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<H2><A NAME="SECTION0004102000000000000000">SLA_GMST - UT to GMST</A>
<A NAME="xref_SLA_GMST"> </A><A NAME="SLA_GMST"> </A>
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<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Conversion from universal time UT1 to Greenwich mean
sidereal time.
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>D = sla_GMST (UT1)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>UT1</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>universal time (strictly UT1) expressed as
modified Julian Date (JD-2400000.5)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>sla_GMST</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>Greenwich mean sidereal time (radians)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The IAU 1982 expression
(see page S15 of the 1984 <I>Astronomical
Almanac</I>) is used, but rearranged to reduce rounding errors. This
expression is always described as giving the GMST at <IMG WIDTH="18" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img49.gif"
ALT="$0^{\rm h}$">UT;
in fact, it gives the difference between the
GMST and the UT, which happens to equal the GMST (modulo
24 hours) at <IMG WIDTH="18" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img49.gif"
ALT="$0^{\rm h}$">UT each day. In sla_GMST, the
entire UT is used directly as the argument for the
canonical formula, and the fractional part of the UT is
added separately; note that the factor <IMG WIDTH="94" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img120.gif"
ALT="$1.0027379\cdots$"> does
not appear.
<DT>2.
<DD>See also the routine sla_GMSTA, which
delivers better numerical
precision by accepting the UT date and time as separate arguments.
</DL></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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