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diff --git a/src/slalib/sun67.htx/node219.html b/src/slalib/sun67.htx/node219.html new file mode 100644 index 0000000..4214aaa --- /dev/null +++ b/src/slalib/sun67.htx/node219.html @@ -0,0 +1,184 @@ +<!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>Universal Time: UTC, UT1</TITLE> +<META NAME="description" CONTENT="Universal Time: UTC, UT1"> +<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="node220.html"> +<LINK REL="previous" HREF="node218.html"> +<LINK REL="up" HREF="node217.html"> +<LINK REL="next" HREF="node220.html"> +</HEAD> +<BODY > +<BR> <HR> +<A NAME="tex2html2645" HREF="node220.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2643" HREF="node217.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2637" HREF="node218.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="tex2html2646" HREF="node220.html">Sidereal Time: GMST, LAST</A> +<BR> +<B>Up:</B> <A NAME="tex2html2644" HREF="node217.html">Timescales</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2638" HREF="node218.html">Atomic Time: TAI</A> +<BR> <HR> <P> +<P><!--End of Navigation Panel--> +<H3><A NAME="SECTION000515200000000000000"> +Universal Time: UTC, UT1</A> +</H3> +<I>Coordinated Universal Time</I> UTC is the basis of civil timekeeping. +Most time zones differ from UTC by an integer number +of hours, though a few (<I>e.g.</I> parts of Canada and Australia) differ +by <I>n</I>+0.5 hours. The UTC second is the same as the SI second, +as for TAI. In the long term, UTC keeps in step with the +Sun. It does so even though the Earth's rotation is slightly +variable (due to large scale movements of water and atmosphere +among other things) by occasionally introducing a <I>leap +second</I>. +<P><I>Universal Time</I> UT, or more specifically UT1, +is in effect the mean solar time. It is continuous +(<I>i.e.</I> there are no leap seconds) but has a variable +rate because of the Earth's non-uniform rotation period. It is +needed for computing the sidereal time, an essential part of +pointing a telescope at a celestial source. To obtain UT1, you +have to look up the value of UT1-UTC for the date concerned +in tables published by the International Earth Rotation +Service; this quantity, kept in the range +<IMG WIDTH="15" HEIGHT="25" ALIGN="MIDDLE" BORDER="0" + SRC="img30.gif" + ALT="$\pm$"><IMG WIDTH="24" HEIGHT="14" ALIGN="BOTTOM" BORDER="0" + SRC="img31.gif" + ALT="$0^{\rm s}\hspace{-0.3em}.9$"> by means of UTC leap +seconds, is then added to the UTC. The quantity UT1-UTC, +which typically changes by 1 or 2 ms per day, +can only be obtained by observation, though seasonal trends +are known and the IERS listings are able to predict some way into +the future with adequate accuracy for pointing telescopes. +<P> +UTC leap seconds are introduced as necessary, +usually at the end of December or June. +On the average the solar day is slightly longer +than the nominal 86,400 SI seconds and so leap seconds are always positive; +however, provision exists for negative leap seconds if needed. +The form of a leap second can be seen from the +following description of the end of June 1994: +<P><A NAME="27722"> </A> +<TABLE CELLPADDING=3> +<CAPTION><STRONG>Figure 1:</STRONG> +Relationship Between Celestial Coordinates</CAPTION> +<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="LEFT" NOWRAP> </TD> +<TD ALIGN="RIGHT" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP> </TD> +</TR> +<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="LEFT" NOWRAP> </TD> +<TD ALIGN="RIGHT" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP>UTC</TD> +<TD ALIGN="CENTER" NOWRAP>UT1-UTC</TD> +<TD ALIGN="CENTER" NOWRAP>UT1</TD> +</TR> +<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="LEFT" NOWRAP> </TD> +<TD ALIGN="RIGHT" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP> </TD> +</TR> +<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP>1994</TD> +<TD ALIGN="LEFT" NOWRAP>June</TD> +<TD ALIGN="RIGHT" NOWRAP>30</TD> +<TD ALIGN="CENTER" NOWRAP>23 59 58</TD> +<TD ALIGN="CENTER" NOWRAP>-0.218</TD> +<TD ALIGN="CENTER" NOWRAP>23 59 57.782</TD> +</TR> +<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="LEFT" NOWRAP> </TD> +<TD ALIGN="RIGHT" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP>23 59 59</TD> +<TD ALIGN="CENTER" NOWRAP>-0.218</TD> +<TD ALIGN="CENTER" NOWRAP>23 59 58.782</TD> +</TR> +<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="LEFT" NOWRAP> </TD> +<TD ALIGN="RIGHT" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP>23 59 60</TD> +<TD ALIGN="CENTER" NOWRAP>-0.218</TD> +<TD ALIGN="CENTER" NOWRAP>23 59 59.782</TD> +</TR> +<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="LEFT" NOWRAP>July</TD> +<TD ALIGN="RIGHT" NOWRAP>1</TD> +<TD ALIGN="CENTER" NOWRAP>00 00 00</TD> +<TD ALIGN="CENTER" NOWRAP>+0.782</TD> +<TD ALIGN="CENTER" NOWRAP>00 00 00.782</TD> +</TR> +<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP> </TD> +<TD ALIGN="LEFT" NOWRAP> </TD> +<TD ALIGN="RIGHT" NOWRAP> </TD> +<TD ALIGN="CENTER" NOWRAP>00 00 01</TD> +<TD ALIGN="CENTER" NOWRAP>+0.782</TD> +<TD ALIGN="CENTER" NOWRAP>00 00 01.782</TD> +</TR> +</TABLE> +<P> +Note that UTC has to be expressed as hours, minutes and +seconds (or at least in seconds for a given date) if leap seconds +are to be taken into account. It is improper to express a UTC as a +Julian Date, for example, because there will be an ambiguity +during a leap second (in the above example, +1994 June 30 <IMG WIDTH="96" HEIGHT="17" ALIGN="BOTTOM" BORDER="0" + SRC="img308.gif" + ALT="$23^{h}\,59^{m}\,60^{s}.0$"> and +1994 July 1 <IMG WIDTH="96" HEIGHT="17" ALIGN="BOTTOM" BORDER="0" + SRC="img309.gif" + ALT="$00^{h}\,00^{m}\,00^{s}.0$"> would <I>both</I> come out as +MJD 49534.00000). Although in the vast majority of +cases this won't matter, there are potential problems in +on-line data acquisition systems and in applications involving +taking the difference between two times. Note that although the routines +sla_DAT +and +sla_DTT +expect UTC in the form of an MJD, the meaning here is really a +whole-number <I>date</I> rather than a time. Though the routines will accept +a fractional part and will almost always function correctly, on a day +which ends with a leap +second incorrect results would be obtained during the leap second +itself because by then the MJD would have moved into the next day. +<P> +<BR> <HR> +<A NAME="tex2html2645" HREF="node220.html"> +<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next_motif.gif"></A> +<A NAME="tex2html2643" HREF="node217.html"> +<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up_motif.gif"></A> +<A NAME="tex2html2637" HREF="node218.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="tex2html2646" HREF="node220.html">Sidereal Time: GMST, LAST</A> +<BR> +<B>Up:</B> <A NAME="tex2html2644" HREF="node217.html">Timescales</A> +<BR> +<B> Previous:</B> <A NAME="tex2html2638" HREF="node218.html">Atomic Time: TAI</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> |