Air mass at given zenith distance (double precision).
D = sla_AIRMAS (ZD)
observed zenith distance (radians)
air mass (1 at zenith)
The observed zenith distance referred to above means “as affected by refraction”.
The routine uses Hardie’s (1962) polynomial fit to Bemporad’s data for the relative
air mass, ,
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
and better than 1% up to .
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
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.