#include "slalib.h"
#include "slamac.h"
double slaAirmas ( double zd )
/*
**  - - - - - - - - - -
**   s l a A i r m a s
**  - - - - - - - - - -
**
**  Air mass at given zenith distance.
**
**  (double precision)
**
**  Given:
**     zd     d     observed zenith distance (radians)
**
**  The result is an estimate of the air mass, in units of that
**  at the zenith.
**
**  Notes:
**
**  1)  The "observed" zenith distance referred to above means "as
**      affected by refraction".
**
**  2)  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.
**
**  3)  The sign of the ZD is ignored.
**
**  4)  At zenith distances greater than about ZD = 87 degrees the
**      air mass is held constant to avoid arithmetic overflows.
**
**  References:
**     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.
**
**  Adapted from original Fortran code by P.W.Hill, St Andrews.
**
**  Last revision:   5 October 1994
**
**  Copyright P.T.Wallace.  All rights reserved.
*/
{
   double w, seczm1;

   w = fabs ( zd );
   seczm1 = 1.0 / ( cos ( gmin ( 1.52, w ) ) ) - 1.0;
   return 1.0 + seczm1 * ( 0.9981833
                      - seczm1 * ( 0.002875 + 0.0008083 * seczm1 ) );
}