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<H2><A NAME="SECTION000460000000000000000">SLA_DT - Approximate ET minus UT</A>
<A NAME="xref_SLA_DT"> </A><A NAME="SLA_DT"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Estimate <IMG WIDTH="16" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img19.gif"
ALT="$\Delta$">T, the offset between dynamical time
and Universal Time, for a given historical epoch.
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>D = sla_DT (EPOCH)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>EPOCH</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TH ALIGN="LEFT" NOWRAP>(Julian) epoch (<I>e.g.</I> 1850D0)</TH>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>sla_DT</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>approximate ET-UT (after 1984, TT-UT1) in seconds</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>Depending on the epoch, one of three parabolic approximations
is used:
<PRE><TT>
before AD 979 Stephenson & Morrison's 390 BC to AD 948 model
AD 979 to AD 1708 Stephenson & Morrison's AD 948 to AD 1600 model
after AD 1708 McCarthy & Babcock's post-1650 model
</TT></PRE>
The breakpoints are chosen to ensure continuity: they occur
at places where the adjacent models give the same answer as
each other.
<DT>2.
<DD>The accuracy is modest, with errors of up to <IMG WIDTH="24" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img86.gif"
ALT="$20^{\rm s}$"> during
the interval since 1650, rising to perhaps <IMG WIDTH="30" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img87.gif"
ALT="$30^{\rm m}$"> by 1000 BC. Comparatively accurate values from AD 1600
are tabulated in
the <I>Astronomical Almanac</I> (see section K8 of the 1995
edition).
<DT>3.
<DD>The use of <TT>DOUBLE PRECISION</TT> for both argument and result is
simply for compatibility with other SLALIB time routines.
<DT>4.
<DD>The models used are based on a lunar tidal acceleration value
of
<IMG WIDTH="52" HEIGHT="35" ALIGN="MIDDLE" BORDER="0"
SRC="img88.gif"
ALT="$-26\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.00$"> per century.
</DL></DL>
<P> <DL>
<DT><STRONG>REFERENCE:</STRONG>
<DD>Seidelmann, P.K. (ed), 1992. <I>Explanatory
Supplement to the Astronomical Almanac,</I> ISBN 0-935702-68-7.
This contains references to the papers by Stephenson & Morrison
and by McCarthy & Babcock which describe the models used here.
</DL>
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<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>
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