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diff --git a/src/slalib/sun67.htx/node222.html b/src/slalib/sun67.htx/node222.html new file mode 100644 index 0000000..7cc8f58 --- /dev/null +++ b/src/slalib/sun67.htx/node222.html @@ -0,0 +1,113 @@ +<!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>Calendars</TITLE> +<META NAME="description" CONTENT="Calendars"> +<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="node223.html"> +<LINK REL="previous" HREF="node217.html"> +<LINK REL="up" HREF="node197.html"> +<LINK REL="next" HREF="node223.html"> +</HEAD> +<BODY > +<BR> <HR> +<A NAME="tex2html2673" HREF="node223.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2671" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2665" HREF="node221.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="tex2html2674" HREF="node223.html">Geocentric Coordinates</A> +<BR> +<B>Up:</B> <A NAME="tex2html2672" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2666" HREF="node221.html">Dynamical Time: TT, TDB</A> +<BR> <HR> <P> +<P><!--End of Navigation Panel--> +<H2><A NAME="SECTION000516000000000000000"> +Calendars</A> +</H2> +The ordinary <I>Gregorian Calendar Date</I>, +together with a time of day, can be +used to express an epoch in any desired timescale. For many purposes, +however, a continuous count of days is more convenient, and for +this purpose the system of <I>Julian Day Number</I> can be used. +JD zero is located about 7000 years ago, well before the +historical era, and is formally defined in terms of Greenwich noon; +Julian Day Number 2449444 began at noon on 1994 April 1. <I>Julian Date</I> +is the same system but with a fractional part appended; +Julian Date 2449443.5 was the midnight on which 1994 April 1 +commenced. Because of the unwieldy size of Julian Dates +and the awkwardness of the half-day offset, it is +accepted practice to remove the leading `24' and the trailing `.5', +producing what is called the <I>Modified Julian Date</I>: +MJD = JD-2400000.5. SLALIB routines use MJD, as opposed to +JD, throughout, largely to avoid loss of precision. +1994 April 1 commenced at MJD 49443.0. +<P> +Despite JD (and hence MJD) being defined in terms of (in effect) +UT, the system can be used in conjunction with other timescales +such as TAI, TT and TDB (and even sidereal time through the +concept of <I>Greenwich Sidereal Date</I>). However, it is improper +to express a UTC as a JD or MJD because of leap seconds. +<P> +SLALIB has six routines for converting to and from dates in +the Gregorian calendar. The routines +sla_CLDJ +and +sla_CALDJ +both convert a calendar date into an MJD, the former interpreting +years between 0 and 99 as 1st century and the latter as late 20th or +early 21st century. The routines sla_DJCL +and +sla_DJCAL +both convert an MJD into calendar year, month, day and fraction of a day; +the latter performs rounding to a specified precision, important +to avoid dates like `<TT>94 04 01.***</TT>' appearing in messages. +Some of SLALIB's low-precision ephemeris routines +(sla_EARTH, +sla_MOON +and +sla_ECOR) +work in terms of year plus day-in-year (where +day 1 = January 1st, at least for the modern era). +This form of date can be generated by +calling +sla_CALYD +(which defaults years 0-99 into 1950-2049) +or +sla_CLYD +(which covers the full range from prehistoric times). +<P> +<BR> <HR> +<A NAME="tex2html2673" HREF="node223.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2671" HREF="node197.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2665" HREF="node221.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="tex2html2674" HREF="node223.html">Geocentric Coordinates</A> +<BR> +<B>Up:</B> <A NAME="tex2html2672" HREF="node197.html">EXPLANATION AND EXAMPLES</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2666" HREF="node221.html">Dynamical Time: TT, TDB</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> |