| 1 | /*
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| 2 | *+
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| 3 | * Name:
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| 4 | * palDe2h
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| 5 |
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| 6 | * Purpose:
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| 7 | * Equatorial to horizon coordinates: HA,Dec to Az,E
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| 8 |
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| 9 | * Language:
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| 10 | * Starlink ANSI C
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| 11 |
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| 12 | * Type of Module:
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| 13 | * Library routine
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| 14 |
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| 15 | * Invocation:
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| 16 | * palDe2h( double ha, double dec, double phi, double * az, double * el );
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| 17 |
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| 18 | * Arguments:
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| 19 | * ha = double * (Given)
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| 20 | * Hour angle (radians)
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| 21 | * dec = double * (Given)
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| 22 | * Declination (radians)
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| 23 | * phi = double (Given)
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| 24 | * Observatory latitude (radians)
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| 25 | * az = double * (Returned)
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| 26 | * Azimuth (radians)
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| 27 | * el = double * (Returned)
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| 28 | * Elevation (radians)
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| 29 |
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| 30 | * Description:
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| 31 | * Convert equatorial to horizon coordinates.
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| 32 |
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| 33 | * Authors:
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| 34 | * PTW: Pat Wallace (STFC)
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| 35 | * TIMJ: Tim Jenness (JAC, Hawaii)
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| 36 | * {enter_new_authors_here}
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| 37 |
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| 38 | * Notes:
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| 39 | * - All the arguments are angles in radians.
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| 40 | * - Azimuth is returned in the range 0-2pi; north is zero,
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| 41 | * and east is +pi/2. Elevation is returned in the range
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| 42 | * +/-pi/2.
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| 43 | * - The latitude must be geodetic. In critical applications,
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| 44 | * corrections for polar motion should be applied.
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| 45 | * - In some applications it will be important to specify the
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| 46 | * correct type of hour angle and declination in order to
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| 47 | * produce the required type of azimuth and elevation. In
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| 48 | * particular, it may be important to distinguish between
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| 49 | * elevation as affected by refraction, which would
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| 50 | * require the "observed" HA,Dec, and the elevation
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| 51 | * in vacuo, which would require the "topocentric" HA,Dec.
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| 52 | * If the effects of diurnal aberration can be neglected, the
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| 53 | * "apparent" HA,Dec may be used instead of the topocentric
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| 54 | * HA,Dec.
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| 55 | * - No range checking of arguments is carried out.
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| 56 | * - In applications which involve many such calculations, rather
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| 57 | * than calling the present routine it will be more efficient to
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| 58 | * use inline code, having previously computed fixed terms such
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| 59 | * as sine and cosine of latitude, and (for tracking a star)
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| 60 | * sine and cosine of declination.
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| 61 |
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| 62 | * History:
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| 63 | * 2012-02-08 (TIMJ):
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| 64 | * Initial version with documentation taken from Fortran SLA
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| 65 | * Adapted with permission from the Fortran SLALIB library.
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| 66 | * {enter_further_changes_here}
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| 67 |
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| 68 | * Copyright:
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| 69 | * Copyright (C) 1995 Rutherford Appleton Laboratory
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| 70 | * Copyright (C) 2012 Science and Technology Facilities Council.
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| 71 | * All Rights Reserved.
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| 72 |
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| 73 | * Licence:
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| 74 | * This program is free software: you can redistribute it and/or
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| 75 | * modify it under the terms of the GNU Lesser General Public
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| 76 | * License as published by the Free Software Foundation, either
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| 77 | * version 3 of the License, or (at your option) any later
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| 78 | * version.
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| 79 | *
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| 80 | * This program is distributed in the hope that it will be useful,
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| 81 | * but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 82 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 83 | * GNU Lesser General Public License for more details.
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| 84 | *
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| 85 | * You should have received a copy of the GNU Lesser General
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| 86 | * License along with this program. If not, see
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| 87 | * <http://www.gnu.org/licenses/>.
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| 88 |
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| 89 | * Bugs:
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| 90 | * {note_any_bugs_here}
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| 91 | *-
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| 92 | */
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| 93 |
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| 94 | #include "pal.h"
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| 95 | #include "palmac.h"
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| 96 | #include <math.h>
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| 97 |
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| 98 | void
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| 99 | palDe2h ( double ha, double dec, double phi, double *az, double *el) {
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| 100 |
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| 101 | double sh;
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| 102 | double ch;
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| 103 | double sd;
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| 104 | double cd;
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| 105 | double sp;
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| 106 | double cp;
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| 107 |
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| 108 | double a;
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| 109 |
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| 110 | double x;
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| 111 | double y;
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| 112 | double z;
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| 113 | double r;
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| 114 |
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| 115 | /* Useful trig functions */
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| 116 | sh = sin(ha);
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| 117 | ch = cos(ha);
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| 118 | sd = sin(dec);
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| 119 | cd = cos(dec);
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| 120 | sp = sin(phi);
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| 121 | cp = cos(phi);
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| 122 |
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| 123 | /* Az,El as x,y,z */
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| 124 | x = -ch * cd * sp + sd * cp;
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| 125 | y = -sh * cd;
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| 126 | z = ch * cd * cp + sd * sp;
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| 127 |
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| 128 | /* To spherical */
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| 129 | r = sqrt(x * x + y * y);
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| 130 | if (r == 0.) {
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| 131 | a = 0.;
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| 132 | } else {
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| 133 | a = atan2(y, x);
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| 134 | }
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| 135 | if (a < 0.) {
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| 136 | a += PAL__D2PI;
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| 137 | }
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| 138 | *az = a;
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| 139 | *el = atan2(z, r);
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| 140 |
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| 141 | return;
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| 142 | }
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