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diff --git a/src/slalib/sun67.htx/node206.html b/src/slalib/sun67.htx/node206.html new file mode 100644 index 0000000..9a94a3d --- /dev/null +++ b/src/slalib/sun67.htx/node206.html @@ -0,0 +1,125 @@ +<!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>Epoch</TITLE> +<META NAME="description" CONTENT="Epoch"> +<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="node207.html"> +<LINK REL="previous" HREF="node205.html"> +<LINK REL="up" HREF="node197.html"> +<LINK REL="next" HREF="node207.html"> +</HEAD> +<BODY > +<BR> <HR> +<A NAME="tex2html2511" HREF="node207.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2509" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2503" HREF="node205.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="tex2html2512" HREF="node207.html">Proper Motion</A> +<BR> +<B>Up:</B> <A NAME="tex2html2510" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2504" HREF="node205.html">Mean Places</A> +<BR> <HR> <P> +<P><!--End of Navigation Panel--> +<H2><A NAME="SECTION00056000000000000000"> +Epoch</A> +</H2> +The word ``epoch'' just means a moment in time, and can be supplied +in a variety of forms, using different calendar systems and timescales. +<P> +For the purpose of specifying the epochs associated with the +mean place of a star, two conventions exist. Both sorts of epoch +superficially resemble years AD but are not tied to the civil +(Gregorian) calendar; to distinguish them from ordinary calendar-years +there is often +a ``.0'' suffix (as in ``1950.0''), although any other fractional +part is perfectly legal (<I>e.g.</I> 1987.5). +<P> +The older system, +<I>Besselian epoch</I>, is defined in such a way that its units are +tropical years of about 365.2422 days and its timescale is the +obsolete <I>Ephemeris Time</I>. +The start of the Besselian year is the moment +when the ecliptic longitude of the mean Sun is +<IMG WIDTH="34" HEIGHT="14" ALIGN="BOTTOM" BORDER="0" + SRC="img167.gif" + ALT="$280^\circ$">; this happens near the start of the +calendar year (which is why <IMG WIDTH="34" HEIGHT="14" ALIGN="BOTTOM" BORDER="0" + SRC="img167.gif" + ALT="$280^\circ$"> was chosen). +<P> +The new system, <I>Julian epoch</I>, was adopted as +part of the IAU 1976 revisions (about which more will be said +in due course) and came formally into use at the +beginning of 1984. It uses the Julian year of exactly +365.25 days; Julian epoch 2000 is defined to be 2000 January 1.5 in the +TT timescale. +<P> +For specifying mean places, various standard epochs are in use, the +most common ones being Besselian epoch 1950.0 and Julian epoch 2000.0. +To distinguish the two systems, Besselian epochs +are now prefixed ``B'' and Julian epochs are prefixed ``J''. +Epochs without an initial letter can be assumed to be Besselian +if before 1984.0, otherwise Julian. These details are supported by +the SLALIB routines +sla_DBJIN +(decodes numbers from a +character string, accepting an optional leading B or J), +sla_KBJ +(decides whether B or J depending on prefix or range) and +sla_EPCO +(converts one epoch to match another). +<P> +SLALIB has four routines for converting +Besselian and Julian epochs into other forms. +The functions +sla_EPB2D +and +sla_EPJ2D +convert Besselian and Julian epochs into MJD; the functions +sla_EPB +and +sla_EPJ +do the reverse. For example, to express B1950 as a Julian epoch: +<P><PRE> + DOUBLE PRECISION sla_EPJ,sla_EPB2D + : + WRITE (*,'(1X,''J'',F10.5)') sla_EPJ(sla_EPB2D(1950D0)) +</PRE> +<P>(The answer is J1949.99979.) +<P> +<BR> <HR> +<A NAME="tex2html2511" HREF="node207.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2509" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2503" HREF="node205.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="tex2html2512" HREF="node207.html">Proper Motion</A> +<BR> +<B>Up:</B> <A NAME="tex2html2510" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2504" HREF="node205.html">Mean Places</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> |