1 /*
2 *+
3 * Name:
4 * palAirmas
5
6 * Purpose:
7 * Air mass at given zenith distance
8
9 * Language:
10 * Starlink ANSI C
11
12 * Type of Module:
13 * Library routine
14
15 * Invocation:
16 * double palAirmas( double zd );
17
18 * Arguments:
19 * zd = double (Given)
20 * Observed zenith distance (radians)
21
22 * Description:
23 * Calculates the airmass at the observed zenith distance.
24
25 * Authors:
26 * PTW: Patrick Wallace (STFC)
27 * TIMJ: Tim Jenness (JAC, Hawaii)
28 * {enter_new_authors_here}
29
30 * Notes:
31 * - The "observed" zenith distance referred to above means "as
32 * affected by refraction".
33 * - Uses Hardie's (1962) polynomial fit to Bemporad's data for
34 * the relative air mass, X, in units of thickness at the zenith
35 * as tabulated by Schoenberg (1929). This is adequate for all
36 * normal needs as it is accurate to better than 0.1% up to X =
37 * 6.8 and better than 1% up to X = 10. Bemporad's tabulated
38 * values are unlikely to be trustworthy to such accuracy
39 * because of variations in density, pressure and other
40 * conditions in the atmosphere from those assumed in his work.
41 * - The sign of the ZD is ignored.
42 * - At zenith distances greater than about ZD = 87 degrees the
43 * air mass is held constant to avoid arithmetic overflows.
44
45 * See Also:
46 * - Hardie, R.H., 1962, in "Astronomical Techniques"
47 * ed. W.A. Hiltner, University of Chicago Press, p180.
48 * - Schoenberg, E., 1929, Hdb. d. Ap.,
49 * Berlin, Julius Springer, 2, 268.
50
51 * History:
52 * 2012-03-02 (TIMJ):
53 * Initial version from the SLA/F version including documentation.
54 * Adapted with permission from the Fortran SLALIB library.
55 * {enter_further_changes_here}
56
57 * Copyright:
58 * Copyright (C) 1999 Rutherford Appleton Laboratory.
59 * Copyright (C) 2012 Science and Technology Facilities Council.
60 * All Rights Reserved.
61
62 * Licence:
63 * This program is free software; you can redistribute it and/or
64 * modify it under the terms of the GNU General Public License as
65 * published by the Free Software Foundation; either version 3 of
66 * the License, or (at your option) any later version.
67 *
68 * This program is distributed in the hope that it will be
69 * useful, but WITHOUT ANY WARRANTY; without even the implied
70 * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
71 * PURPOSE. See the GNU General Public License for more details.
72 *
73 * You should have received a copy of the GNU General Public License
74 * along with this program; if not, write to the Free Software
75 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
76 * MA 02110-1301, USA.
77
78 * Bugs:
79 * {note_any_bugs_here}
80 *-
81 */
82
83 #include "pal.h"
84 #include "palmac.h"
85
palAirmas(double zd)86 double palAirmas ( double zd ) {
87 double seczm1;
88 double airmass;
89
90 /* Have maximum zenith distance of 87 deg */
91 const double MAXZD = 87.0 * PAL__DD2R;
92
93 zd = fabs(zd);
94 zd = ( zd > MAXZD ? MAXZD : zd );
95
96 seczm1 = (1.0 / cos(zd)) - 1.0;
97 airmass = 1.0 + seczm1*(0.9981833 - seczm1*(0.002875 + 0.0008083*seczm1));
98 return airmass;
99 }
100