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DOUBLE PRECISION FUNCTION sla_GMST (UT1)
*+
* - - - - -
* G M S T
* - - - - -
*
* Conversion from universal time to sidereal time (double precision)
*
* Given:
* UT1 dp universal time (strictly UT1) expressed as
* modified Julian Date (JD-2400000.5)
*
* The result is the Greenwich mean sidereal time (double
* precision, radians).
*
* The IAU 1982 expression (see page S15 of 1984 Astronomical
* Almanac) is used, but rearranged to reduce rounding errors.
* This expression is always described as giving the GMST at
* 0 hours UT. In fact, it gives the difference between the
* GMST and the UT, which happens to equal the GMST (modulo
* 24 hours) at 0 hours UT each day. In this routine, the
* entire UT is used directly as the argument for the
* standard formula, and the fractional part of the UT is
* added separately; note that the factor 1.0027379... does
* not appear.
*
* See also the routine sla_GMSTA, which delivers better numerical
* precision by accepting the UT date and time as separate arguments.
*
* Called: sla_DRANRM
*
* P.T.Wallace Starlink 14 September 1995
*
* Copyright (C) 1995 Rutherford Appleton Laboratory
*-
IMPLICIT NONE
DOUBLE PRECISION UT1
DOUBLE PRECISION sla_DRANRM
DOUBLE PRECISION D2PI,S2R
PARAMETER (D2PI=6.283185307179586476925286766559D0,
: S2R=7.272205216643039903848711535369D-5)
DOUBLE PRECISION TU
* Julian centuries from fundamental epoch J2000 to this UT
TU=(UT1-51544.5D0)/36525D0
* GMST at this UT
sla_GMST=sla_DRANRM(MOD(UT1,1D0)*D2PI+
: (24110.54841D0+
: (8640184.812866D0+
: (0.093104D0-6.2D-6*TU)*TU)*TU)*S2R)
END
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