**ACTION:**- Air mass at given zenith distance (double precision).
**CALL:**`D = sla_AIRMAS (ZD)`

**GIVEN:**-

*ZD***D**observed zenith distance (radians)

**RETURNED:**-

*sla_AIRMAS***D**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 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.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk