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slalib/gmst.f

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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 in the
+*  IAU 1982 expression explicitly but in the form of the coefficient
+*  8640184.812866, which is 86400x36525x0.0027379...
+*
+*  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 October 2001
+*
+*  Copyright (C) 2001 Rutherford Appleton Laboratory
+*
+*  License:
+*    This program is free software; you can redistribute it and/or modify
+*    it under the terms of the GNU General Public License as published by
+*    the Free Software Foundation; either version 2 of the License, or
+*    (at your option) any later version.
+*
+*    This program is distributed in the hope that it will be useful,
+*    but WITHOUT ANY WARRANTY; without even the implied warranty of
+*    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*    GNU General Public License for more details.
+*
+*    You should have received a copy of the GNU General Public License
+*    along with this program (see SLA_CONDITIONS); if not, write to the 
+*    Free Software Foundation, Inc., 59 Temple Place, Suite 330, 
+*    Boston, MA  02111-1307  USA
+*
+*-
+
+      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