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DOUBLE PRECISION FUNCTION slZD (HA, DEC, PHI)
*+
* - - -
* Z D
* - - -
*
* HA, Dec to Zenith Distance (double precision)
*
* Given:
* HA d Hour Angle in radians
* DEC d declination in radians
* PHI d observatory latitude in radians
*
* The result is in the range 0 to pi.
*
* Notes:
*
* 1) The latitude must be geodetic. In critical applications,
* corrections for polar motion should be applied.
*
* 2) In some applications it will be important to specify the
* correct type of hour angle and declination in order to
* produce the required type of zenith distance. In particular,
* it may be important to distinguish between the zenith distance
* as affected by refraction, which would require the "observed"
* HA,Dec, and the zenith distance in vacuo, which would require
* the "topocentric" HA,Dec. If the effects of diurnal aberration
* can be neglected, the "apparent" HA,Dec may be used instead of
* the topocentric HA,Dec.
*
* 3) No range checking of arguments is done.
*
* 4) In applications which involve many zenith distance 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, and perhaps sine and cosine of
* declination.
*
* P.T.Wallace Starlink 3 April 1994
*
* 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., 51 Franklin Street, Fifth Floor,
* Boston, MA 02110-1301 USA
*
* Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
*-
IMPLICIT NONE
DOUBLE PRECISION HA,DEC,PHI
DOUBLE PRECISION SH,CH,SD,CD,SP,CP,X,Y,Z
SH=SIN(HA)
CH=COS(HA)
SD=SIN(DEC)
CD=COS(DEC)
SP=SIN(PHI)
CP=COS(PHI)
X=CH*CD*SP-SD*CP
Y=SH*CD
Z=CH*CD*CP+SD*SP
slZD=ATAN2(SQRT(X*X+Y*Y),Z)
END
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