FACT++  1.0
palDh2e.c
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1 /*
2 *+
3 * Name:
4 * palDh2e
5 
6 * Purpose:
7 * Horizon to equatorial coordinates: Az,El to HA,Dec
8 
9 * Language:
10 * Starlink ANSI C
11 
12 * Type of Module:
13 * Library routine
14 
15 * Invocation:
16 * palDh2e( double az, double el, double phi, double * ha, double * dec );
17 
18 * Arguments:
19 * az = double (Given)
20 * Azimuth (radians)
21 * el = double (Given)
22 * Elevation (radians)
23 * phi = double (Given)
24 * Observatory latitude (radians)
25 * ha = double * (Returned)
26 * Hour angle (radians)
27 * dec = double * (Returned)
28 * Declination (radians)
29 
30 * Description:
31 * Convert horizon to equatorial coordinates.
32 
33 * Authors:
34 * PTW: Pat Wallace (STFC)
35 * TIMJ: Tim Jenness (JAC, Hawaii)
36 * {enter_new_authors_here}
37 
38 * Notes:
39 * - All the arguments are angles in radians.
40 * - The sign convention for azimuth is north zero, east +pi/2.
41 * - HA is returned in the range +/-pi. Declination is returned
42 * in the range +/-pi/2.
43 * - The latitude is (in principle) geodetic. In critical
44 * applications, corrections for polar motion should be applied.
45 * - In some applications it will be important to specify the
46 * correct type of elevation in order to produce the required
47 * type of HA,Dec. In particular, it may be important to
48 * distinguish between the elevation as affected by refraction,
49 * which will yield the "observed" HA,Dec, and the elevation
50 * in vacuo, which will yield the "topocentric" HA,Dec. If the
51 * effects of diurnal aberration can be neglected, the
52 * topocentric HA,Dec may be used as an approximation to the
53 * "apparent" HA,Dec.
54 * - No range checking of arguments is done.
55 * - In applications which involve many such calculations, rather
56 * than calling the present routine it will be more efficient to
57 * use inline code, having previously computed fixed terms such
58 * as sine and cosine of latitude.
59 
60 * History:
61 * 2012-02-08 (TIMJ):
62 * Initial version with documentation taken from Fortran SLA
63 * Adapted with permission from the Fortran SLALIB library.
64 * {enter_further_changes_here}
65 
66 * Copyright:
67 * Copyright (C) 1996 Rutherford Appleton Laboratory
68 * Copyright (C) 2012 Science and Technology Facilities Council.
69 * All Rights Reserved.
70 
71 * Licence:
72 * This program is free software: you can redistribute it and/or
73 * modify it under the terms of the GNU Lesser General Public
74 * License as published by the Free Software Foundation, either
75 * version 3 of the License, or (at your option) any later
76 * version.
77 *
78 * This program is distributed in the hope that it will be useful,
79 * but WITHOUT ANY WARRANTY; without even the implied warranty of
80 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
81 * GNU Lesser General Public License for more details.
82 *
83 * You should have received a copy of the GNU Lesser General
84 * License along with this program. If not, see
85 * <http://www.gnu.org/licenses/>.
86 
87 * Bugs:
88 * {note_any_bugs_here}
89 *-
90 */
91 
92 #include "pal.h"
93 #include <math.h>
94 
95 void
96 palDh2e ( double az, double el, double phi, double *ha, double *dec) {
97 
98  double sa;
99  double ca;
100  double se;
101  double ce;
102  double sp;
103  double cp;
104 
105  double x;
106  double y;
107  double z;
108  double r;
109 
110  /* Useful trig functions */
111  sa = sin(az);
112  ca = cos(az);
113  se = sin(el);
114  ce = cos(el);
115  sp = sin(phi);
116  cp = cos(phi);
117 
118  /* HA,Dec as x,y,z */
119  x = -ca * ce * sp + se * cp;
120  y = -sa * ce;
121  z = ca * ce * cp + se * sp;
122 
123  /* To HA,Dec */
124  r = sqrt(x * x + y * y);
125  if (r == 0.) {
126  *ha = 0.;
127  } else {
128  *ha = atan2(y, x);
129  }
130  *dec = atan2(z, r);
131 
132  return;
133 }
palDh2e
void palDh2e(double az, double el, double phi, double *ha, double *dec)
Definition: palDh2e.c:96