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slalib/dh2e.f

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+      SUBROUTINE sla_DH2E (AZ, EL, PHI, HA, DEC)
+*+
+*     - - - - -
+*      D E 2 H
+*     - - - - -
+*
+*  Horizon to equatorial coordinates:  Az,El to HA,Dec
+*
+*  (double precision)
+*
+*  Given:
+*     AZ      d     azimuth
+*     EL      d     elevation
+*     PHI     d     observatory latitude
+*
+*  Returned:
+*     HA      d     hour angle
+*     DEC     d     declination
+*
+*  Notes:
+*
+*  1)  All the arguments are angles in radians.
+*
+*  2)  The sign convention for azimuth is north zero, east +pi/2.
+*
+*  3)  HA is returned in the range +/-pi.  Declination is returned
+*      in the range +/-pi/2.
+*
+*  4)  The latitude is (in principle) geodetic.  In critical
+*      applications, corrections for polar motion should be applied.
+*
+*  5)  In some applications it will be important to specify the
+*      correct type of elevation in order to produce the required
+*      type of HA,Dec.  In particular, it may be important to
+*      distinguish between the elevation as affected by refraction,
+*      which will yield the "observed" HA,Dec, and the elevation
+*      in vacuo, which will yield the "topocentric" HA,Dec.  If the
+*      effects of diurnal aberration can be neglected, the
+*      topocentric HA,Dec may be used as an approximation to the
+*      "apparent" HA,Dec.
+*
+*  6)  No range checking of arguments is done.
+*
+*  7)  In applications which involve many such calculations, rather
+*      than calling the present routine it will be more efficient to
+*      use inline code, having previously computed fixed terms such
+*      as sine and cosine of latitude.
+*
+*  P.T.Wallace   Starlink   21 February 1996
+*
+*  Copyright (C) 1996 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 AZ,EL,PHI,HA,DEC
+
+      DOUBLE PRECISION SA,CA,SE,CE,SP,CP,X,Y,Z,R
+
+
+*  Useful trig functions
+      SA=SIN(AZ)
+      CA=COS(AZ)
+      SE=SIN(EL)
+      CE=COS(EL)
+      SP=SIN(PHI)
+      CP=COS(PHI)
+
+*  HA,Dec as x,y,z
+      X=-CA*CE*SP+SE*CP
+      Y=-SA*CE
+      Z=CA*CE*CP+SE*SP
+
+*  To HA,Dec
+      R=SQRT(X*X+Y*Y)
+      IF (R.EQ.0D0) THEN
+         HA=0D0
+      ELSE
+         HA=ATAN2(Y,X)
+      END IF
+      DEC=ATAN2(Z,R)
+
+      END