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slalib/dt.f

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+      DOUBLE PRECISION FUNCTION sla_DT (EPOCH)
+*+
+*     - - -
+*      D T
+*     - - -
+*
+*  Estimate the offset between dynamical time and Universal Time
+*  for a given historical epoch.
+*
+*  Given:
+*     EPOCH       d        (Julian) epoch (e.g. 1850D0)
+*
+*  The result is a rough estimate of ET-UT (after 1984, TT-UT) at
+*  the given epoch, in seconds.
+*
+*  Notes:
+*
+*  1  Depending on the epoch, one of three parabolic approximations
+*     is used:
+*
+*      before 979    Stephenson & Morrison's 390 BC to AD 948 model
+*      979 to 1708   Stephenson & Morrison's 948 to 1600 model
+*      after 1708    McCarthy & Babcock's post-1650 model
+*
+*     The breakpoints are chosen to ensure continuity:  they occur
+*     at places where the adjacent models give the same answer as
+*     each other.
+*
+*  2  The accuracy is modest, with errors of up to 20 sec during
+*     the interval since 1650, rising to perhaps 30 min by 1000 BC.
+*     Comparatively accurate values from AD 1600 are tabulated in
+*     the Astronomical Almanac (see section K8 of the 1995 AA).
+*
+*  3  The use of double-precision for both argument and result is
+*     purely for compatibility with other SLALIB time routines.
+*
+*  4  The models used are based on a lunar tidal acceleration value
+*     of -26.00 arcsec per century.
+*
+*  Reference:  Explanatory Supplement to the Astronomical Almanac,
+*              ed P.K.Seidelmann, University Science Books (1992),
+*              section 2.553, p83.  This contains references to
+*              the Stephenson & Morrison and McCarthy & Babcock
+*              papers.
+*
+*  P.T.Wallace   Starlink   1 March 1995
+*
+*  Copyright (C) 1995 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 EPOCH
+      DOUBLE PRECISION T,W,S
+
+
+*  Centuries since 1800
+      T=(EPOCH-1800D0)/100D0
+
+*  Select model
+      IF (EPOCH.GE.1708.185161980887D0) THEN
+
+*     Post-1708: use McCarthy & Babcock
+         W=T-0.19D0
+         S=5.156D0+13.3066D0*W*W
+      ELSE IF (EPOCH.GE.979.0258204760233D0) THEN
+
+*     979-1708: use Stephenson & Morrison's 948-1600 model
+         S=25.5D0*T*T
+      ELSE
+
+*     Pre-979: use Stephenson & Morrison's 390 BC to AD 948 model
+         S=1360.0D0+(320D0+44.3D0*T)*T
+      END IF
+
+*  Result
+      sla_DT=S
+
+      END