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80 lines
2.5 KiB
Fortran
80 lines
2.5 KiB
Fortran
DOUBLE PRECISION FUNCTION sla_ZD (HA, DEC, PHI)
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*+
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* - - -
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* Z D
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* - - -
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*
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* HA, Dec to Zenith Distance (double precision)
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*
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* Given:
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* HA d Hour Angle in radians
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* DEC d declination in radians
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* PHI d observatory latitude in radians
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*
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* The result is in the range 0 to pi.
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*
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* Notes:
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*
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* 1) The latitude must be geodetic. In critical applications,
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* corrections for polar motion should be applied.
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*
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* 2) In some applications it will be important to specify the
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* correct type of hour angle and declination in order to
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* produce the required type of zenith distance. In particular,
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* it may be important to distinguish between the zenith distance
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* as affected by refraction, which would require the "observed"
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* HA,Dec, and the zenith distance in vacuo, which would require
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* the "topocentric" HA,Dec. If the effects of diurnal aberration
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* can be neglected, the "apparent" HA,Dec may be used instead of
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* the topocentric HA,Dec.
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*
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* 3) No range checking of arguments is done.
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*
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* 4) In applications which involve many zenith distance calculations,
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* rather than calling the present routine it will be more efficient
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* to use inline code, having previously computed fixed terms such
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* as sine and cosine of latitude, and perhaps sine and cosine of
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* declination.
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*
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* P.T.Wallace Starlink 3 April 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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DOUBLE PRECISION HA,DEC,PHI
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DOUBLE PRECISION SH,CH,SD,CD,SP,CP,X,Y,Z
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SH=SIN(HA)
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CH=COS(HA)
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SD=SIN(DEC)
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CD=COS(DEC)
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SP=SIN(PHI)
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CP=COS(PHI)
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X=CH*CD*SP-SD*CP
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Y=SH*CD
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Z=CH*CD*CP+SD*SP
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sla_ZD=ATAN2(SQRT(X*X+Y*Y),Z)
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END
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