/*
*+
* Name:
* palAirmas
* Purpose:
* Air mass at given zenith distance
* Language:
* Starlink ANSI C
* Type of Module:
* Library routine
* Invocation:
* double palAirmas( double zd );
* Arguments:
* zd = double (Given)
* Observed zenith distance (radians)
* Description:
* Calculates the airmass at the observed zenith distance.
* Authors:
* PTW: Patrick Wallace (STFC)
* TIMJ: Tim Jenness (JAC, Hawaii)
* {enter_new_authors_here}
* Notes:
* - The "observed" zenith distance referred to above means "as
* affected by refraction".
* - Uses Hardie's (1962) polynomial fit to Bemporad's data for
* the relative air mass, X, in units of thickness at the zenith
* as tabulated by Schoenberg (1929). This is adequate for all
* normal needs as it is accurate to better than 0.1% up to X =
* 6.8 and better than 1% up to X = 10. Bemporad's tabulated
* values are unlikely to be trustworthy to such accuracy
* because of variations in density, pressure and other
* conditions in the atmosphere from those assumed in his work.
* - The sign of the ZD is ignored.
* - At zenith distances greater than about ZD = 87 degrees the
* air mass is held constant to avoid arithmetic overflows.
* See Also:
* - Hardie, R.H., 1962, in "Astronomical Techniques"
* ed. W.A. Hiltner, University of Chicago Press, p180.
* - Schoenberg, E., 1929, Hdb. d. Ap.,
* Berlin, Julius Springer, 2, 268.
* History:
* 2012-03-02 (TIMJ):
* Initial version from the SLA/F version including documentation.
* Adapted with permission from the Fortran SLALIB library.
* {enter_further_changes_here}
* Copyright:
* Copyright (C) 1999 Rutherford Appleton Laboratory.
* Copyright (C) 2012 Science and Technology Facilities Council.
* All Rights Reserved.
* Licence:
* 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 3 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; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
* MA 02110-1301, USA.
* Bugs:
* {note_any_bugs_here}
*-
*/
#include "pal.h"
#include "palmac.h"
double palAirmas ( double zd ) {
double seczm1;
double airmass;
/* Have maximum zenith distance of 87 deg */
const double MAXZD = 87.0 * PAL__DD2R;
zd = fabs(zd);
zd = ( zd > MAXZD ? MAXZD : zd );
seczm1 = (1.0 / cos(zd)) - 1.0;
airmass = 1.0 + seczm1*(0.9981833 - seczm1*(0.002875 + 0.0008083*seczm1));
return airmass;
}