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78 lines
2.6 KiB
Fortran
78 lines
2.6 KiB
Fortran
DOUBLE PRECISION FUNCTION sla_GMST (UT1)
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*+
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* - - - - -
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* G M S T
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* - - - - -
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*
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* Conversion from universal time to sidereal time (double precision)
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*
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* Given:
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* UT1 dp universal time (strictly UT1) expressed as
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* modified Julian Date (JD-2400000.5)
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*
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* The result is the Greenwich mean sidereal time (double
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* precision, radians).
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*
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* The IAU 1982 expression (see page S15 of 1984 Astronomical Almanac)
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* is used, but rearranged to reduce rounding errors. This expression
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* is always described as giving the GMST at 0 hours UT. In fact, it
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* gives the difference between the GMST and the UT, which happens to
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* equal the GMST (modulo 24 hours) at 0 hours UT each day. In this
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* routine, the entire UT is used directly as the argument for the
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* standard formula, and the fractional part of the UT is added
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* separately. Note that the factor 1.0027379... does not appear in the
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* IAU 1982 expression explicitly but in the form of the coefficient
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* 8640184.812866, which is 86400x36525x0.0027379...
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*
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* See also the routine sla_GMSTA, which delivers better numerical
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* precision by accepting the UT date and time as separate arguments.
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*
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* Called: sla_DRANRM
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*
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* P.T.Wallace Starlink 14 October 2001
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*
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* Copyright (C) 2001 Rutherford Appleton Laboratory
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*
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* License:
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program (see SLA_CONDITIONS); if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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* Boston, MA 02111-1307 USA
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*
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*-
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IMPLICIT NONE
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DOUBLE PRECISION UT1
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DOUBLE PRECISION sla_DRANRM
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DOUBLE PRECISION D2PI,S2R
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PARAMETER (D2PI=6.283185307179586476925286766559D0,
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: S2R=7.272205216643039903848711535369D-5)
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DOUBLE PRECISION TU
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* Julian centuries from fundamental epoch J2000 to this UT
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TU=(UT1-51544.5D0)/36525D0
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* GMST at this UT
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sla_GMST=sla_DRANRM(MOD(UT1,1D0)*D2PI+
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: (24110.54841D0+
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: (8640184.812866D0+
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: (0.093104D0-6.2D-6*TU)*TU)*TU)*S2R)
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END
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