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