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119 lines
3.4 KiB
Fortran
119 lines
3.4 KiB
Fortran
SUBROUTINE sla_CLYD (IY, IM, ID, NY, ND, JSTAT)
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*+
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* - - - - -
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* C L Y D
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* - - - - -
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*
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* Gregorian calendar to year and day in year (in a Julian calendar
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* aligned to the 20th/21st century Gregorian calendar).
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*
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* Given:
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* IY,IM,ID i year, month, day in Gregorian calendar
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*
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* Returned:
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* NY i year (re-aligned Julian calendar)
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* ND i day in year (1 = January 1st)
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* JSTAT i status:
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* 0 = OK
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* 1 = bad year (before -4711)
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* 2 = bad month
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* 3 = bad day (but conversion performed)
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*
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* Notes:
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*
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* 1 This routine exists to support the low-precision routines
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* sla_EARTH, sla_MOON and sla_ECOR.
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*
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* 2 Between 1900 March 1 and 2100 February 28 it returns answers
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* which are consistent with the ordinary Gregorian calendar.
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* Outside this range there will be a discrepancy which increases
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* by one day for every non-leap century year.
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*
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* 3 The essence of the algorithm is first to express the Gregorian
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* date as a Julian Day Number and then to convert this back to
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* a Julian calendar date, with day-in-year instead of month and
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* day. See 12.92-1 and 12.95-1 in the reference.
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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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* p604-606.
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*
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* P.T.Wallace Starlink 26 November 1994
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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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INTEGER IY,IM,ID,NY,ND,JSTAT
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INTEGER I,J,K,L,N
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* Month lengths in days
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INTEGER MTAB(12)
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DATA MTAB/31,28,31,30,31,30,31,31,30,31,30,31/
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* Preset status
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JSTAT=0
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* Validate year
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IF (IY.GE.-4711) THEN
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* Validate month
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IF (IM.GE.1.AND.IM.LE.12) THEN
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* Allow for (Gregorian) leap year
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IF (MOD(IY,4).EQ.0.AND.
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: (MOD(IY,100).NE.0.OR.MOD(IY,400).EQ.0)) THEN
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MTAB(2)=29
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ELSE
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MTAB(2)=28
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END IF
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* Validate day
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IF (ID.LT.1.OR.ID.GT.MTAB(IM)) JSTAT=3
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* Perform the conversion
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I=(14-IM)/12
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K=IY-I
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J=(1461*(K+4800))/4+(367*(IM-2+12*I))/12
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: -(3*((K+4900)/100))/4+ID-30660
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K=(J-1)/1461
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L=J-1461*K
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N=(L-1)/365-L/1461
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J=((80*(L-365*N+30))/2447)/11
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I=N+J
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ND=59+L-365*I+((4-N)/4)*(1-J)
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NY=4*K+I-4716
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* Bad month
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ELSE
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JSTAT=2
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END IF
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ELSE
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* Bad year
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JSTAT=1
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END IF
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END
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