Air mass at given zenith distance. (double precision)
zd d observed zenith distance (radians) The result is an estimate of the air mass, in units of that at the zenith.
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.
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. Original Fortran code by P.W.Hill, St Andrews This version by P.T.Wallace, Starlink, 30 October 1993