF="node217.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2647" HREF="node219.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="tex2html2656" HREF="node221.html">Dynamical Time: TT, TDB</A> +<BR> +<B>Up:</B> <A NAME="tex2html2654" HREF="node217.html">Timescales</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2648" HREF="node219.html">Universal Time: UTC, UT1</A> +<BR> <HR> <P> +<P><!--End of Navigation Panel--> +<H3><A NAME="SECTION000515300000000000000"> +Sidereal Time: GMST, LAST</A> +</H3> +Sidereal Time is the ``time of day'' relative to the +stars rather than to the Sun. After +one sidereal day the stars come back to the same place in the +sky, apart from sub-arcsecond precession effects. Because the Earth +rotates faster relative to the stars than to the Sun by one day +per year, the sidereal second is shorter than the solar +second; the ratio is about 0.9973. +<P> +The <I>Greenwich Mean Sidereal Time</I> GMST is +linked to UT1 by a numerical formula which +is implemented in the SLALIB routines +sla_GMST +and +sla_GMSTA. +There are, of course, no leap seconds in GMST, but the second +changes in length along with the UT1 second, and also varies +over long periods of time because of slow changes in the Earth's +orbit. This makes the timescale unsuitable for everything except +predicting the apparent directions of celestial sources. +<P> +The <I>Local Apparent Sidereal Time</I> LAST is the apparent right +ascension of the local meridian, from which the hour angle of any +star can be determined knowing its <IMG WIDTH="13" HEIGHT="14" ALIGN="BOTTOM" BORDER="0" + SRC="img24.gif" + ALT="$\alpha$">. It can be obtained from the +GMST by adding the east longitude (corrected for polar motion +in precise work) and the <I>equation of the equinoxes</I>. The +latter, already described, is an aspect of the nutation effect +and can be predicted by calling the SLALIB routine +sla_EQEQX +or, neglecting certain very small terms, by calling +sla_NUTC +and using the expression <IMG WIDTH="61" HEIGHT="27" ALIGN="MIDDLE" BORDER="0" + SRC="img310.gif" + ALT="$\Delta\psi\cos\epsilon$">.<P> +<BR> <HR> +<A NAME="tex2html2655" HREF="node221.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2653" HREF="node217.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2647" HREF="node219.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="tex2html2656" HREF="node221.html">Dynamical Time: TT, TDB</A> +<BR> +<B>Up:</B> <A NAME="tex2html2654" HREF="node217.html">Timescales</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2648" HREF="node219.html">Universal Time: UTC, UT1</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> |