SLA_AIRMAS - Air Mass   

ACTION:

Air mass at given zenith distance (double precision).

CALL:

D = sla_AIRMAS (ZD)

GIVEN:

ZD observed zenith distance (radians)

RETURNED:

sla_AIRMAS air mass (1 at zenith)

NOTES:

1.

The observed zenith distance referred to above means ``as affected by refraction''.

2.

The routine 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 $\zeta = 87^{\circ}$ the air mass is held constant to avoid arithmetic overflows.

REFERENCES:

1.

Hardie, R.H., 1962, in Astronomical Techniques ed. W.A. Hiltner, University of Chicago Press, p180.

2.

Schoenberg, E., 1929, Hdb. d. Ap., Berlin, Julius Springer, 2, 268.